WO2007007899A1

WO2007007899A1 - Radio apparatus

Info

Publication number: WO2007007899A1
Application number: PCT/JP2006/314138
Authority: WO
Inventors: Yasuhiro Tanaka; Seigo Nakao
Original assignee: Sanyo Electric Co., Ltd.
Priority date: 2005-07-14
Filing date: 2006-07-11
Publication date: 2007-01-18
Also published as: CN101248697B; CN101248697A

Abstract

A control unit estimates the time required from when signals are transmitted respectively to a plurality of terminal apparatuses to when responses from them are received. In a plurality of partial periods, partial periods for receiving signals continue after partial periods for transmitting the signals continue, and the order of terminal apparatuses assigned in the partial periods for receiving signals are defined in the order of terminal apparatuses assigned in the partial periods for transmitting signals, and the control unit assigns a terminal apparatus, whose estimated required time is longer, to an early partial period in a series of partial periods for transmitting signals. A radio unit and the like perform communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned.

Description

DESCRIPTION

RADIO APPARATUS

, TECHNICAL FIELD

The present invention relates to radio apparatuses, and it particularly relates to a radio apparatus using multiple subcarriers.

RELATED ART

An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust- in the multipath environment. This OFDM modulation scheme has been applied to the wireless standards such as IEEE802.11a/g and HIPERLAN/2. The packet signals in such a wireless LAN are generally transferred via a time- varying channel environment and are also subjected to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.

In order for the receiving apparatus to carry out the channel estimation, 'two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carriers in the beginning of the packet signal, which is the so-called preamble or training signal. The other is the known signal, provided for part of carriers in the data area of the packet signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance) . Related Art List

- (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, "Channel Estimation Techniques Based on Pilo^'t Arrangement in OFDM Systems", IEEE Transactions on broadcasting, vol. 48, No.3, pp. 223-229, Sept. 2002. In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of ^' antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple- Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a plurality of packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in the packet signal is called "stream") . That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.

Moreover, combining such a MIMO system with the OFDM modulation scheme results in a higher data transmission rate. In such a MIMO system, CSMA (Carrier Sense Multiple Access) is. carried out to allow the base station apparatus to multiplex a plurality of terminal apparatuses. For the purpose of improving the transmission efficiency or reducing the processing delay, the base station apparatus specifies, in partial periods of time, the timing at which the signals are to be transmitted to a plurality of terminal apparatuses (hereinafter referred to as "transmit timing") and the timing at which the signals from a plurality of terminal apparatuses are to be received (hereinafter referred to as "receive timing") . -Then the base station apparatus informs respectively the plurality of terminal apparatuses of said specification, and each of. the plurality of terminal apparatuses carries out a processing in accordance with said specification (hereinafter, such a processing will be referred to as "assignment mode") . Here it is assumed that after a plurality of transmit timings for the plurality of terminals are specified consecutively, a plurality of receive timings are specified consecutively. A terminal apparatus receives a signal at the specified transmit timing. When the receiving has been successful, a terminal apparatus generates an ACK signal and transmits the ACK signal to the base station apparatus at the specified receive timing. ^■When the receiving has failed, the terminal apparatus does not generate the signal.

The inventor of the present invention came to realize _. the following problems to be solved under the circumstances as. described above. That is, even when a terminal apparatus has succeeded in receiving the signal, the terminal apparatus cannot transmit an ACK signal if it fails to generate the ACK signal at or before the receive timing. As a result, there will be a delay in the transmitting of the ACK signal, which will in turn cause also a delay in the subsequent processing at the base station apparatus. On the other hand, the processing speeds in a plurality of terminal apparatuses are generally not the same, and various processing speeds exist. Moreover, the number of streams in a packet signal to be received by a plurality of terminals differs. Generally speaking, the greater the number of streams, the more the amount of processing . involved in the receiving by a terminal apparatus will^' result and the longer the processing period will be.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the foregoing circumstances and an object thereof is to provide a radio apparatus which determines communication timings in such a manner as to realize efficient communication with a plurality of terminal apparatuses. In order to solve the above problems, a radio apparatus according to a preferred embodiment of the present invention comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit. The assigning unit includes: an estimation unit which estimates time required from when signals are transmitted respectively to the plurality of terminal, apparatuses to when responses therefrom are received; and an execution unit which assigns a terminal apparatus, whose required time estimated by the estimation unit is longer,, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals are defined in the order opposite to the order of terminal apparatuses assigned in the partial periods for transmitting signals. According to this embodiment, an early partial period in a series of partial periods for transmitting signals is assigned to a terminal apparatus where time required from the receiving of a signal to the transmission of a response signal is longer. Thus, the permissible period for a receiving processing in said terminal apparatus can be made longer.

Another preferred embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal ^• apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which ^'the respective plurality- of partial periods have been assigned by the assigning unit. The assigning unit includes: an estimation unit which estimates time required from when signals are transmitted respectively to the plurality of terminal apparatuses to when responses therefrom are received; . and an execution unit which assigns a terminal apparatus, whose required time estimated by the estimation unit is longer, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue .after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According- to this embodiment, the order for the assignment of receive timings is set identical to the order for the assignment of transmit timings, so that the processing can be simplified.

Still another preferred embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses^'; .and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit., The assigning unit includes: an estimation unit which estimates time required from when signals are transmitted respectively to the plurality of terminal apparatuses to when responses therefrom are received; and an execution unit which estimates time required, in each order, from when a signal is transmitted to the terminal apparatus to when a response therefrom is received and which assigns a terminal apparatus, whose required time estimated by the estimation unit is longer, to a partial period corresponding to the 'order in which the time required is longer wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According to this embodiment, a terminal apparatus with a longer processing period from the receiving of a signal to the transmission of a response signal is assigned to the transmit timing at which a receive-transmit period is longer. Hence, the possibility that said terminal apparatus can transmit the signal can be raised.

Still another preferred, embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit. The assigning unit includes: an identifying unit which identifies processing speeds for the respective plurality of terminal apparatuses; and an execution unit which' assigns a terminal apparatus, identified by the identifying unit, whose processing speed is low, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial^' periods for receiving signals is defined in the order opposite to the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According to this embodiment, a terminal apparatus whose processing speed is slow is assigned to an early partial period in a series of partial periods for transmitting signals^'. Thus, the permissible period for a receiving processing in said terminal apparatus can be made longer.

Still another preferred embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit.' The assigning unit includes: an identifying unit which identifies processing speeds for the respective plurality of terminal apparatuses; and an execution unit which assigns a terminal apparatus, whose processing speed identified by the identified unit is low, to an early partial period. in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial ^' periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According to this embodiment, the order for the assignment of receive timings is set identical to the order for the assignment of transmit timings, so that the processing can be simplified.

Still another preferred embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit. The assigning unit includes: an identifying unit which identifies processing speeds for the respective plurality of terminal apparatuses; and an execution unit which estimates time required, in each order, from when a signal is transmitted to a terminal apparatus to when a response therefrom is received and. which assigns a terminal apparatus, whose processing speed identified by the identified unit is low, to a partial period corresponding to the order in which the time required is longer . wherein, in the plurality of partial periods, partial periods for -receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is ■ defined in the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According to this embodiment, a terminal apparatus whose processing speed is low is assigned to the transmit timing at which a recei.ve-transmit period is longer. Hence, the possibility that said terminal apparatus can transmit the signal can be raised.

The identifying unit may include: a measurement unit which measures time 'periods from when signals are transmitted respectively to the plurality of terminal apparatuses to when responses to the signals are received, respectively; and an execution . unit which identifies processing speeds, based on the time periods measured by the measurement unit. In this case, the time required from the receiving of a signal until the transmission of a response signal is identified, so that the allocation in accordance with a CPU or the like of terminal apparatus can be realized.

The identifying unit may include: a reception unit which receives information on the processing speeds from the respective plurality of terminal apparatuses; and an execution unit which identifies the processing speeds, based on the information received by the reception unit.

Still another preferred embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication, using at least one stream, with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit. The assigning unit includes: an identifying unit which identifies the number of streams for each of the plurality of terminal apparatuses; and an execution unit which assigns a terminal apparatus, whose number of streams identified by the identifying unit is large, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue . and wherein the. order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order opposite to the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According to this embodiment, a terminal apparatus whose number of streams is large is assigned to an early partial period in a series of partial periods for transmitting signals. Thus, the permissible period for a receiving processing in said terminal apparatus can be made longer.

Still another preferred embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit. The assigning unit includes: an identifying unit which identifies the number of streams for each of the plurality of terminal apparatuses; and an execution unit which assigns a terminal apparatus, whose number of streams identified by the identifying unit is large, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According to this embodiment, the order for the assignment of receive timings is set identical to the order for the assignment of transmit timings, so that the processing can be simplified. Still another preferred embodiment of the present invention relates also to a radio apparatus. This apparatus comprises: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by the assigning unit. The assigning unit includes: an identifying unit which identifies the number of streams for each of the plurality of terminal apparatuses; and an execution unit which estimates time required, in each order, from when a signal is transmitted to the terminal apparatus to when a response therefrom is received and which assigns a terminal apparatus, whose number of streams identified by the identified unit is larger, to a partial period corresponding to the order in which the time required is longer wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order of terminal apparatuses assigned in the partial periods for transmitting signals.

According to this embodiment, a terminal apparatus whose number of streams is large is assigned to the transmit timing at which a receive-transmit period is longer. Hence, the possibility that said terminal apparatus can transmit the signal can be raised.

In the partial periods for receiving signals the communication unit may receive a response to a signal transmitted in the partial periods for transmitting signals from the terminal apparatus, .

Data may be composed of a plurality of streams. A known signal may be composed of a plurality of streams. A control signal may be composed of a plurality of streams. It is to be noted that any arbitrary combination of the above-described structural components and expressions changed among a method, an apparatus, a system, a recording medium, a computer program and so forth are all effective as and encompassed. by the present embodiments. . Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be sub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of examples only, with reference to the accompanying drawings which are meant to be exemplary, not limiting and wherein like elements are numbered alike in several Figures in which: FIG. 1 illustrates a spectrum of a multicarrier signal according to an embodiment, of the present invention.

FIG. 2 ^■ illustrates a structure of a communication system according to an embodiment of the present invention.

FIGS. 3A and 3B illustrate packet formats in the communication system shown in FIG. 2.

FIG. 4 illustrates a structure of a first radio apparatus shown in FIG. 2.

FIG. 5 illustrates a structure of a frequency-domain signal shown in FIG. 4. FIG. 6 illustrates a structure of a baseband processing unit shown in FIG. 4. FIG. 7 illustrates an outline of timing assignment in the communication system shown in FIG. 2.

FIG. 8 is a sequence diagram showing a procedure for specifying processing speed in the communication system shown in FIG. 2.

FIG. 9 illustrates an outline of another modification of timing assignment in the communication system shown in FIG. 2.

FIG. 10 illustrates an outline of still another modification of timing assignment in the communication system shown in FIG. 2..

FIG. 11 illustrates packet formats in the communication system shown in_. FIG. 2;

FIG. 12 illustrates a structure of IF unit and modem unit shown in FIG. 4;

FIG. 13 illustrates another structure of IF unit and modem unit shown in FIG. 4;

FIGS. 14A and 14B illustrate packet formats according to a modification of the present invention; FIGS. 15A tb 15D illustrate packet formats in the communication system of FIG. 2;

FIGS. 16A and 16B illustrate another packet formats in the communication sy'stem of FIG. 2;

FIGS. 17A to 17C illustrate packet formats for training signals in the communication system of FIG. 2;

FIG. 18 illustrates a structure of a baseband processing unit shown in FIG. 4;

FIG. 19 illustrates a structure of a receiving processing unit shown in FIG. 18;

FIG. 20 illustrates a structure of a transmitting processing unit shown in FIG. 15;^' and

FIG. 21 illustrates a packet format of packet signal finally transmitted by the communication system of FIG. 2.

DETAILED DESCRIPTION The invention will now be described based on the following embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiments are- hot necessarily essential to the invention.

First embodiment

An outline of the present invention will be given before describing a detail description thereof. Embodiments of the present invention relate t.o a MIMO system comprised of a plurality of radio apparatuses. One of the radio apparatuses corresponds to a base station apparatus whereas the rest thereof correspond to a plurality of terminal apparatuses. The base station apparatus basically performs CSMA on a plurality of terminal apparatuses. Over a certain period of time, the base station apparatus performs an assignment mode. Under such circumstances, the terminal apparatus receives signals at transmit timing, and the base station apparatus performs processing as follows in order that the terminal apparatus can receive the signals at the transmit timing. and it can generate an ACK signal before or at the receive timing.

Before executing a specification, the base station apparatus identifies the respective processing speeds of a plurality of terminal apparatuses and carries out an assignment mode, which reflects the identified processing speeds. To be more precise, the base station apparatus specifies a plurality of receive timings consecutively after specifying a plurality of transmit timings consecutively. Moreover, the base station apparatus specifies the receive timings for the terminal apparatuses in an order opposite to the order in which the transmit timings have been specified to the terminal apparatuses. In other words, if the terminal apparatuses .are, for instance, -denoted by "1" to "3", the base station apparatus specifies the transmit timings in the order of- "1" to "3" and then specifies the receive timings in the order of "3" to ^λλl". In doing so, the base station apparatus assigns an earlier transmit timing to a terminal apparatus with a lower processing speed. As a result, the duration from a transmit timing to a receive timing will be longer for a terminal apparatus with a lower processing speed.

FIG. 1 illustrates a spectrum of a multicarfier signal according to an embodiment of the present invention. In particular, FIG. 1 shows a spectrum of a signal in the OFDM modulation scheme. One of a plurality of carriers in an OFDM modulation -scheme is generally called .a subcarrier. Herein, however, a subcarrier is designated by a "subcarrier number". In a MIMO system, 56 subcarriers, namely, subcarrier numbers "-28" to "28" are defined. It is to be noted that the subcarrier number "0" is set to null so as to reduce the effect of a direct current component in a baseband signal. On the other hand, 52 subcarriers, namely, subcarrier numbers "-26" to "26" are defined in a communication system which is not compatible with a MIMO (such a communication systems as this -will be hereinafter referred to as a legacy- system) . One example of legacy systems is a wireless LAN complying with the IEEE802.11a standard.

The respective subcarriers are modulated by a modulation scheme which is set variably. Used here is any of modulation schemes among BPSK (Binary Phase-Shift Keying) , QPSK (Quadrature Phase-Shift Keying) , 16-QAM (Quadrature Amplitude Modulation) and 64-QAM.

Convolutional coding is applied, as an error correction scheme, to these signals. The coding rates for the convolutional coding are set to 1/2, 3/4 and so forth. The number of data to be transmitted in parallel is set variably. The data are transmitted as packet signals and each of packet signals to be transmitted in parallel is called "stream" herein. As a result thereof, since the mode of modulation scheme, the values of coding rate and the number of. streams are set variably, the data rate is also set variably. It is to be noted that the "data rates" may^¬ be determined by arbitrary combination of these factors or by one of them.

FIG. 2 illustrates a structure of a communication system 100 according to an embodiment of the present invention. The communication system 100 includes a first radio apparatus 10a and a second radio apparatus 10b, which are generically referred to as "radio apparatus 10". The first radio apparatus 10a includes a first antenna 12a, a second antenna 12b, . a third antenna 12c and a fourth antenna 12d, which are generically referred to as "antennas 12", and the second radio apparatus_. 10b includes a first antenna 14a, a ^"second antenna 14b, a third antenna 14c and a- fourth antenna 14d, which are generically referred to as "antennas 14". Here, the first radio apparatus 10a corresponds to a base station apparatus, whereas the second radio apparatus 10b corresponds to a terminal apparatus. The first radio apparatus 10a may connect to a plurality of terminal apparatuses, not shown. Here the plurality of terminal apparatuses not shown are represented by the third radio apparatus 10c, fourth radio apparatus 1Od and the like.

When connecting with the plurality of terminal apparatuses, the first radio apparatus 10a basically performs CSMA. As described earlier, the first radio apparatus 10a also performs an assignment mode. The assignment mode will be discussed later- in detail. ^■ , An outline of a MIMO system is given before a description of a structure of the communication system 100. Assume herein that data are being transmitted from the first radio apparatus 10a to the second radio apparatus 10b. The first radio apparatus 10a transmits respectively data of multiple streams from the first antenna 12a to the fourth antenna 12d, respectively. As a result, the data rate becomes higher. The second radio apparatus 10b receives the data of multiple streams by the first antenna 14a to the fourth antenna 14d.- The second radio apparatus 10b separates the received signals by adaptive array signal processing and demodulates independently the data of multiple streams.

Since the number of antennas 12 is "4" and the number of antennas 14 is also- "4" here, the number of combinations of channels between the antennas 12 and the antennas 14 is "16". The channel characteristic between from the ith antenna 12i to the jth antenna 14j is denoted by hi_j . In FIG. 2, the channel characteristic between the first antenna 12a and the first antenna 14a is denoted by h_u, that between the first antenna 12a and the second antenna 14b by h₁₂, that between the second antenna 12b and the first antenna 14a by h.₂i_/ that between the second antenna 12b and the second antenna 14b by h₂2^ and that between the fourth antenna 12d and the fourth antenna 14d by h₄₄. For the clarity of illustration, it is omitted to show the other channels in FIG.. 2.

FIGS. 3A and 3B illustrate packet formats used by a communication system 100. FIG. 3A shows packet formats in which preamble signals corresponding to a MIMO system are placed at the top portion. Here it is assumed that data contained in two streams are to be transmitted, and a packet format corresponding to a first stream is shown in the top row and that corresponding to a second stream in the bottom row. In the packet signal corresponding to the first stream, "STSl" and "LTSl" are assigned as preamble signals, whereas in the packet signal corresponding to the second stream, "STS2" and "LTS2" are assigned as preamble signals. Here, "STSl" and "STS2", as well as "LTSl" and "LTS2", are signals having different patterns from each other.

FIG. 3B shows packet formats in which a preamble signal corresponding to a legacy system is additionally placed anterior to a preamble signal corresponding to a MIMO system. Here the STS and LTS of the preamble signal corresponding to a ^'legacy system are denoted as "L-STS" and "L-LTS", respectively, in the first stream. On the other hand, "L-STS" and the like are assigned in the second stream as well. In so doing, the "L-STS" and the like in the second stream are ones to which a CDD (Cyclic Delay

Diversity) processing has been applied, for instance. In other words, the L-STS assigned to the second stream is equal to the L-STS assigned to the first stream which has been given a cyclic timing shift. Here, as shown in the bottom row of FIG. 3B, the L-STS having been subjected to a CDD processing is denoted by "L-STS + CDD". The same applies to a case where "L-STS" and the like are assigned to a third stream and the like. A "Signal" is placed between the preamble signal corresponding to a legacy system and the preamble signal corresponding to a MIMO system. The "Signal" contains information indicating that --a preamble signal corresponding to a MIMO system is assigned posterior thereto. Accordingly, when a communication apparatus of a legacy system has received this packet signal, the communication apparatus may discard this packet signal from the content of the "Signal". The information indicating the assignment of such a preamble signal may be the length of a packet signal. That is, it is only necessary that some signal can be determined to last for a certain period of time. Either of the packet formats shown in FIGS. 3A and 3B may be used. The packet format of FIG. 3A, which has less of redundant signal components, can improve the utilization efficiency. On the other hand, the packet format of FIG. 3B, with the addition of a preamble signal corresponding to a legacy system, allows the detection thereof by a communication apparatus corresponding to a legacy system.

FIG. 4 illustrates a structure of a first radio apparatus 10a. ^'The first radio apparatus 10a includes a first radio unit 20a, a second radio unit 20b, ... and a fourth radio unit 2Od, which are generically referred to as "radio unit 20", a baseband processing unit 22, a modem unit 24, an IF unit 26 and a control unit 30. Signals involved include a first time-domain signal 200a, a second time- domain signal 200b, ... and a fourth time-domain signal 20Od, which are generically referred to a's "time-domain signal 200", and a first frequency-domain signal 202a, a second frequency-domain signal 202b, a third frequency-domain signal 202c and a fourth frequency-domain signal 202d, which are generically referred to as "frequency-domain signal 202", It is to be noted that the second radio apparatus 10b is so structured as to correspond to the first radio apparatus 10a,

As a receiving operation, the radio unit 20 carries out frequency conversion of radiofrequency signal received by the antennas 12 so as to derive baseband signals. The radio unit 20 outputs the baseband signals to the baseband processing unit 22 as the time-domain signals 200. The baseband signal, which is composed of in-phase components and quadrature components, shall generally be transmitted by two signal lines. For the clarity of figure, the baseband signal is presented here by a single signal line^' only. An AGC unit and an A-D conversion unit are also included.

As a transmission operation, the radio unit 20 carries out frequency conversion of baseband signals from the baseband processing unit 22 so as to derive radiofrequency signals. Here, the baseband signal from the baseband processing unit 22 is also indicated as the time-domain signal 200. The radio unit 20 outputs the radiofrequency signals to the antennas 12. A PA (Power Amplifier) and a D- A conversion unit are also included. It is assumed herein that the time-domain signal 200 is a multicarrier signal converted to the time domain and is a digital signal.

As a receiving operation, the baseband processing unit 22 converts a plurality of time-domain signals 200 respectively into the frequency domain and performs adaptive array signal processing on the thus converted frequency- domain signals. Then the baseband processing unit 22 outputs the result of adaptive array signal processing as the frequency-domain signals 202. One frequency-domain signal 202 corresponds ^• to data contained respectively in a plurality of streams transmitted from the second radio apparatus 10b, not shown here. As a transmission operation, the baseband processing unit 22 inputs, from the modem unit 24, the frequency-domain signals 202 serving as signals in the frequency domain, converts the frequency-domain signals into time domain and then outputs the thus converted signals as time-domain signals by associating them respectively to a plurality of antennas 12.

It is assumed that the number of antennas 12 to be used in the transmission processing is specified by the control unit 30; It is assumed herein that the frequency- domain signal 202, which is a signal in the frequency domain, contains a plurality of subcarrier components as shown in FIG. 1. For the clarity of figure, the frequency-domain signal is arranged in the order of the subcarrier numbers, and forms serial signals. FIG. 5 illustrates a structure of a frequency-domain signal. Assume herein that a combination of subcarrier numbers "-28" to "28" shown in FIG. 1 constitutes an "OFDM symbol". An "i"th OFDM symbol is such that subcarriers components are arranged in the order of subcarrier numbers "1" to "28" and subcarrier numbers "-28" to "-1". Assume also that an "(i-1) "th OFDM symbol is placed before the "i"th OFDM symbol", and an "(i+1) "th OFDM symbol is placed after the "i"th OFDM symbol.

