Abstract:
The communication system carries out signal processing adapted for transmission-channel characteristics. In the communication system, a right transmitter detects a transmission-signal-characteristics-correcting coefficient sent from a left receiver and corrects, according to the transmission-signal-characteristics-correcting coefficient, at least one of a transfer function and a spatial frequency characteristic of a transmission signal. The left receiver computes the transmission-signal-characteristics-correcting coefficient and a reception-signal-characteristics-correcting coefficient through the processes of detecting correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together. According to the reception-signal-characteristics-correcting coefficient, the left receiver corrects at least one of a frequency characteristic and a spatial frequency characteristic of the reception signal. The left receiver transmits the transmission-signal-characteristics-correcting coefficient to the right transmitter, so that the right transmitter may correct at least one of a transfer function and a spatial frequency characteristics of a transmission signal according to the transmission-signal-characteristics-correcting coefficient.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims benefit of priority under 35 U.S.C. § 119 to Japanese Patent Application No.2001-343535. filed on Nov. 8. 2001, the entire contents of which are incorporated by reference herein.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to communication methods, communication systems, transmitters, and receivers, to carry out adaptive signal transmission and reception processes.  
           [0004]    2. Description of the Related Art  
           [0005]    A communication system usually compensates the characteristics of a transmission channel on a reception side or on a transmission side. FIG. 1 shows an example of a communication system employing an adaptive equalizer to compensate a delay spread occurring in a transmission channel. The system Includes a transmit information input terminal  1001 , a transmitter  1002 , a transmission channel  1003 , a receiver  1004 , the adaptive equalizer  1005 , and a demodulated signal output terminal  1006 .  
           [0006]    [0006]FIG. 2 shows an example of the adaptive equalizer  1005 . The adaptive equalizer  1005  includes a signal input terminal  1007 , delay elements  1008  to  1010 , multipliers  1011  to  1014 , an adder  1015 , a signal output terminal  1016 , and an adaptive controller  1017 .  
           [0007]    The adaptive controller  3017  estimates a coefficient applied as another input signal to each of the multipliers  1011  to  1014 . The adaptive controller  1017  employs, for example, an LMS (least mean square) algorithm to minimize an ISI (intersymbol interference) component of an output signal provided from the output terminal  1016 .  
           [0008]    [0008]FIG. 3 shows an example of a wireless communication system employing a post-detection selection combining diversity reception method that uses a plurality of antennas to receive signals, demodulates only a signal from one of the antennas that provides a highest reception quality, and outputs the demodulated signal. The system includes a transmit information input terminal  1020 , a transmitter  1021 , antennas  1022  to  1024 , receivers  1025  and  1026 , a level comparator  1027 , a switching circuit  1028 , and a demodulated signal output terminal  1029 .  
           [0009]    In FIG. 3, the transmitter  1021  transmits a radio signal from the antenna  1022 . The transmitted signal is received by the antennas  1023  and  1024 . Levels of the reception signals from the antennas are compared with each other in the level comparator  1027 . One of the antennas providing a higher signal level is selected by the switching circuit  1028 , and the signal from the selected antenna is output through the output terminal  1029 . It can be said that the system weights signals received by the antennas  1023  and  1024  with “1” and “0” on according to levels of the reception signals.  
           [0010]    [0010]FIG. 4 shows an example of a transmission-diversity reception system, which oppositely operates to the diversity reception system of FIG. 3. The system of FIG. 4 employs a plurality of antennas on a transmission side, to transmit a signal only from one of the antennas that may provide a highest reception level. The system includes transmission signal input terminals  1034  and  1048 , transmitters  1035  and  1046 , receivers  1038 ,  1039 , and  1045 , switching circuits  1032  and  1033 , antenna sharing units  1036 ,  1037 , and  1044  for transmission and reception, antennas  3041  to  1043 , demodulated signal output terminals  1031  and  1047 , and a level comparator  1040 .  
           [0011]    In FIG. 4, the transmitter  1046  transmits a signal. The signal is received by the antennas  1041  and  1042 , which send the reception signals to the receivers  1038  and  1039 , respectively. Levels of the reception signals are compared with each other in the level comparator  1040 . One of the reception signals having a higher level is selected by the switching circuit  1032  and is demodulated. When the transmitter  1035  transmits a signal according to data input to the input terminal  1034 , the switching circuit  1033  selects the antenna chosen by the level comparator  1040  so that the signal may be transmitted through the selected antenna. The transmitted signal is received by the antenna  1043  and is demodulated by the receiver  1045 . The demodulated signal is output from the output terminal  1047 .  
           [0012]    This system transmits a signal with the antenna that can receive a signal at a highest level. Due to the reversibility of a transmission channel, the signal transmitted by the antenna that can receive a signal at the highest level can be received by the receiver at a highest level compared with when the same signal is transmitted through any other antenna. The system, therefore, improves communication quality It can be said that the system of FIG. 4 carries out the diversity reception of FIG. 3 on a transmission side, too.  
           [0013]    [0013]FIG. 5 shows an example of an intelligent communication system employing an adaptive array. The system includes a transmit information input terminal  1051 , a transmitter  1052 , antennas  1053  and  1058  to  1061 , frequency converters  1054  to  1057 , the adaptive array  1062 , a demodulator  1063 , and a demodulated signal output terminal  1064 .  
