Patent Publication Number: US-9854539-B2

Title: Radio communication device, radio communication system and beam control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-021491, filed on Feb. 8, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a radio communication device, a radio communication system, and a beam control method. 
     BACKGROUND 
     In a radio communication system using high frequencies as in millimeter wave communication, beamforming that concentrates power by narrowing a beam in the direction of a user terminal may be performed to compensate for a propagation loss. When beamforming is performed, beams are narrowed in specific directions in which user terminals are present, and therefore areas where high-power signal can be received may be spatially isolated from each other, thus providing an effect of suppressing interference between the user terminals present in a same space. In addition, frequency usage efficiency may be increased due to an improvement in a degree of spatial multiplexing. 
     Interference between user terminals present within a same cell may be reduced by thus performing beamforming. However, beam interference between adjacent cells (inter-cell interference) may occur. As a measure against inter-cell interference, a coordinated beamforming (CB) system is proposed which obtains information on a user terminal from a base station of an adjacent cell, and controls a beam so as to direct NULL (direction of the beam in which power is minimized) to the user terminal in the adjacent cell. 
     Incidentally, a technology is proposed which weakens transmission power for user terminals located in the vicinity of a base station, and for user terminals located in the vicinities of cell boundaries, controls frequency allocation such that frequency differs between the user terminals in adjacent beam areas. In addition, a satellite communication system is proposed which sets a plurality of regions (beam areas) on the ground in which regions radio terminals may communicate via a beam output by a satellite, and assigns different frequency bands to adjacent beam areas. 
     CITATION LIST 
     Patent Documents 
     [Patent Document 1] 
     Japanese Laid-open Patent Publication No. 2010-109745 
     [Patent Document 2] 
     Japanese Laid-open Patent Publication No. 2011-087009 
     SUMMARY 
     According to an aspect of the embodiments, a radio communication device including a memory that stores information on a strength of interference between a beam output by a first base station and a beam output by a second base station for each of a plurality of combinations, each of the plurality of combinations including at least one of a plurality of beams output by the first base station and at least one of a plurality of beams output by the second base station, emitting directions of each of the plurality of beams output by the first base station being different each other, emitting directions of each of the plurality of beams output by the second base station being different each other, and a processor coupled to the memory and the processor configured to identify one or more combinations having the interference strength higher than a threshold value among the plurality of combinations, and assign different radio resources to each of beams included in the identified one or more combinations. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a radio communication system according to a first embodiment; 
         FIG. 2  is a diagram illustrating an example of a radio communication system according to a second embodiment; 
         FIG. 3  is a diagram of assistance in explaining beamforming and inter-cell interference; 
         FIG. 4  is a diagram illustrating an example of hardware of a radio terminal according to the second embodiment; 
         FIG. 5  is a diagram illustrating an example of hardware of a base station according to the second embodiment; 
         FIG. 6  is a diagram illustrating an example of hardware of a control station according to the second embodiment; 
         FIG. 7  is a block diagram illustrating an example of functions of a radio terminal according to the second embodiment; 
         FIG. 8  is a block diagram illustrating an example of functions of a base station according to the second embodiment; 
         FIG. 9  is a diagram illustrating an example of cooperative control information according to the second embodiment; 
         FIG. 10  is a diagram illustrating an example of resource information (base station) according to the second embodiment; 
         FIG. 11  is a block diagram illustrating an example of functions of a control station according to the second embodiment; 
         FIG. 12  is a diagram illustrating an example of group information according to the second embodiment; 
         FIG. 13  is a diagram illustrating an example of resource information (control station) according to the second embodiment; 
         FIG. 14  is a sequence diagram illustrating an example of operation of a radio communication system according to the second embodiment; 
         FIG. 15  is a diagram of assistance in explaining identification of adjacent base stations and determination of non-interference resource sets, the identification and the determination being performed by a control station according to the second embodiment; 
         FIG. 16  is a first flowchart illustrating a flow of processing related to grouping of beams, the processing being performed by a control station according to the second embodiment; 
         FIG. 17  is a second flowchart illustrating the flow of processing related to the grouping of beams, the processing being performed by the control station according to the second embodiment; and 
         FIG. 18  is a flowchart illustrating a flow of processing related to assignment of resources, the processing being performed by a base station according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In a case where the CB system is applied, a plurality of base stations controlling beams may each use user information related to user terminals in adjacent cells. For example, each base station may calculate beamforming weights in consideration of channel information related to a user terminal in an adjacent cell in order to direct NULL to the user terminal. Therefore, the base stations may be coupled to each other by high-speed communication interfaces, and the user information of the adjacent cells may be transmitted and received between the base stations when beamforming is performed. 
     Inter-cell interference may be suppressed by applying the CB system. However, processing loads may not be insignificant which are involved in the transmission and reception of the user information, which is performed between the base stations each time beamforming is performed, and the calculation of the beamforming weights. When inter-cell interference may be suppressed by beamforming not using the user information of the adjacent cells, it may contribute to reduction in such processing loads. In addition, when the high-speed communication interfaces may be omitted, it may contribute to reduction in cost involved in system introduction and operation management. 
     According to one aspect, it is an object of the present disclosure to provide a radio communication device, a radio communication system, and a beam control method that may reduce user information obtained from base stations in adjacent cells in order to suppress inter-cell interference at a time of beamforming. 
     Embodiments of the present technology will be described in the following with reference to the accompanying drawings. Incidentally, repeated description of elements having essentially identical functions in the present specification and the drawings may be omitted by identifying the elements by the same reference symbols. 
     1. First Embodiment 
     A first embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating an example of a radio communication system according to the first embodiment. A radio communication system  1  illustrated in  FIG. 1  is an example of the radio communication system according to the first embodiment. 
     The first embodiment relates to a technology that realizes beamforming effective in suppressing inter-cell interference. The CB system adopts a method of calculating beamforming weights so as to direct NULL toward a user terminal in an adjacent cell, using channel information related to the user terminal, and suppressing inter-cell interference by beamforming. The first embodiment, on the other hand, provides a method of suppressing the inter-cell interference without considering the channel information related to the user terminal in the adjacent cell at a time of beamforming. 
     The above-described adjacent cell refers to another cell positionally adjacent to a certain cell. However, an adjacent cell referred to herein includes a micro cell covering a relatively small area and located in a macro cell covering a wide area. For example, in the following, relation between a macro cell and a micro cell located in the macro cell may be referred to as adjacency of the cells for the convenience of description. 
     As illustrated in  FIG. 1 , the radio communication system  1  includes a first base station  11 , a second base station  12 , and a radio communication device  20 . The first base station  11  and the second base station  12  are coupled to the radio communication device  20  via a communication line  5 . 
     Incidentally, while description will be made by taking, as an example, the two base stations adjacent to each other (the first base station  11  and the second base station  12 ) for the convenience of description, the number of base stations included in the radio communication system  1  may be three or more. In addition, description will be made by taking, as an example, a case where there are a first cell (Cell #1) and a second cell (Cell #2) illustrated in (A) in  FIG. 1 . In the present example, the first base station  11  (BS #1) forms Cell #1. The second base station  12  (BS #2) forms Cell #2. There are two radio terminals (UE #1 and UE #2) in Cell #1. 
     The first base station  11  performs beamforming. For example, the first base station  11  includes a plurality of antennas, and changes the direction of a beam by controlling the phase and power of radio waves output from the respective antennas. In the example of (A) in  FIG. 1 , the first base station  11  (BS #1) may switch a beam between beams Bm #11 and Bm #12 having different directions. Incidentally, while the number of beams to which switching may be performed is two for the convenience of description, the number of beams to which switching may be performed may be three or more. 
     The second base station  12  performs beamforming. For example, the second base station  12  includes a plurality of antennas, and changes the direction of a beam by controlling the phase and power of radio waves output from the respective antennas. In the example of (A) in  FIG. 1 , the second base station  12  (BS #2) may switch a beam between beams Bm #21 and Bm #22 having different directions. Incidentally, while the number of beams to which switching may be performed is two for the convenience of description, the number of beams to which switching may be performed may be three or more. 
     The radio communication device  20  includes a storage unit  21  and a control unit  22 . 
     The storage unit  21  is a volatile storage device such as a random access memory (RAM) or a nonvolatile storage device such as a hard disk drive (HDD) or a flash memory. The control unit  22  is a processor such as a central processing unit (CPU) or a digital signal processor (DSP). However, the control unit  22  may be an electronic circuit such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA). The control unit  22  may execute a program stored in the storage unit  21  or another memory. 
