Patent Publication Number: US-2016248495-A1

Title: Radio Communication Apparatus and Radio Communication Method

Description:
TECHNICAL FIELD 
     The present invention relates to a radio communication apparatus and a radio communication method that are used in a radio communication system. 
     BACKGROUND ART 
     In recent years, radio transmission technologies that employ a plurality of antennas are being investigated to effectively use radio frequencies. As one example, there is an adaptive array antenna technology that adaptively controls the directivity formed by the entirety of a plurality of antennas by adjusting the amplitude and phase of the signal processed at each antenna. If the amplitude and phase values are set based on the channels state, power can be radiated focused in the direction of good channel quality and communication quality can be improved. In this technology, the beam width of directivity can be narrowed and the degree of concentration of power that is radiated in a specific direction can be increased in proportion to the number of antennas employed. 
     Information relating to channels is necessary for forming directivity, and there are two methods of acquiring this information by a transmitter: in the first method, the state of the channels are estimated by a transmitter, and in the second method, the state of the channel is estimated by a receiver and then the estimated result is reported to the transmitter. Regardless of the method used, there is a time difference between estimating the state of the channels and executing the transmission that uses the directivity based on the estimated result. If the direction of good channel quality should fluctuate during this time difference, the direction of the main lobe of directivity that was formed at the time of transmission shifts from the direction of good channel quality and the communication quality is therefore degraded compared to the case in which directivity was able to be formed ideally based on the state of the channels at the time of transmission. In particular, when the beam width of directivity is narrow, the shift in direction is not contained within the beam width and tends to orient in the null or side lobe of directivity in the direction of good channel quality, and the amount of degradation of communication quality therefore tends to increase. As a countermeasure, a method can be considered for increasing the frequency of estimating the channels state to use information relating to channels that is as recent as possible, but this method is not preferable due to the increase in the amount of computation. 
     A method is investigated in Patent Document 1 for controlling the number of antennas that are used based on the degree of fluctuation of the reception power of signals exchanged between transmission/reception apparatuses in order to suppress deterioration of the communication quality due to fluctuation of the direction of good channel quality without increasing the frequency of estimating the channels state. In this method, the fluctuation of the direction of good channel quality is determined to be great when there is a large degree of fluctuation of reception power and the number of antennas used is decreased. By decreasing the number of antennas, the degree of concentration of power in a specific direction falls, but because the beam width of directivity broadens, major deterioration of the communication quality due to fluctuation of the direction of good channel quality is avoided. 
     LITERATURE OF THE PRIOR ART 
     Patent Documents 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-278076 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     Nevertheless, the fluctuation of the direction of good channel quality is not necessarily also great when the fluctuation of received power is great. For example, the received power may fluctuate greatly when the angular spread of a channel is small, but the range of fluctuation of the direction of good channel quality is small. As a result, in the method described in Patent Document 1, the fluctuation of the direction of good channel quality cannot be accurately estimated, and the amount of degradation of the communication quality is great compared to a case in which directivity can be formed ideally based on the state of the channels at the time of transmission. 
     It is therefore an object of the present invention to provide a radio communication apparatus and a radio communication method that enable both an accurate estimation of the fluctuation of the direction of good channel quality and a reduction in the amount of deterioration of communication quality for a case in which directivity can be ideally formed based on the channels state at the time of transmission. 
     Means for Solving the Problem 
     The radio communication apparatus according to the present invention is a radio communication apparatus that is provided with a plurality of antennas and includes:
     a channel information acquisition unit that acquires information relating to channels with another radio communication apparatus;   an index calculation unit that uses the information to calculate indices relating to the angular spread of the channels;   a weighting factor generation unit that uses the information and the indices to generate weighting factors corresponding to each of the plurality of antennas; and   a weighting factor multiplication unit that multiplies the signals that are processed by each of the plurality of antennas by the weighting factors that correspond to antennas that process the signals.   

     The radio communication method according to the present invention is a radio communication method in a radio communication apparatus that is provided with a plurality of antennas and includes steps of:
     acquiring information relating to channels with another radio communication apparatus;   

