Patent Publication Number: US-11387872-B2

Title: Wireless base station and wireless communication method

Description:
TECHNICAL FIELD 
     The present invention relates to a radio base station and a radio communication method. 
     BACKGROUND ART 
     In a UMTS (Universal Mobile Telecommunications System) network, Long Term Evolution (LTE) is specified for a higher data rate, lower latency, and the like. For a broader bandwidth and a higher speed based on LTE, successor systems of LTE are also studied (for example, the systems are called LTE-A (LTE-Advanced), FRA (Future Radio Access), 5G (5th generation mobile communication system), 5G+(5G plus), and New-RAT (Radio Access Technology)). 
     In a future radio communication system (for example, 5G), the use of Massive MIMO (Multiple Input Multiple Output) using a large number of antenna elements (for example, more than 100 elements) in a high frequency band (for example, 5 GHz or higher) is studied to further increase the speed and reduce the interference in signal transmission. 
     An example of a technique of controlling beams or streams in MIMO includes a method of combining digital precoding/postcoding (hereinafter, simply referred to as precoding/postcoding in some cases) and beam-forming (BF) (for example, see NPL 1). 
     CITATION LIST 
     Non-Patent Literature 
     
         
         NPL 1 T. Obara et al.: “Joint Processing of Analog Fixed Beamforming and CSI-based Precoding for Super High Bit Rate Massive MIMO Transmission Using Higher Frequency Bands,” IEICE Transactions on Communications VOL. E98-B, NO. 8 Aug. 2015 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, beam control in an environment (multi-site environment) including a plurality of sites (also referred to as cells in some cases) formed by base stations (hereinafter, also referred to as Massive MIMO base stations in some cases) using Massive MIMO is not sufficiently examined. 
     An aspect of the present invention provides a radio base station and a radio communication method that can appropriately control beams in the multi-site environment. 
     Solution to Problem 
     An aspect of the present invention provides a radio base station in a radio communication system that performs MIMO transmission between a plurality of radio base stations and at least one user terminal, the radio base station including: a precoding section that applies precoding to a data signal based on channel information indicating channels between the radio base stations and the user terminal; and a communication section that transmits the precoded data signal, in which the precoding in a first radio base station of the plurality of radio base stations is applied based on the channel information including at least a channel between the first radio base station and a first user terminal connected to a second radio base station other than the first radio base station. 
     Advantageous Effects of Invention 
     According to the aspect of the present invention, the beams can be appropriately controlled in the multi-site environment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a configuration example of a radio communication system according to Embodiment 1; 
         FIG. 2  is a block diagram showing a configuration example of a radio base station according to Embodiment 1; 
         FIG. 3  is a block diagram showing a configuration example of a user terminal according to Embodiment 1; 
         FIG. 4  is a flow chart showing an operation example of the radio base station according to Embodiment 1; 
         FIG. 5  illustrates a configuration example of a radio communication system according to Embodiment 2; 
         FIG. 6  is a flow chart showing an operation example of the radio base station according to Embodiment 2; 
         FIGS. 7A and 7B  illustrate a configuration example of a radio communication system according to Embodiment 3; 
         FIG. 8  is a flow chart showing an operation example of the radio base station according to Embodiment 3; and 
         FIG. 9  illustrates an example of a hardware configuration of the radio base station and the user terminal according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     Streams are distributed and multiplexed for a plurality of user terminals in MU-MIMO (Multi-User MIMO) transmission in the case described below. BF and precoding/postcoding are performed in Massive MIMO in the case described below. Therefore, precoding/postcoding and beam-forming are performed in the MU-MIMO transmission between a radio base station and a plurality of user terminals in a radio communication system according to the present invention. 
     Hereinafter, the precoding will be sorted into precoding for inter-user interference (IUI) and precoding for inter-stream interference (ISI) in each user terminal. Precoding is performed for the inter-user interference, and precoding and postcoding are performed for the inter-stream interference in each user terminal. 
     In an example illustrated below, the radio base station includes N T  antenna elements and performs Massive MIMO transmission in the downlink between the radio base station and N U  user terminals. The number of antenna elements of an i-th (i=1 to N U ) user terminal is N Ri , and the number of streams is M i . 
     In this case, reception signal r received by each user terminal is expressed by the following equation 1. 
     
       
         
           
             
               
                 
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     Here, H i  (i=0 to N U −1) represents channel information (channel matrix) of the i-th user terminal multiplexed in the MU-MIMO transmission, W represents a BF weight, P IUI  represents a precoding matrix for the inter-user interference, P ISI  represents a precoding matrix for the inter-stream interference, d i  (i=0 to N U −1) represents a stream for i-th user terminal, and z represents noise. 
     As shown in equation 1, precoding processing (P IUI ) for the inter-user interference in the precoding (for example, block diagonalization) orthogonalizes the channels between the user terminals and prevents the inter-user interference. 
     Signal y obtained by postcoding of reception signal r received by each user terminal is expressed by the following equation 2. 
     
       
         
           
             
               
                 
                   
                       
