JP4405411B2 - OFDM signal transmitter - Google Patents

Description

  The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) signal transmission apparatus using a plurality of transmission antennas.

  Among OFDM signal transmission apparatuses, the method of simultaneously transmitting different data by using a plurality of transmission antennas has an advantage that a large amount of data can be transmitted at high speed, but the error rate characteristic of the data is likely to deteriorate. Therefore, a pilot subcarrier is formed by superimposing a known signal called a pilot symbol on one or a plurality of specific subcarriers on the transmission side, and propagation path compensation of each subcarrier is performed based on the pilot subcarriers on the reception side. There is known a method of obtaining a received signal with good error rate characteristics by performing frequency offset compensation.

  In this way, when the same known signal is transmitted using a plurality of transmission antennas using pilot subcarriers of the same frequency, the transmission signals of the pilot subcarriers interfere with each other to form a directional beam. When the carrier frequency (5 GHz band) is higher than the pilot subcarrier interval (about 4.4 MHz) as in the OFDM signal based on the IEEE 802.11a standard, the directional beam corresponding to each pilot subcarrier is almost equal. Face in the same direction. In this case, the null point at which the electric field of each directional beam suddenly falls also points in the same direction, so it is almost impossible to receive pilot subcarriers in the null point direction, and the reception characteristics deteriorate rapidly. To do.

In order to cope with such a problem, Patent Document 1 discloses a technique in which a pilot subcarrier is transmitted using only one transmission antenna, and a null signal is transmitted in the frequency band of the pilot subcarrier using the other transmission antennas. Yes. According to this method, the problem of mutual interference between pilot subcarriers due to transmission of pilot subcarriers from a plurality of transmission antennas can be avoided, so that deterioration of reception characteristics due to formation of a directional beam can be prevented.
Japanese Patent Laid-Open No. 2003-304216

  In the method of transmitting pilot subcarriers from only a single transmission antenna as in Patent Document 1, the total transmission power of pilot subcarriers is reduced as compared to the case of transmitting pilot subcarriers from a plurality of transmission antennas. This deteriorates the reception performance of the receiver.

  If the transmission power of pilot subcarriers from a single transmission antenna is made larger than the transmission power of data subcarriers from each transmission antenna, the total transmission power of pilot subcarriers can be increased and reception performance is improved. . On the other hand, increasing the transmission power of pilot subcarriers from a single transmission antenna will cause uneven transmission power within the frequency band of the OFDM signal. This increases the dynamic range of the D / A converter of the receiver (especially the input dynamic range).

  An object of the present invention is to provide an OFDM signal transmission apparatus that reduces the composite third-order distortion without reducing the transmission power of pilot subcarriers. A further object of the present invention is to increase the area where high-quality reception is possible.

  An OFDM signal transmission apparatus according to an aspect of the present invention is an OFDM signal transmission apparatus that transmits an OFDM (Orthogonal Frequency Division Multiplexing) signal using a plurality of transmission antennas, wherein some of the subcarriers of the OFDM signal transmit a pilot signal. A signal to be transmitted on each subcarrier is set so that it is used as a pilot subcarrier for transmission and the remaining subcarriers are used as a data subcarrier for transmitting a data signal, and a pilot is set for each transmission antenna. There is a mode in which a transmission beam is formed regardless of the channel response between the subcarrier setting means for changing the polarity of the signal for subcarrier and the OFDM signal receiving apparatus that receives the OFDM signal. Different weights for carriers and pilot subcarriers Characterized by comprising a beamformer for forming the transmission beam using.

  The beam former may form the transmission beam only for the data subcarrier in the mode.

  Further, the beamformer, for example, with respect to the pilot subcarrier, in the mode, multiplies an input matrix representing the pilot subcarrier by a unit matrix.

  According to the present invention, even if the received power of one pilot subcarrier is small, the possibility that the received power of other pilot subcarriers is increased is increased. Therefore, a so-called dead zone in which the reception power of all pilot subcarriers simultaneously drops can be reduced, and the area where high-quality reception is possible increases. Further, since the transmission power of pilot subcarriers from each transmission antenna can be made uniform, there is no occurrence of complex third-order distortion, and there is no need to particularly increase the input dynamic range of the D / A converter.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
As shown in FIG. 1, in the OFDM system according to the first embodiment of the present invention, an OFDM signal is transmitted from an OFDM signal transmission apparatus 100 having a plurality of transmission antennas 101a and 101b, respectively. The transmitted OFDM signal is received by an OFDM signal receiving apparatus 200 having a plurality of receiving antennas 201a and 201b. Here, a case where the OFDM signal transmission apparatus 100 has two transmission antennas 101a and 101b and the OFDM signal reception apparatus 200 has two reception antennas 201a and 201b will be described, but the present invention is not limited to this. The present invention is also effective when an antenna and a receiving antenna are provided.

  As schematically shown in FIGS. 2A and 2B, in the present embodiment, the OFDM signal transmission apparatus 100 forms two OFDM signals from two different transmission data, and transmits these from different transmission antennas 101a and 101b. To do. The first OFDM signal shown in FIG. 2 (a) is superimposed with transmission data DATA_a (N, K), and the second OFDM signal shown in FIG. 2 (b) is superimposed with transmission data DATA_b (N, K). Yes. Here, DATA_a (N, K) represents data transmitted from the transmission antenna 101a and transmitted on the Kth subcarrier of the Nth symbol. DATA_b (N, K) represents a signal transmitted from the Nth symbol K subcarrier in the data transmitted from the transmission antenna 101b. The pilot subcarrier will be described later.

Now, the transfer function of the propagation path from the transmission antenna 101a to the reception antenna 201a (hereinafter, the transfer function of the propagation path is referred to as a propagation path response value) is Haa, and the propagation path response value from the transmission antenna 101a to the reception antenna 202b is Hab, When the propagation path response value from the transmission antenna 101b to the reception antenna 201a is Hba and the propagation path response value from the transmission antenna 101b to the reception antenna 202b is Hbb, the reception signal RXa of the reception antenna 201a and the reception signal RXb of the reception antenna 201b Can be written as:

TXa and TXb indicate transmission signals from the transmission antennas 101a and 101b, respectively. The transmission signals TXa and TXb can be demodulated by multiplying the reception signals RXa and RXb by the inverse matrix of the matrix formed by the propagation path response values Haa, Hab, Hba, and Hbb.
In the first embodiment, apart from a data subcarrier for transmitting data, a pilot subcarrier for transmitting a known signal used for compensating a residual phase error of a frequency offset or a clock offset is used. That is, at the time of reception, a residual phase error is detected and compensated using a known signal transmitted by a pilot subcarrier.

  Here, for comparison, in Patent Document 1, the OFDM signal shown in FIG. 30 (a) is transmitted from the first transmission antenna, and the OFDM signal shown in FIG. 30 (b) is transmitted from the second transmission antenna. Is sent. That is, as shown in FIG. 30A, pilot subcarriers indicated by diagonal lines are transmitted only from the first antenna. From the second antenna, pilot subcarriers are not transmitted as shown in FIG. 30 (b), and a null signal is transmitted at a frequency corresponding to the pilot subcarriers as indicated by a blank. Therefore, since the pilot subcarriers are transmitted without interfering with each other, the reception characteristics are not deteriorated by the directional beam, but the total transmission power of the pilot subcarriers is reduced.

  On the other hand, according to the first embodiment, good reception characteristics can be obtained while transmitting pilot subcarriers from the two transmission antennas 101a and 101b and sufficiently securing the total transmission power of the pilot subcarriers.

  Next, the OFDM signal transmission apparatus 100 shown in FIG. 1 will be described with reference to FIG. An OFDM signal transmission apparatus 100 includes an encoder 102, a serial / parallel converter 103, modulators 104a and 104b, a serial / parallel converter 105, a pilot subcarrier insertion unit 106, a beam former 107, and an inverse fast Fourier transform (IFFT). Units 108a and 108b are included.

  The input transmission data is a wireless packet having a structure as described later, and is encoded by the encoder 102. The encoded data is subjected to serial / parallel conversion by the serial / parallel converter 103, so that the encoded data is distributed into first transmission data corresponding to the transmission antenna 101a and second transmission data corresponding to the transmission antenna 101b. The first transmission data and the second transmission data are subcarrier modulated by modulators 104a and 104b, respectively. For example, BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), or 64QAM is used as a modulation method of the modulators 104a and 104b.

  The modulated data output from the modulator 104a is distributed to a plurality of first data subcarriers by the serial / parallel converter 105a. Similarly, the modulation data output from the modulator 103b is distributed to a plurality of second data subcarriers by the serial / parallel converter 105b.

