Patent Publication Number: US-2016234694-A1

Title: Communication control method, base station, and user terminal

Description:
TECHNICAL FIELD 
     The present invention relates to a communication control method, a base station, and a user terminal used in a mobile communication system that supports downlink multi-antenna transmission 
     BACKGROUND ART 
     An LTE (Long Term Evolution) system of which the specifications are designed in 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, supports downlink multi-antenna transmission (see Non Patent Literature 1). For example, a base station directs a beam to one user terminal (beamforming) and directs a null to another user terminal (null steering). 
     Further, a mode of the downlink multi-antenna transmission includes CB (Coordinated Beamforming)-CoMP (Coordinated Multi Point). 
     CITATION LIST 
     Non Patent Literature 
     [NPL 1] 3GPP Technical Specification “TS 36.300 V11.6.0” July, 2013 
     SUMMARY OF INVENTION 
     In the CB-CoMP, a base station that manages a cell receives beamforming control information fed back from each of a plurality of terminals subject to beamforming connected with the cell of the base station, and null-steering control information fed back from a terminal subject to null steering connected with a neighboring cell. Then, the base station selects, as a pair terminal that forms a pair with the terminal subject to null steering, a terminal subject to beamforming that feeds back the beamforming control information that matches the null-steering control information. 
     However, when there is no terminal subject to beamforming that feeds back beamforming control information that matches null-steering control information, a base station is not capable of selecting a pair terminal. In this case, there is a problem in that it is not possible to effectively utilize the downlink multi-antenna transmission. 
     Therefore, an object of the present invention is to provide a communication control method, a base station, and a user terminal, with which it is possible to effectively utilize the downlink multi-antenna transmission. 
     A communication control method according to a first aspect is used in a mobile communication system that supports downlink multi-antenna transmission. The communication control method comprises a step A of feeding back for a plurality of number of times, by a user terminal subject to null steering by a base station, null-steering control information for controlling the null steering; and a step B of selecting, by the base station, through a matching process in which the null-steering control information fed back from the user terminal is checked with beamforming control information fed back from other user terminals, a pair terminal that forms a pair with the user terminal from among the other user terminals. The base station manages a history of past null-steering control information fed back prior to a last time from the user terminal. In the step B, the base station applies, in addition to latest null-steering control information fed back this time from the user terminal, the past null-steering control information to the matching process. 
     A base station according to a second aspect is used in a mobile communication system that supports downlink multi-antenna transmission. The base station comprises a receiver configured to receive null-steering control information fed back for a plurality of number of times from a user terminal subject to null steering by the base station; and a controller configured to select, through a matching process in which the null-steering control information fed back from the user terminal is checked with beamforming control information fed back from other user terminals, a pair terminal that forms a pair with the user terminal from among the other user terminals. The controller manages a history of past null-steering control information fed back prior to a last time from the user terminal. The controller applies, in addition to latest null-steering control information fed back this time from the user terminal, the past null-steering control information to the matching process. 
     A user terminal according to a third aspect is a user terminal subject to null steering by a base station, in a mobile communication system that supports downlink multi-antenna transmission. The user terminal comprises a controller configured to feed back for a plurality of number of times null-steering control information for controlling the null steering. The controller adds additional information associated with the priority order of the null-steering control information to be fed back, to the null-steering control information to be fed back. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of an LTE system according to an embodiment. 
         FIG. 2  is a block diagram of a UE according to the embodiment. 
         FIG. 3  is a block diagram of an eNB according to the embodiment. 
         FIG. 4  is a protocol stack diagram of a radio interface according to the embodiment. 
         FIG. 5  is a configuration diagram of a radio frame according to the embodiment. 
         FIG. 6  is a diagram (part  1 ) for describing CB-CoMP according to the embodiment. 
         FIG. 7  is a diagram (part  2 ) for describing CB-CoMP according to the embodiment. 
         FIG. 8  is a diagram for describing an operation pattern  1  according to the embodiment. 
         FIG. 9  is a diagram for describing an operation pattern  2  according to the embodiment. 
         FIG. 10  is a diagram for describing an operation pattern  3  according to the embodiment. 
         FIG. 11  is a diagram for describing MU-MIMO according to a modification of the embodiment (part  1 ). 
         FIG. 12  is a diagram for describing the MU-MIMO according to the modification of the embodiment (part  2 ). 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Overview of Embodiments 
     A communication control method according to embodiments is used in a mobile communication system that supports downlink multi-antenna transmission. The communication control method comprises a step A of feeding back for a plurality of number of times, by a user terminal subject to null steering by a base station, null-steering control information for controlling the null steering; and a step B of selecting, by the base station, through a matching process in which the null-steering control information fed back from the user terminal is checked with beamforming control information fed back from other user terminals, a pair terminal that forms a pair with the user terminal from among the other user terminals. The base station manages a history of past null-steering control information fed back prior to a last time from the user terminal. In the step B, the base station applies, in addition to latest null-steering control information fed back this time from the user terminal, the past null-steering control information to the matching process. 