     The storage unit  21  stores information on the strengths of interference between beams output by the first base station  11  and beams output by the second base station  12 . 
     For example, as illustrated in (B) in  FIG. 1 , the information on the interference strengths described above includes information indicating the strengths of interference between the beams measured in each radio terminal. In the example of (B) in  FIG. 1 , the interference strength corresponding to the combination of Bm #11 and Bm #21 with respect to UE #1 is ten. This indicates that the interference strength based on reception power values of Bm #11 output by BS #1 and Bm #21 output by BS #2, which reception power values are measured by UE #1, is ten. 
     Even in the case of the combination of the same beams, the measured interference strength may differ when measured by radio terminals at different positions. In the example of (B) in  FIG. 1 , an interference strength based on reception power values of Bm #11 output by BS #1 and Bm #21 output by BS #2, which reception power values are measured by UE #2, is six. Incidentally, while the interference strengths with respect to UE #1 and UE #2 are indicated individually in the present example, the storage unit  21  may store an average value obtained by averaging values indicating the interference strengths with respect to the radio terminals in association with the combination of the beams. 
     The control unit  22  identifies combinations of beams whose interference strengths are higher than a threshold value Th. The threshold value Th may be set in advance and stored in the storage unit  21 , or may be set adaptively according to the number of identified combinations. For example, the threshold value Th may be set so as to minimize a difference between the number of identified combinations and the number of remaining combinations. 
     In the example of (B) in  FIG. 1 , the threshold value Th is set to five, and a combination of beams (Bm #11 and Bm #21) is identified whose average value obtained by averaging the values of the interference strengths with respect to the radio terminals is larger than the threshold value Th. It is thus possible to apply a method of identifying a combination of beams based on comparison between an average value and the threshold value Th, a method of identifying a combination of beams whose interference strength with respect to a given number of radio terminals (for example, one radio terminal) or more is higher than the threshold value Th, or the like. 
     The control unit  22  determines the assignment of radio resources such that different radio resources (frequency and time) are assigned to the beam of the first base station  11  and the beam of the second base station  12 , the beams being included in the identified combination. In the example of (B) in  FIG. 1 , Bm #11 and Bm #21 are identified. Thus, as in (C) in  FIG. 1 , when a radio resource Rs #1 is assigned to Bm #11, a radio resource Rs #2 different from Rs #1 is assigned to Bm #21. 
     The control unit  22  notifies the first base station  11  and the second base station  12  of information on the radio resources assigned to the beams. For example, in the example of (C) in  FIG. 1 , the first base station  11  (BS #1) is notified of the identifying information of the beam Bm #11 of BS #1 which beam is included in the combination of the beams (Bm #11 and Bm #21) and information on the radio resource Rs #1 assigned to Bm #11. Meanwhile, the second base station  12  (BS #2) is notified of the identifying information of Bm #21 and information on the radio resource Rs #2 assigned to Bm #21. 
     When the first base station  11  (BS #1) having already received the above-described notification uses Bm #11 in beamforming, the first base station  11  (BS #1) assigns Rs #1 to a radio terminal to which Bm #11 is directed. On the other hand, when the second base station  12  (BS #2) having already received the above-described notification uses Bm #21 in beamforming, the second base station  12  (BS #2) assigns Rs #2 to a radio terminal to which Bm #21 is directed. When such assignment control is performed, there is a small interference between the beams even in conditions where Bm #11 and Bm #21 are used simultaneously. It is therefore possible to suppress inter-cell interference. 
     As described above, the first embodiment identifies a combination of beams that produce a strong effect of inter-cell interference when using the same radio resource, and assigns radio resources different from each other to the beams included in the combination. 
     According to the foregoing, each base station may obtain an effect of suppressing inter-cell interference by appropriately using radio resources assigned to respective beams. For example, it may be possible to suppress inter-cell interference even when channel information related to radio terminals in adjacent cells or the like is not obtained from base stations of the adjacent cells. Therefore, the user information of the adjacent cells may not need to be considered when beamforming weights are calculated, and the user information may not need to be transmitted and received at high speed between the base stations. This may consequently contribute to reduction in processing loads, reduction in cost involved in the installation and operation of high-speed communication interfaces, and the like. 
     The first embodiment has been described above. 
     2. Second Embodiment 
     A second embodiment will next be described. The second embodiment relates to a technology that realizes beamforming effective in suppressing inter-cell interference, and suppresses inter-cell interference without consideration given to channel information related to user terminals in adjacent cells at a time of beamforming. 
     [2-1. System] 
     A radio communication system according to the second embodiment will first be described with reference to  FIG. 2 .  FIG. 2  is a diagram illustrating an example of the radio communication system according to the second embodiment. A radio communication system  50  illustrated in  FIG. 2  is an example of the radio communication system according to the second embodiment. 
     As illustrated in  FIG. 2 , the radio communication system  50  includes radio terminals  101  and  102 , base stations  201 ,  202 , and  203 , and a control station  300 . The base station  201  forms a cell  71 . The base station  202  forms a cell  72 . The base station  203  forms a cell  73 . In addition, the base stations  201 ,  202 , and  203  and the control station  300  are coupled to each other by a communication line. 
     In the following, the radio terminal  101  may be denoted as UE #1, and the radio terminal  102  may be denoted as UE #2. In addition, the base station  201  may be denoted as BS #1, the base station  202  may be denoted as BS #2, and the base station  203  may be denoted as BS #3. In addition, the cell  71  may be denoted as Cell #1, the cell  72  may be denoted as Cell #2, and the cell  73  may be denoted as Cell #3. Incidentally, while the control station  300  is represented in the shape of a computer different from a base station in the example of  FIG. 2 , it is also possible to provide the base stations with the functions of the control station  300 . 
     The base stations  201 ,  202 , and  203  each include a plurality of antennas, and change the direction of a beam by controlling the phase and power of radio waves output from the respective antennas. 
     When the radio terminal  101  is present in the cell  71 , for example, the base station  201  transmits a reference signal (RS) while selecting a plurality of beams, and receives, from the radio terminal  101 , information on a beam from which a maximum reception power is measured. The base station  201  then transmits a data signal to the radio terminal  101  using the beam indicated by the information received from the radio terminal  101 . 
     The base station  202  selects a beam based on beam information received from a radio terminal present in the cell  72 , and transmits a data signal to the target radio terminal using the selected beam. The base station  203  selects a beam based on beam information received from a radio terminal present in the cell  73 , and transmits a data signal to the target radio terminal using the selected beam. 
     When the base stations  201 ,  202 , and  203  each independently perform beamforming toward radio terminals within the cells in which the base stations  201 ,  202 , and  203  themselves are located, as described above, inter-cell interference occurs as illustrated in  FIG. 3 .  FIG. 3  is a diagram of assistance in explaining beamforming and inter-cell interference. 
     (A) in  FIG. 3  illustrates a state in which the base station  201  (BS #1) receives information on a beam (Bm #11) corresponding to a maximum reception power from the radio terminal  101  (UE #1) present within the cell  71 . (B) in  FIG. 3  illustrates a state in which the base station  202  (BS #2) located in the cell  72  adjacent to the cell  71  receives information on a beam (Bm #21) corresponding to a maximum reception power from the radio terminal  102  (UE #2) present within the cell  72 . 
     In a case where the two beams Bm #11 and Bm #21 strongly interfere with each other, desired signal quality may not be obtained even when beamforming is performed. Incidentally, this signal quality may be evaluated by a signal-to-interference plus noise power ratio (SINR), for example. The second embodiment accordingly provides a technology that suppresses inter-cell interference by assigning different radio resources (frequency and time) to beams that cause a strong interference between adjacent cells as in (C) in  FIG. 3 . Incidentally, the technology according to the second embodiment is applicable to both of an analog beamforming system and a digital beamforming system. 
     The radio communication system  50  has been described above. In the following, further description will be made of the radio terminals  101  and  102 , the base stations  201 ,  202 , and  203 , and the control station  300 . 
     [2-2. Hardware] 
     Description will first be made of hardware of the radio terminals  101  and  102 , the base stations  201 ,  202 , and  203 , and the control station  300 . 
     (Radio Terminal) 
     Functions of the radio terminal  101  may be implemented by using hardware illustrated in  FIG. 4 , for example. Incidentally, hardware of the radio terminal  102  is the same as that of the radio terminal  101 .  FIG. 4  is a diagram illustrating an example of the hardware of the radio terminal according to the second embodiment. 
     As illustrated in  FIG. 4 , the radio terminal  101  includes a processor  701 , a memory  702 , a baseband processing circuit  703 , a radio processing circuit  704 , and an antenna  705 . 
     The processor  701  is a processing circuit such as a CPU, a DSP, an ASIC or an FPGA. The memory  702  is a volatile storage device such as a RAM or a nonvolatile storage device such as an HDD or a flash memory. The baseband processing circuit  703  subjects baseband signals to processing such as error correction coding and decoding. 
     The radio processing circuit  704  generates a radio frequency (RF) signal by modulating a carrier wave based on a baseband signal output from the baseband processing circuit  703 , and transmits the RF signal from the antenna  705 . In addition, the radio processing circuit  704  demodulates a baseband signal from an RF signal received from the antenna  705 , and inputs the baseband signal to the baseband processing circuit  703 . Incidentally, suppose that the analog to digital (AD) and digital to analog (DA) conversions of the baseband signals are performed by the radio processing circuit  704 . While the example of  FIG. 4  is provided with one antenna  705 , the number of antennas may be two or more. 