  Modulated data allocated to the first data subcarrier and the second data subcarrier (hereinafter referred to as the first data subcarrier and the second data subcarrier) are input to pilot subcarrier insertion section 106. In pilot subcarrier insertion section 106, some subcarriers of the OFDM signal are allocated to pilot subcarriers for transmitting pilot signals, and the remaining subcarriers are allocated to data subcarriers for transmitting data signals.

  Specifically, pilot subcarrier insertion section 106 transmits a pilot signal transmitted on at least one first pilot subcarrier between first data subcarriers (hereinafter referred to as a first pilot subcarrier in this specification). And pilot signals (hereinafter referred to as second pilot subcarriers) transmitted on at least one second pilot subcarrier are inserted between the second data subcarriers, respectively. . A set of first data subcarriers and first pilot subcarriers is called a first subcarrier signal, and a set of second data subcarriers and second pilot subcarriers is called a second subcarrier signal.

  The first subcarrier signal and the second subcarrier signal output from pilot subcarrier insertion section 106 are input to beam former 107. The beam former 107 performs beam forming processing to form a plurality of transmission beams by weighting and synthesizing the output from the serial / parallel converter 105a and the output from the serial / parallel converter 105b. The beam former 107 will be described in detail later by taking as an example a case where the number of transmission antennas is larger.

  The output signal from the beam former 107 is subjected to inverse fast Fourier transform by the IFFT units 108a and 108b, respectively. The output after IIFFT is output from the transmission antennas 101a and 101b. In FIG. 3, the beam former 107 is inserted before the IFFT units 117a and 108b, but may be inserted after the IFFT units 117a and 108b. As a result of the inverse fast Fourier transform in the IFFT units 108a and 108b, the first subcarrier signal and the second subcarrier signal after the beam forming process are multiplexed by being converted from a signal on the frequency axis to a signal on the time axis. The first OFDM signal a and the second OFDM signal b as shown in FIGS. 2A and 2B are generated. The OFDM signals a and b are sent to the transmission antennas 101a and 101b via a wireless transmission unit (not shown), and transmitted from these antennas 101a and 101b.

Next, pilot subcarrier insertion section 106 will be described using FIG.
In pilot subcarrier insertion section 106, the first data subcarrier from serial / parallel converter 105a and the second data subcarrier from serial / parallel converter 105b are output to beamformer 107 as they are. At this time, at least one first pilot subcarrier and second pilot subcarrier are inserted between the first data subcarriers and between the second data subcarriers, respectively. In the present embodiment, there are four first pilot subcarriers and four second pilot subcarriers.

The sequence generator 110 generates a pseudorandom noise (PN) sequence such as an M sequence as a pseudo-random sequence. The first pilot subcarrier is generated by obtaining the product of the pseudo random sequence PN (i) and the polarity data Sa (j) of the first pilot subcarrier stored in the ROM 121a by the multiplication units 111a to 111d. Similarly, the second pilot subcarrier is generated by obtaining the product of the pseudo random sequence PN (i) and the polarity data Sb (j) of the second pilot subcarrier stored in the ROM 121b by the multiplication units 112a to 112d. . Assuming that the baseband signal of the first pilot subcarrier transmitted from the transmitting antenna 101a is Pa (i, j), Pa (i, j) is expressed as PN (i) and Sa (j) as follows. Expressed as a product.

However, i is a symbol number and is arranged in the time direction. j is the number of the pilot subcarrier, and is arranged in the frequency direction. Similarly, the pilot subcarrier baseband signal Pb (i, j) from the transmitting antenna 101b is represented by the product of PN (i) and Sb (j) as follows.

In the first embodiment, the number of pilot subcarriers transmitted from the transmitting antennas 101a and 101b is four (j = 1 to 4), and the first and second transmitted from the transmitting antennas 101a and 101b, respectively. The pilot subcarrier polarity data Sa (j) and Sb (j) (j = 1, 2, 3, 4) are set as follows.

  That is, the polarity data Sa (j) and Sb (j) multiplied by the pseudo-random sequence are different between the first pilot subcarrier transmitted from the transmission antenna 101a and the second pilot subcarrier transmitted from the transmission antenna 101b. Accordingly, the polarity pattern of the first pilot subcarrier and the polarity pattern of the second pilot subcarrier are different. Here, the polarity pattern of the first pilot subcarrier is a pattern of combinations of the polarities of the first pilot subcarriers. Similarly, the polarity pattern of the second pilot subcarrier is a pattern of combinations of the polarities of the second pilot subcarriers. The effect obtained by making the polarity patterns of the first pilot subcarrier and the second pilot subcarrier different from each other will be described in detail later. Here, the pilot subcarrier polarity data and the pseudo-random sequence are expressed using real numbers, but complex polarity data or complex pseudo-random sequences can also be used.

  Next, the OFDM signal receiving apparatus 200 in FIG. 1 will be described with reference to FIG. The OFDM signal receiving apparatus 200 includes FFT (Fast Fourier Transform) units 202a and 202b, an interference cancellation circuit 203, a residual phase error detector 204, phase compensation units 205a and 205b, a serial / parallel converter 206, and a decoder 207.

  An OFDM signal received by the receiving antenna 201a is input to the FFT unit 202a through a radio receiving unit (not shown), and is divided into signals of each subcarrier by being subjected to Fourier transform. Similarly, the OFDM signal received by the receiving antenna 201b is also divided into subcarrier signals by being subjected to Fourier transform by the FFT unit 202b.

As shown in FIG. 1, the signal received by the receiving antenna 201a is superimposed with the OFDM signal transmitted from the transmitting antennas 101a and 101b, and the signal received by the receiving antenna 201b is also transmitted from the transmitting antennas 101a and 101b. OFDM signals to be superimposed are superimposed. The interference cancellation circuit 203 performs interference cancellation in order to separate and receive the OFDM signals from the transmission antennas 101a and 101b. The interference cancellation method for this purpose is a known technique, but here, a method of multiplying the reception signal by the inverse matrix of the matrix formed by the propagation path response of Equation (1) will be described. The inverse matrix of the matrix created by the propagation path response of Equation (1) can be written as follows:

  The OFDM signal from the transmitting antennas 101a and 101b is separated by multiplying the inverse matrix of Equation (6) by the received signal vector generated from the received signals output from the receiving antennas 201a and 201b, respectively. In the multipath environment, since the channel response value differs for each subcarrier, there are as many channel response values as the number of subcarriers, and the derivation of the inverse matrix coefficient and the multiplication of the inverse matrix are performed for each subcarrier. The signal thus separated by the interference cancellation circuit 203 is sent to the residual phase error detector 204.

  The residual phase error detector 204 detects residual components such as a frequency offset and a clock offset compensated by using a radio packet preamble (not shown). The residual phase error detector 204 further detects the residual phase error of the two received signals RXa and RXb using the known signal transmitted by the pilot subcarrier, and sends this to the phase compensation units 205a and 205b.

FIG. 6 shows the detection principle of the residual phase error detector 204. Here, the detection of the residual phase error will be described by taking as an example a case where it is applied to a signal that is not subjected to interference removal. That is, with respect to pilot subcarriers, the matrix expressed by Equation (6) (the right side of Equation (6)) is represented by a unit matrix, or weighted and combined with the outputs of FFT units 202a and 202b, This is a case where a matrix that maximizes the power-to-noise power ratio, that is, a matrix or a vector that performs reception diversity can be used. When transmitting binary pilot subcarriers generated from a binary pseudo-random sequence using two transmitting antennas 101a and 102b, the OFDM signal receiving apparatus before interference removal uses 2 ² as shown in FIG. There are four candidates for received signal points (1, 1), (1, -1), (-1, 1), (-1, -1). Here, for example, (1, −1) represents that a modulated signal “1” is transmitted from the transmitting antenna 101a and a modulated signal “−1” is transmitted from the transmitting antenna 101b.

  When transmitting the first pilot subcarrier and the second pilot subcarrier generated from the pseudo-random sequence common to the transmission antennas 101a and 101b as in the first embodiment from the transmission antennas 101a and 101b, the received signal point is ( 1, 1) and (-1, -1), or (1, -1) and (-1, 1). This combination does not change during radio packet reception. For example, when the combination of received signal points is (1, −1) and (−1, 1), it looks to the OFDM signal receiving device as if a BPSK signal was transmitted from a single transmitting antenna.