     using the information to calculate indices relating to the angular spread of the channels; 
     using the information and the indices to generate weighting factors corresponding to each of the plurality of antennas; and 
     multiplying the signals that are processed at each of the plurality of antennas by the weighting factors corresponding to the antennas that process the signals. 
     Effect of the Invention 
     The present invention enables a reduction in the amount of deterioration of communication quality for a case in which directivity can be formed ideally based on the channels state at the time of transmission under communication conditions in which the state of the channel fluctuates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural view showing the radio communication system in the first exemplary embodiment of the present invention. 
         FIG. 2  is a block diagram showing the functional configuration of a radio base station in the first exemplary embodiment of the present invention. 
         FIG. 3  is a flow chart describing an example of the reception operation of a radio base station in the first exemplary embodiment of the present invention. 
         FIG. 4  is a flow chart describing an example of the transmission operation of a radio base station in the first exemplary embodiment of the present invention. 
         FIG. 5  is a flow chart describing an example of the operation of the weighting factor generation unit in the first exemplary embodiment of the present invention. 
         FIG. 6  is a structural view showing the radio communication system in the second exemplary embodiment of the present invention. 
         FIG. 7  is a block diagram showing the functional configuration of a radio base station in the second exemplary embodiment of the present invention. 
         FIG. 8  is a flow chart for describing an example of the operation of the weighting factor generation unit in the second exemplary embodiment of the present invention. 
         FIG. 9  is a view showing the configuration of the antennas of a radio base station in the fourth exemplary embodiment of the present invention. 
         FIG. 10  is a view describing an example of setting weighting factors in the fourth exemplary embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Exemplary embodiments of the present invention are next described while referring to the accompanying drawings. Although the radio communication system that is described in each of the following exemplary embodiments is assumed to conform to the OFDM (Orthogonal Frequency Division Multiplexing) mode, the present invention can be applied to radio communication systems that conform to other communication modes. 
     (I) First Exemplary Embodiment 
     (1.1) Explanation of Configuration 
       FIG. 1  is a structural view showing the radio communication system in the first exemplary embodiment of the present invention. As shown in  FIG. 1 , the radio communication system in the present exemplary embodiment includes radio base station  100  and radio terminal  200 . 
     Radio base station  100  and radio terminal  200  are each assumed to be equipped with antennas  101 - 1 - 101 -M and antennas  201 - 1 - 201 -N, respectively. Here, M is an integer equal to or greater than 2 and N is an integer equal to or greater than 1 and satisfies the relation M≧N. 
     Radio base station  100  controls the directivity that is formed by antennas  101 - 1 - 101 -M when transmitting a signal addressed to radio terminal  200  according to the state of the channels with radio terminal  200 . 
       FIG. 2  is a block diagram showing the functional configuration of radio base station  100  in the present exemplary embodiment. As shown in  FIG. 2 , radio base station  100  in the present exemplary embodiment has: antennas  101 - 1 - 101 -M, radio transmission/reception units  102 - 1 - 102 -M, GI (Guard Interval) removal units  103 - 1 - 103 -M, FFT (Fast Fourier Transform) units  104 - 1 - 104 -M, channel information acquisition unit  105 , index calculation unit  106 , weighting factor generation unit  107 , encoding unit  108 , modulation unit  109 , weighting factor multiplication unit  110 , IFFT (Inverse Fast Fourier Transform) units  111 - 1 - 111 -M, and GI insertion units  112 - 1 - 112 -M. 
     Each of antennas  101 - 1 - 101 -M receives a radio frequency signal that was transmitted by radio terminal  200 . 
     Each of radio transmission/reception units  102 - 1 - 102 -M corresponds to a respective antenna of antennas  101 - 1 - 101 -M and converts the reception signal that is the signal received by the corresponding antenna to a baseband signal. 
     Each of GI removal units  103 - 1 - 103 -M corresponds to a respective radio transmission/reception unit of radio transmission/reception units  102 - 1 - 102 -M and removes GI from the reception signal that was converted to a baseband signal in the corresponding radio transmission/reception unit. 
     Each of FFT units  104 - 1 - 104 -M corresponds to a respective GI removal unit of GI removal units  103 - 1 - 103 -M, carries out a FFT upon the received signal from which GI has been removed by a corresponding GI removal unit, and converts the reception signal to the frequency domain signal. 
     Channel information acquisition unit  105  uses the plurality of reception signals that have been converted to signals of a frequency band in each of FFT units  104 - 1 - 104 -M to acquire channel information that is information relating to the channels between radio base station  100  and radio terminal  200  that is another radio communication apparatus. The channel information is, for example, the channels frequency responses between each of antennas  101 - 1 - 101 -M of radio base station  100  and each of antennas  201 - 1 - 201 -N of radio terminal  200 . 
     As methods for acquiring channel information, there are a first method in which radio base station  100  estimates the states of the channels and a second method in which radio base station  100  receives the states of the channels that were estimated by radio terminal  200 . The appropriate method is used by taking into account factors such as the amount of computation required to acquire the channel information and the accuracy of the channel information. 
     Index calculation unit  106  uses the channel information that was acquired by channel information acquisition unit  105  to calculate an index relating to the angular spread of the channels. The index is not assumed to be the channel angular spread itself but a value that can be calculated by an amount of computation that is smaller than the actual angular spread of the channels. The index will be later explained in detail. 
     Weighting factor generation unit  107  uses the channel information that were acquired by channel information acquisition unit  105  and the indices that were calculated by index calculation unit  106  to generate weighting factors corresponding to each of antennas  101 - 1 - 101 -M. A detailed explanation will be later given regarding the method of generating the weighting factors. 
     Encoding unit  108  encodes transmission data addressed to radio terminal  200 . No particular limitations apply to the encoding method. 
     Modulation unit  109  modulates the transmission data that have been encoded in encoding unit  108 . The modulation method is assumed to be a digital modulation method such as Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). 