           [0014]    [0014]FIG. 6 shows an example of the adaptive array  1062 . The adaptive array  1062  includes signal input terminals  1065  to  1068 , multipliers  1069  to  1072 , an adder  1073 , a signal output terminal  1074 , and an adaptive controller  1075 . The adaptive controller  3075  determines coefficients applied as other input signals to the multipliers  1069  to  1072 , respectively. Like the adaptive controller  1017  of the adaptive equalizer  1005  of FIGS. 1 and 2, the adaptive controller  1075  controls the coefficients applied to the multipliers  1069  to  1072  according to an LMS algorithm to optimize the SNR of a transmission signal provided from the output terminal  1074 .  
           [0015]    The related arts explained above carry out equalization only on a transmission side or on a reception side. There are communication systems that carry out equalization both on transmission and reception sides. FIG. 7 shows an example of such dual equalization communication systems. The system includes demodulated signal output terminals  1076  and  1089 , a transmit information input terminal  1085 , tapped delay line filters  1077 ,  1088 ,  1084 , and  1094 , receivers  1078  and  1087 , transmitters  1083  and  1095 , antenna sharing units  1079  and  1093 , antennas  1082  and  1086 , a coefficient computation unit  1090  to estimate coefficients for the tapped delay line filters, delay elements  1080 ,  1091 , and  1092 , and a coefficient setter  1081 . Coefficients for the tapped delay line filters  1077  and  1084  are computed by the coefficient computation unit  1090  and are transmitted through the transmitter  1095  and antenna  1086 . The transmitted coefficients are received by the antenna  1082 , are demodulated by the receiver  1078 , and are output from the tapped delay line filter  1077 . From the tapped delay line filter  1077 , the coefficient setter  1081  fetches the coefficients and sets them for the tapped delay line filters  1077  and  1084 , respectively.  
           [0016]    [0016]FIG. 8 shows an example of one of the tapped delay line filters. The tapped delay line filter includes a signal input terminal  1100 , delay elements  1101  to  1103 , multipliers  1104  to  1107 , an adder  1108 , a signal output terminal  1109 , and a coefficient input terminal  1110 . A tapped delay line filter on a transmission side has a tap coefficient w r  and a tapped delay line filter on a reception side has a tap coefficient w r . Time is represented with “t”, a transmit information signal vector with “s( t),” a reception signal with “y(t),” and received power with “&lt;|y(t)| 2 &gt;.” The received power serves as an evaluation factor, which must be maximized. When the received power is maximized, the tapped delay line filters on the transmission and reception sides form matched filters. The average power &lt;|y(t)| 2 &gt; Will indefinitely increase if the coefficients w r  and w r  are indefinitely increased. To constrict this, the following restrictive condition is set;  
             w   t   T   w   t               =1 , w   r   T   w   r               =1 , &lt;s               ( t ) s ( t )&gt;= I    (1)  
           [0017]    where I Is a unit matrix.  
           [0018]    Ignoring the influence of noise, the reception signal y(t) is expressed as follows:  
                     y        (   t   )       =                w   r   T          x        (   t   )         =       w   r   T          (       Aw   r          s        (   t   )         )                     =                w   r   T          Aw   r          s        (   t   )         =         s   T          (   t   )            w   r   T          A   T          w   r                       (   2   )                               
 
           [0019]    where x( t) is a vector representing a reception signal sequence and A is a transfer function matrix.  
           [0020]    If a transmission channel has an impulse response length L, an impulse response vector of the transmission channel is expressed as [a 0 , a 1 , . . . , a x−1 ] T . Then, the transfer function matrix A is expressed as follows:  
             A   =     [           a   0           a   1         ⋯         a     L   -   1           0       ⋯       0           0         a   0         ⋯         a     L   -   2             a     L   -   1           0       ⋮           ⋮       ⋰       ⋰       ⋮       ⋯         a     L   -   1           0           0       ⋯       0         a   0           a   1         ⋯         a     L   -   1             ]             (   3   )                               
 
           [0021]    Namely, the matrix A becomes vectors of N rows and (N+(L−1)) columns. Then, the average power of the reception signal y( t) is expressed as follows with &lt;·&gt; being an ensemble mean:  
                     〈            y        (   t   )            2     〉     =              w   i   T          Aw   i          〈       s        (   t   )              s   H          (   t   )         〉          w   i   H          A   H          w   i   *                   =              w   i   T          A   I          w   i          w   i   H          A   *          w   i   *          〈         s   H          (   t   )            s        (   t   )         〉                   =              L   i          w   i   T          Aw   i          w   i   H          A   H          w   i   *                   =              L   i          w   i   T          A   T          w   i          w   i   H          A   *          w   i   *                     (   4   )                               
 
           [0022]    where L t  is a transmission filter length.  