     In the embodiments, in the step B, the base station selects, as the pair terminal, the one of the other user terminals that feeds back the beamforming control information that matches either one of the latest null-steering control information or the past null-steering control information. The communication control method further comprises a step C of performing, by the base station, a beamforming to the pair terminal and the null steering to the user terminal, on the basis of the matched beamforming control information. 
     In operation pattern  1  of the embodiments, in the step A, the user terminal feeds back null-steering control information having a highest priority order, from among a plurality of pieces of null-steering control information having a priority order set on the basis of a radio situation of the user terminal. In the step B, the base station sets, as a priority order of the beamforming control information applied to the matching process, a relatively high priority order to the beamforming control information that is relatively new. 
     In operation pattern  2  and  3  of the embodiments, in the step A, the user terminal derives a plurality of pieces of null-steering control information having a priority order set on the basis of a radio situation of the user terminal. When the null-steering control information having a highest priority order is different between during a last feedback and during a current feedback, the user terminal feeds back the null-steering control information having the highest priority order. When the null-steering control information having the highest priority order is the same between during the last feedback and during the current feedback, the user terminal feeds back null-steering control information having a second highest priority order. 
     In operation pattern  2  and  3  of the embodiments, in the step A, when the null-steering control information having the highest priority order and the null-steering control information having the second highest priority order are the same between during the last feedback and during the current feedback, the user terminal feeds back null-steering control information having a third highest priority order. 
     In operation pattern  2  and  3  of the embodiments, in the step A, the user terminal adds additional information associated with the priority order of null-steering control information to be fed back, to the null-steering control information to be fed back. 
     In operation pattern  2  of the embodiments, the additional information is information indicating the priority order of the null-steering control information to be fed back. 
     In operation pattern of the embodiments, the additional information is information indicating whether or not the history managed by the base station should be deleted. 
     In operation pattern  2  and  3  of the embodiments, in the step B, when the latest null-steering control information is the null-steering control information having the highest priority order or when it is indicated that the history should be deleted, as the priority order of the beamforming control information applied to the matching process, the base station sets the latest null-steering control information to the highest priority order, and deletes the history. 
     In operation pattern  2  and  3  of the embodiments, in the step B, when the latest null-steering control information is the null-steering control information having the highest priority order, as the priority order of the beamforming control information applied to the matching process, the base station sets the latest null-steering control information to the highest priority order and moves down by one the priority order of the past null-steering control information included in the history. 
     In operation pattern  2  of the embodiments, in the step B, when the latest null-steering control information is the null-steering control information having the second highest priority order, as the priority order of the beamforming control information applied to the matching process, the base station sets the past null-steering control information having the newest highest priority order of the history, to the highest priority order, sets the latest null-steering control information to the second highest priority order, and deletes the past null-steering control information having a second priority order or lower included in the history. 
     Alternatively, in operation pattern  2  of the embodiments, in the step B, when the latest null-steering control information is the null-steering control information having the second highest priority order, as the priority order of the beamforming control information applied to the matching process, the base station sets the past null-steering control information having the newest highest priority order of the history, to the highest priority order, sets the latest null-steering control information to the second highest priority order, and moves down by one the priority order of the past null-steering control information having a second priority order or lower included in the history. 
     In operation pattern  3  of the embodiments, in the step B, when it is indicated that the history should not be deleted, as the priority order of the beamforming control information applied to the matching process, the base station sets the past null-steering control information having the newest highest priority order of the history, to the highest priority order, and sets the past null-steering control information fed back subsequent to the past null-steering control information having the newest highest priority order, to the second highest priority order. 
     A base station according to the embodiments is used in a mobile communication system that supports downlink multi-antenna transmission. The base station comprises a receiver configured to receive null-steering control information fed back for a plurality of number of times from a user terminal subject to null steering by the base station; and a controller configured to select, through a matching process in which the null-steering control information fed back from the user terminal is checked with beamforming control information fed back from other user terminals, a pair terminal that forms a pair with the user terminal from among the other user terminals. The controller manages a history of past null-steering control information fed back prior to a last time from the user terminal. The controller applies, in addition to latest null-steering control information fed back this time from the user terminal, the past null-steering control information to the matching process. 
