JP5592839B2 - Wireless communication system and wireless communication method - Google Patents

Description

  The present invention relates to a communication technique in a wireless communication system that receives wireless signals simultaneously from a plurality of communication partners using spatial multiplexing.

  In recent years, the IEEE802.11g standard, the IEEE802.11a standard, and the like are remarkable as high-speed wireless access systems using the 2.4 GHz band or the 5 GHz band. These systems use an Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme, which is a technique for stabilizing characteristics in a multipath fading environment, and realize a physical layer transmission rate of 54 Mbps at the maximum. (Non-Patent Document 1).

  However, the transmission rate here is a transmission rate on the physical layer. That is, since the transmission efficiency in the MAC (Medium Access Control) layer is actually about 50 to 70%, the upper limit of the actual throughput is about 30 Mbps. For this reason, if the number of communication partners requiring information increases, this characteristic further deteriorates. On the other hand, in the world of wired LAN (Local Area Network), the high speed of 100 Mbps is expected due to the widespread use of Ethernet (registered trademark) 100Base-T interface and FTTH (Fiber to the home) using optical fiber in each home. Line provision is widespread. In accordance with such widespread use, further increase in transmission speed is demanded in the wireless world.

  As a technique for this purpose, in IEEE 802.11n, a MIMO (Multiple Input Multiple Output) technique has been introduced as a spatial multiplexing transmission technique. Further, in IEEE 802.11ac, a multi-user MIMO (MU-MIMO) transmission method has been studied (Non-Patent Document 2).

FIG. 13 is a diagram illustrating a functional configuration of a wireless communication system that performs conventional MIMO communication. Hereinafter, a conventional uplink using MIMO communication will be described with reference to FIG. In FIG. 13, 9 is a base station, 8-k is a terminal, 9-2 and 8-k-4 are transmission signal generation units, 9-3 and 8-k-2 are radio signal transmission / reception units, and 9-4-1. -9-4-N, 8-k-1-1-1 to 8-k-1- _Mk are antennas, 9-5, 8-k-3 are position detection channel estimation synchronization units, 9-6, 8-k -5 represents a demodulation unit, and 9-7 represents a decoding determination unit. M _k is the number of antennas of the terminal 8-k, and N is the number of antennas of the base station 9. A case will be described in which orthogonal wave division multiplexing (OFDM) is used as secondary modulation and communication is performed using packets.

The processing flow of this system is as follows. When the transmission signal generation unit 8-k-4 of the terminal 8-k generates a transmission signal, the radio signal transmission / reception unit 8-k-2 performs modulation and assigns a guard interval. Further, when the radio signal transmission / reception unit 8-k-2 performs conversion to an analog signal and up-conversion, the radio signal transmission / reception unit 8-k-2 performs at least one of the antennas 8-k-1-1-1 to 8-k-1- _Mk. A radio signal is transmitted. The base station 9 receives a signal via at least one of the antennas 9-4-1 to 9-4-N. The wireless signal transmission / reception unit 9-3 down-converts the received signal and converts it into a digital signal. Next, the position detection / channel estimation synchronization unit 9-5 performs signal position detection, frequency deviation compensation, and channel information estimation. Thereafter, the demodulator 9-6 demodulates the signal and extracts the transmitted bits. The decoding determination unit 9-7 performs error determination on the demodulated bit sequence and determines whether or not the decoding has been normally performed. In the case of normal decoding, the decoding determination unit 9-7 notifies the transmission signal generation unit 9-2 to that effect. In that case, the transmission signal generation unit 9-2 generates an ACK signal notifying that it has been normally decoded. The radio signal transmitting / receiving unit 9-3 modulates the ACK signal, adds a necessary control signal, converts the signal into an analog signal, and upconverts the signal. Thereafter, the ACK signal is transmitted via at least one of the antennas 9-4-1 to 9-4-N. When a signal is transmitted on the downlink, the terminal 8-k receives the signal via the antennas 8-k-1-1-1 to 8-k-1- _Mk . The radio signal transmission / reception unit 8-k-2 down-converts the received ACK signal and converts it to a digital signal. The position detection channel estimation synchronization unit 8-k-3 performs signal position detection, frequency shift compensation, and channel information estimation. The demodulator 2-k-5 demodulates the received signal and detects that the uplink communication is normal.

An operation of the position detection channel estimation synchronization unit 9-5 in the base station 9 in the uplink will be described. In the following description, it is assumed that all antennas are used. However, N and M _k used in the following can be regarded as antennas used as a result of antenna selection. The channel matrix representing the channel response between antennas in the j-th subcarrier between the actual base station 9 and the terminal 8-k is

It is defined as Here, an OFDM system is considered, and the number of subcarriers is F. That is, j is a value between 1 and F. h _{k, j, i, l} are from the l-th antenna 8-k-1-l of the terminal 8-k to the i-th antenna 9-4 of the base station 9 in the j-th subcarrier of the terminal 8-k. Represents the channel response up to -i. Suppose that the transmission signal from the terminal 8-k is x _{k, j} , and the k-th terminal 8-k performs spatial multiplexing of the L _k stream and performs transmission using x _k of the L _k × 1 vector. The reception signal y _j of the j-th subcarrier at the base station 9 is defined as follows.

Here, W _{k, j} is the transmission weight of the M _k × L _k matrix for the j-th frequency channel in the terminal 8-k. This matrix selects L _k row vectors from M _k row vectors of W _{k, j} , sets the elements whose column component positions do not overlap as 1 and sets the other components as 0, and sets L _k An antenna selection matrix for selecting an antenna may be used. A fixed weight may be used in common for all subcarriers. It is also possible to determine by performing computation on the channel matrix for each subcarrier. For example, the base vector obtained by using the orthogonalization method for the row vector of Equation 1 or the right singular vector of Equation 1 can be used as the transmission weight. Also, it can be a diagonal matrix with 1 element, and in this case, it is the same as when W _{k, j} is not used. n _j is an N × 1 noise vector, and the elements can be regarded as a complex Gaussian distribution having an average of 0 and a variance σ ² .

Under such conditions, the base station 9 can estimate the transmission signals x _{1 to} x _K by estimating the transmission weight-included channel matrix H _{k, j} W _{k, j} in advance. When reception is performed using MMSE (Minimum Mean Squared Error) weight, the reception weight is calculated using the channel matrix H _{k, j.}

Can be calculated as The transmission signal x _{k, j} can be estimated by multiplying the received signal by the complex conjugate transpose of R _j .

  In this way, uplink MIMO communication can be performed by a MIMO technique that can increase the transmission capacity in proportion to the smaller number of antennas. In order to realize such communication, it is essential for the terminal 8-k and the base station 9 to establish synchronization. Next, a synchronization method in uplink MIMO communication will be described.

In wireless LAN communication for a single user, a short training symbol is used to detect a signal from a communication partner. In the example of IEEE802.11a, when communication is performed using 52 subcarriers, a specific signal is used for 12 subcarriers, and signal 0, that is, no signal is used for the remaining subcarriers. A short training symbol is formed by connecting 10 signals of 0.8 μs obtained by performing an Inverse Discrete Fourier Transform (IDFT). Here, it is assumed that signals designated for 12 subcarriers are b _{0 to} b ₁₁ . When the RF (Radio Frequency) frequency is arranged from the lower frequency, the short preamble signal p _S before performing IDFT is

It can be expressed. When dropped to the baseband signal, 0 is put in the center corresponding to the DC component. Obtained by performing the IDFT on the signal _s S, _{0 for ~s S, 63} has a a repeat four times _{s S, 0 ~s S, 15} , _{thereafter, s S,} 0 ~s _{S 15} as a representative.

FIG. 11 is a diagram illustrating an outline of an OFDM packet including a short training symbol, a long training symbol, and a data symbol. This 0.8 μs signal (for one short training symbol) can be expressed as a short training symbol s _{S, 0 to} s _{S, 15} consisting of 16 samples with one sample 50 ns. On the receiving side, these s _{0 to} s ₁₅ are received, the correlation value is calculated by convolution of the correlation, and the signal position can be detected. Furthermore, since 10 short training symbols s _{S, 0 to} s _{S, 15} are continuously transmitted, the frequency deviation from the communication partner is estimated by observing the fluctuation of the correlation value of the received signal. be able to.

Here, ρ _t represents the correlation value with the short training symbol at time t, and r _{l, t} represents the received signal at time t at the l-th antenna of the base station. Whereas the received signal y _j is a frequency domain signal, r _{l, t} corresponds to the time domain received signal. When the short training symbol is received, the peak of the correlation value is observed 10 times every 0.8 μs according to Equation 6. Based on this reception position, the signal position can be determined by performing a discrete Fourier transform (DFT) on the OFDM signal. Further, when the time at the detected peak of the 10 correlation values is T _{0 to} T ₉ , the frequency shift can be detected from the phase change of ρ _t (t = T _{0 to} T ₉ ). When the phase is rotated by θ _S for each peak, the frequency is shifted between the base station and the terminal by a magnitude corresponding to θ _S. Therefore, for example, a frequency shift can be compensated by giving a reverse phase rotation to the received signal as shown in Equation 7 below.