           [0023]    The expression (4) is abase of the restrictive condition (1), and a maximum value of the expression (4) must be found. As is well known, a solution of an optimization problem under a restrictive condition can be found according to Lagrange&#39;s method of indeterminate coefficients. According to the Lagrange&#39;s method, a maximum value of &lt;|y(t)| 2 &gt; is a solution of the following expressions:  
             h   =         1     L   i            〈            y        (   t   )            2     〉       -       λ   i          (         w   i   T          w   i   *       -   1     )       -       λ   r          (         w   r   T          w   r   *       -   1     )                 (   5   )                   ∂   h       ∂     w   i         =           Aw   r          w   r   H          A   H          w   t   *       -       λ   t          w   t   *         =   0             (   6   )                   ∂   h       ∂     w   r         =           A   T          w   t          w   t   H          A   *          w   r   *       -       λ   r          w   r   *         =   0             (   7   )                               
 
           [0024]    Solutions of the expressions (6) and (7) are obtained as follows:  
             Aw   r   w   r   H   A   H   w   l               =λ l   w   l                 (8)  
             A   T   w   l   w   l   H   A                 w   r               =λ r   w   r                 (9)  
           [0025]    The tap coefficients in the expressions (8) and (9) are nested. A conventional technique assigns proper initial values w t (0) and w r (0) for the tap coefficients, substitutes w r (0) for w r  of the expression (8), solves an eigenvalue problem related to w r , and sets the solution as w r (1). Then, w t (1) Is substituted for w t  of the expression (9), an eigenvalue problem related to w r  is solved, and the solution is set as w r (1). These operations are repeated to find converged solutions. This technique is called a sequential method in this specification.  
           [0026]    The sequential method has no assurance of convergence. Namely, the sequential method has a risk of divergence. Even if it meets convergence, it needs iterations of operations before providing converged values, with each iteration requiring two eigenvalue decomposition operations. In this way, the sequential method, involves a risk of divergence and needs many operations.  
         SUMMARY OF THE INVENTION  
         [0027]    An object of the present invention is to provide a communication technique capable of surely and stably providing solutions only by solving eigenvalue problems twice, thereby greatly reducing the number of operations.  
           [0028]    To achieve the above object, there is provided a communication method employing a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, comprising the steps of: in the transmitter: detecting a transmission-signal characteristics-correcting coefficient sent from the receiver: and correcting, according to the transmission-signal-characteristics-correcting coefficient, at least one of a transfer function and a spatial frequency characteristic of a transmission signal; and in the receiver: computing the transmission-signal-characteristics-correcting coefficient and a reception-signal-characteristics-correcting coefficient through the processes of detecting correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; correcting, according to the reception-signal-characteristics-correcting coefficient, at least one of a frequency characteristic and a spatial frequency characteristic of the reception signal; and transmitting the transmission-signal-characteristics-correcting coefficient to the transmitter  
           [0029]    Further, to achieve the above object, there is provided a communication method employing a bass station and a terminal each having a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, comprising the steps of; in one of the base station and the terminal: in the receiver; computing a reception-signal-characteristics-correcting coefficient through the processes of detecting a correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; and correcting, according to the reception-signal-characteristics-correcting coefficient, at least one of a frequency characteristic and a spatial frequency characteristic of the reception signal; and in the other of the base station and the terminal: in the receiver: computing a transmission-signal-characteristics-correcting coefficient through the processes of detecting a correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; and in the transmitter: correcting, according to the transmission-signal-characteristics-correcting coefficient, at least a transfer function and a spatial frequency characteristic of a transmission signal.  
           [0030]    Further, to achieve the above object, there is provided a communication method employing a base station and a terminal each having a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, comprising the steps of: in the receiver of each of the base station and terminal: computing a transmission-signal-characteristics-correcting coefficient and a reception-signal-characteristics-correcting coefficient through the processes of detecting a correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; and correcting, according to the reception-signal-characteristics-correcting coefficient, at least one of a frequency characteristic and a spatial frequency characteristic of the reception signal; and in the transmitter of each of the base station and terminal; correcting, according to the transmission-signal-characteristics-correcting coefficient, at least one of a transfer function and a spatial frequency characteristic of a transmission signal.  
           [0031]    In a preferred embodiment of the present invention, the operation of correcting the transmission signal includes: convoluting the transmission signal with the transmission-signal-characteristics-correcting coefficient.  
           [0032]    In a preferred embodiment of the present invention, the operation of correcting the transmission signal includes: changing, according to the transmission-signal-characteristics-correcting coefficient, the frequency characteristics of a signal to be transmitted from an antenna.  
           [0033]    In a preferred embodiment of the present invention, the operation of correcting the transmission signal includes: changing, according to the transmission-signal-characteristics-correcting coefficient, the phase distributions of signals to be transmitted from antenna elements.  
           [0034]    In a preferred embodiment of the present invention, the operation of correcting the transmission signal includes: changing, according to the transmission-signal-characteristics-correcting coefficient, the frequency characteristics and phase distributions of a signal to be transmitted from an antenna.  
           [0035]    In a preferred embodiment of the present invention, the operation of correcting the reception signal includes: convoluting the reception, signal with the reception-signal-characteristics-correcting coefficient.  
           [0036]    In a preferred embodiment of the present Invention, the operation of correcting the reception signal includes: changing, according to the reception-signal-characteristics-correcting coefficient, the frequency characteristic of the reception signal.  
           [0037]    In a preferred embodiment of the present invention, the operation of correcting the reception signal includes: carrying out, according to the reception-signal-characteristics-correcting coefficient, weight and add operations on each of signals received by antenna elements.  
           [0038]    In a preferred embodiment of the present invention, the operation of correcting the reception signal includes: changing, according to the reception-signal-characteristics-correcting coefficient, the frequency characteristics of signals received by antenna elements and adding the frequency-characteristic-changed signals to one another.  