  Next, a case where residual phase error is detected using pilot subcarriers Pa (1) and Pb (1) transmitted on the (−k + 1) th subcarrier will be described. Now, only the FFT 202a connected to the receiving antenna 201a is considered, and the pilot subcarrier propagation path response value from the transmitting antenna 101a to the receiving antenna 201a in the (−k + 1) th subcarrier is assumed to be Haa. Similarly, the propagation path response value from the transmitting antenna 101b to the receiving antenna 201a is assumed to be Hba. When the polarities of the pilot subcarriers are expressed by equations (4) and (5), the polarities corresponding to the (−k + 1) th pilot subcarrier are Sa (1) = 1 and Sb (1) = 1. . Therefore, since the pilot signal in which signals from the two transmitting antennas are multiplexed is multiplied by the propagation path response value of Haa + Hba, two points (1, 1) and (-1, -1) are received. Therefore, the residual phase error detector calculates the propagation path response value Haa + Hba using the propagation path response values Haa and Hba, and creates the reference signal points (1, 1) and (−1, −1).

  Now, assume that (1, 1) is transmitted in the next OFDM symbol, and the received signal point at this time is the “next symbol” shown in FIG. At this time, the residual phase error detector 204 can detect the phase difference θ between the current received signal point (1, 1) and the next symbol as a residual phase error. The residual phase error detection value can be obtained from both the output of the system of the receiving antenna 201a, the output of the system of the receiving antenna 201b, and a plurality of outputs. In this case, it is possible to output both average values and weighted average values to the phase compensators 205a and 205b.

  In the first embodiment, residual phase error detection using pilot subcarriers is performed without using interference cancellation. However, residual phase error can also be detected after interference cancellation is performed. In this case, the reception signal points of the pilot subcarriers appear as many as the transmission signal points from the transmission antennas 101a and 101b. In addition, when applying the detection of residual phase error using pilot subcarriers after performing interference cancellation, the signal power-to-noise power ratio of pilot subcarriers deteriorates, and the estimation accuracy deteriorates.

  The phase compensation units 205a and 205b perform phase compensation by performing phase rotation on the received signal by the residual phase error. The two received signals after phase compensation are converted into serial signals by the parallel / serial converter 206 and decoded by the subsequent decoder 207 to obtain a received signal corresponding to the transmitted signal.

  As described above, pilot subcarriers are used to detect residual phase errors. Since the reception power of the pilot subcarrier is low, for example, when the pilot subcarrier is received in a state where it is buried in noise, the residual phase error may be erroneously detected. In that case, since the phase compensation units 205a and 205b perform phase compensation according to an erroneous residual phase error detection result, there is a high possibility that all data subcarriers will be received in error. Therefore, it is no exaggeration to say that the reception power of the pilot subcarrier determines the reception characteristics of the OFDM signal receiving apparatus. In order to solve this problem, in the present embodiment, as described above, the polarities of the first pilot subcarrier transmitted from the transmission antenna 101a and the second pilot subcarrier transmitted from the transmission antenna 101b are different.

FIG. 7 schematically shows transmission directivity images of the antennas 101a and 101b when transmitting pilot subcarriers of the same polarity pattern from the two transmission antennas 101a and 101b, and a transmission combined beam pattern by the antennas 101a and 101b. Yes. The polarity data Sa (1), Sa (2), Sa (3), Sa (4) of the first pilot subcarrier transmitted from the transmission antenna 101a is expressed by Equation (4), and the second data transmitted from the transmission antenna 101b. The pilot subcarrier polarity data Sb (1), Sb (2), Sb (3), and Sb (4) are expressed by the following equation (7).

  The transmission antennas 101a and 101b are assumed to be omnidirectional antennas as shown on the upper side of FIG. For this reason, when pilot subcarriers having the same polarity are transmitted simultaneously from the antennas 101a and 101b, the transmission signals interfere with each other, and the combined beam pattern forms a directional beam. Further, taking the case of the IEEE802.11a standard as an example, the center frequency (carrier frequency) of the OFDM signal is 5 GHz, whereas the interval between pilot subcarriers is as small as about 4.4 MHz. The directional beams of the two pilot subcarriers are directed in substantially the same direction as shown in the lower side of FIG. As a result, there is a possibility that a dead zone where the reception power of the four pilot subcarriers greatly decreases, that is, a dead band where the reception characteristics of the OFDM signal receiving apparatus are significantly deteriorated may occur.

  On the other hand, FIG. 8 shows the antenna 101a when the polarity pattern of the first pilot subcarrier transmitted from the transmission antenna 101a is different from the polarity pattern of the second pilot subcarrier transmitted from the transmission antenna 101b according to the first embodiment. , 101b and the transmission combined beam pattern by the antennas 101a and 101b. The polarity data Sa (1), Sa (2), Sa (3), Sa (4) of the first pilot subcarrier transmitted from the transmission antenna 101a is expressed by Equation (4), and the second data transmitted from the transmission antenna 101b. The pilot subcarrier polarity data Sb (1), Sb (2), Sb (3), and Sb (4) are expressed by equation (5).

  According to Equations (4) and (5), for example, the phase difference between the first pilot subcarrier controlled according to polarity data Sa (1) and the second pilot subcarrier controlled according to polarity data Sb (1) is zero. In contrast, the phase difference between the first pilot subcarrier controlled according to the polarity data Sa (2) and the second pilot subcarrier controlled according to the polarity data Sb (2) is 180 °. As a result, for example, as shown in the lower side of FIG. 8, the direction of the directional beam formed by the pilot subcarriers of polarity Sa (1) and Sb (1) and the polarity Sa (2) and Sb (2) The direction of the directional beam formed by the pilot subcarriers of the pilot subcarriers differs by 180 °.

  FIG. 9 shows an average normalized reception level of pilot subcarriers at the receiver when pilot subcarriers are transmitted using the embodiment of the present invention. There are two transmitting antennas, and Pa (1), Pa (2), Pa (3), Pa (4) and Pb (1), Pb (2), Pb using equations (4) and (5). (3) Four pilot subcarriers Pb (4) are used. Note that the pilot subcarrier polarity is 5 GHz at the center frequency of the OFDM signal, the signal bandwidth is about 20 MHz, the antenna element spacing is a half wavelength, and each element is omnidirectional. The propagation channel model uses “Channel model D (NLOS)” shown in IEEE802.11-03-940 / r1 “TGn Channel model”. The X axis in FIG. 9 shows the angle seen from the transmission antenna, and the received power of four pilot subcarriers corresponding to a certain transmission angle is shown on the Y axis.

  As can be seen from FIG. 9, at some angles, the pilot subcarrier power may drop compared to other angles. However, since the reception power of other pilot subcarriers increases at that location, the receiver can compensate for the residual phase error using a pilot subcarrier having a high level.

  On the other hand, FIG. 10 shows received power when pilot subcarriers are transmitted using Equations (4) and (7). That is, the characteristics are obtained when pilot signals having the same polarity are used. As can be seen from FIG. 10, at one angle, the power of the pilot subcarrier may drop compared to the other angles. The tendency is the same for other pilot subcarriers, and the power of all pilot subcarriers drops simultaneously. Therefore, in a receiver that exists at such an angle, it is difficult to compensate for the residual phase error even if other pilot subcarriers are used.

  Therefore, even if the reception power of one pilot subcarrier is small on the reception side, there is a high possibility that the reception power of other pilot subcarriers will increase. For this reason, it is possible to reduce the dead zone in which the reception power of all pilot subcarriers falls simultaneously, and the area where high quality reception is possible is expanded.

  Further, in the first embodiment, it is not necessary to particularly increase the transmission power of subcarriers for all antenna transmission antennas 101a and 101b, so that the composite third-order distortion does not increase, and the D / A converter does not increase. There is no need to increase the input dynamic range.

  In the above description, the example in which the OFDM signal transmission apparatus 100 includes the two transmission antennas 101a and 101b has been described. However, the present invention can be extended even when the number of transmission antennas is larger. FIGS. 11A and 11B show examples of two types of polarity patterns of the first to fourth pilot subcarriers transmitted from each transmission antenna when four transmission antennas 101a to 101d are used as an example. Indicates. FIGS. 11C and 11D show examples of two types of polarity patterns of the first to fourth pilot subcarriers transmitted from each transmission antenna when three transmission antennas 101a to 101c are similarly used. Indicates. Here, two types of pilot subcarrier polarities 1 and 2 are prepared for the first to fourth pilot subcarriers. The pilot subcarrier polarity 1 is a polarity pattern when a real number is used, and the pilot subcarrier polarity 2 is a polarity pattern when an imaginary number is used. The pilot subcarrier polarity 2 is generated using a Fourier matrix coefficient.