     (Base Station) 
     Functions of the base station  201  may be implemented by using hardware illustrated in  FIG. 5 . Incidentally, hardware of the base stations  202  and  203  is the same as that of the base station  201 .  FIG. 5  is a diagram illustrating an example of the hardware of the base station according to the second embodiment. 
     As illustrated in  FIG. 5 , the base station  201  includes a processor  801 , a memory  802 , a baseband processing circuit  803 , a radio processing circuit  804 , an antenna group  805 , and an interface circuit  806 . 
     The processor  801  is a processing circuit such as a CPU, a DSP, an ASIC or an FPGA. The memory  802  is a volatile storage device such as a RAM or a nonvolatile storage device such as an HDD or a flash memory. The baseband processing circuit  803  subjects baseband signals to processing such as error correction coding and decoding. 
     The radio processing circuit  804  generates an RF signal by modulating a carrier wave based on a baseband signal output from the baseband processing circuit  803 , and transmits the RF signal from the antenna group  805 . In addition, the radio processing circuit  804  demodulates a baseband signal from an RF signal received from the antenna group  805 , and inputs the baseband signal to the baseband processing circuit  803 . Incidentally, suppose that the AD and DA conversions of the baseband signals are performed by the radio processing circuit  804 . The antenna group  805  is a set of a plurality of antennas. 
     Incidentally, in the case of the digital beamforming system, the adjustment of a phase and an amplitude is made for each antenna in the domain of digital baseband signals (in a stage preceding the DA conversion), and therefore beam switching processing is performed mainly by the baseband processing circuit  803 . In the case of the analog beamforming system, on the other hand, the adjustment of the phase and the amplitude is made in the analog domain using an analog phase shifter or the like, and therefore the beam switching processing is performed mainly by the radio processing circuit  804 . The technology according to the second embodiment is applicable to both of the systems. 
     The interface circuit  806  is a communication interface for communicating with the base stations  202  and  203  and the control station  300 . The interface circuit  806  is, for example, coupled to a backbone network used for communication between the base stations. 
     (Control Station) 
     Functions of the control station  300  may be implemented by using hardware illustrated in  FIG. 6 . For example, the functions of the control station  300  may be implemented by controlling the hardware illustrated in  FIG. 6  using a computer program.  FIG. 6  is a diagram illustrating an example of the hardware of the control station according to the second embodiment. 
     As illustrated in  FIG. 6 , the hardware mainly includes a CPU  902 , a read only memory (ROM)  904 , a RAM  906 , a host bus  908 , and a bridge  910 . The hardware further includes an external bus  912 , an interface  914 , an input unit  916 , an output unit  918 , a storage unit  920 , a drive  922 , a coupling port  924 , and a communicating unit  926 . 
     The CPU  902 , for example, functions as an arithmetic processing device or a control device, and controls the whole or a part of operation of each constituent element based on various kinds of programs recorded in the ROM  904 , in the RAM  906 , in the storage unit  920 , or on a removable recording medium  928 . The ROM  904  is an example of a storage device storing a program read by the CPU  902 , data used for operation, and the like. The RAM  906 , for example, temporarily or permanently stores the program read by the CPU  902 , various kinds of parameters changing when the program is executed, and the like. 
     These elements are, for example, coupled to each other via the host bus  908  capable of high-speed data transmission. The host bus  908  is, for example, coupled to the external bus  912 , which has a relatively low data transmission speed, via the bridge  910 . Used as the input unit  916  are, for example, a mouse, a keyboard, a touch panel, a touch pad, a button, a switch, a lever, and the like. 
     Used as the output unit  918  is, for example, a display device such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display panel (PDP) or an electro-luminescence display (ELD). A printer or the like may also be used as the output unit  918 . 
     The storage unit  920  is a device configured to store various kinds of data. A magnetic storage device such as an HDD is used as the storage unit  920 , for example. A semiconductor storage device such as a solid state drive (SSD) or a RAM disk, an optical storage device, a magneto-optical storage device, or the like may also be used as the storage unit  920 . 
     The drive  922  is a device reading information recorded on the removable recording medium  928 , which is a recording medium capable of being attached and detached, or writing information to the removable recording medium  928 . Used as the removable recording medium  928  is, for example, a magnetic disk, an optical disk, a magneto-optical disk, a semiconductor memory, or the like. 
     The coupling port  924  is, for example, a port for coupling an external coupling apparatus  930 , the port being a universal serial bus (USB) port, an IEEE (Institute of Electrical and Electronics Engineers) 1394 port, a small computer system interface (SCSI), an RS-232C port, or the like. There is, for example, a printer, an auxiliary power supply, or the like as the external coupling apparatus  930 . 
     The communicating unit  926  is a communication device configured to couple to a network. Used as the communicating unit  926  is, for example, a communication circuit for a wired or wireless local area network (LAN), a communication circuit or a router for optical communication, a communication interface for coupling to the backbone network connecting the base stations to each other, or the like. 
     The hardware has been described above. 
     [2-3. Functions] 
     Description will next be made of functions of the radio terminals  101  and  102 , the base stations  201 ,  202 , and  203 , and the control station  300 . 
     (Radio Terminal) 
     Functions of the radio terminal  101  will first be described with reference to  FIG. 7 . Incidentally, functions of the radio terminal  102  are similar to those of the radio terminal  101 .  FIG. 7  is a block diagram illustrating an example of the functions of the radio terminal according to the second embodiment. 
     As illustrated in  FIG. 7 , the radio terminal  101  includes an RS receiving unit  111 , a power measuring unit  112 , and a power value transmitting unit  113 . 
     The RS receiving unit  111  receives a reference signal (RS). The power measuring unit  112  measures the reception power of the reference signal received by the RS receiving unit  111 . The power value transmitting unit  113  transmits the reception power measured by the power measuring unit  112  to the base station in the cell in which the radio terminal  101  itself is located (for example, the base station  201  in the cell  71  or the like). 
     In the following, a jth (j=1, 2) beam output by BS #i (i=1, 2, 3) will be denoted as Bm #ij. In addition, the reception power at UE #k of the reference signal transmitted by the beam Bm #ij will be denoted as rij(k) (k=1, 2). 
     (Base Station) 
     Functions of the base station  201  will next be described with reference to  FIG. 8 . Incidentally, functions of the base stations  202  and  203  are similar to those of the base station  201  (however, the indexes of the cells and the beams corresponding to the respective base stations are different).  FIG. 8  is a block diagram illustrating an example of the functions of the base station according to the second embodiment. 
     As illustrated in  FIG. 8 , the base station  201  includes a storage unit  211 , a BF control unit  212 , a correlation matrix generating unit  213 , and a data transmitting unit  214 . Functions of the storage unit  211  may be implemented by the memory  802 . Functions of the BF control unit  212 , the correlation matrix generating unit  213 , and the data transmitting unit  214  may be implemented mainly by the processor  801 . 
     The storage unit  211  stores cooperative control information  211   a  and resource information  211   b.    
     The cooperative control information  211   a  is, for example, received by the base station  201  from the control station  300  and stored in the storage unit  211  before a start of processing (processing of  FIG. 14 ) related to beam grouping to be described later. As illustrated in  FIG. 9 , the cooperative control information  211   a  is information indicating timing in which the base stations  201 ,  202 , and  203  each switch and output beams.  FIG. 9  is a diagram illustrating an example of the cooperative control information according to the second embodiment. In the example of  FIG. 9 , Bm #11 is set in a period Ts 1 , Bm #21 is set in a period Ts 2 , Bm #31 is set in a period Ts 3 , Bm #12 is set in a period Ts 4 , Bm #22 is set in a period Ts 5 , and Bm #32 is set in a period Ts 6 . Incidentally, the settings are an example, and may be changed. 
     As illustrated in  FIG. 10 , the resource information  211   b  is information associating a combination of beams strongly interfering with each other with resources assigned to the respective beams.  FIG. 10  is a diagram illustrating an example of the resource information (base station) according to the second embodiment. As will be described later, for a combination of beams strongly interfering with each other, assignment resources are set for the respective beams so as to suppress the interference between the beams. Of the setting contents, the resources assigned to the beams used by the base station itself (BS #1 in the example of  FIG. 10 ) are stored as the resource information  211   b  in the storage unit  211 . 
     The BF control unit  212  controls the direction of a beam by controlling the phase and power of radio waves output from the respective antennas. For example, in the case of the digital beamforming system, the BF control unit  212  changes the direction of the beam by controlling mainly the baseband processing circuit  803 . In the case of the analog beamforming system, on the other hand, the BF control unit  212  changes the direction of the beam by controlling mainly the radio processing circuit  804 . 
     The correlation matrix generating unit  213  includes a cooperative RS transmitting unit  213   a , a power value collecting unit  213   b , and a correlation value calculating unit  213   c.    
     The cooperative RS transmitting unit  213   a  selects a beam based on the cooperative control information  211   a , and transmits the reference signal by the selected beam. For example, based on the cooperative control information  211   a  illustrated in  FIG. 9 , the cooperative RS transmitting unit  213   a  transmits the reference signal by Bm #11 in the period Ts 1 , and transmits the reference signal by Bm #12 in the period Ts 4 . 
     The power value collecting unit  213   b  receives reception power values (rij(k)) of the reference signal, which is transmitted from the base stations  201 ,  202 , and  203  based on the cooperative control information  211   a , from a radio terminal (for example, UE #k; k=1, 2) within the cell in which the base station  201  is located. Incidentally, the reception power values received by the power value collecting unit  213   b  may be expressed in a vector form as in the following Equation (1). Incidentally, ri(k) is a vector.
 
