Patent Publication Number: US-2022216900-A1

Title: Wireless communication system, wireless communication method, transmitting station device and receiving station device

Description:
TECHNICAL FIELD 
     The present invention relates to techniques for performing transmit beam forming with a time-domain linear equalizer in a wireless communication system that performs wide-band single carrier MIMO (SC-MIMO (Single Carrier Multiple-Input Multiple-Output)) transmission in a communication environment with frequency selective fading. 
     BACKGROUND ART 
     When wide-band SC-MIMO transmission is performed in a communication environment with frequency selective fading, it is necessary to remove inter-symbol interference caused by temporal spread of communication path characteristics and inter-stream interference caused by spatial spread of multiple antennas. To that end, there has been study on an approach to removing inter-symbol interference and inter-stream interference simultaneously by performing transmit beam forming in time/space directions using a time-domain linear equalizer of FIR (finite impulse response) type (an FIR filter) (see Non-Patent Literature 1, for instance). In transmit beam forming with an FIR filter, a time-domain linear equalizer is constructed using an inverse matrix of a transfer function matrix of propagation path characteristics (CIR (channel impulse response)) resulting from channel estimation as a transmit weight matrix and respective elements of the transmit weight matrix as filter tap coefficients. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: Kuriyama Keita, Fukuzono Hayato, Yoshioka Masafumi, Tatsuta Tsutomu, “FIR-type Transmit Beamforming for Wide-band Single Carrier MIMO Transmission”, IEICE technical report, vol. 118, no. 435, RCS2018-247, pp. 31-36, January 2019. 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Here, in the conventional technique above, an inverse matrix H −1 (z) of a transfer function matrix H(z) is computed as a transmit weight matrix W(z) as shown in Expression (1). 
     
       
         
           
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     In Expression (1), the transmit weight matrix W(z) can be expressed by the product of transfer functions of 1/det(H(z)) and a matrix of adj(H(z)). Note that det(⋅) and adj(⋅) represent a determinant and an adjugate matrix, respectively. The adj is different from an adjoint matrix representing Hermitian transposition. 
     An approach that uses the inverse matrix of the transfer function matrix H(z) for CIR as the transfer functions for a linear equalizer has a problem of the transmit weight matrix W(z) diverging when the determinant det(H(z)) of the transfer function matrix is in a non-minimum phase, making it impossible to remove inter-symbol interference and inter-stream interference with a time-domain linear equalizer. 
       FIG. 6  shows an example of filter tap coefficients when det(Hz) is in the minimum phase versus in a non-minimum phase. In  FIG. 6 , the horizontal axis indicates time corresponding to delay taps (Z −0 , Z −1 , Z −2  . . . ) of an FIR filter, and the vertical axis indicates complex gain corresponding to the filter tap coefficient for each delay tap.  FIG. 6( a )  shows complex gain for each delay tap when det(H(z)) is in the minimum phase, while  FIG. 6( b )  shows complex gain for each delay tap when det(H(z)) is in a non-minimum phase. 
     In  FIG. 6( a ) , when det(H(z)) is in the minimum phase, the complex gain of the delay tap gradually decreases to converge, so that operation is stable and implementation of an equalizer with an FIR filter is possible. By contrast, in  FIG. 6( b ) , when det(H(z)) is in a non-minimum phase, the complex gain of the delay tap gradually increases to diverge, so that the operation becomes unstable and implementation with a time-domain linear equalizer such as an FIR filter is difficult. 
     The present invention is aimed at providing a wireless communication system, a wireless communication method, a transmitting station device and a receiving station device that can remove inter-symbol interference and inter-stream interference using a time-domain linear equalizer by changing antenna combination so that the determinant of the transfer function matrix H(z) for CIR will not be in a non-minimum phase in SC-MIMO transmission. 
     Means for Solving the Problem 
     A first aspect of the present invention is a wireless communication system that performs single carrier MIMO transmission between a transmitting station device and a receiving station device, the transmitting station device including: a time-domain linear equalization unit configured to remove inter-symbol interference and inter-stream interference from a data signal to be transmitted to the receiving station device; a propagation path characteristics estimation unit configured to receive a training signal which is transmitted by the receiving station device and estimate a transfer function matrix of propagation path characteristics; a filter tap calculation unit configured to calculate filter tap coefficients for the time-domain linear equalization unit based on the transfer function matrix by a predefined approach; and a transmission mode determination unit configured to make the filter tap calculation unit calculate the filter tap coefficients when the transfer function matrix meets a predefined condition, and to change a transmission mode and determine the transmission mode that meets the predefined condition when the transfer function matrix does not meet the predefined condition, and the receiving station device including a training signal generation unit configured to generate the training signal for use in estimation of propagation path characteristics and transmit the training signal to the transmitting station device. 
