Abstract:
An electronic device including circuitry configured to perform communication in a MU-MIMO (Multi User-Multiple Input Multiple Output) mode or a SU-MIMO (Single User-Multiple Input Multiple Output) mode. The circuitry is configured to allocate a first resource for the MU-MIMO mode and a second resource for the SU-MIMO mode. The circuitry is further configured to notify a communication device of identification information for discriminating between the first resource and the second resource.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/704,254 filed Dec. 14, 2012, the entire content of which is incorporated herein by reference. U.S. application Ser. No. 13/704,254 is a National Stage of PCT/JP11/064218 filed Jun. 22, 2011, and is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2010-188129 filed Aug. 25, 2010. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a wireless communication device, a wireless communication system, a wireless communication method, and a program. 
     BACKGROUND ART 
     Currently, in third generation partnership project (3GPP), standardization of a wireless communication system for 4G has been promoted. In 4G, a technology such as relay, carrier aggregation, coordinated multiple point transmission and reception (CoMP), and multi user multi input multi output (MU-MIMO) has been attracting attention. 
     Relay is considered as an important technology to improve a throughput of a cell edge. In addition, carrier aggregation is a technology that can handle a band width of 20 MHz×5=100 MHz by handling, for example, five frequency bands each having a band width of 20 MHz, together. By such carrier aggregation, improvement of a maximum throughput can be expected. 
     In addition, CoMP is a technology in which a plurality of base stations transmit and receive data in cooperation in order to improve coverage of a high data rate. In addition, MU-MIMO is a technology that improves a system throughput so that a plurality of users use a resource block of the same frequency and the same time, on which spatial multiplexing is performed. As described above, further improvement of the performance in 4G (LTE-Advanced) by various technologies has been discussed. 
     Here, MU-MIMO is described in detail. In 3.9G (LTE), there are technologies of MU-MIMO and single user MIMO (SU-MIMO). For example, as discussed in Patent literature 1, SU-MIMO is a technology in which a plurality of channels are used so that single user equipment (UE) performs spatial multiplexing of the plurality of channels although spatial multiplexing is not performed between pieces of UE. 
     On the other hand, as described above, MU-MIMO is a technology in which each UE uses a resource block of the same frequency and the same time, on which spatial multiplexing is performed (spatial multiplexing is performed between pieces of UE). However, in MU-MIMO that is realized in 3.9G, each UE handles a mere single channel. On the contrary, in 4G, MU-MIMO in which each UE can handle a plurality of channels is being realized. 
     In order to achieve such MU-MIMO in 4G, it has been studied that two types (V 1  and V 2 ) of transmission weight are used in a base station. The V 1  is transmission weight that realizes directivity, and the V 2  is transmission non-directional weight, the main purpose of which is to adjust a phase. The V 1  and V 2  can be determined, for example, in UE. To be more specific, the UE receives a reference signal that is transmitted from a base station, obtains a channel matrix H from the reception result of the reference signal, and determines optimal V 1  and V 2  for the channel matrix H. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2005-184730A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, high calculation load in UE for determining transmission weight V 1  and transmission weight V 2  is concerned because the transmission weight V 1  and transmission weight V 2  are complex numbers. 
     Therefore, in the present disclosure, there are proposed a new and improved wireless communication device, wireless communication system, wireless communication method, and program that can suppress calculation load in a communication partner for determining transmission weight. 
     Solution to Problem 
     According to an embodiment of the present disclosure, there is provided a wireless communication device including a communication unit that transmits a reference signal, a first multiplication unit that performs multiplication of first transmission weight that is determined based on reception of the reference signal by a communication partner, and a second multiplication unit that performs multiplication of second transmission weight that is determined based on reception of the reference signal by the communication partner. The communication unit transmits a reference signal with weight that is obtained by multiplying the reference signal by the first transmission weight after determination of the first transmission weight. 
     The wireless communication device may further includes a reference signal management unit that manages a resource for transmitting the reference signal with weight. 
     The reference signal management unit may allocate a resource for transmitting the reference signal with weight and a resource for transmitting the reference signal after determination of the first transmission weight. 
     The reference signal management unit may allocate more resources for transmitting the reference signal than the resource for transmitting the reference signal with weight. 
     The reference signal management unit may allocate a resource so that transmission frequency of the reference signal on a time axis becomes higher than transmission frequency of the reference signal with weight on the time axis. 
     The reference signal management unit may allocate a resource so that a density of a resource for transmitting the reference signal on a frequency axis becomes higher than a density of a resource for transmitting the reference signal with weight on the frequency axis. 
     The wireless communication device may further include a scheduler that allocates a resource for communication of a first scheme or a second scheme to each communication partner. The scheduler may allocate a resource within a first frequency range for the communication of the first scheme, and allocate a resource within a second frequency range for the communication of the second scheme. 
     The first frequency range may be a frequency range to which a resource for transmitting the reference signal with weight is allocated. The second frequency range may be a frequency range to which a resource for transmitting the reference signal is allocated. 
     The first scheme may be multi user multi input multi output (MU-MIMO), the second scheme may be single user multi input multi output (SU-MIMO). 
     The wireless communication device may further include a scheduler that allocates a resource for communication of a first scheme or a second scheme to each communication partner. The scheduler may allocate, for the communication of the first scheme, a resource within a frequency range to which a resource for transmitting the reference signal with weight is allocated, and allocate, for the communication of the first scheme or the second scheme, a resource within a frequency range to which a resource for transmitting the reference signal is allocated. 
     Update frequency of the second transmission weight may be higher than update frequency of the first transmission weight. 
     The first transmission weight may be weight for forming directivity, and the second transmission weight may be non-directional weight for adjusting a phase. 
     Further, according to another embodiment of the present disclosure, there is provided a program for causing a computer to function as a wireless communication device that includes a communication unit that transmits a reference signal, a first multiplication unit that performs multiplication of first transmission weight that is determined based on reception of the reference signal by a communication partner, and a second multiplication unit that performs multiplication of second transmission weight that is determined based on reception of the reference signal by the communication partner. The communication unit may transmit a reference signal with weight that is obtained by multiplying the reference signal by the first transmission weight after determination of the first transmission weight. 
     Further, according to another embodiment of the present disclosure, there is provided a wireless communication method including transmitting a reference signal, multiplying the reference signal by first transmission weight that is determined based on reception of the reference signal by a communication partner, and transmitting a reference signal with weight that is obtained by multiplying the reference signal by the first transmission weight. 
     Further, according to another embodiment of the present disclosure, there is provided a wireless communication system including a first wireless communication device, and a second wireless communication device that includes, a communication unit that transmits a reference signal, a first multiplication unit that performs multiplication of first transmission weight that is determined based on reception of the reference signal by the first wireless communication device, and a second multiplication unit that performs multiplication of second transmission weight that is determined based on reception of the reference signal by the first wireless communication device. The communication unit transmits a reference signal with weight that is obtained by multiplying the reference signal by the first transmission weight after determination of the first transmission weight. 