                   
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     In equation 2, B i   ISI  (i=0 to N U −1) represents a postcoding matrix of the inter-stream interference for the i-th user terminal. As shown in equation 2, precoding matrix P ISI  and postcoding matrix B ISI  orthogonalize the channels between the streams of the user terminals and prevent the inter-stream interference. 
     Note that precoding matrices (P IUI , P ISI ) and postcoding matrix (B ISI ) may be computed by, for example, singular value decomposition (SVD) using channel information (channel matrix) between the radio base station and the user terminal. 
     The precoding processing and the beam-forming processing are processing for a plurality of user terminals connected to one radio base station. Therefore, the processing is processing in a site formed by a single Massive MIMO base station. 
     On the other hand, to increase the efficiency and maintain the quality of the high frequency band communication in the future, beam control, such as precoding processing and beam-forming processing, in sites (multiple sites) formed by a plurality of Massive MIMO base stations needs to be examined. 
     Therefore, a method of appropriately controlling the beams in the multi-site environment will be described in the following embodiments. 
     Embodiment 1 
     &lt;Radio Communication System&gt; 
       FIG. 1  shows a configuration example of a radio communication system according to the present embodiment. 
     The radio communication system according to the present embodiment includes a plurality of radio base stations  10  and at least one user terminal  20 . The radio base station  10  is, for example, a Massive MIMO base station. 
     Each user terminal  20  is connected to (accesses) at least one radio base station  10 . In the example illustrated in  FIG. 1 , user terminals  20 - 1  to  20 - 3  are connected to radio base station  10 A, and user terminals  20 - 4  to  20 - 6  are connected to radio base station  10 B. 
     &lt;Radio Base Station&gt; 
       FIG. 2  illustrates an example of an overall configuration of the radio base station according to the present embodiment. Radio base station  10  shown in  FIG. 2  includes discovery signal generation section  101 , candidate weight multiplication section  102 , reference signal generation section  103 , access point selection section  104 , inter-base station communication section  105 , weight selection section  106 , judgement section  107 , precoding matrix generation section  108 , data generation section  109 , precoding section  110 , beam-forming section  111 , communication sections  112 , and antennas  113 . 
     Note that constituent sections (for example, IFFT processing section and CP adding section) and the like for generating an OFDM (Orthogonal Frequency Division Multiplexing) signal in radio base station  10  are not illustrated in  FIG. 2 . Furthermore, the signal waveform of the signal transmitted from radio base station  10  is not limited to the waveform based on OFDM modulation. 
     Discovery signal generation section  101  generates discovery signals (reference signals) for deciding BF weight (W). For example, discovery signal generation section  101  generates at least the same number of discovery signals as the number of candidates of the BF weight. Discovery signal generation section  101  outputs the generated discovery signals to candidate weight multiplication section  102 . 
     Candidate weight multiplication section  102  multiplies the discovery signals input from discovery signal generation section  101  by the candidates of the BF weight, respectively, and outputs the discovery signals multiplied by the BF weight candidates to communication sections  112 . 
     Reference signal generation section  103  generates a reference signal used for channel estimation and outputs the reference signal to beam-forming section  111 . 
     Access point selection section  104  selects user terminal  20  to be connected to radio base station  10  among the plurality of radio base stations  10  based on candidate weight information fed back from each user terminal  20 . Examples of the candidate weight information include an SNR (Signal to Noise Ratio) of the discovery signal multiplied by the candidate weight and received power. 
     For example, for each user terminal  20 , access point selection section  104  selects radio base station  10  that generates the BF weight with the maximum candidate weight information (received power) and sets radio base station  10  as a radio base station as an access point of user terminal  20 . Access point selection section  104  may, for example, notify user terminal  20  connected to radio base station  10  of access point information indicating radio base station  10  as an access point of user terminal  20  (not illustrated). Each user terminal  20  performs a connection operation for radio base station  10  indicated in the access point information. Access point selection section  104  may also, for example, notify other radio base stations  10  of the access point information indicating radio base station  10  as an access point of user terminal  20  through inter-base station communication section  105 . Alternatively, access point selection section  104  may specify user terminal  20  to be connected to radio base station  10  based on the access point information reported from other radio base stations  10  through inter-base station communication section  105 . 
     Inter-base station communication section  105  communicates with other radio base stations  10  or a control station (may also be referred to as a central control station) that controls a plurality of radio base stations  10  through, for example, a backhaul. 
     Weight selection section  106  selects BF weight (W) to be used for beam-forming from the BF weight candidates based on the candidate weight information fed back from each user terminal  20  and outputs BF weight (W) to beam-forming section  111 . For example, weight selection section  106  may select L BF weights not overlapping with each other, in descending order of SNR (or received power) indicated in the candidate weight information. 
     Judgement section  107  judges whether to execute extended precoding based on the candidate weight information fed back from each user terminal  20 . For example, judgement section  107  specifies user terminals  20  other than user terminals  20  connected to radio base station  10  (that is, user terminals  20  not connected to radio base station  10 ) based on the access point information input from access point selection section  104 . Judgement section  107  then judges to execute the extended precoding when there is user terminal  20  in which the SNR indicated in the candidate weight information is smaller than predetermined threshold σ among user terminals  20  not connected to radio base station  10 . On the other hand, judgement section  107  judges to execute normal precoding without executing the extended precoding when there is no user terminal  20  in which the SNR indicated in the candidate weight information is smaller than predetermined threshold σ among user terminals  20  not connected to radio base station  10 . Judgement section  107  outputs the judgement result to precoding matrix generation section  108 . 
     Note that the normal precoding is precoding applied based on channel estimation values fed back from user terminals  20  connected to single radio base station  10 . On the other hand, the extended precoding is precoding applied based on channel estimation values including channels between radio base station  10  and user terminals  20  connected to radio base station  10  and channels between radio base station  10  and user terminals  20  connected to radio base stations  10  other than radio base station  10 , in which the SNR of user terminals  20  is smaller than predetermined threshold σ. 
     Hereinafter, a precoding matrix generated in the normal precoding based on the channel estimation values indicating the channels between single radio base station  10  and user terminals  20  connected to radio base station  10  will be referred to as a “normal precoding matrix”. A precoding matrix generated in the extended precoding based on the channel estimation values including the channels between radio base station  10  and user terminals  20  connected to radio base station  10  and the channels between radio base station  10  and user terminals  20  connected to other radio base stations  10  will be referred to as an “extended precoding matrix”. 
     Based on the judgement result input from judgement section  107 , precoding matrix generation section  108  uses the channel estimation values fed back from user terminals  20  to generate the normal precoding matrix or the extended precoding matrix. 
     Note that the channel estimation value is, for example, channel information (HW) indicating the equivalent channel matrix including BF weight (W). 
     Specifically, precoding matrix generation section  108  generates precoding matrix (P IUI ) for removing the interference between a plurality of user terminals  20  (inter-user interference) multiplexed in the MU-MIMO and precoding matrix (P ISI ) for removing the interference between a plurality of streams (inter-stream interference) in each user terminal  20 . Precoding matrix generation section  108  also generates an extended precoding matrix for removing interference of user terminals  20  that are interfered by radio base station  10  and that are connected to radio base stations other than radio base station  10 . 
     Precoding matrix generation section  108  outputs generated precoding matrices (P IUI , P ISI ) (hereinafter, collectively referred to as “P” in some cases) to precoding section  110 . 
     Data generation section  109  generates data (downlink signals) for a plurality of user terminals  20 . Note that  FIG. 2  shows a configuration of data generation section  109  for one user terminal  20  (i-th user terminal  20 ). However, radio base station  10  includes data generation section  109  for each of a plurality of (N U ) user terminals  20 . 
     Data generation section  109  includes coding sections  191  and modulation sections  192 . Coding sections  191  and modulation sections  192  are provided according to the number of streams (M i ) for user terminal i. Each coding section  191  encodes data signals of M i  streams. Each modulation section  192  modulates the encoded data signals and outputs the modulated data signals to precoding section  110 . Note that the code rate and the modulation scheme in each coding section  191  and each modulation section  192  may be different in each stream. 
     Precoding section  110  multiplies the data signals input from data generation section  109  by precoding matrix (P) and outputs the precoded data signals to beam-forming section  111 . For example, precoding section  110  applies precoding to the data signals of M streams to generate L (the number of beams, for example, L&gt;M) signals. 
     In the channel estimation, beam-forming section  111  multiplies the reference signal input from reference signal generation section  103  by BF weight W input from weight selection section  106  and outputs the reference signal after the BF weight multiplication to communication section  112 . According to the processing, each user terminal  20  can use the reference signal multiplied by BF weight (W) decided based on the candidate weight information (SNR) to estimate equivalent channel information (HW) including the BF weight. 
     At data transmission, beam-forming section  111  multiplies the data signals input from precoding section  110  by BF weight (W) input from weight selection section  106  and outputs the data signals (N T  signals) after the BF weight multiplication to communication section  112 . 
     Communication sections  112 - 1  to  112 -N T  are provided according to N T  antennas  113  (antenna elements). Each communication section  112  applies transmission processing, such as D/A conversion and up-conversion, to the input signals. Each communication section  112  multiplexes the signals after the transmission processing based on, for example, time-division, frequency-division, or code-division multiplexing and transmits the signals from N T  antennas  113 , respectively. Specifically, each communication section  112  transmits the discovery signal input from candidate weight multiplication section  102  to each user terminal  20  through antenna  113 . In the channel estimation, communication section  112  transmits the reference signal input from beam-forming section  111  to each user terminal  20  through antenna  113 . At data transmission, communication section  112  transmits the signal of the stream input from beam-forming section  111  to each user terminal  20  through antenna  113 . 
     &lt;User Terminal&gt; 
       FIG. 3  illustrates an example of an overall configuration of the user terminal according to the present embodiment. User terminal  20  shown in  FIG. 3  includes antennas  201 , communication sections  202 , candidate weight information measurement section  203 , channel estimation section  204 , postcoding matrix generation section  205 , postcoding section  206 , and data reception section  207 . 
     Note that  FIG. 3  shows an example of the configuration of i-th user terminal  20 . Constituent sections (for example, CP removing section and FFT processing section) and the like for receiving the OFDM signal in user terminal  20  are not illustrated in  FIG. 3 . The signal waveform of the signal received by user terminal  20  is not limited to the waveform based on the OFDM modulation. 
     Communication sections  202 - 1  to  202 -N Ri  are provided according to N Ri  antennas  201 , respectively. Each communication section  202  applies reception processing, such as down-conversion and A/D conversion, to the reception signals received through antenna  201 . Here, examples of the reception signals include the discovery signal, the reference signal, and the data signal. Communication section  202  outputs the discovery signal to candidate weight information measurement section  203 , outputs the reference signal to channel estimation section  204 , and outputs the data signal to postcoding section  206 . 
     Candidate weight information measurement section  203  uses the discovery signals input from communication sections  202  to measure the candidate weight information (for example, SNR or received power). The discovery signals are multiplied by the BF weight candidates, respectively. Therefore, candidate weight information measurement section  203  measures the SNR or the received power for each BF weight candidate used. The candidate weight information indicating the measured SNR or received power is fed back to radio base station  10  through, for example, communication section  202 . 
     Channel estimation section  204  uses the reference signal input from communication section  202  to estimate the channel estimation value (channel information) indicating the channel between radio base station  10  and user terminal  20 . The reference signal is multiplied by BF weight (W) in radio base station  10  (beam-forming section  111 ). Therefore, channel estimation section  204  estimates equivalent channel information including the BF weight (equivalent channel matrix HW). Estimated channel information (HW) is fed back to radio base station  10  (precoding matrix generation section  108 ) through, for example, communication section  202 . Channel estimation section  204  also outputs the estimated channel information to postcoding matrix generation section  205 . 
     Postcoding matrix generation section  205  uses channel information (HW) input from channel estimation section  204  to generate postcoding matrix (B ISI ). Postcoding matrix generation section  205  outputs the generated postcoding matrix to postcoding section  206 . Note that postcoding matrix generation section  205  may use channel information HWP estimated by using the reference signal multiplied by the precoding matrix (extended precoding matrix) and the BF weight to generate the postcoding matrix. 
     Postcoding section  206  uses postcoding matrix (B ISI ) input from postcoding matrix generation section  205  to perform postcoding of the data signal input from communication section  202 . Postcoding section  206  outputs the data signal after the postcoding to data reception section  207 . 
     Data reception section  207  applies reception processing (including demodulation processing and decoding processing) to the data signal input from postcoding section  206  and obtains a plurality of streams for i-th user terminal  20 . Data reception section  207  includes demodulation sections  271  and decoding sections  272 . Demodulation sections  271  and decoding sections  272  are provided according to the number of streams (M i ) for i-th user terminal  20 . Each demodulation section  271  demodulates the data signals of M i  streams, and each decoding section  272  decodes the demodulated data signals to obtain M i  streams. Note that the code rate and the modulation scheme in each demodulation section  271  and each decoding section  272  may be different in each stream. 
     &lt;Operation of Radio Base Station  10  and User Terminal  20 &gt; 
     Next, operation of radio base station  10  and user terminal  20  will be described in detail. 
     Note that in the following description, the radio communication system includes AP radio base stations  10  and AP×N U  user terminals  20 . 
       FIG. 4  is a flow chart showing an operation of radio base station  10  according to the present embodiment. 
     First, radio base station  10  selects one of the candidates for BF weight (W) (ST 101 ). Radio base station  10  then transmits the discovery signal multiplied by the selected candidate for BF weight (W) to user terminal  20  (ST 102 ). If radio base station  10  has not transmitted the discovery signals corresponding to all of the candidates for BF weight (W) (ST 103 : No), radio base station  10  returns to the processing of ST 101  and ST 102  and transmits the discovery signal multiplied by another candidate for BF weight (W). 
     The discovery signal multiplied by the candidate for the BF weight is not precoded and is transmitted to all antennas  201  of all user terminals  20 . The discovery signal may be assigned to, for example, radio resources (subcarriers) in one symbol (for example, one OFDM symbol) based on frequency-division multiplexing or may be assigned to a plurality of symbols based on time-division multiplexing. The discovery signal may also be transmitted between a plurality of radio base stations  10  based on time-division multiplexing, frequency-division multiplexing, or code-division multiplexing. In this way, the method of multiplexing and transmitting the discovery signals to the radio resources allows radio base station  10  to efficiently select the BF weight described later. 
     For example, a reception signal (reception signal vector) of the discovery signal transmitted from i-th radio base station  10  in each user terminal  20  is expressed by the following equation 3. 
     