Now refer back to FIG. 4. The baseband processing unit 22 performs CDD to generate packet signals corresponding to FIG. 3B. CDD is performed as a matrix C expressed by the following Equation (1) .

CCO = dia(l,exp(-j2π^δ/Nout),•••,exp(-j2π^δ(Nout-1)/Nout))— (1)

where δ indicates a shift amount and i indicates the subcarrier number. The multiplication of C with streams is carried out per subcarrier. That is, the baseband processing unit 22 performs a cyclic time shifting within L-

STS or the like on a stream-by-stream basis. The shift amounts ^'are each set to a different value per stream.

As a receiving processing, the modem unit 24 demodulates and decodes the frequency-domain signal 202 outputted from the baseband processing unit 22. The demodulation and decoding are carried out per subcarrier. The modem unit 24 outputs the decoded signal to the IF unit 26. As a transmission processing, the modem unit 24 carries out coding and modulation. The modem unit 24 outputs the modulated signal to the baseband processing unit 22 as a frequency-domain signal 202. When 'the transmission processing is carried out, the modulation scheme and coding rate are specified by the control unit 30. As a receiving processing, the IF unit 26 combines signals outputted from a plurality of modem units 24 and then forms one data stream. The IF unit 26 outputs the data stream. As a transmission processing, the IF unit 26 inputs one data stream and then separates it. Then the IF unit 26 outputs the thus separated data to the plurality of modem units 24.

The control unit 30 controls the timing and the like of the first radio apparatus 10a. When it multiplexes a plurality of terminal apparatuses, the control unit 30 performs CSMA. CSMA is a known technology and the description thereof is omitted here. In addition to CSMA, the control unit 30 performs an assignment mode. The assignment mode is performed in a given period. Before performing the assignment mode, the control unit 30 notifies a plurality of terminal apparatuses of the start of assignment mode via the baseband processing unit 22 and the like. In addition to the terminal apparatus contained in the assignment mode, terminal apparatuses which are not contained in the assignment mode are also counted as those which receive the notice of start. In the assignment mode, the control unit 30 transmits control information in a leading portion. Following the control information, packet signals to be sent to a plurality of terminal apparatuses are assigned. The packet signals to be assigned are formed, for instance, by a series of a plurality of packet signals. Also, at least one of the packet signals to be assigned is time-divided, and the time- divided parts may be assigned to the terminal apparatuses, respectively. In either structure, the control unit 30 allocates partial periods, for transmitting signals to a plurality of terminal apparatuses. It is to be noted that the partial periods may show individual periods for individual terminal apparatuses or show integrally a period for a plurality of terminal apparatuses. Herein, however, no distinction is made as to the mode of. assignment of partial periods .

Furthermore, following the partial periods ^' for transmitting signals, the control unit 30 assigns partial periods for receiving signals from a plurality of terminal apparatuses, respectively. The terminal apparatuses transmit packet- signals to a first radio apparatus 10a in their respectively assigned partial periods. For this, a plurality of packets are assigned consecutively. That is, the control unit 30 divides a given period into a plurality of partial periods and assigns the plurality of respective partial periods correspondingly to a plurality of terminal apparatuses. It is to be noted that the control signal contains information indicating the correspondence between the partial periods and the terminal apparatuses.

Here, a description is- given of a method for assigning and allocating terminal apparatuses to the partial periods. Before executing^' an^' "assignment mode, the control unit 30 identifies respective processing speeds for a plurality of terminal apparatuses. Here the processing speed is a concept that includes the speed of processing by a terminal apparatus from its receiving a packet signal and generating an ACK signal until its transmitting the ACK signal. Generally, the processing speed is dependent on the processing speed of CPU included in the terminal apparatus and so forth. The determination or identifying of the ^•processing speeds by the control unit 30 is carried out as follows. The control unit 30 transmits predetermined packet signals respectively to a plurality of terminal apparatuses via a baseband processing unit 22 or the like. The predetermined packet signals are transmitted as normal data at the time of CSMA.

The control unit 30 measures the period from its transmitting a packet signal to its receiving an ACK signal corresponding to said packet signal for each of the terminal apparatuses. The control unit 30 identifies a processing speed based on the period thus measured. For example, the control unit 30 identifies a terminal apparatus with a shorter measured period as a terminal apparatus with a higher processing speed. It is to be noted that the control unit 30 may process the measured periods statistically and identify the processing speeds based on the periods processed statistically. Such determination is equivalent to the estimation of the required period from the transmission of respective signals to a plurality of terminal apparatuses to the receiving of their ACK signals. In the assignment mode, as described above, there are a plurality of partial periods, which are composed of a series of partial periods for transmitting signals and a subsequent series of partial periods for receiving signals. A "series of" or "being contiguous" meant here is not a series without breaks between partial periods, but is rather a series with breaks to which no other partial period having another function is assigned. What is meant here, in other words, is a series that has no partial period for receiving a signal assigned to the break between partial periods for transmitting signals. It is to be noted that the order of the terminal apparatuses assigned to the partial periods for receiving signals is so defined to be opposite to the order 5 of' the terminal apparatuses assigned to the partial periods for transmitting signals. As described earlier, for connecting with three terminal apparatuses designated as the second radio apparatus 10b to the fourth radio apparatus 1Od, the base station apparatus specifies transmit timings in the

10 order of the second radio apparatus 10b to the fourth radio apparatus" 1Od and specifies receive timings in the order of the fourth radio apparatus 1Od to the second radio apparatus 10b.

Moreover, the^' control unit 30 assigns terminal

15. apparatuses with lower processing speeds to the earlier or front periods of the series of partial periods for transmitting signals,. -That- is, the previous example is a case where the second radio apparatus 10b is the terminal apparatus with the lowest processing speed. By an

20 assignment like this, the second radio apparatus 10b has a longer period from its receiving a packet signal and generating an ACK signal to its transmitting the ACK signal. It is to be noted here that the terminal apparatuses transmit their ACK signals in the partial periods when they

25 are to transmit signals. The baseband processing unit 22 and the like of the first radio apparatus 10a receive the ACK signals. Upon recognizing the receipt of the ACK signals, the control unit 30 has the baseband processing unit 22 and the like prepare the next packet signals to be transmitted to the terminal -apparatuses. . In terms of hardware, this structure can be realized by a CPU, a memory and other LSIs of an arbitrary computer. In terms of software, it is realized by memory-loaded programs which have communication functions and the like, but drawn and described herein are function blocks that are realized in cooperation with those. Thus, it is understood by those skilled in the art that these function blocks can be realized in a variety of forms such as by hardware only, software only or the combination thereof.

FIG. 6 illustrates a structure of a baseband processing unit 22. The baseband processing unit 22 includes a processing unit for use with receiving 50 and a processing unit for use with transmission 52. The receiving processing unit 50 executes a part, corresponding to a receiving operation, of operations by the baseband processing unit 22. That is, the receiving processing unit 50 performs adaptive array signal processing on time-domain signals 200 and, for this purpose, derives receiving weight vectors. Then the receiving processing unit 50 outputs the result of array synthesis as the frequency-domain signal 202 A processing of receiving processing unit 50 will now be described in a specific manner. The receiving processing unit 50 inputs a plurality of time-domain signals 200 and then performs Fourier transform on them, respectively, so as to derive frequency-domain signals. As described earlier, a frequency-domain signal is such that signals corresponding to^' subcarriers are arranged serially in the order of subcarrier numbers .

The receiving processing unit 50 weights the frequency-domain signals with receiving weight vectors, and a plurality of weighted signals are added up. Since the frequency-domain signal is composed of a plurality of ■ subcarriers, the above processing is also executed on a subcarrier-by-subcarrier basis. As a result, the signals summed up are also arranged serially, as shown in FIG. 5, in the order of subcarrier numbers. The signals summed up are the aforementioned frequency-domain signals 202.

The receiving processing unit 50 derives receiving weight vectors by use of an adaptive algorithm, for example, LMS algorithm. Alternatively, receiving response vectors are derived by correlation processing and then the receiving weight vectors may be derived from the receiving response vectors. Here, the latter case will be described. If a frequency-domain signal corresponding to the first time- domain signal 200a is denoted by xχ(t), a frequency-domain signal corresponding to the second time-domain signal 200b by X₂ (t), a reference signal in the first stream by Si (t) and a reference signal in the second stream by S₂ (t), then Xχ(t) and X2(t) will be expressed by the following Equation (2)

The noise is ignored here. A first correlation matrix Ri, with E as an ensemble average, is expressed by the following Equation (3) :

A second correlation matrix R₂ among the reference signals is given by the following Equation (4) .

E[S₁S₁ ] E[S₁S₂]

R₉ = — (4: [E[S₂Sj] E[S₂S₂]

Finally, the first correlation matrix Ri is multiplied by the inverse matrix of the second correlation matrix R₂ so as to derive a receiving response vector, which is expressed by the following Equation (5) .

Then the receiving processing unit 50 computes a receiving weight vector from the receiving response vector.

The transmitting processing unit 52 executes a part, corresponding to a transmission operation, of operations by the baseband processing unit 22. The transmitting processing unit may perform beamforming or eigenmode transmission. These are known techniques and therefore the description thereof is omitted here.

FIG. 7 illustrates an outline of timing assignment in a communication system 100. More specifically, FIG. 7 shows the timings assigned by the control unit 30 and shows the transmit timing of packet signals by the first radio apparatus 10a to fourth radio apparatus 1Od. The transmit timing of packet signal by the second radio apparatus 10b to fourth radio apparatus 1Od are indicated as the receive timing of packet signal in the first radio apparatus 10a. Here, "controlling timing period", "transmit timing period" and "receive timing period" are placed in this order with a focus on the processing by the first radio apparatus 10a. The control unit 30 has already identified, as the processing speed of terminal apparatuses, that the second radio apparatus 10b is slowest and the fourth radio apparatus 1Od is fastest.

The control unit 30 transmits control signals in a controlling timing period. In a transmit timing period, the control unit 30 assigns the transmit timing in the order of "Data 2", "Data 3" and "Data 4". Here, "Data 2" is the data transmitted from the first radio apparatus 10a to the second radio apparatus 10b. "Data 3" is the data transmitted from the first radio apparatus 10a to the third radio apparatus 10c. "Data 4" is the data transmitted from the first radio apparatus 10a to the fourth radio apparatus 1Od. That is, as described above, the control unit 30 assigns the leading or earlier transmit timing to the second radio apparatus 10b whose processing speed is lowest. Here, the respective "Data 2", "Data 3" and "Data 4" may have burst formats shown in FIGS. 3A and' 3B, respectively. In other words, these may be ^' formed as distinct packet signals.

"Data 2", "Data 3" and "Data 4" may be contained in a part where "Data 1" and "Data 2" are time-shared among the burst format shown in FIGS. 3A and 3B. In other words, these may be formed as a single packet signal. In a receive timing period, the control unit 30 assigns the receive timing in the order opposite to the order of the terminal apparatuses that have transmitted data. That is, the control unit 30 assigns "Data 4'" and "ACK", which are the packet signals transmitted from the fourth radio apparatus 1Od, to the beginning of the receive timing period.

Following this, the control unit 30 assigns "Data 3'" and "ACK" which are the packet signals from the third radio apparatus 10c. Finally, the control unit 30 assigns "Data 2'" and "ACK" which are the packet signals from the second radio apparatus 10b. It is to be noted "ACK" alone may be transmitted.

FIG. 8 is a sequence diagram showing a procedure for specifying processing speed in a communication system 100. The first radio apparatus 10a transmits a packet signal to the second radio apparatus 10b (SlO) and, at the same time, starts a timer (S12) . When the second- radio apparatus 10b receives the packet signal, the second radio apparatus 10b ^■ generates an ACK signal (S14). The second radio apparatus 10b transmits the ACK signal to the first radio apparatus 10a (Slβ) . When the first radio apparatus 10a receives the ACK. signal, the timer is stopped (S18) so as to measure the time from its transmitting the packet signal to its receiving the ACK signal. Then the first radio apparatus 10a identifies the processing speed, based on the thus measured period. It is to be noted that the above processing is performed on not only the second radio apparatus 10b but also the third radio apparatus 10c and the like.

A modification will now be described. Thus far, the control unit 30 specifies^' respectively processing speeds of a plurality of terminal apparatus, as time required from when the signals are sent -respectively to the plurality of terminals and to when the ACK signals are received. Moreover, the control unit allocates transmit timings according to the processing speeds. Here, the number of streams to be transmitted to a plurality of terminal apparatuses are specified respectively. If the number of streams contained in a packet signal is large at the time when a terminal apparatus receives the packet signal, the period during which said packet signals are being processed by the terminal apparatus will be generally longer. This is because the receiving processings for a plurality of streams need to be carried out concurrently. Hence, irrespective of the processing speed of terminal apparatus, the control unit 30 assigns terminal apparatuses, having a large number of streams to be transmitted, in a earlier period of a series of^' partial periods for transmitting signals. The second radio apparatus 10b shown in Fig.7 corresponds to the terminal apparatus with a large number of streams to be transmitted.

According to the present embodiments of the present invention, the terminal apparatuses having the longer required time from its receiving the signals to their transmitting ACK signals are assigned in an front period of a series of partial periods -for transmitting signals. Thus, the permissible period for a receiving processing in said terminal apparatus can be made longer. Since the permissible period can be made longer, the probability that the ACK signal can be produced in time for when the ACK signal must be sent can be raised. Since the base station apparatus receives the -ACK signal earlier, the base station apparatus can perform the subsequent processing earlier.

The base station apparatus can so determine the timing as to efficiently communicate with a plurality of terminal apparatuses. The processing speed is identified as the required time from when the signal is received to when the ACK signal is transmitted. Thus, the allocation in accordance with a CPU or the like of terminal apparatus can be realized. Since the processing speed of terminal apparatus is measured in the midst of data communication, the increase in signals to be transmitted can be restricted. Since the increase in signals to be transmitted can be suppressed, the transmission efficiency can be improved.

Next, a description of another modification is given below. In the assignment mode of this modification, partial periods for transmitting signals and partial periods for receiving signals are assigned the same way as for the above-described modification. In this modification, however, the base station apparatus specifies the receive timings for the terminal apparatuses in an order equal to the order in which the transmit timings have been specified to the terminal apparatuses. In other words, if the terminal apparatuses are, for instance, denoted by "1" to "3", the base station apparatus specifies the transmit timings in the order of ^λλl" to "3" and then specifies the receive timings also in the order of "1" to ^λλ3". The period during which signals are being transmitted from the base station apparatus is generally longer than the period during which signals are being received by the base station apparatus. Hence, a terminal apparatus assigned for an earlier transmit timing can have a longer duration from its receiving a signal to its transmitting a signal. Accordingly, the base station apparatus assigns earlier transmit timings to the terminal apparatuses for which the required duration from their transmitting a signal to their receiving a response is longer.

The structure of a radio apparatus 10 according to this modification is of the same type as the radio apparatus 10 of FIG. 4. What is different therefrom is the processing at the control unit 30, which is explained here. The control unit 30 defines the order of terminal apparatuses to be assigned to the partial periods for receiving signals to be the same as the order of terminal apparatuses to be assigned to the partial periods for transmitting signals. Furthermore, the control unit 30 assigns terminal apparatuses with lower processing speed to the earlier partial periods in the series of partial periods for transmitting signals. FIG. 9 is an illustration for explaining the outline of the another modification of the assignment of timings in a communication system 100. FIG. 9 is the same type of illustration as FIG. 7. A control unit 30 transmits a control signal in the controlling timing period. The control unit 30 also assigns transmit timings in the order of "Data 2", "Data 3" and "Data 4" in the transmit timing period. As mentioned earlier, the control unit 30 assigns the leading transmit timing to a second radio apparatus 10b which has the lowest processing speed. The control unit 30 assigns receive timings, in the order of the terminal apparatuses to which data have been transmitted, ^' in the receive timing period. That is, the control unit 30 assigns

"Data 2'" and "ACK" in the beginning of the receive timing period. Following that, the control unit 30 assigns "Data . 3'" and "ACK". -Finally, the control unit 30 assigns "Data 4'". and "ACK".

In a modification like this, the control unit 30 specifies the respective processing speeds for a plurality of terminal apparatuses by way of a required duration from transmission of the respective signals to a plurality of terminal apparatuses to the receiving of the ACK signals. Furthermore, the control unit 30 assigns transmit timings according to the processing speeds. Otherwise, the control unit 30 may specify the respective numbers of streams to be transmitted to a plurality of terminal apparatuses, as a required duration from transmission of respective signals to the plurality of terminal apparatuses to receiving of the ACK signals. In so doing, the control unit- 30 assigns the terminal apparatuses with larger numbers of streams to be .transmitted to the earlier periods in the series of partial periods for transmitting signals, irrespectively of the processing speeds of the terminal apparatuses. In other words, the second radio apparatus 10b shown in FIG. 9 is equivalent to a terminal apparatus with a larger number of streams to be transmitted. Next, a description of still another modification is given below. In the assignment mode of this modification, partial periods for transmitting signals and partial periods for receiving signals are arranged the same way as for the above-described modifications. The receive timings for the. terminal apparatuses are specified in an order equal to the order in which the transmission timings have been specified to the terminal apparatuses. That is, once the order of transmit timings is determined, the order of receive timings is also determined. The following point, however, differs from the embodiment and modifications described above. The base station apparatus changes the combination of the order of terminal apparatuses to which the order of transmit timings is to.be assigned, and estimates the period from the end of transmit timing for the terminal apparatuses to the start ' of receive timing (hereinafter referred to as "receive-transmit period") for each of the combinations.

Generally speaking, the length of a packet signal transmitted by a base station apparatus and the length of a packet signal received thereby vary with the terminal apparatus. Hence, a change in the order of terminal apparatuses results in a change in the receive-transmit period for each of the terminal apparatuses. The base station apparatus specifies a combination, from among a variety of combinations, that provides a longer receive- transmit period. In so doing, the base station apparatus assigns transmit timings in such a manner that terminal apparatuses with longer receive-transmit periods may be terminal apparatuses with lower processing speeds.

The structure of a radio apparatus 10 according to this modification is of the same type as the radio apparatus 10 of FIG. 4. What is different herein is the processing at the., control unit 30, which is explained below. The control unit 30 estimates the receive-transmit period for each of different orders. To facilitate the explanation thereof, two terminal apparatuses are assumed and they are denoted by "1" and "2", respectively. The control unit 30 deals with a combination of ^λλl" and "2" (hereinafter referred to as "first combination") as the order of assignment of transmission timings. In so doing, the control unit 30 derives the receive-transmit period for each of the terminal apparatuses ^Λλl" and^' "2". It is to be noted here that the partial periods for transmitting signals and the partial periods for receiving signals for each of the terminal apparatuses "1" and "2" are recognized beforehand. Following this, the control unit 30 addresses a combination of "2" and "1" (hereinafter referred to as "second combination") as the order of assignment of transmission timings .

In so doing, the control unit 30 derives the receive- transmit period for^' each of the terminal apparatuses ^λλ2" and "1". It is to be noted also that by a similar processing as in the embodiment, the control unit 30 acquires the respective processing speeds of terminal apparatuses "1" and "2" and is in recognition of the processing speed of terminal apparatus "1" being slower. The control unit 30 compares the receive-transmit periods of terminal apparatus . ^λλl" for the first combination and the second combination. As ^' a result, if the receive-transmit period in the first combination is longer, the control unit 30 will assign the transmit timings in the order of terminal apparatuses "1" and ^λλ2". On the other hand, ^• if the receive-transmit period in the second combination is longer, the control unit 30 will assign the transmit timings in the order of terminal apparatuses "2" and "1". That is, the control unit 30 assigns terminal apparatuses with lower processing speeds to the partial periods corresponding to the order in which the receive-transmit period is longer. FIG. 10 is an illustration for explaining the outline of still another modification of the assignment of timings in a communication system 100. FIG. 10 is the same type of illustration as FIG. 9, so that the different points only will be explained. In -FIG. 9, which shows one of a plurality of combinations, the control unit 30 assigns the transmit timings in the order of the second radio apparatus 10b, the third radio apparatus 10c and the fourth radio apparatus 1Od. Here "A", ^XλB" and ^λλC" denote the receive- transmit periods for the second radio apparatus 10b to fourth radio apparatus 1Od, respectively. Furthermore, the control unit 30 carries out a similar processing^' to other combinations not shown. As a result, the control unit 30 identifies the receive-transmit period for each radio apparatus 10 in each of the combinations. The control unit 30 acquires the processing speed for each of the second radio apparatus 10b to fourth radio apparatus 1Od. Finally, the control unit 30 selects a combination that provides a longer receive-transmit period for the radio apparatus 10 with lower processing speed. As a result, the selected combination determines the orders of the transmit timings and the receive timings.

In a modification like this, the control unit 30 specifies respective processing speeds of a plurality of terminal apparatuses by way of required durations from the transmission of respective signals to the plurality of terminal apparatuses to the receiving of the ACK signals. Furthermore, the control unit 30 specifies assigns transmission timings according to the processing speeds. Otherwise, the control unit 30 may specify the respective numbers of streams to be transmitted to a plurality of terminal apparatuses by way of the required durations from the transmission of the respective signals to the plurality of terminal apparatuses to the receiving of the ACK signals. In so doing, the control unit 30 assigns the terminal apparatuses with larger numbers of streams to be transmitted to the earlier periods in the series of partial periods for transmitting signals, irrespectively of the processing speeds of the terminal apparatuses.

According to the embodiments of the present invention, terminal apparatuses with longer required durations from the receiving of a signal to the transmission of an ACK signal are. assigned to the earlier periods in the series of partial periods for transmitting signals, so that the period permissible for receiving processing by the terminal apparatuses can be made longer. And this longer period permissible can increase the possibility of generating an ACK signal before the timing for transmitting the ACK signal. Moreover, the base station apparatus, which receives an ACK signal earlier, can carry out subsequent processing earlier. The base station apparatus can determine communication timings in such a manner as to communicate efficiently with a plurality of terminal apparatuses. Since the processing speeds of terminal apparatuses are specified as the required durations from receiving of a signal to transmission of an ACK signal, the assignment -can be accomplished that suits the CPU and the like of the terminal apparatuses. Since the processing speed of a terminal apparatus is measured during data communication, it is possible to restrict the increase of signals to be transmitted. And this restriction of the increase of signals to be transmitted may improve the transmission efficiency. Since terminal apparatuses with the larger numbers of streams of signals to be transmitted are assigned to the earlier periods in the series of partial periods for transmitting signals, the period permissible for receiving processing by the terminal apparatuses can be made longer. And this longer period permissible can increase the possibility of generating an ACK signal before the timing for transmitting ^■ the ACK signal even without the recognition of the processing speeds of the terminal apparatuses.