[Expression 1]
 
 ri ( k )=[ ri 1( k ), ri 2( k )] T  ( i= 1, 2, 3;  T  is transposition)  (1)
 
     The correlation value calculating unit  213   c  calculates correlation values indicating the strengths of interference between beams, using the reception power values received by the power value collecting unit  213   b . The correlation values are, for example, given by respective elements of matrices R12 and R13 (correlation matrices) expressed by the following Equation (2). The correlation value calculating unit  213   c  transmits the calculated correlation matrices R12 and R13 to the control station  300 . 
     
       
         
           
             
               
                 
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     Incidentally, while the correlation matrices illustrated in the above-described Equation (2) are calculated in the base station  201 , correlation matrices R21 and R23 illustrated in the following Equation (3) are calculated in the base station  202 , and correlation matrices R31 and R32 illustrated in the following Equation (4) are calculated in the base station  203 . The correlation matrices R21, R23, R31, and R32 are also transmitted to the control station  300 . 
     
       
         
           
             
               
                 
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     The control station  300  determines radio resources to be assigned to the beams based on the transmitted correlation matrices R12, R13, R21, R23, R31, and R32. Then, information on the determined radio resources is notified to the base stations  201 ,  202 , and  203 . The correlation matrix generating unit  213  stores the information on the radio resources (resource information  211   b ) notified from the control station  300  in the storage unit  211 . The resource information  211   b  is used by the data transmitting unit  214 . 
     The data transmitting unit  214  includes a user selecting unit  214   a , an RS transmitting unit  214   b , an assignment beam determining unit  214   c , and a resource assigning unit  214   d.    
     The user selecting unit  214   a  selects a radio terminal as the transmission destination of a data signal by a scheduling method such as a round robin method. The RS transmitting unit  214   b  transmits the reference signal to the selected radio terminal while switching the beam. The assignment beam determining unit  214   c  obtains information indicating reception power values of the reference signal transmitted by the RS transmitting unit  214   b  from the radio terminal selected by the user selecting unit  214   a , and assigns a beam corresponding to a maximum reception power value to the radio terminal. 
     The resource assigning unit  214   d  checks whether the beam whose assignment is determined by the assignment beam determining unit  214   c  is included in the resource information  211   b . When the beam is included in the resource information  211   b , the resource assigning unit  214   d  assigns a radio resource assigned to the beam to the radio terminal. 
     When the beam whose assignment is determined by the assignment beam determining unit  214   c  is not included in the resource information  211   b , on the other hand, the resource assigning unit  214   d  assigns an arbitrary usable radio resource to the radio terminal. The data transmitting unit  214  transmits a data signal to the selected radio terminal by the beam assigned by the assignment beam determining unit  214   c  using the radio resource assigned by the resource assigning unit  214   d.    
     (Control Station) 
     Functions of the control station  300  will next be described with reference to  FIG. 11 .  FIG. 11  is a block diagram illustrating an example of the functions of the control station according to the second embodiment. 
     As illustrated in  FIG. 11 , the control station  300  includes a storage unit  301 , a correlation matrix obtaining unit  302 , a grouping processing unit  303 , an assignment resource determining unit  304 , and a control information providing unit  305 . Functions of the storage unit  301  may be implemented by using the RAM  906 , the storage unit  920 , or the like. Functions of the correlation matrix obtaining unit  302 , the grouping processing unit  303 , the assignment resource determining unit  304 , and the control information providing unit  305  may be implemented by using mainly the CPU  902  or the like. 
     The storage unit  301  stores correlation matrix information  301   a , group information  301   b , and resource information  301   c.    
     The correlation matrix information  301   a  is information representing the correlation matrices R12, R13, R21, R23, R31, and R32 received from the base stations  201 ,  202 , and  203 . As illustrated in  FIG. 12 , the group information  301   b  is information indicating a result of grouping combinations of beams according to interference strength.  FIG. 12  is a diagram illustrating an example of the group information according to the second embodiment. 
     In the example of  FIG. 12 , a combination of Bm #11 and Bm #22 and a combination of Bm #12 and Bm #21 belong to a group G #1. A combination of Bm #11 and Bm #31 belongs to a group G #2. A combination of Bm #22 and Bm #31 belongs to a group G #3. The beam Bm #32 not belonging to the groups G #1 to G #3 belongs to a group G #4. A method of the grouping will be described later. 
     As illustrated in  FIG. 13 , the resource information  301   c  is information on radio resources assigned to the respective beams.  FIG. 13  is a diagram illustrating an example of the resource information (control station) according to the second embodiment. In the example of  FIG. 13 , Rs #1 is associated with the beams Bm #11 and Bm #12 of BS #1, Rs #2 is associated with the beams Bm #21 and Bm #22 of BS #2, and Rs #3 is associated with the beam Bm #31 of BS #3. As for Bm #32, a setting is made such that any of the radio resources Rs #1 to Rs #3 may be assigned to Bm #32. A method of setting the assignment resources will be described later. 
     The correlation matrix obtaining unit  302  obtains correlation matrices from the base stations  201 ,  202 , and  203 . In addition, the correlation matrix obtaining unit  302  averages correlation matrices whose index combinations are the same (for example, R12 and R21) among the obtained correlation matrices, and generates an averaged correlation matrix. In addition, the grouping processing unit  303  generates the group information  301   b  by grouping the beams based on averaged correlation matrices. Incidentally, an averaged correlation matrix Qmn is given by the following Equation (5), for example. 
     
       
         
           
             
               
                 
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                     5 
                   
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                   Qmn 
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                       Rmn 
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                         Rnm 
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     As an example, the grouping processing unit  303  refers to each element of a correlation matrix Q12, and identifies an element whose value is larger than a preset threshold value. The grouping processing unit  303  then includes a combination of beams corresponding to the identified element in a group. In a case where an element in a pth row and a qth column of the correlation matrix Qmn is denoted as Qmn(p, q), the correlation matrix Q12 is the following Equation (6), and the threshold value is six, the grouping processing unit  303  identifies Q12(1, 1) and Q12(2, 2). 
     
       
         
           
             
               
                 