     A second aspect of the present invention is a wireless communication system that performs single carrier MIMO transmission between a transmitting station device and a receiving station device, the transmitting station device including: a time-domain linear equalization unit configured to remove inter-symbol interference and inter-stream interference from a data signal to be transmitted to the receiving station device; a training signal generation unit configured to generate a training signal for use in estimation of propagation path characteristics and transmit the training signal to the receiving station device, and the receiving station device including: a propagation path characteristics estimation unit configured to receive the training signal which is transmitted by the transmitting station device and estimate a transfer function matrix of propagation path characteristics; a filter tap calculation unit configured to calculate filter tap coefficients for the time-domain linear equalization unit of the transmitting station device based on the transfer function matrix by a predefined approach; and a transmission mode determination unit configured to make the filter tap calculation unit calculate the filter tap coefficients when the transfer function matrix meets a predefined condition, and to change a transmission mode and determine the transmission mode that meets the predefined condition when the transfer function matrix does not meet the predefined condition. 
     A third aspect of the present invention is the first aspect or the second aspect of the invention, in which the predefined approach is an approach that determines a transmit weight matrix W(z) with a product of 1/det(H(z)) and adj(H(z)) using a determinant det(H(z)) of a transfer function matrix H(z) for CIR and an adjugate matrix adj(H(z)); the predefined condition is whether the determinant det(H(z)) of the transfer function matrix H(z) is in a minimum phase or not; and changing of the transmission mode is changing of antenna combination including a multiplex factor of antennas of the transmitting station device and the receiving station device. 
     A fourth aspect of the present invention is a wireless communication method in a wireless communication system that performs single carrier MIMO transmission between a transmitting station device and a receiving station device, the transmitting station device performing: time-domain linear equalization processing for removing inter-symbol interference and inter-stream interference from a data signal to be transmitted to the receiving station device; propagation path characteristics estimation processing for receiving a training signal which is transmitted by the receiving station device and estimating a transfer function matrix of propagation path characteristics; filter tap calculation processing for calculating filter tap coefficients for the time-domain linear equalization processing based on the transfer function matrix by a predefined approach; and transmission mode determination processing for calculating the filter tap coefficients by the filter tap calculation processing when the transfer function matrix meets a predefined condition, and for changing a transmission mode and determining the transmission mode that meets the predefined condition when the transfer function matrix does not meet the predefined condition, and the receiving station device performing training signal generation processing for generating the training signal for use in estimation of propagation path characteristics and transmitting the training signal to the transmitting station device. 
     A fifth aspect of the present invention is a wireless communication method in a wireless communication system that performs single carrier MIMO transmission between a transmitting station device and a receiving station device, the transmitting station device performing: time-domain linear equalization processing for removing inter-symbol interference and inter-stream interference from a data signal to be transmitted to the receiving station device; and training signal generation processing for generating a training signal for use in estimation of propagation path characteristics and transmitting the training signal to the receiving station device, and the receiving station device performing: propagation path characteristics estimation processing for receiving the training signal which is transmitted by the transmitting station device and estimating a transfer function matrix of propagation path characteristics; filter tap calculation processing for calculating filter tap coefficients for the time-domain linear equalization processing at the transmitting station device based on the transfer function matrix by a predefined approach; and transmission mode determination processing for calculating the filter tap coefficients by the filter tap calculation processing when the transfer function matrix meets a predefined condition, and for changing a transmission mode and determining the transmission mode that meets the predefined condition when the transfer function matrix does not meet the predefined condition. 
     A sixth aspect of the present invention is the fourth aspect or the fifth aspect of the invention, in which the predefined approach is an approach that determines a transmit weight matrix W(z) with a product of 1/det(H(z)) and adj(H(z)) using a determinant det(H(z)) of a transfer function matrix H(z) for CIR and an adjugate matrix adj(H(z)); the predefined condition is whether the determinant det(H(z)) of the transfer function matrix H(z) is in a minimum phase or not, and changing of the transmission mode is changing of antenna combination including a multiplex factor of antennas of the transmitting station device and the receiving station device. 
     A seventh aspect of the present invention is a transmitting station device that performs single carrier MIMO transmission with a receiving station device, the transmitting station device including: a time-domain linear equalization unit configured to remove inter-symbol interference and inter-stream interference from a data signal to be transmitted to the receiving station device; a propagation path characteristics estimation unit configured to receive a training signal which is transmitted from the receiving station device for use in estimation of propagation path characteristics and to estimate a transfer function matrix of propagation path characteristics; a filter tap calculation unit configured to calculate filter tap coefficients for the time-domain linear equalization unit based on the transfer function matrix by a predefined approach; and a transmission mode determination unit configured to make the filter tap calculation unit calculate the filter tap coefficients when the transfer function matrix meets a predefined condition, and to change a transmission mode and determine the transmission mode that meets the predefined condition when the transfer function matrix does not meet the predefined condition. 