  In the long training symbol portion, it is possible to perform further detailed estimation of phase rotation and channel information. As shown in FIG. 11, the long training symbol is a long training symbol (1) structured by bundling two OFDM symbols of 8 μs, or a long training symbol (2) of 4 μs, or its Both can be used.

First, frequency offset compensation using a long training symbol will be described. The long training symbol defines a specific signal for the data subcarrier. Here, in accordance with the IEEE 802.11a format, signals p _{0 to} p ₅₁ used for 52 subcarriers are generated. Next, DFT is performed on 64 signals obtained by inserting 0 in the direct current component into p _{0 to} p ₅₁ and 0 in the end of the frequency band, and the obtained time domain signals s _{L, 0 to} s _{L, 63} is used to generate a long training symbol. Long training symbols before DFT are in order from low frequency subcarriers at RF frequency.

By performing IDFT as described above, 64 time domain signals s _{L, 1 to} s _{L, 63} are obtained. The long training symbol (1) is composed of a time-series signal of 160 samples and can be expressed as Equation 9 below.

  The long training symbol (2) is composed of a time-series signal of 80 samples and can be expressed as Equation 10 below.

That is, the long training symbol is a repetitive signal of at least a part of s _{L, 0 to} s _{L, 63} . On the receiving side, this periodicity is used to compensate for the frequency shift from the phase shift of the received signal. In the example of s _L1 , since s _{L, 0 to} s _{L, 63} are repeated twice, if the timing at which the long training symbol is received is _TL , the first s _{L, 0} is _TL +31, the second s _{L, 0} is obtained by T _L +95. Therefore, due to the autocorrelation of the received signal,

Get. The phase φL _{, l} of ρL _{, l} is the phase rotation in 64 samples. This value is obtained for each analog-to-digital converter (ie, antenna). However, the value of a specific receiving device is used, a receiving device having a high reception level is selected, or ρ _L, or averaging the _l, it can be used phase phi _L obtained by or average phase φ _{L, l.} In the case of the long training symbol (2) s _L2 , s _{L, 58 to} s _{L, 63} are repeated twice. Therefore, when the timing of first _{s L, 58} is measured and _{T L,} the second _{s L, 58} will be obtained by _{T L + 64.} Therefore, due to the autocorrelation of the received signal,

Phase theta _{L, l} of the [rho _{L, l} is the phase rotation obtained by the l th reception apparatus in 64 samples. This likewise, or selected by or averaged, it is possible to calculate the phase phi _L. The received signals r ′ _{l, t} are received signals at the timing t of the l-th antenna that has undergone frequency compensation by the short training symbol. The obtained here theta _L, can be compensated as follows frequency deviation.

Thus, the frequency shift can be compensated by applying the reverse phase rotation.
Furthermore, channel information can be estimated by the long training symbol. DFT is performed on the received signals r ″ _{l, TL to} r ″ _{l, TL + 63} corresponding to s _{L, 0 to} s _{L, 63} of the long training symbol, and 52 components corresponding to the data subcarriers are represented by y. Received signals corresponding to 0th to _51st subcarriers are obtained as _{l, 0 to} y1,51. The received signal vector at antennas 1 to N of the j-th subcarrier obtained by performing DFT on the received signal corresponding to the m-th long training symbol is

It can be expressed. a _{m, 1 to} a _{m, Lk} are coding coefficients used for the long training symbol,

  It can be expressed. here,

A is determined so that The role of the coding coefficient matrix A is to transmit a pilot signal enabling channel estimation from all antennas and to effectively use transmission power. If only channel estimation is performed, a signal is output from each antenna, that is, a diagonal matrix of element 1 can be used as the matrix A. However, since one antenna cannot transmit another antenna during transmission, the efficiency is increased. It ’s not right. As A, a Walsh code can be used, or a DFT transformation matrix can be used. When the received signal matrix Y _j is expressed using received signal vectors y _{1, j to} y _{Lk, j} of L _k long training symbols, it can be expressed as Expression 17 shown below.

That is, the base station 9 knows p _j and A in advance, so that the estimation matrix H ′ _{W, k, j} of H _{k, j} W _{k, j} can be expressed as in Equation 18 below. it can.

If the thermal noise is sufficiently small, H ′ _{W, k, j} = H _{k, j} W _{k, j} .
The operation of the long training symbol will be described with reference to FIG. FIG. 12 is a diagram of training symbols when the number of antennas of the terminal is two. The long training symbols are transmitted from 2 antennas by 2 OFDM symbols, and from stream 1, the coefficients a _{1, 1} and a ₁ , ₂ are multiplied by the known signal p _j of each subcarrier and then transmitted. To do. From the stream 2, the coefficients a _{2, 1} and a ₂ , ₂ are multiplied by the base signal p _j of each subcarrier, and then transmitted. At the time of reception, an OFDM symbol multiplied by a ₁ , 1 of stream 1 and an OFDM symbol multiplied by a _{2, 1 of} stream 2 are simultaneously received (time 1), and a _{1, 2 of} stream 1 is multiplied. It can be seen that the OFDM symbol and the OFDM symbol multiplied by a _{2 and} 2 of stream 2 are received simultaneously (time 2). Therefore, in the j-th subcarrier, the received signal matrix Y _j can be generated as [y _{1, j} y _{2, j} ] from Equation 17 from the received signal vectors y _{1, j} and y _{2, j} corresponding to time 1. From Equation 18, a 2 × 2 channel matrix can be obtained.

  In this way, the conventional single user MIMO technique can perform synchronization and channel estimation, calculate the reception weight as shown in Equation 3, and decode the data symbol as shown in Equation 4.

Masahiro Morikura "Wireless LAN Textbook" IEEE, “Proposed specification framework for TGac,” doc .: IEEE 802.11-09 / 0992r21, Jan. 2011.

However, the following problems have occurred in the uplink of MU-MIMO communication. When the base station receives transmission signals with the same frequency and the same time from a plurality of terminals, the frequency shift and the signal arrival time differ between the transmission signals. In addition, if there is a gap in the level of the signal received by the base station at the same time, the communication quality of the terminal with low received power is significantly reduced due to quantization error.
As described above, in uplink MU-MIMO communication in which the base station receives data simultaneously from a plurality of terminals, communication quality deteriorates due to a frequency shift, arrival time shift, and reception level difference for each terminal. There was a problem that communication could not be established.

  In view of the above circumstances, an object of the present invention is to provide a technique for reducing deterioration in communication quality in uplink MU-MIMO communication in which a base station simultaneously receives data from a plurality of terminals.

  One aspect of the present invention is a wireless communication system in which a plurality of terminals simultaneously transmit data to a base station using the same frequency, wherein the base station includes a receiving unit that receives signals from the plurality of terminals. The signal position for detecting the signal position shift of the signal transmitted from each terminal by the correlation between the training symbol transmitted from the terminal and received by the receiving unit and the training symbol held by itself A frequency shift estimation unit that estimates a frequency shift for each of the terminals based on a phase difference between the detection unit, the training symbol, and the training symbol designated by the own device for the terminal; On the other hand, a control for generating a control signal including an identifier and information specifying a training symbol used for uplink transmission at the same frequency and the same time. And a downlink transmission unit that generates and transmits a downlink transmission signal including the control signal, and the terminal apparatus receives the signal from the base station, A synchronization / timing setting unit that extracts the control signal from the downlink transmission signal and calculates a frequency shift obtained from the signal received by the reception unit when the identifier included in the control signal indicates the own device And a transmission unit that corrects and transmits a transmission signal to which a unique training symbol specified by the control signal is added based on the frequency shift calculated by the synchronization / timing setting unit.

  One aspect of the present invention is the above-described wireless communication system, wherein the control signal generation unit simultaneously communicates terminals that do not exceed a predetermined reception level difference between terminals using the same frequency. Select as a group.

  One aspect of the present invention is the above-described wireless communication system, wherein the control signal generation unit is configured such that a deviation in received power from a terminal in a group that simultaneously performs communication using the same frequency is a predetermined allowable power. The amount of power for reducing the transmission power for each terminal is determined so as to be equal to or less than the difference, and designation of the amount of transmission power reduction is included in the control signal.

  One aspect of the present invention is the above-described wireless communication system, wherein the control signal generation unit is configured such that a deviation in received power from a group of terminals that simultaneously perform communication using the same frequency is a predetermined maximum power. Terminals that do not exceed the sum of the reduction amount and the allowable power difference are selected as a group of terminals that simultaneously communicate using the same frequency.