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     From the definitions of Equation (1), Equation (2), and Equation (5) described above, Q12(1, 1) represents a correlation between Bm #11 and Bm #21, and Q12(2, 2) represents a correlation between Bm #12 and Bm #22. In addition, Q12 corresponds to a combination of the beams of BS #1 and the beams of BS #2. The grouping processing unit  303  accordingly generates the group G #1 corresponding to a combination of BS #1 and BS #2. The grouping processing unit  303  then registers the combination of Bm #11 and Bm #21 and the combination of Bm #12 and Bm #22 in the group G #1. 
     For Q13 and Q23, the grouping processing unit  303  similarly identifies an element whose value is larger than the threshold value, and performs the generation of a group and registration in the group. For example, for Q13, the grouping processing unit  303  generates the group G #2 corresponding to a combination of BS #1 and BS #3, and registers a combination of Bm #11 and Bm #31 in the group G #2. In addition, for Q23, the grouping processing unit  303  generates the group G #3 corresponding to a combination of BS #2 and BS #3, and registers a combination of Bm #22 and Bm #31 in the group G #3. 
     In addition, the grouping processing unit  303  generates the group G #4 for registering a beam not included in any of the groups G #1, G #2, and G #3. Then, the grouping processing unit  303  identifies the beam Bm #32 not included in any of the groups G #1, G #2, and G #3, and registers the beam Bm #32 in the group G #4. In the present example, the group information  301   b  illustrated in  FIG. 12  is obtained. 
     Incidentally, while the method of using the averaged correlation matrices Qmn has been illustrated, the group information  301   b  may be generated using correlation matrices Rmn (m&lt;n) having m smaller than n among the correlation matrices Rmn in place of Qmn (modification). In addition, when a certain beam is included in a plurality of groups, the beam may be included in a group in which the beam has a highest correlation (modification). 
     The assignment resource determining unit  304  determines assignment resources to be assigned to the respective beams based on the group information  301   b . At this time, the assignment resource determining unit  304  determines the assignment resources such that different radio resources are assigned to beams of different base stations among the beams belonging to the same group. 
     For example, in the case of the group information  301   b  illustrated in  FIG. 12 , the group G #1 includes the beams Bm #11 and Bm #12 of BS #1 and the beams Bm #21 and Bm #22 of BS #2. In this case, the assignment resource determining unit  304  determines the assignment resources such that the resource assigned to the beams Bm #11 and Bm #12 of BS #1 is different from the resource assigned to the beams Bm #21 and Bm #22 of BS #2. 
     For example, the assignment resource determining unit  304  assigns Rs #1 to Bm #11 and Bm #12, and assigns Rs #2 to Bm #21 and Bm #22. Also for the groups G #2 and G #3, the assignment resource determining unit  304  similarly determines assignment resources. At this time, when there is a beam whose assignment resource is previously determined, the assignment resource determining unit  304  sets the resource assigned to the beam as a reference, and determines a resource to be assigned to another beamforming a set with the beam in question. 
     In addition, the assignment resource determining unit  304  sets an arbitrary radio resource (for example, all of the assignment resources set to the beams in the groups G #1 to G #3) as a resource assigned to the beam included in the group G #4. In the present example, the resource information  301   c  illustrated in  FIG. 13  is obtained. 
     The control information providing unit  305  provides information indicating the relations between the beams and the assignment resources to the respective base stations  201 ,  202 , and  203  based on the resource information  301   c . For example, the control information providing unit  305  transmits information on the assignment resources related to the beams Bm #11 and Bm #12 of BS #1 (both of the assignment resources are Rs #1) to the base station  201  (BS #1). 
     In addition, the control information providing unit  305  transmits information on the assignment resources related to the beams Bm #21 and Bm #22 of BS #2 (both of the assignment resources are Rs #2) to the base station  202  (BS #2). In addition, the control information providing unit  305  transmits information on the assignment resource (Rs #3) related to the beam Bm #31 of BS #3 to the base station  203  (BS #3). 
     Incidentally, the control information providing unit  305  may transmit, or may not transmit, information on the assignment resource (Rs #1 to Rs #3) related to the beam (Bm #32), to which an arbitrary radio resource is assigned. In addition, the information on the assignment resources may be transmitted to the respective base stations  201 ,  202 , and  203  according to the beams, as described above, or the resource information  301   c  may be provided to all of the base stations  201 ,  202 , and  203  (modification). 
     Functions of the radio terminals  101  and  102 , the base stations  201 ,  202 , and  203 , and the control station  300  have been described above. 
     As described above, inter-cell interference may be suppressed by assigning radio resources different from each other to beams between adjacent cells causing a strong interference, and assigning the radio resources to radio terminals using the beams. For example, inter-cell interference may be suppressed by checking whether beams suitable for use by radio terminals as data transmission destinations are registered as the beams causing the strong interference, and using the designated assignment resources when the beams are registered. 
     Hence, inter-cell interference may be suppressed more easily as compared with a method such as the CB system that suppresses inter-cell interference using channel information obtained from the base stations of adjacent cells through high-speed communication interfaces. This may consequently contribute to a reduction in cost involved in the installation, operation, and the like of the high-speed communication interfaces and a reduction in a load involved in processing of calculating beamforming weights in consideration of the channel information of the adjacent cells. 
     [2-4. Flow of Processing] 
     A flow of processing in a radio communication system will next be described. 
     (Operation of Radio Communication System) 
     A reference will be made to  FIG. 14 .  FIG. 14  is a sequence diagram illustrating an example of operation of a radio communication system according to the second embodiment. The radio communication system described with reference to  FIG. 14  may be the radio communication system  50  illustrated in  FIG. 2 . 
     The example of  FIG. 14  includes processing in a first stage related to the grouping of beams and processing in a second stage related to the control of assignment of radio resources to radio terminals based on information (resource information  211   b ) on the radio resources assigned to the respective beams.  FIG. 14  illustrates a processing sequence in which the processing in the first stage (S 101  to S 116 ) and the processing in the second stage (S 117  to S 120 ) are performed successively for the convenience of description. However, it suffices to perform the processing in the first stage in preset timing (for example, once a day, once every week, or the like), and the processing in the first stage does not need to be performed each time a base station transmits a data signal. 
     (S 101 ) 
     The cooperative RS transmitting unit  213   a  of the base station  201  transmits the reference signal (RS) using the beam Bm #11 based on the cooperative control information  211   a  (see  FIG. 9 ). The base station  202  similarly transmits the reference signal (RS) using the beam Bm #21. The base station  203  similarly transmits the reference signal (RS) using the beam Bm #31. 
     (S 102 ) 
     The radio terminal  101  receives the reference signal transmitted from the base stations  201 ,  202 , and  203  by the RS receiving unit  111 , and measures reception power values (r11(1), r21(1), and r31(1)) by the power measuring unit  112 . 
     (S 103 ) 
     The radio terminal  101  transmits the reception power values (r11(1), r21(1), and r31(1)) measured by the power measuring unit  112  to the base station  201  by the power value transmitting unit  113 . Incidentally, suppose that the radio terminal  101  is present within the cell  71  formed by the base station  201 . 
     (S 104 ) 
     The cooperative RS transmitting unit  213   a  of the base station  201  transmits the reference signal (RS) using the beam Bm #12 based on the cooperative control information  211   a  (see  FIG. 9 ). The base station  202  similarly transmits the reference signal (RS) using the beam Bm #22. The base station  203  similarly transmits the reference signal (RS) using the beam Bm #32. 
     (S 105 ) 
     The radio terminal  101  receives the reference signal transmitted from the base stations  201 ,  202 , and  203  by the RS receiving unit  111 , and measures reception power values (r12(1), r22(1), and r32(1)) by the power measuring unit  112 . 