           [0039]    In a preferred embodiment of the present invention, the communication method further comprises: in the transmitter: storing training signals for antenna elements, respectively; and transmitting the training signals from the antenna elements; and in the receiver: storing the same training signals as those stored in the transmitter; and In the operation of computing at least one of the signal-characteristics-correcting coefficients, finding correlation values between the training signals and signals received by antenna elements, and carrying out eigenvalue decomposition on the product matrix obtained by multiplying the correlation matrix having the found correlation values as elements and the transported matrix of the correlation matrix together.  
           [0040]    In a preferred embodiment of the present invention, the communication method further comprises: in the transmitter: spreading an information signal according to different code words for the antenna elements, respectively; and in the receiver: In the operation of computing at least one of signal-characteristics-correcting coefficients, branching each of signals received by antenna elements, finding correlation values between the branched signals and the code words sent from the transmitter, and carrying out eigenvalue decomposition on the product matrix obtained by multiplying the correlation matrix having the found correlation values as elements and the transported matrix of the correlation matrix together.  
           [0041]    Further, to achieve the above object, there is provided a communication system employing a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, comprising: the transmitter having: coefficient detection means for detecting a transmission-signal-characteristics-correcting coefficient sent from the receiver; and transmission signal correction means for correcting, according to the transmission-signal-characteristics-correcting coefficient, at least one of a transfer function and a spatial frequency characteristic of a transmission signal; and the receiver having: coefficient computation means for the transmission-signal-characteristics-correcting coefficient and a reception-signal-characteristics-correcting coefficient through the processes of detecting correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; reception signal correction means for correcting, according to the reception-signal-characteristics-correcting coefficient, at least one of a frequency characteristic and a spatial frequency characteristic of a reception signal; and coefficient transmission means for transmitting the transmission-signal-characteristics-correcting coefficient to the transmitter.  
           [0042]    Further, to achieve the above object, there is provided a communication system employing a base station and a terminal each having a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, comprising: in one of the base station and terminal: the receiver having: coefficient computation means for computing a reception-signal-characteristics-correcting coefficient through the processes of detecting a correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together: and reception signal correction means for correcting, according to the reception-signal-characteristics-correcting coefficient, at leas one of a frequency characteristic and a spatial frequency characteristic of the reception signal; and in the other of the bass station and terminal: the receiver having: coefficient computation means for computing a transmission-signal-characteristics-correcting coefficient through the processes of detecting a correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; and the transmitter having: transmission signal correction means for correcting, according to the transmission-signal-characteristics-correcting coefficient, at least one of a transfer function and a spatial frequency characteristic of a transmission signal.  
           [0043]    Further, to achieve the above object, there is provided a communication system employing a base station and a terminal each having a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, comprising: the receiver of each of the base station and terminal having: coefficient computation means for computing a transmission-signal-characteristics-correcting coefficient and a reception-signal-characteristics-correcting coefficient through the process of detecting a correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; and reception signal correction means for correcting, according to the reception-signal-characteristics-correcting coefficient, at least one of a frequency characteristic and a spatial frequency characteristic of the reception signal; and the transmitter of each of the base station and terminal having: transmission signal correction means for correcting, according to the transmission-signal-characteristics-correcting coefficient, at least one of a transfer function and a spatial frequency characteristic of a transmission signal.  
           [0044]    In a preferred embodiment of the present invention, the transmitter has: antenna elements: training signal storage means for storing training signals for signals to be transmitted from the antenna elements, respectively; and training signal transmission means for transmitting the training signals; and the receiver has: antenna elements; training signal storage means for storing the same training signals as those stored in the transmitter; and the coefficient computation means computing the transmission-signal-characteristics-correcting coefficient and the reception-signal-characteristics-correcting coefficient through the processes of finding correlation values between the training signals and signals received by antenna elements, and carrying out eigenvalue decomposition on the product matrix obtained by multiplying the correlation matrix having the found correlation values as elements and the transported matrix of the correlation matrix together.  
           [0045]    In a preferred embodiment of the present invention, the transmitter has: antenna elements: and spread means for spreading an information signal according to different code words for the antenna elements, respectively; and the receiver has: antenna elements; signal branch means for branching each of signals received by the antenna elements; and the coefficient computation means for computing the transmission-signal-characteristics-correcting coefficient and the reception-signal-characteristics-correcting coefficient through the processes of finding correlation values between the branched signals and the code words sent from the transmitter, and carrying out eigenvalue decomposition on the product matrix obtained by multiplying the correlation matrix having the found correlation values as elements and the transported matrix of the correlation matrix together.  
           [0046]    Further, to achieve the above object, there is provided, in a communication system employing a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, the transmitter comprising: coefficient detection means for detecting a transmission-signal-characteristics-correcting coefficient sent from the receiver; and transmission signal correction means for correcting, according to the transmission-signal-characteristics-correcting coefficient, at least one of a transfer function and a spatial frequency characteristic of a transmission signal,  
           [0047]    Further, to achieve the above object, there is provided, in a communication system employing a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, the transmitter comprising: transmission signal correction means for correcting, according to a transmission-signal-characteristics-correcting coefficient found by the receiver, at least one of a transfer function and a spatial frequency characteristic of a transmission signal.  
           [0048]    In a preferred embodiment of the present invention, the transmission signal correction means convolutes the transmission signal with the transmission-signal-characteristics-correcting coefficients  
           [0049]    In a preferred embodiment of the present invention, the transmitter further comprises: antenna elements; and the transmission signal correction means changing, according to the transmission-signal-characteristics-correcting coefficient, the frequency characteristics of signals to be transmitted from the antenna elements.  