     A user terminal according to the embodiments is a user terminal subject to null steering by a base station, in a mobile communication system that supports downlink multi-antenna transmission. The user terminal comprises a controller configured to feed back for a plurality of number of times null-steering control information for controlling the null steering. The controller adds additional information associated with the priority order of the null-steering control information to be fed back, to the null-steering control information to be fed back. 
     Embodiments 
     An embodiment of applying the present invention to the LTE system will be described below. 
     (System Configuration) 
       FIG. 1  is a configuration diagram of an LTE system according to an embodiment. As illustrated in  FIG. 1 , the LTE system includes a plurality of UEs (User Equipments)  100 , E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network)  10 , and EPC (Evolved Packet Core)  20 . 
     The UE  100  corresponds to a user terminal. The UE  100  is a mobile communication device and performs radio communication with a cell (a serving cell) with which a connection is established. Configuration of the UE  100  will be described later. 
     The E-UTRAN  10  corresponds to a radio access network. The E-UTRAN  10  includes a plurality of eNBs (evolved Node-Bs)  200 . The eNB  200  corresponds to a base station. The eNBs  200  are connected mutually via an X2 interface. Configuration of the eNB  200  will be described later. 
     The eNB  200  manages one or a plurality of cells and performs radio communication with the UE  100  which establishes a connection with the cell of the eNB  200 . The eNB  200  has a radio resource management (RRM) function, a routing function for user data, and a measurement control function for mobility control and scheduling, and the like. It is noted that the “cell” is used as a term indicating a minimum unit of a radio communication area, and is also used as a term indicating a function of performing radio communication with the UE  100 . 
     The EPC  20  corresponds to a core network. A network of the LTE system is configured by the E-UTRAN  10  and the EPC  200 . The EPC  20  includes a plurality of MME (Mobility Management Entity)/S-GWs (Serving-Gateways)  300 . The MME performs various mobility controls and the like for the UE  100 . The S-GW performs control to transfer user. MME/S-GW  300  is connected to eNB  200  via an S1 interface. 
       FIG. 2  is a block diagram of the UE  100 . As illustrated in  FIG. 2 , the UE  100  includes plural antennas  101 , a radio transceiver  110 , a user interface  120 , a GNSS (Global Navigation Satellite System) receiver  130 , a battery  140 , a memory  150 , and a processor  160 . The memory  150  and the processor  160  constitute a controller. The UE  100  may not have the GNSS receiver  130 . Furthermore, the memory  150  may be integrally formed with the processor  160 , and this set (that is, a chip set) may be called a processor  160 ′. 
     The plural antennas  101  and the radio transceiver  110  are used to transmit and receive a radio signal. The radio transceiver  110  converts a baseband signal (a transmission signal) output from the processor  160  into the radio signal and transmits the radio signal from the antenna  101 . Furthermore, the radio transceiver  110  converts a radio signal received by the antenna  101  into a baseband signal (a received signal), and outputs the baseband signal to the processor  160 . 
     The user interface  120  is an interface with a user carrying the UE  100 , and includes, for example, a display, a microphone, a speaker, various buttons and the like. The user interface  120  accepts an operation from a user and outputs a signal indicating the content of the operation to the processor  160 . The GNSS receiver  130  receives a GNSS signal in order to obtain location information indicating a geographical location of the UE  100 , and outputs the received signal to the processor  160 . The battery  140  accumulates power to be supplied to each block of the UE  100 . 
     The memory  150  stores a program to be executed by the processor  160  and information to be used for a process by the processor  160 . The processor  160  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on the baseband signal, and CPU (Central Processing Unit) that performs various processes by executing the program stored in the memory  150 . The processor  160  may further include a codec that performs encoding and decoding on sound and video signals. The processor  160  executes various processes and various communication protocols described later. 
       FIG. 3  is a block diagram of the eNB  200 . As illustrated in  FIG. 3 , the eNB  200  includes plural antennas  201 , a radio transceiver  210 , a network interface  220 , a memory  230 , and a processor  240 . The memory  230  and the processor  240  constitute a controller. 
     The plural antennas  201  and the radio transceiver  210  are used to transmit and receive a radio signal. The radio transceiver  210  converts a baseband signal (a transmission signal) output from the processor  240  into the radio signal and transmits the radio signal from the antenna  201 . Furthermore, the radio transceiver  210  converts a radio signal received by the antenna  201  into a baseband signal (a received signal), and outputs the baseband signal to the processor  240 . 
     The network interface  220  is connected to the neighboring eNB  200  via the X2 interface and is connected to the MME/S-GW  300  via the S1 interface. The network interface  220  is used in communication over the X2 interface and communication over the S1 interface. 
     The memory  230  stores a program to be executed by the processor  240  and information to be used for a process by the processor  240 . The processor  240  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on the baseband signal and CPU that performs various processes by executing the program stored in the memory  230 . The processor  240  executes various processes and various communication protocols described later. 