     Further, according to another embodiment of the present disclosure, there is provided a wireless communication device including a communication unit that receives a reference signal from a communication partner, and a weight determination unit that determines first transmission weight and second transmission weight based on a reception result of the reference signal by the communication unit. When a reference signal with weight that is obtained by multiplying the reference signal by the first transmission weight is received by the communication unit, the weight determination unit determines the second transmission weight based on a reception result of the reference signal with weight. 
     Further, according to another embodiment of the present disclosure, there is provided a wireless communication device including a scheduler that allocates a resource for communication of a first scheme or a second scheme to each communication partner. The scheduler allocates a resource within a first frequency range for the communication of the first scheme, and allocates a resource within a second frequency range for the communication of the second scheme. 
     The first scheme may be multi user multi input multi output (MU-MIMO), and the second scheme may be single user multi input multi output (SU-MIMO). 
     Advantageous Effects of Invention 
     As described above, according to the present disclosure, calculation load in a communication partner for determining transmission weight can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an illustrative diagram illustrating a configuration of a wireless communication system according to an embodiment of the present disclosure. 
         FIG. 2  is an illustrative diagram illustrating an example of multiplication order of transmission weight. 
         FIG. 3  is an illustrative diagram illustrating relationship of V 1  and V 2 . 
         FIG. 4  is an illustrative diagram illustrating a determination method using a comparative example of the transmission weight V 1  and transmission weight V 2 _MU. 
         FIG. 5  is an illustrative diagram illustrating a determination method using a comparative example of transmission weight in a case in which MU-MIMO and SU-MIMO are present. 
         FIG. 6  is an illustrative diagram illustrating a configuration of a base station according to an embodiment of the present disclosure. 
         FIG. 7  is an illustrative diagram illustrating of a configuration of a weight multiplication unit. 
         FIG. 8  is an illustrative diagram illustrating a configuration of a weight multiplication unit according to a variant. 
         FIG. 9  is an illustrative diagram illustrating a configuration of a mobile station according to an embodiment. 
         FIG. 10  is an illustrative diagram illustrating a first embodiment of the present disclosure. 
         FIG. 11  is an illustrative diagram illustrating a second embodiment of the present disclosure. 
         FIG. 12  is an illustrative diagram illustrating a third embodiment of the present disclosure. 
         FIG. 13  is an illustrative diagram illustrating a resource allocation example of a V 1 *CSI_RS and a CSI_RS according to a fourth embodiment. 
         FIG. 14  is an illustrative diagram illustrating a specific example of resource allocation according to a fifth embodiment. 
         FIG. 15  is an illustrative diagram illustrating a specific example of resource allocation according to a sixth embodiment. 
         FIG. 16  is an illustrative diagram illustrating a specific example of resource allocation according to a seventh embodiment. 
         FIG. 17  is a flowchart illustrating an operation of a base station according to the embodiments of the present disclosure. 
         FIG. 18  is a flowchart illustrating an operation of a mobile station according to the embodiments of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the appended drawings. Note that, in this specification and the drawings, elements that have substantially the same function and structure are denoted with the same reference signs, and repeated explanation is omitted. 
     In addition, in this specification and drawings, a plurality of elements having substantially the same function and structure may be distinguished so as to be denoted with different alphabets after the same reference numeral. For example, a plurality of configurations having substantially the same function and structure such as mobile stations  20 A,  20 B, and  20 C may be distinguished as appropriate. However, when there is no particular need to distinguish a plurality of elements having substantially the same function and structure individually, the plurality of elements are denoted with the mere same reference numeral. For example, when there is no particular need to distinguish mobile stations  20 A,  20 B, and  20 C, the mobile stations are merely referred to as mobile station  20 . 
     In addition, “Description of Embodiments” is made in accordance with the order of the following items. 
     1. Outline of a wireless communication system
         1-1. Configuration of the wireless communication system   1-2. Transmission weight (V 1  and V 2 )   1-3. Feedback scheme of transmission weight   1-4. Dynamic switching   1-5. Comparative example       

     2. Basic configuration of a base station 
     3. Basic configuration of a mobile station 
     4. Description of each embodiment
         4-1. First embodiment   4-2. Second embodiment   4-3. Third embodiment   4-4. Fourth embodiment   4-5. Fifth embodiment   4-6. Sixth embodiment   4-7. Seventh embodiment       

     5. Operation of the base station and the mobile station 
     6. Conclusion 
     1. OUTLINE OF A WIRELESS COMMUNICATION SYSTEM 
     Currently, in 3GPP, standardization of a wireless communication system for 4G has been promoted. An embodiment of the present disclosure can be applied to the wireless communication system for 4G as an example, and, first, the outline of the wireless communication system for 4G is described. 
     1-1. Configuration of a Wireless Communication System 
       FIG. 1  is an illustrative diagram illustrating a configuration of a wireless communication system  1  according to an embodiment of the present disclosure. As illustrated in  FIG. 1 , the wireless communication system  1  according to the embodiment of the present disclosure includes a base station  10  and a plurality of mobile stations  20 . Note that the base station  10  may a wireless communication device such as eNodeB, a relay node, or home eNodeB that is a household small base station in 4G. In addition, the mobile station  20  may be a wireless communication device such as a relay node or UE in 4G. 
     The base station  10  controls communication with the mobile station  20  in a cell. In addition, the base station  10  is operated using three sectors so that each of the sectors has, for example, an angle of 120 degrees as illustrated in  FIG. 1 . In addition, the base station  10  includes a plurality of antennas, and can form directivity in a plurality of directions in each of the sectors (four directions in the example illustrated in  FIG. 1 ) by multiplying a transmission signal from each of the antennas by transmission weight V 1  that is described later. 
     Therefore, the base station  10  can perform multiplexing so that mobile stations  20 A and  20 B that exist in different directions when viewed from the base station  10  are spatially separated. That is, the base station  10  can communicate with the plurality of the mobile stations  20  by MU-MIMO. Note that the base station  10  can also communicate with the mobile stations  20  by SU-MIMO. 
     The mobile station  20  is a wireless communication device that communicates with the base station  10  by MU-MIMO or SU-MIMO. The mobile station  20  moves in accordance with the movement of a moving body such as a user and a vehicle. Note that, in the embodiment, the mobile station  20  is described as an example of a wireless communication device that wirelessly communicates with the base station  10 , and the embodiment can be also applied to a wireless communication device that is installed in a fixed manner. 
     1-2. Transmission Weight (V 1  and V 2 ) 
     In 4G, in the realization of the MU-MIMO, it is has been studied that transmission weight that is referred to as V 2  is used in addition to the V 1  that is described above (double codebook scheme). The V 1  is transmission weight that realizes directivity as described above. Such V 1  has a characteristic such as coverage of a wide frequency area and lower update frequency than that of the V 2 . 