     An eighth aspect of the present invention is a receiving station device that performs single carrier MIMO transmission with a transmitting station device, the receiving station device including: a propagation path characteristics estimation unit configured to receive a training signal which is transmitted by the transmitting station device for use in estimation of propagation path characteristics and to estimate a transfer function matrix of propagation path characteristics; a filter tap calculation unit configured to calculate filter tap coefficients for a time-domain linear equalization unit of the transmitting station device based on the transfer function matrix by a predefined approach; and a transmission mode determination unit configured to make the filter tap calculation unit calculate the filter tap coefficients when the transfer function matrix meets a predefined condition, and to change a transmission mode and determine the transmission mode that meets the predefined condition when the transfer function matrix does not meet the predefined condition. 
     Effects of the Invention 
     The wireless communication system, the wireless communication method, the transmitting station device and the receiving station device according to the present invention can remove inter-symbol interference and inter-stream interference using a time-domain linear equalizer by changing antenna combination so that the determinant of the transfer function matrix H(z) for CIR will not be in a non-minimum phase in SC-MIMO transmission. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of overall configuration of a wireless communication system in various embodiments. 
         FIG. 2  shows an example of a transmitting station device and a receiving station device. 
         FIG. 3  shows an example of the transmitting station device and the receiving station device according to a first embodiment. 
         FIG. 4  shows an example of the transmitting station device and the receiving station device according to a second embodiment. 
         FIG. 5  shows an example of processing performed by the wireless communication system according to the first embodiment or the wireless communication system according to the second embodiment. 
         FIG. 6  shows an example of filter tap coefficients when det(Hz) is in a minimum phase versus in a non-minimum phase. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the wireless communication system, wireless communication method, the transmitting station device and the receiving station device according to the present invention are described below with reference to drawings. 
       FIG. 1  shows an example of overall configuration of a wireless communication system  100 , which is common to various embodiments. In  FIG. 1 , the wireless communication system  100  includes a transmitting station device  101  and a receiving station device  102  and performs wireless communication between the transmitting station device  101  and the receiving station device  102 . The transmitting station device  101  has multiple (N T , an integer N T ≥2) antennas from an antenna ATt( 1 ) through an antenna ATt(N T ). The receiving station device  102  has multiple (N N , an integer N R ≥2) antennas from an antenna ATr( 1 ) through an antenna ATr(N R ). In the following description, for discussion common to the antenna ATt( 1 ) through the antenna ATt(N T ) of the transmitting station device  101 , they are indicated as antenna ATt omitting “(number)” at the end of their reference signs, and are indicated as antenna ATt( 1 ), for example, with addition of “(number)” at the end of the reference sign when a specific antenna is referred to. The antenna ATr( 1 ) through the antenna ATr(N R ) of the receiving station device  102  are also indicated in a similar manner. Also, multiple same blocks, if provided, are indicated in a similar manner. 
     The wireless communication system  100  according to the present embodiment performs wireless communication of wide-band SC-MIMO scheme using multiple antennas between the transmitting station device  101  and the receiving station device  102 . As shown in  FIG. 1( a ) , inter-stream interference due to spatial spread occurs between the N T  antennas of the transmitting station device  101  and the N R  antennas of the receiving station device  102 . Also, signals transmitted and received between the respective antennas of the transmitting station device  101  and the receiving station device  102  undergo frequency selective fading due to multiple delayed waves with different delay times such as multipath, giving rise to inter-symbol interference due to temporal spread such as shown in  FIG. 1( b ) . Thus, inter-stream interference and inter-symbol interference need to be suppressed in the wireless communication system  100 . 
     Here, the CIR of a wireless communication path between the transmitting station device  101  and the receiving station device  102  can be represented by a matrix of transfer functions (referred to as a transfer function matrix) H(z), which has N T ×N R  elements according to the number of multiple antennas. Expression (2) represents a transfer function matrix H(z) for N×N MIMO, where N R =N T  (=N). The transfer function matrix H(z) is an Nth-order polynomial matrix. 
     
       
         
           
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     In Expression (2), a transfer function H ij (z) at each element of the transfer function matrix H(z) is represented by Expression (3). The transfer function H ij (z) is an Lth-order polynomial. 
     
       
         
           
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     Here, i is an integer 1≤i≤N, and j is an integer 1≤j≤N. Also, h (l)   ij  indicates the CIR for the l-th path between the i-th receive antenna and the j-th transmit antenna. L is CIR length (equivalent to the number of paths a signal propagates through), and z −1  is a delay operator for the transfer function. 
     Then, by constructing a linear equalization unit with an inverse matrix H −1 (z) of the transfer function matrix H(z) as the transmit weight matrix W(z), inter-symbol interference and inter-stream interference can be removed simultaneously. 
     Expression (4) shows the transmit weight matrix W(z). Note that Expression (4) is the same as the Expression (1) described in the prior art, and the transmit weight matrix W(z) can be represented by the adjugate matrix det(H(z)) and the determinant adj(H(z)). 
     
       
         
           
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     As shown in Expression (4), the transmit weight matrix W(z) can be represented by the product of the transfer functions of 1/det(H(z)) and the matrix of adj(H(z)). 