           [0050]    In a preferred embodiment of the present invention, the transmitter further comprises: antenna elements; and the transmission signal correction means changing, according to the transmission-signal-characteristics-correcting coefficient, the phase distributions of signals to be transmitted from the antenna elements.  
           [0051]    In a preferred embodiment of the present invention, the transmitter further comprising: antenna elements; and the transmission signal correction means changing, according to the transmission-signal-characteristics-correcting coefficient, the frequency characteristics and phase distributions of signals to be transmitted from the antenna elements.  
           [0052]    In a preferred embodiment of the present invention, the transmitter further comprises: training signal storage means for storing training signals for signals to be transmitted from the antenna elements, respectively; and training signal transmission means for transmitting the training signals.  
           [0053]    In a preferred embodiment of the present invention, the transmitter further comprises: spread means for spreading an information signal according to different code words for the antenna elements, respectively.  
           [0054]    Further, to achieve the above object, there is provided, in a communication system employing a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, the receiver comprising; coefficient computation means for computing a transmission-signal-characteristics-correcting coefficient and a reception-signal-characteristics-correcting coefficient through the processes of detecting correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; reception signal correction means for correcting, according to the reception-signal-characteristics-correcting coefficient, at least one of a frequency characteristic and a spatial frequency characteristic of the reception signal; and coefficient transmission means for transmitting the transmission-signal-characteristics-correcting coefficient to the transmitter.  
           [0055]    Further, to achieve the above object, there is provided, in a communication system employing a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, the receiver comprising; coefficient computation means for computing a reception-signal-characteristics-correcting coefficient through the processes of detecting a correlation of a reception Signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together; and reception signal correction means for correcting, according to the reception-signal-characteristics-correcting coefficient, at least one of a frequency characteristic and a spatial frequency characteristic of the reception signal,  
           [0056]    Further, to achieve the above object, there is provided, in a communication system employing a transmitter and a receiver, for carrying out signal processing adapted for transmission-channel characteristics, the receiver comprising: coefficient computation means for computing a transmission-signal-characteristics-correcting coefficient through the processes of detecting a correlation of a reception signal, and carrying out eigenvalue decomposition on a product matrix obtained by multiplying a correlation matrix having the detected correlation as elements and a transported matrix of the correlation matrix together.  
           [0057]    In a preferred embodiment of the present invention, the reception signal correction means convolutes the reception signal with the reception-signal-characteristics-correcting coefficient.  
           [0058]    In a preferred embodiment of the present invention, the receiver further comprises: antenna elements: and the reception signal correction means carrying out, according to the reception-signal-characteristics-correcting coefficient, weight and add operations on each of signals received by the antenna elements.  
           [0059]    In a preferred embodiment of the present invention, the receiver further comprises: antenna elements; and the reception signal correction means changing, according to the reception-signal-characteristics- correcting coefficient, the frequency characteristics of signals received by the antenna elements and adding the frequency-characteristic-changed signals to one another.  
           [0060]    In a preferred embodiment of the present invention, the receiver further comprises: antenna elements; training signal storage means for storing the same training signals as those stored in the transmitter: and the coefficient computation means for computing the transmission-signal-characteristics-correcting coefficient and the reception-signal-characteristics-correcting coefficient through the processes of finding correlation values between the training signals and signals received by antenna elements, and carrying out eigenvalue decomposition on the product matrix obtained by multiplying the correlation matrix having the found correlation values as elements and the transported matrix of the correlation matrix together.  
           [0061]    In a preferred embodiment of the present invention, the receiver further comprising: antenna elements; and the coefficient computation means for computing the transmission-signal-characteristics-correcting coefficient and the reception-signal-characteristics-correcting coefficient through the processes of branching each of signals received by the antenna elements, finding correlation values between the training signals and signals received by antenna elements, and carrying out eigenvalue decomposition on the product matrix obtained by multiplying the correlation matrix having the found correlation values as elements and the transported matrix of the correlation matrix together.  
           [0062]    The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0063]    In the accompanying drawings:  
         [0064]    [0064]FIG. 1 is a block diagram showing a communication system according to a related art;  
         [0065]    [0065]FIG. 2 is a block diagram showing an adaptive equalizer according to the related art;  
         [0066]    [0066]FIG. 3 is a block diagram showing a diversity reception system according to a related art;  
         [0067]    [0067]FIG. 4 is a block diagram showing a transmission-diversity reception system according to a related art;  
         [0068]    [0068]FIG. 5 is a block diagram showing an adaptive array reception system according to a related art;  
         [0069]    [0069]FIG. 6 is a block diagram showing an adaptive array according to the related art of FIG. 5;  
         [0070]    [0070]FIG. 7 is a block diagram showing a dual equalizing communication system according to a related art;  
         [0071]    [0071]FIG. 8 Is a block diagram showing a tapped delay line filter according to the related art of FIG. 7;  
         [0072]    [0072]FIG. 9 is a block diagram showing a basic configuration of a communication system according to the present invention;  
         [0073]    [0073]FIG. 10 is a block diagram showing a dual beam forming system according to a first embodiment of the present invention;  
         [0074]    [0074]FIG. 11 is a block diagram showing a transmission beam forming unit according to the first embodiment;  
         [0075]    [0075]FIG. 12 is a block diagram showing a dual beam forming system employing spatio-temporal encoding according to a second embodiment of the present invention;  
         [0076]    [0076]FIG. 13 is a block diagram showing a dual spatio-temporal beam forming system according to a third embodiment of the present invention;  
         [0077]    [0077]FIG. 14 is a block diagram showing a spatio-temporal beam forming unit according to the third embodiment; and  
         [0078]    [0078]FIG. 15 is a block diagram showing a spatio-temporal transmission beam forming unit according to the third embodiment. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0079]    [0079]FIG. 1 is a block diagram showing a basic configuration of a communication system according to the present invention. The system includes a signal input terminal  1 , a transmission signal corrector  2  to correct the transfer function and spatial frequency characteristic of a transmission signal, a transmission channel  3 , a reception signal corrector  4  to correct the transfer function and spatial frequency characteristic of a reception signal, an output terminal  5 , and a coefficient estimator  6  to estimate coefficients for the correctors  2  and  4 .  