     (S 106 ) 
     The radio terminal  101  transmits the reception power values (r12(1), r22(1), and r32(1)) measured by the power measuring unit  112  to the base station  201  by the power value transmitting unit  113 . 
     (S 107 ) 
     The radio terminal  102  (UE #2) measures reception power values (r11(2), r21(2), r31(2), r12(2), r22(2), and r32(2)) as in S 102  and S 105 , and transmits a result of the measurement to the base station  201 . The base station  201  receives the reception power values (r11(2), r21(2), r31(2), r12(2), r22(2), and r32(2)) measured by the radio terminal  102 . Incidentally, suppose that the radio terminal  102  is present within the cell  71  formed by the base station  201 . 
     (S 108  and S 109 ) 
     The base station  201  calculates, by the correlation value calculating unit  213   c , correlation matrices R12 and R13 (see Equation (2) described above) having correlation values indicating beam correlations as elements based on the reception power values received in S 103 , S 106 , and S 107 . The base station  201  then transmits the correlation matrices R12 and R13 calculated by the correlation value calculating unit  213   c  to the control station  300 . 
     (S 110 ) 
     A radio terminal present in the cell  72  in which the base station  202  is located measures reception power values of the reference signal (RS) transmitted from the base stations  201 ,  202 , and  203  as in S 102  and S 105 , and transmits a result of the measurement to the base station  202 . The base station  202  calculates correlation matrices R21 and R23 (see Equation (3) described above) based on the received reception power values as in S 108 , and transmits the calculated correlation matrices R21 and R23 to the control station  300 . 
     (S 111 ) 
     A radio terminal present in the cell  73  in which the base station  203  is located measures reception power values of the reference signal (RS) transmitted from the base stations  201 ,  202 , and  203  as in S 102  and S 105 , and transmits a result of the measurement to the base station  203 . The base station  203  calculates correlation matrices R31 and R32 (see Equation (4) described above) based on the received reception power values as in S 108 , and transmits the calculated correlation matrices R31 and R32 to the control station  300 . 
     (S 112 ) 
     The control station  300  generates the group information  301   b  (see  FIG. 12 ) by grouping the beams based on the correlation matrices R12, R13, R21, R23, R31, and R32 obtained from the base stations  201 ,  202 , and  203  by the grouping processing unit  303 . 
     As an example, the grouping processing unit  303  calculates the correlation matrices Q12, Q13, and Q23 (Qmn is an average of Rmn and Rnm) based on Equation (5) described above. In addition, the grouping processing unit  303  generates the group G #1 corresponding to the correlation matrix Q12. The correlation matrix Q12 corresponds to a combination of the beams output by the base stations  201  and  202  (UE #1 and UE #2). For example, the group G #1 is a group corresponding to a combination of the base stations  201  and  202 . 
     In addition, the grouping processing unit  303  compares each element of the correlation matrix Q12 with a threshold value, and includes a combination of beams corresponding to an element whose value is larger than the threshold value in the group G #1. For example, an element Q12(1, 1) located in a first row and a first column of the correlation matrix Q12 corresponds to a combination of Bm #11 and Bm #21. When the value of the element Q12(1, 1) is larger than the threshold value, the grouping processing unit  303  includes Bm #11 and Bm #21 in the group G #1. 
     The grouping processing unit  303  similarly generates the group G #2 corresponding to the correlation matrix Q13, compares each element of the correlation matrix Q13 with the threshold value, and includes a combination of beams corresponding to an element whose value is larger than the threshold value in the group G #2. In addition, the grouping processing unit  303  generates the group G #3 corresponding to the correlation matrix Q23, compares each element of the correlation matrix Q23 with the threshold value, and includes a combination of beams corresponding to an element whose value is larger than the threshold value in the group G #3. 
     In addition, the grouping processing unit  303  generates the group G #4 for registering a beam not included in any of the groups G #1 to G #3. The grouping processing unit  303  then identifies a beam not included in any of the groups G #1 to G #3, and includes the identified beam in the group G #4. By such processing, the grouping processing unit  303  generates the group information  301   b  as illustrated in  FIG. 12 , for example. Incidentally, the above-described threshold value may be set in advance or may, for example, be set for each correlation matrix so as to minimize a difference between the number of elements included in the group and the number of remaining elements. 
     (S 113 ) 
     The control station  300  determines radio resources (assignment resources) to be assigned to the respective beams based on the group information  301   b  by the assignment resource determining unit  304 . At this time, the assignment resource determining unit  304  determines the assignment resources such that, of the beams included in the same group, beams of different base stations are assigned different radio resources. 
     For example, in the case of the group information  301   b  illustrated in  FIG. 12 , the group G #1 includes Bm #11, Bm #12, Bm #21, and Bm #22. In this case, the assignment resource determining unit  304  determines the assignment resources such that the assignment resource of Bm #11 and Bm #12 corresponding to the base station  201  (BS #1) is different from the assignment resource of Bm #21 and Bm #22 corresponding to the base station  202  (BS #2). 
     For example, the assignment resource determining unit  304  sets Rs #1 as the assignment resource of Bm #11 and Bm #12, and sets Rs #2 as the assignment resource of Bm #21 and Bm #22 (see  FIG. 13 ). Incidentally, it suffices for the two assignment resources to be different from each other at least on a frequency axis or on a time axis. 
     (S 114 , S 115 , and S 116 ) 
     The control station  300  transmits information (resource information) indicating the assignment resources of the respective beams to the base stations  201 ,  202 , and  203  by the control information providing unit  305 . At this time, the control information providing unit  305  may transmit the resource information  301   c  to the base stations  201 ,  202 , and  203 , or may transmit, to each base station, information indicating the assignment resources of the beams corresponding to the base station, the information being extracted from the resource information  301   c.    
     (S 117 ) 
     The base station  201  selects a radio terminal as the transmission destination of a data signal by the user selecting unit  214   a . For example, the user selecting unit  214   a  selects a radio terminal present within the cell  71  by using a round robin method or the like. Incidentally, suppose in this case that the radio terminal  101  is selected. 
     (S 118 ) 
     The base station  201  transmits the reference signal (RS) to the radio terminal  101  selected in S 117  by the RS transmitting unit  214   b , and obtains reception power values measured by the radio terminal  101 . Then, the base station  201  selects a beam whose reception power value is a maximum based on the obtained reception power values, and sets the beam as a beam assigned to the radio terminal  101 , by the assignment beam determining unit  214   c.    
     (S 119 ) 
     The base station  201  determines a radio resource to be assigned to the radio terminal  101  based on the resource information  301   c  (see  FIG. 10 ) received from the control station  300  by the resource assigning unit  214   d.    
     For example, the resource assigning unit  214   d  refers to the resource information  301   c , and determines whether the beam selected in S 118  is included in the resource information  301   c . When the selected beam is included in the resource information  301   c , the resource assigning unit  214   d  assigns the assignment resource indicated in the resource information  301   c  to the radio terminal  101 . When the selected beam is not included in the resource information  301   c , on the other hand, the resource assigning unit  214   d  assigns an arbitrary radio resource usable by the base station  201  to the radio terminal  101 . 
     (S 120 ) 
     The base station  201  transmits a data signal to the radio terminal  101  using the beam selected in S 118  and the radio resource assigned in S 119 . The series of processing illustrated in  FIG. 14  is ended when the processing of S 120  is completed. Inter-cell interference may be suppressed without the use of channel information of adjacent cells, by assigning respective different radio resources to beams causing a strong interference between the adjacent cells in advance and assigning the radio resources to respective radio terminals based on the assignment, as described above. 