     Here, an approach that uses the inverse matrix of the transfer function matrix H(z) for CIR as the transfer functions for a linear equalizer has a problem of the transmit weight matrix W(z) diverging when the determinant det(H(z)) of the transfer function matrix is in a non-minimum phase, making it impossible to remove inter-symbol interference and inter-stream interference with a time-domain linear equalizer. 
     For example, in the case of 2×2 MIMO, the determinant det(H(z)) of the transfer function matrix H(z) is represented by Expression (5). 
       [Math. 5] 
       det( H ( z ))= H   11 ( z ) H   22 ( z )− H   12 ( z ) H   21 ( z )  (5)
 
     Here, applicability of the transmit weight matrix W(z) of Expression (4) will vary depending on a condition of whether the determinant det(H(z)) of the transfer function matrix is in a non-minimum phase or in the minimum phase in Expression (5). 
     Accordingly, in the wireless communication system according to each embodiment described below, when the determinant det(H(z)) of the transfer function matrix H(z) is in a non-minimum phase, a condition is found under which the determinant det(H(z)) will be in the minimum phase by changing the combination of antennas ATt of the transmitting station device  101  and antennas ATr of the receiving station device  102 , and control is effected so that the transmit weight matrix W(z) does not diverge. How to change the antennas combination is described in detail later. 
       FIG. 2  shows an example of the transmitting station device  101  and the receiving station device  102 . In  FIG. 2 , the transmitting station device  101  includes a QAM modulation unit  201 , a linear equalization unit  202 , an RF unit  203 , and antennas ATt. 
     The QAM modulation unit  201  outputs a data signal S(n), generated by quadrature amplitude modulation (QAM) of a bit string of data information bits which are to be transmitted to the receiving station device  102 . The QAM modulation unit  201  has a QAM modulation unit  201 ( 1 ) through a QAM modulation unit  201 (N) and outputs data signals corresponding to N streams. 
     The linear equalization unit  202  performs equalization processing for inter-symbol interference and inter-stream interference with the transmit weight matrix W(z) computed based on the CIR between the transmitting station device  101  and the receiving station device  102 . How to compute the transmit weight matrix W(z) is described in detail later. At the same time with the equalization processing, processing for normalizing transmission power is also performed. 
     The RF unit  203  has N RF units: an RF unit  203 ( 1 ) through an RF unit  203 (N) corresponding to the N antennas ATt respectively, and frequency-converts a signal outputted by the linear equalization unit  202  into a transmit signal of a high frequency and sends it from each antenna ATt for each stream. 
     The antennas ATt include N antennas: the antenna ATt( 1 ) through antenna the ATt(N), and radiates the high-frequency signal for each stream outputted by the RF unit  203  into space as an electromagnetic wave. 
     In this manner, the transmitting station device  101  can transmit signals from which inter-symbol interference and inter-stream interference have been removed by the linear equalization unit  202  to the receiving station device  102 . 
     In  FIG. 2 , the receiving station device  102  includes the antennas ATr, an RF unit  301  and a QAM demodulation unit  302 . 
     The antennas ATr include N antennas for transmission and reception: the antenna ATr( 1 ) through the antenna ATr(N), and converts an electromagnetic wave in space transmitted from the transmitting station device  101  into a high-frequency signal. 
     The RF unit  301  has N RF units: an RF unit  301 ( 1 ) through an RF unit  301 (N) respectively corresponding to the N antennas ATr, and frequency-converts the respective high-frequency signals outputted by the antenna ATr( 1 ) through the antenna ATr(N) into baseband data signals. 
     The QAM demodulation unit  302  demodulates data signals S{circumflex over ( )}(n) for N streams outputted by the RF unit  301  into information bits and outputs a bit string. Since the RF unit  301  outputs data signals S{circumflex over ( )}(n) for N streams according to the number of antennas ATr, the QAM demodulation unit  302  demodulates the data signal S{circumflex over ( )}(n) for each stream. 
     In this manner, the receiving station device  102  can receive signals from which inter-symbol interference and inter-stream interference have been removed on the transmitting station device  101  side and demodulate the data signals. 
     In  FIG. 2 , when representing the data signal outputted by the QAM modulation unit  201  as S(n), the transmit weight matrix of the linear equalization unit  202  as W(Z), the transfer function matrix for CIR as H(z), the data signal outputted by the RF unit  301  as S{circumflex over ( )}(n), and additive noise as η(n), relationship of a signal transmitted/received in the wireless communication system  100  according to the present embodiment can be represented by Expression (6): 
       [Math. 6] 
         Ŝ ( n )= H ( z ) W ( z ) S ( n )+η( n )  (6)
 
     First Embodiment 
       FIG. 3  shows an example of the transmitting station device  101  and the receiving station device  102  according to the first embodiment. In  FIG. 3 , the wireless communication system  100  includes the transmitting station device  101  and the receiving station device  102 . 