     Weighting factor multiplication unit  110 , after duplicating the modulated signals that were generated in modulation unit  109  to generate signals that are to be processed at each of antennas  101 - 1 - 101 -M, multiplies each of these signals by the weighting factors corresponding to each of antennas  101 - 1 - 101 -M that were generated in weighting factor generation unit  107 . 
     Each of IFFT units  111 - 1 - 111 -M, corresponding to each of antennas  101 - 1 - 101 -M, carries out an IFFT upon, from among the transmission signals that have been multiplied by weighting factors in weighting factor multiplication unit  110 , the transmission signal that is to be processed at the corresponding antenna. 
     Each of GI insertion units  112 - 1 - 112 -M, corresponding to IFFT units  111 - 1 - 111 -M, inserts a GI in the transmission signal that has undergone the inverse fast Fourier transform in the corresponding IFFT unit. 
     Radio transmission/reception units  102 - 1 - 102 -M, corresponding to each of GI insertion units  112 - 1 - 112 -M, converts the transmission signals into which GI has been inserted by a corresponding GI insertion unit to a signal of a radio frequency band. 
     Each of antennas  101 - 1 - 101 -M transmits a transmission signal that has been converted to a signal of a radio frequency band in the radio transmission/reception unit that corresponds to that antenna. 
     (1.2) Explanation of the Operation 
       FIG. 3  is a flow chart describing an example of the reception operation in radio base station  100 . 
     Each of antennas  101 - 1 - 101 -M of radio base station  100  first receives a signal of a radio frequency band that was transmitted from each of antennas  201 - 1 - 201 -N of radio terminal  200  and supplies the signal as a reception signal to the radio transmission/reception unit that corresponds to that antenna (Step S 301 ). 
     Each of radio transmission/reception units  102 - 1 - 102 -M receives the reception signal from the antenna that corresponds to that radio transmission/reception unit, converts the reception signal to a baseband signal, and supplies the baseband signal to the GI removal unit that corresponds to that radio transmission/reception unit (Step S 302 ). 
     Each of GI removal units  103 - 1 - 103 -M receives the reception signal from the radio transmission/reception unit that corresponds to that GI removal unit, removes the GI from the reception signal, and supplies the reception signal from which the GI was removed to the FFT unit that corresponds to that GI removal unit (Step S 303 ). 
     Each of FFT units  104 - 1 - 104 -M receives the reception signal from the GI removal unit that corresponds to that FFT unit, subjects the reception signal to a FFT, and supplies the reception signal that has undergone the FFT to channel information acquisition unit  105  (Step S 304 ). 
     Channel information acquisition unit  105  receives the reception signals from each of FFT units  104 - 1 - 104 -M and uses the reception signals to acquire channel information. Channel information acquisition unit  105  then supplies the channel information that was acquired to index calculation unit  106  and weighting factor generation unit  107  (Step S 305 ). 
     Index calculation unit  106  receives the channel information from channel information acquisition unit  105  and uses the channel information to calculate an index. Index calculation unit  106  then supplies the calculated index to weighting factor generation unit  107  (Step S 306 ). 
     Weighting factor generation unit  107  receives the channel information from channel information acquisition unit  105  and receives the index from index calculation unit  106 . Weighting factor generation unit  107  then uses the channel information and index that were received to generate a plurality of weighting factors corresponding to each of antennas  101 - 1 - 101 -M and supplies the plurality of weighting factors to weighting factor multiplication unit  110  (Step S 307 ), whereby the reception operation is completed. 
       FIG. 4  is a flow chart describing an example of the transmission operation in radio base station  100 . 
     Encoding unit  108  first receives transmission data addressed to radio terminal  200 , encodes the transmission data, and then supplies the encoded data to modulation unit  109  (Step S 401 ). 
     Modulation unit  109  receives the transmission data from encoding unit  108 , modulates the transmission data, and supplies the modulated data to weighting factor multiplication unit  110  (Step S 402 ). 
     Weighting factor multiplication unit  110  receives the modulated signal from modulation unit  109  and receives the weighting factors that were supplied from weighting factor generation unit  107  in Step S 307  of  FIG. 3 . Weighting factor multiplication unit  110  duplicates the modulated signal to generate transmission signals that are to be processed at each of antennas  101 - 1 - 101 -M and calculates weighting factors corresponding to the antennas that are to process the transmission signals for each of the transmission signals. Weighting factor multiplication unit  110  then supplies each of the transmission signals that were multiplied by the weighting factors to the IFFT units that correspond to the antennas that are to process the transmission signals (Step S 403 ). 
     Each of IFFT units  111 - 1 - 111 -M receives a transmission signal from weighting factor multiplication unit  110 , subjects the transmission signal to IFFT, and supplies the transmission signal that has undergone the IFFT to the GI insertion unit that corresponds to that IFFT unit (Step S 404 ). 
     Each of GI insertion units  112 - 1 - 112 -M receives a transmission signal from the IFFT unit of IFFT units  111 - 1 - 111 -M that corresponds to that GI insertion unit, inserts a GI into the transmission signal, and supplies the transmission signal into which the GI was inserted to the radio transmission/reception unit that corresponds to that GI insertion unit (Step S 405 ). 
     Radio transmission/reception units  102 - 1 - 102 -M each receive the transmission signals from corresponding GI insertion units  112 - 1 - 112 -M, convert the transmission signals to radio frequency band signals, and supply the radio frequency band signals to the corresponding antennas of the radio transmission/reception units (Step S 406 ). 
     Each of antennas  101 - 1 - 101 -M receives a radio frequency band signal from the radio transmission/reception unit that corresponds to that antenna of radio transmission/reception units  102 - 1 - 102 -M and transmits the signal (Step S 407 ), thereby completing the transmission process. 
     (1.3) Calculation of Indices 
     Actual examples of indices that relate to the angular spread of the channels that are calculated by index calculation unit  106  are next described. In each of the following examples, the index increases in proportion to an increase in the angular spread of the channel. 
     In the first example, index calculation unit  106  uses the channel information to find the correlation between any of antennas  101 - 1 - 101 -M of radio base station  100  and calculates an index based on this correlation. More specifically, if the channel frequency response between antenna  101 - m  of radio base station  100  and antenna  201 - n  of radio terminal  200  is h n,m , index calculation unit  106  calculates index p using the following Equation (1). 
     