         [0080]    The coefficient for the transmission signal corrector  2  is represented with w r  and the coefficient for the reception signal corrector  4  with w r . Under the restrictive condition of the expression (1) , a solution to maximize a reception signal level is a solution of the expression (5). In practice, the solution is obtained by solving the eigenvalue problems of the expressions (8) and (9).  
         [0081]    The left side of the expression (8) is multiplied by a vector w t   T  as follows:  
           w   t   T   Aw   r   w   l   H   A   H   w   l               =λ l   w   t   l   w   l               =λ l    (10)  
         [0082]    Similarly, the left side of the expression (9) is multiplied by a vector w r   T  as follows;  
           w   l   T   A   T   w   l   w   r   H   A                 w   r               =λ r   w   r   T   w   l               =λ r    (11)  
         [0083]    Due to the expression (1), the expressions (10) and (11) have the following relationships:  
                       w   r   T          Aw   r          w   r   H          A   H          w   i   *       =       (       w   i   T          Aw   r       )            (       w   r   T          A   T          w   i       )     *                   =       (       w   r   T          A   T          w   t       )            (       w   r   T          A   T          w   i       )     *                   =              w   r   T          Aw   r            2                   (   12   )                         w   r   T          A   T          w   i          w   i   H          A   H          w   r   *       =       (       w   r   T          Aw   i       )            (       w   i   T          Aw   r       )     *                   =       (       w   i   T          Aw   r       )            (       w   i   T          Aw   r       )     *                   =              w   i   T          Aw   r            2                   (   13   )                               
 
         [0084]    Naturally, w r   T A T w r =w r   T Aw r . Accordingly, the right aides of the expressions (12) and (13) are equal to each other.  
         λ r =λ r =λ  (14)  
         [0085]    Then, the following is obtained:  
           w   r   T   A   T   w   l   =w   t   T   Aw   r ={square root}{square root over (λ)}  (15)  
         [0086]    The expression (15) is substituted Into the expression (8) as follows;  
           Aw   r   w   r   H   A   H   w   l                 =Aw   r ( w   r   T   A   T   w   t )               =λw   t                 (16.1)  
         [0087]    Then, the following is obtained:  
           Aw   l   ={square root}{square root over (λw l               )}   (16.2)  
         [0088]    The expression (15) is substituted into the expression (9) as follows:  
           A   T   w   t   w   l   H   A                 w   r                 =A   T   w   t ( w   r   T   Aw   r )               =λw   r                 (17.1)  
         [0089]    Then, the following is obtained:  
           A   T   w   l   ={square root}{square root over (λw l               )}   (17.2)  
         [0090]    The complex conjugate of w t  in the expression (16.2) is substituted into the expression (17.2) as follows:  
                 A   T          w   r       =         1     λ            A   T          A   *          w   r   *       =       λ          w   r   *                 (   18.1   )                               
 
         [0091]    Then, the following is obtained:  
           A   T   A                 w   l                 =λw   r                 (18.2)  
         [0092]    The complex conjugate of w r  in the expression (17.2) is substituted into the expression (16.2) as follows;  
               Aw   r     =         1     λ            AA   H          w   i   *       =       λ          w   i   *                 (   19.1   )                               
 
         [0093]    Then, the following is obtained:  
           AA   H   w   l                 =λw   l                 (19.2)  
         [0094]    The coefficient w t  for the transmission signal corrector  2  and the coefficient w r  for the reception signal corrector  4  are obtained by solving the eigenvalue problems of the expressions (18.2) and (19.2). Namely, the solutions can be obtained only by solving the eigenvalue problems expressed with the expressions (18.2) and (19.2).  
         [0095]    In this way, the present invention is capable of surely finding the solutions only by solving eigenvalue problems twice. The present invention can stably provide the solutions and greatly reduce the number of operations compared with the related arts in practice, the eigenvalue problems are solved according to, for example, Jacobi method and QR method.  
         [0096]    Various embodiments of the present Invention will be explained in detail with reference to the accompanying drawings. Systems described in the embodiments are based on the principle mentioned above. When the present invention is applied to the dual equalizing communication system of FIG. 7, solutions of the expressions (18.2) and (19.2) serve as tap coefficients for the tapped delay lines on the transmission and reception sides.  