     In  FIG. 3 , the transmitting station device  101  includes an information bit generation unit  401 , a data signal modulation unit  402 , an FIR filter unit  403 , a transmit signal conversion unit  404 , a received-signal conversion unit  405 , an CIR estimation unit  406 , a transmission mode determination unit  407 , a filter tap calculation unit  408 , and N antennas from the antenna ATt( 1 ) through the antenna ATt(N). Also in  FIG. 3 , the receiving station device  102  includes N antennas from the antenna ATr( 1 ) through the antenna ATr(N), a received-signal conversion unit  501 , a data signal demodulation unit  502 , an information bit detection unit  503 , a training signal generation unit  504 , and a transmit signal conversion unit  505 . 
     First, the configuration of the transmitting station device  101  is described. 
     The information bit generation unit  401  generates data information bits for transmission to the receiving station device  102 . The data information bits are a bit string corresponding to a data signal inputted from outside (not shown), a data signal internally generated and the like, for example. The information bit generation unit  401  changes the number of signal streams in accordance with the number of antennas ATt which are used in transmission determined by the transmission mode determination unit  407  discussed later, and outputs each signal stream to the data signal modulation unit  402 . The information bit generation unit  401  may also have an error correction coding feature for generating an error correction code with a predetermined coding rate, an interleaving feature and the like. 
     The data signal modulation unit  402  outputs a data signal S(n) generated by modulating a bit string for each signal stream outputted by the information bit generation unit  401  by a predetermined modulation scheme (e.g., quadrature amplitude modulation (QAM)). Since in the present embodiment the information bit generation unit  401  outputs data signals which have been modulated for each of signal streams divided according to the number of antennas ATt, the data signal modulation unit  402  is provided for each one stream. 
     The FIR filter unit  403  outputs to the transmit signal conversion unit  404  a signal generated by removing inter-symbol interference and inter-stream interference from the data signal S(n) outputted by the data signal modulation unit  402  using a filter tap coefficient computed by the filter tap calculation unit  408  discussed later. Here, the FIR filter unit  403  corresponds to a linear equalization unit which performs time-domain linear equalization processing. The FIR filter unit  403  has, for example, delay taps for holding a data signal outputted by the data signal modulation unit  402  and shifting it at certain intervals, and outputs a sum of signals obtained by multiplying the signals from the respective delay taps by a predetermined filter tap coefficient. In this manner, the FIR filter unit  403  performs time-domain linear equalization processing for removing inter-symbol interference and inter-stream interference. The FIR filter unit  403  also performs processing for normalizing transmission power at the same time with the time-domain linear equalization processing. 
     The transmit signal conversion unit  404  frequency-converts a data signal outputted by the FIR filter unit  403  into a transmit signal of a high frequency for transmission from the antenna ATt. For example, the transmit signal conversion unit  404  up-converts a data signal in 20 MHz band into a high-frequency signal in 5 GHz band and sends it from the antenna ATt. Here, respective ones of multiple divided signal streams are converted into high-frequency signals and sent from the antennas ATt of a combination to be used for transmission determined by the transmission mode determination unit  407  discussed later. 
     The antennas ATt include N antennas for transmission and reception from the antenna ATt( 1 ) through the antenna ATt(N), and radiates the high-frequency signal outputted by the transmit signal conversion unit  404  into space as an electromagnetic wave. Alternatively, the antennas ATt convert an electromagnetic wave in space transmitted from the receiving station device  102  into high-frequency signals and output them to the received-signal conversion unit  405 . For example, a combination of M (M being a positive integer, M&lt;N) antennas ATt out of the N antennas ATt is selected by the transmission mode determination unit  407  discussed later and M signal streams are transmitted from the selected M antennas ATt, respectively. 
     The received-signal conversion unit  405  converts frequency-converts the high-frequency received signals received by the respective ones of the antenna ATt( 1 ) through the antenna ATt(N) into a baseband signal of a low frequency. For example, the received-signal conversion unit  405  down-converts a high-frequency signal in 5 GHz band and outputs a baseband signal in 20 MHz band. Here, in the present embodiment, the received-signal conversion unit  405  receives a training signal from the receiving station device  102  and outputs it to the CIR estimation unit  406 . 
     The CIR estimation unit  406  estimates the CIR based on the training signal transmitted from the receiving station device  102 . The CIR estimation unit  406  corresponds to a propagation path characteristics estimation unit for executing propagation path characteristics estimation processing. 