  One aspect of the present invention is the above-described wireless communication system, in which the control signal generation unit measures a signal arrival time estimated for each terminal and generates a delay after making a signal transmission request from the own device. By measuring the time and including in the control signal a control signal for specifying the transmission timing so that the delay time until each response signal arrives at its own device from each terminal is within the predetermined range, the transmission of the terminal Control timing.

  One aspect of the present invention is the above-described wireless communication system, wherein the control signal generation unit is configured such that the frequency shift, the clock shift, and the delay time for one terminal exceed the predetermined range. The terminal is not selected as a terminal that performs simultaneous communication using the same frequency.

を用いて送信されている場合に、各ストリームに対し推定された周波数ずれから得られる位相回転情報θ_１〜θ_Ｌ、シンボル間時間Ｔ_ＬＤを用いて、

となるように直交符号に補正を行い、チャネル推定を行うことで、チャネル推定精度を向上するチャネル情報推定手段をさらに備える。つまり、数式１８のAとして、数式２０を用いることができる。 One aspect of the present invention is the above-described wireless communication system, which is channel information estimation means for estimating channel information for a terminal from received training symbols based on information of a training symbol designated from the apparatus to the terminal. The training symbol is L × L orthogonal code

When using the phase rotation information θ _{1 to} θ _L obtained from the frequency shift estimated for each stream and the inter-symbol time T _LD ,

Channel information estimation means for improving channel estimation accuracy is further provided by correcting the orthogonal code so that That is, Equation 20 can be used as A in Equation 18.

  One aspect of the present invention is the above-described wireless communication system, in which the transmission unit of the terminal apparatus uses the training used by the base station to know signal reception timing from each terminal apparatus as a unique training symbol. Transmit symbols at a different time from other terminal devices.

  One aspect of the present invention is the above-described wireless communication system, in which the transmission unit of the terminal apparatus uses the training used by the base station to know signal reception timing from each terminal apparatus as a unique training symbol. The symbol is transmitted at a time different from that of the other terminal apparatus, and the other terminal apparatus transmits the training symbol for signal position estimation at the timing when the other terminal apparatus transmits the training symbol for signal position estimation. A channel estimation training symbol is transmitted using a frequency channel other than the frequency channel used in the above.

  One aspect of the present invention is a wireless communication method performed by a wireless communication system in which a plurality of terminals simultaneously transmit data to a base station using the same frequency, and the base station receives signals from the plurality of terminals. The signal position of the signal transmitted from each terminal due to the correlation between the receiving step of receiving and the training symbol transmitted from the terminal and received by the base station in the receiving step and the training symbol held by itself. A signal position detecting step for detecting a shift for each terminal, and the base station detects a frequency shift based on a phase difference between the training symbol and a training symbol designated by the own apparatus to the terminal. A frequency shift estimation step that is estimated every time, and the base station determines the identifier and the same frequency at the same time for each of a plurality of terminals. A control signal generating step for generating a control signal including information specifying a training symbol used in uplink transmission, and a downlink in which the base station generates and transmits a downlink transmission signal including the control signal. A transmitting step, a receiving step in which the terminal apparatus receives a signal from the base station, and the terminal apparatus extracts the control signal from the received downlink transmission signal, and the identifier included in the control signal Indicates a self-device, a synchronization / timing setting step for calculating a frequency shift obtained from the signal received in the reception step, and the terminal device adds a unique training symbol specified by the control signal. The transmitted signal is corrected based on the frequency deviation calculated in the synchronization / timing setting step and transmitted. And a transmission step.

  According to the present invention, when receiving data from a plurality of communication partners using the same time and the same frequency using the spatial multiplexing method, a terminal capable of communication is selected, synchronized to each terminal, channel information is estimated, and demodulation is performed. Do. Therefore, it is possible to reduce deterioration in communication quality in uplink MU-MIMO communication in which reception is performed simultaneously from a plurality of terminals.

It is a figure which shows the function structure of the radio | wireless communications system in embodiment of this invention. It is a figure which shows an example of the communication flow of uplink MU-MIMO. It is a figure which shows that each terminal is each transmitting the short training symbol. It is a figure which shows the outline of the OFDM packet containing a short training symbol, a long training symbol, and a data symbol. It is a figure which shows the outline of the OFDM packet containing a short training symbol, a long training symbol, and a data symbol. It is a figure which shows the outline of the OFDM packet containing a short training symbol, a long training symbol, and a data symbol. It is a figure which shows a 2nd communication sequence. It is a figure which shows a 3rd communication sequence. It is a figure which shows a 4th communication sequence. It is a figure which shows the effect in this embodiment. It is a figure which shows the outline of the OFDM packet which consists of a short training symbol, a long training symbol, and a data symbol. It is a figure of a training symbol when the number of antennas of a terminal is 2. It is a figure which shows the function structure of the radio | wireless communications system which performs the conventional MIMO communication.

FIG. 1 is a diagram illustrating a functional configuration of a wireless communication system according to an embodiment of the present invention. Hereinafter, various embodiments will be described with reference to FIG.
In FIG. 1, 1 is a base station, 2-1 to 2-K are terminals, 1-2, 2-1-4 to 2-K-4 are transmission signal generators, 1-3, 2-1-2. 2-K-2 is a radio signal transmission / reception unit, 1-4-1-1-4-N, 2-1-1-1-2-K-1-M _K is an antenna, and 1-5 is a position detection / channel. Estimated user separation and synchronization unit, 2-1-3 to 2-K-3 are position detection channel estimation synchronization units, 1-6 and 2-1-5 to 2-K-5 are demodulation units, and 2-1-6 to 2-K-6 represents a synchronization / timing setting unit, and 1-7 represents a decoding determination unit. M ₁ is the number of antennas of terminal 2-1, M _k is the number of antennas of terminal 2-k, and N is the number of antennas of base station 1. A case will be described in which orthogonal wave division multiplexing (OFDM) is used as secondary modulation and communication is performed using packets.

  Main functions of the units constituting the base station 1 and the terminals 2-1 to 2-k in the present embodiment will be described. First, the transmission signal generation unit 1-2 of the base station 1 determines a terminal that performs uplink communication, and performs a group ID (terminal identifier) of a target terminal and a transmission time for the terminal that performs the uplink. A signal (Sig1) including a control signal including at least one of the section, the type of training symbol to be used, transmission timing setting, and transmission power setting is generated. The wireless signal transmission / reception unit 1-3 transmits the signal (Sig1) generated by the transmission signal generation unit 1-2 via at least one of the antennas 1-4-1 to 1-4-N.

The radio signal transmitting / receiving unit 2-k-2 of the terminal 2-k receives a signal via at least one of the antennas 2-k-1-1-1 to 2-k-1- _Mk . The position detection channel estimation synchronization unit 2-k-3 performs synchronization, and the demodulation unit 2-k-5 performs demodulation. The demodulator 2-k-5 receives at least one of the group ID of the target terminal, the time interval for transmission, the type of training symbol used, the transmission timing setting, and the transmission power setting from the received control signal of Sig1. Is output to the synchronization / timing setting unit 2-k-6. At this time, for information that is not described in the control signal, information on frequency shift and clock shift with the base station 1 obtained from the synchronization block of the received signal from the base station 1 is acquired, and a synchronization / timing setting unit You may output to 2-k-6.

  The synchronization / timing setting unit 2-k-6 of the terminal 2-k receives a frequency shift, a clock shift, transmission timing setting, transmission power setting, training power setting, training power, Using at least one of the types of symbols, the transmission signal generation unit 2-k-4 is instructed to output a transmission signal. The transmission signal generation unit 2-k-4 generates a transmission signal, adds a designated training symbol, and outputs the transmission signal to the radio signal transmission / reception unit 2-k-2. The radio signal transmission / reception unit 2-k-2 compensates the notified frequency shift for the input transmission signal, and outputs it as a radio packet from the antenna. Here, the training symbols are given in advance for each terminal before the terminals 2-1 to 2-k transmit, and are held in a storage unit included in the radio signal transmitting / receiving units 2-1 to 2-k-2. The The training symbol may be specified by the control signal of Sig1, the training symbol to be used may be determined according to the order in which the group ID or the communication partner is specified, or fixed during past communication with the base station 1. Training symbols may be assigned. When the synchronization / timing setting unit 2-k-6 detects that information for controlling transmission power is received from the base station 1, the radio signal transmitting / receiving unit 2-k-2 is based on the information. Then, transmission power control is performed.

  On the other hand, the base station 1 detects a frame synchronization shift and a frequency shift by using a training symbol added to each wireless packet for wireless packets received simultaneously from a plurality of terminals. That is, the signal received via the antenna is input to the position detection / channel estimation user separation / synchronization unit 1-5 as a digital signal by the radio signal transmission / reception unit 1-3. The position detection / channel estimation user separation / synchronization unit 1-5 detects a signal position shift and a frequency shift by using a training symbol specified for each terminal.