In pilot subcarrier polarity 1 in the case of the four transmitting antennas shown in FIG. 11A, the polarity of the first pilot subcarrier is Sa (1) = 1, Sa (2) = 1, Sa (3) = 1, Sa (4) = − 1, the polarity of the second pilot subcarrier is Sb (1) = 1, Sb (2) = − 1, Sb (3) = 1, Sb (4) = 1, and the third pilot subcarrier The carrier polarity is Sc (1) = 1, Sc (2) = − 1, Sc (3) = − 1, Sc (4) = − 1, and the polarity of the fourth pilot subcarrier is Sd (1) = 1. , Sd (2) = 1, Sd (3) = − 1, and Sd (4) = 1. Here, consider a vector having each polarity as an element for pilot subcarriers transmitted from a certain frequency. Since four pilot subcarriers are transmitted from each antenna and there are four transmission antennas, the following four vectors having four polar data elements can be defined as the pilot subcarrier polarity pattern.

  s (1) to s (4) are vectors different from each other. For example, no matter how many times the vector of s (1) is multiplied, it does not become another vector. In this way, because the pilot subcarrier vector transmitted from a certain frequency is different from the pilot subcarrier vector transmitted from another frequency, the directional beam of each pilot subcarrier points in a different direction. It is possible to reduce the dead zone. Although s (1) to s (4) are orthogonal to each other, the directional beams can be directed in different directions even if they are not necessarily orthogonal.

  In the pilot subcarrier polarity 2 in the case of the four transmitting antennas shown in FIG. 11B, the polarity of the first pilot subcarrier is Sa (1) = 1, Sa (2) = 1, Sa (3) = 1, Sa (4) = − 1, and the polarity of the second pilot subcarrier is Sb (1) = 1, Sb (2) = − j, Sb (3) = − 1, Sb (4) = − j, The pilot subcarrier polarity is Sc (1) = 1, Sc (2) = − 1, Sc (3) = 1, Sc (4) = 1, and the fourth pilot subcarrier polarity is Sd (1) = 1. , Sd (2) = j, Sd (3) = − 1, and Sd (4) = j. Here, j is an imaginary unit. In this case as well, as described above, regarding pilot subcarriers transmitted from a certain frequency, when considering vectors having respective polarities as elements, the respective vectors are different from each other in the complex domain, and are orthogonal in the complex domain. Although there is a relationship, the relationship is not necessarily orthogonal.

  On the other hand, in the pilot subcarrier polarity 1 in the case of the three transmitting antennas shown in FIG. 11C, the polarity of the first pilot subcarrier is Sa (1) = 1, Sa (2) = 1, Sa (3) = 1. , Sa (4) = − 1, and the polarity of the second pilot subcarrier is Sb (1) = 1, Sb (2) = − 1, Sb (3) = 1, Sb (4) = 1, The polarities of the pilot subcarriers are Sc (1) = 1, Sc (2) = − 1, Sc (3) = − 1, and Sc (4) = − 1. Also in this case, as described above, when considering vectors having respective polarities as elements for pilot subcarriers transmitted from a certain frequency, the respective vectors are different from each other.

  Next, in the pilot subcarrier polarity 2 in the case of the three transmitting antennas shown in FIG. 11D, the polarity of the first pilot subcarrier is Sa (1) = 1, Sa (2) = 1, Sa (3) = 1, Sa (4) =-1, and the polarity of the second pilot subcarrier is Sb (1) = 1, Sb (2) =-j, Sb (3) =-1, Sb (4) =-j The polarities of the third pilot subcarriers are Sc (1) = 1, Sc (2) = − 1, Sc (3) = 1, and Sc (4) = 1. In this case as well, as described above, regarding the pilot subcarriers transmitted from a certain frequency, considering the vectors having the respective polarities as elements, the respective vectors are different from each other, and each vector is multiplied by a number in the complex domain. But it won't be another vector.

  By determining the arrangement of the pilot subcarriers in this way, the directional beams of the pilot subcarriers are directed in different directions, and thus it is possible to reduce the dead zone.

By the way, the pilot subcarrier polarities shown in FIGS. 11A, 11B, 11C, and 11D can be expressed by the following formula (9) in the complex domain.

Where s _k (i) is the polarity of the pilot subcarrier, j is an imaginary unit, i is the number of the pilot subcarrier, k is the antenna number of the transmission antenna, and the first element of k is transmitted from the transmission antenna 101a, for example. The second element of k indicates a signal transmitted from the transmission antenna 101b.

  According to Equation (9), the phase difference between the subcarriers of the first to fourth pilots transmitted from the antenna 101b with k = 2 is −90 °. The phase difference between pilot subcarriers transmitted from the antenna 101c with k = 3 is −180 °, and the phase difference between pilot subcarriers transmitted from the antenna 101d with k = 4 is −270 °. Thus, when the transmission antennas are different, the phase difference between pilot subcarriers is different. Accordingly, since the directional beams corresponding to the pilot subcarriers are directed in different directions as before, the dead zone can be reduced.

  The phase difference −90 ° and the phase difference 270 ° are the same, and the phase difference −270 ° and the phase difference 90 ° are also the same. Therefore, in order to faithfully represent FIGS. 11 (a), (b), (c), and (d), the exponent term in Equation (9) requires a minus sign, but there is no minus sign from the above discussion. Since the same applies to the case, the symbol is omitted in Equation (9).

Further, according to FIGS. 11A and 11B, the polarity of the fourth pilot subcarrier among the pilot subcarriers transmitted from the first transmitting antenna 101a is first to first as shown in the following formula (10). Inverted with respect to the third pilot subcarrier.

In other words, the polarities of the first to fourth pilot subcarriers are expressed by the following formula (11).

Furthermore, when generalized, the polarity of the pilot subcarrier is expressed by Equation (12) or (13) depending on the pilot subcarrier number i.

In the above description, i is the number of the pilot subcarrier, but i can be replaced with the frequency of the pilot subcarrier. Specifically, for example, -21, -7, +7, 21 is used as the value of i. At this time, in consideration of the periodicity of the Fourier transform function, it is also possible to express the pilot subcarrier polarity as in the following formula (14).

Here, s _k (i) is the polarity of the pilot subcarrier, j is an imaginary unit, i is the frequency of the pilot subcarrier, k is the antenna number of the transmitting antenna, and N is the number of input points in the inverse Fourier transform.

  Even using Equation (14), if the transmission antenna is different, the phase difference between the pilot subcarriers is different, so that the dead zone can be reduced. In consideration of the Fourier transform pair, expression (14) is equivalent to shifting the transmission signal by one sample for each transmission antenna on the time axis.

  In the above description, a case where a different signal is transmitted for each transmission antenna has been described. However, in the present embodiment, a different signal is actually transmitted for each of a plurality of transmission beams formed by the beam former 107. Hereinafter, the beam former 107 will be described in detail.

  The beam former 107 is an apparatus for performing processing (beam forming) for forming a plurality of transmission beams, and a known technique can be used for the realization method. The beam forming method is roughly classified into two modes. The first mode is a beamforming mode where the channel response between the wireless transmitter and the wireless receiver is fully known, and the weight for beamforming (in accordance with the channel response to direct the beam to the receiver) ( Weight) is calculated. The second mode is a beam forming mode when the channel response is not completely known, and a predetermined weight is used for beam forming. Therefore, the beam does not necessarily face the receiving device.

  John Ketchum et al. "Ftp: // ieee: wireless@ftp.802wirelessworld.com/11/04/11-04-0870-00-000n-802-11-ht-system-description-and-operating-principles. doc "describes the Eigenvector steering (ES) method, which is one of the first beam forming modes, and the Spatial spreading (SS) method, which is one of the second beam forming modes. Also, John Ketchum et al. Describes that the diversity effect can be obtained by using beam forming.

  The beam former 107 has, for example, an ES method and an SS method in order to correspond to the first beam forming mode and the second beam forming mode. For data subcarriers, it is effective to perform beam forming as described by John Ketchum et al. Regardless of whether the ES method or the SS method is used. On the other hand, it is not preferable to perform beam forming on the pilot subcarrier by the technique taught by John Ketchum et al. This is because the pilot subcarrier is known information on the receiving side and must be received accurately on the receiving side.

Next, a configuration method of pilot subcarriers when the ES scheme and the SS scheme are used will be described.
In the ES system, as described on page 10 of the document by John Ketchum et al., The channel response between the wireless transmission device and the wireless reception device is measured, and the transmission beam is directed toward the wireless reception device. Therefore, no matter what pilot subcarrier is generated in pilot subcarrier insertion section 106, the pilot signal reaches the radio reception apparatus, so that no dead band occurs. Therefore, when beam forming is performed using the ES method, the same settings as those in the first to eighth embodiments may be performed regarding the polarity of the pilot subcarrier. On the other hand, the SS method is a beam forming mode when the channel response is not completely known, and a predetermined weight coefficient (weight) for beam forming is used. Do not guarantee to reach.

Now, it is assumed that pilot subcarriers when the number of transmission antennas is four are represented by the following matrix P.