     The transmission mode determination unit  407  assesses a predefined condition, and if the condition is not met, changes the transmission mode (e.g., antenna combination) and iteratively performs similar processing until an antenna combination including a multiplex factor of antennas satisfying the condition (hereinafter referred to as antenna combination unless otherwise required) is found (corresponding to transmission mode determination processing). For the antenna combination, when selecting nine antennas out of ten antennas for example, there are ten possible combinations, hence  100  possible combinations across the transmitting station device  101  and the receiving station device  102 , and an antenna combination meeting the determination condition is found from those combinations. If none is found, similar processing is repeated with a reduced number of antennas for selection (multiplex factor). Here, in the present embodiment, the determination condition is whether det(H(z)) is in the minimum phase or in a non-minimum phase when the transmit weight matrix W(z) is represented by the product of the transfer functions of 1/det(H(z)) and the matrix of adj(H(z)) as described in Expression (4). Then, if det(H(z)) is in a non-minimum phase, the antenna combination is changed and an antenna combination with which det(H(z)) is in the minimum phase is determined. Changing of the antenna combination can be effected by selecting the transmit signal conversion units  404  corresponding to the antennas ATt to be used, for example. Specifically, when the antenna ATt( 1 ) through an antenna ATt(M) are used, a transmit signal conversion unit  404 ( 1 ) through a transmit signal conversion unit  404 (M) may be selected. Here, a norm for determining the transmission mode is not limited to the determinant det(H(z)); any application condition appropriate for the way of computing the transmit weight matrix W(z) may be used. For example, in a case of transforming the transmit weight matrix W(z) into an expression for the time-domain linear equalizer according to a different condition rather than representing it as the product of the transfer functions of 1/det(H(z)) and the matrix of adj(H(z)) as in Expression (4), determination may be made by that condition. The transmission mode determination unit  407  notifies the receiving station device  102  of a determination result via the transmit signal conversion unit  404  and the antennas ATt, enabling the antenna combination to be changed on the receiving station device  102  side as well. Notification of the determination result to the receiving station device  102  can use a method that prepares indices recording antenna combinations in the transmitting station device  101  and the receiving station device  102  in advance and gives a notification of an index number before starting data transmission, for example. 
     The filter tap calculation unit  408  computes the transmit weight matrix W(z) for the transmission mode determined by the transmission mode determination unit  407  based on the CIR estimated by the CIR estimation unit  406 . The filter tap calculation unit  408  also calculates filter tap coefficients for use in the FIR filter unit  403  based on the respective elements of the transmit weight matrix W(z) and outputs them to the FIR filter unit  403  (corresponding to filter tap calculation processing). 
     In this manner, the transmitting station device  101  can transmit data signals from which inter-symbol interference and inter-stream interference have been removed by the FIR filter unit  403  to the receiving station device  102 . 
     Next, the configuration of the receiving station device  102  shown in  FIG. 3  is described. 
     The antennas ATr include N antennas for transmission and reception: the antenna ATr(l) through the antenna ATr(N), and radiate a high-frequency signal outputted by the transmit signal conversion unit  505  discussed later into space as an electromagnetic wave. Alternatively, the antennas ATr convert an electromagnetic wave in space transmitted from the transmitting station device  101  into high-frequency signals and outputs them to the received-signal conversion unit  501  discussed later. In the present embodiment, a combination of M (M&lt;N) antennas ATr out of the N antennas ATr is selected based on an antenna combination indicated from the transmitting station device  101 , and signals are received by the selected M antennas ATr. Changing of the antenna combination can be effected by selecting the received-signal conversion units  501  corresponding to the antennas ATr to be used, for example. Specifically, when the antenna ATr( 1 ) through an antenna ATr(M) are used, received signals of the received-signal conversion unit  501 ( 1 ) through a received-signal conversion unit  501 (M) may be selected. 
     The received-signal conversion unit  501  frequency-converts the high-frequency signals received from the respective ones of the antenna ATr( 1 ) through the antenna ATr(N) into a baseband signal, as with the received-signal conversion unit  405  of the transmitting station device  101 . Here, data signals received from the transmitting station device  101  are outputted to the data signal demodulation unit  502 . 
     The data signal demodulation unit  502  demodulates the data signal S{circumflex over ( )}(n) outputted by the received-signal conversion unit  501  into information bits and outputs a bit string. Since the received-signal conversion unit  501  outputs the data signal S{circumflex over ( )}(n) for multiple streams according to the number of antennas ATr, the data signal demodulation unit  502  demodulates the data signal S{circumflex over ( )}(n) for each stream. Then, the data signal demodulation unit  502  outputs a bit string formed by concatenating bit strings that have been divided into multiple streams on the transmitting station device  101  side to the information bit detection unit  503 . The data signal demodulation unit  502  may have an error correction decoding feature and/or a deinterleaving feature as appropriate for the functionality of the transmitting station device  101  side. Here, for example, the received-signal conversion units  501  corresponding to the antennas ATr to be used are selected based on the antenna combination, and data signals are demodulated by the data signal demodulation units  502  corresponding to the selected received-signal conversion units  501 . Specifically, when the antenna ATr( 1 ) through the antenna ATr(M) are used, data signals are demodulated by the data signal demodulation unit  502 ( 1 ) through the data signal demodulation unit  502 (M) corresponding to the received-signal conversion unit  501 ( 1 ) through the received-signal conversion unit  501 (M). 