Use of the same order for the assignment of transmission timings and for the assignment of reception timings can make the processing simpler. Since the terminal apparatuses with lower processing speeds are assigned to the transmission timings for longer receive-transmit periods, it is possible to raise the possibility of transmitting ACK signals by said terminal apparatuses. This assignment of terminal apparatuses with lower processing speeds to the transmission timings for longer receive-transmit periods can raise the possibility of transmitting ACK signals by said terminal apparatuses.

The present invention has been described based on the embodiments and modifications which are only exemplary. It is therefore understood by those skilled in the art that still other various modifications to the combination of each component and process are possible and that such modifications are also within the scope of the present invention.

According to the first embodiment of the present invention, in order to identify the processing speeds, the control unit 30 measures the period from the transmission of a packet signal to the receiving of an ACK signal corresponding to said packet signal, for each of the terminal apparatuses. The arrangement, however, is not limited thereto, and the control unit 30 may, for instance, receive information on the processing speed from each of a plurality of terminal apparatuses via a baseband processing unit 22 or the like. The information on the processing speed may correspond to the clock frequency of the CPU provided in the terminal apparatus, for instance. The processing speed may be classified into a plurality of stages, and the information may indicate the stages to which the terminal apparatuses correspond. The control unit 30 may specify the processing speeds based on the thus received information. According to this modification, the accuracy of identifying the processing speeds can be improved. That is, the accuracy is satisfactory if the processing speeds of the terminal apparatuses can be grasped. In the present embodiments according to the present invention, the communication system 100 uses multi-carriers. However, the present invention is not limited thereto and, for instance, single carrier may be used. As evident from these modifications, the present invention can be applied to various types of communication systems.

While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims .

Second embodiment

A problem to be solved by a second embodiment of the present invention will be stated as follows. In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in a packet signal is called "stream") . That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates. Moreover, combining this MIMO system with the OFDM modulation scheme results in a higher data transmission rate.

For the purpose of enhancing the transmission efficiency in this MIMO system, the data signals to be transmitted respectively in a plurality of packets are aggregated into a single packet. In so doing, the control signals are appended to the respective data signals. In other words, a plurality of combinations of control signals and data signals are contained in the packet signals. It is generally the case that the number of subcarries necessary for transmitting the control signal is smaller than the number of subcarriers necessary for transmitting the data signal. Accordingly, if the number of subcarriers used for the transmission of the control signal differs from that used for the transmission of the data signal, the signal strength varies periodically at the time of transmitting packets. That is, the signal strength is attenuated in part of the control signal. When such a fluctuation as this occurs, the signals received by the receiving apparatus also varies. As a result, the power of estimated channel characteristics do not match the power of control signals and thereby the receiving characteristics may possibly deteriorate as will be discussed.

In a MIMO system like this, it is generally the case that the number of subcarries necessary for transmitting the control signal is smaller than the number of subcarriers necessary for transmitting the data signal. The number of subcarriers in the known signal for use in estimating the channel characteristics is made equal to the number of subcarriers in the data signal. If the number of subcarriers used for the transmission of the control signal differs from that used for the transmission of the known signal, the power of estimated channel characteristics do not correspond to the power of control signals and thereby the receiving characteristics may possibly deteriorate as will be discussed. An outline of the present invention will be given before a detailed description thereof. Embodiments of the present invention relate to a MIMO system comprised of at least two radio apparatuses. One of the radio apparatuses corresponds to a transmitting apparatus whereas the other thereof corresponds to a receiving apparatus. The transmitting apparatus generates one packet signal in such a manner as to contain a plurality of combinations of control signal and data signal. One packet signal is composed of a plurality of streams. As mentioned earlier, when the number of subcarriers necessary for transmitting a control signal differs from that of subcarriers necessary for transmitting a data signal, the strength of packet signals transmitted fluctuates. In the^' second embodiment, the following processing is executed to restrict the variation in the signal strength.

The transmitting apparatus performs interleaving of a size defined by the number of subcarriers corresponding to a control signal (hereinafter referred to as the "first number of first subcarriers") on the control signal. The transmitting apparatus performs interleaving of a size defined by the number of subcarriers corresponding to a data signal (hereinafter referred to as the "second number of first subcarriers") on the data signal. It is assumed here that the first number of subcarriers is "48" and the second number of subcarriers is "52". Of a plurality of combinations, the transmitting apparatus attaches additional signals to the control signa.ls contained in the second and the subsequent combinations. Hereinafter, a control signal to which an additional signal is appended or control signals to which additional signals are appended will be referred to as a "control signal with an additional signal" or "control signals with their respective additional signals", respectively.

If the number of subcarriers corresponding to an additional signal is set to "4", the number of subcarriers used for a control signal with an additional signal" will be "52". Hence the number of subcarriers used for the control signal with the additional signal is now equal to the number of subcarriers used for a data signal. As a result, the variation in the signal strength is restricted. In a plurality of combinations, no additional signal is appended to a control signal contained in a combination in the beginning. This is because it is arranged that a radio apparatus in a communication system which is not compatible with a MIMO system (such a communication system will be hereinafter referred to as a "legacy system") can receive packet signals according to the second embodiment.

FIG. 1 illustrates a spectrum of a multicarrier signal according to an embodiment of the present invention. In particular, FIG. 1 shows a spectrum of a signal in the OFDM modulation scheme. One of a plurality of carriers in an OFDM modulation scheme is generally called a subcarrier.

Herein, however, a subcarrier is designated by a "subcarrier number". In a MIMO system, 56 subcarriers, namely, subcarrier numbers "-28" to "28" are defined herein. It is to be noted that the subcarrier number "0" is set to null so as to reduce the effect of a direct current component in a baseband signal. On the other hand, in a legacy system, 52 subcarriers, namely, subcarrier numbers "-26" to "26" are defined. One example of legacy systems is a wireless LAN complying with the IEEE802.11a standard. The respective subcarriers are modulated by a modulation scheme which is set variably. Used here is any of modulation schemes among BPSK (Binary Phase-Shift Keying) , QPSK (Quadrature Phase-Shift Keying) , 16-QAM (Quadrature Amplitude Modulation) and 64-QAM. Convolutional coding is applied, as an error correction scheme, to these signals. The coding rates for the convolutional coding are set to 1/2, 3/4 and so forth.

The number of data to be transmitted in parallel is set variably. The data are transmitted as packet signals and each of packet signals to be transmitted in parallel is called "stream" herein. As a result thereof, since the mode of modulation scheme and the values of coding rate and the number of streams are set variably, the data rate is also set variably. It is to be noted that the "data rates" may be determined by arbitrary combination of these factors or by one of them. If the modulation scheme is BPSK and the coding rate is 1/2 in a legacy system, the data rate will be 6 Mbps. If, on the other hand, the modulation scheme is BPSK and the coding rate is -3/4, the date rate will be 9 Mbps . FIG. 2 illustrates a structure of a communication system 100 according to an embodiment of the present invention. The communication system 100 includes a first radio apparatus 10a and a second radio apparatus 10b, which are generically called- "radio apparatus 10". The first radio apparatus 10a includes a first antenna 12a, a second antenna 12b, a third antenna 12c and a fourth antenna 12d, which are generically referred to as "antennas 12", and the second radio apparatus 10b includes a first antenna 14a, a second antenna 14b, a third antenna 14c and a fourth antenna 14d, which are generically referred to as "antennas 14". Here, the first radio apparatus 10a corresponds to a transmitting apparatus, whereas the second radio apparatus 10b corresponds to a receiving apparatus.

An outline of a MIMO system will be given before a description of a structure of the communication system 100. Assume herein that data are being transmitted from the first radio apparatus 10a to the second radio apparatus 10b. The first radio apparatus 10a transmits respectively data of a plurality of streams from the first antenna 12a to fourth antenna 12d, respectively. As^' a result, the data rate becomes higher. The second radio apparatus 10b receives the data of a plurality of streams by the first antenna 14a to fourth antenna 14d. The second radio apparatus 10b separates the received signals by adaptive array signal processing and demodulates independently the data of a plurality of streams.

Since the number of antennas 12 is "4" and the number of antennas 14 is also "4" here, the number of combinations of channels between the antennas 12 and the antennas 14 is ^λλlβ". The channel characteristic between from the ith antenna 12i to the jth antenna 14j is denoted by hi_j . In FIG. 2, the channel characteristic between the first antenna 12a and the first antenna 14a is denoted by hn, that between from the first antenna 12a to the second antenna 14b by h₁₂, that between the second antenna 12b and the first antenna 14a by h₂i, that between from the second antenna 12b to the second antenna 14b by h₂₂, and that between from the fourth antenna 12d to the fourth antenna 14d by h₄₄. For the clarity of illustration, it is omitted to show the other channels in FIG. 2.

FIG. 11 illustrates packet formats in a communication system 100. For the simplicity of explanation, it is assumed here that the number of streams contained in the packet formats is "2". The stream transmitted from the first antenna 12a is shown in the top row whereas the stream transmitted from the second antenna 12b is shown in the bottom row. In the top row of FIG. 11, "L-STF", "L-LTF",

"L-SIG" and "HT-SIG" correspond to a known signal for timing estimation, a known signal for channel estimation, a control signal compatible with a legacy system, and a control signal compatible with a MiMO system, respectively. In the bottom row of FIG. 11, "L-STF + CDD", "L-LTF + CDD", "L-SIG + CDD" and "HT-SIG + CDD" correspond to the results obtained when CDD (Cyclic Delay Diversity) is implemented to "L-STF", "L- LTF", "L-SIG" and "HT-SIG", respectively. That is, "L-STF + CDD" is such that "L-STF" has undergone the cyclic timing shifting. ^•

"HT-STF" and "HT-STF'" correspond to known signals, for timing estimation, compatible with a MIMO system, and they are so defined^' as to use different subcarriers from each other. "HT-LTFl", "HT-LTFl'", "HT-LTF2" and "HT-LTF2"' correspond to known signals, for channel characteristics, compatible with a MIMO system. "HT-LTFl" and "HT-LTFl'" are so defined as to use different subcarriers from each other.

The same applies to "HT-LTF2" and "HT-LTF2"'. On the other hand, "HT-LTF2" is so defined as to use the subcarriers that have not been used in "HT-LTFl". "HT-DATAl" and "HT-DATA2" are. data signals. The control signals for "HT-DATAl" and "HT-DATA2" correspond to "HT-SIG" and "HT-SIG + CDD", respectively. Accordingly, a set of "HT-SIG", "HT-SIG + CDD", "HT-DATAl" and "HT-DATA2" is called a "first combination". "HT-SIGl" and "HT-SIGl'" are control signals for "HT- DATA3" and "HT-DATA4" which are assigned posterior to the "HT-SIGl" and "HT-SIGl'", respectively. "HT-SIGl" and "HT- SIGl'" are so defined as to use subcarriers different from each other. "HT-DATA3" and "HT-DATA4" are data signals. A set of "HT-SIGl" and "HT-SIGl'", "HT-DATA3" and "HT-DATA4" is called a "second combination". The same holds for "HT- SIG2" and "HT-SIG2"', "HT-DATA5" and "HT-DATA6", and a set of them is called a "third combination".

The portions from the beginning up to "HT-SIG" and "HT-SIG + CDD" use "52" subcarriers in the same way as in a legacy system. Of "52" subacarriers, "4" subcarriers correspond to the pilot signals. On the other hand, the portions corresponding to "HT-STF" and "HT-STF'" use "24" subcarriers in the total of a plurality of streams. The portions corresponding to "HT-LTFl", "HT-LTFl'", "HT-SIGl", "HT-SIGl'" and so forth use "56" subcarriers in the total of a plurality of streams. The portions corresponding to "HT-

DATAl", "HT-DATA2" and so forth use "56" subcarriers . The control signals in "HT-SIGl" and the like correspond to the aforementioned control signals with their respective additional signals.

"HT-SIG" and the like are demodulated based on "L-LTF" The both use the same number of carriers, namely "52", and a processing for adjusting to the power at a posterior part of "56" subcarriers is carried out. On the other hand, "HT- SIGl" and the like are demodulated based on "HT-LTFl" and the like. If "HT-SIGl" and the like use "52" subcarriers in the same way as in "HT-SIG" and the like, the number of subacarriers used does not agree with the number of subcarriers, namely, "56", used in "HT-LTFl" and the like, so that the powers at the both parts do not coincide. Thus, according to the present invention, the number of subcarriers used in "HT-SIG" and the like is extended to "56" as was explained above.

FIG. 4 illustrates a structure of a first radio apparatus 10a. The first radio apparatus 10a includes a first radio unit 20a, a second radio unit 20b, ... and a fourth radio unit 2Od, which are generically referred to as "radio unit 20", a baseband processing unit 22, a modem unit 24, an IF unit 26 and a control unit 30. Signals involved include a first time-domain signal 200a, a second time- domain signal 200b, ... and a fourth time-domain signal 20Od, which are generically referred to as "time-domain signal

200", and a first frequency-domain signal 202a, a second frequency-domain signal 202b, a third frequency-domain signal 202c and a fourth frequency-domain signal 202d, which are. generically referred to as "frequency-domain signal 202" The second radio apparatus 10b has a structure similar to that of the first radio apparatus 10a.

As a receiving operation, the radio unit 20 carries out frequency conversion of radiofrequency signal received by the antennas 12 so as to derive baseband signals. The radio unit 20 outputs the baseband signals to the baseband processing unit 22 as the time-domain signals 200. The baseband signal, which is composed of in-phase components and quadrature components, shall generally be transmitted by two signal lines. For the clarity of figure, the baseband signal is presented here by a single signal line only. An AGC unit and an A-D conversion unit are also included.

As a transmission operation, the radio unit 20 carries out frequency conversion of baseband signals from the baseband processing unit 22 so as to derive radiofrequency signals. Here, the baseband signal from the baseband processing unit 22 is also indicated as the time-domain signal 200. The radio unit 20 outputs the radiofrequency signals to the antennas 12. A PA (power .amplifier) and a D- A conversion unit are also included. It is assumed herein that the time-domain signal 200 is a multicarrier signal converted to the time domain and is a digital signal.

As a receiving operation, the baseband processing unit 22 converts a plurality of time-domain signals 200 respectively into the frequency domain and performs adaptive array signal processing on the thus converted frequency- domain signals. Then the baseband processing unit 22 outputs the result of adaptive array signal processing as the frequency-domain signals 202. One frequency-domain signal 202 corresponds to data contained in each of a plurality of streams transmitted from the second radio apparatus 10b, not shown here. As a transmission operation, the baseband processing unit 22 inputs, from the modem unit 24, the frequency-domain signals 202 serving as signals in the frequency domain, converts the frequency-domain signals into time domain and then outputs the thus converted signals as time-domain signals 200 by associating them respectively with a plurality of antennas 12.

It is assumed that the number of antennas 12 to be used in the transmission processing is specified by the control unit 30. It is assumed herein that the frequency- domain signal 202, which is a signal in the frequency domain, contains a plurality of subcarrier components as shown in FIG. 1. For the clarity of figure, the frequency-domain signal is arranged in the order of the subcarrier numbers, and forms serial signals.

FIG. 5 illustrates a structure of a frequency-domain signal. Assume herein that a combination of subcarrier numbers "-28" to "28" shown in FIG. 1 constitutes an "OFDM symbol". An "i"th OFDM symbol is such that subcarrier components are arranged in the order of subcarrier numbers "1". to "28" and subcarrier numbers "-28" to "-1". Assume also that an "(i-2)"th OFDM symbol is placed before the "i"th OFDM symbol, and an "(i+l) "th OFDM symbol is placed after the "i"th OFDM symbol. It is to be noted here that in the portions such as "L-STF" shown in FIG. 11 a combination of from the subcarrier number "-26" to the subcarrier number "-26" is used.

Now refer back to FIG. 4. To produce the packet format corresponding to FIG.- 11, the baseband processing unit 22 carries out^' CDD. CDD is expressed as a matrix C in the following Equation (2-1) .

CCO=diag(l, exp(-]2π£δINout), •••, exp(-j2π^δ(Nout-1)/Nout)) - (2-

D where δ indicates a shift amount ^and H a subcarrier number. The multiplication ^• of the matrix C by a stream is done on a subcarrier-by-subcarrier basis. That is, the baseband processing 22 carries out a cyclic time shifting within the LTF and so forth per stream. The shift amount is set to a different value for each stream.

As a receiving processing, the modem unit 24 demodulates and deinterleaves the frequency-domain signal 202 outputted from the baseband processing unit 22. The demodulation is carried out per subcarrier. The modem unit

24 outputs the demodulated signal to the IF unit 26. As a transmission processing, the modem unit 24 carries out interleaving arid modulation. In so doing, the modem unit 24 generates a control signal with an additional signal by- appending an additional signal to a control signal. The modem unit 24 outputs the modulated signal to the baseband processing unit 22 as a frequency-domain signal 202. When the transmission processing is carried out, the modulation scheme is specified by the control unit 30.

As a receiving processing, the IF unit 26 combines signals outputted from a plurality of modem units 24 and then forms one data stream. The IF unit 26 decodes the one data stream. The IF unit 26 outputs the decoded data stream. As a transmission processing, the IF unit 26 inputs one data stream, then codes it and_/ thereafter, separates the coded data stream. Then the IF unit 26 outputs the thus separated data to the plurality of modem units 24. When the transmission processing is carried out, the coding rate is specified by the control unit 30. The control unit 30 controls the timing and the like of the first radio apparatus 10a.

In terms of hardware, this structure can be realized by a CPU, a memory and other LSIs of an .arbitrary computer. In terms of software, it is realized by memory-loaded programs which have communication functions and the like, but drawn and described herein are function blocks that are realized in cooperation with those. Thus, it is understood by those skilled in the art that these function blocks can • be realized in a variety of forms such as by hardware only, software only or the combination thereof.

FIG. 6 illustrates a structure of a baseband processing unit 22. The baseband processing unit 22 includes a processing unit for use with receiving 50 and a processing unit for use with transmission 52. The receiving processing unit 50 executes a part, corresponding to a receiving operation, of operations by the baseband processing unit 22. That is, the receiving processing unit 50 performs adaptive array signal processing on time-domain signals 200 and therefore derives receiving weight vectors. Then the receiving processing unit 50 outputs the result of array synthesis as the frequency-domain signal 202.

A processing of receiving processing unit 50 will now be described in a specific manner. The receiving processing unit 50 inputs a plurality of time-domain signals 200 and then performs Fourier transform on them, respectively, so as to derive frequency-domain signals. As described earlier, a frequency-domain signal is such that signals corresponding to subcarriers are arranged serially in the order of subcarrier numbers. The receiving processing unit 50 weights the frequency-domain signals with receiving weight vectors, and a plurality of weighted signals are added up. Since the frequency-domain signal is composed of a plurality of subcarriers, the above processing is also executed on a subcarrier-by-subcarrier basis. As a result, the signals summed up are also arranged serially, as shown in FIG. 5, • in the order of subcarrier numbers. The signals summed up are the aforementioned frequency-domain signals 202.

The receiving processing unit 50 derives receiving weight vectors by use of an adaptive algorithm, for example, LMS algorithm. Alternatively, receiving response vectors are derived by correlation processing and then the receiving weight vectors may be derived from the receiving response vectors. Here, the latter case will be described. If a frequency-domain signal corresponding to the first time- domain signal 200a is denoted by xi(t), a frequency-domain signal corresponding to the second time-domain signal 200b by X₂(t), a reference signal in the first stream by Si (t) and a reference signal in the second stream by S₂ (t) , then Xi(t) and x₂(t) will be expressed by the following Equation (2-2):

x₁(t) = h₁₁S₁(t)+h₂₁S₂(t)

The noise is ignored here. A first correlation matrix R₁, with E as an ensemble average, is expressed by the following Equation (2-3) :

A second correlation matrix R₂ among the reference signals is given by the following Equation (2-4).

Finally, the first correlation matrix Ri is multiplied by the inverse matrix of the second correlation matrix R₂ so as to derive a receiving response vector, which is expressed by the following Equation (2-5) .

It is to be noted that the receiving processing unit 50 computes plural kinds of receiving weight vectors. A first kind of receiving weight vector is a receiving weight vector to receive HT-SIG and the like, and is derived from L-LTF and the like. A second kind of receiving weight vector is a receiving weight vector to receive HT-DATAl and the like and is derived from HT-LTFl, HT-LTF2 and the like. A third kind of receiving weight vector is a receiving weight vector to receive HT-SIGl and the like and is derived from HT-LTFl and the like. Using such plural kinds of receiving weight vectors as above, the receiving processing unit 50 carries out array synthesis. Under such a condition, the modem unit 24 provided at a stage subsequent to the baseband processing unit 22 carries out demodulation using . the pilot signals. ^■ _ The transmitting processing unit 52 executes a part, corresponding to a transmission operation, of operations by the baseband processing unit 22. The transmitting processing unit 52 may perform beamforming or eigenmode transmission. Any known technique may be used for these and therefore the description thereof is omitted here.

FIG. 12 illustrates a structure of IF unit 26 and modem unit 24. Shown here is a portion concerning the transmission function in the IF unit 26 and the modem unit 24. The IF unit 26' ^'includes an FEC (Forward Error- Correcting) unit 60 and a separation unit 62. The modem unit 24 includes a first interleave unit 64a ... and a fourth interleave unit 64d, which are generically referred to as "interleave unit 64", a first adding unit 66a ... and a fourth adding unit 66d, which- are generically referred to as "adding unit 66", and a first mapping unit 68a ... and a fourth mapping unit 68d, which are generically referred to as "mapping unit 68".

A plurality of combinations of control signal and data signal, which are to use a plurality of subcarriers, are inputted to the FEC unit 60. The combinations meant here are equal to the "first combination" to the "third combination" as shown in FIG. 11. The control signal corresponds to "HT-SIG", "HT-SIGl" and the like in FIG. 11. The FEC unit 60 performs coding on each of the plurality of combinations. Note that the coding rate may be set for the control signal and the data signal independently of each other.