The channel analysis unit 1-8 stores the received power from each terminal detected from the signal received by the base station 1 in order to determine the terminal that performs the uplink simultaneously and at the same frequency. The channel analysis unit 1-8 may determine and store information for controlling the transmission power for each terminal so that the reception power from each terminal of the terminal group transmitting at the same time is approximately the same. . Channel analysis section 1-8 individually stores control signals for transmission power to each terminal according to the combination of uplink terminals. The transmission power control signal is determined so that the signal difference of the received signal level at each base station 1 from each terminal becomes a certain level or less in the combination of terminals performing uplink. For example, the channel analysis unit 1-8 sets a transmission signal power reduction value for each terminal so that the reception level of the base station 1 is equal to or less than P _d [dB]. When the transmission power reduction value needs to be set to a value greater than P ₀ [dB], the channel analyzer 1-8 does not include the terminal in the combination of terminals performing simultaneous uplink. . For example, if P _d is 3 and P ₀ is 10, if the difference in received power at the base station 1 when no transmission power control is performed is greater than 13 [dB], this terminal will simultaneously transmit at the same frequency. It is not specified as a combination for line connection. Further, the channel analysis unit 1-8 detects a signal position shift and a frequency shift for each terminal. And channel analysis part 1-8 may notify each terminal of these so that each terminal can precompensate. Such information can be included in the control information of Sig1 described above.

Also in this system, the same single user communication as before is performed. Consider uplink communication from terminal 2-k. When the transmission signal generation unit 2-k-4 generates a transmission signal, the radio signal transmission / reception unit 2-k-2 performs modulation and assigns a guard interval. Then, the radio signal transmission / reception unit 2-k-2 performs conversion to an analog signal, up-conversion, and transmits via at least one of the antennas 2-k-1-1-1 to 2-k-1- _Mk. To do. The radio signal transmission / reception unit 1-3 of the base station 1 receives a signal via at least one of the antennas 1-4-1 to 1-4-N, and performs down-conversion and conversion into a digital signal. The position detection / channel estimation user separation / synchronization unit 1-5 performs signal position detection, frequency shift detection and compensation, and channel information estimation. At this time, the position detection / channel estimation user separation / synchronization unit 1-5 does not perform user separation when handling a received signal from one terminal. In this case, the position detection / channel estimation user separation / synchronization unit 1-5 outputs the channel size, the frequency shift value, and the delay amount for the terminal 2-k to the channel analysis unit 1-8. The demodulator 1-6 demodulates the received signal and extracts the original transmitted bits.

  The decoding determination unit 1-7 performs error determination on the demodulated bit sequence and determines whether or not the signal has been normally received. When the decoding determination unit 1-7 determines that the decoding has been normally performed, the decoding determination unit 1-7 notifies the transmission signal generation unit 1-2 of that fact. In response to this notification, the transmission signal generation unit 1-2 generates an ACK signal notifying that the decoding has been normally performed. The radio signal transmitting / receiving unit 1-3 modulates the ACK signal, adds a necessary control signal, converts it to an analog signal, up-converts it, and at least one of the antennas 1-4-1 to 1-4-N. Send via the above.

When a signal is transmitted on the downlink, the radio signal transmitting / receiving unit 2-k-2 of the terminal 2-k receives the signal via the antennas 2-k-1-1-1 to 2-k-1- _Mk . The radio signal transmitting / receiving unit 2-k-2 performs down-conversion on the received ACK signal and conversion into a digital signal. The position detection channel estimation synchronization unit 2-k-3 performs signal position detection, frequency shift compensation, and channel information estimation. The demodulator 2-k-5 demodulates the received signal and detects that the uplink communication is normal. Further, the channel analysis unit 1-8 of the base station 1 stores the channel size, frequency shift, clock shift, delay amount, and stability thereof for each terminal.
The frequency shift can be obtained from the above-described θ _S and θ _L as the amount of phase rotation for each symbol, as shown in Equation 21 below.

The amount of delay is the amount of delay after RTS / CTS (Request to Send / Clear to Send) communication or when a signal requesting a reply is sent from the base station when an ACK is returned from the terminal. It is a value indicating whether it can be confirmed. For example, in IEEE 802.11, after the base station 1 completes communication, ACK signals are returned at SIFS (Short InterFrame Space) intervals. However, the signal is received with a time delay corresponding to the SIFS interval + the apparatus signal processing delay + the propagation path delay from when the transmission is actually completed to when it is received. Therefore, it is possible to determine how much delay time exists for each terminal by storing, for each terminal, the time D until the reply is received after transmitting the control signal. Specifically, the time from transmission until it detects the reply is D, if the waiting time determined by the control and D _S, the delay time D _P can be expressed as (D-D _S) . Further, the clock shift can be determined by the amount of rotation of the phase of the pilot subcarrier into which the known signal is inserted when demodulating the data symbol. The stability information will be described later. The frequency shift, clock shift, and delay amount of the terminal 2-k stored in this way are converted into bits as a terminal control signal by the transmission signal generation unit 1-2 together with the identifier of the terminal 2-k. The terminal control signal is included in the control signal (Sig1). The radio signal transmitting / receiving unit 1-3 transmits a control signal to the terminal 2-k via the antennas 1-4-1 to 1-4-N.

The radio signal transmission / reception unit 2-1-k of the terminal 2-k receives a control signal via at least one of the antennas 2-k-1-1-1 to 2-k-1- _Mk . The position detection channel estimation synchronization unit 2-k-3 performs signal position detection, synchronization, and channel estimation. The demodulator 2-k-5 demodulates the received control signal. When the terminal control signal is included in the demodulated bit information, the synchronization / timing setting unit 2-k-6 determines whether or not the identifier included in the terminal control signal matches the identifier of the own device. judge. If they match, the synchronization / timing setting unit 2-k-6 performs correction so that the frequency and clock of the carrier wave are the same as those of the base station. Here, the synchronization / timing setting unit 2-k-6 was obtained when performing synchronization with the received signal from the base station 1 regardless of the control signal when correcting the carrier frequency and clock. Information may be used. The synchronous timing setting unit 2-k-6, when the control signal is returned after a control time is transmitted from the terminal 2-k from the base station 1, so the delay time becomes a constant value D ₀ The control time may be adjusted. This delay time D _0, from the delay time required in an uplink single user or may be set larger. The correction of the frequency shift may be performed by adjusting a reference signal generator that generates the frequency of the carrier wave, or the received signal is compensated for the frequency shift with respect to the time domain transmission signal u _{l, t} transmitted from the l-th antenna. Just like when you do

As described above, by generating the compensated transmission signal u ′ _{l, t} as a baseband signal _, a transmission signal in which the frequency shift is compensated in advance can be generated.
By performing uplink communication with each terminal, the channel analysis unit 1-8 can collect information on the frequency shift, clock shift, channel size, and delay time of the communication partner terminal. Further, by transmitting the terminal control signals as described above to each terminal, the deviation frequency, close the clock shift to 0., it is possible to make the delay time to a constant value D _0. However, there are errors in these estimated values, and there are cases where the terminal cannot always be controlled because there are individual differences and instability with respect to time. The above-described stability can be used as a parameter indicating how much the terminal can be controlled. That is, as a result of inserting the terminal control signal and resetting the frequency shift, clock shift, and delay time by the synchronization / timing setting units 2-1-6 to 2-k-6 of the terminal, the frequency shift and clock shift are again performed thereafter. The delay time is measured to determine whether it is within the expected value. If the stability is low, it is determined that uplink MU-MIMO communication is not possible, and uplink MU-MIMO can not be performed on the terminal. For example, when the frequency shift θ is equal to or less than θ ₀ and the delay amount D is in the range of D ₀ ± ΔD, it can be determined as a terminal capable of communication by uplink MU-MIMO.

  Also, the combination of uplink MU-MIMO can be determined in advance by the group ID in the channel analyzer 1-8. This group ID can be the same as that used during downlink MU-MIMO communication. The group ID of the downlink MU-MIMO can be specified for a terminal that needs to send downlink data at the same time or a terminal with a high downlink frequency. By specifying the group ID, the terminal can be assigned to the own terminal. It becomes possible to quickly detect that a signal arrives by MU-MIMO communication. However, uplink MU-MIMO has more conditions imposed on the terminal than downlink MU-MIMO communication, and is more difficult to establish. Therefore, group IDs can be specified independently for uplink and downlink. . Alternatively, a terminal combination capable of uplink MU-MIMO can be determined as a subgroup within a group ID designated on the downlink. Alternatively, the number of streams used on the uplink of each terminal may be specified for each terminal including 0 using the group ID of the downlink. With this configuration, it can be determined that the number of streams is 0 as being excluded from the group. First, in uplink MU-MIMO, when a difference occurs in the received power from each terminal, the communication quality of the terminal corresponding to the low received power at the time of reception in the base station 1 decreases. This is because the number of bits of a digital signal that can be used in the analog-to-digital converter in the base station 1 is finite, so that the number of bits allocated to a terminal having low reception power is reduced and the quantization error is increased. It is because it ends. Therefore, in uplink MU-MIMO, it is necessary to prevent an increase in quantization error by either one of the following two approaches for imbalance of received power.