       FIG. 4  is a protocol stack diagram of a radio interface in the LTE system. As illustrated in  FIG. 4 , the radio interface protocol is classified into a layer  1  to a layer  3  of an OSI reference model, wherein the layer  1  is a physical (PHY) layer. The layer  2  includes a MAC (Medium Access Control) layer, an RLC (Radio Link Control) layer, and a PDCP (Packet Data Convergence Protocol) layer. The layer  3  includes an RRC (Radio Resource Control) layer. 
     The PHY layer performs encoding and decoding, modulation and demodulation, antenna mapping and demapping, and resource mapping and demapping. The PHY layer of the eNB  200  applies a precoder matrix (transmission antenna weight) and a rank (number of signal sequences) to perform downlink multi-antenna transmission. The downlink multi-antenna transmission according to the embodiment will be described later. Between the PHY layer of the UE  100  and the PHY layer of the eNB  200 , use data and control signal are transmitted via the physical channel. 
     The MAC layer performs priority control of data, a retransmission process by hybrid ARQ (HARQ), and the like. Between the MAC layer of the UE  100  and the MAC layer of the eNB  200 , user data and control signal are transmitted via a transport channel. The MAC layer of the eNB  200  includes a scheduler that determines a transport format of an uplink and a downlink (a transport block size and a modulation and coding scheme (MCS)) and a resource block to be assigned to the UE  100 . 
     The RLC layer transmits data to an RLC layer of a reception side by using the functions of the MAC layer and the PHY layer. Between the RLC layer of the UE  100  and the RLC layer of the eNB  200 , user data and control signal are transmitted via a logical channel. 
     The PDCP layer performs header compression and decompression, and encryption and decryption. 
     The RRC layer is defined only in a control plane dealing with control signal. Between the RRC layer of the UE  100  and the RRC layer of the eNB  200 , control message (RRC messages) for various types of configuration are transmitted. The RRC layer controls the logical channel, the transport channel, and the physical channel in response to establishment, re-establishment, and release of a radio bearer. When there is an RRC connection between the RRC of the UE  100  and the RRC of the eNB  200 , the UE  100  is in a connected state (an RRC connected state), otherwise the UE  100  is in an idle state (an RRC idle state). 
     A NAS (Non-Access Stratum) layer positioned above the RRC layer performs a session management, a mobility management and the like. 
       FIG. 5  is a configuration diagram of a radio frame used in the LTE system. In the LTE system, OFDMA (Orthogonal Frequency Division Multiplex Access) is applied to a downlink, and SC-FDMA (Single Carrier Frequency Division Multiple Access) is applied to an uplink, respectively. 
     As illustrated in  FIG. 5 , the radio frame is configured by 10 subframes arranged in a time direction, wherein each subframe is configured by two slots arranged in the time direction. Each subframe has a length of 1 ms and each slot has a length of 0.5 ms. Each subframe includes a plurality of resource blocks (RBs) in a frequency direction, and a plurality of symbols in the time direction. The resource block includes a plurality of subcarriers in the frequency direction. Among radio resources assigned to the UE  100 , a frequency resource can be specified by a resource block and a time resource can be specified by a subframe (or slot). 
     In the downlink, an interval of several symbols at the head of each subframe is a control region used as a physical downlink control channel (PDCCH) for mainly transmitting a control signal. Furthermore, the other interval of each subframe is a region available as a physical downlink shared channel (PDSCH) for mainly transmitting user data. 
     In the uplink, both ends in the frequency direction of each subframe are control regions used as a physical uplink control channel (PUCCH) for mainly transmitting a control signal. Furthermore, the central portion in the frequency direction of each subframe is a region available as a physical uplink shared channel (PUSCH) for mainly transmitting user data. 
     (CB-CoMP) 
     The LTE system according to the embodiment supports CB-CoMP which is a mode of the downlink multi-antenna transmission. In the CB-CoMP, a plurality of eNBs  200  work together to perform the beamforming and the null steering. 
       FIG. 6  and  FIG. 7  are diagrams for describing the CB-CoMP. As shown in  FIG. 6 , an eNB  200 - 1  and an eNB  200 - 2  manage cells adjacent to each other. Further, the cell of the eNB  200 - 1  and the cell of the eNB  200 - 2  belong to the same frequency. 
     The UE  100 - 1  is in a state of establishing connection with a cell of the eNB  200 - 1  (connected state). That is, the UE  100 - 1  uses, as a serving cell, the cell of the eNB  200 - 1  to perform communication. 