         [0097]    [0097]FIG. 10 is a block diagram showing an exemplary configuration of a dual beam forming system according to the first embodiment of the present invention. The system includes reception signal output terminals  101  and  146 , a signal input terminal  123 , demodulators  102  and  145 , modulators  121  and  148 , transmission beam forming units  120  and  144 , adaptive arrays  103  and  143 , frequency converters  104  to  109  and  137  to  142 , antenna sharing units  110  to  112  and  134  to  136 , antennas  113  to  115  and  131  to  133 , switching circuits  116  to  118 , a training sequence input terminal  119 , a coefficient computation unit  147  for computing coefficients or weights for the adaptive array  143  and transmission beam forming unit  144 , and a coefficient setter  122  for setting the coefficients or weights, which have been computed by the coefficient computation unit  147  and transmitted, for the adaptive array  103  and transmission beam forming unit  120 .  
         [0098]    The coefficient or weight for the transmission beam forming units  120  and  144  is w t  and the coefficient or weight for the adaptive arrays  103  and  143  is w r . Then, a reception signal y(t) is expressed as follows:  
                     y        (   t   )       =         w   i   T          x        (   t   )         =       w   i   T          (       Aw   r          u        (   t   )         )                     =         w   i   T          Aw   r          u        (   t   )         =       w   r   T          A   I          w   t          u        (   t   )                         (   20   )                               
 
         [0099]    where u(r) is a transmission signal expressed in scalar.  
         [0100]    Like the expression (1), the following restrictive condition is set:  
         &lt; u               ( t ) u ( t )&gt;=1   (21)  
         [0101]    The restrictive condition of the expression (21) is employed instead of the restrictive condition of the expression (1) related to a transmission signal vector. The weights w t  and w r  to maximize a reception signal level are obtainable according to the expressions (6) and (7) whose solutions are provided by the expressions (18.2) and (19.2). In this case, the transfer function matrix A of the expression (3) becomes as follows:  
             A   =     [           a     0   ,   0             a     0   ,   1           ⋯         a     0   ,     M   -   1                   a     1   ,   0             a     1   ,   1           ⋯         a     1   ,     M   -   1                 ⋮       ⋰       ⋰       ⋮             a       N   -   1     ,   0             a       N   -   1     ,   1           ⋯         a       N   -   1     ,     M   -   1               ]             (   22   )                               
 
         [0102]    where N and M are the numbers of transmission and reception antennas, respectively, and a ij  is response between a left “i”th antenna and a right “j”th antenna.  
         [0103]    To obtain response between the transmission and reception antennas, the system of FIG. 10 employs a training sequence. A training sequence[b 1.1 .b 1.2 , . . . , b 1.I2 ] (i 1, . . . , N−1) is prepared for each antenna, where Ls is a training sequence length. Each training sequence is provided with orthogonal vectors. Namely, each training sequence satisfies the following;  
                 ∑     k   =   1     LS            b     i   ,   k            b     i   ,   k     *         =     {             Ls                 i     =   l                 0                 i     ≠   l                     (   23   )                               
 
         [0104]    During a training sequence transmission period, the switching circuits  116  to  118  select the training signals, and the antennas transmit the different training signals, respectively. A “j”th antenna receives the following signal y j (t) if the influence of noise is ignored:  
               y     j        (   t   )         =       ∑     i   =   0       N   -   1                         ∑     k   =              1       L   s                         a     t   ,   j            b     1   ,   1            h        (     t   -     k                 T       )                     (   24   )                 j   =   1     ,   …   ,   M                             l   =   1     ,     ...   N                                             
 
         [0105]    where T is symbol cycle and h(r) a transfer function of a waveform shaping filter.  
         [0106]    Array response is estimated according to the following correlation operation:  
                 ∑     n   =              1       L   s              b     1   ,     s            y   j          (     t   +     n                 T       )           =           (   25   )                 ∑     k   =              1       L   s              b     1   ,     s          y   j            ∑     i   =   0       N   -   1              ∑     k   =   0       L   s              a     i   ,   j            b     i   ,   k            h        (       (     n   -   k     )        T     )                                         j   =   1     ,   ...   ,   M                             l   =   1     ,   ...   ,   N                                           
 
         [0107]    If the waveform shaping filter is a Nyquist filter employed by, for example, a QPSK modulation system, it satisfies the following:  
               h   (     k                 T                )     =     {           1           0                    k   =   0               k   ≠   0                       (   26   )                               
 
         [0108]    According to the expression (26), the expression (25) is written as follows:  
                 ∑     n   =              1       L   s              b                  l   .   a       s            y   j          (     t   +     n                 T       )           =           (   27   )                   ∑     i   =   0       N   -   1              ∑     k   =   0       L   s              a     i   ,   j            b     i   ,   k             =         L                s     |     b     l   ,   k            |   2          a     l        .   j                                           j   =   1     ,   ...   ,   M                 l   =   1     ,   ...   ,   N                                                 
 
         [0109]    A correlation operation based on the expression (27) provides array response and generates a transfer function matrix.  
         [0110]    The system of FIG. 10 transmits the training signals during a training period. The coefficient computation unit  147  finds a transfer function matrix according to the expression (27). According to the found transfer function matrix, the coefficients w t  and w r  are estimated. After the training period, the right transmitter-receiver generates a transmission beam and reception beam and transmits the coefficient w t  and w r . The left receiver demodulates one of signals from the frequency converters  104  to  106  instead of carrying out an adaptive array process and stores the coefficients contained in the signal in the coefficient setter  122 .  