The first method is a method of assigning a group ID to a pair of terminals whose difference in magnitude of received power from the terminals is equal to or less than Pd [dB]. Specifically, it is as follows. In the first method, the channel analysis unit 1-8 combines terminals having similar stored channel sizes as simultaneous communication terminals in uplink MU-MIMO. Assuming that the received power of the terminals 2-1 to 2-k is P _{1 to} P _k [dBm], the size allowed as the level deviation is set in advance as P _d [dB], and the selected combination The difference between the maximum received power and the minimum received power can be set to be equal to or less than P _d [dB]. For example, when the terminals 2-1 to 2-4 correspond to the downlink group ID 1, the received power is 55 dBm, 58 dBm, 41 dBm, and 37 dBm, and P _d is set to 5 dB. Then, an uplink signal can be requested from the terminal as an uplink terminal combination capable of simultaneously communicating with the terminals 2-1 and 2-2 and the terminals 2-3 and 2-4.

In the second method, a change in transmission power is specified for the terminal. Specifically, it is as follows. In the second method, the terminal is requested to reduce transmission power. That is, the terminal 2-k is designated to lower the transmission power by P _{r, k} [dB] _, compared to the case of the uplink for a single user. In other words, the received power of the above terminal 2-1 to 2-4 are 55dBm, 58dBm, 41dBm, when was 37dBm, the transmission power reduction _{factor, P r, 1 ~P r,} 4 and 18dB, 21 dB, 4dB , 0 dB, all received power can be set to 37 dB so that the problem of quantization error does not occur.

Further, more detailed conditions are imposed on the second method, and when the transmission power reduction amount P _{r, k} [dB] exceeds P ₀ , this terminal is not designated as a terminal combination of uplink MU-MIMO. You can also. In other words, if you were to P ₀ and 20 [dB], in the second method, the terminal 22 is not included in the combination of simultaneously communicable uplink terminal. For example, if the terminals 2-1 to 2-4 belong to the same group ID in the downlink MU-MIMO communication, the terminals 2-1 to 2-3 are designated as the uplink group ID. 2-4 is applicable (subgroup of downlink group ID), but terminal 2-2 is not applicable. In correspondence with downlink MU-MIMO, when uplink MU-MIMO is performed by transmitting an ACK signal or the like, since terminal 2-2 is not included in uplink MU-MIMO communication, an independent time slot is prepared. There is a need to. Further, if the terminal 2-4 is not included, the terminals 2-1, 2-2, and 2-3 can be made an uplink MU-MIMO group.

From here, the case where uplink MU-MIMO communication is designated to the terminals 2-1 to 2-U among terminals will be described. FIG. 2 is a diagram illustrating an example of a communication flow of uplink MU-MIMO. The base station 1 detects in advance that the terminals 2-1 to 2-U have uplink data, and inquires in the channel analysis unit that these terminals are combinations capable of performing uplink MU-MIMO at the same time. To do. At the same time it is possible combinations uplink MU-MIMO is that the difference between the received power of the signal from the terminal 2-1 to 2-U has become a less P _d, when performing power control, the maximum The stability of detecting whether the difference in received power is smaller than the sum of the power correction amounts P ₀ and P _d , and the signal from the terminal is corrected after the above-described frequency, clock, and transmission timing are specified. This is determined by confirming that the control specified by the base station 1 is performed in the terminal by measurement. When starting the uplink MU-MIMO, an uplink MU-MIMO start control signal Sig1 designating these terminals is transmitted. The uplink MU-MIMO start control signal can include correction values for frequency deviation, clock, and transmission timing, and a transmission prohibition interval (NAV: Network Allocation Vector) can also be set. When Sig1 is received, each terminal detects a signal by the position detection channel estimation synchronization unit 2-k-3 via the radio signal transmission / reception unit 2-k-2, and the demodulator 2-k-5 causes the Sig1 to detect the signal. Read. If Sig1 includes bits for frequency shift, clock shift, and timing compensation for the terminal itself, correction is performed in the synchronization / timing setting unit. Furthermore, a data packet used on the uplink is generated. Here, if NAV is specified in Sig1, a packet signal is generated so as to be equal to or shorter than the specified length, or a dummy is added after the data symbol so as to be the specified length. Insert data. Each terminal starts transmission of an uplink communication signal after a predetermined time after receiving Sig1.

  FIG. 2 shows a state in which the terminals 2-1 to 2-U transmit data after receiving Sig1. P in front of the data symbol represents a control signal including a short training symbol and a long training symbol. A dummy signal can be inserted into a frame surrounded by a dotted line shown after the data symbol, or no signal can be sent. When inserting a dummy signal, it is possible to select a signal having a low correlation with data or a dummy signal transmitted from another terminal transmitted at the same time. For example, a different PN sequence can be selected for each terminal.

  Here, the short training symbol and the long training symbol to be inserted into P are not conventional ones but are used for uplink MU-MIMO communication. From here, the short training symbol and the long training symbol will be described.

In the example of IEEE 802.11a, when communication is performed using 52 subcarriers in a 64-point DFT, a specific signal is used for 12 subcarriers among them, and a signal 0, that is, a signal is used for the remaining subcarriers. The short training symbol is obtained by connecting 10 signals of 0.8 μs obtained by performing IDFT without using. In uplink MU-MIMO, the signal position and the frequency shift can be estimated independently for each terminal. In order to generate a signal in this way, the base station 1 uses Sig1 to specify the number of spatial streams and the order of the terminals in advance for each terminal. From here, it is assumed that the number of terminals U is 3, and the spatial multiplexing number L _k used by each terminal is 2. Therefore, the total number of spatial streams is 6, and the order specified for the terminal 2-1 is 1, 2, 3 is assigned to the terminal 2-2, and 5 and 6 are assigned to the terminal 2-3. Six or fewer types of short training symbols are generated as the short training symbols. It is possible to generate three types for each user, to be common to the terminals 2-1 and 2-2, and to be one type for the terminal 2-3, for a total of two types. In addition, a short training symbol can be used as one type and a conventional format can be used. The type of these short training symbols is designated by Sig1. As a basic rule, _Q when generating the _S type of short training symbols, to generate a short training symbol _{p S, 1 ~p} S, _Qs prior to the IDFT, and, of these The vector is determined to be orthogonal. That is, when n and m are not equal, it is determined so that p _{S, n} ^H p _{S, m} = 0. Alternatively, the value obtained by performing the convolution operation of s _{S, n} and s _{S, m} obtained by IDFT of p _{S, n} and p _{S, m} is set so as to be significantly smaller than | s _{S, m} |. it can.

Next, three methods for generating a short training symbol are shown.
<Short training symbol 1>
The n-th short training symbol is generated from b _{n, 0 to} b _{n, 11} to specify signals for 12 subcarriers. When arranged from the lower RF frequency, the signal p _{S, n} of the n-th short preamble 1 before IDFT is obtained.

  It can be expressed. However, when n and m are different,

B _{n, 0 to} b _{n, 11} are determined so as to satisfy the above. Here, when the number of subcarriers using a known signal is 12, 12 types of orthogonal short training symbols can be generated. Therefore, by expanding the bandwidth and increasing the number of subcarriers for short training symbols, or by reducing the number of subcarriers for inserting zeros, the number of signals used increases and is generated as short training symbols. You can also increase the number of possible types.

<Short training symbol 2>
The short training symbol 2 can generate orthogonal short training symbols by changing the positions of subcarriers while b _{0 to} b ₁₁ are common signals for 12 subcarriers. According to the IEEE 802.11a standard, up to four types can be generated,

It can be expressed as. In this way, they are determined to be orthogonal to each other by the shift of the subcarrier position. In addition, since the above example is an example in which one known signal is used for four subcarriers, only four types can be generated, but more types can be generated by reducing the frequency of signal insertion. Here, b ₁₂ may be not used as zero for the signal corresponding to the DC component in the first kind. b ₁₃ from the fact that the edge of the band, it is also possible not to use as zero.

<Short training symbol 3>
The short training symbol 3 is a combination of the short training symbol 1 and the short training symbol 2. That is, for each signal sequence in short training symbols 2, _{Q S} number of signals _{b n, 0 ~b} n, ₁₁ satisfying Equation 25 _{or,, b n, 0 ~b n} , 12 or,, _{b n, 0 to} b _{n, 13} are generated. If the number of short training symbols generated in the short training symbol 2 is Q _S ′, the short training symbol 3 can generate Q _S × Q _S ′ types of short training symbols. Also, the plurality of short training symbols can be stored in the department in advance and used.