       
         
           
             
               
                 
                   [ 
                   3 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     r 
                     i 
                   
                   = 
                   
                     
                       
                         
                           [ 
                           
                             
                               
                                 
                                   r 
                                   
                                     i 
                                     , 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     
                                       
                                         r 
                                         
                                           i 
                                           , 
                                           1 
                                         
                                       
                                     
                                   
                                   
                                     
                                       
                                         
                                           
                                             ⋮ 
                                           
                                         
                                         
                                           
                                             
                                               r 
                                               
                                                 i 
                                                 , 
                                                 
                                                   
                                                     N 
                                                     
                                                       AP 
                                                       - 
                                                       1 
                                                     
                                                   
                                                   ⁢ 
                                                   
                                                     N 
                                                     
                                                       U 
                                                       - 
                                                       1 
                                                     
                                                   
                                                 
                                               
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   H 
                                   
                                     i 
                                     , 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     
                                       
                                         H 
                                         
                                           i 
                                           , 
                                           1 
                                         
                                       
                                     
                                   
                                   
                                     
                                       ⋮ 
                                     
                                   
                                   
                                     
                                       
                                         H 
                                         
                                           i 
                                           , 
                                           
                                             
                                               
                                                 N 
                                                 
                                                   AP 
                                                   - 
                                                   1 
                                                 
                                               
                                               ⁢ 
                                               
                                                 N 
                                                 U 
                                               
                                             
                                             - 
                                             1 
                                           
                                         
                                       
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         w 
                         
                           i 
                           , 
                           x 
                         
                       
                       ⁢ 
                       p 
                     
                     + 
                     
                       z 
                       i 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     In equation 3, r i,j  (j=0 to N AP-1 N U −1) represents a reception signal from i-th radio base station  10  in j-th user terminal  20 , H i,j  (j=0 to N AP-1 N U −1) represents channel information (channel matrix) between j-th user terminal  20  and i-th radio base station  10 , w i,x  represents x-th BF weight candidate (vector) in i-th radio base station  10 , p represents a discovery signal, and z i  represents noise corresponding to i-th radio base station  10 . 
     Each user terminal  20  uses the reception signal (discovery signal) indicated in equation 3 to measure the candidate weight information (for example, SNR or received power) and feeds back the candidate weight information to radio base station  10 . 
     After transmitting the discovery signals corresponding to all of the candidates for BF weight (W) (ST 103 : Yes), radio base station  10  uses the candidate weight information fed back from each user terminal  20  to select radio base station  10  as the access point of each user terminal  20  (ST 104 ). For example, radio base station  10  follows a maximum received power standard indicated in the following equation 4 to select radio base station  10  (i j   opt -th radio base station  10 ) that generates BF weight w i,x  with maximum received power (∥H i,j w i,x p+z i ∥ 2 ) as the access point of j-th user terminal  20 . 
     
       
         
           
             
               
                 
                   [ 
                   4 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     i 
                     j 
                     opt 
                   
                   = 
                   
                     
                       
                         arg 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         max 
                       
                       
                         i 
                         , 
                         x 
                       
                     
                     ⁢ 
                     
                       
                          
                         
                           
                             
                               H 
                               
                                 i 
                                 , 
                                 j 
                               
                             
                             ⁢ 
                             
                               w 
                               
                                 i 
                                 , 
                                 x 
                               
                             
                             ⁢ 
                             p 
                           
                           + 
                           
                             z 
                             
                               i 
                               , 
                               j 
                             
                           
                         
                          
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     For example, radio base station  10 A is selected as the access point of user terminals  20 - 1  to  20 - 3 , and radio base station  10 B is selected as the access point of user terminals  20 - 4  to  20 - 6  in the example of the radio communication system shown in  FIG. 1 . 
     Next, radio base station  10  selects (decides) BF weight (W) to be used for beam-forming from the BF weight candidates based on the candidate weight information used to select the access point (ST 104 ) of user terminal  20  (ST 105 ). Radio base station  10  may select, for example, L BF weights not overlapping with each other, in descending order of SNR or received power indicated in the candidate weight information. For example, BF weight vector W i   opt  including L BF weights of i-th radio base station  10  is expressed by the following equation 5.
 
[5]
 
 W   i   opt =[ w   i,1   . . . w   i,L ]  (Equation 5)
 
     Next, radio base station  10  uses the candidate weight information fed back from each user terminal  20  to compute SIR (Signal to Interference power Ratio) indicating a signal to site interference power ratio of user terminal  20  other than user terminals  20  connected to radio base station selected in ST 104  (that is, user terminal  20  connected to another radio base station  10 ). For example, i-th radio base station  10  computes SIR j  of j-th user terminal  20  according to the following equation 6. 
     
       
         
           
             
               
                 
                   [ 
                   6 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     SIR 
                     j 
                   
                   = 
                   
                     
                       
                          
                         
                           
                             
                               H 
                               
                                 i 
                                 , 
                                 j 
                               
                             
                             ⁢ 
                             
                               W 
                               i 
                               opt 
                             
                             ⁢ 
                             p 
                           
                           + 
                           
                             z 
                             
                               i 
                               , 
                               j 
                             
                           
                         
                          
                       
                       2 
                     
                     
                       
                         ∑ 
                         
                           k 
                           ≠ 
                           i 
                         
                       
                       ⁢ 
                       
                         
                            
                           
                             
                               
                                 H 
                                 
                                   k 
                                   , 
                                   j 
                                 
                               
                               ⁢ 
                               
                                 W 
                                 k 
                                 opt 
                               
                               ⁢ 
                               p 
                             
                             + 
                             
                               z 
                               
                                 k 
                                 , 
                                 j 
                               
                             
                           
                            
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
     More specifically, a numerator component of equation 6 indicates the received power of the signal of i-th radio base station  10  (that is, desired signal of j-th user terminal  20 ), and a denominator component of equation 6 indicates the received power of the signal (that is, interference signal for j-th user terminal  20 ) of radio base station  10  (k(≠i)-th radio base station  10 ) other than i-th radio base station  10 . 
     Next, radio base station  10  determines whether to execute the extended precoding based on whether there is user terminal  20  in which the SIR computed in ST 106  is smaller than threshold σ (ST 107 ). 
     For example, when the SIR for j-th user terminal  20  connected to i-th radio base station  10  is smaller than threshold σ in equation 6, k(≠i)-th radio base station  10  that most interferes with j-th user terminal  20  may determine to generate the extended precoding matrix. Note that radio base station  10  that generates the extended precoding matrix is not limited to radio base station  10  that most interferes with j-th user terminal  20 . A predetermined number of radio base stations  10  in descending order of interference with j-th user terminal  20  or all radio base stations  10  that interfere with j-th user terminal  20  may generate the extended precoding matrix. 
     If k-th radio base station  10  determines to execute the extended precoding (ST 107 : Yes), k-th radio base station  10  multiplies the reference signal by BF weight (W k   opt ) selected in ST 105  and transmits the reference signal multiplied by the BF weight to user terminal  20  (ST 108 ). 
     User terminal  20  uses the received reference signal to estimate channel information (HW) and feeds back estimated channel information HW (channel estimation value) to radio base station  10 . User terminal  20  also uses estimated channel information HW to generate a postcoding matrix. 
     Radio base station  10  uses channel information HW fed back from user terminal  20  to generate extended precoding matrix (P opt ) (ST 109 ). For example, k-th radio base station  10  uses channel information (equivalent channel matrix) H k   ext W k   ext  including channel information HW of (kN U )-th to ((k+1)N U −1)-th user terminals  20  connected to k-th radio base station  10  and channel information HW of j-th user terminal  20  with SIR smaller than threshold σ that is connected to i-th radio base station  10  to compute extended precoding matrix P k   opt  according to the following equation 7. 
     
       
         
           
             
               
                 
                   [ 
                   7 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       H 
                       k 
                       ext 
                     
                     ⁢ 
                     
                       W 
                       k 
                       ext 
                     
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               H 
                               
                                 k 
                                 , 
                                 
                                   kN 
                                   U 
                                 
                               
                             
                           
                         
                         
                           
                             
                               
                                 
                                   
                                     H 
                                     
                                       k 
                                       , 
                                       
                                         
                                           kN 
                                           U 
                                         
                                         + 
                                         1 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   ⋮ 
                                 
                               
                               
                                 
                                   
                                     H 
                                     
                                       k 
                                       , 
                                       
                                         
                                           
                                             ( 
                                             
                                               k 
                                               + 
                                               1 
                                             
                                             ) 
                                           
                                           ⁢ 
                                           
                                             N 
                                             U 
                                           
                                         
                                         - 
                                         1 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     H 
                                     
                                       k 
                                       , 
                                       j 
                                     
                                   
                                 
                               
                             
                           
                         
                       
                       ] 
                     
                     ⁢ 
                     
                       W 
                       k 
                       opt 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ) 
                 
               
             
           
         
       
     
     Therefore, k-th radio base station  10  computes extended precoding matrix P k   opt  in consideration of the channel information of user terminal  20  connected to k-th radio base station  10  as well as the channel information of j-th user terminal  20  with respect to k-th radio base station  10 , j-th user terminal  20  being interfered by k-th radio base station  10  and connected to i-th radio base station  10 . The k-th radio base station  10  uses the equivalent channel matrix indicated in equation 7 to generate extended precoding matrix P k   opt  for removing the interference for j-th user terminal  20  interfered by k-th radio base station  10 . 
     Therefore, the extended precoding in k-th radio base station  10  is applied based on the channel information including the channels between k-th radio base station  10  and user terminals  20  connected to k-th radio base stations  10  and the channel between k-th radio base station  10  and user terminal  20  connected to radio base station  10  (i-th radio base station  10 ) other than k-th radio base station  10 . 
     Radio base station  10  then multiplies the signal (information) of the stream by extended precoding matrix (P k   opt ) and BF weight (W k   opt ) and transmits the signal of the stream to user terminal  20  (ST 110 ). In this case, radio base station  10  does not transmit, to user terminal  20  connected to another radio base station  10  (user terminal  20  in which the SIR is smaller than threshold σ), the signal in which the channel information is considered for the signal when the extended precoding matrix is generated. 
     For example, reception signal r k   ext  received by (kN U )-th to ((k+1)N U −1)-th user terminals  20  connected to k-th radio base station  10  is expressed by the following equation 8. 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     [ 
                     8 
                     ] 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           r 
                           k 
                           ext 
                         