     On the other hand, the V 2  is transmission non-directional weight, the main purpose of which is to adjust a phase. More specifically, the V 2  is used for maximizing reception power by adjusting a phase of each path between antennas of the mobile station  20  and the base station  10 . In addition, the V 2  has a characteristic such as coverage of a narrow frequency area and higher update frequency than that of the V 1 . 
     The base station  10  according to the embodiment realizes MU-MIMO by multiplying transmission data by such transmission weight V 1  and transmission weight V 2 . Note that, as illustrated in  FIG. 2 , the base station  10  may multiply transmission data by transmission weight in order of V 2  and V 1 , and may multiply transmission data by transmission weight in order of V 1  and V 2 . 
       FIG. 3  is an illustrative diagram illustrating a relationship of V 1  and V 2 . As illustrated in  FIG. 3 , when the base station  10  includes 8 antennas, these antennas operate as two set of linear array antennas  4 A and  4 B each of which is constituted of four elements. Note that the linear array antennas  4 A and  4 B operate as array antennas having the same directivity as illustrated in  FIG. 3 . 
     In addition, the V 2  operates so that two code words of transmission data are distributed into the two set of linear array antennas  4 A and  4 B by changing the phase. That is, the V 2  operates so as to change the phase of a transmission signal to be supplied to the linear array antennas  4 A and  4 B that perform transmission in the same direction. On the other hand, the V 1  is applied to each antenna as illustrated in  FIG. 3  and operates so that the linear array antennas  4 A and  4 B form directivity. 
     Specific examples of the above-described V 1  and V 2  are described below. Note that “d” in “Formula 1” that represents the V 1  indicates a distance from a reference antenna, “λ” indicates a wavelength, “θ” indicates a direction of beam, and “i” indicates an antenna number. In addition, “H” in “Formula 2” that represents V 2  indicates a channel matrix. 
     
       
         
           
             
               
                 
                   
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     As illustrated in “Formula 2”, the V 2  is transmission weight that is represented as plus or minus 1, or plus or minus j. Note that the j indicates an imaginary number. Thus, a load for multiplying a certain matrix by the V 2  is small. On the other hand, the V 1  is transmission weight that is described by a directional vector, and is not a matrix that is represented by plus or minus 1 and plus or minus j. Therefore, in calculation using the V 1 , calculation load is increased. 
     Note that when transmission data of the base station  10  is “S” and reception data of the mobile station  20  is “R”, the reception data R of the mobile station  20  can be represented as the following “Formula 3” or “Formula 4”.
 
 R=H·V 1· V 2· S   [Math. 3]
 
 R=H·V 2· V 1· S   [Math. 4]
 
     1-3. Feedback Scheme of Transmission Weight 
     As a feedback scheme of MIMO for determining the above-described transmission weight V 1  and transmission weight V 2 , three schemes of implicit feedback, explicit feedback, and SRS-based feedback are conceivable. In 4G, as a feedback scheme of MIMO for determining the transmission weight V 1  and transmission weight V 2 , the use of the implicit feedback is determined because a load on a feedback circuit is small. For reference, each of the feedback schemes in 3.9G (LTE) is described below. 
     (1) Implicit Feedback 
     In a base station, 16 types of transmission weight (V 1 ) to transmission weight (V 16 ) are prepared (pre-coded) for a coodbook that has been designed in beforehand. A mobile station that receives a reference signal from the base station obtains a channel matrix H between the base station and mobile station. In addition, the mobile station pre-determines HV having the highest reception power from among HV ( 1 ), HV ( 2 ), . . . , HV ( 16 ). After that, the mobile station provides feedback of an index number that indicates V that makes reception power maximum, to the base station. The base station transmits data using the V corresponding to the index that is fed back. 
     (2) Explicit Feedback 
     The base station transmits a reference signal, and the mobile station that receives the reference signal from the base station obtains a channel matrix H between the base station and the mobile station similarly to the case of the implicit feedback. In addition, the mobile station provides feedback of the channel matrix H as-is, to the base station. The base station calculates and creates a desired transmission weight from the channel matrix H in downlink that is fed back from the mobile station. In addition, the base station transmits data using the created transmission weight. In this explicit feedback, there is a problem that a resource that is used for feedback becomes larger than that of the implicit feedback because a channel matrix H is transmitted as-is at the time of feedback. 
     (3) SRS-Based Feedback 
     The mobile station transmits a reference signal, and the base station that receives the reference signal from the mobile station obtains a channel matrix in uplink between the mobile station and the base station. When the reversibility of a channel can be established (in a case of a TDD mode), the base station can makes a virtual channel matrix in downlink from the channel matrix. A scheme in which a virtual channel matrix in downlink is made as described above is the SRS-based feedback. In the SRS-based feedback, there is a problem such that, when calibration is not performed in which variations of analog circuits in the base station are compensated, the reversibility of channels in uplink and downlink (channel matrix that includes a characteristic of the analog circuit) is not established. 
     1-4. Dynamic Switching 
     In 4G (LTE-Advanced), it is has been studied that setting of MIMO is dynamically switched between MU-MIMO and SU-MIMO. In addition, in MU-MIMO in 4G, the use of eight streams has been studied. In the case of eight streams, one matrix for phase adjustment of V 2  as described in “1-2. Transmission weight (V 1  and V 2 )” is used. 
     The example is described above in which MU-MIMO is realized by combining V 1  having a 4×4 matrix and V 2  having a 2×2 matrix. On the other hand, mere V 2  having an 8×8 matrix is used for SU-MIMO. In addition, each element of the V 2  having the 8×8 matrix is represented by the plus or minus 1 and plus or minus j, similarly to the V 2  having the 2×2 matrix. Note that j indicates an imaginary number. 
     As described above, different V 2  are used for MU-MIMO and SU-MIMO, and, in this specification, V 2  for MU-MIMO is referred to as V 2 _MU, and weight for SU-MIMO is referred to as V 2 _SU, thereby distinguishing the two of V 2 . 
     1-5. Comparative Example 
     In 4G and the embodiments, as described in “1-3. Feedback scheme of transmission weight,” the transmission weight V 1  and transmission weight V 2 _MU are determined by implicit feedback. Here, in order to clarify the technical significance of the embodiments, a determination method using a comparative example of the transmission weight V 1  and transmission weight V 2 _MU is described with reference to  FIG. 4 . 
       FIG. 4  is an illustrative diagram illustrating the determination method using a comparative example of the transmission weight V 1  and transmission weight V 2 _MU. In  FIG. 4 , the horizontal axis indicates a time. In addition, CSI indicates a channel state information reference signal (CSI_RS). 