The separation unit 62 partitions and separates a signal inputted from the FEC unit 60 into a plurality of streams. The interleave unit 64 carries out an interleaving of a size defined by the first number of subcarriers, namely, 48, on the control signal, and carries out an interleaving of a size defined by the second number of subcarriers, namely, 52, on the data signal. Here, the amount of data contained in the size defined by the number of subcarriers "52" is changed by the modulation scheme or the like used by the modem unit 24. It is -assumed that the interleaving pattern is predetermined.

The adding unit 66 adds additional signals to control signals contained in the second and subsequent combinations of the plurality of combinations interleaved by the interleaving unit 64. As a result, control signals with their respective additional signals are generated. Here the control signals contained in the second and subsequent combinations correspond to "HT-SIGl", "HT-SIGl'", "HT-SIG2" and "HT-SIG2"' shown in FIG. 11. It is to be noted that the amount of additional signal to be added by the adding unit 66 is determined by the difference of the second number of subcarriers from the first number of subcarriers. In other words, the amount of additional signal is determined by the • difference "4" between the second number of subcarriers and the_ first number of subcarriers and the modulation scheme. As a result of the processing as described above, the number of subcarriers used by control signals with their respective additional signals becomes the same as the number of subcarriers used by the data signals. It is to be understood here that the additional signal is a dummy signal,

The mapping unit 68 performs mappings of BPSK, QPSK, 16-QAM and 64-QAM on the signals from the adding unit 66. Mapping, which is a known technology, is not explained here. The mapping unit 68 ^' outputs a mapped signal as a frequency- domain signal 202. The insertion of known signals, such as "L-STF" as shown in FIG. 11, or the insertion of pilot signals is done by the modem unit 24.

On the other hand, the receiving function for receiving the packet signals generated as described above performs operation opposite to that explained above. That is, the modem unit 24 receives an input of frequency-domain signals 202. The frequency domain signal 202, which is a combination of control signal and data signal, is equal to a combination using a plurality of subcarriers. Here the control signals contained in the second and subsequent combinations correspond to control signals with their respective additional signals. The excluding unit (not shown) in the modem unit 24 excludes additional signals from the control signals with their respective additional signals contained in the second and subsequent combinations out of a plurality of combinations. In other words, the excluding unit outputs control signals and data signals by excluding the dummy signals therefrom. Note that the excluding unit excludes additional signals according to the difference between the second number of subcarriers and the first number of subcarriers.

A deinterleave unit (not shown) in the modem unit 24 performs a deinterleaving of a size defined by the first number of subcarriers, namely, 48, on the control signal, of the plurality of combinations with the additional signals excluded, and performs a deinterleaving of a size defined by the second number of subcarriers, namely, 52, on the data signal.

In the description thus far, an additional signal is added to an interleaved control signal. In this condition, the number of subcarriers used for "HT-LTSl" and the like is equal to the number of subcarriers used for a control signal with additional signal. In other words, the variation in the number of subcarriers and the variation in the signal strength of packet signals are subject to restriction. On the other hand, the size of interleaving, when based on the number of subcarriers, is different between the control signal with an additional signal and the data signal. As a result, a switching in the size of interleaving is done between the two. A modification to be described later aims to restrict the change in size to be used in the interleaving.

FIG. 13 illustrates another structure of IF unit 26 and modem unit 24. Shown here is a portion concerning the transmission function in the IF unit 26 and the modem unit 24. The IF unit 26 includes an adding unit 66, an FEC (Forward Error-Correcting) unit 60 and a separation unit 62. The modem unit 24 includes a first interleave unit 64a ... and a fourth interleave unit 64d, which are generically referred to as "interleave unit ^'64", and a first mapping unit 68a ... and a fourth mapping unit 68d, which are generically referred to as "mapping unit 68". The components having the function equivalent to those in FIG. 12 are given the same reference numerals and therefore their repeated explanation will be omitted as appropriate. Compared with the above structure, the arrangement of the adding unit 66 differs from that in FIG. 12.

A plurality of combinations, of control signal and data signals, which are to use a plurality of subcarriers are inputted to the adding unit 66. The adding unit 66 appends additional signals to the control signal contained in the second combination and the subsequent combinations in a plurality of combinations. Accordingly, control signals with their respective additional signals are produced. Here, the amount of additional signals appended by the adding unit 66 is determined according to the difference between the first number of subcarriers and the second number of subcarriers. It is assumed herein that the additional signals are for use with CRC (Cyclic Redundancy Check) . The signals for CRC are generated by the FEC unit 60. As a result, the bit number used for CRC increases and therefore the data error characteristics improves. The additional signal may be a signal for use with parity check.

The interleave unit 64 carries out an interleaving of a size defined by the first number of subcarriers on the control signal contained in the first combination, and carries out an interleaving of a size defined by the second number of subcarriers on the remaining signals. That is, the number of interleave size switching can be reduced.

On the other hand, the receiving function of receiving the packet signals thus generated executes an operation opposite to the operation in the above description. That is, the modem unit 24 inputs the frequency-domain signals 202.

The frequency-domain signal corresponds to a combination, of control signal and data signal, which uses a plurality of subcarriers. Here,^' control signals contained in the second combination and the subsequent combinations are control signals with their respective additional signals.

A deinterlieave unit (not shown) in the modem unit 24 performs a deinterleaving of a size defined by the first number of subcarriers on control signals contained in the first combination among a plurality of combinations, and performs a deinterleaving of a size defined by the second number of subcarriers on the remaining signals.

The excluding unit (not shown) in the modem unit 24 excludes additional signals from the control signals with their respective additional signals contained in the second and subsequent combinations out of a plurality of combinations. That is, the excluding unit outputs control signals and data signals by excluding the signals for CRC. Note that the excluding unit excludes additional signals according to the difference between the second number of subcarriers and the^' first number of ' subcarriers . The IF unit 26 executes the detection by CRC.

A .modification will be explained hereinbelow. In this modification, an additional signal is added to a control signal in the same way as in the second embodiment. However, the packet format in the modification differs from that in the second embodiment. In the second embodiment, a plurality of combinations are included in a packet signal. In the modification, however, it may be such that only one combination is included in a single packet signal. A control signal is placed between known signals for use with channel estimation. Accordingly, the control signal. is demodulated based on the known signal for use with channel estimation. At this time, if there is a difference between the number of subcarriers used for the control signal and the number of subcarriers used for the known signal for channel estimation, the same problem as mentioned earlier will arise. Hence, in the modification, too, an additional signal is added to a control signal as mentioned above (hereinbelow, such a control signal is also referred to as a "control signal with an additional signal") .

The structure of a radio apparatus 10 according to the modification is of the same type as that of the first radio apparatus 10a of FIG. 4, whereas the structures of an IF unit 26 and a modem unit 24 thereof are of the same type as those of the IF unit 26 and the modem unit 24 of FIG. 12. Thus the repeated explanation thereof is omitted here. It is to be noted that the radio apparatus 10 generates a packet signal, with control signals placed in the intermediate intervals, which is to use ^Ja plurality of subcarriers, ^'and transmits the packet -.signal thus generated. It is so defined that the number of subcarriers necessary for transmitting a control signal is smaller than that of subcarriers used in a known signal for channel estimation, which is placed in the preceding interval.

In the modification, an adding unit 66 adds an additional signal to a control signal so that the number of subcarriers to be used in the control signal becomes equal to that of subcarriers used in the known signal for channel estimation. Here, the additional signal is to be a pilot signal, namely, a known signal. By the processing as described above, the aforementioned problem is solved in the modification in a similar manner as in the second embodiment. _ FIGS. 14A and 14B illustrate packet formats according to a modification of the present invention. FIG. 14A shows the first packet format according to the modification. Here, data contained in four streams are to be transmitted, and the packet formats corresponding to the first to fourth streams are shown in order from the top to the bottom level . In a packet signal corresponding to the first stream, "L- STF", "HT-LTF" and the like are assigned as preamble signals. In a packet signal corresponding to the second stream, "L- STF-400ns", "HT-LTF-400ns" and the like are assigned as preamble signals. In a packet signal corresponding to the third stream, "L-STF-200ns", "HT-LTF-200ns" and the like are assigned as preamble signals. And in a packet signal corresponding to the fourth stream, "L-STF-βOOns", "HT-LTF- 600ns" and the like are assigned as preamble signals. Here, "400ns" and the like represent the amounts of shift by CDD. Note that "HT-SIG" in the packet formats is equivalent to a control signal. The HT-LTF in the first stream is placed in the order of "HT-LTF", "-HT-LTF", "HT- LFT" and "-HT-LTF" from the top. Here these are called the "first component", the "second component", the "third component" and the "fourth component" in order in all the streams. A desired signal for the first stream is extracted at the receiving apparatus by carrying out an operation of "first component minus (-) second component plus (+) third component minus (-) fourth component" for the received signals of all the streams.

By performing an operation of "first component + second component + third component + fourth component" for the received signals of all the streams, desired signals for the second stream are extracted at the receiving apparatus. By performing an operation of "first component - second component - third component + fourth component" for the received signals of all the streams, desired signals for the third stream are extracted at the receiving apparatus . By performing an operation of "first component + second component - third component - fourth component" for the received signals of all the streams, desired signals for the fourth stream are extracted at the receiving apparatus. It is to be noted that the additions and subtractions are done by the vector operation. The number of subcarriers used for "HT-LTF" and the like is "56". As for "HT-SIG", the number of subcarriers necessary for transmitting a control signal is "52". Hence, if the number of subcarriers used for "HT-SIG" is "52", then the "HT-SIG" will have a smaller number of subcarriers than that used in the preceding interval. As a result, the same problem as in the embodiment arises as mentioned above. Accordingly, the adding unit 66 adds a pilot signal composed of 4 subcarriers to "HT-SIG". This makes the number of subcarriers used for "HT-SIG" "56", thus making the number of subcarriers equal to that in the preceding interval. Here the reason why the number of subcarriers necessary for transmitting a control signal, which is an "HT-SIG", is "52" will be explained by referring to FIG. 14B.

FIG. 14B shows the second packet format according to the modification. Referring to FIG. 14B, "L-LTF" and "L- SIG" are assigned posterior to "L-STF", in the first stream. "HT-SIG" is assigned posterior to the "L-SIG". "HT-STF", "HT-LTF" and so forth are assigned posterior to the "HT- SIG". , on the other hand, the CDD with the shift amounts of "50 ns", "100 ns" and "150 ns" is implemented in "L-STF", "L-LTF" and "L-SIG" in the second stream to fourth stream, respectively. Similar to FIG. 14A, the CDD with the shift amounts of "400 ns", "200 ns" and "600 ns" is implemented in "HT-SIGs" in the second stream to fourth stream, respectively. Placed following the above signals are "HT-STF", "HT- LTF" and the like. Here "L-STF", "L-LTF" and "L-SIG" are signals placed therein to retain compatibility with legacy systems. Accordingly, the number of subcarriers used in "L- LTF" and "L-SIG" is "52" in the same manner as in the legacy system. Thus, the number of subcarriers used in "HT-SIG", which follows these, is also "52". It is to be rioted that "56" subcarriers are used in "HT-STF", "HT-LTF" and the like in order to realize a high transmission rate in a MIMO system.

In order to simplify the processing for the packet formats as shown in FIGS. 14A and 14B, there is a demand that the same interleave unit and the same deinterleave unit be placed in the radio apparatus 10 and the processing be done on "HT-SIG" which has the same information bit arrangement. Normally, however, the number of subcarriers for "HT-SIG" in FIG. 14A is necessarily "52" in accordance with the number of subcarriers for "HT-SIG" in FIG. 14B. And this gives rise to a power variation in the case of FIG. 14A. According to the present invention, however, the addition of an additional signal compensates for the power variation while meeting the aforementioned demand.

Here, any of the packet formats shown in FIGS. 14A and 14B may be used. The packet formats of FIG. 14A, with fewer redundant signal components, can improve the utilization efficiency. On the other hand, the packet formats of FIG. 14B, for which preamble signals compatible with the legacy system are added, allow detection by communication apparatuses compatible with the legacy system. The adding unit 66 adds pilot signals when the packet format of FIG. 14B is used, and does not add pilot signals when the packet format of FIG. 14A is used. That is, the adding unit 66 stops adding additional signals when a packet signal is generated in such a manner that the number of subcarriers necessary for a control signal is the same as that used in the preceding interval.

Hereinbelow, a description will be given of a processing carried out by the radio apparatus 10 when the packet formats shown in FIGS. 14A and 14B are received. The baseband processing unit 22 identifies the format of a received packet format. In the packet format shown in FIG. 14A (hereinafter referred to as "first format") , additional signals are attached to the control signal whose number of subcarriers required therefor is smaller than the number of subcarriers used in an early interval so that the number of subcarriers required becomes identical to that of subcarriers used in the early interval. In the packet format shown in FIG. 14B (hereinafter referred to as "second format") , on the other hand, the number of subcarriers required therefor is identical to that of subcarriers used in an early interval. Whether a received packet signal is the first packet or the second packet is identified in the baseband processing unit 22.

More specifically, the baseband processing unit 22 estimates the channel characteristics based on L-LTF, using a known technique. The shift amount for CDD is defined in the range of "200 ns" to "600 ns" in the first format, whereas it is defined in the rage of "50 ns" to "150 ns" in the second format. Accordingly, in the estimated channel characteristic the delay time of delayed waves in the first format is longer than that in the second format. The baseband processing unit 22 identifies the packet format by comparing the delay time of delayed waves with a threshold value. For instance, if the delay time of delayed waves is greater than the threshold value, it will be identified that the packet format is the first format.

The baseband processing unit 22 and the modem unit 24 process the packet signals in accordance with the identified format of a packet signal. When the packet format is the first format, the baseband processing unit 22 and the modem unit 24 exclude the additional signals from the control signals with additional signals. Then a processing similar to the above processing will be performed on the control signals. If the additional signal is the pilot signal, the modem unit 24 will correct the phase based on the pilot signal. When, on the other hand, the packet format is the second format, the baseband processing unit 22 and the modem unit 24 do not exclude the additional signals. Then a processing similar to that carried out in the case of the first format will be executed.

According to the second embodiment, the number of subcarriers used in a data signal is made equal to the number of subcarriers used in a control signal with additional signals by attaching additional signals to the control signal which is inserted among data signals. Thus, the variation in signal strength can be suppressed and controlled. And because of this controlled variation in signal strength, the time constant of AGC at the receiving . apparatus can be made longer. Also, because of this controlled variation in signal strength, the dynamic range¹ at the receiving apparatus can be made smaller. In addition, the receiving characteristics thereof can be improved. Moreover, since drops in signal strength in the course of a packet signal can be avoided, any transmission from a third party communication apparatus multiplexed by CSMA can be prevented. And since any transmission from a third party communication apparatus multiplexed by CSMA can be prevented, the probability of signal collisions can be lowered. Furthermore, since a ^' dummy signal is added as an additional signal, complexity of processing can be reduced. Since a receiving apparatus, once additional signals are removed from control signals with additional signals, can perform normal functions, additional processing can be reduced.

The number of subcarriers used for data signals and the number of subcarriers used for control signals with additional signals are made equal to each other by adding an additional signal to each control signal inserted between data signals before 'interleaving. Thus, the number of interleave size switching can be reduced. And variation in signal strength can be suppressed and controlled while reducing the number of interleave size switching.^' Since a signal for CRC is added as an additional signal, the receiving characteristics can be improved.

Furthermore, since the number of subcarriers used for known signals for channel estimation and the number of subcarriers used for control signals with additional signals are made equal to each other by adding an additional signal to each control signal inserted between known signals for channel estimation, variation in signal strength can be suppressed and controlled. And since a pilot signal is added as an additional signal, the receiving characteristics at a receiving apparatus can be improved. And the addition of pilot signals only helps reduce the complexity of processing. Since the addition of an additional signal is stopped when a packet signal is generated such that the number of subcarriers necessary for a control signal is the same as that used in the preceding interval, it is possible to adjust the number of subcarriers in such a manner as to suit the packet format .

Since the variation in signal strength can be suppressed, the time constant of AGC at the receiving apparatus can be made longer. Since the variation in signal strength can be suppressed, the dynamic range at the receiving apparatus' can be made smaller. In addition, the receiving characteristics thereof can be improved. Moreover, since drops in signal strength in the course of a packet signal can be avoided, any transmission from a third party communication apparatus multiplexed by CSMA can be prevented.

And since any transmission from a third party communication apparatus multiplexed by CSMA can be prevented, the probability of signal collisions can be lowered. Furthermore, since a dummy signal is attached as an additional signal, the complexity of processing can be reduced. The receiving apparatus can perform normal functions if the additional signals are removed from control signals with additional signals, so that additional processing can be reduced.

Whether a received packet signal is a packet format where a control signal with an additional signal is assigned or a packet format where a control signal is assigned is identified, and a processing is executed according to the identified result. Hence, the packet signals can be received independently of whether the additional signals are attached or not. Since the number of subcarriers used for a known signal is made equal to that of subcarriers used for a control signal irrespective of whether the additional signals are attached or not, the deterioration of receiving qualities can be prevented. Since the packet format can be identified automatically, a plurality of packet formats can be accommodated even without any other signal attached thereto. When the additional signal is a pilot signal, said pilot signal can be used to correct the phase. Thus, the receiving characteristics can be improved. Since the processing of excluding the additional signals is added, the increase in processing amount due to the addition can be suppressed.

Third embodiment

A problem to be solved by a third embodiment of the present invention will be stated as follows. Varying the number of antennas to be used for data communication in a MIMO system enables adjusting a data rate, too. The data rate can be adjusted in greater detail by use of an adaptive modulation. To perform such an adjustment of data rates more reliably it is desired that a transmitting apparatus acquire from a receiving apparatus the information on data rates suited for a radio channel between the transmitting apparatus and the receiving apparatus (hereinafter referred to as "rate information") .^■ TO enhance the accuracy of such rate information, it is desirable that the receiving apparatus acquire the channel characteristics between a plurality of antennas contained in the transmitting apparatus and those contained in the receiving apparatus, respectively.

Examples of the combinations of directivity patterns in the antennas of the transmitting apparatus and receiving apparatus in a MIMO system are as follows. One example is a case where the antennas of a transmitting apparatus have omni patterns and the antennas of ^' a receiving apparatus have patterns in adaptive array signal processing. Another example is a case where both the antennas of the transmitting apparatus and those of the receiving apparatus have patterns in adaptive array signal processing. This is also called the beamforming. The system can be simplified in the former case. In the latter case, however, the directivity patterns of antennas can be controlled in greater detail, so that the characteristics thereof can be improved. Since in the latter case the transmitting apparatus performs adaptive array signal processing for transmission, it is necessary to receive beforehand from the receiving apparatus the known signals by which to estimate channels .

To improve the accuracy of rate information and the accuracy of beamforming in the above-mentioned requirements, it is necessary that the channel characteristics be acquired with high accuracy. To improve the accuracy in the acquisition of channel characteristics, it is desirable that the channel characteristics between a plurality of antennas contained in the transmitting apparatus and those in the receiving apparatus be acquired respectively. For this reason, the transmitting apparatus or the receiving apparatus transmits from all of antennas the known signals for use in channel estimation. Hereinafter, the known signals, for use in channel estimation, transmitted from a plurality of antennas will be referred to as "training signals" independently of the number of antennas to be used for data communication.

Under these circumstances, the inventor of the present invention came to recognize the following problems to be solved. When the training signals are transmitted, the number of streams containing known signals for use in channel estimation (hereinafter referred to as "channel estimation known signals") differs from that containing data, A known signal for setting AGC (Automatic Gain Control) , hereinafter referred to as "AGC known signal", at the receiving side is assigned anterior to the channel estimation known signals. When an AGC known signal is assigned only in a stream where data is assigned, one of the channel estimation known signals is received in a state where the AGC known signal has not been received anterior thereto. In particular, when the strength of AGC known signal doesn't get larger at the receiving side, the gain of AGC is set to a value which is large to a certain degree. In so doing, when the -strength of channel estimation known signal of a stream where the AGC known signal is not assigned is larger, there is a strong possibility that said channel estimation known signal may be amplified to such a degree that distortion is caused by AGC. As a result thereof, the error in channel estimation based on said channel estimation known signal becomes larger.

On the other hand, when an AGC known signal is

-I ΛO CTUTn assigned in a stream where a channel estimation known signal is assigned, the number of streams in which the AGC known signal is assigned differs from that in which data is assigned. Hence, there is a possibility that the gain set by the AGC known signal is not suitable for the demodulation of data. AS a result, the demodulated data are subject to errors. The present invention has been made under such circumstances and a general purpose thereof is to provide a radio apparatus that prevents the degradation in receiving characteristics when transmitting known signals for use in channel estimation.

An outline of the present invention will be given before a detailed description thereof. Embodiments of the present invention relate to a MIMO system comprised of at least two radio apparatuses. One of the radio apparatuses corresponds to a transmitting apparatus whereas the other thereof corresponds to a receiving apparatus. The transmitting apparatus generates one packet signal composed of a plurality of streams. In particular, a description will be given here of a processing performed when the transmitting apparatus transmits training signals. Any known technique may be used for the adaptive modulation processing using the aforementioned rate information and the beamforming and therefore the repeated explanation will be omitted here.

The transmitting apparatus assigns to a header portion of a packet signal a known signal for use in channel estimation ^'in a legacy system (hereinafter referred to as "legacy known signal") , and assigns a control signal, a channel estimation known signal and a data signal to positions posterior to the legacy known signal. Since the number of subcarriers used for a MIMO system is greater than that used for a legacy system, the number of subcarriers used for the channel estimation known signal and data signal is greater than that used for conventional known signal. On the other hand, to improve the transmission efficiency of packet signals it is desirable that the length of known signals contained in a packet signal be shorter. Accordingly, the legacy known signal is used as part of the channel estimation known signal. Subcarrier parts running short in the legacy known signals, among the channel estimation known signals, are contained in the control signal .