     (Identification of Adjacent Base Stations and Determination of Non-Interference Resource Sets) 
     The description thus far has been made by taking as an example a system including three base stations adjacent to each other for the convenience of the description. In the following, an extension to a system including more base stations will be assumed, and referring to  FIG. 15 , description will be made of identification of adjacent base stations and determination of non-interference resource sets. 
     A non-interference resource set referred to herein is a set of assignment resources (non-interference resources) in a case where each base station is assigned a radio resource such that the radio resources assigned to adjacent cells are different from each other.  FIG. 15  is a diagram of assistance in explaining identification of adjacent base stations and determination of non-interference resource sets, the identification and the determination being performed by a control station according to the second embodiment. The control station described with reference to  FIG. 15  may be the control station illustrated in  FIG. 2 . 
     In  FIG. 15 , circles represent base stations, and numbers included within the circles indicate identification numbers identifying the base stations. For example, a circle including #1 therewithin represents a base station BS #1. In the example of (A) in  FIG. 15 , six base stations BS #i (i=1, 2, . . . , 6) are depicted. 
     Combinations of adjacent base stations may be determined more or less subjectively from the arrangement of the base stations. For example, a region obtained by applying Voronoi tessellation to the position of each base station is regarded as a cell formed by the base station, and a base station that the cell is adjacent to may be determined as an adjacent base station. In (B) in  FIG. 15 , BS #1 and BS #2 are in adjacent relation to each other, BS #1 and BS #4 are in adjacent relation to each other, BS #2 and BS #3 are in adjacent relation to each other, BS #2 and BS #4 are in adjacent relation to each other, BS #2 and BS #6 are in adjacent relation to each other, BS #3 and BS #4 are in adjacent relation to each other, BS #3 and BS #5 are in adjacent relation to each other, BS #3 and BS #6 are in adjacent relation to each other, BS #4 and BS #5 are in adjacent relation to each other, and BS #5 and BS #6 are in adjacent relation to each other. 
     As a method of assigning mutually different radio resources (non-interference resources) to base stations in adjacent relation to each other, a full search method may be applied which searches for non-interference resource sets while increasing the number of radio resources Rs #I (I=1, . . . , L) in order, for example. The full search method tries assignment in combinations of (Number of Radio Resources)^(Number of Base Stations−1) while increasing L, and checks whether a state is obtained in which adjacent base stations do not use a same radio resource. Incidentally, (Number of Base Stations−1) is set because a first assignment may be fixed. 
     However, the larger the number of base stations, the higher the processing load of the full search method. Thus, when there are a large number of base stations, it is preferable to determine non-interference resource sets by using a graph coloring algorithm (for example, the Welsh-Powell algorithm) or the like. The graph coloring algorithm is an algorithm that performs painting in a small number of different colors in a graph having a plurality of demarcated regions such as a map such that regions adjacent to each other have different colors. 
     When this algorithm is applied, the base stations may be painted in different colors as illustrated in (C) in  FIG. 15 , for example (different hatchings in (C) in  FIG. 15 ). For example, when different radio resources are assigned to base stations having different colors, the different radio resources (non-interference resources) may be assigned to the base stations in adjacent relation to each other, and thus non-interference resource sets are obtained. In the example of (C) in  FIG. 15 , Rs #1 is assigned as a non-interference resource to BS #1 and BS #3 not adjacent to each other, Rs #2 is assigned as a non-interference resource to BS #2 and BS #5 not adjacent to each other, and Rs #3 is assigned as a non-interference resource to BS #4 and BS #6 not adjacent to each other. 
     Inter-cell interference is suppressed when non-interference resources are used according to the above-described assignment. The following description will be made of an example in which non-interference resource sets are used for the assignment of radio resources. 
     (Grouping of Beams) 
     The description thus far has been made supposing that the number of beams used by each base station is two for the convenience of the description. In the following, referring to  FIG. 16  and  FIG. 17 , description will be made of processing related to the grouping of beams in a case where each base station may use N beams (N is a natural number). Incidentally, the number of beams may differ between the base stations. In this case, the loops of S 202  and S 203  to be described later are modified according to the number of beams of each base station. 
     Incidentally, in the case where non-interference resource sets are used, the control station  300  groups beams, and provides group information to the base stations  201 ,  202 , and  203 . In addition, the base stations  201 ,  202 , and  203  determine the assignment of radio resources to radio terminals based on the group information and the non-interference resource sets. 
       FIG. 16  is a first flowchart illustrating a flow of processing related to grouping of beams, the processing being performed by a control station according to the second embodiment.  FIG. 17  is a second flowchart illustrating the flow of processing related to the grouping of beams, the processing being performed by the control station according to the second embodiment. The control station described with reference to  FIG. 16  and  FIG. 17  may be the control station  300  illustrated in  FIG. 2 . Incidentally,  FIG. 16  and  FIG. 17  illustrate processing related to a correlation matrix Rij. The grouping processing unit  303  performs the processing of  FIG. 16  and  FIG. 17  for all combinations of i and j. 
     (S 201  and S 212 ) 
     The grouping processing unit  303  repeatedly performs processing from S 202  to S 211  while changing a parameter Th from Th1 to Th2 by a fixed step value. Th is a parameter representing a threshold value compared with elements of the correlation matrix Rij to determine beams to be included in a group. Th1 is a lower limit value of a possible range of the threshold value Th. Th2 is an upper limit value of the possible range of the threshold value Th. Th1, Th2, and the fixed step value are set in advance. When the iterative processing is ended, the processing proceeds to S 213 . 
     (S 202  and S 208 ) 
     The grouping processing unit  303  repeatedly performs processing from S 203  to S 207  while changing a parameter m from  1  to N. When the iterative processing is ended, the processing proceeds to S 209 . 
     (S 203  and S 207 ) 
     The grouping processing unit  303  repeatedly performs processing from S 204  to S 206  while changing a parameter n from  1  to N. When the iterative processing is ended, the processing proceeds to S 208 . 
     (S 204 ) 
     The grouping processing unit  303  determines whether or not an element Rij(m, n) located in an mth row and an nth column of the correlation matrix Rij is larger than the threshold value Th. When the element Rij(m, n) is larger than the threshold value Th, the processing proceeds to S 205 . When the element Rij(m, n) is not larger than the threshold value Th, on the other hand, the processing proceeds to S 206 . 
     (S 205 ) 
     The grouping processing unit  303  includes the beam Bm #im of BS #i in a beam set Lij. In addition, the grouping processing unit  303  includes the beam Bm #jn of BS #j in a beam set Lji. The sets Lij and Lji are sets of beams having a strong correlation (the element Rij(m, n) is larger than the threshold value Th). When the processing of S 205  is completed, the processing proceeds to S 207 . 
     (S 206 ) 
     The grouping processing unit  303  includes the beam Bm #im of BS #i in a beam set Sij. In addition, the grouping processing unit  303  includes the beam Bm #jn of BS #j in a beam set Sji. The sets Sij and Sji are sets of beams having a weak correlation (the element Rij(m, n) is not larger than the threshold value Th). When the processing of S 206  is completed, the processing proceeds to S 207 . 
     (S 209 ) 
     The grouping processing unit  303  calculates an evaluation value Eij based on the following Equation (7), where N[X] is the number of beams included in a set X, and Nb is the number of resources that may be used as non-interference resources (total number of resources in the present example). The evaluation value Eij is an evaluation value for evaluating a balance between the number of elements having values larger than the threshold value Th and the number of elements having values not larger than the threshold value Th. The closer the ratio between these two numbers of elements comes to a given value, the smaller the evaluation value Eij becomes. Incidentally, | . . . | denotes an absolute value.
 