       
         
           
             
               
                 
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     Here, Δm is an integer equal to or greater than 1 but less than M that is determined in advance. In addition, the second term on the right side of Equation (1) corresponds to the correlation between, from among antennas  101 - 1 - 101 -M of radio base station  100 , antennas that are separated by Δm. Index ρ that is calculated by equation (1) is equal to or greater than 0 and equal to or less than 1. 
     In the second example, index calculation unit  106 , based on the channel information, constructs a matrix that takes as components the frequency responses of the channels between antennas  101 - 1 - 101 -M of radio base station  100  and antennas  201 - 1 - 201 -N of radio terminal  200 , finds eigenvalues of the products of this matrix and an Hermitian transpose, and calculates indices based on these eigenvalues. More specifically, if the N×M′ matrix in which the elements of n rows and m columns are frequency responses h n,m , is assumed to be H, index calculation unit  106  uses the eigenvalue λ i  (where 1≦i≦N) of the product of matrix H and the Hermitian transpose of matrix H to calculate index ρ based on the following equation (2). 
     
       
         
           
             
               
                 
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     Eigenvalue λ i  satisfies the relation λ 1 ≧λ 2 ≧ . . . ≧λ N . In addition, the index realized by Equation (2) can be calculated only when N is at least 2. Here, M′ corresponds to the number of antennas that contribute to the formation of directivity and is an integer equal to 2 or more and no greater than M. In addition, the value M′ is not necessarily limited to one value and a plurality of indices may be computed corresponding to each of the plurality of M′ by index calculation unit  106 . 
     Although the elements of matrix H were the frequency responses corresponding to antennas  101 - 1 - 101 -M′ of radio base station  100  in the above-described example, the elements may also be the frequency responses corresponding to M′ antennas that are aligned continuously. In addition, the indices computed by Equation (2) are equal to or greater than 0 and no greater than 1. 
     In the third example, index calculation unit  106  finds vectors that take as elements the frequency responses of the channels with antennas  101 - 1 - 101 -M of radio base station  100  for each of antennas  201 - 1 - 201 -N of radio terminal  200  based on channel information, and computes indices based on the angles formed between each of the vectors. More specifically, if the M′-dimension vector for which the m th  element is frequency response h n,m  is h n , index ρ is calculated from the next Equation (3). 
     