When the training signals are produced from the packet signals defined by the -packet format as above, the number of subbcarriers runs short if the legacy known signals are also used for sub-streams. Thus, the required channel estimation cannot be carried out. As a result, there is a possibility that the estimation of channel characteristics will be degraded. Also, since the number of streams to which the AGC known signals are assigned differs from that to which the channel estimation known signals are assigned, there is o y a possibility that the error in estimation of channel characteristics in the receiving apparatus will deteriorate. For these reasons, the following processing will be carried out in the third embodiment. ^' . A transmitting apparatus according to the third embodiment separates a channel known signal into a part of streams where data signals are assigned and a part of streams where no data signal is assigned. Here, part corresponding to a stream where data signals are assigned (hereinafter referred to as "main stream") is called a first known signal, whereas part corresponding to a stream where no data signal is assigned (hereinafter referred to as "sub- stream") will be called a second known signal. The transmitting apparatus assigns signals in the order of an AGC known signal, a legacy known signal, a control signal, a first known signal, ^"a second known signal and a data signal. In other words, the transmitting apparatus sets, in a main stream, a blank period after the first known signal and sets a data signal after the bland period. Here, the blank period corresponds to a period in which the second known signal is assigned in a sub-stream.

As described earlier, for a main stream, a known component for channel estimation is composed of a first known signal and a component assigned in part of subcarriers of a control signal. On the other hand, the number of subcarriers used in the second known signal is .so defined as to equal that used in the data signal. Accordingly, even when the control signal is not assigned to a sub-stream, the use of only the second known signal makes it possible to estimate the channel characteristics for the sub-stream. It is assumed herein that training signals are transmitted from the first radio apparatus 10a to the second radio apparatus 10b of FIG. 2.

FIGS. 15A to 15D show packet formats for a communication system 100. The packet formats shown in the in FIGS. 15A to 15D are not the formats of training signals but those of ordinary packet signals. FIG. 15A represents a case where the number of streams is "4", and FIG. 15B a case where the number of streams is "2". FIG. 15C has the same format as FIG. 15A, and shows a case where the timing shift amounts are represented by "Ans", "Bns" and "Cns". FIG. 15D has the same format as FIG. 15B, and shows a case where the timing shift amount is represented by "Ans". In FIG. 15A, it is assumed that data contained in four streams are to be transmitted, and packet formats corresponding to the first to fourth streams are shown in order from top row to bottom row. In the packet signal corresponding to the first stream, "L-STF", "HT-LTF" and the like are assigned as preamble signals.

"L-STF", "L-LTF", "L-SIG", "HT-SIGl" and "HT-SIG2" are a known signal for AGC setting, a known signal for channel estimation and a control signal compatible with a legacy system, and a control signal compatible with a MIMO system, respectively. "HT-SIGl" and "HT-SIG2" will be generically referred to as "HG-SIG". The control signal compatible with a MIMO system, for example, has information on the number of streams included therein. "HT-STF" and "HT-LTF" are a known signal for AGC setting and a known signal for channel estimation for a MIMO system, respectively. On the other hand, "Data 1" is a data signal. Note that L-LTF and HT-LTF are used not only for AGC setting but also for timing setting.

In the packet signal corresponding to the second stream, "L-STF (-50ns) ", "HT-LTF (-400ns) " and the like are assigned as preamble signals. In the packet signal corresponding to the third stream, "L-STF (-100ns) ", "HT- LTF (-200ns)" and the like are assigned as preamble signals. In the packet signal corresponding to the fourth stream, "L- STF (-150ns)", "HT-LTF (-600ns) " and the like are assigned as preamble signals.

Here, ^λλ-400ns" and the like indicate the amounts of timing shift in CDD. The CDD is a processing where in a predetermined interval a time-domain waveform is shifted, by a shift amount, in a posterior direction and then the waveform pushed out from the rearmost part in the predetermined interval is assigned cyclically in a header portion of the predetermined interval. In other words, "L- STF (-400ns)" is "L-STF" given a cyclic timing shift by a delay of -400ns. Assume that L-STF and HT-STF is each composed of a repetition of an 800 ns duration and that the other HT-LTF and the like are each constituted by a repetition of a 3.2 μs duration. It is also to be noted

that "Data 1" to "Data 4" are also subjected to CDD and the amounts of timing shift are of the same values as those for HT-LTFs assigned anterior thereto.

In the first stream, HT-LTFs are assigned in the order of "HT-LTF", "-HT-LTF", "HT-LTF" and "-HT-LTF" from the top. Here, these in this order are called "a first component", "a second component", "a third component" and "a fourth component" in all the streams. A receiving apparatus extracts a desired signal for the first stream by computing "first component minus (-) second component plus (+) third component minus (-) fourth component" for received signals of all the streams. The receiving apparatus extracts a desired signal for the second stream by computing "first component + second component + third component + fourth component" for received signals of all the streams. The receiving apparatus extracts a desired signal for the third stream by computing "first component - second component - third component + fourth component" for received signals of all the streams. The receiving apparatus extracts a desired signal for the fourth stream by computing "first component + second component - third component - fourth component" for received signals of all the streams. Note that the addition and subtraction processing is done by vector operation.

As with a legacy system, "52" subcarriers are used for the part from "L-LTF" to "HT-SIGl" and so forth. Note that "4" subcarriers out of the "52" subcarriers correspond to pilot signals. On the other hand, the part of "HT-LTF" or the like and thereafter uses "56" subcarriers.

FIG. 15B is similar to the first stream and second stream of the packet formats shown in FIG. 15A. Here, the assignment of "HT-LTFs" in FIG. 15B differs from that of "HT-LTFs" in FIG. 15A. That is, there are only the first components and the second components of HT-LTFs. In the first stream, HT-LTFs are assigned in the order of "HT-LTF" and "HT-LTF" from the top. A receiving apparatus extracts a desired signal for the first stream by computing "first component + second component" for received signals of all the streams. Also, the receiving apparatus extracts a desired signal for the second stream by computing "first component - second component" for received signals of all the streams.

FIGS. 16A and 16B show another packet formats for a communication system 100. The packet formats shown in the in FIGS. 16A and 16B correspond to those for improving the transmission efficiency in the packet formats in FIGS. 15A and 1OB. Hereinafter, those for improving the transmission efficiency in the packet formats in FIG. 15A and '10B will be referred to as "short formats", and in association with this "short format" the packet formats as shown in FIGS. 15A and 1OB will be called "long formats". In other words, those shown in FIGS. 16A and 16B are such that part of "HT-STF" are .shared with "L-STF" and "L-STF" is used as a substitute for "HT-LTF". As a result, the length of known signal is made shorter than that in the case of FIGS. 15A and 15B. FIG. 16A shows a short format over FIG. 15A. Comparing FIG. 16A with FIG. 15A, the header portions and HT-STFs are removed in FIG. 16A from among the four "HT-LTFs" and so forth in each of stream shown in FIG. 15A.

In FIG. 16A, "L-LTF" is used as a header portion of the four "HT-LTFs" and so forth. Here, as described above, "HT-LTF" uses 56 subcarriers. That is, the subcarriers corresponding to the subcarrerier numbers "-28" to "28" shown in FIG. 1 are used. On the other hand, "L-LTF" uses 52 subcarriers as described above. That is, the subcarriers corresponding to the subcarrerier numbers "-26" to "26" shown in FIG. 1 are used. Note that the values at subcarriers corresponding to the subcarriers numbers "-26" to "26" are common to both "HT-LTF" and "L-LTF". Accordingly, when "L-LTF" is used as a substitute for "HT- LTF", values corresponding to the subcarriers numbers "-28", "-27", "27" and "28" are missing. In order to cope with this, in FIG. 16A the subcarriers corresponding to the subcarrier numbers "-28", "-27", "27" and "28" are added for "L-SIG'" and then the values corresponding to those of "HT-LFT" are assigned to the added subcarriers . As a result, if "HT-LTF" is counted . as one unit, the one unit will be also constructed by "L- LTF'-' and part of "L-SIG'".

FIG. 16B shows a short format associated with FIG. 15B. Comparing FIG. 16B with FIG. 15B, the header portions and HT-STFs are removed in FIG. 16B from among the two "HT-LTFs" and so forth in each of stream shown in FIG. 15B. Now, since the header portion of FIG. 16B is constructed similarly to FIG. 16A, the repeated description will be omitted here.

FIGS. 17A to 17C show packet formats for use with training signals in^' a communication system 100. Note that FIGS. 17A to 17C show training signals corresponding to short formats. FIG. 17A represents a case where the number of streams to which a data signal is assigned is "2", and FIGS. 17B and 17C a case where the number of streams to ^' which a data signal is ^'assigned is "1". That is, a data signal is assigned to each of the first stream and the second stream in FIG. 17A, whereas a data signal is assigned to the first stream in FIGS. 17B and 17C. The assignment up to HT-LTF in the first stream and the second stream in FIG. 17A is the same as that of FIG. 16B. In a position posterior thereto, however, a blank duration is provided in the first stream and the second stream. In the third and fourth streams, on the other hand, HT-LTFs are assigned to the position corresponding to the blank duration in the first and second streams. Following the HT-LTFs assigned in the third and fourth streams, Data are assigned to the first and second streams.

- The assignment as described above makes the number of streams to which "HT-STF" is assigned equal to the number of main streams, so that the error contained in the gain set by "L-STF" becomes small at a receiving apparatus, thus preventing the worsening of data signal receiving characteristics. Also, since the "HT-STFs" assigned to the third and fourth streams are only assigned to these two streams, the error contained in the gain set by "L-STF", becomes small at a receiving apparatus, thus preventing a drop in the accuracy of channel estimation.

In the first stream and second stream, namely, in the main stream, the structure of known signals for channel estimation is the same as that shown in FIG. 16B. "L-LTF", part of "L-SIG'" and "HT-LTF" form known signal for channel estimation. In the third stream and fourth stream, namely, in the sub-stream, the structure of known signals for channel estimation is the same as that shown in FIG. 15B. The sub-stream is formed by one stream composed of "HT-LTF" and HT-LTF" and the other stream composed of "HT-LTF" and "- HT-LTF". In the main stream the channel efficiency is enhanced by use of short formats and, at the same time, in the sub-stream a channel characteristic corresponding to the sub-stream can be derived by use of "HT-LTF" constituted by "56" subcarriers .

Note that -the amounts of timing shift for the third and fourth stream shown in FIG. 17A are represented by "Ans" and "Bns", respectively. Here it is assumed that the degrees of priority for the amounts of timing shift are defined in the descending order of "0 ns", "-200 ns", "-100 ns" and "100 ns". In other words, "0 ns" has the highest degree of priority, and "100 ns" the lowest. Here, for each of the main stream and the sub-stream, the amounts of timing shift are used in descending degree of priority. Accordingly, the values of "0 ns" and "-200 ns" are used as timing shift amounts in the first stream and the second stream, respectively. In such a case, the values of "0 ns" and "-200 ns" are also used as timing shift amounts, respectively, in the third stream and the fourth stream, so that "Ans" becomes "0 ns" and "Bns" becomes "-200 ns". As a result, when "HT-LTF" in the first stream and "-HT-LTF" (- 200 ns) in the second stream are deformed and modified, the thus deformed and modified fields are also used in the third and fourth streams, thus making the processing simpler.

Different amounts of timing shifts may also be set respectively to the amounts of timing shift for a plurality of streams. The timing shift amount of "0 ns" is set for the first stream. The timing shift amount "-200 ns" is set for the second stream. The timing shift amount of "-100 ns" is set for the third stream. The timing shift amount of "- 100 ns" is set for the fourth stream. Accordingly, the timing shift amounts of ^λλ-100 ns" and "100 ns" are used in the third stream and the fourth stream, respectively, instead of the above-described timing shift amounts of "0 ns" and "-200 ns" in the third stream and the fourth stream, respectively. And ^λλAns" is substituted by "-100" and "Bns" is substituted by "100 ns". The structure of a known signal for channel estimation in the first stream, namely, the main stream in FIG. 17B is the same as that described so far. Since the main stream is composed of a single stream, a single "HT-LTF" only should be contained in the main stream. As described above, one "HT-LTF" is substituted by part of "L-SIG'" and "L-LTF", so that "HT-LTF" is not contained in the main stream of FIG. 17B. In a position posterior to "HT-SIG" of the first stream, a blank duration is provided in the first stream. In the second to fourth streams, on the other hand, HT-LTFs are assigned to the positions corresponding to the blank duration in the first stream. And, following the HT-LTFs assigned in the second to fourth streams, Data is assigned to the first stream.'

Here it is assumed that the degrees of priority are given to the combinations of sign of "HT-LFT". That is, the combination of signs in the first stream has the highest degree of priority, and the combination of signs in the fourth stream has the lowest degree of priority. For the main stream, the combination of signs is used in descending degree of priority and at the same time, for the sub-streams, the combination of signs is used in descending degree of priority. In this manner, the combinations of signs for the main stream and the sub-stream are set identical to each other. As a result, when the receiving apparatus carries out the operation of plus (+) and minus (-) and then retrieves each component, the same common circuit can be used for both the calculation of channel characteristics for "HT-LTF" in the main stream and that for "HT-LTF" in the sub-stream.

The packet format of FIG. 17C is structured the same way as for that of FIG. 17B. However, the combination of the signs of "HT-LTF" in FIG. 17C differs from that in FIG. 17B. Here the combination of signs of "HT-LTF" is s.o defined that an orthogonal relationship holds among the streams. Furthermore, in FIG. 17C, the combination of the signs of "HT-LTF" is so defined as to be fixed for each of a plurality of streams. Note that the amounts of timing shift in FIG. 17B and FIG. 17C are represented by "Ans", "Bns" and "Cns". The values therefor were described above and the description thereof is omitted here. FIG. 4 illustrates a structure of a first radio apparatus 10a. The first radio apparatus 10a includes a first radio unit 20a, a second radio unit 20b, ... and a fourth radio unit 2Od, which are generically referred to as "radio unit 20", a baseband processing unit 22, a modem unit. 24, an IF unit 26 and a control unit 30. Signals involved include a first time-domain signal 200a, a second time- domain signal 200b, ... and a fourth time-domain signal 20Od, which are generically referred to as "time-domain signal 200", and a first frequency-domain signal 202a, a second frequency-domain signal 202b, a third frequency-domain signal 202c and a fourth frequency-domain signal 202d, which are generically referred to as "frequency-domain signal 202". The second radio apparatus 10b has a structure similar to that of the first radio apparatus 10a. Accordingly, in the following description, the description on the receiving operation corresponds to the processing by the second radio apparatus 10b, whereas the description on the transmission operation corresponds to the processing by the first radio apparatus 10a. This correspondence may be reversed, too. As a receiving operation, the radio unit 20 carries out frequency conversion of radiofrequency signal received by the antennas 12 so as to derive baseband signals. The radio unit 20 outputs the baseband signals to the baseband processing unit 22 as the time-domain signals 200. The baseband signal, which is composed of in-phase components and quadrature components, shall generally be transmitted by two signal lines. For the clarity of figure, the baseband signal is presented here by a single signal line only. An

AGC unit and an A-D conversion unit are also included. The AGC unit sets the gain in "L-STF" and "HT-STF".

As a transmission operation, the radio unit 20 carries out .frequency conversion of baseband signals from the baseband processing unit 22 so as to derive radiofrequency signals. Here, the baseband signal from the baseband processing unit 22 is also indicated as the time-domain signal 200. The radio unit 20 outputs the radiofrequency signals to the antennas 12. That is, the radio unit 20 transmits radio-frequency packet signals from the antennas 12. A PA (power amplifier) and a D-A conversion unit are also included. It is assumed herein that the time-domain signal 200 is a multicarrier signal converted to the time domain and is a digital signal.

As a receiving operation, the baseband processing unit 22 converts a plurality of time-domain signals 200 respectively into the frequency domain and performs adaptive array signal processing on the thus converted frequency- domain signals. Then the baseband processing unit 22 outputs the result of adaptive array signal processing as the frequency-domain signals 202. One frequency-domain signal 202 corresponds to data contained in each of a plurality of streams transmitted from the second radio apparatus 10b, not shown here. As a transmission operation, the baseband processing unit 22 inputs, from the^' modem unit 24, the frequency-domain signals 202 serving as signals in the frequency domain, converts the frequency-domain signals into time domain and then outputs the thus converted signals as time-domain s-ignals 200 by associating them respectively with a plurality of antennas 12.

FIG. 5 illustrates a structure of a frequency-domain signal. Assume herein that a combination of subcarrier numbers "-28" to "28" shown in FIG. 1 constitutes a so- called "OFDM symbol". An "i"th OFDM symbol is such that subcarrier components are arranged in the order of subcarrier numbers "1" to "28" and subcarrier numbers "-28" to "-1". Assume also that an "(i-1) "th OFDM symbol is placed before the "i"th OFDM symbol, and an "(i-/-l) "th OFDM symbol is placed after the "i"th OFDM symbol. It is to be noted here that, in the portions such as "L-SIG" shown in FIG. 15A or the like, a combination of from the subcarrier number "-26" to the subcarrier number "-26" is used.

Now refer back to FIG. 4. To produce the . packet formats corresponding to FIGS. 15A and 15B, FIGS. 16A and

16B and FIGS. 17A to 17C, the baseband processing unit 22 carries out CDD. The baseband processing unit 22 may perform the multiplication of a steering matrix to deform or modify the packet format produced. Such processing will be discussed later.

As a receiving processing, the modem unit 24 demodulates and deinterleaves the frequency-domain signal 202 outputted from the baseband processing unit 22. The demodulation is carried out per subcarrier. The modem unit 24 outputs the demodulated signal to the IF unit 26. As a transmission processing, the modem unit 24 carries out interleaving and modulation.- The modem unit 24 outputs the modulated signal to' the baseband processing unit 22 as a frequency-domain signal 202. When the transmission processing is carried out,, the modulation scheme is specified by the control unit 30.

As a receiving processing, the IF unit 26 combines signals outputted from- a plurality of modem units 24 and then forms one data stream. The IF unit 26 decodes the one data stream. The IF unit 26 outputs the decoded data stream. As a transmission processing, the IF unit 26 inputs one data stream, then codes ^'it and, thereafter, separates the coded data stream. Then the IF unit 26 outputs the thus separated data to the plurality of modem units 24. When the transmission processing is carried out, the coding rate is specified by the control unit 30. Here, an example of the coding is convolutional coding, whereas an example of decoding is Viterbi decoding.

The control unit 30 controls the timing and the like of the first radio apparatus 10a. The control unit 30 produces packet signals composed of a plurality of streams as shown in FIGS. 15A and 15B and FIGS. 17A to 17C in cooperation with the IF unit 26, the modem unit 24 and the baseband processing unit 22. Though the description of the processing for generating the packet signals shown in FIGS. 15A and 15B and FIGS. 16A and 16B is omitted here, it is preferred that the relevant part of the processing corresponding to that for generating the packet signals shown in FIGS. 17A to 17C be executed. For the baseband processing unit 22, the control unit 30 assigns Data to at least one main stream in a plurality of streams, and assigns HT-LTF to a position anterior to the Data in the main stream. This corresponds to the arrangement in the first main stream and second main stream shown in FIG. 17A. When the main stream is composed of a single stream, HT-LTF is not assigned therein. Accordingly, L-LTF and L-SIG' are assigned anterior to Data. For the sub-streams, the control unit 30 assigns HT-LTFs to the timings other than those at which the respective signals in the main stream are assigned. This corresponds to the arrangement in the third sub-stream and fourth sub-stream shown in FIG. 17A. As a result of the above, the baseband processing unit 22 produces the packet signals of the packet format shown in FIG. 17A.

The control unit 30 so defines that the number of subcarriers in one of known signals assigned to the main stream, namely, L-LTF is smaller than the number of subcarriers in Data. As described above, the number of subcarriers for L-LTF is defined to be "52", whereas the number of subcarriers for Data is defined to be "56". The known components corresponding to the subcarriers not contained in L-LTF among a plurality of subcarriers which are to constitute Data is so defined as to be contained in L-SIG' . Accordingly, of "56"- subacarriers that constitute HT-LTF, the components equivalent to "52" subcarriers are assigned to L-LTF and those equivalent to "4" subcarriers are assigned to L-SIG' . Thus, L-SIG' is composed of "56" subcarriers, too. On the other hand, the control unit 30 so defines that the number of subcarriers in HT-LTF in the sub- stream is made equal tσ the number of subcarriers in Data. Further explanation from a different perspective will now be given of the above processing. The control unit 30 defines short formats as shown in FIGS. 16A and 16B and long formats as shown in FIGS. 15C and 15D. The control unit 30 uses L-LTF, L-SIG' and HT-LTF defined by the short formats as shown in FIGS. 16A and 16B to transmit the known components in the main streams. And the control ^'unit 30 uses HT-LFT defined by the long formats as shown in FIGS.

15C and 15D to transmit the known components in the sub- streams.

For the baseband processing unit 22, the control unit 30 applies CDD to HT-LTF and the like assigned to the main, stream. Where one stream serves as a reference, the CDD is equivalent to applying a cyclic shifting within HT-LFT to HT-LFT assigned to the other streams. The control unit 30 applies CDD also to HT-LTF assigned to sub-streams. The control unit 30 sets, in advance, degrees of priority for the amounts of timing shift. As described above, here the amount of timing shift ^λλ0 ns" is given the maximum degree of priority, and following this the degree of priority is so set that it decreases in the order of "-200 ns", "-100 ns" and "100 ns".

For the main streams-, the control unit 30 has the baseband processing unit 22 use the amounts of timing shift sequentially in order from that having a high degree of priority. For example,- referring to FIG. 17A, "0 ns" is used for the first stream and ^Λλ-200 ns" is* used for the second stream. For the sub-streams, too, the control unit 30 has the baseband processing unit 22 use the amounts of timing shift sequentially from that having the high degree of priority. For example, referring to FIG. 17A, "0 ns" is used for the third stream and ^λλ-200 ns" is used for the fourth stream. The control unit 30 also has the 'baseband processing unit apply CDD to Data and has it use the timing shift amounts for the main streams. It is to be noted that the control unit 30 may set different amounts of timing shift to a plurality of streams, respectively. In the case of FIG. 17A, for example, "0 ns" is used for the first stream, ^λλ-200 ns" is used for the second stream, "-100 ns" is used for the third stream, and "100 ns" is sued for the fourth stream.

By the above processing, after the generation of packet formats as shown in FIGS. 17A to 17C, the control unit 30 may have the baseband processing unit 22 deform or modify the packet signals such as these and then transmit the deformed or modified packet signals to the radio unit 20. The baseband processing unit 22 extends the number of main streams to the number of a plurality of streams, and then applies CDD to the extended stream. The baseband processing unit 22 also extends the number of sub-streams to the number of a plurality of streams, and then applies CDD to the extended stream. Here, the control unit 30 sets the amounts of timing shift in a manner that the absolute value of a timing shift amount when the packet signals shown in FIGS. 17A to 17C are generated is larger than the absolute value of a timing shift amount when the packet signals shown in FIGS. 17A and 17B are deformed. In terms of hardware, this structure described as above can be realized by a CPU, a memory and other LSIs of an arbitrary computer. In terms of software, it can be realized by memory-loaded programs which have communication functions and the like, but drawn and described herein are function blocks that are realized in cooperation with those. Hence, it is understood by those skilled in the art that these function blocks can be realized in a variety of forms such as by hardware only, software only or the combination thereof .