     On the other hand, the UE  100 - 2  is in a state of establishing connection with a cell of the eNB  200 - 2  (connected state). That is, the UE  100 - 2  uses, as a serving cell, the cell of the eNB  200 - 2  to perform communication. In  FIG. 6 , only one UE  100 - 2  is shown which establishes the connection with the cell of the eNB  200 - 2 ; however, in a real environment, a plurality of UEs  100 - 2  establish the connection with the cell of the eNB  200 - 2 . 
     The UE  100 - 1  is located at a boundary area of the cell of the eNB  200 - 1  and the cell of the eNB  200 - 2 . In this case, the UE  100 - 1  is influenced by interference from the cell of the eNB  200 - 2 . When the CB-CoMP is applied to the UE  100 - 1 , it is possible to suppress the interference received in the UE  100 - 1 . 
     A communication procedure of the CB-CoMP when the CB-CoMP is applied to the UE  100 - 1  will be described, below. It is noted that the UE  100 - 1  to which the CB-CoMP is applied may be called a “CoMP UE”. That is, the UE  100 - 1  corresponds to a terminal subject to null steering. The serving cell of the UE  100 - 1  (CoMP UE) may be called an “anchor cell”. 
     Each of the UE  100 - 1  and the UE  100 - 2  feeds beamforming control information for directing a beam to the UE  100 - 1  and the UE  100 - 2 , back to the serving cell, on the basis of a reference signal received from the serving cell, for example. In the embodiment, the beamforming control information includes a precoder matrix indicator (PMI) and a rank indicator (RI). The PMI is an indicator indicating a precoder matrix (transmission antenna weight) recommended to the serving cell. The RI is an indicator indicating a rank (signal sequence number) recommended to the serving cell. Each of the UE  100 - 1  and the UE  100 - 2 , which holds a table (code book) in which the precoder matrix and the indicator are associated, selects the precoder matrix that improves communication quality of a desired wave, and feeds back, as the PMI, the indicator corresponding to the selected precoder matrix. 
     The UE  100 - 1  further feeds null-steering control information for directing a null to the UE  100 - 1 , back to the serving cell, on the basis of a reference signal received from a neighboring cell, for example. In the embodiment, the null-steering control information includes a BCI (Best Companion PMI) and the RI. The BCI is an indicator indicating a precoder matrix (transmission antenna weight) recommended to the neighboring cell. The UE  100 - 1 , which holds a table (code book) in which the precoder matrix and the indicator are associated, selects the precoder matrix that reduces a reception level of an interference wave or reduces influence to a desired wave, and feeds back, as the BCI, the indicator corresponding to the selected precoder matrix. 
     The eNB  200 - 1  transfers the null-steering control information (BCI, RI) fed back from the UE  100 - 1 , to the eNB  200 - 2 . 
     The eNB  200 - 2  receives the beamforming control information (PMI, RI) fed back from each of the plurality of UEs  100 - 2  connected with a cell of the eNB  200 - 2  and the null-steering control information (BCI, RI) fed back from the UE  100 - 1  connected with the neighboring cell. Then, the eNB  200 - 2  selects the UE  100 - 2  that feeds back the beamforming control information that matches the null-steering control information, as a pair UE (pair terminal) that forms a pair with the UE  100 - 1 . In the embodiment, the “beamforming control information that matches the null-steering control information” is beamforming control information that includes a combination of the PMI and the RI that matches a combination of the BCI and the RI included in the null-steering control information. 
     When selecting the pair UE (UE  100 - 2 ), the eNB  200 - 2  assigns the same radio resource as the radio resource assigned to the UE  100 - 1 , to the pair UE. Then, the eNB  200 - 2  applies the beamforming control information (PMI, RI) fed back from the pair UE, and performs a transmission to the pair UE. As a result, as shown in  FIG. 7 , the eNB  200 - 2  is capable of performing a transmission to the pair UE by directing a beam to the pair UE while directing a null to the UE  100 - 1 . 
     (Operation According to Embodiment) 
     (1) Operation Overview 
     As described above, the eNB  200 - 2  selects, as the pair UE that forms a pair with the UE  100 - 1 , the UE  100 - 2  that feeds back the beamforming control information (PMI, RI) that matches the null-steering control information (BCI, RI) fed back from the UE  100 - 1 . Here, the UE  100 - 1  corresponds to a terminal subject to null steering and the UE  100 - 2  corresponds to a terminal subject to beamforming. 
     However, when there is no UE  100 - 2  that feeds back the beamforming control information that matches the null-steering control information, the eNB  200 - 2  is not capable of selecting a pair UE that forms a pair with the UE  100 - 1 . A communication control method for resolving such a problem will be described, below. 