       
         
           
             
               
                 
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     Here, the subscript n′ is: 
     
       
         
           
             
               
                 
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     In the index calculated from Equation (3), as in the index calculated from Equation (2), M′ corresponds to the number of antennas that contribute to the formation of directivity and is an integer equal to or greater than 2 and no greater than M. In addition, M′ is not necessarily limited to one value, and index calculation unit  106  may calculate indices corresponding to each of a plurality of M′. 
     Although the elements of vector h n  were the frequency responses corresponding to antennas  101 - 1 - 101 -M′ of radio base station  100  in the above-described example, the elements of vector h n  may also be the frequency responses that correspond to M′ antennas that are aligned continuously. In addition, the indices calculated in Equation (3) are equal to or greater than 0 and lower than 1. 
     In the first to third examples described hereinabove, indices were calculated based on channel information that was acquired at a particular time, i.e., based on an instantaneous state of the channels. However, when the state of the channels changes violently, the generation of weighting factors from indices that are averaged over time is thought to be more appropriate than from indices that are based on an instantaneous state of the channels. 
     For example, if the instantaneous index at time i is ρ i , and the forgetting coefficient is α (where 0≦α&lt;1), index calculation unit  106  calculates the index at time i: 
       [Numerical Expression 5] 
     
       
      
       ρ 
       i  
      
     
     based on the following Equation (5). 
       [Numerical Expression 6] 
         ρ   i =(1−α)ρ i +α ρ   i−1    (5)
 
     The past information that is necessary for calculating an index according to Equation (5) is only the index that was previously calculated, and there is consequently no need to store past channel frequency responses. 
     In addition, the indices described hereinabove can be calculated in a subcarrier that can acquire channel frequency responses. As a result, when indices relating to the angular spread of channels can be calculated for a plurality of subcarriers, index calculation unit  106  may calculate the average value or the maximum value of the indices for the plurality of subcarriers as the index. 
     (1.4) Generation of Weighting Factor 
       FIG. 5  is a flow chart describing an actual example of the operation of weighting factor generation unit  107 . 
     Weighting factor generation unit  107  uses the indices that were calculated in index calculation unit  106  to determine the number M (0)  of antennas that take 0 as the weighting factor (Step S 501 ). At this time, weighting factor generation unit  107  increases the value of M (0)  in proportion to the angular spread of the channels. 
     Weighting factor generation unit  107  next constructs a channel from the elements of antennas, which take 0 as the weighting factor, have been eliminated based on the channel information that was acquired in channel information acquisition unit  105  and the number M (0)  of antennas that take 0 as the weighting factor that was determined in Step S 501  (Step S 502 ). At this time, weighting factor generation unit  107  constructs a channel matrix such that the elements of (M−M (0) ) antennas from which antennas that take 0 as the weighting factor have been eliminated are aligned continuously. 
     Weighting factor generation unit  107  then uses the channel matrix that was constructed in Step S 502  to generate a weighting factor that is used when transmitting signals addressed to radio terminal  200  (Step S 503 ) and thus completes the process. 
     Details regarding the processes of Step S 501  and Step S 503  are next explained. 
     Actual examples of the method of determining the number M (0)  of antennas that take 0 as the weighting factor in Step S 501  are first described. 
     In the first example, weighting factor generation unit  107  uses one index to determine the number M (0)  of antennas that take 0 as the weighting factor. More specifically, when the index is ρ and the maximum value of M (0)  is M (0)   max , weighting factor generation unit  107  determines M (0)  using the following Equation (6). It is assumed that M (0)   max  has been determined in advance. 
       [Numerical Expression 7] 
         M   (0)   =└M   max   (0) ρ┘  (6)
 
     However, in Equation (6), 
       [Numerical Expression 8] 
       └x┘
 
     indicates the maximum integer among integers equal to or less than x. 
     In the second example, weighting factor generation unit  107  uses a plurality of indices to determine the number M (0)  of antennas that take 0 as the weighting factor. Here, the plurality of indices is assumed to refer to the plurality of indices that are calculated using Equation (2) or (3) for each number of antennas that contribute to the formation of directivity. When the index for a case in which the number of antennas that contribute to the formation of directivity is (M−m (0) ) is: 
       [Numerical Expression 9] 
       ρ M−m     (0)    
 
     and the threshold value is ρ th , weighting factor generation unit  107  determines the minimum m (0)  that satisfies the following Equation (7) when m (0)  is changed from 0 to M−2 to be the number M (0)  of antennas that take 0 as the weighting factor. 
       [Numerical Expression 10] 
       ρ M−m     (0)   &lt;ρ th    (7)
 
     When the number M (0)  of antennas that take 0 as the weighting factor is large, the beam width of directivity that is formed spreads and the degree of concentration of the power of a transmission signal in a specific direction decreases. As a result, weighting factor generation unit  107  preferably controls M (0)  according to the channel information such that the received signal power in radio terminal  200  does not become excessively weak. For example, weighting factor generation unit  107  determines the maximum m (0)  that satisfies the following Equation (8) when m (0)  is changed from 0 to M−2 as the upper limit of the number M (0)  of antennas that take 0 as the weighting factor. 
     