[Expression 7]
 
 dN 1= N[Lij ]×( Nb− 1)− N[Sij],  
 
 dN 2= N[Lji ]×( Nb− 1)− N[Sji],  
 
 Eij=|dN 1|+| dN 2|  (7)
 
     (S 210 ) 
     The grouping processing unit  303  determines whether the threshold value Th is equal to Th1 and whether the evaluation value Eij is smaller than a parameter Em. When the threshold value Th is equal to Th1 or the evaluation value Eij is smaller than the parameter Em, the processing proceeds to S 211 . When the threshold value Th is not equal to Th1 nor is the evaluation value Eij smaller than the parameter Em, on the other hand, the processing proceeds to S 212 . 
     (S 211 ) 
     The grouping processing unit  303  sets the evaluation value Eij as the parameter Em. In addition, the grouping processing unit  303  sets the threshold value Th as a candidate threshold value Th0. In addition, the grouping processing unit  303  sets a set {Lij, Lji} as a candidate group GO. When the processing of S 211  is completed, the processing proceeds to S 212 . 
     (S 213 ) 
     The grouping processing unit  303  sets the candidate group GO as a group Gij corresponding to the correlation matrix Rij. Incidentally, the grouping processing unit  303  may store the candidate threshold value Th0 in the storage unit  301 , and use the candidate threshold value Th0 when performing the grouping processing next. 
     When groups Gij are obtained for all of correlation matrices Rij, the grouping processing unit  303  transmits information on the groups Gij to each base station. At this time, the grouping processing unit  303  transmits information on ∪ l Gil (where I≠i) to BS #i (i=1, 2, . . . ). ∪ denotes a sum of sets. When the processing of S 213  is completed, the series of processing illustrated in  FIG. 16  and  FIG. 17  is ended. 
     Incidentally, while the beams are grouped using the correlation matrices Rij in this case, the beams may be grouped using the correlation matrices (averages) Qij illustrated in Equation (5) described above. 
     (Assignment of Resources to Radio Terminals) 
     The following description will be made of processing of assigning resources to radio terminals in the case where non-interference resource sets are used.  FIG. 18  is a flowchart illustrating a flow of processing related to assignment of resources, the processing being performed by a base station according to the second embodiment. The base station described with reference to  FIG. 18  may be the base station  201 ,  202  or  203  illustrated in  FIG. 2 . Incidentally, the processing at BS #i will be described as an example. In addition, suppose that BS #i has already received the information on ∪ l Gil from the control station  300 . 
     (S 301 ) 
     The user selecting unit  214   a  sets a user index k to mod (k0+1, Nu). k0 denotes the user index of a radio terminal last selected by BS #i in a previous time slot. Incidentally, Nu is the number of radio terminals (number of users) present within the cell of BS #i. 
     (S 302 ) 
     The user selecting unit  214   a  selects a radio terminal (UE #k) having the user index selected in S 301 . 
     (S 303 ) The assignment beam determining unit  214   c  identifies a beam (Bm #iq) for UE #k. For example, the assignment beam determining unit  214   c  identifies a beam whose reception power value is a maximum at UE #k. 
     (S 304 ) 
     The resource assigning unit  214   d  determines whether or not the beam Bm #iq selected in S 303  is included in ∪ l Gil (I≠i). When the beam Bm #iq is included in ∪ l Gil, the processing proceeds to S 305 . When the beam Bm #iq is not included in ∪ l Gil, on the other hand, the processing proceeds to S 306 . 
     (S 305 ) 
     The resource assigning unit  214   d  selects a non-interference resource set for UE #i. When the processing of S 305  is completed, the processing proceeds to S 307 . 
     (S 306 ) The resource assigning unit  214   d  selects a usable radio resource other than the non-interference resource set for UE #i. 
     (S 307 ) 
     The resource assigning unit  214   d  determines whether or not the radio resource selected in S 305  or S 306  (selected resource) is unused (not assigned to another radio terminal). When the selected resource is unused, the processing proceeds to S 308 . When the selected resource is not unused, on the other hand, the processing proceeds to S 309 . 
     (S 308 ) 
     The resource assigning unit  214   d  assigns the selected resource to UE #k. 
     (S 309 ) The resource assigning unit  214   d  determines whether or not all of assignable radio resources have already been assigned. When all of the radio resources have already been assigned, the series of processing illustrated in  FIG. 18  is ended. When there is an unassigned radio resource, on the other hand, the processing proceeds to S 310 . 
     (S 310 ) 
     The resource assigning unit  214   d  resets the user index k to mod (k+1, Nu). Incidentally, mod denotes modulo arithmetic. In addition, Nu is the number of radio terminals (number of users) present within the cell of BS #i. When the processing of S 310  is completed, the processing proceeds to S 302 . 
     As described above, based on a group of beams causing a strong interference between cells, non-interference resources are assigned to the beams included in the group. Inter-cell interference may be thereby suppressed. In addition, each base station refers to beam group information already obtained from the control station  300 , and determines the assignment of the non-interference resources. Therefore inter-cell interference may be suppressed without channel information or the like being obtained from adjacent cells at a time of beamforming. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.