     The communication control method according to the embodiment includes a step A of feeding back for a plurality of number of times, by the UE  100 - 1  subject to the null steering by the eNB  200 - 2 , null-steering control information for controlling the null steering. “Feeding back for a plurality of number of times” is a periodic feedback, for example. However, in addition to the periodic feedback, an unperiodic feedback may also be possible. 
     Further, the communication control method according to the embodiment includes a step B of selecting, by the eNB  200 - 2 , through a matching process in which the null-steering control information fed back from the UE  100 - 1  is checked with beamforming control information fed back from the UE  100 - 2 , a pair UE that forms a pair with the UE  100 - 1  from among the UEs  100 - 2 . 
     The eNB  200 - 2  manages a history of past null-steering control information fed back prior to the last time from the UE  100 - 1 . In the step B, the eNB  200 - 2  applies, in addition to latest null-steering control information fed back this time from the UE  100 - 1 , the past null-steering control information to the matching process. 
     Therefore, even when there is no UE  100 - 2  that feeds back the beamforming control information that matches the latest null-steering control information, it is possible to select the UE  100 - 2 , as the pair UE, that feeds back the beamforming control information that matches the past null-steering control information. Thus, it is possible to apply the CB-CoMP to the UE  100 - 1 . 
     In the embodiment, in the step B, the eNB  200 - 2  selects, as the pair UE, the UE  100 - 2  that feeds back the beamforming control information that matches either one of the latest null-steering control information or the past null-steering control information. The communication control method according to the embodiment further includes a step C of performing, by the eNB  200 - 2 , a beamforming to the pair UE and a null steering to the UE  100 - 1 , on the basis of the matched beamforming control information. 
     (2) Operation Specific Example 
     Next, as an operation specific example according to the embodiment, operation patterns  1  to  3  will be described. 
     (2.1) Operation Pattern  1   
       FIG. 8  is a diagram for explaining an operation pattern  1  according to the embodiment. 
     As shown in  FIG. 8 , in the operation pattern  1  according to embodiment, in the step A, the UE  100 - 1  feeds back null-steering control information having the highest priority order, from among a plurality of pieces of null-steering control information having a priority order set on the basis of a radio situation of the UE  100 - 1 . 
     For example, for each of a plurality of PMIs (and RIs) included in a code book, the UE  100 - 1  sets the priority order for each combination of PMIs and RIs, by using, as an evaluation index, a degree by which a reception level of an interference wave is reduced or a degree by which an influence to a desired wave is reduced. Specifically, a highest priority order is set to a combination of PMI and RI having the highest degree by which the reception level of the interference wave is reduced or degree by which the influence to the desired wave is reduced. Then, the UE  100 - 1  feeds back the combination (null-steering control information) of the PMI and the RI having the highest priority order. 
     In an example of  FIG. 8 , at a time T 1 , the null-steering control information having the highest priority order is “A” and, the null-steering control information having the second highest priority order is “B”, and thus, the UE  100 - 1  feeds back “A”. After a time T 2 , the operation is performed according to a similar rule. 
     In the operation pattern  1  according to the embodiment, in the step B, the eNB  200 - 2  sets, as the priority order of the beamforming control information applied to the matching process, a relatively high priority order to beamforming control information that is relatively new. 
     In an example of  FIG. 8 , the eNB  200 - 2  treats the two consecutive different feedbacks as the second highest priority order and the highest priority order, in order of time sequence. For example, on the basis of the feedback at a time T 3 , the eNB  200 - 2  sets the highest priority order to “B” that is the latest null-steering control information, and sets the second highest priority order to 
     “A” that is the past feedback information corresponding to the time T 2  (previous feedback). However, when the same feedback continues for a long period of time, the previous feedback may be ignored. 
     (2.2) Operation Pattern  2   
       FIG. 9  is a diagram for describing an operation pattern  2  according to the embodiment. Here, description proceeds with a focus on a difference from the operation pattern  1 . 
     As shown in  FIG. 9 , in the operation pattern  2  according to the embodiment, in the step A, when the null-steering control information having the highest priority order is different between during the last feedback and during the current feedback, the UE  100 - 1  feeds back the null-steering control information having the highest priority order. Further, when the null-steering control information having the highest priority order is the same between during the last feedback and during the current feedback, the UE  100 - 1  feeds back the null-steering control information having the second highest priority order. Further, when the null-steering control information having the highest priority order and the null-steering control information having the second highest priority order are the same between during the last feedback and during the current feedback, the UE  100 - 1  feeds back the null-steering control information having a third highest priority order. Thereafter, the operation is performed according to a similar rule. 