                         = 
                         
                           
                             [ 
                             
                               
                                 
                                   
                                     r 
                                     
                                       k 
                                       , 
                                       
                                         kN 
                                         U 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       
                                         ⋮ 
                                       
                                     
                                     
                                       
                                         
                                           
                                             
                                               
                                                 r 
                                                 
                                                   k 
                                                   , 
                                                   
                                                     
                                                       
                                                         ( 
                                                         
                                                           k 
                                                           + 
                                                           1 
                                                         
                                                         ) 
                                                       
                                                       ⁢ 
                                                       
                                                         N 
                                                         U 
                                                       
                                                     
                                                     - 
                                                     1 
                                                   
                                                 
                                               
                                             
                                           
                                           
                                             
                                               0 
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                               
                             
                             ] 
                           
                           = 
                           
                             
                               
                                 H 
                                 k 
                                 ext 
                               
                               ⁢ 
                               
                                 W 
                                 k 
                                 opt 
                               
                               ⁢ 
                               
                                 
                                   P 
                                   k 
                                   opt 
                                 
                                 ⁡ 
                                 
                                   [ 
                                   
                                     
                                       
                                         
                                           d 
                                           
                                             k 
                                             , 
                                             
                                               kN 
                                               U 
                                             
                                           
                                         
                                       
                                     
                                     
                                       
                                         
                                           
                                             
                                               ⋮ 
                                             
                                           
                                           
                                             
                                               
                                                 
                                                   
                                                     
                                                       d 
                                                       
                                                         k 
                                                         , 
                                                         
                                                           
                                                             
                                                             ( 
                                                             
                                                             k 
                                                             + 
                                                             1 
                                                             
                                                             ) 
                                                             
                                                             ⁢ 
                                                             
                                                             N 
                                                             U 
                                                             
                                                           
                                                           - 
                                                           1 
                                                         
                                                       
                                                     
                                                   
                                                 
                                                 
                                                   
                                                     0 
                                                   
                                                 
                                               
                                             
                                           
                                         
                                       
                                     
                                   
                                   ] 
                                 
                               
                             
                             + 
                             
                               z 
                               k 
                               ext 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             
                               [ 
                               
                                 
                                   
                                     
                                       
                                         H 
                                         ~ 
                                       
                                       
                                         k 
                                         , 
                                         
                                           kN 
                                           U 
                                         
                                       
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     0 
                                   
                                 
                                 
                                   
                                     
                                         
                                     
                                   
                                   
                                     ⋱ 
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                 
                                 
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                       
                                         H 
                                         ~ 
                                       
                                       
                                         k 
                                         , 
                                         
                                           
                                             
                                               ( 
                                               
                                                 k 
                                                 + 
                                                 1 
                                               
                                               ) 
                                             
                                             ⁢ 
                                             
                                               N 
                                               U 
                                             
                                           
                                           - 
                                           1 
                                         
                                       
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                 
                                 
                                   
                                     0 
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                       
                                         H 
                                         ~ 
                                       
                                       
                                         k 
                                         , 
                                         j 
                                       
                                     
                                   
                                 
                               
                               ] 
                             
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       d 
                                       
                                         k 
                                         , 
                                         
                                           kN 
                                           U 
                                         
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       
                                         
                                           ⋮ 
                                         
                                       
                                       
                                         
                                           
                                             
                                               
                                                 
                                                   d 
                                                   
                                                     k 
                                                     , 
                                                     
                                                       
                                                         
                                                           ( 
                                                           
                                                             k 
                                                             + 
                                                             1 
                                                           
                                                           ) 
                                                         
                                                         ⁢ 
                                                         
                                                           N 
                                                           U 
                                                         
                                                       
                                                       - 
                                                       1 
                                                     
                                                   
                                                 
                                               
                                             
                                             
                                               
                                                 0 
                                               
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           + 
                           
                             z 
                             k 
                             ext 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ) 
                 
               
             
           
         
       
     
     As indicated in equation 8, k-th radio base station  10  does not transmit the data to j-th user terminal  20  interfered by k-th radio base station  10  (that is, d k,j =0). 
     The data multiplied by extended precoding matrix (P k   opt ) is transmitted by avoiding the interference of j-th user terminal  20 . User terminal  20  multiplies the received signal of the stream by postcoding matrix (B ISI ) to demodulate the signal (data) of the stream (not illustrated). The inter-stream interference is rejected in the data multiplied by precoding matrix (P ISI ) and postcoding matrix (B ISI ). 
     On the other hand, if radio base station  10  determines not to execute the extended precoding in  FIG. 4  (ST 107 : No), radio base station  10  multiplies the reference signal by BF weight (W k   opt ) selected in ST 105  and transmits the reference signal multiplied by the BF weight to user terminal  20  (ST 111 ). User terminal  20  uses the received reference signal to estimate channel information (HW) and feeds back estimated channel information HW (channel estimation value) to radio base station  10 . User terminal  20  also uses estimated channel information HW to generate a postcoding matrix. 
     Radio base station  10  uses channel information HW fed back from user terminal  20  to generate normal precoding matrix (P) (ST 112 ). For example, k-th radio base station  10  uses channel information (equivalent channel matrix) HW including channel information HW of (kN U )-th to ((k+1)N U −1)-th user terminals  20  connected to k-th radio base station  10  to compute precoding matrix P. 
     Radio base station  10  then multiplies the signal (information) of the stream by precoding matrix (P) and BF weight (W k   opt ) and transmits the signal of the stream to user terminal  20  (ST 113 ). 
     An example will be described, in which the SIR of user terminal  20 - 3  with respect to radio base station  10 A is smaller than threshold σ, and radio base station  10 B most interferes with user terminal  20 - 3  in the radio communication system shown in  FIG. 1 . The SIRs of all of user terminals  20 - 4  to  20 - 6  with respect to radio base station  10 B are equal to or greater than threshold σ. 
     In this case, radio base station  10 A determines not to execute the extended precoding. More specifically, radio base station  10 A uses channel information HW of user terminals  20 - 1  to  20 - 3  connected to radio base station  10 A to generate the normal precoding matrix. Therefore, spatially separated group A as a group to be precoded (spatially separated) in radio base station  10 A includes user terminals  20 - 1  to  20 - 3  connected to radio base station  10 A as shown in  FIG. 1 . 
     On the other hand, radio base station  10 B determines to execute the extended precoding. More specifically, radio base station  10 B uses channel information HW of user terminal  20 - 3  connected to radio base station  10 A in addition to channel information HW of user terminals  20 - 4  to  20 - 6  connected to radio base station  10 B to generate the extended precoding matrix. Therefore, the group (spatially separated group B) to be precoded (spatially separated) in radio base station  10 B includes user terminal  20  interfered by radio base station  10 B in addition to user terminals  20 - 4  to  20 - 6  connected to radio base station  10 B as shown in  FIG. 1 . 
     In other words, user terminal  20 - 3  shown in  FIG. 1  is included in both of spatially separated group A and spatially separated group B. Therefore, the channel information of user terminal  20 - 3  connected to radio base station  10 A and significantly interfered by radio base station  10 B is used to generate the precoding matrix in both radio base station  10 A and radio base station  10 B. In this case, a precoding matrix for removing the interference (ISI or IUI) with respect to the stream for user terminal  20 - 3  is generated in radio base station  10 A, and an extended precoding for removing signal components interfering with user terminal  20 - 3  is generated in radio base station  10 B. 
     According to the processing, user terminal  20 - 3  can efficiently receive the stream from radio base station  10 A while rejecting the interference from radio base station  10 B. 
     Advantageous Effects of Present Embodiment 
     In this way, the extended precoding in radio base station  10 B of radio base stations  10  shown in  FIG. 1  is applied in the present embodiment based on, for example, the channel information including the channels between radio base station  10 B and user terminals  20 - 4  to  20 - 6  connected to radio base station  10 B and the channel between radio base station  10 B and user terminal  20 - 3  connected to radio base station  10 A other than radio base station  10 B. 
     Therefore, the spatial separation processing for user terminals  20  is executed in all of the sites in consideration of the interference between the plurality of sites formed by radio base stations  10 , and the interference between the sites can be reduced. More specifically, radio base station  10 A and radio base station  10 B cooperate to execute the spatial separation processing for user terminal  20 - 3  connected to radio base station  10 A in the present embodiment. 
     Therefore, according to the present embodiment, the beams can be appropriately controlled in the multi-site environment, and the efficiency of the MU-MIMO transmission can be improved. 
     Modifications of Embodiment 1 
     Note that the parameter used as a reference in selecting the BF weight or selecting the access point of user terminal  20  is not limited to the SNR or the received power in the present embodiment, and some candidate weight information (for example, reception correlation) measured by using the discovery signal multiplied by the BF weight candidate may be used. 
     In the case described in the present embodiment, the discovery signal transmitted from radio base station  10  to user terminal  20  is used to measure the candidate weight information (for example, received power of BF weight candidate). However, the present embodiment is not limited to this, and for example, user terminal  20  may transmit the reference signal to radio base station  10 , and radio base station  10  may use the received reference signal to select the BF weight, select the access point of user terminal  20 , or generate the precoding matrix. According to the processing, the feedback of the discovery signal, the candidate weight information, and channel information (HW) are not necessary. Therefore, the use of the radio resources in the channel estimation processing can be reduced. 
     In the case described in the present embodiment, the access point of user terminal  20  is selected based on the maximum received power standard regarding radio base stations  10  as indicated in equation 4. However, the present embodiment is not limited to this, and another method may be used to select the access point of user terminal  20 . In the present embodiment, the access point of user terminal  20  may be selected based on another standard, such as a maximum signal to interference power ratio standard, instead of the maximum received power standard. 
     In the case described in the present embodiment, the BF weight is selected based on the maximum received power standard in each radio base station  10 . However, the present embodiment is not limited to this, and the BF weight may be selected based on another standard, such as a maximum signal to interference power ratio standard. 
     In the example described in the present embodiment, there is one user terminal  20  in which the SIR is smaller than threshold σ in equation 8. However, when there are a plurality of user terminals  20  in which the SIR is smaller than threshold σ, radio base station  10  may use the equivalent channel matrix including the channel information of the plurality of user terminals  20  to compute the extended precoding matrix. According to the processing, the extended precoding matrix can be used to reduce the interference for each user terminal  20  in which the SIR is smaller than threshold σ. 
     In the case described in the present embodiment, whether the SIR of user terminal  20  is smaller than threshold σ is judged to determine whether radio base station  10  executes the extended precoding. However, the present embodiment is not limited to this, and whether to execute the extended precoding matrix may be determined based on another standard. For example, radio base station  10  may apply the extended precoding to user terminals  20  with SIR in bottom x % among user terminals  20 . According to the process, radio base station  10  can surely reduce the interference for user terminals  20  with SIR in bottom x %. 
     In the case described in the present embodiment, radio base station  10  interfering with user terminal  20  (radio base station  10  executing the extended precoding) does not transmit the user data to user terminal  20  (for example, d k,j =0 in equation 8). However, the present embodiment is not limited to this, and when the user data can be transmitted between radio base stations  10  in the backhaul, radio base station  10  may receive the user data from another radio base station  10  that is the access point of user terminal  20  interfered by radio base station  10  and transmit the user data to user terminal  20 . Therefore, radio base station  10  interfering with user terminal  20  generates the extended precoding matrix for removing the interference of the user data for user terminal  20  in this case. According to the processing, the interference can be rejected for user terminal  20 , and the throughput can be improved. 
     In the case described in the present embodiment, the SIR is used as a determination criterion for the execution of the extended precoding. However, the determination criterion for the execution of the extended precoding is not limited to this, and other parameters indicating the status of interference between sites may be used. 
     Embodiment 2 
     The spatial separation processing for the space influenced by a plurality of sites is described in Embodiment 1. In contrast, a plurality of radio base stations apply beams in the space influenced by a plurality of sites to transmit data in a case described in the present embodiment. 
     Note that the basic configurations of the radio base station and the user terminal according to the present embodiment are the same as the basic configurations of radio base station  10  and user terminal  20  according to Embodiment 1, and the radio base station and the user terminal will be described with reference to  FIGS. 2 and 3 . 
       FIG. 5  shows a configuration example of a radio communication system according to the present embodiment. 
     As in Embodiment 1, the radio communication system according to the present embodiment includes a plurality of radio base stations  10  (for example, Massive MIMO base stations) and at least one user terminal  20 . Each user terminal  20  is connected to at least one radio base station  10 . In the example illustrated in  FIG. 5 , user terminals  20 - 1  to  20 - 3  are connected to radio base station  10 A, and user terminals  20 - 3  to  20 - 5  are connected to radio base station  10 B. Therefore, user terminal  20 - 3  is connected to both of radio base station  10 A and radio base station  10 B. 
       FIG. 6  is a flow chart illustrating operation of radio base station  10  according to the present embodiment. Note that in  FIG. 6 , the same processing as in Embodiment 1 ( FIG. 4 ) are provided with the same reference signs, and the description will not be repeated. 
     Radio base station  10  (access point selection section  104  and weight selection section  106 ) transmits the discovery signals corresponding to all of the candidates for BF weight (W) (ST 103 : Yes) and then uses the candidate weight information (received power or SNR) fed back from each user terminal  20  to select radio base station  10  as the access point of each user terminal  20  and the BF weight (ST 201 ). In this case, radio base station  10  selects the access point of user terminal  20  and the BF weight based on a maximum received power standard obtained by adding the candidate weight information for a plurality of radio base stations  10 . For example, each radio base station  10  may receive the candidate weight information for other radio base stations  10  from other radio base stations  10  through inter-base station communication section  105 . 
     For example, i-th radio base station  10  selects, for example, user terminal (j) and BF weight (x) with maximum received power (∥H i,j w i,x p+z i,j ∥ 2 ) according to the maximum received power standard indicated in the following equation 9. 
     