The number of columns of the matrix P is the same as the number of pilot subcarriers on the frequency axis. The first column indicates the polarity of the first pilot subcarrier transmitted from each antenna. The number of rows of the matrix P is the same as the number of input ports of the beamformer 107. That is, the matrix P represents FIG. On the other hand, the beamforming matrix Q using the Walsh matrix exemplified in John Ketchum et al.

Here, the number of columns of the matrix Q is equal to the number of input ports of the beam former 107, that is, the number of transmission beams. The number of rows in the matrix Q is equal to the number of transmit antennas. The i-th row of the matrix Q represents the weight corresponding to the i-th transmission beam (i = 1, 2, 3, 4 in this example). The beam former 107 multiplies the matrix P and the matrix Q to obtain the following matrix QP.

  The number of columns of the matrix QP is the same as the number of pilot subcarriers. The number of rows in the matrix QP is equal to the number of transmit antennas. For example, paying attention to the first column of the matrix QP, this corresponds to transmitting a pilot whose power is “16” (that is, the amplitude is 4) only from the first transmitting antenna. When attention is paid to the second column of the matrix QP, this corresponds to transmitting a pilot whose power is “8” (that is, the amplitude is √8) only from the third transmission antenna and the fourth transmission antenna. Thus, the first pilot subcarrier is transmitted with high power only from the first transmitting antenna. For the second pilot subcarrier, a pilot signal is transmitted only from the third and fourth transmission antennas. As a result, the transmission power becomes uneven within the frequency band of the OFDM signal, and the problems pointed out in the prior art such as generation of complex third-order distortion and increase in the dynamic range of the transmission signal occur.

  That is, when beam forming based on the SS scheme is applied to pilot subcarriers, the orthogonality of pilot subcarriers is lost. Also, for a certain pilot subcarrier, a transmission antenna that does not transmit a pilot signal is generated, so that the effect of spatial diversity is reduced.

Therefore, in the ninth embodiment, when beam forming according to the SS method is used in the beam former 107, the SS method is used for data subcarriers as in the literature of John Ketchum et al., But beam forming is performed for pilot subcarriers. Do not do. Specifically, the matrix Q is used as a unit matrix for pilot subcarriers (this is referred to as Q ′).

  The i-th row of the matrix Q ′ represents the weight corresponding to the i-th transmission beam (i = 1, 2, 3, 4 in this example). Thus, since the matrix P appears as it is in the matrix Q′P, the pilot signal is transmitted by pilot subcarriers orthogonal to each other on the frequency axis and the space axis. When the SS method is used, cyclic delayed diversity (CDD) is applied to the transmission signals from the respective antennas. Specifically, a signal that is cyclically shifted by, for example, 50 nsec from the signal transmitted from the first transmission antenna is transmitted from the second transmission antenna. Since the CDD method is described in the literature of John Ketchum et al., Description thereof is omitted.

  Thus, according to the second beam forming mode that does not use channel response information, for example, when beam forming is performed by the SS method, pilot signals are transmitted without applying beam forming to pilot subcarriers. As a result, the pilot signal is not transmitted from only a single antenna and the dynamic range of the transmission signal is not increased. Furthermore, it is possible to avoid a dead zone on the receiving side due to the effect of space diversity.

  In the above-described example, the pilot subcarrier is not beam-formed (multiplied by the unit matrix) in the second beam forming mode. However, the following method may be used instead.

A beam forming matrix Q ′ for multiplying the matrix P by the beam former 107 is

The resulting matrix Q′P is

It becomes. As can be seen from the matrix Q′P, pilot subcarriers orthogonal to each other on the frequency axis and the space axis are transmitted from each antenna. This is the pilot subcarrier polarity pattern shown in FIG. As a result, the pilot signal is not transmitted from only a single antenna and the dynamic range of the transmission signal is not increased.

  In this way, in the second mode, which is a mode for forming a transmission beam regardless of the channel response, the transmission beam weight for the data subcarrier and the transmission beam weight for the pilot subcarrier are set to different values. Signals are not transmitted in a biased manner from a single antenna, and pilot subcarriers that are orthogonal to each other on the frequency axis and spatial axis are transmitted from each antenna, thus avoiding the generation of dead bands on the receiving side. Can do.

  Next, another embodiment of the present invention will be described. In other embodiments described below, the pilot subcarrier insertion unit 106 in the OFDM signal transmitting apparatus 100 is different from that in the first embodiment.

(Second Embodiment)
As shown in FIG. 12, the pilot subcarrier insertion unit 106 according to the second embodiment includes ROMs 121a and 121b storing polarity data for the first pilot subcarrier and the second pilot subcarrier, and subcarrier pattern control. A device 122 is added. In the ROMs 121a and 121b, as illustrated in FIG. 13, the polarity data representing the polarities Sa (1) to Sa (4) of the first pilot subcarrier and the polarities Sb (1) to Sb ( Polarity data representing 4) is stored for each of three patterns (pattern A, pattern B, and pattern C). Which polarity data of pattern A, pattern B, and pattern C is read from the ROMs 121a and 121b is determined by address data given to the ROMs 121a and 121b.

  In the second embodiment, the polarities of the first pilot subcarrier and the second pilot subcarrier transmitted from each of the transmission antennas 101a and 101b are not fixed but change for each radio packet. That is, polarity data of a different pattern for each wireless packet is read from the ROMs 121a and 121b, and multiplied by the pseudo random sequence generated by the pseudo random sequence generator 110 by the multipliers 111a to 111d and 112a to 112d.

  Radio packet counter 123 provided outside pilot subcarrier insertion section 106 shown in FIG. 12 counts the number of radio packets in transmission data input to encoder 102 in FIG. It passes to the carrier pattern controller 122. The subcarrier pattern controller 122 changes the address data given to the ROMs 121a and 121b each time the count value of the wireless packet counter 123 is incremented by one. Thereby, the subcarrier pattern controller 122 changes the patterns of the polarity data of the first and second pilot subcarriers read from the ROMs 121a and 121b, respectively.

  For example, pattern A polarity data is read when a certain wireless packet is transmitted, pattern B polarity data is read when the next wireless packet is transmitted, and pattern C polarity data is read when the next wireless packet is transmitted. As a result, the pilot subcarrier polarity pattern is changed for each radio packet. The change of the polarity data pattern by the subcarrier pattern controller 122 is performed at random for each radio packet, for example.

  The polarity data read out from the ROMs 121a and 121b in this manner is input to the multipliers 111a to 111d and multiplied by the pseudo random sequence generated by the pseudo random sequence generator 110 in the same manner as in the first embodiment. A subcarrier and a second pilot subcarrier are generated. The generated first pilot subcarrier and second pilot subcarrier are inserted between the first data subcarrier and the second data subcarrier, respectively, thereby generating the first subcarrier and the second subcarrier. Is done.

  The first subcarrier and the second subcarrier are input to IFFT units 108a and 108b shown in FIG. 3 to generate a first OFDM signal and a second OFDM signal. The first OFDM signal and the second OFDM signal are respectively sent to the transmission antennas 101a and 101b via a wireless transmission unit (not shown) and transmitted from these antennas 101a and 101b.

  According to the second embodiment, for example, when transmitting pilot subcarriers controlled by pattern A polarity data and when transmitting pilot subcarriers controlled by pattern B polarity data, antennas 101a and 101b are used. The pattern of the directional beam formed is different.

  Here, for example, in the OFDM signal receiving apparatus located at a place where the received power is lower than that of the pattern A pilot subcarrier, when a pilot subcarrier of a pattern B different from the pattern A is transmitted, There is a high possibility that the received power of the carrier will recover. Therefore, by changing the pilot subcarrier pattern for each radio packet, it is possible to reduce places where reception is not possible at all because the reception power of the pilot subcarrier is low.

  When changing the pattern of the polarity data of the pilot subcarrier for each radio packet, it is not always necessary to make the change random. For example, among the various patterns of the polarity data of the pilot subcarrier, a pattern in which the reception characteristic is improved for each OFDM signal receiving device is stored, and stored in correspondence with the OFDM signal receiving device of the transmission destination It is also possible to transmit pilot subcarriers using a certain pattern.

(Third embodiment)
Next, a third embodiment of the present invention will be described. In the third embodiment, an error occurs in the previously transmitted radio packet, and the polarity pattern of the pilot subcarrier is changed only when a retransmission packet is transmitted. As shown in FIG. 14, in the pilot subcarrier insertion unit 106 according to the third embodiment, except that the radio packet counter 123 in FIG. 12 is replaced with the retransmission detector 124, the pilot subcarrier shown in FIG. The same as the carrier insertion unit 106.