       
         
           
             
               
                 
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                      
                     
                         
                     
                      
                     11 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         M 
                         - 
                         
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                             ( 
                             0 
                             ) 
                           
                         
                       
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                         ∑ 
                         
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                           = 
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                             = 
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                               h 
                               
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                           2 
                         
                       
                     
                   
                   &gt; 
                   β 
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In Equation (8), h n,m  is the channel frequency response between antenna  101 - m  of radio base station  100  and antenna  201 - n  of radio terminal  200 , and β is a threshold value that is determined in advance. 
     An example of the method of generating a weighting factor in Step S 503  is next described. In the interest of simplifying the explanation in the following explanation, it is assumed that the antennas that take 0 as the weighting factor are antennas  101 -(M-M (0) +1)- 101 -M and the elements of the n th  row m th  column of N×(M-M (0) ) matrix H that was constructed in Step S 502  is h n,m . 
     In the first example, weighting factor generation unit  107  first performs singular value decomposition of matrix H as in the following Equation (9): 
       [Numerical Expression 12] 
       H=UΣN H    (9)
 
     Here, U is an N-dimension unitary matrix having the left singular vectors of matrix H as column vectors, Σ is an N×(M−M (0) ) matrix in which the diagonal elements are singular values of H, and moreover, the off-diagonal elements are 0, and V is a (M−M (0) )-dimension unitary matrix having the right singular vectors of matrix H as the column vectors. 
     Weighting factor generation unit  107  then uses the right singular vector v 1  that corresponds to the maximum singular value to find the M-dimension weighting factor vectors w having weighting factors corresponding to each antenna as elements as in the following Equation (10): 
     
       
         
           
             
               
                 
                   [ 
                   
                     Numerical 
                      
                     
                         
                     
                      
                     Expression 
                      
                     
                         
                     
                      
                     13 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   w 
                   = 
                   
                     ( 
                     
                       
                         
                           
                             v 
                             1 
                           
                         
                       
                       
                         
                           
                             0 
                             
                               M 
                               
                                 ( 
                                 0 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Here, 
       [Numerical Expression 14] 
       0 M   (0)    
     is the M (0) -dimension zero vector. The right singular vector v 1  can be derived from the eigenvalue decomposition of the products of matrix H H  and matrix H. 
     In the second example, matrix H is represented as in the following Equation (11): 
       [Numerical Expression 15] 
         H   T =( h   1  . . . h N )   (11)
 
     and weighting factor generation unit  107  uses, from among vectors h n  (where 1≦n≦N), vector h n′  at which the norm is a maximum to find weighting factor vector w based on the following Equation (12). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Numerical 
                      
                     
                         
                     
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                     Expression 
                      
                     
                         
                     
                      
                     16 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   w 
                   = 
                   
                     ( 
                     
                       
                         
                           
                             
                               h 
                               
                                 n 
                                 ′ 
                               
                               * 
                             
                             
                                
                               
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                                   ′ 
                                 