<Short training symbol 4>
The short training symbol 4 is characterized in that any one of the short training symbols 1 to 3 is used and the transmission timing of each terminal is shifted. FIG. 3 shows that the terminal 2-1 has the spatial multiplexing number 2 (L ₁ = 2), the terminal 2-2 has the spatial multiplexing number 1 (L ₁ = 1), and the terminal 2-3 has the spatial multiplexing number 1 (L ₁ = 1). ) Is a diagram showing that each terminal transmits a short training symbol under the condition (1).

By shifting the timing in this way, it is possible to improve the accuracy of detection of the signal position and frequency shift of each terminal. Also, some signals before and after the short training symbols can be converted to 0 in order to prevent the preceding and following short training symbols from overlapping at times T _{S, 2} and T _{S, 3} . For example, the conventional short training symbol is obtained by repeating s _{S, 0 to} s _{S, 15} 10 times, and the correlation value peak is detected 10 times, but the leading s _{S, 0 to} s is detected. It is possible to reduce the levels of _{S and 15} so that the peak of the correlation value is detected nine times, thereby preventing overlap between users with different head portions. In addition, signals can be not transmitted in A11 to A31 and B11 to B22 indicated by dotted lines. Alternatively, in A11 and A21, a long training symbol can be transmitted using subcarriers other than the short training symbol subcarrier of terminal 2-2. Similarly, in A12, A22, and A31, a long training symbol can be transmitted using subcarriers other than the short training symbol subcarrier of terminal 2-2. Also, in B11 and B21, a long training symbol can be transmitted using a subcarrier other than the short training symbol subcarrier of terminal 2-1. However, by doing so, there is a risk that the signal of the terminal 2-1 becomes difficult to detect. The positions of the short training symbols of the terminals 2-2 and 2-3 are easy to estimate because the signal position of the terminal 2-1 has already been detected. Similarly, in B22, a long training symbol can be transmitted using a subcarrier other than the short training symbol subcarrier of terminal 2-2.

That is, by performing an operation for obtaining a correlation value on the reception side for 1 to Q _S short training symbols, even if the short training symbols are received in an overlapping manner, it can be demodulated. A method for detecting the signal of the nth short training symbol will be described. 4 to 6 are diagrams showing an outline of an OFDM packet including a short training symbol, a long training symbol, and a data symbol. Here, FIG. 6 shows that the terminal 2-1 has the spatial multiplexing number 2 (L ₁ = 2), the terminal 2-2 has the spatial multiplexing number 1 (L ₁ = 1), and the terminal 2-3 has the spatial multiplexing number 1 (L ₁ = 1), a short training symbol is generated for each terminal. Each stream is assigned with a unique number of training symbols to be used, 1 for stream 1 of terminal 2-1, 2 for stream 2, 3 for terminal 2-2, and 4 for terminal 2-3. A symbol is specified. This can be designated by Sig1 transmitted to each terminal, or the terminal may determine in the order designated as the simultaneous communication terminal. For example, in Sig1, if the simultaneous communication uplink terminal is designated in the order of the terminals 2-1, 2-2, 2-3, and the number of streams of the terminal 2-1 is described as 2, The terminal assigns training symbols in the designated order, and can determine the unique number of the training symbol to which the terminal's stream corresponds. The generated _{pn, 1 to pn, 3} are subjected to IDFT and converted into a time domain signal vector, s _{S, n, 1 to} s _{Sn, 3} . Where these signals 0.8 .mu.s, 1 as a sample 50 ns, the short training symbols _s S each consisting of 16 _{samples, 1,0 ~s S, 1,15, s} S, 2,0 ~s _It can be expressed as a repetitive signal of _S, 2,15, s _{S, 3,0 to} s _S, 3,15. On the receiving side, the short training symbol is received, the correlation value is calculated by convolution of the correlation, and the signal position can be detected. Since the tenth nth short training symbols s _{S, n, 0 to} s _{S, n, 15} are transmitted continuously, the fluctuations in the correlation value of the received signal are observed, so Can be estimated.

Here, ρ _{n, t} is a correlation value with the n-th short training symbol at time t, and r _{l, t} represents a received signal at time t at the l-th antenna of the base station. When a short training symbol is received, the correlation value peak is observed 10 times every 0.8 μs according to Equation 27 for each short training symbol. Based on this reception position, a signal position for performing DFT on the OFDM signal can be determined. In the case of uplink MU-MIMO, the position where DFT is performed for each terminal can be made common, or can be set separately. In the case of common setting, the DFT position is set based on the start position of the entire correlation value peak or the end position of the entire correlation value.

Also, assuming that the time at the peak of the 10 correlation values at which the n-th short training symbol is detected is T _{n, 0 to} T _{n, 9} ρ _{n, t} (t = T _{n, 0 to} T _{n , 9} ) can detect a frequency shift from the phase change. When the phase is rotated by θ _{S, n for} each peak, the frequency is shifted between the base station and the terminal transmitting the nth short training symbol by an amount corresponding to θ _{S, n.} Will be. However, since this frequency shift exists as many as the number of terminals with which it communicates, it is necessary to branch the received signal for each terminal and to correlate each branched training signal with each short training symbol. There is. The received signal for terminal 2-k that is transmitting the nth short training symbol,

  As described above, the frequency shift can be compensated by applying the reverse phase rotation. Compensation for frequency deviation is performed after the received signal is branched. Therefore, one received signal is branched by the number of terminals. Alternatively, if a plurality of terminals have high frequency synchronization accuracy and can be corrected with a common frequency deviation compensation coefficient, if the number of received signals to be branched is G and the number of terminals sharing frequency deviation compensation is (G -1) can be reduced.

Next, the long training symbol will be explained. In the long training symbol, signals p _{0 to} p ₅₁ used for 52 subcarriers are generated as in the conventional case, for example, in accordance with the IEEE 802.11a format. Next, DFT is performed on 64 signals obtained by inserting 0 in the direct current component into p _{0 to} p ₅₁ and 0 in the end of the frequency band, and the obtained time domain signals s _{L, 0 to} s _{L, 63} is used to generate a long training symbol. The long training symbols before DFT are the same as those in Equation 8, and 64 time domain signals s _{L, 1 to} s _{L, 63} are obtained by performing DFT. The generation methods of the long training symbols (1) and (2) are the same as those in Equations 9 and 10.

The estimation and compensation of the frequency shift using the long training symbol is shown. The long training symbol is a repetitive signal of at least a part of s _{L, 0 to} s _{L, 63.} However, when receiving simultaneously from all terminals, the frequency shift of each terminal is mixed, The frequency shift cannot be separated. In order to prevent this, for example, as shown in FIG. 4, the transmission timing of the long training symbol is shifted for each terminal, each independent OFDM symbol is received, and the amount of phase rotation at 64 sample intervals as in the conventional method To extract. The long training symbol shown in FIG. 4 may be either the long training symbol (1) or the long training symbol (2). On the receiving side, this periodicity is used to compensate for the frequency shift from the phase shift of the received signal. Referring to FIG. 4, consider a case where U long training symbols are received by TL, 1 to TL, U, and one each from each terminal. In this case, the long training symbol received at time _{TL, k} is transmitted from the terminal 2-k, and the estimated frequency shift is the frequency shift between the terminal 2-k and the base station 1. Corresponding to

First, an example of a long training symbol (1) consisting of 160 samples is shown. The frequency shift with respect to the terminal 2-k is obtained by using the received signal r ′ ^(k) _{l, t} of Equation 28 in which the frequency shift with the terminal 2-k is compensated by using the received signal of the short training symbol.

Get. The phase θ _{L, k, l of} ρ _{L, k, l is} the phase rotation for 64 samples corresponding to the l-th analog-digital converter (or antenna) for the terminal 2-k. As the amount of phase rotation, the value of a specific analog-to-digital converter is used, an analog-to-digital converter having a high reception level is selected, or ρ _{L, k,} obtained by all analog-to-digital converters the asking for phase correlation values are E are averaged _l, and or an average phase theta _{L, l,} can be used phase theta _L. In the case of the long training symbol (2), s _{L, 58 to} s _{L, 63} are repeated twice. Therefore, when the timing of first _{s L, 58} is measured and _{T L,} the second _{s L, 58} will be obtained by _{T L + 64.} Therefore, the correlation value can be obtained as follows by autocorrelation of the received signal.

Using this ρ _{L, k, l} , the phase θ _{L, k} can be determined similarly to the result of Equation 32. The frequency shift can be compensated as follows by θ _{L, k} obtained here.