     As illustrated in  FIG. 4 , the base station transmits a CSI_RS (step  1 ), and the mobile station obtains a channel matrix H from the CSI_RS received from the base station. In addition, the mobile station evaluates optimal V 1  for the obtained channel matrix H, among four types of V 1  candidate. For example, the mobile station selects V 1  that makes reception power maximum, among four types of V 1  candidate. In addition, the mobile station evaluates and selects optimal V 2 _MU. After that, the mobile station provides feedback of Index_V 1  that indicates the selected V 1  and Index_V 2  that indicates V 2 _MU to the base station (step  2 ). The base station determines V 1  and V 2 _MU on the basis of the feedback from the mobile station. 
     When the base station and the mobile station determines V 1  and V 2 _MU, the base station and the mobile station updates the only V 2 _MU multiple times (step  3 ) followed by updating the V 1  and V 2 _MU (step  4 ). As described above, update frequency of V 2 _MU is higher than update frequency of V 1 . 
     Here, the mobile station performs calculation using a plurality of types of V 1  when the mobile station selects V 1 . As described in “1-2. Transmission weight (V 1  and V 2 )”, load of the mobile station in the case of selecting V 1  becomes large because load of calculation using V 1  is larger than load of calculation using V 2 _MU. 
     On the other hand, it is conceived that calculation using V 1  is not desired in the case of selecting V 2 . However, the idea is wrong, and the mobile station performs calculation using V 1  in the case of selecting V 2 . This is because the mobile station obtains a channel matrix H from a newly received CSI_RS, multiplies the channel matrix H by already determined V 1 , and evaluates optimal V 2 _MU for the channel matrix H that is multiplied by the V 1 . As described above, in the determination method of transmission weight using the comparative example, the amount of calculation in the mobile station is increased undesirably because it is desirable that the mobile station performs calculation using V 1  in any update of V 1  and V 2 . 
     Next, a determination method using a comparative example of transmission weight in a case in which MU-MIMO and SU-MIMO are present is described with reference to  FIG. 5 . 
       FIG. 5  is an illustrative diagram illustrating the determination method using a comparative example of transmission weight in the case in which MU-MIMO and SU-MIMO are present. As illustrated in  FIG. 5 , in the case in which MU-MIMO and SU-MIMO are present, the base station and the mobile station updates V 2 _SU for all a CSI_RS in addition to V 1  and V 2 _MU. Therefore, calculation load in the mobile station is further increased undesirably because the V 2 _SU is updated. However, in order to realize dynamic switching of MU-MIMO and SU-MIMO, it is important to evaluate both of the V 2 _MU and V 2 _SU all the time. 
     The above-described determination method of transmission weight by a comparative example is summarized as follows: 
     (1) Calculation Load in the Mobile Station is High 
     Reason: As described with reference to  FIG. 4 , calculation using already determined V 1  is performed even in the case of evaluating V 2 _MU. 
     (2) Calculation Load in the Mobile Station is Further Increase when Dynamic Switching of MU-MIMO and SU-MIMO is Tried to be Realized. 
     Reason: As described with reference to  FIG. 5 , both of the V 2 _MU and V 2 _SU are evaluated all the time. 
     In addition, when dynamic switching is performed in a communication system using a plurality of subcarriers of an OFDM modulation scheme, etc., there has been no an allocation method of a frequency subcarrier that can effectively reduce the amount of calculation. 
     Therefore, the embodiments of the present disclosure have been led to creation by regarding the above circumstances as a point of view. According to each embodiment of the present disclosure, calculation load in the mobile station  20  for determining transmission weight can be suppressed. Each of such embodiments of the present disclosure is described below in detail. 
     2. BASIC CONFIGURATION OF A BASE STATION 
     A technology according to the present disclosure can be implemented in various forms as described in detail in “4-1. First embodiment” to “4-7. Seventh embodiment” as examples. In addition, the base station  10  according to each of the embodiments includes: 
     A: a communication unit (an antenna  110 , an analog processing unit  120 , etc.) that transmits a reference signal (CSI_RS), 
     B: a first multiplication unit (V 1  multiplication unit  154 ) that performs multiplication of first transmission weight (V 1 ) that is determined on the basis of reception of the reference signal by a communication partner (the mobile station  20 ), and 
     C: a second multiplication unit (V 2 _MU multiplication unit  156 ) that performs multiplication of second transmission weight (V 2 _MU) that is determined on the basis of reception of the reference signal by the communication partner. In addition, 
     D: the communication unit transmits a reference signal with weight (V 1 *CSI_RS) obtained by multiplying the reference signal by the first transmission weight after determination of the first transmission weight. 
     First, a common basic configuration in the base station  10  according to such embodiments is described below with reference to  FIGS. 6 to 8 . 
       FIG. 6  is an illustrative diagram illustrating a configuration of the base station  10  according to the embodiment of the present disclosure. As illustrated in  FIG. 6 , the base station  10  according to the embodiment of the present disclosure includes the plurality of antennas  110 , a switch SW  116 , an analog processing unit  120 , an AD/DA conversion unit  124 , a demodulation processing unit  128 , an upper layer signal processing unit  132 , a scheduler  136 , a modulation processing unit  140 , and a weight multiplication unit  150 . 
     The antennas  110 A to  110 N function as a reception unit that converts a radio signal that is transmitted from the mobile station  20  into an electrical reception signal and supplies the converted signal to the analog processing unit  120 , and a transmission unit that converts a transmission signal supplied from the analog processing unit  120  into a radio signal and transmits the converted signal to the mobile station  20 . Note that the number of the antennas  110  is not particularly limited, and may be, for example, 8 or 16. 
     The switch SW  116  is a switch for switching a transmission operation and a reception operation by the base station  10 . The base station  10  performs the transmission operation when the antennas  110 A to  110 N are connected to a transmission circuit of the analog processing unit  120  through the switch SW  116 , and performs the reception operation when the antennas  110 A to  110 N are connected to a reception circuit of the analog processing unit  120  through the switch SW  116 . 
     The analog processing unit  120  includes the transmission circuit that performs analog processing for a transmission signal, and the reception circuit that performs analog processing for a reception signal. In the transmission circuit, for example, up-conversion, filtering, gain control, etc. of a transmission signal, in an analog form, which is supplied from the AD/DA conversion unit  124  are performed. In the reception circuit, for example, down-conversion, filtering, etc. of a reception signal that is supplied from the antenna  110  through the switch SW  116  are performed. 
     The AD/DA conversion unit  124  performs analogue/digital (AD) conversion of a reception signal that is supplied from the analog processing unit  120 , and performs digital/analogue (DA) conversion of a transmission signal that is supplied from the weight multiplication unit  150 . 
     The demodulation processing unit  128  performs demodulation processing of a reception signal that is supplied from the AD/DA conversion unit  124 . The demodulation processing that is performed by the demodulation processing unit  128  may include OFDM demodulation processing, MIMO demodulation processing, error correction, etc. 