       
         
           
             
               
                 
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                   9 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     ( 
                     
                       
                         j 
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                     ⁢ 
                     
                         
                     
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     For example, i-th radio base station  10  decides i-th radio base station  10  as the access point of j opt -th user terminal  20  selected according to equation 9. As indicated in the following equation 10, i-th radio base station  10  further decides 1-th BF weight wo of i-th radio base station  10  as BF weight (W i,xopt ) selected according to equation 9 (for example, 1=1 to L).
 
[10]
 
 w   i,l   =w   i,x   opt   (Equation 10)
 
     Therefore, each radio base station  10  selects L beams (combinations of user terminal (j) and BF weight (x)) with maximum received power. Radio base station  10  may select, for example, L BF weights in descending order of received power and user terminals  20  corresponding to the BF weights not overlapping with each other. BF weight vector W i   opt  constituted by L BF weights of i-th radio base station  10  is expressed by, for example, the following equation 11.
 
[11]
 
 W   i   opt =[ w   i,1   . . . w   i,L ].  (Equation 11)
 
     As a result, a plurality of radio base stations  10  apply beams to user terminal  20  selected by radio base stations  10 , and the data is transmitted. 
     For example, in the example of the radio communication system shown in  FIG. 5 , radio base station  10 A selects user terminals  20 - 1  to  20 - 3  as user terminals  20  to be connected to radio base station  10 A, and radio base station  10 B selects user terminals  20 - 3  to  20 - 5  as user terminals  20  to be connected to radio base station  10 B. Therefore, user terminal  20 - 3  is connected to both of radio base station  10 A and radio base station  10 B. 
     Next, radio base station  10  (judgement section  107 ) determines whether to execute cooperative precoding based on the access point of user terminal  20  selected in ST 201  (ST 202 ). 
     Here, the cooperative precoding is precoding executed for user terminal  20  connected to a plurality of radio base stations  10  by applying an equivalent channel matrix constituted by channel estimation values between radio base stations  10  connected to user terminal  20  and user terminal  20 . Hereinafter, the precoding matrix generated by using the channel estimation values between radio base stations  10  connected to user terminal  20  and user terminal  20  will be referred to as a “cooperative precoding matrix”. 
     Specifically, radio base station  10  determines to execute the cooperative precoding for user terminal  20  when user terminal  20  connected to radio base station  10  is also connected to another radio base station  10 . For example, each radio base station  10  may receive the information indicating user terminal  20  connected to another radio base station  10  from radio base station  10  through inter-base station communication section  105 . For example, radio base station  10 A and radio base station  10 B determine to execute the cooperative precoding in  FIG. 5  because user terminal  20 - 3  is connected to both. 
     If radio base station  10  (precoding matrix generation section  108 ) determines to execute the cooperative precoding (ST 202 : Yes), radio base station  10  uses channel information HW fed back from user terminal  20  in ST 108  to generate the cooperative precoding matrix (ST 203 ). 
     Specifically, of the precoding matrices, radio base station  10  computes precoding matrix (P IUI ) for rejecting the inter-user interference based on the channel information between radio base station  10  and user terminal  20  connected to radio base station  10 . 
     On the other hand, of the precoding matrices, radio base station  10  computes precoding matrix (P ISI ) for rejecting the inter-stream interference based on the channel information between user terminal  20  and a plurality of radio base stations  10  (including radio base station  10 ) connected with user terminal  20 . 
     For example, when j-th user terminal  20  is connected to i #1 -th radio base station  10  and i #2 -th radio base station  10 , each of i #1 -th radio base station  10  and i #2 -th radio base station  10  uses channel information {tilde over (H)} i   cmp  indicated in the following equation 12 to generate cooperative precoding matrix (P ISI ) for j-th user terminal  20 .
 