     The information bit detection unit  503  outputs received data generated by converting the bit string outputted by the data signal demodulation unit  502  into digital data. An error correction decoding feature and/or a deinterleaving feature may be implemented on the information bit detection unit  503  side. 
     The training signal generation unit  504  generates a training signal for the CIR estimation unit  406  of the transmitting station device  101  to estimate the CIR (corresponding to training signal generation processing). The training signal is a predetermined signal generated by modulating a predefined information, such as a preamble for signal detection (e.g., a specific pattern such as an alternating pattern of “01”), with a modulation scheme resistant to interference such as PSK (Phase Shift Keying), and is used for estimating the CIR on the transmitting station device  101  side. Information on the training signal which is transmitted by the receiving station device  102  is known by the transmitting station device  101  in advance. 
     The transmit signal conversion unit  505  converts the training signal outputted by the training signal generation unit  504  into a high-frequency signal and sends it from the antennas ATr. 
     In this manner, the receiving station device  102  can transmit the training signal for estimating the CIR on the transmitting station device  101  side and receive data signal transmitted from the transmitting station device  101  in which inter-symbol interference and inter-stream interference have been equalized. 
     Second Embodiment 
       FIG. 4  shows an example of a transmitting station device  101   a  and a receiving station device  102   a  according to a second embodiment. In  FIG. 4 , a wireless communication system  100   a  includes the transmitting station device  101   a  and the receiving station device  102   a.    
     In  FIG. 4 , the transmitting station device  101   a  includes an information bit generation unit  401 , a data signal modulation unit  402 , an FIR filter unit  403 , a transmit signal conversion unit  404 , a received-signal conversion unit  405 , a training signal generation unit  410 , and N antennas from an antenna ATt( 1 ) through an antenna ATt(N). Also in  FIG. 4 , the receiving station device  102   a  includes N antennas from an antenna ATr( 1 ) through an antenna ATr(N), a received-signal conversion unit  501 , a data signal demodulation unit  502 , an information bit detection unit  503 , a transmit signal conversion unit  505 , an CIR estimation unit  510 , a transmission mode determination unit  511  and a filter tap calculation unit  512 . 
     Here, the wireless communication system  100   a  shown in  FIG. 4  is different from the wireless communication system  100  described in  FIG. 3  in the following two respects. A first difference is that the training signal is transmitted from the transmitting station device  101   a . A second difference is that the receiving station device  102   a  performs CIR estimation, determination of the transmission mode and calculation of filter tap coefficients and transmits the calculated filter tap coefficients to the FIR filter unit  403  of the transmitting station device  101   a . In  FIG. 4 , primary operations of the blocks with the same reference signs as in  FIG. 3  are the same as in  FIG. 3 . 
     Differences from  FIG. 3  are described below. 
     First, the configuration of the transmitting station device  101   a  is described. 
     The training signal generation unit  410  operates in a similar manner to the training signal generation unit  504  in the first embodiment and generates a training signal for the CIR estimation unit  510  of the receiving station device  102   a  to estimate the CIR (corresponding to training signal generation processing). Information on the training signal which is transmitted by the transmitting station device  101   a  is known by the receiving station device  102   a  in advance. 
     The FIR filter unit  403  outputs to the transmit signal conversion unit  404  signals generated by removing inter-symbol interference and inter-stream interference from the data signal S(n) outputted by the data signal modulation unit  402  using the filter tap coefficients received from the receiving station device  102   a  (corresponding to FIR filter processing). A difference of the FIR filter unit  403  in the second embodiment from the FIR filter unit  403  in the first embodiment is use of the filter tap coefficients received from the receiving station device  102   a  via the received-signal conversion unit  405  and antennas ATt, and its chief operation is the same as in the first embodiment. 
     Next, the configuration of the receiving station device  102   a  is described. 
     The CIR estimation unit  510  operates in a similar manner to the CIR estimation unit  406  in the first embodiment and estimates the CIR based on the training signal transmitted from the transmitting station device  101   a . The CIR estimation unit  510  corresponds to a propagation path characteristics estimation unit for executing propagation path characteristics estimation processing. 
     The transmission mode determination unit  511  operates in a similar manner to the transmission mode determination unit  407  in the first embodiment, and determines an antenna combination with which det(H(z)) is in the minimum phase by changing the antenna combination. The transmission mode determination unit  511  notifies the transmitting station device  101   a  of a determination result via the transmit signal conversion unit  505  and the antennas ATr, enabling the antenna combination to be changed on the transmitting station device  101   a  side as well. Notification of the determination result to the transmitting station device  101   a  can use a method that prepares indices recording antenna combinations and gives a notification of an index number before starting data transmission, as described in the first embodiment. 