FIG. 18 illustrates a structure of a baseband processing unit 22. The baseband processing unit 22 includes a processing unit for use with receiving 50 and a processing unit for use with transmission 52. The receiving processing unit 50 executes a part, corresponding to a receiving operation-,^' of operations by the baseband processing unit 22. That is, the receiving processing unit 50 performs adaptive array signal processing on time-domain signals 200 and therefore derives receiving weight vectors. Then the receiving processing unit 50 outputs the result of array synthesis as the frequency-domain signal 202. It is to be noted here that the receiving processing unit 50 may generate rate information based on the frequency-domain signals 202. As for the generation of rate information, a known technique se'rves the purpose as mentioned above and the explanation thereof is omitted here. The transmitting processing unit 52 executes a part, corresponding to a transmission operation, of operations by

SA-70257WO the baseband processing unit 22. That is, the transmitting processing unit 52 converts the frequency-domain signal 202 so as to generate the time-domain signal 200. The transmitting unit 52 associates a plurality of streams with a plurality of antennas 12, respectively. The transmitting processing unit 52 applies CDD as shown in FIGS. 17A to 17C. -The transmitting processing unit 52 may execute an operation using a steering matrix. The transmitting processing unit 52 outputs finally the time-domain signals 200. On the other hand, the transmitting processing unit 52 may execute beamforming when transmitting the packet signals. As for the beamforming, a known technique serves the purpose as mentioned above and the explanation thereof _^is omitted here. FIG. 19 illustrates a structure of the receiving processing unit 50. The receiving processing unit 50 includes an FFT unit 74, a weight vector derivation unit 76, and a first combining unit 80a, a second combining unit 80b, a third combining unit 80c and a fourth combining unit 8Od, which are generically referred to as "combining unit 80". The FFT unit 74 performs FFT on the time-domain signal 200 so as to convert the time-domain signal 200 into a frequency-domain value. It is assumed here that the frequency-domain value is structured as shown in FIG. 5. That is, a frequency-domain value for one time-domain signal 200 is outputted via one signal line.

The weight vector derivation unit 76 derives a weight vector from a frequency-domain value, on a subcarrier-by- subcarrier basis. The weight vector is so derived as to correspond to each of a plurality of streams, and a weight vector for one stream contains factors corresponding to the number of antennas, for each stream. HT-LTF and the like are used in deriving a weight vector corresponding to each of a plurality of streams. To derive the weight vector, an adaptive algorithm may be used or a channel characteristics may be used. Since a known technique may be employed in the processing for the adaptive algorithm and so forth, the explanation thereof is omitted here. When deriving the weight vector, the weight vector derivation unit 76 executes an operation of the first component minus (-) the second component plus (+) the third component minus (-) the fourth component or the like, as described earlier. As also described above, the weights are derived finally for each of subcarriers, antennas 12 and streams, respectively.

The combining unit 80 combines the frequency-domain value converted by .the ^'FFT unit 74 and the weight vector from the weight vector derivation unit 76. For example, as the weight vector to which a multiplication is to be executed, a weight which corresponds to both one subcarrier and the first stream is selected from among the weight vectors from the weight vector derivation unit 76. The selected weight has a value corresponding to each antenna 12. As another weight vector to which a multiplier is to be executed, a value corresponding to one subcarrier is selected from among the frequency-domain values converted by the FFT unit 74. The selected value contains a value corresponding to each antenna 12. Note that both the selected weight and the selected value belong to the same subcarrier. While being associated respectively with the antennas 12, the selected weight and the selected value are respectively multiplied and the multiplication results are summed up. As a result, a value corresponding to one subcarrier in the first stream is derived. In the first combining unit 80a, the above-described processing is performed on the other subcarriers so as to derive data corresponding to the first stream. The similar processing is carried out to derive data corresponding respectively to the second to fourth streams. The derived first to fourth streams are outputted as the first frequency-domain signal 202a to the fourth frequency-domain signal 202d, respectively.

FIG. 20 illustrates a structure of the transmitting processing unit 52. The transmitting processing unit 52 includes a distribution unit 166 and an IFFT unit 168. The IFFT unit 168 performs IFFT on the frequency-domain signals 202 and then outputs time-domain signals. As a result thereof, the IFFT unit 168 outputs the time-domain signal corresponding to each stream.

The distribution unit 166 associates the streams from the IFFT unit 168 with the antennas 12. To produce the packet signals corresponding to FIGS. 15A and 15B, FIGS. 16A and 16B and FIGS. 17A to 17C, the distribution unit 166 carries out CDD.^' CDD is expressed as a matrix C in the following Equation (3-1) .

CCO = <ϋag(l, exρ(- }2π£b I Nout), ^{• • •} , exp(- j2π^δ(Nout - 1) / Nout) — ( 3 -

D where δ indicates a shift amount and £ a subcarrier number. The multiplication of the matrix C by a stream is done on a subcarrier-by-subcarrier basis. That is, the distribution unit 166 carries out a cyclic time shifting within the L-STF and so forth per stream. The amount of timing shift is set to the above-described degree of priority.

The distribution unit 166 may multiply respectively the training signals produced, as in FIGS. 17A to 17C, by a steering matrix so as to increase the number of streams for training signal up to the number of a plurality of streams. FIG. 21 shows a packet format of packet signals finally transmitted in the communication system 100. FIG. 21 is equivalent to a case where the packet signals of FIG. 17A are deformed. The first stream and the second stream in FIG. 17A undergo the operation by an orthogonal matrix described later. As a result, "HT-LFTl" to "HT-LTF12" and so forth are produced. The CDD in timing shift amounts of "0 ns", ^ΛΛ- 50 ns", "-100 ns" and "-150 ns" is applied to the first to the fourth stream, respectively. The absolute value of timing shift amount in the CDD for the second is so set as to be smaller than the absolute value of timing shift amount in the initial CDD applied. The similar processing is carried out to "ΗT-LTFs" and so forth assigned to the third >and .the fourth stream. The similar processing is carried out to the packet format of signals shown in FIG. 17B so as to produce packet signals using the first to- fourth streams.

Now, before carrying out multiplication, the distribution unit 166 extends the degree of inputted signals to the number of a plurality of streams. In the case of FIG. 17A, the number of signals inputted is "2" in "HT-STF" and the like assigned to the first and the second stream, and this will be represented by ^λλNin" here. Accordingly, the inputted data are indicated by a vector of ^NλNinXl". The number of a plurality of streams is "4" and this is represented by ^λλNout" here. The distribution unit 166 extends the degree of inputted data to Nout from Nin. In other words, the vector of ^λλNinXl" is extended to the vector of "Noutxl". In so doing, "0" is inserted to components from the (Nin+l)th row to the Nout-th row. On the other hand, the component up to Nin are "O's" for ^λλHT-LTF", and HT-LTF (-200 ns) and the like are inserted to the components from (Nin+l)th row to the Nout-th row

A steering matrix is expressed by the following Equation ( 3-2) .

SCO = CCOW -— O-2)

^QA_i n? s7WO The steering matrix is a matrix of "NoutXNout". W is an orthogonal matrix of "NoutXNout". An example of the

orthogonal matrices is a Walsh Matrix. Here, £ is the subcarrier number, and the multiplication by a steering matrix is done on a subcarreri-by-subcarrier basis. C denotes CDD as described above. Here, the amounts of timing shift are so defined as to differ for a plurality of streams, respectively.

According to the third embodiment, even in the case when one of known signals for use in channel estimation is to be assigned in L-SIG' and L-LTF in the main streams, all of the known signals for use in channel estimation are assigned to HT-LTF in the sub-streams. Hence, the channel corresponding to the sub-streams can be estimated without using L-SIG' . One of known signals for use in channel estimation is assigned to L-SIG' and L-LTF in the main streams, thus improving the transmission efficiency. All of the known signals for use in channel estimation are assigned to HT-LTF, so that the degradation in channel estimation associated with the sub-streams can be restricted. The short format and the long format are defined beforehand. And L-LTF and the like in the short format are used for the main streams whereas HT-LTF and the like in the long format are used for the sub-streams, so that the processing can be simplied.

Also, the number of streams to which HT-STF is assigned is the same as the number of streams to which data is assigned when generating a training signal. Hence, the gain set by HT-STF is in correspondence to data, thus preventing the worsening of data receiving characteristics . In generating a training signal, the timings at which L-LTF, L-SIG' , HT-LTF and Data are assigned respectively in a main stream are shifted from the timing of HT-LTF assigned to a sub-stream so as to get the received powers of both the streams closer to each other. As a result of this getting the received powers of both the streams closer to each other, even when HT-STF is not assigned to the stream where data is not assigned, it is possible to prevent any worsening of estimation of channel characteristics by said stream.

More of the same timing shift amounts can be used by defining the degrees of priority for the timing shift amounts and using the timing shift amounts in order from one with the highest degree of priority for both the stream where data is assigned and the stream where data is not assigned. Moreover, the processing may be made simpler by using more of the same timing shift amounts. Further, when the number of a plurality of streams is ^λλ2" and the number of streams to which data is assigned is "1", a receiving apparatus may instruct a transmitting apparatus which of the plurality of streams is to have data assigned, according to the receiving condition of L-LTF and/or HT-LTF. In other words, it is possible to execute transmission diversity. Since the timing shift amounts for the respective HT-

LTFs assigned to a plurality of streams are of the same values, a receiving apparatus can cope easily when there are changes in streams that have data assigned. Since different timing shift amounts are set for a plurality of streams, respectively, the processing can be carried out uniformly. Moreover, such a uniformly performed processing makes the processing simpler. Even when the number of streams where data is assigned increases in the subsequent packet signal, the HT-LTF for the stream to have the increase thereof has already been transmitted with the same timing shift amount, so that the receiving apparatus can use the already derived timing and the like. Since it can use the already derived timing and the like, the receiving apparatus can easily cope with the increase in the number of streams where data is assigned.

The present invention has been described based on the embodiments. These embodiments are merely exemplary, and it is understood by those skilled in the art that various modifications to the combination of each component and process thereof are possible and that such modifications are also within the scope of the present invention.

According to the second embodiment of the present invention, an adding unit 66 appends dummy signals as additional signals. The embodiment, however, is not limited thereto, and the adding unit 66 may, for instance, add signals for parity check as additional signals instead.

This modification may contribute to a more effective use of additional signals and improved receiving characteristics. For this modification, it is only required that additional signals be added whose number of subcarriers is equal to the difference between the number of subcarriers used for data signals and the number of subcarriers used for control signals .

According to the second embodiment of the present invention, an adding unit 66 adds dummy signals as additional signals. The embodiment, however, is not limited thereto, and the adding unit 66 may, for instance, add pilot signals as additional signals instead. The pilot signals are known signals. In this modification, the adding unit 66 assigns pilot signals to subcarrriers with subcarrier numbers "-28", "-27", "27" and "28" as shown in FIG. 1. Also, the receiving apparatus uses the pilot signals in carrying out demodulation. It is to be noted that where there are already pilot signals inserted in a plurality of subcarriers with subcarrier numbers from "-26" to "26", the addition of pilot signals by the adding unit 66 is equivalent to the addition of pilot signals. This modification may contribute to improved receiving characteristics. For this modification, the only requirement is such that additional signals be added whose number of subcarriers is equal to the difference between the number of subcarriers used for data signals and the number of subcarriers used for control signals.

According to the third embodiment of the present invention, a signal compatible with the legacy system is added in a leading part of a packet format. As a result, the adding unit 66 does not add an additional signal to the leading control signal "HT-SIG". The arrangement, however, is not limited thereto, and it may be that no signal compatible with the legacy system is added in the leading part of a packet format. Accordingly, the adding unit 66 may add additional signals to all the control signals. According to this modification, the same processing is done on all the control signals, so that the processing can be simpler. For this modification, therefore, the only requirement is such that additional signals be added whose number of subcarriers is equal to the difference between the number of subcarriers used for data signals and the number of subcarriers used for control signals.

According to the -second embodiment of the present invention, it is assumed that the communication system 100 is a MIMO system. The arrangement, however, is not limited thereto, and it may be that the communication system 100 is not a MIMO system. ^' In other words, the arrangement may be such that signals of a single stream are transmitted from a single antenna 12. According to this modification, the present invention can be applied to a variety of' communication systems. That is, the only requirement is that a plurality of subcarriers are used and there is a need to control the variation in the number of subcarriers in the course of a packet signal. _ According to the third embodiment of the present invention, the description has been given of a case when the number of multistreams is "4". However, the present invention is not limited thereto and, for example, the number of a plurality of streams may be less than "4" or may be greater than ^λX4". Along with this example, the number of antennas 12 may be less than "4" in the former case and may be greater than 4" in the latter case. According to this modification, the present invention can be applied to a variety of the number of streams. The present invention described in the first and the third embodiment may be described by the following Item 1 to Item 11, Item 1-1 to Item 1-15, Item 2-1 and Item 3-1 to Item 3-9: Item 1

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; an interleave unit which performs interleaving of a size defined by a first number of subcarriers on a control signal and performs interleaving of a size defined by a second number of subcarriers on a data signal in the plurality of combinations inputted to said input unit; and an adding unit which adds an additional signal to a control signal contained in a second combination and the subsequent combination, wherein said adding unit ^' adds additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers. Item 2

A radio apparatus according to Item 1, wherein the additional signal inserted by said adding unit is a dummy signal . Item 3

A radio apparatus according to Item- 1, wherein the additional signal inserted by said adding unit is a signal for parity check. Item 4

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; an adding unit which adds an additional signal to a control signal contained in a second combination and the subsequent combination; and an interleave unit which performs interleaving of a size defined by a first number of subcarriers on ^'a control signal contained in a first combination and performs interleaving of a size defined by a second number of subcarriers on the remaining signals among a plurality of combinations in 'which the additional signal has been added by s_.aid adding unit, wherein said adding unit adds additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers. Item 5

A radio apparatus according to Item 4, wherein the additional signal inserted by said adding unit is a signal for cyclic redundancy check (CRC) . Item 6

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; an excluding unit which excludes an additional signal from a control signal contained in a second combination and the subsequent combination among the plurality of combinations received by said receiver; and a deinterleave unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal and performs deinterleaving of a size defined by a second number of subcarriers on a data signal in a plurality of combinations in which the additional signal has been excluded by said excluding unit, wherein said excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 7

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; a deinterleave unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal contained in a first combination and performs deinterleaving of a size defined by a second number of subcarriers on the remaining signals among the plurality of combinations received by said receiver; and an excluding unit which excludes an additional signal from a control signal contained in a second and the subsequent combination among the plurality of combinations deinterleaved by said deinterleave unit, wherein said excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 8

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; an interleave unit which performs interleaving of a size defined by a first number of subcarriers on a control signal and performs interleaving of a size defined by a second number of subcarriers on a data signal in the plurality of combinations inputted to said input unit; and an adding unit which adds an additional signal to a control signal in a plurality of combinations interleaved by said interleave unit, wherein said adding unit adds additional signals whose amount corresponds to a difference between^' the second number of subcarriers and the first number of subcarriers. Item 9

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; an adding unit which adds an additional signal to a control signal in the plurality of combinations inputted to said input unit; and an interleave unit which performs interleaving of a size defined by a predetermined number of subcarriers in a plurality of combinations in which the additional signal has been added by said adding unit, wherein said adding unit adds additional signals whose amount corresponds to a difference between the number of subcarriers corresponding to control signals other than the additional signal and the number of subcarriers corresponding to the data signal. Item 10

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; an excluding unit which excludes an additional signal from a control signal in the plurality of combinations received by said receiver; and a deinterleave unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal and performs deinterleaving of a size defined by a second number of subcarriers on a data signal in the plurality of combinations in which the additional signal has been excluded by said excluding unit, wherein said excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 11

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein a combination uses a plurality of subcarriers; a deinterleave unit which performs deinterleaving of a size defined by a predetermined number of subcarriers in the plurality of combinations received by said receiver; and an excluding unit which excludes an additional signal from a control signal in a plurality of combinations deinterleaved by said deinterleave unit, wherein said excluding unit excludes additional signals whose amount corresponds to a difference between the number of subcarriers corresponding to the control signals other than the additional signal and the number of subcarriers corresponding to a data signal. Item 1-1

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; an interleave unit which performs interleaving of a size defined by a first number of subcarriers on a control signal and performs interleaving of a size defined by a second number of subcarriers on a data signal in the plurality of combinations inputted to the input unit; and an adding unit which adds an additional signal to a control signal contained in a second combination and the subsequent combination, wherein the adding unit adds additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers. Item 1-2 A radio apparatus as described in Item 1-1, wherein the additional signal inserted by the adding unit is a dummy signal . Item 1-3

A radio apparatus as described in Item 1-1, wherein the additional signal inserted by the adding unit is a signal for parity check. Item 1-4

A radio apparatus as described in Item 1-1, wherein the additional signal inserted by the adding unit is a known signal. Item 1-5

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; an adding unit which adds an additional signal to a control signal contained in a second combination and the subsequent combination; and an interleave unit which performs interleaving of a size defined by a first number of subcarriers on a control signal contained in a first combination and performs interleaving of a size defined by a second number of subcarriers on the remaining signals among a plurality of combinations to which the additional signal has been added, wherein the adding unit adds additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers. Item 1-6

A radio apparatus as described in Item 1-5, wherein the additional signal inserted by the adding unit is a signal for cyclic redundancy check (CRC) . Item 1-7

A radio apparatus as described in Item 1-5, wherein the additional signal inserted by the adding unit is a known signal. Item 1-8

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; an excluding unit which excludes an additional signal from a control signal contained in a second combination and the subsequent combination among the plurality of combinations received by the receiver; and a deinterleave unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal and performs deinterleaving of a size defined by a second number of subcarriers on a data signal in a plurality of combinations in which the additional signal has been excluded by the excluding unit, wherein the excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 1-9

A radio apparatus, comprising: a receiver which receives a plurality .of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; a deinterleave unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal contained in ^' a first combination and performs deinterleaving of a size defined by a second number of subcarriers on the remaining signals among the plurality of combinations received by the receiver; and an excluding unit which excludes an additional signal from a control signal contained in a second and the subsequent combination among the plurality of combinations deinterleaved by the deinterleave unit, wherein the excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers. Item 1-10

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; an interleave unit which performs interleaving of a size defined by a first number of subcarriers on a control signal and performs interleaving of a size defined by a second number of subcarriers on a data signal in the plurality of combinations inputted to said input unit; and an adding unit which adds an additional signal to a control signal in a plurality of combinations interleaved by the interleave unit, wherein the adding unit adds additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers. Item 1-11

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; an adding unit which adds an additional signal to a control signal in the plurality of combinations inputted to the input unit; and an interleave^' unit which performs interleaving of a size defined by a predetermined number of subcarriers in a plurality of combinations to which the additional signal has been added by the adding unit, wherein the adding unit adds additional signals whose amount corresponds to a difference between the number of subcarriers corresponding to control signals other than the additional signal and the number of subcarriers corresponding to the data signal. Item 1-12

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; an excluding unit which excludes an additional signal from a control signal in the plurality of combinations received by the receiver; and a deinterleave^' unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal and performs deinterleaving of a size defined by a second number of subcarriers on a data signal in the plurality of combinations in which the additional signal has been excluded by the excluding unit, wherein the excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 1-13

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; a deinterleave unit which performs deinterleaving of a size defined by a predetermined number of subcarriers in the plurality of combinations received by the receiver; and an excluding unit which excludes an additional signal from a control signal in a plurality of combinations deinterleaved by the deinterleave unit, wherein the excluding unit excludes additional signals whose amount corresponds to a difference between the number of subcarriers corresponding to the control signals other than the additional signal and the number of subcarriers corresponding to a data signal. Item 1-14

A radio apparatus, comprising: a generator which generates a packet signal in which a control signal is assigned to any internal within the packet signal, the packet signal using a plurality of subcarriers; and a transmitter which transmits the packet signal generated by the generator, wherein the generator adds an additional signal to a control signal, whose number of subcarriers required is less than the number of subcarriers used in a preceding interval, in such a manner that the number of subcarriers required equals the number of subcarriers used in the preceding interval. Item 1-15

A radio apparatus as described in Item 1-14, wherein the generator generates also a packet signal such that the number of subcarriers required of a control signal is equal to the number of subcarriers used in a preceding interval, and wherein when generating said packet signal, the generator stops adding the additional signal. Item 2-1 A radio apparatus, comprising: a receiver which receives a packet signal in which a control signal is assigned to any internal within the packet signal, the packet signal using a plurality of subcarriers; an identifying unit which identifies a form of the packet signal received by the receiver; and a processing unit which processing the packet signal received by the receiver, according to the format of the packet signal identified by the identifying unit, wherein, in a first format of the packet signal identified by the identifying unit, an additional signal is added to a control signal, whose number of subcarriers required is less than the number of subcarriers used in a preceding interval, in such a manner that the number of subcarriers required equals the number of subcarriers used in the preceding interval; in a second format of the packet signal, the number of subcarriers required by the control signal equals the number of subcarriers used in the preceding interval, and wherein for the first format the processing unit excludes the additional signal and then performs processing on the control signal, whereas for the second format the processing unit performs processing on the control signal without excluding the additional signal.