     In an example of  FIG. 9 , the null-steering control information having the highest priority order corresponding to the time T 2  is “A” and the null-steering control information having the highest priority order corresponding to the time T 1  that is the last feedback time is also “A”, and thus, the UE  100 - 1  feeds back, at the time T 2 , the null-steering control information “B” having the second highest priority order. Further, the null-steering control information having the highest priority order corresponding to the time T 3  is “B” and the null-steering control information having the highest priority order corresponding to the time T 2  that is the last feedback time is “A”, and thus, the UE  100 - 1  feeds back, at the time T 3 , the null-steering control information “B” having the highest priority order. 
     Thus, in the operation pattern  2  according to the embodiment, the UE  100 - 1  avoids overlapping of the last feedback and the current feedback, and preferentially feeds back the null-steering control information having a higher priority order. 
     Further, in the operation pattern  2  according to the embodiment, in the step A, the UE  100 - 1  adds additional information (field) associated with the priority order of the null-steering control information to be fed back, to the null-steering control information to be fed back. Specifically, the additional information is information indicating the priority order of the null-steering control information to be fed back. As a result, the eNB  200 - 2  is capable of grasping the priority order set to the null-steering control information to be fed back. 
     In the operation pattern  2  according to the embodiment, in the step B, when the latest null-steering control information is the null-steering control information having the highest priority order, the eNB  200 - 2  sets the latest null-steering control information to the highest priority order, as the priority order of the beamforming control information applied to the matching process, and deletes the history. Here, the reason why the history is deleted is explained as follows: The null-steering control information having the highest priority order is changed in the UE  100 - 1 , which means that it is possible to consider that the radio environment in the UE  100 - 1  is changed. Therefore, the reliability of the history is low, and thus, the history is deleted. 
     In the operation pattern  2  according to the embodiment, in the step B, when the latest null-steering control information is the null-steering control information having the second highest priority order, the eNB  200 - 2  sets, as the priority order of the beamforming control information applied to the matching process, the past null-steering control information having the newest highest priority order of the history, to the highest priority order, sets the latest null-steering control information to the second highest priority order, and deletes the past null-steering control information having a second highest order or lower included in the history. Thereafter, the operation is performed according to a similar rule. 
     Alternatively, the operation in which the history is deleted in the eNB  200 - 2  may be changed to an operation as follows: In the step B, when the latest null-steering control information is the null-steering control information having the highest priority order, as the priority order of the beamforming control information applied to the matching process, the eNB  200 - 2  sets the latest null-steering control information to the highest priority order and moves down by one the priority order of the past null-steering control information included in the history. In this case, the history is not deleted, and thus, it is possible to effectively utilize the history. 
     Further, in the step B, when the latest null-steering control information is the null-steering control information having the second highest priority order, as the priority order of the beamforming control information applied to the matching process, the eNB  200 - 2  sets the past null-steering control information having the newest highest priority order of the history to the highest priority order, sets the latest null-steering control information to the second highest priority order, and moves down by one the priority order of the second highest priority order or lower of the past null-steering control information included in the history. 
     (2.3) Operation Pattern  3   
       FIG. 10  is a diagram for describing an operation pattern  3  according to the embodiment. The operation pattern  3  resembles the operation pattern  2  in operation, and thus, description proceeds with a focus on a difference from the operation pattern  2 . 
     As shown in  FIG. 10 , in the operation pattern  3  according to the embodiment, the method of selecting the null-steering control information fed back in the UE  100 - 1  is similar to that of the operation pattern  2 . 
     However, in the operation pattern  3  according to the embodiment, additional information added to the null-steering control information to be fed back is that is information indicating whether or not the history managed by the eNB  200 - 2  should be deleted. When feeding back the null-steering control information having the highest priority order, the UE  100 - 1  adds, as additional information, information (new) indicating that the history should be deleted, to the null-steering control information. Further, when feeding back the second null-steering control information or lower, the UE  100 - 1  adds, as the additional information, information (hold) indicating that the history should not be deleted, to the null-steering control information. 
     In the operation pattern  3  according to the embodiment, in the step B, when it is indicated that the history should be deleted, as the priority order of the beamforming control information applied to the matching process, the eNB  200 - 2  sets the latest null-steering control information to the highest priority order, and deletes the history. 
     Further, in the operation pattern  3  according to the embodiment, in the step B, when it is indicated that the history should not be deleted, as the priority order of the beamforming control information applied to the matching process, the eNB  200 - 2  sets the past null-steering control information having the newest highest priority order of the history, to the highest priority order, and sets the past null-steering control information fed back subsequent to the past null-steering control information having the newest highest priority order, to the second highest priority order. 
     Thus, in the operation pattern  3  according to the embodiment, the additional information has only two types that are new and hold, and thus, as compared to the operation pattern  2 , it is possible to reduce an information amount of the additional information. 