     The upper layer signal processing unit  132  performs processing for inputting and outputting transmission data and reception data between the upper layer signal processing unit  132  and an upper layer, control processing of the scheduler  136 , the modulation processing unit  140 , and the weight multiplication unit  150 , determination processing of each transmission weight based on feedback information from the mobile station  20 , etc. 
     In addition, the base station  10  according to the embodiment transmits a V 1 *CSI_RS (reference signal with weight) obtained by multiplying a CSI_RS by V 1  in addition to a CSI_RS (reference signal) after determination of the transmission weight V 1  on the basis of feedback information from the mobile station  20  as described later in detail. The upper layer signal processing unit  132  includes a function as a reference signal management unit that manages a resource for transmitting the CSI_RS and a V 1 *CSI_RS. In addition, the upper layer signal processing unit  132  controls the weight multiplication unit  150  so that transmission of the CSI_RS or V 1 *CSI_RS is performed in the allocated resource. 
     The scheduler  136  allocates a resource for data communication to each of the mobile stations  20 . The resource that is allocated by the scheduler  136  is reported to each of the mobile stations  20  by a control channel, and each of the mobile stations  20  performs data communication in uplink or downlink using the reported resource. 
     The modulation processing unit  140  performs modulation processing such as mapping based on a constellation on transmission data that is supplied from the upper layer signal processing unit  132 . The transmission signal obtained after modulation by the modulation processing unit  140  is supplied to the weight multiplication unit  150 . 
     The weight multiplication unit  150  multiplies the transmission signal that is supplied from the modulation processing unit  140  by the transmission weight V 1  and transmission weight V 2 _MU that are determined by the upper layer signal processing unit  132  at the time of execution of MU-MIMO. On the other hand, the weight multiplication unit  150  multiplies the transmission signal that is supplied from the modulation processing unit  140  by the transmission weight V 2 _SU that is determined by the upper layer signal processing unit  132  at the time of execution of SU-MIMO. In addition, the weight multiplication unit  150  multiplies a CSI_RS by V 1  in a resource that is allocated for transmission of a V 1 *CSI_RS (the “*” is complex multiplication) by the upper layer signal processing unit  132 . Such configuration of the weight multiplication unit  150  is described below in more detail with reference to  FIG. 7 . 
       FIG. 7  is an illustrative diagram illustrating a configuration of the weight multiplication unit  150 . As illustrated in  FIG. 7 , the weight multiplication unit  150  includes selectors  151 ,  157 , and  158 , a V 2 _SU multiplication unit  152 , the V 1  multiplication unit  154 , and the V 2 _MU multiplication unit  156 . 
     The selector  151  supplies a transmission signal that is supplied from the modulation processing unit  140  to the V 2 _MU multiplication unit  156  or the V 2 _SU multiplication unit  152 . More specifically, the selector  151  supplies a transmission signal to the V 2 _MU multiplication unit  156  when setting of MIMO is MU-MIMO, and supplies a transmission signal to the V 2 _SU multiplication unit  152  when setting of MIMO is SU-MIMO. 
     The V 2 _SU multiplication unit  152  multiplies the transmission signal that is supplied from the selector  151  by V 2 _SU that is determined by the upper layer signal processing unit  132 . 
     On the other hand, the V 2 _MU multiplication unit  156  multiplies the transmission signal that is supplied from the selector  151  by V 2 _MU that is determined by the upper layer signal processing unit  132 . In addition, the V 1  multiplication unit  154  multiplies the transmission signal that is multiplied by the V 2 _MU, by V 1 . 
     The selector  157  selectively outputs the multiplication result by the V 1  multiplication unit  154 , or the multiplication result by the V 2 _SU multiplication unit  152 . More specifically, the selector  157  outputs the multiplication result by the V 1  multiplication unit  154  when setting of MIMO is MU-MIMO and outputs the multiplication result by the V 2 _SU multiplication unit  152  when setting of MIMO is SU-MIMO. 
     A selector  158  supplies a CSI_RS to the former part or the latter part of the V 1  multiplication unit  154 . More specifically, the selector  158  supplies a CSI_RS to the latter part of the V 1  multiplication unit  154  in a resource that is allocated for transmitting the CSI_RS. In this case, the base station  10  transmits a CSI_RS that is not multiplied by V 1 . 
     On the other hand, the selector  158  supplies a CSI_RS to the former part of the V 1  multiplication unit  154  in a resource that is allocated for transmitting a V 1 *CSI_RS. In this case, the base station  10  transmits a V 1 *CSI_RS because the CSI_RS is multiplied by V 1  in the V 1  multiplication unit  154 . 
     Note that, in  FIG. 7 , the example is described in which the V 1  multiplication unit  154  is arranged in the latter part of the V 2  multiplication unit  156 , however, the configuration of the weight multiplication unit  150  is not limited to such example. For example, as described below with reference to  FIG. 8 , the V 1  multiplication unit  154  may be arranged in the former part of the V 2  multiplication unit  156 . 
       FIG. 8  is an illustrative diagram illustrating a configuration of a weight multiplication unit  150 ′ according to a variant. As illustrated in  FIG. 8 , the weight multiplication unit  150 ′ according to the variant includes the selectors  151 ,  155 ,  157 , and  159 , the V 2 _SU multiplication unit  152 , the V 1  multiplication unit  154 , and the V 2 _MU multiplication unit  156 . 
     In the weight multiplication unit  150 ′ according to the variant, as illustrated in  FIG. 8 , the V 1  multiplication unit  154  is arranged in the former part of the V 2 _MU multiplication unit  156 . In addition, in the weight multiplication unit  150 ′ according to the variant, the selector  159  supplies a CSI_RS to the former part of the V 1  multiplication unit  154  or the latter part of the V 2 _MU multiplication unit  156 . 
     More specifically, the selector  159  supplies a CSI_RS to the latter part of the V 2 _MU multiplication unit  156  in a resource that is allocated for transmitting a CSI_RS. In this case, the base station  10  transmits a CSI_RS that is not multiplied by V 1 . 
     On the other hand, the selector  159  supplies a CSI_RS to the former part of the V 1  multiplication unit  154  in a resource that is allocated for transmitting a V 1 *CSI_RS. In this case, the CSI_RS is multiplied by V 1  in the V 1  multiplication unit  154 , and the V 1 *CSI_RS that is the multiplication result is supplied from the selector  155  to the selector  157  so as to bypass the V 2 _MU multiplication unit  156 . As a result, the base station  10  transmits the V 1 *CSI_RS. 
     As described above, the base station  10  according to the embodiment starts to transmit a V 1 *CSI_RS after determination of transmission weight V 1 . By such configuration, calculation load of V 2 _MU, etc. in the mobile station  20  that is described below can be suppressed. 