Item 3-1

A radio apparatus for transmitting a packet signal formed by a plurality of streams wherein the packet signal is formed by a plurality of carriers, the apparatus comprising: a generator which generates the packet signal in a manner that, while a data signal is assigned to at least one main stream of a plurality of streams and a known signal and a control signal are assigned anterior to the data signal in the main stream, for a sub-stream to which no data signal is assigned, an extensional known signal is assigned to timing, other than timing at which a known signal, a control signal and a data signal in the main stream are assigned respectively; and a transmitter which transmits the packet signal generated by the generator, wherein while defining in a manner that the number of carriers in one of known signals assigned to the main stream is made smaller than the number of carriers in a^' data signal, the generator defines in a manner that a known component corresponding to carriers not contained in the one of known signals is contained in a control signal, and defines in a manner that the number of carriers in an extensional known signal assigned to the sub-stream is made equal to the number of carriers in a data signal. Item 3-2

A radio apparatus as described in Item 3-1, wherein the generator defines a first packet format arranged in the order of one of known signals, a control signal and a data signal and defines a second packet format arranged in the order of a known signal, defined by the same number of carriers as that in a data signal, and the data signal, and uses the known signal and the control signal defined by the first format to transmit a known component in the main streams and uses the known signal defined by the second packet format to transmit a known component in the sub- stream. Item 3-3 A radio apparatus as described in Item 3-1, wherein, with a known signal, assigned to one of main streams, serving as a reference, the generator performs cyclic timing shift within a known signal on known signals assigned to the other streams and at the same time performs also timing shift on an extensional signal assigned to a sub-stream and wherein a timing shift amount is given a predetermined degree of priority whereby the timing shift amount is used for a main stream in order of decreasing degrees of priority and the timing shift amount is used for a sub-stream also in_. order of decreasing degrees of priority. Item 3-4

A radio apparatus as described in Item 3-1, wherein, with a known signal, assigned to one of main streams, serving as a reference, the generator performs cyclic timing shift within a known signal on known signals assigned to the other streams and at the same time performs also timing shift on an extensional signal assigned to a sub-stream and wherein a different value of timing shift amount is set to each of a plurality of streams. Item 3-5 A radio apparatus as described in Item 3-3 or Item 3-4, wherein the known signal and the extensional known signal are formed by repeating a predetermined unit in time domain and wherein a combination of signs of predetermined units is defined so that orthogonality relation holds among streams and at the same time the combination of signs of predetermined units is fixed.

The "predetermined unit" may be defined not only in the time domain but' also in the frequency domain. In the latter case, the periods corresponding respectively to a plurality of units may differ when the predetermined unit is converted to the time domain. Item 3-6

A radio apparatus as described in Item 3-3 or Item 3-4, wherein the known signal and the extensional known signal are formed by repeating a predetermined unit in time domain and. wherein a combination of signs of predetermined units is defined so that orthogonality relation holds among streams and at the same time the combination of signs of predetermined units is given a predetermined degree of priority whereby the combination of sign is used, for a stream to which a data signal is assigned, in order of decreasing degrees of priority and the combination of signs is used, for a stream to which no data signal is assigned, also in order of decreasing degrees of priority. Item 3-7 A radio apparatus as described in any of Item 3-3 to Item 3-6, wherein the generator performs cyclic timing shift on the data signal and wherein a timing shift amount for a main stream is used as the timing shift amount. Item 3-8 A radio apparatus as described in any of Item 3-3 to Item 3-7, further comprising a deformation unit which deforms or modifies the packet signal generated by the generator and the deformed or modified packet signal to the transmitter, the deformation unit including: a first processing unit which extends the number of 14138

L37 main streams to the number of a plurality of streams and then applies cyclic timing shift within a known signal to known signals assigned to the other streams wherein a known . signal assigned ^'to one of the extended streams serves as a reference; and a second processing unit which extends the number of sub-streams to the number of a plurality of streams and then applies cyclic timing shift within an extensional known signal to extensional known signals assigned to the other streams wherein an extensional known signal assigned to one of the extended streams serves as a reference, wherein each of timing shift amounts used for the streams extended by the first processing unit is so set as to equal each of timing shift amounts used for the streams extended by the second processing unit. Item 3-9

A radio apparatus as described in Item 3-8, wherein the absolute value of timing shift amount in the generator is so set as to be larger than the absolute value of timing shift amount in the deformation unit.

While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims. Fourth embodiment

A problem to be solved by a fourth embodiment of the present invention will be stated as follows. In wireless communications, -adaptive array antenna technology is one of the .technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in a packet signal is called "stream") . That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.

Moreover, combining this MIMO system with the OFDM modulation scheme results in a higher data transmission rate. For the purpose of ^'enhancing the transmission efficiency in this MIMO system, the data signals to be transmitted respectively in a plurality of packets are aggregated into a single packet. In so doing, the control signals 'are appended to the respective data signals. In other words, a plurality of combinations of control signals and data signals are contained in the packet signals. It is generally the case that the information amount of control signals is smaller than that of data signals. Here, MIMO is carried out between a plurality of streams to transmit the data signals. On the other hand, the subcarriers to be used respectively by a plurality of streams are so defined as to be varied, and the control signals are divided into streams, respectively, so as to be transmitted.

Under the above circumstances, the weight in the receiving apparatus differs between when the data signal is received and when the control signal is received. In the case where known signals are appended to the header portion of packet signals and the weights are derived, using said known signals, by the receiving apparatus, there is a possibility that the error rate will worsen in the rear combination. Since the control signal contains important information therein, it is required that the control signals be transmitted more reliably than the data signal.

An outline of the present invention will be given before a detailed description thereof. The Embodiments of the present invention relate to a MIMO system comprised of at least two radio apparatuses. One of the radio apparatuses corresponds to a transmitting apparatus whereas the other thereof corresponds to a receiving apparatus. The transmitting apparatus generates one packet signal in such a manner as to contain a plurality of combinations of control signal and data signal. Note that one packet signal is composed of a plurality of streams. As mentioned earlier, if the weight at the time of receiving a control signal differ from the weight at the time of receiving a data signal, the receiving apparatus must derive the weights for them, respectively. It is desired that the degradation of error rates for the control signals contained in the combination placed in an anterior part of the packet signal be prevented. In the present embodiment, the following processing is executed to solve the above problems.

The transmitting apparatus appends a known signal for use in channel estimation (hereinafter referred to as "first known signal") to a header portion of a packet signal, and appends a known signal for channel estimation (hereinafter referred to as "second known signal") to front portions of the second and the subsequent combinations, respectively. Here, the first known signal is composed of a plurality of symbols, and a subcarrier used in any of the plurality of symbols coincides with a subcarrier used in the control signal. The second known signal is so defied as to be identical to part of the first known signal corresponding to any of the plurality of symbols. When the receiving apparatus receives a packet signal, the receiving apparatus receives data contained respectively in a plurality of combinations while using the weights derived from the first known signal. On the other hand, the receiving apparatus control signal contained in the second and the subsequent combinations, respectively, while using the weights derived from the second known signal. In this manner, the weights at the time of receiving the control signals are derived based on the second known signal which has been assigned immediately prior thereto, so that the degradation of error rates for the control signals assigned to a posterior part of the packet signal can be prevented. Since the second known signal is defined as part of the first known signal, the drop in transmission efficiency can be restricted.

FIG. 1 illustrates a spectrum of a multicarrier signal according to an embodiment of the present invention. In particular, FIG. 1 shows a spectrum of a signal in the OFDM modulation scheme. One of a plurality of carriers in an OFDM modulation scheme is generally called a subcarrier. Herein, however, a subcarrier is designated by a "subcarrier number". In a MIMO system, 56 subcarriers, namely, subcarrier numbers "-28" to "28" are defined. It is to be noted that the subcarrier number "0" is set to null so as to reduce the effect of a direct current component in a baseband signal. On the other hand, in a legacy system, 52 subcarriers, namely, subcarrier numbers "-26" to "26" are defined. One example of legacy systems is a wireless LAN complying with the IEEE802.11a standard. The respective subcarriers are modulated by a modulation scheme which is set variably. Used here is any of modulation schemes among BPSK (Binary Phase-Shift Keying) , QPSK (Quadrature Phase-Shift Keying) , lβ-QAM (Quadrature Amplitude Modulation) and 64-QAM.

Convolutional coding is applied, as an error correction scheme, to these signals. The coding rates for the convolutional coding are set to 1/2, 3/4 and so forth. The number of data to be transmitted in parallel is set variably. The data are transmitted as packet signals and each of packet signals to be transmitted in parallel is called "stream" herein. As a result thereof, since the mode of modulation scheme and the values of coding rate and the number of streams are set variably, the data rate is ^' also set variably. It is to be noted that the "data rates" may be determined by arbitrary combination of these factors or by one of them.

FIG. 2 illustrates a structure of a communication system 100 according to an embodiment of the present invention. The communication system 100 includes a first radio apparatus 10a and a second radio apparatus 10b, which are generically called "radio apparatus 10". The first radio apparatus 10a includes a first antenna 12a, a second antenna 12b, a third antenna 12c and a fourth antenna 12d, which are generically referred to as "antennas 12", and the second radio apparatus 10b includes a first antenna 14a, a second antenna 14b, a third antenna 14c and a fourth antenna

14d, which are generically referred to as "antennas 14". Here, the first radio apparatus 10a corresponds to a transmitting apparatus, whereas the second radio apparatus 10b- corresponds to a receiving apparatus.

An outline of a MIMO system will be given before a description of a structure of the communication system 100. Assume herein that data are being transmitted from the first radio apparatus 10a to the second radio apparatus 10b. The first radio apparatus 10a transmits respectively the data of a plurality of streams from the first antenna 12a to the fourth antenna 12d, respectively. As a result, the data rate becomes higher. The second radio apparatus 10b receives the data of- a plurality of streams by the first antenna 14a to the fourth antenna 14d. The second radio apparatus 10b separates the received signals by adaptive array signal processing and demodulates independently the data of a plurality of streams.

Since the number of antennas 12 is "4" and the number of antennas 14 is also "4" here, the number of combinations of channels between the antennas 12 and the antennas 14 is ^λλ16". The channel characteristic between from the ith antenna 12i to the jth antenna 14j is denoted by hi_j . In FIG. 2, the channel characteristic between the first antenna 12a and the first antenna 14a is denoted by hu, that between from the first antenna 12a to the second antenna 14b by hχ₂, that between the second antenna 12b and the first antenna 14a by h2i, that between from the second antenna 12b to the second antenna 14b by h.22r and that between from the fourth antenna 12d to the fourth antenna 14d by h₄₄. For the clarity of illustration, the other channels are omitted in FIG. 2.

FIG. 11 illustrates packet formats in a communication system 100. For the simplicity of explanation, it is assumed here that the number of streams contained in the packet formats is "2". The stream transmitted from the first antenna 12a is shown in the top row whereas the stream transmitted from the second antenna 12b is shown in the bottom row. In the top row of FIG. 11, "L-STF", "L-LTF", "L-SIG" and "HT-SIG" correspond to a known signal for timing estimation, a known signal for channel estimation, a control signal compatible with a legacy system, and a control signal compatible with a MIMO system, respectively. In the bottom row of FIG. 11, "L-STF + CDD", "L-LTF + CDD", "L-SIG + CDD" and "HT-SIG + CDD" correspond to the results obtained when CDD (Cyclic Delay Diversity) is implemented to "L-STF", "L- LTF", "L-SIG" and "HT-SIG", respectively. The CDD is a processing where in a predetermined interval a time-domain waveform is shifted,' by a shift amount, in a posterior direction and then the waveform pushed out of the rearmost part in the predetermined interval is assigned cyclically to a header portion of the predetermined interval. That is, "L- STF + CDD" is such that "L-STF" has undergone the cyclic timing shifting.

"HT-STF" and "HT-STF'" correspond to known signals, for timing estimation, compatible with a MIMO system, and they are so defined as to use different subcarriers from each other. The both symbols, namely, "HT-STF" and "HT- STF'" are so defined as to use different subcarriers from each other. For example, "HT-STF" uses subcarriers whose subcarrier number is odd, whereas "HT-STF'" uses those whose subcarrier number is even. "HT-LTFl", "HT-LTFl'", "HT-LTF2" and "HT-LTF2'" correspond to known signals, for channel estimation, compatible with a MIMO system. Here, "HT-LTFl" and "HT-LTFl'" are so defined as to use different subcariers from each other, similarly to "HT-STF" and "HT-STF'". The same applies to "HT-LTF2" and "HT-LTF2"'. On the other hand, "HT-LTF2" is so defined as to use the subcarriers that have not been used in "HT-LTFl".

"HT-DATAl" ^'and "HT-DATA2" are data signals. The control signals for "HT-DATAl" and "HT-DATA2" correspond to "HT-SIG" and "HT-SIG + CDD", respectively. Accordingly, a set of "HT-SIG", "HT-SIG + CDD", "HT-DATAl" and "HT-DATA2" is called a "first combination".

"HT-SIGl" and "HT-SIGl'" are control signals for "HT- DATA3" and "HT-DATA4" which are assigned posterior to the "HT-SIGl" and "HT-SIGl'", respectively. "HT-SIGl" and "HT- SIGl'" are so defined as to use subcarriers different from each other, similarly to "HT-STF" and "HT-STF'". Note that the subcarriers used for "HT-SIGl" are the same as those used for "HT-LTFl", and the subcarriers used for "HT-SIGl'" are the same as those used for "HT-SIGl'". Here, "HT-LTFl" and "HT-LTFl'" are assigned anterior to "HT-SIGl" and "HT- SIGl'". "HT-DATA3" and "HT-DATA4" are data signals. A set of "HT-SIGl" and "HT-SIGl'", "HT-DATA3" and "HT-DATA4" is called a "second combination".

The same holds for "HT-SIG2" and "HT-SIG2"', "HT- DATA5" and "HT-DATA6", and a set of these is called a "third combination". "HT-LTFl" and "HT-LTFl'" are assigned anterior to "HT-SIG2" and "HT-SIG2"'

For the above packet formats, the receiving apparatus performs receiving processing on "HT-SIG" contained in the first combination, by use of the weights derived from "L-

LTS". The receiving apparatus performs receiving processing on "HT-DATAl" and so forth, by use of the weights derived from "HT-LTFl", "HT-LTF2", "HT-LTFl'" and "HT-LTF2'". The receiving apparatus performs receiving processing on "HT- SIGl" and "HT-SIGl'", by use of the weights derived from "HT-LTFl" and "HT-LTFl'" immediately prior thereto. The receiving apparatus performs receiving processing on "HT- SIG2" and "HT-SIG2''', by use of the weights derived from "HT-LTFl" and "HT-LTFl'" immediately prior thereto. The portions from the beginning up to "HT-SIG" and

"HT-SIG + CDD" use "52" subcarriers in the same Way as in a ^' legacy system (hereinafter this number will be referred to as "first number of subcarriers") . Of "52" subacarriers, "4" subcarriers correspond to the pilot signals. On the other hand, the portions corresponding to "HT-STF" and "HT- STF'" use "24" subcarriers in the total of a plurality of streams. The portions corresponding to "HT-LTFl", "HT- LTFl'", . "HT-SIGl", "HT-SIGl'" and so forth use "56" subcarriers in the total of a plurality of streams (hereinafter this number will be referred to as "second number of subcarriers") . The portions corresponding to "HT- DATAl", "HT-DATA2" and so forth use "56" subcarriers.

In the receiving apparatus, "HT-SIG" and the like are demodulated based on "L-LTF", as described earlier. The both use the same number of carriers, namely "52", and a processing for adjusting to the power at a posterior part of "56" subcarriers is carried out. On the other hand, "HT- SIGl" and the like are demodulated based on "HT-LTFl" and the like immediately prior thereto, as described above. Note that the amount of ^■ data such as "HT-SIGl" is the same as the amount of data such as "HT-SIGl". Accordingly, if

"HT-SIGl" and the like use "52" subcarriers in the same way as in "HT-SIG" and the like, the number, of subacarriers used does not agree with the number of subcarriers, namely, "56", used in "HT-LTFl" and the like, so that the powers at the both fields do not coincide. Thus, according to the present invention, the number of subcarriers used in "HT-SlG" and the like is extended to "56". In so doing, an "additional signal is appended to the "control signal". Hereinafter, a control signal to which an additional signal is appended or . control signals 'to which additional signals are appended will be referred to as a "control signal with an additional signal" or "control signals with their respective additional signals", respectively.

The above described packet formats are structured in the light of following grounds. The strong error resistance is required of "HT-SIGl" and the like and therefore the execution of spatial multiplexing as with "DATAl" and the like is not desirable. Hence, the same subcarriers as with "HT-LTFl" and "HT-LTFl'" are used so as to make the^' error resistance of "HT-SIGl" stronger. As a result, the weights for "DATAl" and the like will differ from the weights for "HT-SIGl" and the like. Since "DATAl" and the like are received continuously to some extent, the receiving suited to the change in channel characteristics can be done by referring to the pilot signals in "DATAl" and the like while using "HT-LTFl", "HT-LTF2", "HT-LTFl'" and "HT-LTF2"'. On the other hand, "HT-SIGl" and the like are of discrete nature, and there is little chance of updating the receiving weights for "HT-SIGl" and the like. Hence, it is difficult to perform the receiving operation suited to the change in channel characteristics. Thus, it is preferred that "HT-LTFl" and "HT-LTFl'" be inserted immediately before "HT-SIGl" and the like. Since "HT-LTFl" and "HT-LTFl'" are part of "HT-LTFl", "HT-LTF2", "HT-LTFl'" and "HT-LTF2"', the increased cost of the radio apparatus ^'10 can be restricted. .

For the above-described "HT-SIG", "HT-SIGl" and so forth, there has been a demand that the same interleave unit and the same deinterleave unit are installed in the radio apparatus 10 for the simplicity of processing and the processing is performed on "HT-SIGs" to which the same information bits have been arranged. Normally, since the number of subcarriers for "HT-SIGl" and the like is adjusted to the number of subcarriers for "HT-SIG", that of "HT-SIGl" and the like becomes "52". Thus, the power fluctuation occurs in "HT-SIGl" and the like. According to the present invention, the power variation can be compensated while meeting the above demand by appending the additional signals

FIG. 4 illustrates a ■ structure of a first radio apparatus 10a. The first radio apparatus 10a includes a first radio unit 20a, a second radio unit 20b, ... and a fourth radio unit 2Od, which are generically referred to as "radio unit 20", a baseband processing unit 22, a modem unit 24, an IF unit 26 and a control unit 30. Signals involved include a first time-domain signal 200a, a second time- domain signal 200b, ^' ... and a fourth time-domain signal 20Od, which are generically referred to as "time-domain signal 200", and a first frequency-domain signal 202a, a second frequency-domain signal 202b, a third frequency-domain signal 202c and a fourth frequency-domain signal 202d, which are generically referred to as "frequency-domain signal 202" The second radio apparatus 10b has a structure similar to that of the first radio apparatus 10a. As a receiving operation, the radio unit 20 carries out frequency conversion of radiofrequency signal received by the antennas 12 so as to derive baseband signals. The radio unit 20 outputs the baseband signals to the baseband processing unit 22 as the time-domain signals 200. The baseband signal, which is composed of in-phase components and quadrature components, shall generally be transmitted by two signal lines. For the clarity of figure, the baseband signal is presented here by a single signal line only. An AGC unit and an A-D conversion unit are also included. As a transmission operation, the radio unit 20 carries out frequency conversion of baseband signals from the baseband processing unit 22 so as to derive radiofrequency signals. Here, the baseband signal from the baseband processing unit 22 is also indicated as the time-domain signal 200. The radio unit 20 outputs the radiofrequency signals to the antennas 12. A PA (power amplifier) and a D- A conversion unit are also included. It is assumed herein that the time-domain signal 200 is a multicarrier signal converted to the time domain and is a digital signal. As a receiving operation, the baseband processing unit 22 converts a plurality of time-domain signals 200 respectively into the frequency domain and performs adaptive array signal processing on the thus converted frequency- domain signals. The detailed description of adaptive array signal processing will be given alter. The baseband processing unit 22 outputs the result of adaptive array signal processing as the frequency-domain signals 202. One frequency-domain signal 202 corresponds to data contained in each of a plurality of streams transmitted from the second radio apparatus 10b, not shown here. As a transmission operation, the baseband processing unit 22 inputs, from the modem unit 24, the frequency-domain signals 202 serving as signals in the frequency domain, converts the frequency- domain signals into time domain and then outputs the thus converted signals as time-domain signals by associating them respectively with a plurality of antennas 12.

FIG. 5 illustrates a structure of a frequency-domain signal. Assume herein that a combination of subcarrier numbers "-28" to "28" shown in FIG. 1 constitutes an "OFDM symbol". An "i"th OFDM symbol is such that subcarrier components are arranged in the order of subcarrier numbers "1" to "28" and subcarrier numbers "-28" to "-1". Assume also that an "(i-2)"th OFDM symbol is placed before the "i"th OFDM symbol, and an " (i+1) "th OFDM symbol is placed after the "i"th OFDM symbol. It is to be noted here that in the portions such as "L-SIG" shown in FIG. 11 a combination of from the subcarrier number "-26" to the subcarrier number "-26" is used for each "OFDM symbol". Now refer back to FIG. 4. To produce the packet formats corresponding to FIG. 11, the baseband processing unit 22 carries out CDD. CDD is expressed as a matrix C in the following Equation (4-1) . CCO= diag(l, exρ(-fi^'πiSINout), •••, exp(-j2π^δ(Nout-1)/Nout) - (4-1)

where δ indicates a shift amount and I a subcarrier number. The multiplication of the matrix C by a stream is done on a subcarrier-by-subcarrier basis. That is, the baseband processing unit 22 carries out a cyclic time shifting within the STF and so forth per stream. The shift amount is set to a different value for each stream.

As a receiving processing, the modem unit 24 demodulates and deinterleaves the frequency-domain signal 202 outputted from the baseband processing unit 22. The demodulation is carried out per subcarrier. The modem unit 24 outputs the demodulated signal to the IF unit 26. As a transmission processing, the modem unit 24 carries out interleaving and modulation. In so doing, the modem unit 24 generates a control signal with an additional signal by appending an additional signal to a control signal. The modem unit 24 outputs the modulated signal to the baseband processing unit 22 as a frequency-domain signal 202. When the transmission processing is carried out, the modulation scheme is specified by the control unit 30.

As a receiving processing, the IF unit 26 combines signals outputted from a plurality of modem units 24 and then forms one data stream. The IF unit 26 decodes the one data stream. The IF unit 26 outputs the decoded data stream. As a transmission processing, the IF unit 26 inputs one data stream, then codes it and, thereafter, separates the coded data stream. Then the IF unit 26 outputs the thus separated data to the plurality of modem units 24. When the transmission processing is . carried out, the coding rate is specified by the control unit 30.

The control unit 30 controls the timing and the like of the first radio apparatus 10a. The control unit 30 controls the modem unit 24 and the like so that the packet signals to be transmitted form the packet formats as shown in FIG. 11. That is, the control unit 30 appends "HT-LTSl", "HT-LTS2" or the like to the data signals contained in at least the first combination among a plurality of combinations, and appends "HT-LTSl" or the like to anterior parts of the second and the subsequent combinatio'ns, respectively, among a plurality of combinations. Here, as described earlier, part of a plurality of subcarriers are used for "HT-SIGl" and 'the like contained in the second and _. the subsequent combinations, respectively. And "HT-LTSl" and_. the like for the second and the subsequent combinations, respectively, are defined in a manner such that part corresponding to said part of a plurality of subcarriers is extracted from "HT-LTSl" and the like.