                               
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                                 ( 
                                 0 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     (1.4) Effects 
     According to the present exemplary embodiment as described hereinabove, weighting factors that correspond to each of antennas  101 - 1 - 101 -M of radio base station  100  are generated based on indices that are related to the angular spread of the channels. As a result, fluctuation of the direction of good channel quality can be accurately estimated to form directivity and the amount of deterioration of communication quality for a case in which ideal directivity is formed can be decreased. 
     In the present exemplary embodiment, moreover, indices are calculated based on the correlation with antennas  101 - 1 - 101 -M of radio base station  100 , whereby fluctuation of the direction of good channel quality can be accurately estimated. 
     Further, in the present exemplary embodiment, indices are calculated based on the eigenvalues of the products of a matrix that takes as its components the frequency responses between radio base station  100  and radio terminal  200  and an Hermitian transpose of the matrix, whereby fluctuation of the direction of good channel quality can be accurately estimated. 
     In the present exemplary embodiment, moreover, indices are calculated based on the angles formed between vectors that take as elements the channels frequency responses that are constructed for each of antennas  201 - 1 - 201 -N of radio terminal  200  with antennas  101 - 1 - 101 -M of radio base station, whereby the fluctuation of the direction of good channel quality can be accurately estimated. 
     In addition, in the present exemplary embodiment, indices that are related to the angular spread of channels and that are averaged over time or indices that are related to the angular spread of channels and that are averaged over frequencies are calculated as the indices, whereby the fluctuation in the direction of good channel quality can be estimated with greater accuracy. 
     In the present exemplary embodiment, moreover, the beam width of directivity that is formed by antennas  101 - 1 - 101 -M of radio base station  100  is adjusted based on indices, whereby the amount of deterioration of communication quality for a case in which ideal directivity is formed can be decreased. 
     In the present exemplary embodiment, moreover, because the weighting factor is set to 0 only for a number that accords with the indices and because the beam width of directivity that is formed by antennas  101 - 1 - 101 -M of radio base station  100  is adjusted, and then the beam width can be easily adjusted. 
     In the present exemplary embodiment, moreover, because weighting factors are increasingly set to 0 with increasing angular spread of channels, the adjustment of the beam width can be appropriate for the fluctuation of the direction of good channel quality. 
     Finally, in the present exemplary embodiment, an upper limit of the number of weighting factors that are set to  0  is determined based on channel information, whereby excessive weakening of the received signal power in radio terminal  200  can be prevented. 
     (2) Second Exemplary Embodiment 
     In the second exemplary embodiment of the present invention, explanation regards a radio communication system in which a plurality of data blocks are transmitted by spatial multiplexing. 
     (2.1) Explanation of Configuration 
       FIG. 6  is a structural diagram showing the radio communication system in the second exemplary embodiment of the present invention. In  FIG. 6 , constructions that are identical to those of  FIG. 1  are given the same reference numbers and redundant explanation is omitted. 
     In contrast with the radio communication system in the first exemplary embodiment shown in  FIG. 1 , the radio communication system in the present exemplary embodiment shown in  FIG. 6  has radio base station  600  in place of radio base station  100  and a plurality of radio terminals  200 . In  FIG. 6 , there are K radio terminals  200 , each of these K radio terminals  200  being referred to as radio terminals  200 - 1 - 200 -K. 
       FIG. 7  is a block diagram showing the functional configuration of radio base station  600  in the present exemplary embodiment. In  FIG. 7 , constructions identical to those of  FIG. 2  are given the same reference numbers and redundant explanation is omitted. 
     Radio base station  600  shown in  FIG. 7  differs from radio base station  100  in the first exemplary embodiment shown in  FIG. 2  in that it is provided with weighting factor generation unit  601  in place of weighting factor generation unit  107 , is newly provided with data block construction unit  602 , is provided with a plurality of each of encoding units  108  and modulation units  109 , and is provided with weighting factor multiplication unit  603  in place of weighting factor multiplication unit  110 . In  FIG. 7 , there are a number Q of each of encoding units  108  and modulation units  109 , these Q encoding units  108  and Q modulation units  109  being respectively referred to as encoding units  108 - 1 - 108 -Q and modulation units  109 - 1 - 109 -Q. 
     Weighting factor generation unit  601  determines a number L of data blocks that are to undergo spatial multiplexing and transmission based on indices that are calculated in index calculation unit  106 . Here, L is equal to or less than Q. 
     Data block construction unit  602  constructs the number L of data blocks that was determined in weighting factor generation unit  601  from a plurality of items of transmission data for radio terminals  200 - 1 - 200 -K. 
     The L encoding units  108 - 1 - 108 -L from among encoding units  108 - 1 - 108 -Q encode respective data blocks that were constructed in data block construction unit  602 . 
     Each of modulation units  109 - 1 - 109 -Q corresponds to a respective encoding unit of encoding units  108 - 1 - 108 -Q, and L modulation units  109 - 1 - 109 -L from among modulation units  109 - 1 - 109 -Q modulate respective data blocks that were encoded in the corresponding encoding unit. 
     Weighting factor multiplication unit  603  makes M duplications (for the number M of antennas) of the modulated signals corresponding to each of the data blocks that were generated in modulation units  109 - 1 - 109 -L, and multiplies these (L×M) modulated signals by the weighting factors that were generated in weighting factor generation unit  107 . Weighting factor multiplication unit  603  then adds together the L modulated signals after the multiplication of the corresponding weighting factors for each of antennas  101 - 1 - 101 -M. 
     (2.2) Determination of Data Blocks for Spatially Multiplexed Transmission and Generation of Weighting Factors 
       FIG. 8  is a flow chart for describing an example of the operation of weighting factor generation unit  601 . 
     As shown in  FIG. 8 , weighting factor generation unit  601  first gives the order of priority to radio terminals  200 - 1 - 200 -K based on channel information and the frequency of assignment of radio resources to each of radio terminals  200 - 1 - 200 -K (Step S 801 ). Weighting factor generation unit  601  is here assumed to give a terminal number to each of radio terminals  200 - 1 - 200 -K as the order of priority in the preferential order of data transmission. 
     Weighting factor generation unit  601  next sets initial value 1 that is the highest order of priority as terminal number k and sets initial value 0 as the number L of data blocks that are to be transmitted by spatial multiplexing (Step S 802 ). 
     Weighting factor generation unit  601  then, when transmitting signals addressed to the radio terminal of terminal number k, determines the number M (0) (k) of antennas that take 0 as the weighting factor based on the indices that were calculated in index calculation unit  106  (Step S 803 ). A method identical to the method of determining the number M (0)  of antennas that take 0 as the weighting factor in the first exemplary embodiment can be used as the method of determining M (0) (k). 
     Weighting factor generation unit  601  then uses the channel information that was acquired by channel information acquisition unit  105  and the number M (0) (k) of antennas that take 0 as the weighting factor that was determined in Step S 803  to determine the number L(k) of data blocks when transmitting signals addressed to the radio terminal of terminal number k (Step S 804 ). For example, if the maximum value of the number of data blocks is L max , weighting factor generation unit  601  finds L(k) using the following Equation (13). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Numerical 
                      
                     
                         
                     
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                     Expression 
                      
                     
                         
                     
                      
                     17 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     L 
                      
                     
                       ( 
                       k 
                       ) 
                     
                   
                   = 
                   
                     min 
                      
                     
                       ( 
                       
                         N 
                         , 
                         
                           
                             L 
                             max 
                           
                           - 
                           L 
                         
                         , 
                         
                           ⌊ 
                           
                             M 
                             
                               M 
                               - 
                               
                                 
                                   M 
                                   
                                     ( 
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                                   ( 
                                   k 
                                   ) 
                                 
                               
                             
                           
                           ⌋ 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     Because the transmission power per data block decreases and the reception power declines in proportion to the increase of the number of data blocks L(k), weighting factor generation unit  601  may limit the number of data blocks based on the channel quality. In addition, the maximum value L max  of the number of data blocks is, for example, the number Q of modulation units  109  and encoding units  108 . 
     Weighting factor generation unit  601  next constructs a channel matrix in which the components of antennas that take 0 as the weighting factor are removed (Step S 805 ). At this time, weighting factor generation unit  601  continuously lines up elements of (M-M (0) (k)) antennas in which antennas that take 0 as the weighting factor are removed, and moreover, constructs a channel matrix such that there is no repetition among L(k) data blocks. 
     Weighting factor generation unit  601  next uses the channel matrix that was constructed in Step S 805  to generate M-dimension weighting factor vectors w(k, j) (where 1≦j≦L(k)) that are used when transmitting each data block to the radio terminal of terminal number k (Step S 806 ). For example, as in the first exemplary embodiment, weighting factor generation unit  601  uses the right singular vector that was acquired from the singular value decomposition of the channel matrix to generate M-dimension weighting factor vectors w(k, j). 
     Weighting factor generation unit  601  then uses the channel information and weighting factor vectors to judge whether the predetermined condition is satisfied when it is assumed that the signals addressed to the radio terminal of terminal number k are spatially multiplexed and transmitted (Step S 807 ). The predetermined conditions are, for example, that the communication quality corresponds to data blocks for which spatially multiplexed transmission has already been determined and that each of the data blocks addressed to the radio terminal of terminal number k surpasses a predetermined threshold value. More specifically, when the terminal number of the radio terminal corresponding to data blocks for which spatially multiplexed transmission has already been determined and terminal number k are consolidated as i, the communication quality for data blocks j′ of the radio terminal of terminal number i′ is γ(i′, j′), the threshold value is γ th , the N×M matrix in which the element of the n th  row and m th  column is the frequency response between antenna  101 - m  of radio base station  600  and antenna  201 - n  of radio terminal  200 - i ′ is and the noise power is N 0 , the predetermined condition is represented as shown in the following Equation (14). 
     