     3. BASIC CONFIGURATION OF A MOBILE STATION 
       FIG. 9  is an illustrative diagram illustrating a configuration of the mobile station  20  according to the embodiment. As illustrated in  FIG. 9 , the mobile station  20  according to the embodiments includes a plurality of antennas  210 , a switch SW  216 , an analog processing unit  220 , an AD/DA conversion unit  224 , a demodulation processing unit  228 , an upper layer signal processing unit  232 , a modulation processing unit  240 , a channel matrix obtaining unit  244 , and a weight determination unit  248 . 
     The antennas  210 A and  210 B function as a reception unit that converts a radio signal that is transmitted from the base station  10  into an electrical reception signal and supplies the converted signal to the analog processing unit  220 , and function as transmission unit that converts a transmission signal that is supplied from the analog processing unit  220  into a radio signal and transmits the converted signal to the base station  10 . Note that the number of antennas  210  is not limited, and for example, may be four, or eight. 
     The switch SW  216  is a switch for switching a transmission operation and a reception operation of the mobile station  20 . The mobile station  20  performs the transmission operation when the antennas  210 A and  210 B are connected to a transmission circuit of the analog processing unit  220  through the switch SW  216 , and the mobile station  20  performs the reception operation when the antennas  210 A and  210 B are connected to a reception circuit of the analog processing unit  220  through the switch SW  216 . 
     The analog processing unit  220  includes a transmission circuit that performs analog processing on a transmission signal and a reception circuit that performs analog processing on a reception signal. In the transmission circuit, for example, up-conversion, filtering, gain control, etc. of a transmission signal in an analog form, which is supplied from the AD/DA conversion unit  224  are performed. In the reception circuit, for example, down-conversion, filtering, etc. of a reception signal that is supplied from the antenna  210  through the switch SW  216  are performed. 
     The AD/DA conversion unit  224  performs AD conversion of a reception signal that is supplied from the analog processing unit  220  and performs DA conversion of a transmission signal that is supplied from the modulation processing unit  240 . 
     The demodulation processing unit  228  performs demodulation processing of a reception signal that is supplied from the AD/DA conversion unit  224 . The demodulation processing that is performed by the demodulation processing unit  228  may include OFDM demodulation processing, MIMO demodulation processing, and error correction. 
     The upper layer signal processing unit  232  performs processing for inputting and outputting transmission data and reception data between the upper layer signal processing unit  232  and an upper layer. In addition, the upper layer signal processing unit  232  supplies feedback information that indicates transmission weight that is determined by the weight determination unit  248  to the modulation processing unit  240 , as transmission data. 
     The modulation processing unit  240  performs modulation processing such as mapping based on a constellation on transmission data that is supplied from the upper layer signal processing unit  232 . The transmission signal obtained after modulation by the modulation processing unit  240  is supplied to the AD/DA conversion unit  224 . 
     The channel matrix obtaining unit  244  obtains a channel matrix H between the base station  10  and the mobile station  20  when a CSI_RS is received from the base station  10 . 
     The weight determination unit  248  determines transmission weight of V 1 , V 2 _MU, V 2 _SU, etc. on the basis of the channel matrix H obtained by the channel matrix obtaining unit  244 . Here, as described above with reference to  FIG. 4 , when V 2 _MU is updated on the basis of the channel matrix H obtained from the CSI_RS, the mobile station according to a comparative example multiplies the channel matrix H by already determined V 1  and evaluates optimal V 2 _MU for the channel matrix H that is multiplied by the V 1 . Therefore, in the mobile station according to the comparative example, calculation using V 1  is performed even at the time of update of V 2 _MU. 
     On the contrary, in the embodiment, after determination of V 1 , V 1 *CSI_RS that is a CSI_RS multiplied by the V 1  is received from the base station  10 . A channel matrix H that is obtained from a V 1 *CSI_RS by the channel matrix obtaining unit  244  is already in a form of being multiplied by V 1 . Thus, the weight determination unit  248  can update V 2 _MU on the basis of the channel matrix H that is obtained from the V 1 *CSI_RS without performing calculation using V 1 . As a result, calculation load in the mobile station  20  for update of V 2 _MU can be significantly reduced. 
     4. DESCRIPTION OF EACH OF THE EMBODIMENTS 
     The basic configurations of the base station  10  and the mobile station  20  according to each of the embodiments of the present disclosure are described above. Next, each of the embodiments of the present disclosure is described in detail. 
     4-1. First Embodiment 
       FIG. 10  is an illustrative diagram illustrating a first embodiment of the present disclosure. As illustrated in  FIG. 10 , the base station  10  transmits a V 1 *CSI_RS to update (determine) V 2 _MU when V 1  is determined after transmitting a CSI_RS. As described above, the mobile station  20  that has received a V 1 *CSI_RS can evaluate optimal V 2 _MU without performing calculation using V 1 . 
     In addition, the base station  10  transmits a CSI_RS to update V 1  after transmitting a V 1 *CSI_RS multiple times. After that, the base station  10  transmits a V 1 *CSI_RS to update V 2 _MU. 
     In  FIG. 10 , an example is described in which the update frequency of V 2  is about 4 to 5 times the update frequency of V 1 , however relationship of update frequency is not limited to the example. In practice, it is conceivable that the update frequency of V 1  is more than 10 times the update frequency of V 2 . 
     4-2. Second Embodiment 
     As described in the first embodiment, when the base station  10  transmits a V 1 *CSI_RS, the mobile station  20  can evaluate optimal V 2 _MU without calculation using V 1 . Here, in order to realize dynamic switching of MU-MIMO and SU-MIMO, it is desirable that the mobile station  20  obtains V 2 _SU. However, it is difficult for the mobile station  20  to evaluate V 2 _SU from the V 1 *CSI_RS. 
     Therefore, the upper layer signal processing unit  132  of the base station  10  according to a second embodiment allocates a resource for transmitting a CSI_RS to update (determine) V 2 _SU in addition to allocation of a resource for transmitting a V 1 *CSI_RS to update (determine) V 2 _MU. An operation of the base station  10  according to such second embodiment is described in detail with reference to  FIG. 11 . 
       FIG. 11  is an illustrative diagram illustrating the second embodiment of the present disclosure. As illustrated in  FIG. 11 , the base station  10  according to the second embodiment transmits a V 1 *CSI_RS to update V 2 _MU after determination of V 1 , and transmits a CSI_RS to update (determine) V 2 _SU. By such configuration, the dynamic switching of MU-MIMO and SU-MIMO can be realized because V 2 _MU is obtained on the basis of the V 1 *CSI_RS and V 2 _SU is obtained on the basis of the CSI_RS. 
     Note that the mobile station  20  can determines that a radio signal that is received from the base station  10  is a CSI_RS or a V 1 *CSI_RS, for example, by a method that is described below. 