         [0111]    According to the transmitted coefficients w r  and w r , the transmission beam forming unit  120  and adaptive array  103  are operated to achieve dual beam forming communication.  
         [0112]    In the system of FIG. 10, the coefficient computation unit  147  is arranged only in the right transmitter-receiver, and the coefficients w t  and w r  computed by the coefficient computation unit  147  are transmitted to the left receiver. In the left receiver, the coefficient setter  122  extracts the coefficients w t  and w r  from a demodulated signal and sets the extracted coefficients for the transmission beam forming unit  120  and adaptive array  103 . Instead, each of the left and right transmitter-receivers may have a coefficient computation unit to form dual beams.  
         [0113]    [0113]FIG. 11 shows an exemplary configuration of any one of the transmission beam forming units  120  and  144 . The transmission beam forming unit includes a signal input terminal  300 , multipliers  301  to  304 , signal output terminals  305  to  308 , and coefficient input terminals  309  to  312 .  
         [0114]    [0114]FIG. 12 is a block diagram showing a dual beam forming system employing spatio-temporal encoding according to the second embodiment of the present invention. The system employs the dual beam forming system of the first embodiment shown in FIG. 10. The system of FIG. 12 includes demodulated signal output terminals  151  and  198 , a signal input terminal  174 , demodulators  152  and  197 , modulators  173  and  202 , transmission beam forming units  172  and  203 , adaptive arrays  153  and  196 , despread circuits  154  to  156  and  193  to  195 , spread circuits  169  to  171  and  204  to  206 , frequency converters  157  to  162  and  187  to  192 , antenna sharing units  163  to  165  and  184  to  186 , antennas  166  to  168  and  381  to  183 , delay elements  175 ,  199 , and  200 , a coefficient computation unit  201 , and a coefficient setter  176 .  
         [0115]    Generally, a spatio-temporal encoding system carries out a spreading process with a different spread code for each antenna. Accordingly, a receiver carries out the correlation operation (despreading operation) of the expression (25) to estimate a transfer function matrix in real time. Namely, dual beams are formed without a training sequence.  
         [0116]    [0116]FIG. 13 is a block diagram showing a dual spatio-temporal beam forming system according to the third embodiment of the present invention. The system employs the dual beam forming system of the first embodiment shown in FIG. 10.  
         [0117]    The system of FIG. 13 includes demodulated signal output terminals  211  and  254 , a signal input terminal  232 , demodulators  212  and  253 , modulators  231  and  256 , spatio-temporal beam forming units  213  and  252 , spatio-temporal transmission beam forming units  230  and  257 , frequency converters  214  to  219  and  246  and  251 , antenna sharing units  220  to  222  and  243  to  245 , antennas  223  to  225  and  240  to  242 , switching circuits  226  to  228 , a training sequence input terminal  229 , a coefficient computation unit  255 , and a coefficient setter  233 .  
         [0118]    [0118]FIG. 14 shows an exemplary configuration of any one of the spatio-temporal beam forming units  213  and  252  with two antennas. The spatio-temporal beam forming unit includes input terminals  320  and  330 , delay elements  321  to  323  and  331  to  333 , multipliers  324  to  327  and  334  to  337 , adders  328  and  329 , an adder  338 , an adaptive controller  339  to provide the multipliers  324  to  327  and  334  to  337  each with a coefficient as another input signal, and a signal output terminal  340 .  
         [0119]    [0119]FIG. 15 shows an exemplary configuration of any one of the spatio-temporal transmission beam forming units  230  and  257  with two antennas. The spatio-temporal transmission beam forming unit includes a signal input terminal  351 , delay elements  352  to  357 , multipliers  358  to  361  and  367  to  370 , adders  362  and  363 , signal output terminals  364  and  365 , and a coefficient input terminal  350  to provide each another input of the multipliers  358  to  361  and  367  to  370  with a coefficient.  
         [0120]    In the system of FIG. 13, an “i”th antenna provides an input signal x i (t). A reception signal vector x(t) is expressed as x(t)=[x 1 (r), x 1 (t-T), . . . , x 1 (t), . . . , x M (t−(L r −1)T)] T . Similarly,a transmission signal vectors(r) is defined as s(t)=[u(t), u(t−T), . . . , u(t−(L t −1)T), u(t), . . . , u(t−(L r −1)T), . . . ] T . As a result, transmission and reception signals are expressed with the expressions (1), (2), and (4). A transfer function matrix may slightly be different but it can be estimated with the use of a training sequence as mentioned above.  
         [0121]    In this way, the present invention can provide a dual spatio-temporal beam forming communication system.  
         [0122]    The present invention is capable of optimizing communication quality irrespective of transmission-channel characteristics and maximizing the transmission-channel characteristics. In particular, the present invention is capable of stabilizing and optimizing communication even under widely varying transmission environment such as mobile communication environment. The present invention is capable of estimating coefficients at high speed, to speedily follow changes in a transmission channel. Even with high-speed Doppler variations occurring in, for example, a bullet train cabin, the present invention is capable of achieving high-quality communication.  
         [0123]    The present invention is capable of improving voice quality, data transmission throughput, and communication service quality.  
         [0124]    It should be understood that many modifications and adaptations of the invention will become apparent to those skilled in the art and it is intended to encompass such obvious modifications and changes in the scope of the claims appended hereto.