     The filter tap calculation unit  512  operates in a similar manner to the filter tap calculation unit  408  in the first embodiment and computes the transmit weight matrix W(z) for the transmission mode determined by the transmission mode determination unit  407  based on the CIR estimated by the CIR estimation unit  510 , and calculates filter tap coefficients for use in the FIR filter unit  403  of the transmitting station device  101   a  based on the respective elements of the transmit weight matrix W(z) (corresponding to filter tap calculation processing). In the present embodiment, the filter tap calculation unit  512  is transmitted to the transmitting station device  101   a  via the transmit signal conversion unit  505  and the antennas ATr and set in the FIR filter unit  403  of the transmitting station device  101   a.    
     In this manner, the wireless communication system  100   a  according to the present embodiment can transmit data signals free from inter-symbol interference and inter-stream interference to the receiving station device  102   a  by performing CIR estimation, determination of the transmission mode and calculation of filter tap coefficients on the receiving station device  102   a  side by means of the training signal transmitted by the transmitting station device  101   a , and transmitting and setting the calculated filter tap coefficients to the FIR filter unit  403  of the transmitting station device  101   a.    
     [Processing Method in the First Embodiment and the Second Embodiment] 
       FIG. 5  shows an example of processing performed by the wireless communication system  100  according to the first embodiment or the wireless communication system  100   a  according to the second embodiment. The process shown in  FIG. 5  is executed by the components of the transmitting station device  101  and the receiving station device  102  shown in  FIG. 3  or the transmitting station device  101   a  and the receiving station device  102   a  shown in  FIG. 4 . 
     In step S 101 , the CIR estimation unit  406  (or the CIR estimation unit  510 ) receives a training signal transmitted from the receiving station device  102   a  (or the transmitting station device  101   a ) and estimates the CIR(H(z)). Here, the transfer function matrix H(z) of the CIR is a matrix having the number of multiple antennas N×N as its elements. 
     In step S 102 , the transmission mode determination unit  407  (or the transmission mode determination unit  511 ) sets an initial value to M=N, with a variable M being the number of antennas ATt of the transmitting station device  101  (or the transmitting station device  101   a ) and the number of antennas ATr of the receiving station device  102  (or the receiving station device  102   a ), respectively. 
     In step S 103 , the transmission mode determination unit  407  (or the transmission mode determination unit  511 ) generates a transfer function matrix H ˜ (z) of all the combinations that are possible from the M transmit antennas and M receive antennas, and computes a determinant det(H ˜ (z)). Here, H ˜ (z) is a matrix having elements of M×M corresponding to the multiplex factor of antennas. 
     In step S 104 , the transmission mode determination unit  407  (or the transmission mode determination unit  511 ) determines whether the respective det(H ˜ (z)) corresponding to all the combinations of M×M antennas are in the minimum phase. Then, if there is a combination with which det(H ˜ (z)) is in the minimum phase, the flow proceeds to processing in step S 106 . When there is no combination with which det(H ˜ (z)) is in the minimum phase, the flow proceeds to processing in step S 105 . 
     In step S 105 , the transmission mode determination unit  407  (or the transmission mode determination unit  511 ) decreases the number of antennas by one and sets it in M=M−1, and returns to step S 103  to repeat processing in a similar manner. 
     In step  3106 , the transmission mode determination unit  407  (or the transmission mode determination unit  511 ) selects the multiplex factor M of antennas and antenna combination with which det(H ˜ (z)) is in the minimum phase. Respective blocks of the transmitting station device  101  and the receiving station device  102  (or the transmitting station device  101   a  and receiving station device  102   a ) are configured to process signals for M streams so that the number of signal streams will also be the same as the multiplex factor M of antennas. 
     In step S 107 , M×M MIMO transmission is carried out via antennas with the multiplex factor of antennas and the combination that were set in step S 106 . 
     In this manner, the wireless communication system  100  according to the first embodiment or the wireless communication system  100   a  according to the second embodiment selects a multiplex factor of antennas and an antenna combination with which the determinant of the transfer function matrix is in the minimum phase and performs communication. Thus, they can remove inter-symbol interference and inter-stream interference with a time-domain linear equalizer without causing divergence of the transmit weight matrix. 
     As has been described in the respective embodiments, the wireless communication system, wireless communication method, transmitting station device and the receiving station device according to the present invention can remove inter-symbol interference and inter-stream interference with a time-domain linear equalizer by changing the transmission mode so that the determinant of the transfer function matrix for the CIR will not be in a non-minimum phase in SC-MIMO transmission. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100 ,  100   a  wireless communication system 
               101 ,  101   a  transmitting station device 
               102 ,  102   a  receiving station device 
               201  QAM modulation unit 
               202  linear equalization unit 
               203 ,  301  RF unit 
               302  QAM demodulation unit 
               401  information bit generation unit 
               402  data signal modulation unit 
               403  FIR filter unit 
               404 ,  505  transmit signal conversion unit 
               405 ,  501  received-signal conversion unit 
             ATt, ATr antenna 
               406 ,  510  CIR estimation unit 
               407 ,  511  transmission mode determination unit 
               408 ,  512  filter tap calculation unit 
               410 ,  504  training signal generation unit 
               502  data signal demodulation unit 
               503  information bit detection unit