     (1) The base station  10  reports timing, order, etc. of transmission of a CSI_RS or a V 1 *CSI_RS through RRC signaling beforehand, to the mobile station  20 . 
     (2) The base station  10  reports timing, order, etc. of transmission of a CSI_RS or a V 1 *CSI_RS to the mobile station  20  by broadcasting system information. 
     (3) The base station  10  transmits a CSI_RS and a V 1 *CSI_RS after performing addition of identification information that indicates a CSI_RS or a V 1 *CSI_RS. 
     4-3. Third Embodiment 
     As described in “1-4. Dynamic switching”, in SU-MIMO, for example, MIMO transmission of eight independent streams is performed. On the other hand, in MU-MIMO, for example, MIMO transmission of two independent streams is performed for each of the four different mobile stations  20 . Thus, V 2 _SU and V 2 _MU are different in terms that V 2 _SU is used for eight streams and V 2 _MU is used for two streams. 
     In this case, it is effective to set update frequency of V 2 _SU higher than update frequency of V 2 _MU because higher accuracy is desired for V 2 _SU that is used for eight streams. 
     Therefore, the upper layer signal processing unit  132  of the base station  10  according to a third embodiment allocates more resources for transmitting a CSI_RS to update (determine) V 2 _SU than that for transmitting of a V 1 *CSI_RS to update (determine) V 2 _MU. An operation of the base station  10  according to such third embodiment is described in detail with reference to  FIG. 12 . 
       FIG. 12  is an illustrative diagram illustrating the third embodiment of the present disclosure. As illustrated in  FIG. 12 , the base station  10  according to the third embodiment transmits, on a time direction, a CSI_RS to update (determines) V 2 _SU at higher frequency than that of a V 1 *CSI_RS to update (determines) V 2 _MU after determination of V 1 . By such configuration, highly accurate V 2 _SU can be obtained while suppressing calculation load in the mobile station  20  at the time of update of V 2 _MU. 
     4-4. Fourth Embodiment 
     In the third embodiment, the description is made in which the base station  10  transmits, in the time direction, a CSI_RS at higher frequency than that of a V 1 *CSI_RS in order to make the update frequency of V 2 _SU higher than the update frequency of V 2 _MU. In a fourth embodiment, similarly to the third embodiment, arrangement of a V 1 *CSI_RS and a CSI_RS on the frequency direction in a subcarrier of OFDM has been devised in order to make the update frequency of V 2 _SU higher than the update frequency of V 2 _MU. A resource allocation example according to the fourth embodiment is described below in detail with reference to  FIG. 13 . 
       FIG. 13  is an illustrative diagram illustrating a resource allocation example of a V 1 *CSI_RS and a CSI_RS according to the fourth embodiment. As illustrated in  FIG. 13 , the upper layer signal processing unit  132  of the base station  10  according to fourth embodiment arranges, on the frequency direction, a CSI_RS more densely than a V 1 *CSI_RS. As described above, similarly to the third embodiment, highly accurate V 2 _SU can be obtained while suppressing calculation load in the mobile station  20  at the time of update of V 2 _MU, by devising the arrangement of a V 1 *CSI_RS and a CSI_RS on the frequency direction. 
     4-5. Fifth Embodiment 
     In a fifth embodiment, resource allocation for data communication using a determined transmission weight is described. 
       FIG. 14  is an illustrative diagram illustrating a specific example of resource allocation according to the fifth embodiment. The horizontal axis in  FIG. 14  indicates a time, and the vertical axis indicates a frequency. In addition, the time width of a square block in  FIG. 14  may be one resource block or one subframe. In addition, the frequency width of the square block may be one resource block ( 12  subcarrier portions) or another band width. 
     As illustrated in  FIG. 14 , when the base station  10  transmits a CSI_RS first, the mobile station  20  obtains V 1 , V 2 _MU, and V 2 _SU for each frequency on the basis of the reception of a CSI_RS. In addition, the mobile station  20  provides feedback of V 1 , V 2 _MU, and V 2 _SU to the base station  10 . 
     After that, as illustrated in  FIG. 14 , the scheduler  136  of the base station  10  allocates four resource blocks from the bottom included in a frequency range B for MU-MIMO (first scheme) with the mobile stations  20 A to  20 C. On the other hand, as illustrated in  FIG. 14 , the scheduler  136  of the base station  10  allocates two resource blocks from the top included in a frequency range A for SU-MIMO (second scheme) with the mobile station  20 D. 
     Here, the scheduler  136  according to the fifth embodiment keeps the resource blocks that are included in the frequency range B as an area for MU-MIMO and keeps the resource blocks that are included in the frequency range A as an area for SU-MIMO. 
     Therefore, for example, when the scheduler  136  according to the fifth embodiment performs dynamic switching of setting of MIMO of the mobile station  20 C from MU-MIMO to SU-MIMO, the resource block that is allocated to the mobile station  20 C is moved to the resource block that is included in the frequency range A, as illustrated in  FIG. 14 . 
     As described above, according to the fifth embodiment, dynamic switching of MU-MIMO and SU-MIMO can be realized by moving a resource block of the mobile station  20  in a frequency direction. 
     4-6. Sixth Embodiment 
       FIG. 15  is an illustrative diagram illustrating a specific example of resource allocation according to a sixth embodiment. As illustrated in  FIG. 15 , the upper layer signal processing unit  132  according to the sixth embodiment allocates resource blocks that are included in the frequency range B for MU-MIMO that is described in the fifth embodiment, for transmitting a V 1 *CSI_RS, after determination of V 1 . In addition, the upper layer signal processing unit  132  allocates resource blocks that are included in the frequency range A for SU-MIMO that is described in the fifth embodiment, for transmitting a CSI_RS. 
     By such configuration, V 2 _SU can be updated in frequency range A while suppressing the amount of calculation in the mobile station  20  and updating V 2 _MU in the frequency range B. Therefore, the frequency range B can be used for communication by MU-MIMO, and the frequency range A can be used for communication by SU-MIMO. 
     4-7. Seventh Embodiment 
     In the above-described fifth embodiment and sixth embodiment, the example is described in which a frequency range for MU-MIMO and a frequency range for SU-MIMO are fixed, and alternatively, as described below with reference to a seventh embodiment, a frequency range for MU-MIMO and a frequency range for SU-MIMO can be dynamically changed. 
       FIG. 16  is an illustrative diagram illustrating a specific example of resource allocation according to a seventh embodiment. As illustrated in  FIG. 16 , it is assumed that, in the time t 1 , resource blocks in a frequency range Z and a frequency range X are allocated for transmitting a CSI_RS, resource blocks in a frequency range Y are allocated for transmission of a V 1 *CSI_RS. 