  As illustrated in FIG. 15, the wireless packet includes a unique word used for synchronization by the OFDM signal receiving apparatus, a transmission source field for specifying a transmission source (OFDM signal transmission apparatus), and a transmission destination (OFDM signal reception apparatus). ), A retransmission field indicating whether the wireless packet is a retransmission packet or not, and an error detection field for determining whether an error has occurred in each field, followed by a plurality of data symbols Followed.

  The transmission signal input to the encoder 102 in FIG. 3 is also input to the retransmission detector 124. The retransmission detector 124 analyzes the retransmission field in the wireless packet that is the transmission signal, and if the wireless packet is a retransmission packet, notifies the subcarrier pattern controller 122 to that effect. Upon receiving the notification that the radio packet is a retransmission packet, the subcarrier pattern controller 122 changes the address data given to the ROMs 121a and 121b, and reads the first and second pilot subcarriers read from the ROMs 121a and 121b, respectively. Controls the pattern of polarity data. As a result, a radio packet including a pilot subcarrier having a polarity pattern different from that of the previously transmitted radio packet is transmitted to the same transmission partner.

  In the third embodiment, it is determined by analyzing the retransmission field that the wireless packet is a retransmission. However, an upper layer that performs wireless access control (for example, Medium Access Control (MAC layer) in the IEEE802.11a standard) directly It is also possible to notify the carrier pattern control device that the wireless packet is a retransmission packet.

  As described above, in the third embodiment, retransmission packets are transmitted to the same transmission partner according to a polarity pattern different from the pilot subcarrier of the wireless packet transmitted by the OFDM signal transmission apparatus 100 last time. As a result, the pattern of the directional beam formed by the plurality of transmission antennas changes at the time of retransmission, and the probability that the OFDM signal receiving apparatus can correctly receive the retransmission packet increases.

(Fourth embodiment)
In pilot subcarrier insertion section 106 according to the fourth embodiment of the present invention, two pseudo-random sequence generators 110a and 110b are provided as shown in FIG. The first pseudo-random sequence generator 110a generates a first pseudo-random sequence PNa that modulates the first pilot subcarrier transmitted from the transmission antenna 101a. The second pseudo random sequence generator 110b generates a second pseudo random sequence PNb that modulates the second pilot subcarrier transmitted from the transmission antenna 101b.

The polarity patterns of the first pilot subcarrier and the second pilot subcarrier may all be the same or different. Here, a case where the same polarity data S is used will be described. The first pilot subcarrier is modulated as follows according to the pseudo-random sequence PNa and the pilot subcarrier polarity data S.

Similarly, the second pilot subcarrier is modulated as follows according to the pseudo-random sequence PNb and the pilot subcarrier polarity data S.

  FIG. 17 shows the first OFDM signal and the second OFDM signal including data subcarriers and pilot subcarriers thus modulated. The first OFDM signal and the second OFDM signal are transmitted to IFFT units 108a and 108b, respectively, and transmitted from transmission antennas 101a and 101b.

FIG. 18 shows the detection principle of the residual phase error detector 204 shown in FIG. 5 according to the fourth embodiment. Here, a case where the residual phase error is estimated before performing interference cancellation will be described. When binary pseudo-random sequences and binary pilot subcarriers are transmitted by the two transmitting antennas 101a and 101b, 2 ² = 4 received signal points (in the same way as in FIG. 6 in the first embodiment) 1, 1), (1, -1), (-1, 1), (-1, -1) candidates exist. In the first embodiment, the received signal point has a combination of (1, 1) and (-1, -1), or a combination of (1, -1) and (-1, 1). In contrast, in the fourth embodiment, pilot subcarriers transmitted from the transmitting antennas 101a and 101b are modulated by different pseudo-random sequences, so that all four received signal points may appear for each OFDM symbol. There is sex.

  For example, in the fourth embodiment, a combination of (1, -1) and (-1, 1) is transmitted, and a combination of (1, 1) and (-1, -1) is transmitted. There are two possible ways. In the former combination, the phase difference between the signals from the transmitting antenna 101a and the transmitting antenna 101b is 180 °, while the latter combination is 0 °. Therefore, when the combination of the former is transmitted and when the combination of the latter is transmitted, the directional beam for transmission is different, so that the received power changes. Note that the four received signal point candidates (1, -1), (-1, 1), (1, 1), and (-1, -1) are as described in the first embodiment. It can be obtained by combining the channel coefficient to each transmitting antenna and receiving antenna with the signal transmitted by the pilot subcarrier.

  Next, a method for measuring the residual phase error will be described. Assume that the received symbol of the pilot subcarrier is (1, 1). In this case, in the first embodiment, (1, 1) is also transmitted as the next symbol or (-1, -1) is transmitted, so that the receiving side receives a BPSK signal from a single antenna. The received power does not change.

  On the other hand, in the fourth embodiment, (−1, 1) may be transmitted as the next symbol. Therefore, the received symbols including the phase error are “next symbol 1” and “next symbol 2” shown in FIG. There are two possible ways. When the reception signal point of the next symbol is (1, 1) or (-1, -1), the residual phase error detector 204 detects the phase difference θ1 as a residual phase error. On the other hand, when the reception signal point of the next symbol is (-1, 1) or (1, -1), the residual phase error detector 204 determines the reception signal of (-1, 1) from the current propagation path response value. And a phase difference θ2 between (−1, 1) and “next symbol 2” is detected as a residual phase error detector 204.

  The phase compensation units 205a and 205b perform phase compensation by performing phase rotation on the received signal by the residual phase error. The two received signals after phase compensation are converted into serial signals by the parallel / serial converter 206 and decoded by the subsequent decoder 207, whereby a received signal corresponding to the transmitted signal is obtained. In the fourth embodiment, the method for measuring the residual phase error before performing the interference removal has been described. However, it is also possible to detect the residual phase error after performing the interference removal. When interference cancellation is performed, pilot subcarriers transmitted from a single antenna appear in the output after interference cancellation, so that only two received signal points appear.

As described above, according to the fourth embodiment, a combination of (1, 1) and (-1, -1) is transmitted in the OFDM symbol as shown in FIG. -1,1) may be transmitted. Therefore, the received power changes for each OFDM symbol. Therefore, even if the received power of the pilot subcarrier decreases in a certain OFDM symbol and the residual phase error cannot be detected, the received power may be recovered in the next symbol. As a result, it is possible to eliminate a dead zone due to a decrease in all received power of pilot subcarriers.
(Fifth embodiment)
The pilot subcarrier insertion unit 106 according to the fifth embodiment of the present invention includes, as shown in FIG. 19, the second embodiment shown in FIG. 12 or the third embodiment shown in FIG. The fourth embodiment shown is combined. That is, in the fifth embodiment, the polarity of pilot subcarriers is different for each transmission antenna, and the pseudo-random sequence for modulating pilot subcarriers is different for each transmission antenna. Therefore, the baseband signal of the pilot subcarrier can be written as follows for the transmitting antennas 101a and 101b.

  In this case, the pilot subcarriers are as shown in FIG. 20, and the directional beams formed by the transmission antennas 101a and 101b for the pilot subcarriers are directed in different directions for each frequency and in different directions in the symbol time direction. Therefore, even if the reception level of the pilot subcarrier is low at a certain frequency or a certain time, the pilot subcarrier can be received at another frequency or another symbol, and the dead zone can be reduced.

(Sixth embodiment)
In pilot subcarrier insertion section 106 according to the sixth embodiment of the present invention, pseudo random sequences from pseudo random sequence generators 110a and 110b are input to transmission diversity circuits 125a and 125b as shown in FIG. The pilot subcarriers are transmitted as follows.

  However, “*” indicates a complex conjugate. As shown in equations (19)-(22), the pseudo-random sequence is transmitted using transmission diversity using two transmission antennas 101a and 101b and two symbols. The transmission diversity method expressed by the equations (19) to (22) is the same as the transmission diversity method disclosed in US Pat. No. 6,185,258B1.

The specific pilot subcarrier signals shown in Equations (19)-(22) are applied to the -k + 1th subcarrier and the k-4th subcarrier in FIG. Equations (19)-(22) are shown for the j-th pilot subcarrier, and the transmission method can be changed for other subcarriers. Specifically, it can be described as follows.

  The specific pilot subcarriers shown in Expressions (23) to (26) are applied to the −k + 4th subcarrier and the k−1th subcarrier in FIG. The residual phase error detector 204 in the OFDM signal receiving apparatus shown in FIG. 5 performs decoding corresponding to transmission diversity using, for example, a decoding method disclosed in US Pat. No. 6,185,258B1. It is possible to maximize the signal-to-noise power ratio of the pilot subcarrier.