[12]
 
 {tilde over (H)}   j   cmp =[ {tilde over (H)}   i     #1     ,j   {tilde over (H)}   i     #2     ,j ]  (Equation 12)
 
     In equation 12, {tilde over (H)} i #1,j  represents the channel information between i #1 -th radio base station  10  and j-th user terminal  20 , and {tilde over (H)} i #2,j  represents the channel information between i #2 -th radio base station  10  and j-th user terminal  20 . 
     Therefore, for user terminal  20  connected to a plurality of radio base stations  10  including radio base station  10 , each radio base station  10  generates cooperative precoding matrix (P ISI ) for removing the interference between a plurality of streams transmitted from radio base stations  10  to user terminal  20 . 
     For example, in  FIG. 5 , each of radio base station  10 A and radio base station  10 B uses the channel information (for example, equation 12) including the channel information between radio base station  10 A and user terminal  20 - 3  and the channel information between radio base station  10 B and user terminal  20 - 3  to generate cooperative precoding matrix (P ISI ) Therefore, as shown in  FIG. 5 , user terminal  20 - 3  is included in both of the group (spatially separated group A) to be precoded (spatially separated) in radio base station  10 A and spatially separated group B of radio base station  10 B. 
     Radio base station  10 A and radio base station  10 B shown in  FIG. 5  then transmit the data multiplied by precoding matrices (P IUI , P ISI ) to user terminal  20 - 3 . User terminal  20 - 3  multiplies the received signals of the streams by postcoding matrix (B ISI ) to demodulate the signals (data) of the streams. In the data multiplied by cooperative precoding matrix (P ISI ) and postcoding matrix (B ISI ), the inter-stream interference is rejected for all of the streams from radio base station  10 A and the streams from radio base station  10 B. 
     According to the processing, user terminal  20 - 3  can efficiently receive the data of the beams applied from radio base stations  10 A and  10 B while rejecting the inter-stream interference from radio base stations  10 A and  10 B. 
     Advantageous Effects of Present Embodiment 
     In this way, when user terminal  20  is connected to a plurality of radio base stations  10  in the present embodiment, the cooperative precoding in radio base stations  10  is applied to remove the interference between a plurality of streams transmitted from radio base stations  10 . 
     Therefore, when a plurality of radio base stations  10  cooperate to apply beams to one user terminal  20  to transmit data, the spatial separation processing is executed for user terminal  20  in consideration of the interference between the streams transmitted from radio base stations  10 , and the interference between the streams can be reduced. 
     According to the present embodiment, the beams can be appropriately controlled in the multi-site environment, and the efficiency of the MU-MIMO transmission can be improved. 
     Modifications of Embodiment 2 
     Note that the parameter used as a reference in selecting the BF weight or selecting the access point of user terminal  20  is not limited to the SNR or the received power in the present embodiment, and some candidate weight information (for example, reception correlation) measured by using the discovery signal multiplied by the BF weight candidate may be used as in Embodiment 1. 
     In the case described in the present embodiment, the discovery signal transmitted from radio base station  10  to user terminal  20  is used to measure the candidate weight information (for example, received power of BF weight candidate). However, the present embodiment is not limited to this, and for example, user terminal  20  may transmit the reference signal to radio base station  10 , and radio base station  10  may use the received reference signal to select the BF weight, select the access point of user terminal  20 , or generate the precoding matrix. According to the processing, the feedback of the discovery signal, the candidate weight information, and channel information (HW) is not necessary. Therefore, the use of the radio resources in the channel estimation processing can be reduced. 
     In the case described in the present embodiment, the access point of user terminal  20  and the BF weight are selected based on the maximum received power standard for a plurality of radio base stations  10  as indicated in equation 9. However, the present embodiment is not limited to this, and another method may be used to select the access point of user terminal  20  and the BF weight. In the present embodiment, the access point of user terminal  20  and the BF weight may be selected based on another standard, such as a maximum signal to interference power ratio standard, instead of the maximum received power standard. 
     In the present embodiment, the transmission speed may be improved by processing of multiplexing and transmitting different data streams transmitted from a plurality of radio base stations  10  to one user terminal  20 , or the quality may be improved by processing of diversity transmission of the same stream. 
     In the case described in the present embodiment, cooperative precoding matrix (P ISI ) is generated for user terminal  20  connected to a plurality of radio base stations  10  in consideration of the inter-stream interference in user terminal  20 . However, the present embodiment is not limited to this, and the channel information (for example, see equation 12) between user terminal  20  and a plurality of radio base stations  10  may be used to generate the entire precoding matrices, that is, P ISI  and precoding matrix (P IUI ) for the inter-user interference, for user terminal  20  connected to a plurality of radio base stations  10 . 
     Embodiment 3 
     Beam-forming control according to the deviation in the amount of data transmission between a plurality of sites will be described in the present embodiment. 
     Note that the basic configurations of a radio base station and a user terminal according to the present embodiment are the same as the basic configurations of radio base station  10  and user terminal  20  according to Embodiment 1, and the radio base station and the user terminal will be described with reference to  FIGS. 2 and 3 . 
       FIGS. 7A and 7B  illustrate configuration examples of a radio communication system according to the present embodiment. 
     The radio communication system according to the present embodiment includes a plurality of radio base stations  10  (for example, Massive MIMO base stations), at least one user terminal  20 , and control station  30 . Each user terminal  20  is connected to at least one radio base station  10 . In the example illustrated in  FIG. 5 , user terminals  20 - 1  to  20 - 3  are connected to radio base station  10 A, and user terminals  20 - 4  and  20 - 5  are connected to radio base station  10 B. 
     Control station  30  is connected to a plurality of radio base stations  10 . Control station  30  monitors the amount of data transmission of each radio base station  10 . Control station  30  instructs, for example, radio base station  10  with the amount of data transmission smaller than predetermined threshold T (that is, radio base station  10  with an extra assignable resource) to form beams for a peripheral site. Note that the processing similar to the processing by control station  30  may be executed by any of radio base stations  10 . 
     For example, control station  30  detects that the amount of data transmission of radio base station  10 B is smaller than threshold T in  FIG. 7A . Therefore, control station  30  instructs radio base station  10 B to change the BF weight to form beams for the site of neighboring radio base station  10 A (details will be described later). For example, control station  30  here instructs radio base station  10 B to form beams for user terminal  20 - 3  connected to radio base station  10 A. 
     As a result of the processing, radio base station  10 B forms beams for user terminal  20 - 3  connected to radio base station  10 A as shown in  FIG. 7B . Therefore, user terminal  20 - 3  is connected to both of radio base station  10 A and radio base station  10 B. Note that the beam control for user terminal  20 - 3  in radio base station  10 A and radio base station  10 B is, for example, the same as the processing described in Embodiment 2, and the details will not be described here. 
     According to the processing, the extra resources in radio base station  10 B can be effectively used to improve the system throughput. 
       FIG. 8  is a flow chart showing operation of radio base station  10  and control station  30  according to the present embodiment. Note that in  FIG. 8 , the same reference signs are provided to the same processing as in Embodiment 1 ( FIG. 4 ) or Embodiment 2 ( FIG. 6 ), and the description will not be repeated. 
     In  FIG. 8 , each radio base station  10  first executes the beam-forming processing and the precoding processing in the site formed by each radio base station  10 . More specifically, each radio base station  10  selects the BF weight in ST 105  and computes the precoding matrix in ST 112  to communicate with user terminals  20  connected to radio base station  10  (ST 113 ). 
     If radio base station  10  has transmitted all of the data (information) to be transmitted to user terminals  20  (ST 301 : Yes), the processing illustrated in  FIG. 8  ends. 
     On the other hand, if radio base station  10  has not transmitted all of the data (information) to be transmitted to user terminals  20  (ST 301 : No), control station  30  monitors the amount of transmission of a plurality of radio base stations  10  (ST 302 ). Control station  30  then determines whether there is radio base station  10  in which the monitored amount of data transmission is smaller than threshold T (ST 303 ). 
     Control station  30  does not do anything for radio base station  10  with the amount of data transmission equal to or greater than threshold T (ST 303 : No) and continues to monitor the amount of data transmission (ST 302 , ST 303 ). Therefore, radio base station  10  with the amount of data transmission equal to or greater than threshold T continues the processing of ST 111 , ST 112 , and ST 113 . 
     On the other hand, control station  30  instructs radio base station  10  with the amount of data transmission smaller than threshold T (ST 303 : Yes) to change the BF weight to form beams for a peripheral site. Radio base station  10  instructed to change the BF weight changes the BF weight (ST 304 ). Radio base station  10  then generates, for example, the cooperative precoding matrix (ST 203 ) as in Embodiment 2 and transmits the data (information) to user terminal  20  (ST 113 ). 
     Hereinafter, an example of the method of changing the BF weight by i-th radio base station  10  will be described. 
     Specifically, i-th radio base station  10  selects an l del -th beam according to a minimum received power standard indicated in the following equation 13 from currently selected L (l=1 to L) beams. The i-th radio base station  10  may delete L Ind  beams selected according to equation 13, for example. 
     
       
         
           
             
               
                 
                   [ 
                   13 
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     Then, i-th radio base station  10  selects an l add -th beam to be formed in a peripheral site (site formed by k-th radio base station  10  here) according to a maximum received power standard indicated in the following equation 14. The i-th radio base station  10  may newly form L Ind  beams selected according to equation 14, for example. 
     
       
         
           
             
               
                 