As shown in the above formula, the frequency shift can be compensated by applying an opposite phase rotation for each terminal.
Next, a case where long training symbols are received simultaneously will be described. FIG. 5 shows an example in which long training symbols are received simultaneously from U terminals. The number of streams from each terminal is 1. As in the case of single user MIMO, the long preamble is multiplied by the coefficients a _{i, j,} and the matrix A having a _{i, j} as elements is transmitted under the condition of A ^H A = I. It is. Each terminal stores A in advance, and can determine which row component element of A is used to form a long training symbol based on a signal from the base station 1 (Sig1 in FIG. 2). Which part of A corresponds to the stream of its own terminal can be determined by the above-mentioned unique number specified by Sig1. This can be designated by Sig1 transmitted to each terminal, or the terminal may determine in the order designated as the simultaneous communication terminal. For example, in Sig1, if the simultaneous communication uplink terminal is designated in the order of the terminals 2-1, 2-2, 2-3, and the number of streams of the terminal 2-1 is described as 2, The terminal assigns training symbols in the designated order, and can determine the unique number of the training symbol to which the terminal's stream corresponds.

In FIG. 5, long training symbols starting from reception times T _{L, 1 to} T _{L, U} are respectively multiplied by coefficients a _{i, j} . By multiplying the received signal at each timing by the complex conjugate a _{i, j} * of coefficient a _{i, j} and taking the sum, it is possible to cancel or reduce the received signal from a terminal other than an arbitrary terminal. . In order to compensate the frequency shift for the terminal 2-k using the long training symbol,

Calculate as Here, TLT is the period of the long training symbol, and can be set to 160 for the long training symbol (1) and 80 for the long training symbol (2). In order to satisfy A ^H A = I, only the long training symbol corresponding to the terminal 2-k can be extracted by Equation 31. However, although transmission is performed so as to satisfy the condition of A ^H A = I, since the frequency shift with each terminal is different, the signals transmitted from terminals other than the terminal 2-k are actually transmitted. If the long training symbols from each terminal are transmitted so as not to overlap as shown in FIG. However, there are two merits of performing such control. First, no signal-free section is created, and second, transmission power can be used effectively. The first point can prevent the hidden terminal problem, the interference of other cell communication due to the difference in the terminal position condition, and the problem of interference. In the second point, since all terminals always transmit with constant power, it is advantageous to be able to increase transmission power used for a signal for channel estimation. By using the obtained values as r ^(k) _{l, t} in Equations 29 and 30, the frequency shift can be estimated for each terminal as in the case of FIG.
4 to 6, the short training symbols can be transmitted with a time shift as shown in FIG.

  Next, channel information estimation using a long training symbol will be described. Note that, unlike the case of single-user MIMO communication, in uplink multi-user MIMO communication, the overall channel information apparently changes depending on which terminal is synchronized with the frequency. First, a parameter representing a frequency shift estimated by the short training symbol and the long training symbol is defined. The estimated residual frequency deviation between terminal 2-k and base station 1 is

The received signal r ″ ^(k) _{l, t} is a received signal compensated for the estimated frequency shift with respect to the terminal 2-k. At an appropriate position excluding the guard interval of the OFDM symbol, F The received signal in the frequency domain of the l-th antenna obtained by performing the point DFT and compensated for the frequency shift of the terminal 2-k is y ^(k) _{l, m, 1} , y ^(k) _{l , M, 2} ,..., Y ^(k) _{l, m, F} , where m is the number of the OFDM symbol and represents the mth OFDM symbol from the beginning, and F is in IEEE 802.11. When the bandwidth of 20 MHz is set, 64 is used, and the received signal of the l-th antenna of the j-th subcarrier of the received signal subjected to frequency compensation for the terminal 2-k is y ^(k) _{l, m, j} and Table That. The received signal vector corresponding to all the antennas of the j-th subcarrier of the received signal subjected to frequency compensation to the terminal 2-k is

  It can be expressed. Assuming that there are U OFDM symbols corresponding to the long training symbols, a received signal matrix corresponding to the long training symbols of the j-th subcarrier that has compensated the frequency shift for the terminal 2-k.

It can be expressed as. Here, it is assumed that the long training symbols are from the first OFDM symbol to the Lth OFDM symbol. L is the number of long training symbols, and in the case of FIGS. 4 and 5, L = U is set. When each terminal uses a plurality of antenna elements, L can also be defined as the sum of the number of antennas of the terminal serving as a communication partner. The case of FIG. 6 corresponds to this, and the terminal 2-1 has two streams. For use, L is set to 4 for 3 terminals. The codes a _{i, j} for orthogonalization used for the long training symbols are given in the same manner as in Equation 15. If there is no frequency shift between all terminals and the base station, L long Representing the received signal matrix Y _j using the received signal vector of training symbols,

It can be expressed as. Here, H _j and W _j are set matrices of channel matrices H _{1, j to} H _{U, j} and W _{1, j to} W _{U, j} corresponding to terminals 2-1 to 2-U,

It can be expressed. In this case, in the base _{station, p} j, that advance know _A, H j _{W j} of the estimated matrix H _{'W, j} is

  As obtained. However, in uplink multi-user MIMO communication, frequency deviation remains in each terminal, so that the method of multiplying the reception signal matrix by the inverse matrix of A degrades the channel estimation accuracy. The received signal matrix for terminal 2-k is

Here, A ′ _k represents a matrix A including a frequency shift viewed from a received signal sequence obtained by compensating the frequency shift for the terminal 2-k, and is defined as follows.

That is, in uplink multiuser MIMO communication, channel estimation accuracy can be improved by using A ′ _k for each terminal instead of A. That is, the entire channel matrix including the transmission weight is

Can be obtained as Here, H ′ ^(k) _{W, j} is a channel matrix that appears when the frequency is matched to the terminal 2-k. In this way, with respect to the received signal matrices Y _j ^{(1) to} Y _j ^(U) compensated for the frequency shift for the terminals 2-1 to 2-U, respectively, an inverse matrix of A ′ _k or a pseudo inverse By multiplying the matrix, channel information H ′ ⁽¹⁾ _{W, j to} H ′ ^(U) _{W, j} for a plurality of terminals can be estimated. H ′ ^(k) _{W, j} is composed of channel information from the terminals 2-1 to 2-U when the frequency is matched to the terminal 2-k. From this information, Zero forcing (ZF ) Method, Minimum mean square error (MMSE) method, and Maximum likelihood detection (MLD), each terminal can demodulate by the demodulator 1-6-1 to 1-6-U. In the case of using MMSE, the reception weight used for the reception signal in which the frequency shift to the terminal 2-k is compensated for the terminal 2-k is

Can be calculated as Here, [X] _(k) is a function for extracting a column vector corresponding to the terminal 2-k from the matrix X. When there are U terminals in each terminal single stream as shown in FIG. 5, [X] _(k) is calculated as the k-th column vector, and the number of streams becomes plural depending on the terminal as shown in FIG. In this case, when k = 1, the first and second column vectors of X are extracted, and when k = 2 and k = 3, the third and fourth column vectors of X are extracted, respectively. The reception weight is R _{k, j} is an N × L _k matrix. The transmission signal x _{k, j} can be estimated by multiplying the reception signal whose frequency shift is compensated for the terminal 2-k by the complex conjugate transpose of R _{k, j} .

Also, as another method, frequency compensation is not performed on the received signal in the time domain for each terminal, so that the number of DFTs can be reduced and the load of the reception algorithm can be reduced. In the method described so far, each terminal needs to compensate for the frequency shift as shown in Equation 28 and Equation 31, and the DFT is performed on the received signal obtained in each terminal. It is necessary to perform the processing by the number of devices × the number of terminals to communicate, which increases the calculation load. Such control has an advantage that interference between subcarriers in OFDM can be minimized for each terminal. However, if the variation in frequency deviation for each terminal is small, the influence of interference between subcarriers is small even if DFT is commonly used for all terminals. In this way, the received signal, without frequency compensation, or used as is used reception signal obtained by performing the compensation at the representative value theta _A for all terminals. In the latter case, the received signal of the 1st analog-digital converter is

As obtained. By setting θ _A = 0, it is possible to obtain a received signal without compensation. As θ _A , for example, an average value of θ _{1 to} θ _U can be used. The received signal matrix corresponding to the long training symbol obtained by performing DFT on the received signal sequence obtained here is

It can be expressed. Here, Y _j is a received signal matrix made up of received signals without frequency compensation or compensated for frequency shift by θ _A.