     Here, in a frequency in which a CSI_RS is transmitted, V 1 , V 2 _MU, and V 2 _SU can be obtained. On the other hand, in a frequency in which a V 1 *CSI_RS is transmitted, V 2 _MU can be obtained, however, V 2 _SU is difficult to be obtained. That is, the frequency in which a V 1 *CSI_RS is transmitted can be used for MU-MIMO, the frequency in which a CSI_RS is transmitted can be used for both of MU-MIMO or SU-MIMO. 
     Therefore, the scheduler  136  according to the seventh embodiment handles resource blocks in the frequency range X and the frequency range Z to which a CSI_RS is transmitted as an area in which switching of SU-MIMO and MU-MIMO can be performed. On the other hand, the scheduler  136  handles resource blocks in the frequency range Y to which a V 1 *CSI_RS is transmitted as a MU-MIMO-dedicated area. 
     For example, as illustrated in  FIG. 16 , at the time t 2 , the scheduler  136  allocates resource blocks in the frequency range X for communication by SU-MIMO, and allocates resource blocks in the frequency range Y and the frequency Z for communication by MU-MIMO. After that, at the time t 3 , the scheduler  136  can switches resource blocks for MU-MIMO to resource blocks for SU-MIMO in the frequency range Z, and can switches at resource blocks for SU-MIMO to at resource blocks for MU-MIMO in the frequency range X. 
     5. OPERATION OF THE BASE STATION AND THE MOBILE STATION 
     Each of the embodiments of the present disclosure is described above. Next, operations of the base station  10  and the mobile station  20  according to the embodiments of the present disclosure are described with reference to  FIGS. 17 and 18 . 
       FIG. 17  is a flowchart illustrating an operation of the base station  10  according to the embodiments of the present disclosure. Note that  FIG. 17  particularly corresponds to the operation of the base station  10  according to the seventh embodiment. 
     As illustrated in  FIG. 17 , first, the base station  10  determines update frequency of V 1  and update frequency of V 2 _MU in the time direction (S 304 ). After that, the base station  10  determines update frequency of V 2 _SU in the time direction (S 308 ). 
     After that, the base station  10  determines a density of a resource for MU-MIMO and a density of a resource for SU-MIMO in the frequency direction (S 312 ). In addition, the base station  10  determines a ratio, which is illustrated in  FIG. 16 , of the MU-MIMO-dedicated area and the area in which dynamic switching can be performed in the frequency direction (S 316 ). Note that in the example illustrated in  FIG. 16 , the ratio of the MU-MIMO-dedicated area and the in which dynamic switching can be performed in the frequency direction is 1:2, and a density ratio of a resource for MU-MIMO and a resource for SU-MIMO in the frequency direction is 2:1. 
     After that, the base station  10  allocates a resource for transmitting a CSI_RS and a resource for transmitting a V 1 *CSI_RS (S 320 ). More specifically, in S 316 , the base station  10  allocates a resource of a frequency that is determined as the MU-MIMO-dedicated area for transmitting a V 1 *CSI_RS and allocates a resource of a frequency that is determined as the area in which dynamic switching can be performed for transmitting a CSI_RS. In addition, the base station  10  allocates a resource in the time direction to a V 1 *CSI_RS and a CSI_RS on the basis of the determination results of S 304  and S 308 . In addition, the base station  10  transmits a CSI_RS and a V 1 *CSI_RS in accordance with the determined resource. 
       FIG. 18  is a flowchart of an operation of the mobile station  20  according to the embodiments. As illustrated in  FIG. 18 , in a case in which the mobile station  20  receives a radio signal from the base station  10  (S 404 ), when the radio signal is a CSI_RS (S 408 ), a channel matrix H is obtained from the reception result of the CSI_RS (S 412 ). In addition, the mobile station  20  determines transmission weight such as V 1 , V 2 _MU, and V 2 _SU on the basis of the channel matrix H obtained in S 412  (S 416 ). In addition, the mobile station  20  provides feedback of V 1 , V 2 _MU, and V 2 _SU to the base station  10  (S 420 ). 
     On the other hand, when the received radio signal is a V 1 *CSI_RS (S 408 ), the mobile station  20  obtains a channel matrix H that is multiplied by V 1  from the reception result of a V 1 *CSI_RS (S 424 ). In addition, the mobile station  20  determines V 2 _MU on the basis of the channel matrix H that is multiplied by V 1  without performing calculation using V 1  (S 428 ). In addition, the mobile station  20  provides feedback of V 2 _MU to the base station  10  (S 432 ). 
     In addition, when the received radio signal is a data signal (S 408 ), the mobile station  20  demodulates the data signal and obtains data that is transmitted from the base station  10  (S 436 ). 
     6. CONCLUSION 
     As described above, the base station  10  according to the embodiments of the present disclosure starts to transmit a V 1 *CSI_RS after determination of transmission weight V 1 . By such configuration, calculation load such as V 2 _MU in the mobile station  20  that is described below can be suppressed. In addition, the base station  10  according to the embodiments of the present disclosure continues to transmit a CSI_RS. By such configuration, the mobile station  20  can determine V 2 _SU on the basis of reception of a CSI_RS. As a result, dynamic switching of MU-MIMO and SU-MIMO can be realized. 
     The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, whilst the present invention is not limited to the above examples, of course. A person skilled in the art may find various alternations and modifications within the scope of the appended claims, and it should be understood that they will naturally come under the technical scope of the present invention. 
     For example, two or more of the first embodiment to the seventh embodiment may be combined. For example, the resource allocation in the time direction that is described in the third embodiment, the resource allocation in the frequency direction that is described in the fifth embodiment, and the resource allocation for SU-MIMO and MU-MIMO that is described in the sixth embodiment can be combined. 
     In addition, the steps in the processing of the base station  10  or the processing of the mobile station  20  in this specification are not necessarily processed in chronological order in accordance with the order that is described as the flowchart. For example, the steps in the processing of the base station  10  or the processing of the mobile station  20  may be processed in order different from the order that is described as the flowchart, or may be processed in parallel. 
     In addition, a computer program can be created that exerts hardware such as a CPU, a ROM, and a RAM, which is built in the base station  10  or the mobile station  20  as a function equivalent to each configuration of the above-described base station  10  or the mobile station  20 . In addition, a storage medium that stores the computer program is also provided. 
     REFERENCE SIGNS LIST 
     
         
           10  base station 
           20 ,  20 A,  20 B mobile station 
           110 ,  210  antenna 
           116 ,  216  switch SW 
           120 ,  220  analog processing unit 
           124 ,  224  AD/DA conversion unit 
           128 ,  228  demodulation processing unit 
           132 ,  232  upper layer signal processing unit 
           136  scheduler 
           140 ,  240  modulation processing unit 
           150  weight multiplication unit 
           152  V 2 _SU multiplication unit 
           154  V 1  multiplication unit 
           156  V 2 _MU multiplication unit 
           244  channel matrix obtaining unit 
           248  weight determination unit