  As can be seen from the equations (16)-(19), diversity transmission is performed using two symbol times here, so that diversity gain cannot be obtained by receiving only one symbol. FIG. 23 shows a signal when the −k + 4th subcarrier is modulated using Expressions (16) − (19). As shown in FIG. 23, the residual phase error is detected with the signal that received only one symbol in symbol 1, but since the symbol received last time can be used from the second symbol, the received signals of symbol 1 and symbol 2 are By using this, it is possible to detect the residual phase error of symbol 2. Similarly, the residual phase error of symbol 3 can be detected using the received signals of symbol 2 and symbol 3. That is, it is possible to detect a phase error by overlapping two symbols. It is also possible to perform all residual phase error detection using only the currently received single symbol.

  As described above, according to the sixth embodiment, by transmitting pilot subcarriers using transmission diversity, it is possible to detect an accurate residual phase error and improve reception performance.

(Seventh embodiment)
In pilot subcarrier insertion section 106 according to the seventh embodiment of the present invention, as shown in FIG. 24, subcarrier arrangement apparatus 126a for changing the positions of pilot subcarriers and data subcarriers between transmission antennas 101a and 101b and 126b is provided. In the subcarrier arrangement devices 126a and 126b, the locations of pilot subcarriers and data subcarriers are different at the transmission antennas. Next, the pilot subcarrier insertion unit 607 will be specifically described with reference to FIG.

  Modulated signals obtained by multiplying the pseudo-random sequence and the polarity Sa (1) -Sa (4) for the transmission antenna 101a are respectively input to the subcarrier arrangement device 126a as pilot subcarriers Pa (1) -Pa (4). . In subcarrier arrangement apparatus 126a, data subcarriers and pilot subcarriers are rearranged and input to IFFT unit 108a. Since the transmission antenna 101b is the same, description thereof is omitted.

  In FIG. 25, as shown in the subcarrier arrangement in the seventh embodiment, pilot subcarriers of the subcarriers are transmitted only from a single transmission antenna. For example, a pilot subcarrier PN (1) modulated with a pseudo-random sequence is transmitted from the transmission antenna 101a as a -k-th subcarrier, and a data subcarrier (DATA) is transmitted from the transmission antenna 101b. Similarly, for the −k + 2nd subcarrier, a pilot subcarrier is transmitted from the transmission antenna 101a, and a data subcarrier (DATA) is transmitted from the transmission antenna 101b.

  Since the data signal is a random signal, the correlation between the first data subcarrier transmitted from the transmission antenna 101a and the second data subcarrier transmitted from the transmission antenna 101b is generally low. For this reason, the phase difference between the subcarriers transmitted from the transmission antenna 101a and the subcarriers transmitted from the transmission antenna 101b is different between the −kth subcarrier and the −k + 2nd subcarrier. There is a high possibility that the directional beams of the pilot subcarrier transmitted on the carrier and the pilot subcarrier transmitted on the −k + 2nd subcarrier are different.

  Therefore, according to the seventh embodiment, since the probability that the received power of all pilot subcarriers simultaneously falls due to the influence of null is very small, there is no dead zone. Also, even if the pilot subcarrier power happens to fall in one symbol period, there is a high possibility that the data signal is different between the current symbol and the next symbol. . As described above, according to the seventh embodiment, it is possible to increase the probability of receiving pilot subcarriers and reduce the dead zone.

  The change of the arrangement of pilot subcarriers can also be performed in a radio packet.

For example, in the IEEE 802.11a standard, a pilot signal for estimating the transfer function of all subcarriers is inserted in the unique word shown in FIG. It is possible to ask for a response. However, since a pilot signal is transmitted using only pilot subcarriers in a data symbol, it is difficult to follow the propagation path response when the temporal variation of the propagation path is fast. However, by changing the arrangement of pilot subcarriers between radio packets and transmitting pilot signals from other subcarriers, it becomes possible to follow the channel response of other subcarriers. Therefore, accurate reception can be performed by using this method. In addition, although all the polarities of the pilot subcarriers are the same in the seventh embodiment, it can be the same as in the first to sixth embodiments.

(Eighth embodiment)
According to the present invention, even when receiving a wireless packet in which a part for transmitting data from a single antenna and a part for transmitting data from a plurality of antennas are received in one wireless packet, the residual phase error is compensated. It can be done accurately. According to the radio communication preamble signal proposal proposed by Jan Boer et al. In “Backwards compatibility,” IEEE 802.11-03 / 714r0, as shown in FIG. 26, first, from one transmission antenna 101a, time synchronization, frequency synchronization and AGC A short preamble sequence x01 used for transmission, a long preamble sequence x02 for channel response estimation, and a first signal field x03 including a field indicating a modulation scheme and length of a radio packet, and a second signal field used subsequently in IEEE 802.11n Send x04. The second signal field describes the number of transmission antennas to be multiplexed, the multiplexing method, and the like. Next, a long preamble sequence x05 for propagation path response estimation is sequentially transmitted from the transmission antenna 101b. After transmission of the preamble signal is thus completed, transmission data x08 and x09 are simultaneously transmitted from the plurality of transmission antennas 101a and 101b.

  The wireless communication preamble signal shown in FIG. 26 is the same as the IEEE 802.11a standard wireless communication preamble signal shown in FIG. 27 based on transmission from the transmission antenna 101a from the short preamble x01 to the first signal field x03. is there. Accordingly, the wireless reception device based on the IEEE 802.11a standard that has received the preamble signal shown in FIG. 26 can recognize the received packet as a wireless packet based on the IEEE 802.11a standard. Therefore, the preamble signal shown in FIG. 26 enables IEEE 802.11n that transmits different data from a plurality of antennas simultaneously on one radio to coexist with the IEEE 802.11a standard that transmits data from a single antenna. To do.

  In the wireless packet of IEEE 802.11a shown in FIG. 27, pilot subcarriers are inserted after the SIGNAL field, and it is possible to compensate for the residual phase error based on this. On the other hand, when the present invention is applied to the radio packet shown in FIG. 26, a configuration is considered in which pilot subcarriers are inserted in the SIGNAL field and SIGNAL2 field, and thereafter pilot subcarriers are also arranged in the DATA portion of X08 to X09. In the subsequent X08 to X09 and later, a configuration in which the pilot subcarriers described in the first to seventh embodiments of the present invention are arranged can be considered. In the eighth embodiment, a case where pilot subcarriers using the first embodiment are transmitted will be described.

  Therefore, specific control when the wireless packet shown in FIG. 26 is received will be described with reference to FIG. FIG. 28 shows a receiving apparatus when receiving the wireless packet shown in FIG. 28 differs from FIG. 5 in that the output of the decoder 207 is input to the SIGNAL analysis unit 208, and the residual phase error detection unit 204 is controlled based on the result of the SIGNAL analysis unit 208.

  The receiver that has received the short preamble x01 detects the beginning of the long preamble sequence x02 using AGC and time synchronization means (not shown) and detects the FFT window. At the same time, the frequency offset is estimated and compensated. The receiver that has received the long preamble sequence x02 measures the propagation path responses of all subcarriers using known pilot subcarriers. In particular, the propagation path response of the pilot subcarrier is passed to the residual phase error detector 204. Since the above processing can be realized by a known technique, description thereof is omitted.

  Next, the wireless receiver receives the SIGNAL field x03. The SIGNAL field is subjected to FFT in FFT 202a and FFT 2b. The FFT output is input to the interference cancellation circuit, but since the SIGNAL field is transmitted from a single antenna, it is not necessary to perform interference cancellation. Therefore, the process performed by the interference cancellation circuit is a process of multiplying the unit matrix, or a process of improving the signal power to noise power ratio by weighted synthesis of the outputs of the FFTs 202a and 202b. Next, the output of the interference cancellation circuit is input to the residual phase error detector 204.

  FIG. 29 is a conceptual diagram of processing performed by the residual phase error detector according to the eighth embodiment. Now, assume that the propagation path response when receiving the long preamble transmitted from the transmitting antenna 101a and received by the receiving antenna 201a is Haa shown in FIG. When the pilot subcarrier is transmitted by BPSK, the values that can be taken by the pilot subcarrier received by the data part are the points (1) or (-1) shown in FIG. Detect phase error.

  Next, when the reception point of the pilot subcarrier of the SIGNAL part is the “next symbol at the time of single antenna transmission” shown in FIG. 29, the residual phase error detector indicates the point (1) and “at the time of single antenna transmission”. The phase difference θ1 from the point of “next symbol” is measured as a residual phase error, and this is corrected by the phase compensator. Similarly, when the SIGNAL 2 part is received, it is possible to detect the residual phase error.

  Note that the demodulator 207 that has demodulated the SIGNAL2 unit passes the decoding result to the SIGNAL analysis unit 208. The SIGNAL analysis unit 208 analyzes the second signal field, analyzes the number of multiplexed transmission antennas and the multiplexing method, and passes them to the residual phase error detection unit 204.