     [Modification] 
     In the above-described embodiment, an example is described where the present invention is applied to the CB-CoMP which is a mode of the downlink multi-antenna transmission; however, the present invention may be applied to MU (Multi-User)-MIMO (Multiple-Input And Multiple-Output) which is another mode of the downlink multi-antenna transmission. In a modification of the embodiment, a case will be described where the present invention is applied to the MU-MIMO. 
       FIG. 11  and  FIG. 12  are diagrams for describing the MU-MIMO. As shown in  FIG. 11 , each of the UE  100 - 1  and the UE  100 - 2  is in a state of establishing connection with a cell of the eNB  200  (connected state). That is, each of the UE  100 - 1  and the UE  100 - 2  uses, as a serving cell, the cell of the eNB  200  to perform communication. In  FIG. 11 , only two UEs  100  are shown which establish the connection with the cell of the eNB  200 ; however, in a real environment, three or more UEs  100  establish the connection with the cell of the eNB  200 . 
     A communication procedure of the MU-MIMO when the MU-MIMO is applied to the UE  100 - 1  will be described, below. Here, the UE  100 - 1  corresponds to a terminal subject to null steering and the UE  100 - 2  corresponds to a terminal subject to beamforming. It is noted that a description duplicated with the above-described embodiment will be omitted. 
     Each of the UE  100 - 1  and the UE  100 - 2  feeds beamforming control information for directing a beam to the UE  100 - 1  and the UE  100 - 2 , back to the serving cell, on the basis of a reference signal received from the serving cell, for example. The beamforming control information includes the PMI and the RI. 
     The UE  100 - 1  further feeds null-steering control information for directing a null to the UE  100 - 1 , back to the serving cell, on the basis of a reference signal received from the serving cell, for example. The null-steering control information includes a BCI (Best Companion PMI) and an RI. 
     The eNB  200  receives the beamforming control information (PMI, RI) fed back from each of the plurality of UEs  100 - 2  connected with a cell of the eNB  200  and the null-steering control information (BCI, RI) fed back from the UE  100 - 1  connected with a cell of the eNB  200 . Then, the eNB  200  selects the UE  100 - 2  that feeds back the beamforming control information that matches the null-steering control information, as a pair UE (pair UE) that forms a pair with the UE  100 - 1 . 
     When selecting the pair UE (UE  100 - 2 ), the eNB  200  assigns the same radio resource as the radio resource assigned to the UE  100 - 1 , to the pair UE. Then, the eNB  200  applies the beamforming control information (PMI, RI) fed back from the pair UE, and performs a transmission to the pair UE. As a result, as shown in  FIG. 12 , the eNB  200  is capable of performing a transmission to the pair UE by directing a beam to the pair UE while directing a null to the UE  100 - 1 . 
     In the present modification, in the communication control method according to the above-described embodiment, when the eNB  200 - 1  and the eNB  200 - 2  are regarded as one eNB  200 , it is possible to appropriately select the pair UE to forma a pair with the UE  100 - 1 , even in the MU-MIMO. 
     Other Embodiments 
     In the above-described embodiment, the null-steering control information transmitted by the UE  100 - 1  is indirectly fed back to the eNB  200 - 2  via the eNB  200 - 1 ; however, the null-steering control information may be directly fed back to the eNB  200 - 2  without passing through the eNB  200 - 1 . 
     In the above-described embodiment and modification thereof, the BCI is described as an example of the null-steering control information; however, a WCI (Worst Companion PMI) may be used instead of the BCI. The WCI is an indicator indicating the precoder matrix in which an interference level from an interference source is high. The eNB  200  receives the beamforming control information (PMI, RI) fed back from each of the plurality of UEs  100 - 2  and the null-steering control information (WCI, RI) fed back from the UE  100 - 1 . Then, the eNB  200  selects the UE  100 - 2  that feeds back the beamforming control information that matches the null-steering control information, as a pair UE (pair terminal) that forms a pair with the UE  100 - 1 . In this case, the “beamforming control information that matches the null-steering control information” is beamforming control information that includes the PMI that does not match the WCI included in the null-steering control information, or that includes the RI that matches the RI included in the null steering control information. 
     In the above-described embodiments, as one example of cellular communication system, the LTE system is described; however, the present invention is not limited to the LTE system, and the present invention may be applied to systems other than the LTE system. 
     It is noted that the entire content of Japanese Patent Application No. 2013-199876 (filed on Sep. 26, 2013) is incorporated in the present application by reference. 
     INDUSTRIAL APPLICABILITY 
     Thus, according the present invention, it is possible to provide a communication control method, a base station, and a user terminal, with which it is possible to effectively utilize downlink multi-antenna transmission.