"HT-LTSl", "HT-LTS2" and the like are each formed by a plurality of symbols. The subcarrier used for each symbol is changed and defined so that the subcarrier used for any of symbols is identical to the subcarrier used for "HT-SIG" and the like contained in the second and the subsequent combinations, respectively. Further, "HT-LTFl" for "HT- SIGl" is so defined as to be identical to "HT-LTSl" in "HT- LTSl" and "HT-LTS2".

In terms of hardware, this structure can be realized by a CPU, a memory and other LSIs of an arbitrary computer. In terms of software, it is realized by memory-loaded programs which have communication functions and the like, but drawn and described herein are function blocks that are realized in cooperation with those. Thus, it is understood by those skilled in the art that these function blocks can be realized in a variety of forms such as by hardware only, software only or the combination thereof.

A processing of receiving processing unit 50 will now be described in a specific manner. The receiving processing unit 50 inputs a plurality of time-domain signals 200 and then performs Fourier transform on them, respectively, so as to derive frequency-domain signals. As described earlier, a frequency-domain signal is such that signals corresponding to subcarriers are arranged serially in the order of subcarrier numbers. The receiving processing unit 50 weights the frequency-domain signals with receiving weight vectors, and a plurality of weighted signals are added up.

Since the frequency-domain signal is composed of a plurality of subcarriers, the above processing is also executed on a subcarrier-by-subcarrier basis. As a result, the signals summed up are also arranged serially, as shown in FIG. 5, in the order of subcarrier numbers. The signals summed up are the aforementioned frequency-domain signals 202. Here, the receiving processing unit 50 computes plural kinds of receiving weight vectors. A first kind of receiving weight vector is a receiving weight vector to receive HT-SIG and the like, and is derived from L-LTF and the like. In this case, the receiving processing unit 50 estimates a channel characteristic from L-LTF and the like, and derives a receiving weight vector by calculating the reciprocal of the estimated channel characteristic. A second kind of receiving weight vector is a receiving weight vector to receive HT-DATAl and the like and is derived from HT-LTFl, HT-LTFl', HT-LTF2, HT-LTF2' and the like. In this case, the receiving processing unit 50 estimates a channel characteristic from HT-LTFl, HT-LTFl', HT-LTF2 and HT-LTF2' . Furthermore, based on the estimated channel characteristic, the receiving processing unit 50 derives a receiving weight vector with which the interference among a plurality of streams gets small.

A third kind of receiving weight vector is a receiving weight vector to receive HT-SIGl, HT-SIGl' and the like and is derived from HT-LTFl and HT-LTFl' placed immediately prior thereto. In this case, the receiving processing unit 50 estimates a channel characteristic from the HT-LTFl and

HT-LTFl' immediately prior thereto and derives a receiving weight vector by calculating the reciprocal of the estimated channel characteristic. A known technique may be used to derive the above receiving weight vectors. Using such plural kinds of receiving weight vectors as above, the receiving processing unit 50 carries out array synthesis. Under such a condition, the modem unit 24 provided at a subsequent stage carries out demodulation using the pilot signals.

The receiving processing unit 50 estimates channel characteristics by use of correlation processing. If a frequency-domain signal corresponding to the first time- domain signal 200a is denoted by xi(t), a frequency-domain signal corresponding to the second time-domain signal 200b by X₂ (t), a reference signal in the first stream by Si (t) and a reference signal in the second stream by S₂Ct), then Xi(t) and x₂(t) will be expressed by the following Equation (4-2):

X₁Ct) = h_nS₁(t)+h₂₁S₂(t) ___ x₂(t)= ^₂S₁(t)+h₂₂S₂(t) The noise is ignored here. A first correlation matrix Ri, with E as an ensemble average, is expressed by the following Equation (4-3) :

A second correlation matrix R₂ among the reference signals is given by the following Equation (4-4) .

Finally, the first correlation matrix Ri is multiplied by the inverse matrix of the second correlation matrix R₂ so as to derive a receiving response vector, which is expressed by the following Equation (4-5) .

Then the receiving processing unit 50 computes a receiving weight vector from the channel characteristics.

The transmitting processing unit 52 executes a part, corresponding to a transmission operation, of operations by the baseband processing unit 22. The transmitting processing unit 52 may perform beamforming or eigenmode transmission. Any known technique may be used for these and therefore the description thereof is omitted here.

FIG. 12 illustrates a structure of IF unit 26 and modulation unit 24. Shown here is a portion concerning the transmission function in the IF unit 26 and the modulation unit 24. The IF unit 26 includes an FEC (Forward Error- Correcting) unit 60 and a separation unit 62. The modulation unit 24 includes a first interleave unit 64a ... and a fourth interleave unit 64d, which are generically referred to as "interleave unit 64", a first adding unit 66a ... and a fourth adding unit 66d, which are generically referred to as "adding unit 66", and a first mapping unit 68a ... and a fourth mapping unit 68d, which are generically referred to as "mapping unit 68". A plurality of combinations of control signal and data signal, which are to use a plurality of subcarriers, are inputted to the FEC unit 60. The combinations meant here are equal to the "first combination" to the "third combination" as -shown in FIG. 11. The control signal corresponds to "HT-SIG", "HT-SIGl" and the like in FIG. 11. The FEC unit 60 performs coding on each of the plurality of combinations. Note that the coding rate may be set for the control signal and the data signal independently of each other. The separation unit 62 partitions and separates a signal inputted from the FEC unit 60 into a plurality of streams. The interleave unit 64 carries out an interleaving of a size defined by the first number of subcarriers, namely, 48, on the control signal, and carries out an interleaving of a size defined by the second number of subcarriers, namely, 52, on the data signal. Here, the amount of data contained in the size defined by the number of subcarriers "52" is changed by the modulation scheme or the like used by the modem unit 24. It is assumed that the interleaving pattern is predetermined.

The adding unit 66 adds additional signals to control signals contained in the second and subsequent combinations of the plurality of^' combinations interleaved by the interleaving unit 64. As a result, control signals with their respective additional signals are generated. Here the control signals contained in the second and subsequent combinations correspond to "HT-SIGl", "HT-SIGl'", "HT-SIG2" and "HT-SIG2"' shown in FIG. 11. It is to be noted that the amount of additional signal to be added by the adding unit 66 is determined by the difference of the second number of subcarriers from the first number of subcarriers. In other words, the amount of additional signal is determined by the difference "4" between the second number of subcarriers and the first number of subcarriers and the modulation scheme. As a result of the processing as described above, the number of subcarriers used by control signals with their respective additional signals becomes the same as the number of subcarriers used by the data signals. It is to be understood here that the additional signal is a dummy signal, The mapping unit 68 performs mappings of BPSK, QPSK, 16-QAM and 64-QAM on the signals from the adding unit 66.

Mapping, which is a known technology, is not explained here. The mapping unit 68 outputs a mapped signal as a frequency- domain signal 202. The insertion of known signals, such as "L-STF" as shown in FIG. 11, or the insertion of pilot signals is done by the modem unit 24.

On the other hand, the receiving function for receiving the packet signals generated as described above performs operation opposite to that explained above. That is, the modem unit 24 receives an input of frequency-domain signals 202. The frequency domain signal 202, which is a combination of control signal and data signal, is equal to a combination using a plurality of subcarriers . Here the control signals contained in the second and subsequent combinations correspond to control signals with their respective additional signals. The excluding unit (not shown) in the modem unit 24 excludes additional signals from the control signals with their respective additional signals contained in the second and subsequent combinations out of a plurality of combinations. In other words, the excluding unit outputs control signals and data signals by excluding the dummy signals therefrom. Note that the excluding unit excludes additional signals according to the difference between the second number of subcarriers and the first number of subcarriers.

A deinterleave unit (not shown) in the modem unit 24 performs a deinterleaving of a size defined by the first number of subcarriers, namely, "48", on the control signal, of the plurality of combinations with the additional signals excluded, and performs a deinterleaving of a size defined by the second number of subcarriers, namely, "52", on the data signal.

In the description thus far, an additional signal is added to an interleaved control signal. In this condition, the number of subcarriers used for "HT-LTSl" and the like is equal to the number of subcarriers used for a control signal with additional signal. In other words, the variation in the number of subcarriers and the variation in the signal strength of packet signals are subject to restriction. On the other hand, the size of interleaving, when based on the number of subcarriers, is different between the control signal with an additional signal and the data signal. As a result, a switching in the size of interleaving is done between the two. A modification to be described later aims to restrict the change in size to be used in the interleaving .

FIG. 13 illustrates another structure of IF unit 26 and modulation unit 24. Shown here is a portion concerning the transmission function in the IF unit 26 and the modulation unit 24. The IF unit 26 includes an adding unit 66, an FEC (Forward Error-Correcting) unit 60 and a separation unit 62. The modulation unit 24 includes a first interleave unit 64a ... and a fourth interleave unit 64d, which are generically referred to as "interleave unit 64", and a first mapping unit 68a ... and a fourth mapping unit 68d, which are generically referred to as "mapping unit 68". The components having the function equivalent to those in FIG. 12 are given the same reference numerals and therefore their repeated explanation will be omitted as appropriate. Compared with the above structure, the arrangement of the adding unit 66 differs from that in FIG. 12.

A plurality of combinations, of control signal and data signals, which are to use a plurality of subcarriers are inputted to the adding unit 66. The adding unit 66 appends additional signals to the second combination and the subsequent combinations in a plurality of combinations. Accordingly, control signals with their respective additional signals are produced. Here, the amount of additional signals appended by the adding unit 66 is determined by the adding unit according to the difference between the first number of subcarriers and the second number of subcarriers. It is assumed herein that the additional signals are for use with CRC (Cyclic Redundancy Check) . The signals for CRC are generated by the FEC unit 60. As a result, the bit number used for CRC increases and therefore the data error characteristics improves. The additional signal may be a signal for use with parity check. The interleave unit 64 carries out an interleaving of a size defined by the first number of subcarriers on the control signal contained in the first combination, and carries out an interleaving of a size defined by the second number of subcarriers on the remaining signals. That is, the number of interleave size switching can be reduced. On the other hand, the receiving function of receiving the packet signals thus generated executes an operation opposite to the operation in the above description. That is, the modem unit 24 inputs the frequency-domain signals 202. The frequency-domain signal corresponds to a combination, of control signal and data signal, which uses a plurality of subcarriers. Here, control signals contained in ^'the second combination and the subsequent combinations are control signals with their respective additional signals.

The excluding unit (not shown) in the modem unit 24 excludes additional signals from the control signals with their respective additional signals contained in the second and subsequent combinations out of a plurality of combinations. That is, the excluding unit outputs control signals and data signals by excluding the signals for CRC. Note that the excluding unit excludes additional signals according to the difference between the second number of subcarriers and the first number of subcarriers. The IF unit 26 executes the detection by CRC.

According to the embodiments of the present invention, a known signal is appended to a part immediately before a control signal contained in the second and the subsequent combinations, so that the degradation in the error rates of control signals can^' be suppressed. Since the degradation in the error rated of control signals can be suppressed, the receiving quality can be improved. The known signal appended to a part immediately before the control signal contained in the second and the subsequent combinations is equivalent to part of a known signal corresponding to a data signal, the drop in transmission efficiency can be prevented. The length of a ^' known signal appended immediately anterior to a control signal contained in' the second and the subsequent combinations is equivalent to part of the length of a known signal corresponding to a data signal, so that the drop in transmission efficiency can be prevented.

By appending an additional signal to a control signal inserted between data signals, the number of subcarriers used for a data signal is made equal to the number of subcarriers used for a control signal with an additional signal. As a result thereof, the variation in signal strength can be restricted. Since the variation in signal strength can be restricted, the time constant of AGC at the receiving apparatus can be made longer. Because of this restricted and thus controlled variation in signal strength, the dynamic range at the receiving apparatus can be made smaller. Also, the receiving characteristics thereof can be improved. Since drops in signal strength in the course of a packet signal can be avoided, any transmission from a third party communication apparatus multiplexed by CSMA can be prevented. Since any transmission from a third party communication apparatus multiplexed by CSMA can be prevented, the probability of signal collisions can be lowered. Since a dummy signal is added as an additional signal, complexity of processing can be reduced. Since a receiving apparatus, once additional signals are removed from control signals with additional signals, can perform normal functions, extra processing can be reduced. ^• The number of subcarriers used for data signals and the number of subcarriers used for control signals with additional signals are made equal to each other by adding an additional signal to each control signal inserted between data signals before interleaving. Thus, the number of interleave size switching can be reduced. And variation in signal strength can be suppressed and controlled while reducing the number of interleave size switching. Since a signal for CRC is appended as an additional signal, the receiving characteristics can be improved. The present invention has been described based on the embodiments. These embodiments are merely exemplary, and it is understood by those skilled in the art that various modifications to the combination of each component and process thereof are possible and that such modifications are also within the scope of the present invention.

According to the embodiments of the present invention, the adding unit 66 appends dummy signals as additional signals. The embodiment, however, is not limited thereto, and the adding unit 66 may, for instance, add signals for parity check as additional signals instead. According to this modification, the additional signals can be Utilized effectively and can improve the receiving characteristics. That is, it is only required that additional signals be added whose number of subcarriers is equal to the difference- between the number of subcarriers used for data signals and the number of subcarriers used for control signals.

According to the embodiments of the present invention, the adding unit 66 adds dummy signals as the additional signals. The embodiment, however, is not limited thereto, and the adding unit 66 may, for instance, append pilot signals as the additional signals instead. The pilot signals are known signals. In this modification, the adding unit 66 assigns pilot signals to subcarrriers with subcarrier numbers "-28", "-27", "27" and "28" as shown in FIG. 1. Also, the receiving apparatus uses the pilot signals in carrying out demodulation. It is to be noted that where there are already pilot signals inserted in a plurality of subcarriers with subcarrier numbers from "-26" to "26", the addition of pilot signals by the adding unit 66 is equivalent to the addition of pilot signals. According to this modification, the receiving characteristics can be improved. That is, the only requirement is such that additional signals be added whose number of subcarriers is equal to the difference between the number of subcarriers used for data signals and the number of subcarriers used for control signals.

According to the embodiments of the present ^' invention, a signal compatible with the legacy system is appended to a leading part of a packet format. Accordingly, the adding unit 66 does not append an additional signal to the leading control signal "HT-SIG". The arrangement, however, is not limited thereto, and an arrangement may be such that the signal compatible with a legacy system is not appended to the leading part of a packet format. In such a case, the adding unit 66 may add additional signals to all of the control signals. According to this modification, the same processing is done on all of the control signals, so that the processing can be simplified. For this modification, therefore, the only requirement is such that additional signals be added whose number of subcarriers is equal to the difference between the number of subcarriers used for data signal and the number of subcarriers used for control signal,

According to the embodiments of the present invention, it is assumed that the communication system 100 is a MIMO system. The arrangement, however, is not limited thereto, and an arragement may be such that the communication system 100 is not a MIMO system. In other words, the arrangement may be such that signals of a single stream are transmitted from a single antenna 12. According to this modification, the present invention can be applied to the various types of communication systems. That is, the only requirement is that a plurality of subcarriers are used and there is a need to control the variation in the number of subcarriers in the course of a packet signal.

The present invention described in the fourth embodiment may be described by the following Item 4-1 to Item 4-13: Item 4-1

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; a generation unit which generates a packet signal from the plurality of combinations inputted to said input unit in a manner that a first known signal is added to a data signal at least contained in a first combination among the plurality of combinations inputted to said input unit and a second known signal is added to a second combination and the subsequent combinations, respectively, among the plurality of combinations inputted to said input unit, in an anterior part thereof; and a transmitter which transmits the packet signal generated by said generation unit, wherein said generation unit uses part of a plurality of subcarriers for control signals contained respectively in the second combination and the subsequent combinations, and defines the second known signal in a manner such that part corresponding to said part of a plurality of subcarriers is extracted from the first known signal. Item 4-2

A radio apparatus according to Item 4-1, wherein, for the first known signal composed of a plurality of symbols, said generation unit changes subcarriers used in the symbols, respectively, so defines a subcarrier used in any of the symbols as to be identical to a subcarrier used in a control signal contained in each of the second and the subsequent combinations, and so defines the second known signal as to be identical to part of the first known signal corresponding to the any of the symbols. Item 4-3

A radio apparatus according to Item 4-1, said generation unit including: an interleave unit which performs interleaving of a size defined by a first number of subcarriers on a control signal contained in a first combination and performs interleaving of a size defined by a second number of subcarriers on a data signal among a plurality of combinations inputted by said input unit; and an adding unit which adds an additional signal to a control signal contained in the second combination and the subsequent combinations; wherein said adding unit adds additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 4-4

A radio apparatus according to Item 4-3, wherein the additional signal inserted by said adding unit is a dummy signal. Item 4-5

A radio apparatus according to Item 4-3, wherein the additional signal inserted by said adding unit is a signal for parity check. Item 4-6

A radio apparatus according to Item 4-3, wherein the additional signal inserted by said adding unit is a known signal. Item 4-7

A radio apparatus according to Item 4-1, said generation unit including: an adding unit which adds an additional signal to a control signal contained in the second combination and the subsequent combinations among a plurality of combinations inputted by said input unit; and an interleave unit which performs interleaving of a size defined by a first number of subcarriers on a control signal contained in a first combination and performs interleaving of a size defined by a second number of subcarriers on the remaining signals among a plurality of combinations in which the additional signal has been added by said adding unit, wherein said adding unit adds additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers. Item 4-8

A radio apparatus according to Item 4-7, wherein the additional signal inserted by said adding unit is a signal for cyclic redundancy check (CRC) . Item 4-9

A radio apparatus according to Item 4-7, wherein the additional signal inserted by said adding unit is a known signal. Item 4-10

A radio apparatus, comprising: a receiver which receives a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; and a demodulation unit which demodulates a packet signal from the plurality of combinations received by said receiver in a manner that while using a first known signal the demodulation is performed on a data signal at least contained in a first combination among the plurality of combinations received by said receiver and while using a second known signal placed in an anterior part the demodulation is performed on a second combination and the subsequent combinations, respectively among the plurality of combinations received by said receiver, wherein, in said receiver, the control signals contained respectively in the second combination and the subsequent combinations use part of a plurality of subcarriers and the second known signal is defined in a manner such that part corresponding to said part of a plurality of subcarriers is extracted from the first known signal . Item 4-11

A radio apparatus according to Item 4-10, said demodulation unit including: an excluding unit which excludes an additional signal from the control signal contained in the second combination and the subsequent combination among the plurality of combinations received by said receiver; and a deinterleave unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal and performs deinterleaving of a size defined by a second number of subcarriers on a data signal in a plurality of combinations in which the additional signal has been excluded by said excluding unit, wherein said excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 4-12

A radio apparatus according to Item 4-10, said modulation unit including: a deinterleave unit which performs deinterleaving of a size defined by a first number of subcarriers on a control signal conatained in a first combination and performs deinterleaving of a size defined by a second number of subcarriers on the remaining signals in the plurality of combinations received by said receiver; and an excluding unit which excludes an additional signal from the control signal contained in the second combination and the subsequent combinations among a plurality of combinations deinterleaved by said deinterleave unit, wherein said excluding unit excludes additional signals whose amount corresponds to a difference between the second number of subcarriers and the first number of subcarriers . Item 4-13

A radio apparatus, comprising: an input unit which inputs a plurality of combinations of control signal and data signal wherein the combination uses a plurality of subcarriers; a generation unit which generates a packet signal from the plurality of combinations inputted to said input unit in a manner that a first known signal is added to a data signal- at least contained in a first combination among the plurality of combinations inputted to said input unit and a second known signal is added to a second combination and the subsequent combinations, respectively, among the plurality of combinations inputted to said input unit, in an anterior part thereof; and ' a transmitter which transmits the packet signal generated by said generation unit, wherein said generation unit defines the second known signal in such a manner as to extract part of the first known signal.

Claims

1. A radio apparatus, comprising: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by said assigning unit, said assigning unit including: an estimation unit which estimates time required from when signals are transmitted respectively to the plurality of terminal apparatuses to when responses therefrom are received; and an execution unit which assigns a terminal apparatus, whose required time estimated by said estimation unit is longer, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order opposite to the order of terminal apparatuses assigned in the partial periods for transmitting signals .

2. A radio apparatus, comprising: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by said assigning unit, said assigning unit including: an identifying unit which identifies processing speeds for the respective plurality of terminal apparatuses; and an execution unit which assigns a terminal apparatus, whose processing speed identified by the identifying unit is low, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order opposite to the order of terminal apparatuses assigned in the partial periods for transmitting signals .

3. A radio apparatus according to Claim 2, wherein the identifying unit includes: a measurement unit which measures time periods from when signals are transmitted respectively to the plurality of terminal apparatuses to when responses to said signals are received, respectively; and an execution unit which identifies processing speeds, based on the time periods measured by the measurement unit.

4. A radio apparatus according to Claim 2, wherein the identifying unit includes: a reception unit which receives information on the processing speeds from the respective plurality of terminal apparatuses; and an execution unit which identifies the processing speeds, based on the information received by the reception unit .

5. A radio apparatus, comprising: an assigning unit which partitions a given period into a plurality of partial periods and assigns the plurality of partial periods to a plurality of terminal apparatuses by associating the partial periods with the terminal apparatuses; and a communication unit which performs communication, using at least one stream, with the plurality of terminal apparatuses to which the respective plurality of partial periods have been assigned by said assigning unit, said assigning unit including: an identifying unit which identifies the number of streams for each of the plurality of terminal apparatuses; and an execution unit which assigns a terminal apparatus, whose number of streams identified by the identifying unit is large, to an early partial period in a series of partial periods for transmitting signals wherein, in the plurality of partial periods, partial periods for receiving signals continue after the partial periods for transmitting signals continue and wherein the order of terminal apparatuses assigned in the partial periods for receiving signals is defined in the order opposite to the order of terminal apparatuses assigned in the partial periods for transmitting signals .

6. A radio apparatus according to Claim 1, wherein, in the partial periods for receiving signals, said communication unit receives responses to the signals transmitted in the partial periods for transmitting signals from the terminal apparatuses .