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     Therefore, i-th radio base station  10  deletes the beams with little influence of deletion on the data transmission in the site of radio base station  10  (here, beams selected according to the minimum received power standard indicated by equation 13). Furthermore, i-th radio base station  10  changes the beams to beams highly efficient for the data transmission in the peripheral site (here, beams selected according to the maximum received power standard indicated in equation 14). 
     Then, i-th radio base station  10  and k-th radio base station  10  use, for example, the channel information (for example, equation 12) between both of radio base stations  10  and user terminal  20 - 3  to generate cooperative precoding matrix (P ISI ) for j-th user terminal  20  (ST 203  shown in  FIG. 8 ) as in Embodiment 2. 
     As for the site in which i-th radio base station  10  forms the beams, a site with a larger amount of data transmission (that is, a site that requires more resources) may be selected from sites around the site formed by i-th radio base station  10 , or a site may be selected based on another standard. Furthermore, the data transmitted from i-th radio base station  10  to j-th user terminal  20  may be transferred from k-th radio base station  10  through inter-base station communication section  105  (backhaul), for example. 
     In this way, a plurality of radio base stations  10  can assign the resources (beams) that can be used by radio base stations  10  to user terminals  20  connected to other radio base stations  10  according to the usage of the resources (for example, deviation in amount of data transmission). Therefore, the resources of all of the sites can be efficiently used to control the beams. 
     Note that in the case described in the flow chart illustrated in  FIG. 8 , the processing shifts to the transmission of the reference signal (ST 111  or ST 305 ) after the judgement of the amount of data transmission in ST 302  (and after the change in the BF weight). However, radio base station  10  may shift to the transmission of the discovery signal (ST 101 ) after the judgement of the amount of data transmission in ST 302  and change the BF weight based on the candidate weight information measured by using the discovery signal in ST 304 . According to the processing, radio base station  10  can use the candidate weight information (SNR or received power) reflecting the current channel status to change the BF weight, and the BF weight can be accurately deleted and selected. 
     Advantageous Effects of Present Embodiment 
     In this way, radio base station  10  with the amount of data transmission smaller than threshold T among radio base stations  10  executes the beam-forming (BF weight changing) processing to form the beams for user terminal  20  connected to radio base station  10  other than the radio base station  10  in the present embodiment. Therefore, radio base station  10  with the amount of data transmission smaller than threshold T executes the beam-forming (BF weight changing) processing to connect radio base station  10  and the user terminal connected to radio base station  10  other than the radio base station  10 . 
     Thus, radio base station  10  with a small amount of data transmission among radio base stations  10  cooperates to apply beams to the site formed by radio base station  10  with a large amount of data transmission to transmit data. According to the processing, the beams can be appropriately controlled in all of the sites in the multi-site environment, and the efficiency of the MU-MIMO transmission can be improved. 
     Furthermore, when radio base stations  10  cooperate to transmit data to one user terminal  20 , the spatial separation processing for user terminal  20  is executed in consideration of the interference between the streams transmitted from radio base stations  10  as in Embodiment 2. Therefore, the interference between the streams can be reduced. 
     Modifications of Embodiment 3 
     In the case described in the present embodiment, the amount of data transmission is used as the amount of used resources of each radio base station  10  monitored by control station  30 . However, the amount of used resources of radio base station  10  is not limited to the amount of data transmission in the present embodiment, and another parameter indicating the usage of resources in each radio base station  10 , the deviation in the amount of used resources between radio base stations  10 , or the like may also be used. Examples of the other parameter include the number of streams transmitted by radio base station  10  and the difference between the amounts of data transmitted between radio base stations  10 . 
     In the case described in the present embodiment, the standard based on the received power is used to change (delete and add) the BF weight as indicated in equation 13 and equation 14. However, the present embodiment is not limited to this, and for example, another standard based on the signal to interference power ratio may be used to change the BF weight. 
     This completes the description of embodiments. 
     In the case described in embodiments, the reference signal is used to perform the channel estimation. However, the channel estimation value (channel information) may be acquired without using the reference signal in the channel estimation. In other words, it is only necessary to acquire the channel information indicating equivalent channel matrix (HW) including the BF weight in the channel estimation. 
     Furthermore, the operation in two radio base stations  10  is described in embodiments as shown in  FIGS. 1, 5, and 7 . However, embodiments are not limited to this, and an operation similar to the operation in embodiments can be performed in a multi-site environment including three or more radio base stations  10 . 
     (Hardware Configuration) 
     Note that the block diagrams used to describe embodiments illustrate blocks on the basis of functions. The functional blocks (constituent sections) are realized by an arbitrary combination of hardware and/or software. Means for realizing the functional blocks is not particularly limited. More specifically, the functional blocks may be realized by one physically and/or logically coupled apparatus. Two or more physically and/or logically separated apparatuses may be directly and/or indirectly (for example, wired and/or wireless) connected, and the plurality of apparatuses may realize the functional blocks. 
     For example, the radio base station, the user terminal, and the like according to one embodiment of the present invention may function as a computer that executes processing of a radio communication method of the present invention.  FIG. 9  illustrates an example of a hardware configuration of the radio base station and the user terminal according to one embodiment of the present invention. Radio base station  10  and user terminal  20  may be physically constituted as a computer apparatus including processor  1001 , memory  1002 , storage  1003 , communication apparatus  1004 , input apparatus  1005 , output apparatus  1006 , bus  1007 , and the like. 
     Note that the term “apparatus” in the following description can be replaced with a circuit, a device, a unit, or the like. The hardware configurations of radio base station  10  and user terminal  20  may include one or a plurality of apparatuses illustrated in the drawings or may not include part of the apparatuses. 
     For example, although only one processor  1001  is illustrated, there may be a plurality of processors. The processing may be executed by one processor, or the processing may be executed by one or more processors at the same time, in succession, or by other methods. Note that processor  1001  may be provided by one or more chips. 
     The functions of radio base station  10  and user terminal  20  are realized by loading predetermined software (program) on the hardware of processor  1001 , memory  1002 , or the like. Processor  1001  performs operation, and the communication by communication apparatus  1004  or reading and/or writing of data in memory  1002  and storage  1003  is controlled. 
     Processor  1001  operates, for example, an operating system to control the entire computer. Processor  1001  may be constituted by a central processing unit (CPU) including an interface for peripheral apparatus, a control apparatus, an operation apparatus, a register, and the like. For example, discovery signal generation section  101 , candidate weight multiplication section  102 , reference signal generation section  103 , access point selection section  104 , weight selection section  106 , judgement section  107 , precoding matrix generation section  108 , data generation section  109 , precoding section  110 , beam-forming section  111 , candidate weight information measurement section  203 , channel estimation section  204 , postcoding matrix generation section  205 , postcoding section  206 , data reception section  207 , and the like may be realized by processor  1001 . 
     Processor  1001  executes various types of processing according to a program (program code), a software module, or data loaded from storage  1003  and/or communication apparatus  1004  to memory  1002 . The program used is a program for causing the computer to execute at least part of the operation described in embodiments. For example, at least part of the functional blocks constituting radio base station  10  and user terminal  20  may be realized by a control program stored in memory  1002  and operated by processor  1001 , and the other functional blocks may also be similarly realized. Although the various types of processing are executed by one processor  1001  in the description, the various types of processing may be executed by two or more processors  1001  at the same time or in succession. Processor  1001  may be provided by one or more chips. Note that the program may be transmitted from a network through a telecommunication line. 
     Memory  1002  is a computer-readable recording medium and may be constituted by, for example, at least one of a ROM (Read Only Memory), an EPROM (Erasable Programmable ROM), an EEPROM (Electrically Erasable Programmable ROM), and a RAM (Random Access Memory). Memory  1002  may be called a register, a cache, a main memory (main storage apparatus), or the like. Memory  1002  can save a program (program code), a software module, and the like that can be executed to carry out the radio communication method according to one embodiment of the present invention. 
     Storage  1003  is a computer-readable recording medium and may be constituted by, for example, at least one of an optical disk such as a CD-ROM (Compact Disc ROM), a hard disk drive, a flexible disk, a magneto-optical disk (for example, a compact disc, a digital versatile disk, or a Blue-ray (registered trademark) disk), a smart card, a flash memory (for example, a card, a stick, or a key drive), a floppy (registered trademark) disk, and a magnetic strip. Storage  1003  may be called an auxiliary storage apparatus. The storage medium may be, for example, a database, a server, or other appropriate media including memory  1002  and/or storage  1003 . 
     Communication apparatus  1004  is hardware (transmission and reception device) for communication between computers through a wired and/or wireless network and is also called, for example, a network device, a network controller, a network card, or a communication module. For example, communication sections  112  and  202 , antennas  113  and  201 , inter-base station communication section  105 , and the like may be realized by communication apparatus  1004 . 
     Input apparatus  1005  is an input device (for example, a keyboard, a mouse, a microphone, a switch, a button, or a sensor) that receives input from the outside. Output apparatus  1006  is an output device (for example, a display, a speaker, or an LED lamp) for output to the outside. Note that input apparatus  1005  and output apparatus  1006  may be integrated (for example, a touch panel). 
     The apparatuses, such as processor  1001  and memory  1002 , are connected by bus  1007  for communication of information. Bus  1007  may be set by a single bus or may be set by different buses between the apparatuses. 
     Furthermore, radio base station  10  and user terminal  20  may include hardware, such as a microprocessor, a digital signal processor (DSP), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), and an FPGA (Field Programmable Gate Array), and the hardware may realize part or all of the functional blocks. For example, processor  1001  may be provided by at least one of these pieces of hardware. 
     (Notification and Signaling of Information) 
     The notification of information is not limited to the modes and embodiments described in the present specification, and the information may be notified by other methods. For example, the notification of information may be carried out by one or a combination of physical layer signaling (for example, DCI (Downlink Control Information) and UCI (Uplink Control Information)), higher layer signaling (for example, RRC (Radio Resource Control) signaling, MAC (Medium Access Control) signaling, broadcast information (MIB (Master Information Block), and SIB (System Information Block))), and other signals. The RRC signaling may be called an RRC message and may be, for example, an RRC connection setup message, an RRC connection reconfiguration message, or the like. 
     (Adaptive System) 
     The aspects and embodiments described in the present specification may be applied to a system using LTE (Long Term Evolution), LTE-A (LTE-Advanced), SUPER 3G, IMT-Advanced, 4G, 5G, FRA (Future Radio Access), W-CDMA (registered trademark), GSM (registered trademark), CDMA2000, UMB (Ultra Mobile Broadband), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, UWB (Ultra-WideBand), Bluetooth (registered trademark), or other appropriate systems and/or to a next-generation system expanded based on these. 
     (Processing Procedure and the like) 
     The orders of the processing procedures, the sequences, the flow charts, and the like of the aspects and embodiments described in the present specification may be changed as long as there is no contradiction. For example, elements of various steps are presented in exemplary orders in the methods described in the present specification, and the methods are not limited to the presented specific orders. 
     (Operation of Base Station) 
     Specific operations performed by the base station (radio base station) in the specification may be performed by an upper node depending on the situation. Various operations performed for communication with a terminal in a network constituted by one or a plurality of network nodes including a base station can be obviously performed by the base station and/or a network node other than the base station (examples include, but not limited to, MME (Mobility Management Entity) and S-GW (Serving Gateway)). Although there is one network node other than the base station in the case illustrated above, a plurality of other network nodes may be combined (for example, MME and S-GW). 
     (Direction of Input and Output) 
     The information, the signals, and the like can be output from a higher layer (or a lower layer) to a lower layer (or a higher layer). The information, the signals, and the like may be input and output through a plurality of network nodes. 
     (Handling of Input and Output Information and the Like) 
     The input and output information and the like may be saved in a specific place (for example, memory) or may be managed by a management table. The input and output information and the like can be overwritten, updated, or additionally written. The output information and the like may be deleted. The input information and the like may be transmitted to another apparatus. 
     (Judgement Method) 
     The judgement may be made based on a value expressed by 1 bit (0 or 1), based on a Boolean value (true or false), or based on comparison with a numerical value (for example, comparison with a predetermined value). 
     (Software) 
     Regardless of whether the software is called software, firmware, middleware, a microcode, or a hardware description language or by other names, the software should be broadly interpreted to mean an instruction, an instruction set, a code, a code segment, a program code, a program, a subprogram, a software module, an application, a software application, a software package, a routine, a subroutine, an object, an executable file, an execution thread, a procedure, a function, and the like. 
     The software, the instruction, and the like may be transmitted and received through a transmission medium. For example, when the software is transmitted from a website, a server, or other remote sources by using a wired technique, such as a coaxial cable, an optical fiber cable, a twisted pair, and a digital subscriber line (DSL), and/or a wireless technique, such as an infrared ray, a radio wave, and a microwave, the wired technique and/or the wireless technique is included in the definition of the transmission medium. 
     (Information and Signals) 
     The information, the signals, and the like described in the present specification may be expressed by using any of various different techniques. For example, data, instructions, commands, information, signals, bits, symbols, chips, and the like that may be mentioned throughout the entire description may be expressed by one or an arbitrary combination of voltage, current, electromagnetic waves, magnetic fields, magnetic particles, optical fields, and photons. 
     Note that the terms described in the present specification and/or the terms necessary to understand the present specification may be replaced with terms with the same or similar meaning. For example, the channel and/or the symbol may be a signal. The signal may be a message. The component carrier (CC) may be called a carrier frequency, a cell, or the like. 
     (“System” and “Network”) 
     The terms “system” and “network” used in the present specification can be interchangeably used. 
     (Names of Parameters and Channels) 
     The information, the parameters, and the like described in the present specification may be expressed by absolute values, may be expressed by values relative to predetermined values, or expressed by other corresponding information. For example, radio resources may be indicated by indices. 
     The names used for the parameters are not limited in any respect. Furthermore, the numerical formulas and the like using the parameters may be different from the ones explicitly disclosed in the present specification. Various channels (for example, PUCCH and PDCCH) and information elements (for example, TPC) can be identified by any suitable names, and various names assigned to these various channels and information elements are not limited in any respect. 
     (Base Station) 
     The base station (radio base station) can accommodate one or a plurality of (for example, three) cells (also called sectors). When the base station accommodates a plurality of cells, the entire coverage area of the base station can be divided into a plurality of smaller areas, and each smaller area can provide a communication service based on a base station subsystem (for example, small base station for indoor, RRH: Remote Radio Head). The term “cell” or “sector” denotes the base station that performs the communication service in the coverage and/or part or all of the coverage area of the base station subsystem. Furthermore, the terms “base station,” “eNB,” “cell,” and “sector” can be interchangeably used in the present specification. The base station may be called a fixed station, a NodeB, an eNodeB (eNB), an access point, a femto cell, a small cell, or the like. 
     (Terminal) 
     The user terminal may be called, by those skilled in the art, a mobile station, a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communication device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a wireless terminal, a remote terminal, a handset, a user agent, a mobile client, a client, or UE (User Equipment) or by some other appropriate terms. 
     (Meaning and Interpretation of Terms) 
     As used herein, the term “determining” may encompasses a wide variety of actions. For example, “determining” may be regarded as judging, calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” may be regarded as receiving (e.g., receiving information), transmitting (e.g., transmitting information), inputting, outputting, accessing (e.g., accessing data in a memory) and the like. Also, “determining” may be regarded as resolving, selecting, choosing, establishing and the like. That is, “determining” may be regarded as a certain type of action related to determining. 
     The terms “connected” and “coupled” as well as any modifications of the terms mean any direct or indirect connection and coupling between two or more elements, and the terms can include cases in which one or more intermediate elements exist between two “connected” or “coupled” elements. The coupling or the connection between elements may be physical or logical coupling or connection or may be a combination of physical and logical coupling or connection. When used in the present specification, two elements can be considered to be “connected” or “coupled” to each other by using one or more electrical wires, cables, and/or printed electrical connections or by using electromagnetic energy, such as electromagnetic energy with a wavelength of a radio frequency domain, a microwave domain, or an optical (both visible and invisible) domain that are non-limited and non-inclusive examples. 
     The reference signal can also be abbreviated as RS and may also be called a pilot depending on the applied standard. The correction RS may be called a TRS (Tracking RS), a PC-RS (Phase Compensation RS), a PTRS (Phase Tracking RS), or an additional RS. The demodulation RS and the correction RS may be called by other corresponding names. The demodulation RS and the correction RS may be prescribed by the same name (for example, demodulation RS). 
     The description “based on” used in the present specification does not mean “only based on,” unless otherwise specifically stated. In other words, the description “based on” means both of “only based on” and “at least based on.” 
     The “section” in the configuration of each apparatus may be replaced with “means,” “circuit,” “device,” or the like. 
     The terms “including”, “comprising”, and modifications of these are intended to be inclusive just like the term “having”, as long as the terms are used in the present specification or the appended claims. Furthermore, the term “or” used in the present specification or the appended claims is not intended to be an exclusive or. 
     The radio frame may be constituted by one or a plurality of frames in the time domain. One or each of a plurality of frames may be called a subframe, a time unit, or the like in the time domain. The subframe may be further constituted by one or a plurality of slots in the time domain. The slot may be further constituted by one or a plurality of symbols (OFDM (Orthogonal Frequency Division Multiplexing) symbols, SC-FDMA (Single Carrier-Frequency Division Multiple Access) symbols, or the like) in the time domain. 
     The radio frame, the subframe, the slot, a mini slot, and the symbol indicate time units in transmitting signals. The radio frame, the subframe, the slot, the mini slot, and the symbol may be called by other corresponding names. 
     For example, in the LTE system, the base station creates a schedule for assigning a radio resource to each mobile station (such as frequency bandwidth that can be used by each mobile station and transmission power). The minimum time unit of scheduling may be called a TTI (Transmission Time Interval), or one mini slot may be called a TTI. 
     For example, one subframe may be called a TTI. A plurality of continuous subframes may be called a TTI. One slot may be called a TTI. 
     The resource unit is a resource assignment unit of the time domain and the frequency domain, and the resource unit may include one or a plurality of continuous subcarriers in the frequency domain. One or a plurality of symbols may be included in the time domain of the resource unit, and the length may be one slot, one mini slot, one subframe, or one TTI. One TTI and one subframe may be constituted by one or a plurality of resource units. The resource unit may be called a resource block (RB), a physical resource block (PRB: Physical RB), a PRB pair, an RB pair, a scheduling unit, a frequency unit, or a subband. The resource unit may be constituted by one or a plurality of REs. For example, it is only necessary that one RE be a resource in a unit (for example, minimum resource unit) smaller than the resource unit serving as a resource assignment unit, and the naming is not limited to RE. 
     The structure of the radio frame is exemplary only, and the number of subframes included in the radio frame, the number of slots included in the subframe, the number of mini slots included in the subframe, the numbers of symbols and resource blocks included in the slot, and the number of subcarriers included in the resource block can be changed in various ways. 
     When articles, such as “a,” “an,” and “the” in English, are added by translation in the entire disclosure, the articles include plural forms unless otherwise clearly indicated by the context. 
     (Variations and the Like of Aspects) 
     The aspects and embodiments described in the present specification may be independently used, may be combined and used, or may be switched and used along the execution. Furthermore, notification of predetermined information (for example, notification indicating “it is X”) is not limited to explicit notification, and the notification of the predetermined information may be implicit (for example, by not notifying the predetermined information). 
     Although the present invention has been described in detail, it is obvious for those skilled in the art that the present invention is not limited to the embodiments described in the present specification. Modified and changed modes of the present invention can be carried out without departing from the spirit and the scope of the present invention defined by the description of the appended claims. Therefore, the description of the present specification is intended for exemplary description and does not limit the present invention in any sense. 
     The present patent application claims the benefit of priority based on Japanese Patent Application No. 2017-026241 filed on Feb. 15, 2017, and the entire content of Japanese Patent Application No. 2017-026241 is hereby incorporated by reference. 
     INDUSTRIAL APPLICABILITY 
     An aspect of the present invention is useful for a mobile communication system. 
     REFERENCE SIGNS LIST 
     
         
           10  Radio base station 
           20  User terminal 
           30  Control station 
           101  Discovery signal generation section 
           102  Candidate weight multiplication section 
           103  Reference signal generation section 
           104  Access point selection section 
           105  Inter-base station communication section 
           106  Weight selection section 
           107  Judgement section 
           108  Precoding matrix generation section 
           109  Data generation section 
           110  Precoding section 
           111  Beam-forming section 
           112 ,  202  Communication section 
           113 ,  201  Antenna 
           191  Coding section 
           192  Modulation section 
           203  Candidate weight information measurement section 
           204  Channel estimation section 
           205  Postcoding matrix generation section 
           206  Postcoding section 
           207  Data reception section 
           271  Demodulation section 
           272  Decoding section