       
         
           
             
               
                 
                   [ 
                   
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                     18 
                   
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                       th 
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     When the predetermined condition is satisfied (“YES” in Step S 807 ), weighting factor generation unit  601  determines to perform spatially multiplexed transmission of the data blocks addressed to the radio terminal of terminal number k and adds L(k) to the number L of data blocks (Step S 808 ), and then judges whether terminal number k or data block number L is the maximum value (Step S 809 ). 
     On the other hand, when the predetermined condition is not satisfied (NO in Step S 807 ), weighting factor generation unit  601  carries out Step S 809  without carrying out the process of Step S 808 . 
     When terminal number k and the number L of data blocks are not the maximum values (NO in Step S 809 ), weighting factor generation unit  601  adds 1 to the terminal number (Step S 810 ) and returns to the process of Step S 803 . 
     On the other hand, when the terminal number k or the number L of data blocks is the maximum value (YES in Step S 809 ), weighting factor generation unit  601  ends the process. 
     (2.3) Effects 
     As described hereinabove, according to the present exemplary embodiment, the spatially multiplexed transmission of a plurality of data blocks in accordance with indices enables an improvement in the communication speed in addition to an improvement in the effects of the first exemplary embodiment. 
     (3) Third Exemplary Embodiment 
     In the third exemplary embodiment of the present invention, the antennas of a radio base station are constituted by antennas for vertical polarization and antennas for horizontal polarization, and weighting factors for multiplying signals that are processed at each antenna are generated with consideration given to the angular spread of channels for each vertically and horizontally polarization. 
     (3.1) Configuration 
     The radio communication system and radio base station in the present exemplary embodiment have the same configuration as the radio communication system and radio base station  100  in the first exemplary embodiment shown in  FIG. 1  and  FIG. 2 , respectively, but antennas  101 - 1 - 101 -M in radio base station  100  are divided between antennas for vertical polarization and antennas for horizontal polarization, and index calculation unit  106  and weighting factor generation unit  107  carry out processing with consideration given to the angular spread of channels for each of vertically polarized waves and horizontally polarized waves. 
     Index calculation unit  106  uses the channel information that was acquired by channel information acquisition unit  105  to calculate indices for each of the vertically polarized wave component and the horizontally polarized wave component. In the calculation of the indices, any of Equations (1), (2), and (3) may be used, as in the first exemplary embodiment. At this time, when calculating indices for a vertically polarized wave component, index calculation unit  106  uses frequency responses that correspond to the vertical polarization antennas and uses frequency responses that correspond to horizontal polarization antennas when calculating indices for a horizontally polarized wave component. 
     Weighting factor generation unit  107  generates weighting factors based on the indices for each polarized wave component. At this time, weighting factor generation unit  107  may generate weighting factors by carrying out the same operations as the operations described using  FIG. 5 , but determines the number of antennas that take 0 as the weighting factor for each polarized wave component and continuously aligns the elements of antennas that do not take 0 as the weighting factor in the antennas for each polarization in the channel matrix. 
     (3.2) Effect 
     Due to the calculation of indices for each vertically and horizontally polarization as described hereinabove, the present exemplary embodiment enables the generation of weighting factors that multiply signals processed at each antenna with consideration given to the angular spread of channels for each vertically and horizontally polarization. Accordingly, the amount of deterioration of communication quality can be reduced for a case in which ideal directivity is formed in an environment in which antennas are configured for vertical polarization and for horizontal polarization. 
     (4) Fourth Exemplary Embodiment 
     In the fourth exemplary embodiment of the present invention, antennas are used that are two-dimensionally disposed in the horizontal direction and vertical direction, and weighting factors that multiply signals processed in each antenna are generated with consideration given to the angular spread of channels for each direction. 
     (4.1) Configuration 
     The radio communication system and radio base station in the present exemplary embodiment have the same configuration as the radio communication system and radio base station in the first exemplary embodiment shown in  FIG. 1  and  FIG. 2 , respectively, but antennas  101 - 1 - 101 -M in the radio base station of the present exemplary embodiment are arranged two-dimensionally, and index calculation unit  106  and weighting factor generation unit  107  carry out processing based on indices for each of the horizontal direction and vertical direction. 
       FIG. 9  shows the configuration of antennas of the present exemplary embodiment. As shown in  FIG. 9 , antennas  101 - 1 - 101 -M are arranged two-dimensionally (M=M x ×M y ) with M x  antennas in the horizontal direction, which is the first direction, and M y  antennas in the vertical direction, which is the second direction. 
     Index calculation unit  106  calculates indices for each of the horizontal direction and vertical direction based on channel information. When calculating indices, index calculation unit  106  may use any of Equations (1), (2), and (3) shown in the first exemplary embodiment. 
     When using Equation (1), index calculation unit  106  calculates indices for the horizontal direction based on the correlation between antennas separated in the horizontal direction and calculates indices for the vertical direction based on the correlation between antennas separated in the vertical direction. 
     When using Equation (2) or (3), index calculation unit  106  calculates indices for the horizontal direction based on frequency responses corresponding to antennas that are continuous in the horizontal direction and calculates indices for the vertical direction based on frequency responses corresponding to antennas continuous in the vertical direction. 
     Weighting factor generation unit  107  uses the indices for each of the horizontal direction, and vertical direction to generate weighting factors. At this time, weighting factor generation unit  107  may generate weighting factors similarly to the operations of the first exemplary embodiment that were described using  FIG. 5 , but the number of antennas that take 0 as the weighting factor is determined for each of the horizontal direction and vertical direction and the components of antennas in which the weighting factor is not set to 0 are aligned continuously in each of the horizontal direction and vertical direction in the channel matrix, as shown in  FIG. 10 . In  FIG. 10 , M x   (0)  and M y   (0)  are the numbers of antennas that take 0 as the weighting factor for each of the horizontal direction and vertical direction, respectively. 
     (4.2) Effect 
     As described hereinabove, because indices are calculated for each of the horizontal direction and vertical direction, the present exemplary embodiment allows weighting factors that multiply signals that are processed at each antenna to be generated with consideration given to the angular spread of channels with respect to each of the horizontal direction and vertical direction. Accordingly, the amount of deterioration of communication quality can be reduced for a case in which ideal directivity is formed in an environment of using antennas arranged two-dimensionally in the horizontal direction and vertical direction. 
     In each of the exemplary embodiments described above, the configurations shown in the figures are merely examples, and the present invention is not limited to these configurations. 
     In addition, although all or a portion of each of the above-described exemplary embodiments can be described as shown in following notes, the present invention is not limited to the following description. 
     Note 1 
     The radio communication apparatus is a radio communication apparatus provided with a plurality of antennas and has: 
     a channel information acquisition unit that acquires information relating to channels with another radio communication apparatus; 
     an index calculation unit that uses the information to calculate indices relating to the angular spread of the channels; 
     a weighting factor generation unit that uses the information and the indices to generate weighting factors corresponding to each of the plurality of antennas; and 
     a weighting factor multiplication unit that multiplies the signals that are processed by each of the plurality of antennas by the weighting factors that correspond to the antennas that process the signals. 
     Note 2 
     In the radio communication apparatus described in Note 1, the index calculation unit uses the information to find a correlation between any of the antennas from among the plurality of antennas and calculates the indices based on the correlation. 
     Note 3 
     In the radio communication apparatus described in Note 1, the index calculation unit uses the information to find eigenvalues of the products of a matrix that takes the channels frequency responses as elements and the Hermitian transpose of the matrix and calculates the indices based on the eigenvalues. 
     Note 4 
     In the radio communication apparatus described in Note 1, the index calculation unit uses the information to find vectors that take as elements the channels frequency responses for each antenna provided in the other radio communication apparatus and calculates the indices based on the angles formed between the vectors. 
     Note 5 
     In the radio communication apparatus described in any one of Notes 1 to 4, the index calculation unit calculates indices relating to the angular spread of the channels that is averaged over time or indices relating to the angular spread of the channels that has been averaged across frequencies. 
     Note 6 
     In the radio communication apparatus described in any one of Notes 1 to 5, the weighting factor generation unit generates the weighting factors such that the beam width of directivity that is formed by the plurality of antennas corresponds to the indices. 
     Note 7 
     In the radio communication apparatus described in Note 6, the weighting factor generation unit sets a number of weighting factors to 0, this number according with the indices, and uses the information to generate weighting factors corresponding to antennas in which the weighting factor is not 0. 
     Note 8 
     In the radio communication apparatus described in Note 7, the weighting factor generation unit uses the indices to increase the number of the weighting factors that are set to 0 in proportion to the angular spread of the channel. 
     Note 9 
     In the radio communication apparatus described in Notes 7 or 8, the weighting factor generation unit uses the information to set an upper limit for the number of the weighting factors that are set to 0. 
     Note 10 
     In the radio communication apparatus described in any one of Notes 1 to 9, the weighting factor generation unit uses the indices to determine the number of data blocks to be subjected to spatially multiplexed transmission, and the weighting factor multiplication unit processes signals corresponding to the data blocks that are subjected to spatially multiplexed transmission. 
     Note 11 
     In the radio communication apparatus described in Note 10, the weighting factor generation unit uses the indices to increase the number of the data blocks in proportion to the increase of angular spread of the channels. 
     Note 12 
     In the radio communication apparatus described in any one of Notes 1 to 11, the plurality of antennas include antennas for vertically polarized waves and antennas for horizontally polarized waves, and the index calculation unit calculates the indices corresponding to vertically polarized waves and horizontally polarized waves. 
     Note 13 
     In the radio communication apparatus described in Notes 1 to 12, the plurality of antennas are arranged two-dimensionally in a first direction and a second direction, and the index calculation unit calculates the indices corresponding to each of the first direction and the second direction. 
     Note 14 
     The radio communication method is a radio communication method in a radio communication apparatus that is provided with a plurality of antennas, the method including steps of: 
     acquiring information relating to channels with another radio communication apparatus; 
     using the information to calculate indices relating to the angular spread of the channels; 
     using the information and the indices to generate weighting factors corresponding to each of the plurality of antennas; and 
     multiplying the signals that are processed at each of the plurality of antennas by the weighting factors corresponding to the antennas that process the signals. 
     This application claims the benefits of priority based on Japanese Patent Application No. 2013-247676 for which application was submitted on Nov. 29, 2013 and incorporates by citation all of the disclosures of that application. 
     EXPLANATION OF REFERENCE NUMBERS 
     
         
           100 , 600  radio base stations 
           101 - 1 - 101 -M,  201 - 1 - 201 -N antennas 
           102 - 1 - 102 -M radio transmission/reception units 
           103 - 1 - 103 -M Guard Interval removal units 
           104 - 1 - 104 -M Fast Fourier Transform units 
           105  channel information acquisition unit 
           106  index calculation unit 
           107 , 601  weighting factor generation unit 
           108 ,  108 - 1 - 108 -Q encoding unit 
           109 , 109 - 1 - 109 -Q modulation unit 
           110 , 603  weighting factor multiplication unit 
           111 - 1 - 111 -M Inverse Fast Fourier Transform units 
           112 - 1 - 112 -M Guard Interval insertion units 
           200 ,  200 - 1 - 200 -K radio terminals 
           602  data block construction unit what is claimed is: