Patent Publication Number: US-9853705-B2

Title: Communication system, base station, mobile station, and reception quality determination method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/JP2013/063082, filed on May 9, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a communication system, a base station, a mobile station, and a reception quality determination method. 
     BACKGROUND 
     A “cooperative communication” technology in a plurality of cells is considered as a method for improving the reception quality of a mobile station at the edge of a cell in Long Term Evolution-Advanced (LTE-A) by the 3rd Generation Partnership Project (3GPP). The cooperative communication is a technique in which a plurality of points (base stations or antennas) cooperates with each other to communicate with one or more mobile stations. For example, the same data is transmitted from the points to a terminal in the cooperative communication in the downlink. The cooperative communication in the downlink increases the gain by the effect of spatial diversity. The increase improves the reception quality of a mobile station at the edge of a cell. The cooperative communication in LTE-A is referred to as Coordinated Multi-Point (CoMP) communication. 
     Herein, all of the cells around a mobile station are candidates for the cell of the cooperative communication (hereinafter, sometimes referred to merely as a “candidate cell”). Selecting a cell to be related to the cooperative communication (namely, a cooperative cell) from the candidate cells increases the number of processes for determination in the mobile station and the amount of information about the determination result from the mobile station to the base station. In light of the foregoing, a small number of cells are previously selected as the candidate cells. A method for selecting the candidate cells is that Reference Signal Received Power (RSRP) of the reference signal to be transmitted to the mobile station in each cell is used. In LTE-A Rel. 11, the candidate cells are selected with the RSRP determined with a Cell-specific Reference Signal (CRS) and sets of the selected candidate cells, namely, CoMP Resource Management (CRM) sets are formed. 
     Note that the “cell” is defined based on the “communication area” and “channel frequency” of a base station. The “communication area” can be the whole of the area that the radio waves transmitted from the base station reach (hereinafter, sometimes referred to as a “range area”) or can be divided areas obtained by dividing the range area (so-called, sectors). The “channel frequency” is a unit of frequencies that the base station uses for the communication, and is defined based on the center frequency and the bandwidth. 
     Alternatively, it is considered for LTE-A Rel. 12 that three-dimensional beam-forming in which vertical beam-forming is added to horizontal beam-forming is used in transmission from a base station. 
     Related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 2012-135051, in Chao Shen, Tsung-Hui Chang, Member, IEEE, Kun-Yu Wang, Zhengding Qiu, and Chong-Yung Chi, Senior Member, IEEE, “Distributed Robust Multicell Coordinated Beamforming With Imperfect CSI: An ADMM Approach”,  IEEE TRANSACTIONS ON SIGNAL PROCESSING , VOL. 60, NO. 6, June, 2012, and in Pengfei Xia, Shengli Zhou, and Georgios B. Giannakisl, Dept. of Electrical and Computer Engr., University of Minnesota 200 Union St. SE, Minneapolis, Minn. 55455, “MIMO OFDM with ST Coding and Beamforming Adapted to Partial CSI”, 2003  Conference on Information Sciences and Systems , The Johns Hopkins University, Mar. 12-14, 2003. 
     When performing beam-forming transmission, the base station uses a plurality of antennas to form transmission beams to a mobile station. When the mobile station determines RSRP using a reference signal transmitted from one of the antennas, the RSRP is lower than the received power of the data signal transmitted by the beam-forming transmission. It is assumed that the beam-forming is used for the cooperative communication. In such a case, when the RSRP of a reference signal transmitted from one of the antennas is used to select a candidate cell, the candidate cell is selected based on the received power different from the received power when the data signal is actually transmitted. This adversely reduces the gain in the cooperative communication. The adverse reduction becomes further pronounced in three-dimensional beam-forming in comparison with in beam-forming in a horizontal direction or a vertical direction (namely, in two-dimensional beam-forming) because the gain is further increased in three-dimensional beam-forming. 
     SUMMARY 
     According to an aspect of an embodiment, a communication system includes a base station including a plurality of antennas that separate from each other and a mobile station. The base station maps a plurality of reference signals separable from each other onto the antennas to transmit the mapped reference signals. The mobile station determines, using the received reference signals, reception quality in consideration of gain of beam-forming transmission performed by the base station with the antennas, and notifies the base station of the determined reception quality. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of an exemplary communication system according to a first embodiment. 
         FIG. 2  is a block diagram of an exemplary base station according to the first embodiment. 
         FIG. 3  is a block diagram of an exemplary mobile station according to the first embodiment. 
         FIG. 4  is a diagram of exemplary mapping of reference signals according to the first embodiment. 
         FIG. 5  is a diagram of exemplary mapping of reference signals according to a second embodiment. 
         FIG. 6  is a diagram of exemplary arrangement of antennas according to the second embodiment. 
         FIG. 7  is a diagram of exemplary reception quality determination modes according to a third embodiment. 
         FIG. 8  is a block diagram of an exemplary mobile station according to the third embodiment. 
         FIG. 9  is a diagram of exemplary mapping of reference signals according to a fourth embodiment. 
         FIG. 10  is a diagram of an exemplary hardware configuration of the base station. 
         FIG. 11  is a diagram of an exemplary hardware configuration of the mobile station. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The embodiments of the communication system, the base station, the mobile station, and the reception quality determination method that are disclosed herein will be described hereinafter with reference to the appended drawings. Note that the communication system, the base station, the mobile station, and the reception quality determination method that are disclosed herein are not limited to the embodiments. The configurations having the same functions will be put with the same reference signs and the overlapping descriptions will be omitted in the embodiments. Communication systems that are an LTE system and an LTE-A system will be described hereinafter as examples. However, the systems are not limited to the examples. 
     First Embodiment 
     &lt;Outline of Communication System&gt; 
       FIG. 1  is a diagram of an exemplary communication system according to a first embodiment.  FIG. 1  illustrates that a communication system  1  includes base stations N 1  to N 7 , and a mobile station U 1 . 
     The base station N 1  manages cells C 11  to C 13 . Similarly, the base stations N 2  to N 7  manage cells C 21  to C 23 , cells C 31  to C 33 , cells C 41  to C 43 , cells C 51  to C 53 , cells C 61  to C 63 , and cells C 71  to C 73 , respectively. 
     In the example, the mobile station U 1  is located at the edge of the cell C 41 , and the cell C 41  works as a serving cell of the mobile station U 1  (the cell in which the mobile station U 1  is registered).  FIG. 1  further illustrates that cooperative communication in the downlink is performed with respect to the mobile station U 1 , with the cell C 13  and the cell C 23  together with the cell C 41  that is the serving cell of the mobile station U 1 . This means that the cell C 13  and the cell C 23  work as cooperative cells. 
     The base stations N 1  to N 7  are connected to each other through wired communication channels. The base stations N 1  to N 7  can be connected through an upper station or directly connected without an upper station. For example, the base stations N 1  to N 7  are directly connected to each other with an X2 interface. 
     &lt;Exemplary Configuration of Base Station&gt; 
       FIG. 2  is a block diagram of an exemplary base station according to the first embodiment.  FIG. 2  illustrates that a base station  100  includes a transmission process unit  101 , a precoding unit  102 , a mapping unit  103 , a reference signal generation unit  104 , wireless transmission units  105 - 1  to  105 -N, and antennas  106 - 1  to  106 -N. The antennas  106 - 1  to  106 -N are placed separately from each other. The base station  100  further includes an antenna  107 , a wireless reception unit  108 , a reception process unit  109 , a communication control unit  110 , and a network interface (IF)  111 . The base station  100  corresponds to the base station N 4  that manages the serving cell C 41  illustrated in  FIG. 1 . 
     The transmission process unit  101  forms a baseband signal by processing the user data and the control data input from the communication control unit  110  in a predetermined transmission process. In other words, the transmission process unit  101 , for example, codes or modulates the user data and the control data in order to form a baseband signal. Then, the transmission process unit  101  outputs the formed baseband signal to the precoding unit  102 . 
     The precoding unit  102  multiplies the baseband signal by a precoding vector, and outputs the baseband signal multiplied by the precoding vector to the mapping unit  103 . The multiplication by the precoding vector weights the beam-forming transmission on the baseband signal. Transmitting the signal weighted by the precoding vector from some of the antennas  106 - 1  to  106 -N is the beam-forming transmission. 
     The reference signal generation unit  104  generates a plurality of reference signals perpendicular to each other as the reference signals separable from each other. The reference signal generation unit  104  then outputs the generated reference signals to the mapping unit  103 . The reference signals are assigned to communication resources that are specific to each cell. 
     The mapping unit  103  maps the baseband signals multiplied by the precoding vector onto some of the antennas  106 - 1  to  106 -N to be used for the beam-forming transmission. In other words, the mapping unit  103  outputs the baseband signals multiplied by the precoding vector to wireless transmission units corresponding to the antennas to be used for the beam-forming transmission among the wireless transmission units  105 - 1  to  105 -N. The mapping unit  103  maps a plurality of reference signals onto some of the antennas  106 - 1  to  106 -N, respectively, and outputs the reference signals to the wireless transmission units corresponding to the mapped antennas among the wireless transmission units  105 - 1  to  105 -N. Note that the antennas used for the beam-forming transmission can be the same as the antennas for transmitting the reference signals, or can be different from the antennas for transmitting the reference signals. 
     Each of the wireless transmission units  105 - 1  to  105 -N generates a wireless signal by processing the baseband signal and reference signal input from the mapping unit  103  in a predetermined wireless transmission process. In other words, each of the wireless transmission units  105 - 1  to  105 -N, for example, performs a digital-analog conversion of the baseband signal and reference signal or up-converts the baseband signal and reference signal in order to generate a wireless signal. Then, the wireless transmission units  105 - 1  to  105 -N transmit the formed wireless signals through the antennas  106 - 1  to  106 -N. 
     The wireless reception unit  108  generates a baseband signal by processing the wireless signal received through the antenna  107  in a predetermined wireless reception process. In other words, the wireless reception unit  108  down-converts the wireless signal or performs an analog-digital conversion of the wireless signal in order to generate a baseband signal. Then, the wireless reception unit  108  outputs the formed baseband signal to the reception process unit  109 . 
     The reception process unit  109  obtains received data by processing the baseband signal in a predetermined reception process. In other words, the reception process unit  109 , for example, demodulates or decodes the baseband signal in order to obtain received data. Then, the reception process unit  109  outputs the obtained received data to the communication control unit  110  and a function unit on the downstream of the reception process unit  109 . The received data includes the reception quality information notified from a mobile station  200  to be described below. 
     The communication control unit  110  extracts (obtains) the reception quality information from the received data. The reception quality information includes a cell identifier in order to indicate which cell is indicated in the reception quality information. Then, the communication control unit  110  selects the candidate cells based on the extracted (obtained) reception quality information. For example, the communication control unit  110  selects a predetermined number of cells as candidate cells in descending order of reception quality. The communication control unit  110  notifies the other base stations that manage the candidate cells of the results from the selection of the candidate cells, in other words, of the sets of the candidate cells through the network IF  111 . The communication control unit  110  assigns the reference signal for each cell to the communication resource specific to each cell, and outputs, to the mapping unit  103 , the assignment resource information indicating which communication resource the reference signal for each cell is assigned to. The mapping unit  103  maps the reference signals input from the reference signal generation unit  104  onto the communication resources in accordance with the assignment resource information. The communication control unit  110  outputs the assignment resource information as the control data to the transmission process unit  101  in order to notify the mobile station  200  to be described below of the assignment resource information. For example, a unit of communication resource is defined by a unit of time and a unit of frequency, and is sometimes referred to as a Resource Element (RE). 
     &lt;Exemplary Configuration of Mobile Station&gt; 
       FIG. 3  is a block diagram of an exemplary mobile station according to the first embodiment.  FIG. 3  illustrates that the mobile station  200  includes an antenna  201 , a wireless reception unit  202 , a reception process unit  203 , a reference signal acquisition unit  204 , a reception quality determination unit  205 , a transmission process unit  206 , and a wireless transmission unit  207 . The mobile station  200  corresponds to the mobile station U 1  illustrated in  FIG. 1 . 
     The wireless reception unit  202  generates a baseband signal by processing the wireless signal received through the antenna  201  in a predetermined wireless reception process. In other words, the wireless reception unit  202 , for example, down-converts the wireless signal or performs an analog-digital conversion of the wireless signal in order to generate a baseband signal. Then, the wireless reception unit  202  outputs the formed baseband signal to the reception process unit  203  and the reference signal acquisition unit  204 . 
     The reception process unit  203  obtains received data by processing the baseband signal in a predetermined reception process. In other words, the reception process unit  203 , for example, demodulates or decodes the baseband signal in order to obtain received data. Then, the reception process unit  203  outputs the obtained received data to the reference signal acquisition unit  204  and a function unit on the downstream of the reception process unit  203 . The received data includes the assignment resource information notified from the base station  100 . 
     The reference signal acquisition unit  204  extracts (obtains) the assignment resource information from the received data. Then, the reference signal acquisition unit  204  obtains the reference signals among the baseband signals in accordance with the extracted (obtained) assignment resource information, and outputs the obtained reference signals to the reception quality determination unit  205 . As described above, the reference signal of each cell is assigned to the communication resource specific to each cell. Thus, the reference signal acquisition unit  204  can determine from the assignment resource information which cell the obtained reference signal is for. Subsequently, the reference signal acquisition unit  204  attaches a cell identifier to the obtained reference signal and outputs the reference signal to the reception quality determination unit  205 . 
     The reception quality determination unit  205  determines the reception quality in consideration of the gain of the beam-forming transmission performed by the base station  100 , using the reference signals input from the reference signal acquisition unit  204 . The reception quality determination unit  205  outputs the reception quality information to the transmission process unit  206  in order to notify the base station  100  of the reception quality information including the determination result and the cell identifier. For example, the reception quality determination unit  205  determines the RSRP as the reception quality. 
     The transmission process unit  206  forms a baseband signal by processing the user data and the reception quality information input from the reception quality determination unit  205  in a predetermined transmission process. In other words, the transmission process unit  206 , for example, codes or modulates the user data and the reception quality information in order to form a baseband signal. Then, the transmission process unit  206  outputs the formed baseband signal to the wireless transmission unit  207 . 
     The wireless transmission unit  207  forms a wireless signal by processing the baseband signal in a predetermined wireless transmission process. In other words, the wireless transmission unit  207 , for example, performs a digital-analog conversion of the baseband signal or up-converts the baseband signal in order to form a wireless signal. Then, the wireless transmission unit  207  transmits the formed wireless signal to the antenna  201 . As a result, the reception quality information is notified to the base station  100 . 
     &lt;Exemplary Mapping of Reference Signals and Exemplary Determination of Reception Quality&gt; 
       FIG. 4  is a diagram of an exemplary mapping of reference signals according to the first embodiment. The antennas A 1  to A 16  in  FIG. 4  correspond to the antennas  106 - 1  to  106 -N of the base station  100 . Thus, N=16 holds in the example. In other words, the base station  100  includes four antennas in a vertical direction and four antennas in a horizontal direction and the total number of antennas is 16, such that the base station  100  can perform three-dimensional beam-forming transmission to transmit the user data and the control data. In other words, the base station  100  can change the direction of beam in the vertical direction and in the horizontal direction. The reference signal generation unit  104  generates four reference signals p 1 , p 2 , p 3 , and p 4  that are perpendicular to each other. For example, the reference signal generation unit  104  generates the four reference signals such that the perpendicular patterns of the reference signals are p 1 =(s, s, s, s), p 2 =(s, −s, s, −s), p 3 =(−s, −s, s, s), and p 4 =(s, −s, −s, s). The mapping unit  103  maps, for example, the reference signal p 1  onto the antenna A 1 , the reference signal p 2  onto the antenna A 13 , the reference signal p 3  onto the antenna A 4 , and the reference signal p 4  onto the antenna A 16 , respectively. 
     The mobile station  200  determines the reception quality as described below, using the reference signals p 1  to p 4  mapped as illustrated in  FIG. 4 . 
     The reception quality determination unit  205  calculates a channel estimation vector H^ indicated in an expression (1). In the expression (1), h 1 ^ indicates a channel estimation value based on the reference signal p 1 . Similarly, h 2 ^ to h 4 ^ indicate channel estimation values based on the reference signals p 2  to p 4 , respectively. In other words, h 1 ^ to h 4 ^ correspond to the antennas A 1 , A 13 , A 4 , and A 16  onto which the reference signals p 1  to p 4  are mapped, respectively. 
     
       
         
           
             
               
                 
                   
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     Next, the reception quality determination unit  205  calculates a channel correlation matrix R of the base station  100  in accordance with an expression (2). The channel correlation matrix R corresponds to the correlation matrix among the antennas A 1 , A 13 , A 4 , and A 16 . In the expression (2), H^ H  indicates the Hermitian matrix of H^, “*” indicates the complex conjugation, and “&lt; &gt;” indicates the time average in a given time. 
     
       
         
           
             
               
                 
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     Next, the reception quality determination unit  205  calculates a weight vector W of the beam-forming in accordance with an expression (3). The calculation of the weight vector W corresponds to the estimation of the precoding vector used in the base station  100 . In the expression (3), “eig(R)” indicates that the eigenvector corresponding to the largest fixed value in the matrix R (namely, the largest eigenvector) is calculated. In other words, W corresponding to the largest gain obtained from the three-dimensional beam-forming performed by the base station  100  is calculated with the expression (3).
 
 W =eig( R )  Expression (3)
 
     Then, the reception quality determination unit  205  calculates P BF  that is RSRP in accordance with an expression (4). In the expression (4), W H  indicates the Hermitian matrix of W. 
     
       
         
           
             
               
                 
                   
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     As expressed in the expression (4), the expression for calculating P BF  includes WW H . The W is the weight vector W of the beam-forming. Thus, by calculating P BF  in accordance with the expression (4), the reception quality determination unit  205  can determine the reception quality in consideration of the gain of the three-dimensional beam-forming transmission performed by the base station  100 . 
     As described above in the first embodiment, the base station  100  maps a plurality of reference signals separable from each other onto a plurality of antennas that separate from each other and transmits the reference signals. The mobile station  200  determines the reception quality in consideration of the gain of the beam-forming transmission performed by the base station  100 , using the received reference signals. Then, the mobile station  200  notifies the base station  100  of the reception quality. Thus, when the mobile station  200  determines the reception quality, the gain of the beam-forming transmission is taken into consideration. Accordingly, when the beam-forming is used for the cooperative communication, the reception quality optimum for selecting a candidate cell is determined. The reception quality in consideration of the gain of the beam-forming transmission is notified to the base station  100 . Thus, without using the beam-forming transmission to transmit the reference signal, the base station  100  can select a candidate cell in accordance with the same reception quality as the reception quality when a data signal is actually transmitted. Thus, the base station  100  can select the optimal candidate cells, and thus can prevent the gain of the cooperative communication from decreasing. 
     In the first embodiment, the base station  100  maps a plurality of reference signals onto the antennas that separate from each other in a vertical direction and a horizontal direction. The mobile station  200  determines the reception quality in consideration of the gain of the three-dimensional beam-forming transmission. This enables the mobile station  200  to determine the reception quality optimum for selecting a candidate cell when the three-dimensional beam-forming is used for the cooperative communication. 
     Second Embodiment 
     &lt;Exemplary Mapping of Reference Signals and Exemplary Determination of Reception Quality&gt; 
       FIG. 5  is a diagram of exemplary mapping of reference signals according to the second embodiment. The antennas A 1  to A 16  in In  FIG. 5  correspond to the antennas  106 - 1  to  106 -N of the base station  100 . Thus, N=16 holds in the example. In other words, the base station  100  includes, for example, four antennas in a vertical direction and four antennas in a horizontal direction and the total number of antennas is 16. Wide horizontal intervals are set among the antennas such that the four antennas placed in the horizontal direction are non-correlated to each other. Thus, the user data and the control data can be transmitted with the two-dimensional beam-forming transmission in a vertical direction and with spatial multiplex transmission or diversity transmission by precoding in a horizontal direction. In other words, the base station  100  can change the beam direction only in the vertical direction. The reference signal generation unit  104  generates two reference signals p 1  and p 2  that are perpendicular to each other. For example, the reference signal generation unit  104  generates the reference signals such that the perpendicular patterns of the reference signals are p 1 =(s, s), and p 2 =(s, −s). The mapping unit  103  maps, for example, the reference signal p 1  onto the antenna A 1 , and the reference signal p 2  onto the antenna A 4 , respectively. 
     The antenna A 1  and the antenna A 4  are relatively arranged as illustrated in  FIG. 6 .  FIG. 6  is a diagram of an exemplary arrangement of antennas according to the second embodiment. In other words, the antenna A 1  and the antenna A 4  separate from each other at a distance d in a vertical direction. Thus, when the transmission beam is emitted in a direction inclined at an angle θ from the vertical direction, a delay time t 2  of the antenna A 1  from the antenna A 4 , and a phase difference φ between the reference signals of the antenna A 1  and the antenna A 4  are expressed as an expression (5). In the expression (5), c indicates a speed of light, and f indicates a frequency. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                               ⁢ 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θ 
                             
                             c 
                           
                         
                       
                     
                   
                   
                     
                       
                         ϕ 
                         = 
                           
                         ⁢ 
                         
                           e 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ft 
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           e 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             f 
                             ⁢ 
                             
                               
                                 d 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θ 
                               
                               c 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
     The mobile station  200  determines the reception quality as described below, using the reference signals p 1  and p 2  mapped as illustrated in  FIG. 5 . 
     Reference signals r 1  and r 2  obtained by the reference signal acquisition unit  204  are expressed as an expression (6). The reference signal r 1  corresponds to the reference signal p 1 , and the reference signal r 2  corresponds to the reference signal p 2 . In the expression (6), h 1  indicates a channel estimation value based on the reference signal p 1 , h 2  indicates a channel estimation value based on the reference signal p 2 , and n 1  and n 2  indicate a noise.
 
 r   1   =h   1   +n   1  
 
 r   2   =h   2   +n   2   ≅h   1   e   jφ   +n   2   Expression (6)
 
     The reception quality determination unit  205  calculates an average complex correlation value between r 1  and r 2  expressed in an expression (7). As expressed in the expression (7), the average complex correlation value can approximately be regarded as the phase difference between the reference signals p 1  and p 2 . In other words, the phase difference between the reference signals p 1  and p 2  is estimated from the expression (7). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             〈 
                             
                               
                                 r 
                                 1 
                                 * 
                               
                               ⁢ 
                               
                                 r 
                                 2 
                               
                             
                             〉 
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             〈 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       h 
                                       1 
                                     
                                     + 
                                     
                                       n 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 * 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   
                                     h 
                                     2 
                                   
                                   + 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             〉 
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         ≅ 
                           
                         ⁢ 
                         
                           
                             〈 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       h 
                                       1 
                                       * 
                                     
                                     + 
                                     
                                       n 
                                       1 
                                       * 
                                     
                                   
                                   ) 
                                 
                                 * 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   
                                     
                                       h 
                                       1 
                                     
                                     ⁢ 
                                     
                                       e 
                                       
                                         j 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         ϕ 
                                       
                                     
                                   
                                   + 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             〉 
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         ≅ 
                           
                         ⁢ 
                         
                           
                             
                               〈 
                               
                                 
                                   
                                      
                                     
                                       h 
                                       1 
                                     
                                      
                                   
                                   2 
                                 
                                 ⁢ 
                                 
                                   e 
                                   
                                     j 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     ϕ 
                                   
                                 
                               
                               〉 
                             
                             + 
                             
                               〈 
                               
                                 
                                   h 
                                   1 
                                   * 
                                 
                                 ⁢ 
                                 
                                   n 
                                   2 
                                 
                               
                               〉 
                             
                             + 
                             
                               〈 
                               
                                 
                                   n 
                                   1 
                                   * 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       h 
                                       1 
                                     
                                     ⁢ 
                                     
                                       e 
                                       
                                         j 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         ϕ 
                                       
                                     
                                   
                                   ) 
                                 
                               
                               〉 
                             
                             + 
                             
                               〈 
                               
                                 
                                   n 
                                   1 
                                   * 
                                 
                                 ⁢ 
                                 
                                   n 
                                   2 
                                 
                               
                               〉 
                             
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         ≅ 
                           
                         ⁢ 
                         
                           e 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ϕ 
                           
                         
                         ≡ 
                         
                           e 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ϕ 
                               ^ 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     7 
                     ) 
                   
                 
               
             
           
         
       
     
     Subsequently, the reception quality determination unit  205  calculates P BF  that is RSRP in accordance with an expression (8). In other words, the reception quality determination unit  205  calculates P BF  from the reference signals r 1  and r 2  obtained by the reference signal acquisition unit  204 , and the average complex correlation value between r 1  and r 2 . 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           P 
                           BF 
                         
                         = 
                           
                         ⁢ 
                         
                           
                              
                             
                               
                                 r 
                                 1 
                               
                               + 
                               
                                 
                                   r 
                                   2 
                                 
                                 ⁢ 
                                 
                                   e 
                                   
                                     
                                       - 
                                       j 
                                     
                                     ⁢ 
                                     
                                       ϕ 
                                       _ 
                                     
                                   
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                              
                             
                               
                                 ( 
                                 
                                   
                                     h 
                                     1 
                                   
                                   + 
                                   
                                     n 
                                     1 
                                   
                                 
                                 ) 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       
                                         h 
                                         1 
                                       
                                       ⁢ 
                                       
                                         e 
                                         
                                           j 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           ϕ 
                                         
                                       
                                     
                                     + 
                                     
                                       n 
                                       2 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   e 
                                   
                                     
                                       - 
                                       j 
                                     
                                     ⁢ 
                                     
                                       
                                         
                                             
                                         
                                         ⁢ 
                                         ϕ 
                                       
                                       _ 
                                     
                                   
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                              
                             
                               
                                 2 
                                 ⁢ 
                                 
                                   h 
                                   1 
                                 
                               
                               + 
                               
                                 n 
                                 1 
                               
                               + 
                               
                                 
                                   n 
                                   2 
                                 
                                 ⁢ 
                                 
                                   e 
                                   
                                     
                                       - 
                                       j 
                                     
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       ϕ 
                                       _ 
                                     
                                   
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     8 
                     ) 
                   
                 
               
             
           
         
       
     
     The expression for calculating P BF  includes the phase difference between the reference signals p 1  and p 2  as expressed in the expression (8). The phase difference corresponds to the average complex correlation value expressed in the expression (7). In other words, adding the noises that are non-correlated to each other reduces the noises and desired signals are added in phase in the expression (8). Thus, the reception quality determination unit  205  calculates P BF  in accordance with the expression (8). This calculation enables the reception quality determination unit  205  to determine the reception quality in consideration of the gain of the two-dimensional beam-forming transmission performed by the base station  100  in a vertical direction. 
     As described above in the second embodiment, the base station  100  maps a plurality of reference signals onto a plurality of antennas that separate from each other in a vertical direction. The mobile station  200  determines the reception quality in consideration of the gain of the two-dimensional beam-forming transmission in the vertical direction. This enables to the mobile station  200  to determine the reception quality optimum for selecting a candidate cell when the two-dimensional beam-forming in the vertical direction is used for the cooperative communication. 
     Third Embodiment 
     &lt;Exemplary Configuration of Base Station&gt; 
     The exemplary configuration of the base station according to the third embodiment is similar to the configuration in the first embodiment. However, a communication control unit  110  performs a process to be described below in addition to the processes in the first embodiment. In other words, the communication control unit  110  outputs the information instructing a method for determining the reception quality of each cell (hereinafter, sometimes referred to as “determination method instruction information”) together with the assignment resource information as the control data to the transmission process unit  101 . 
     For example, the communication control unit  110  selects one of modes  0 ,  1 , and  2  illustrated in  FIG. 7  for each cell as the determination method instruction information about the cell that the base station  100  manages and each of the candidate cells.  FIG. 7  is a diagram of an exemplary reception quality determination mode according to the third embodiment. The mode  0  is selected when the beam-forming transmission is not performed, and instructs that RSRP be determined without considering the gain of the beam-forming in a conventional way. The mode  1  is selected when three-dimensional beam-forming transmission is performed, and instructs that RSRP be determined by calculating the largest eigenvector as described in the first embodiment. The mode  2  is selected when two-dimensional beam-forming transmission in a vertical direction is performed, and instructs that RSRP be determined by calculating the average complex correlation value as described in the second embodiment. In other words, the base station  100  selects any one of the modes  0  to  2  in accordance with the type of the beam-forming transmission. The modes can be determined in accordance with the arrangement of the transmission antennas in each cell. 
     &lt;Exemplary Configuration of Mobile Station&gt; 
       FIG. 8  is a block diagram of an exemplary mobile station according to the third embodiment.  FIG. 8  illustrates that a mobile station  300  includes a reception quality determination unit  301 . The reception quality determination unit  301  extracts (obtains) the determination method instruction information about each cell from the received data, and switches the method for determining the reception quality in accordance with the extracted (obtained) determination method instruction information. In other words, the reception quality determination unit  301  determines RSRP without considering the gain of the beam-forming in a conventional way when the determination method instruction information is the mode  0 . Alternatively, the reception quality determination unit  301  determines RSRP by calculating the largest eigenvector as described in the first embodiment when the determination method instruction information is the mode  1 . Alternatively, the reception quality determination unit  301  determines RSRP by calculating the average complex correlation value as described in the second embodiment when the determination method instruction information is the mode  2 . In other words, the mobile station  300  can determine the reception quality in an optimum determination method in accordance with the type of the beam-forming transmission performed by the base station  100 . 
     As described above in the third embodiment, the base station  100  instructs a method for determining the reception quality to the mobile station  300  in accordance with the type of the beam-forming transmission. The mobile station  300  determines the reception quality in accordance with the determination method instructed by the base station  100 . The mobile station  300  performs a smaller amount of process in the determination by the calculation of the average complex correlation value than the amount of process in the determination by the calculation of the largest eigenvector. The mobile station  300  performs an even smaller amount of process in the determination of RSRP without considering the gain of the beam-forming in a conventional way than the amount of process in the determination by the calculation of the average complex correlation value. Switching the method for determining the reception quality in accordance with the type of the beam-forming transmission can reduce the amount of process and power consumption by the mobile station  300 . 
     Fourth Embodiment 
     &lt;Exemplary Mapping of Reference Signals and Exemplary Determination of Reception Quality&gt; 
       FIG. 9  is a diagram of exemplary mapping of reference signals according to a fourth embodiment. The arrangement of antennas illustrated in  FIG. 9  is identical to the arrangement illustrated in  FIG. 4  (the first embodiment). However, the reference signal generation unit  104  according to the fourth embodiment generates two reference signals p 1  and p 2  that are perpendicular to each other. For example, the reference signal generation unit  104  generates the reference signals such that the perpendicular patters of the reference signals are p 1 =(s, s), and p 2 =(s, −s), respectively. The mapping unit  103  maps, for example, the reference signal p 1  onto an antenna A 13 , and the reference signal p 2  onto an antenna A 4 . 
     The mobile station  200  determines the reception quality as described below, using the reference signals p 1  and p 2  mapped as illustrated in  FIG. 9 . 
     Reference signals r 1  and r 2  obtained by a reference signal acquisition unit  204  are expressed as an expression (9). The reference signal r 1  corresponds to the reference signal p 1 , and the reference signal r 2  corresponds to the reference signal p 2 . In the expression (9), h 2,c  indicates the components correlated to h 1  among the channel estimation values based on the reference signal p 2 , and h 2,u  indicates the components non-correlated to h 1  among the channel estimation values based on the reference signal p 2 . 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           r 
                           1 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             h 
                             1 
                           
                           + 
                           
                             n 
                             1 
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           r 
                           2 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             h 
                             2 
                           
                           + 
                           
                             n 
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 h 
                                 
                                   2 
                                   , 
                                   c 
                                 
                               
                               + 
                               
                                 h 
                                 
                                   2 
                                   , 
                                   u 
                                 
                               
                             
                             ) 
                           
                           + 
                           
                             n 
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         ≅ 
                           
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 
                                   h 
                                   1 
                                 
                                 ⁢ 
                                 
                                   ⅇ 
                                   
                                     j 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     ϕ 
                                   
                                 
                               
                               + 
                               
                                 h 
                                 
                                   2 
                                   , 
                                   u 
                                 
                               
                             
                             ) 
                           
                           + 
                           
                             n 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
     The reception quality determination unit  205  calculates an average complex correlation value between r 1  and r 2  expressed in an expression (10). As expressed in the expression (10), the average complex correlation value can approximately be regarded as the phase difference between the reference signals p 1  and p 2 . In other words, the phase difference between the reference signals p 1  and p 2  is estimated from the expression (10). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             〈 
                             
                               
                                 r 
                                 1 
                                 * 
                               
                               ⁢ 
                               
                                 r 
                                 2 
                               
                             
                             〉 
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             〈 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       h 
                                       1 
                                     
                                     + 
                                     
                                       n 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 * 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   
                                     h 
                                     2 
                                   
                                   + 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             〉 
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   r 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         ≅ 
                           
                         ⁢ 
                         
                           
                             〈 
                             
                               
                                 ( 
                                 
                                   
                                     h 
                                     1 
                                     * 
                                   
                                   + 
                                   
                                     n 
                                     1 
                                     * 
                                   
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   
                                     ( 
                                     
                                       
                                         
                                           h 
                                           1 
                                         
                                         ⁢ 
                                         
                                           e 
                                           
                                             j 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             ϕ 
                                           
                                         
                                       
                                       + 
                                       
                                         h 
                                         
                                           2 
                                           , 
                                           u 
                                         
                                       
                                     
                                     ) 
                                   
                                   + 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             〉 
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         ≅ 
                           
                         ⁢ 
                         
                           
                             
                               
                                 
                                   
                                     〈 
                                     
                                       
                                         
                                            
                                           
                                             h 
                                             1 
                                           
                                            
                                         
                                         2 
                                       
                                       ⁢ 
                                       
                                         e 
                                         
                                           j 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           ϕ 
                                         
                                       
                                     
                                     〉 
                                   
                                   + 
                                   
                                     〈 
                                     
                                       
                                         h 
                                         1 
                                         * 
                                       
                                       ⁢ 
                                       
                                         h 
                                         
                                           2 
                                           , 
                                           u 
                                         
                                       
                                     
                                     〉 
                                   
                                   + 
                                   
                                     〈 
                                     
                                       
                                         h 
                                         1 
                                         * 
                                       
                                       ⁢ 
                                       
                                         h 
                                         2 
                                       
                                     
                                     〉 
                                   
                                   + 
                                   
                                     〈 
                                     
                                       
                                         n 
                                         1 
                                         * 
                                       
                                       ⁡ 
                                       
                                         ( 
                                         
                                           
                                             h 
                                             1 
                                           
                                           ⁢ 
                                           
                                             e 
                                             
                                               j 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               ϕ 
                                             
                                           
                                         
                                         ) 
                                       
                                     
                                     〉 
                                   
                                   + 
                                 
                               
                             
                             
                               
                                 
                                   
                                     〈 
                                     
                                       
                                         n 
                                         1 
                                         * 
                                       
                                       ⁢ 
                                       
                                         h 
                                         
                                           2 
                                           , 
                                           u 
                                         
                                       
                                     
                                     〉 
                                   
                                   + 
                                   
                                     〈 
                                     
                                       
                                         n 
                                         1 
                                         * 
                                       
                                       ⁢ 
                                       
                                         n 
                                         2 
                                       
                                     
                                     〉 
                                   
                                 
                               
                             
                           
                           
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   1 
                                 
                                  
                               
                               〉 
                             
                             ⁢ 
                             
                               〈 
                               
                                  
                                 
                                   h 
                                   2 
                                 
                                  
                               
                               〉 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         ≅ 
                           
                         ⁢ 
                         
                           e 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ϕ 
                           
                         
                         ≡ 
                         
                           e 
                           
                             j 
                             ⁢ 
                             
                               ϕ 
                               _ 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
     The reception quality determination unit  205  subsequently calculates P BF  that is RSRP in accordance with the expression (8), similarly to the second embodiment. 
     As described above in the fourth embodiment, RSRP can be determined by calculating the average complex correlation value, similarly to the second embodiment, even when the base station  100  performs the three-dimensional beam-forming transmission. This can further reduce the amount of process and power consumption for determining RSRP when the base station  100  performs the three-dimensional beam-forming transmission than the first embodiment (namely, the determination of RSRP by the calculation of the Largent eigenvector). 
     Other Embodiments 
     [1] The physical configurations of the base station  100  and the mobile stations  200  and  300  are not necessarily as illustrated in the drawings. In other words, a concrete aspect of the separation and incorporation of each unit is not limited to the aspects illustrated in the drawings. All or some of the units can functionally or physically be separated or incorporated in an arbitrary unit, for example, depending on various loads or usage conditions. 
     Various processing functions of the base station  100  and the mobile stations  200  and  300  can entirely or partially be performed on a Central Processing Unit (CPU) or a microcomputer such as a Micro Processing Unit (MPU) or a Micro Controller Unit (MCU). Alternatively, the processing functions can entirely or partially be performed on a program analyzed and executed by the CPU (or the microcomputer such as an MPU or an MCU), or on hardware by wired logic. 
     [2] The base station  100  and the mobile stations  200  and  300  can be implemented, for example, with the following hardware configuration. 
       FIG. 10  is a diagram of an exemplary hardware configuration of the base station. As illustrated in  FIG. 10 , a base station  10  includes a network IF  11 , a processor  12 , a memory  13 , a Radio Frequency (RF) circuit  14 , and antennas  15 - 1  to  15 -N. For example, a CPU, a Digital Signal Processor (DSP), or a Field Programmable Gate Array (FPGA) can be cited as an example of the processor  12 . For example, a Random Access Memory (RAM) such as a Synchronous Dynamic Random Access Memory (SDRAM), a Read Only Memory (ROM), or a flash memory can be cited as an example of the memory  13 . 
     By executing programs stored in various memories such as a non-volatile storage medium with the processor  12 , various processing functions of the base station  100  can be implemented. In other words, programs corresponding to the processes performed by the transmission process unit  101 , the precoding unit  102 , the mapping unit  103 , the reference signal generation unit  104 , the reception process unit  109 , and the communication control unit  110  can be stored in the memory  13 , and each of the programs can be executed by the processor  12 . The network IF  111  is implemented with the network IF  11 . The wireless transmission units  105 - 1  to  105 -N and the wireless reception unit  108  are implemented with the RF circuit  14 . The antennas  106 - 1  to  106 -N are implemented with the antennas  15 - 1  to  15 -N. The antenna  107  is implemented with the antenna  16 . 
       FIG. 11  is a diagram of an exemplary hardware configuration of the mobile station. As illustrated in  FIG. 11 , a mobile station  20  includes an antenna  21 , an RF circuit  22 , a processor  23 , and a memory  24 . 
     For example, a CPU, a DSP, or an FPGA is cited as an example of the processor  23 . For example, a RAM such as an SDRAM, a ROM, or a flash memory can be cited as an example of the memory  24 . 
     By executing programs stored in various memories such as a non-volatile storage medium with the processor  23 , various processing functions of the mobile stations  200  and  300  can be implemented. In other words, programs corresponding to the processes performed by the reception process unit  203 , the reference signal acquisition unit  204 , the reception quality determination units  205  and  301 , and the transmission process unit  206  can be stored in the memory  24 , and each of the programs can be executed by the processor  23 . The wireless reception unit  202  and the wireless transmission unit  207  are implemented with an RF circuit  22 . The antenna  201  is implemented with the antenna  21 . 
     [3] In the first to fourth embodiments, each of the antennas is described as a physical antenna. However, the disclosed technique can be applied to a logical antenna such as an antenna port as well. 
     According to one of the disclosed aspects, when beam-forming is used for cooperative communication, an optimal reception quality for selecting a candidate cell can be determined. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.