It can be expressed. y _{l, m, 1} , y _{l, m, 2} ,..., y _{l, m, F} are received signals in the frequency domain of the l-th antenna corresponding to the m-th OFDM symbol from the head. It is obtained by performing DFT of F point at an appropriate position excluding the OFDM symbol guard interval with respect to r ″ ′ _{l, t} . The channel information including the transmission weight is

Is estimated as However, since this channel information has a frequency shift with respect to each terminal, it is necessary to compensate for a frequency shift remaining in a reception weight for performing demodulation. In addition, θ _i of the i-th stream defined for each stream in Equations 20, 41, and 46 can be a common value for all streams corresponding to the same terminal. In the case of using MMSE, the reception weight used for the reception signal in which the frequency shift to the terminal 2-k is compensated for the terminal 2-k is

Can be calculated as Unlike Equation 43, there is no need to calculate for each terminal, and the reception weight can be obtained by performing Equation 49 for all terminals. However, since the frequency shift remains for each terminal, it is necessary to multiply the reception weight by the frequency shift compensation coefficient. The transmission signal x _{k, j} corresponding to the terminal 2-k is

Can be calculated as
FIG. 7 is a diagram illustrating a second communication sequence. FIG. 8 is a diagram showing a third communication sequence. FIG. 9 is a diagram illustrating a fourth communication sequence. As described above, according to the method of the present invention, it is possible to estimate the frequency shift in each terminal and estimate the channel information using the matrix A ′ or A ′ _k, but an error occurs in the estimated value of the frequency shift. For this reason, it is important to start communication with a small frequency deviation in advance. A sequence for this will be described.

  In FIG. 7, the base station first designates terminals 2-1 to 2-U to perform uplink communication. Thereafter, a response (Sig21 to Sig2U) is obtained from each terminal that the communication can be performed without any problem. At this time, frequency deviation, clock deviation, and reception level are measured from the received signals Sig21 to Sig2U by a conventional method. When the uplink MU-MIMO is designated again, it is possible to notify information on any of frequency shift, clock shift, and reception level, and improve the transmission quality of data received from each terminal.

  In the system of FIG. 8, every time Sig21 to Sig2U are received, the fact that they are normally received is notified by ACK1 to ACCU. By doing in this way, when there is no reply from each terminal, it is possible to generate a non-signal period and prevent inter-cell interference. Also, in this method, any information of frequency shift, clock shift, and reception level can be notified using ACK1 to ACCU, and Sig3 can be omitted.

  In the system of FIG. 9, when it is notified by Sig1 that uplink MU-MIMO communication is performed, terminals 2-1 to 2-U transmit Sig21 to Sig2U as response signals by uplink MU-MIMO communication. . The base station can estimate the frequency shift by the method of the present invention and notify it by Sig3, thereby improving the transmission quality at the time of data communication.

Further, by introducing CSD (Cyclic Shift Diversity) into each stream having the above configuration, it is possible to prevent the characteristics from deteriorating in a specific fading environment.
FIG. 10 is a diagram illustrating an effect in the present embodiment. The horizontal axis corresponds to the frequency shift and indicates how many times the phase rotates for each OFDM symbol. The vertical axis corresponds to the error between the estimated channel information and the actual channel information. The channel matrix is i. i. d. Given as a complex Gaussian distribution. The number of users was 4, and the channel was estimated with 4 long training symbols. The thermal noise is set to 0, and only the deterioration of the channel estimation accuracy due to the frequency shift is extracted. When A ′ is used according to the configuration of the present invention, the channel is estimated with almost no error. However, in the conventional calculation using A, the phase is rotated once for each OFDM symbol by −26 dB. It can be confirmed that MSE occurs.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

  A communication system that dramatically increases system throughput is realized by allowing multiple terminals to perform uplink communication using the same frequency at the same time.

2-1 to 2-K ... terminal (terminal device), 1-2 ... transmission signal generation unit, 1-3 ... radio signal transmission / reception unit, 1-4-1 to 1-4-N ... antenna, 1-5 ... Position detection / channel estimation user separation / synchronization section, 1-6 ... demodulation section, 1-7 ... decoding determination section, 1-8 ... channel analysis section, 2-k-1- _Mk ... antenna, 2-k-2 ... Wireless signal transmission / reception unit, 2-k-3 ... position detection channel estimation synchronization unit, 2-k-4 ... transmission signal generation unit, 2-k-5 ... demodulation unit, 2-k-6 ... synchronization / timing setting unit

Claims (9)

A wireless communication system in which a plurality of terminals simultaneously transmit data to a base station using the same frequency,
The base station
A receiver for receiving signals from a plurality of terminals;
Signal position detection for detecting, for each terminal, a signal position shift of a signal transmitted from each terminal based on a correlation between a training symbol transmitted from the terminal and received by the receiving unit and a training symbol held by itself. And
A frequency shift estimator for estimating a frequency shift for each terminal based on a phase difference between the training symbol and a training symbol designated by the own device for the terminal;
For each of the plurality of terminals, a control signal generation unit that generates a control signal including an identifier and information specifying a training symbol used in uplink transmission at the same frequency and the same time;
A downlink transmission unit that generates and transmits a downlink transmission signal including the control signal; and
With
The terminal device
A receiver for receiving a signal from the base station;
Synchronization / timing for extracting the control signal from the received downlink transmission signal and calculating a frequency shift obtained from the received signal in the receiving unit when the identifier included in the control signal indicates the own device A setting section;
A transmission unit that corrects a transmission signal to which a unique training symbol specified by the control signal is added based on the frequency shift calculated by the synchronization / timing setting unit, and transmits the transmission signal .
The control signal generation unit selects the one terminal as a terminal that performs simultaneous communication using the same frequency when the frequency deviation, clock deviation, and delay time of the one terminal exceed a predetermined range. do not do,
Wireless communication system.   2. The wireless communication system according to claim 1, wherein the control signal generation unit selects a terminal that does not exceed a predetermined reception level difference between terminals as a group of terminals that simultaneously perform communication using the same frequency. .   The control signal generator is configured to reduce the transmission power to each terminal so that a difference in received power from terminals in a group that simultaneously communicate using the same frequency is equal to or less than a predetermined allowable power difference. The wireless communication system according to claim 1, wherein an amount is determined and designation of a reduction amount of transmission power is included in the control signal.   The control signal generation unit is configured to determine a terminal in which a reception level difference from a terminal of a group that performs simultaneous communication using the same frequency does not exceed a sum of a predetermined maximum power reduction amount and an allowable power difference. The wireless communication system according to claim 1, wherein the wireless communication system is selected as a group of terminals that perform simultaneous communication using a frequency.   The control signal generator measures the estimated signal arrival time for each terminal, measures the delay time that occurs after making a signal transmission request from its own device, and receives a response signal from each terminal to its own device The wireless communication system according to claim 1, wherein the transmission timing of the terminal is controlled by including in the control signal a control signal for designating transmission timing so that a delay time until the delay time is within a predetermined range. Channel information estimation means for estimating channel information for a terminal from received training symbols based on information of training symbols designated from the own device to the terminal, wherein the training symbol is an L × L orthogonal code

Is transmitted using the phase rotation information θ1 to θL obtained from the frequency shift estimated for each stream, and the inter-symbol time TLD,

The wireless communication system according to claim 1, further comprising channel information estimation means for correcting channel orthogonality so as to improve channel estimation accuracy by performing channel estimation.   The transmission unit of the terminal device transmits a training symbol used by the base station to know signal reception timing from each terminal device as a unique training symbol at a time different from that of other terminal devices. Item 2. The wireless communication system according to Item 1. The transmission unit of the terminal device transmits, as a unique training symbol, a training symbol used by the base station to know signal reception timing from each terminal device at a different time from other terminal devices. Channel estimation using a frequency channel other than the frequency channel used by the other terminal apparatus for the signal position estimation training symbol at the timing at which the terminal apparatus transmits the signal position estimation training symbol. The wireless communication system according to claim 7 , wherein the training symbol is transmitted. A wireless communication method performed by a wireless communication system in which a plurality of terminals simultaneously transmit data to a base station using the same frequency,
A receiving step in which the base station receives signals from a plurality of terminals;
Based on the correlation between the training symbol transmitted from the terminal and received in the reception step, and the training symbol held by the base station, the base station determines the signal position deviation of the signal transmitted from each terminal for each terminal. A signal position detecting step to detect;
The base station estimates a frequency shift for each terminal based on a phase difference between the training symbol and a training symbol designated by the own device for the terminal; and
The base station generates a control signal generation step for generating a control signal including, for each of a plurality of terminals, an identifier and information specifying a training symbol used in uplink transmission at the same frequency and same time;
A downlink transmission step in which the base station generates and transmits a downlink transmission signal including the control signal; and
A receiving step in which the terminal apparatus receives a signal from the base station;
When the terminal apparatus extracts the control signal from the received downlink transmission signal and the identifier included in the control signal indicates the own apparatus, a frequency shift obtained from the signal received in the reception step is calculated. Synchronization / timing setting step to be calculated;
A transmission step in which the terminal apparatus corrects a transmission signal to which a unique training symbol specified by the control signal is added based on the frequency shift calculated in the synchronization / timing setting step, and transmits the transmission signal. And
In the control signal generation step, when the frequency shift, clock shift, and delay time of one terminal exceed a predetermined range, the one terminal is selected as a terminal that performs simultaneous communication using the same frequency. do not do,
Wireless communication method.

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