  Next, the receiver receives the long preamble from the transmission antenna 101b and measures the propagation path response from the transmission antenna 101b. Next, a case where the DATA parts X08 to X09 are received will be described. Here, the case where the DATA section is multiplexed with signals from the two transmission antennas 101a and 101b and the multiplexed pilot subcarriers have the polarities of equations (4) and (5) will be described. As described above, the number of multiplexed transmission antennas can be recognized from the signal from the SIGNAL analysis unit 208.

  Here, among the four pilot subcarriers, attention is paid to pilot subcarriers having polarities of Sa (1) and Sb (1) described on the leftmost side of the equations (4) and (5). The polarities of pilot subcarriers transmitted at this frequency are Sa (1) = 1 and Sb (1) = 1. Therefore, when the propagation path response from the transmission antenna 101a to the reception antenna 201a is Haa and the propagation path response from the transmission antenna 101b to the reception antenna 201a is Hba, the propagation path response of the pilot subcarrier received by the DATA unit. The value is Haa + Hba shown in FIG. Therefore, the residual phase error detector obtains a reference point using the synthesized channel response value Haa + Hba based on the measured channel response values Haa and Hba and the multiplexed information from the SIGNAL analysis unit 208, and this A deviation from the reference point is detected.

  Specifically, since the pilot subcarriers are transmitted as BPSK signals modulated with pseudo-random sequences, the points (1, 1) and (-1, -1) shown in FIG. 29 are received. That is, when a single antenna is received, the residual phase error can be detected with reference to points (1) and (-1) in FIG. 29, but the transmission is switched to transmission from a plurality of antennas. In this case, it is necessary to detect the residual phase error with reference to the points (1, 1) and (-1, -1).

  Thus, since it is necessary to switch the operation of the residual phase error detector in the case of single antenna transmission and in the case of multiple antenna transmission, in the eighth embodiment, the second signal field X04 (SIGNAL2 shown in FIG. ) Is analyzed. In the second signal field, the number of signals multiplexed on the transmission side or the multiplexing method is described. By analyzing this, the reference point of the propagation path response value can be obtained, and a single signal field can be obtained. It is possible to switch between a reference point when receiving a transmission signal from an antenna and a reference point when receiving a signal transmitted from a plurality of antennas. Therefore, even when receiving a signal from a single antenna or receiving signals from multiple antennas, it is possible to switch the reference signal point appropriately in either case. Appropriate residual phase error can be detected and guaranteed.

  The present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

1 is a block diagram of an OFDM communication system according to an embodiment of the present invention. The figure which shows the subcarrier arrangement | positioning of the OFDM signal transmitted from two transmission antennas according to the 1st Embodiment of this invention. Block diagram of the OFDM signal transmission apparatus in FIG. Block diagram of pilot subcarrier insertion unit according to the first embodiment of the present invention Block diagram of the OFDM signal receiver in FIG. The figure explaining the residual phase error detection principle corresponding to the 1st Embodiment of this invention The figure which shows typically the transmission directivity image and transmission synthetic beam pattern of each transmission antenna which transmits the pilot subcarrier of the same polarity pattern from two transmission antennas The figure which shows typically the transmission directivity image and transmission synthetic | combination beam pattern of each transmission antenna which transmits the pilot subcarrier of a different polarity pattern from two transmission antennas according to 1st Embodiment. The figure which shows the average normalized reception level of the pilot subcarrier in a receiver at the time of transmitting a pilot subcarrier using embodiment of this invention The figure which shows the reception power when transmitting the 1st and 2nd pilot subcarrier using a certain polarity data The figure which shows the polar pattern of the pilot subcarrier transmitted from each transmitting antenna when extending the 1st Embodiment of this invention to the OFDM apparatus which has four transmitting antennas Block diagram of pilot subcarrier insertion unit according to second embodiment of the present invention The figure which shows the various polarity patterns of the pilot subcarrier transmitted from each transmission antenna in the 2nd Embodiment of this invention Block diagram of pilot subcarrier insertion section according to third embodiment of the present invention The figure which shows an example of the structure of the radio | wireless packet of an OFDM signal Block diagram of pilot subcarrier insertion unit according to the fourth embodiment of the present invention The figure which shows the subcarrier arrangement | positioning of the OFDM signal transmitted from two transmission antennas according to the 4th Embodiment of this invention. The figure explaining the residual phase error detection principle corresponding to the 4th Embodiment of this invention Block diagram of pilot subcarrier insertion unit according to the fifth embodiment of the present invention The figure which shows the subcarrier arrangement | positioning of the OFDM signal transmitted from two transmission antennas according to the 5th Embodiment of this invention. Block diagram of pilot subcarrier insertion unit according to sixth embodiment of the present invention The figure which shows the subcarrier arrangement | positioning of the OFDM signal transmitted from two transmission antennas according to the 6th Embodiment of this invention. The figure explaining the residual phase error detection principle corresponding to the 6th Embodiment of this invention Block diagram of pilot subcarrier insertion unit according to seventh embodiment of the present invention The figure which shows the subcarrier arrangement | positioning of the OFDM signal transmitted from two transmission antennas according to the 7th Embodiment of this invention. The figure which shows an example of the preamble signal for radio | wireless communication The figure which shows the preamble signal for the wireless communication of IEEE 802.11a standard FIG. 26 is a block diagram of an OFDM signal receiving apparatus when receiving the wireless packet shown in FIG. The figure explaining the process performed with the residual phase error detector in the 8th Embodiment of this invention. The figure which shows the subcarrier arrangement | positioning of the OFDM signal transmitted from two transmission antennas based on a prior art

Explanation of symbols

100: OFDM signal transmitting apparatus;
101a, 101b ... transmitting antennas;
102 ... Encoder;
103 ... serial-parallel converter;
104a, 104b ... modulators;
105a, 105b ... serial-parallel converter;
106 ... pilot subcarrier insertion part;
107 ... beam former;
108a, 108b ... IFFT unit;
201a, 201b ... receiving antennas;
202a, 202b ... FFT unit;
203 ... interference canceling circuit;
204 ... residual phase error detector;
205a, 205b ... phase compensation units;
206 ... parallel-serial converter;
207 ... Decoder

Claims (8)

In an OFDM signal transmission apparatus that transmits an OFDM (Orthogonal Frequency Division Multiplexing) signal using a plurality of transmission antennas,
The OFDM signal is transmitted on each subcarrier such that some subcarriers are used as pilot subcarriers for transmitting pilot signals and the remaining subcarriers are used as data subcarriers for transmitting data signals. signal to generate a that, a generation unit to vary the polarity of the pilot subcarriers for each of the transmit antennas,
A beam forming unit that forms a transmission beam regardless of a channel response with an OFDM signal receiving apparatus that receives the OFDM signal, and in the mode, forms a transmission beam only for the data subcarrier ; An OFDM signal transmitting apparatus comprising:   The OFDM signal transmission apparatus according to claim 1, wherein each of the pilot subcarriers has a polarity pattern orthogonal to each other between the transmission antennas. The generating means sets the data indicating the polarity so that the polarity of the pilot subcarrier is different for each transmission antenna, and uses the product of the data indicating the polarity and a specific signal sequence, 2. The OFDM signal transmitting apparatus according to claim 1, wherein the pilot signal transmitted on a pilot subcarrier is generated . The generating means includes: a first storage unit that stores first polarity data corresponding to the first polarity pattern among a plurality of polarity patterns; and a second storage unit that corresponds to a second polarity pattern different from the first. a second storage unit that stores the polarity data, a first multiplier unit for generating a signal for the first pilot sub-carrier by multiplying the first polarity data to the signal sequence, the second to the signal sequence The OFDM signal transmission apparatus according to claim 3, further comprising: a second multiplication unit that multiplies the polarity data of 2 to generate a pilot signal for the second pilot subcarrier. Transmission data to be transmitted by the OFDM signal transmission apparatus is packetized, and the generation means includes subcarrier pattern control means for changing the first polarity pattern and the second polarity pattern for each packet of the transmission data. The OFDM signal transmission apparatus according to claim 3. The transmission data includes a retransmission packet, and the generation means includes subcarrier pattern control means for changing the first polarity pattern and the second polarity pattern when the transmission data is the retransmission packet. The OFDM signal transmitting apparatus according to the description. The OFDM signal transmitting apparatus according to claim 1, wherein the beam setting unit uses different weights for data subcarriers and pilot subcarriers in the mode. The OFDM signal transmitting apparatus according to claim 1, wherein the beamformer multiplies an input matrix representing the pilot subcarrier in the mode by a unit matrix with respect to the pilot subcarrier.

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