Patent Publication Number: US-2015087324-A1

Title: Base station, wireless communication system, and wireless communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Japanese Patent Application No. 2013-194816, filed on Sep. 20, 2013, which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless communication system. 
     2. Description of the Related Art 
     As a result of the widespread use of smartphones, tablet terminals and the like, there is a concern for an explosive increase in wireless traffic. In order to accommodate such increasing wireless traffic, the capacity for wireless traffic (wireless communication capacity) needs to be increased. As a technology for increasing the wireless communication capacity, a small cell configuration is gaining attention in which a service area is covered by a number of low transmission power base stations with narrow communication areas. In the long term evolution (LTE) standard, which is a next-generation wireless communication standard, the base station may be referred to as an E-UTRAN NodeB (eNB), and a terminal as user equipment (UE). 
     The small cell is referred to as a microcell, a picocell, or a femtocell, for example. The base station covering the small cell is referred to as a micro base station (micro eNB), a pico base station (pico eNB), or a femto base station (femto eNB), for example. The femto base station may also be referred to as a Home eNB (HeNB). Meanwhile, a base station with high transmission power and a wide communication area is referred to as a macro base station (macro eNB), and the communication area of the macro base station is referred to as a macrocell. 
     Generally, the wireless communication capacity can be increased by making the cell smaller and providing a number of such small cells. When the cell is made smaller, the distance between terminals and the base station is decreased. As a result, attenuation of radio waves may be reduced, resulting in improved communication quality. 
     SUMMARY OF THE INVENTION 
     On the other hand, when a number of small cells are disposed at high density, communication quality may be significantly decreased by radio wave interference between different cells at the boundary of the communication areas of the cells (referred to as a “cell boundary”). Particularly, in the small cell, because the area of one cell is narrow compared with the macrocell, it is believed difficult to adopt, prior to the installation of the base station, a cell design such that an area in which terminals are less likely to be distributed is located at the cell boundary. Thus, it can be expected that the distribution of the terminals in the cell will be greatly varied depending on the time. For example, in a certain cell at a certain time, a number of terminals may be distributed near the cell boundary. As a result, communication quality may be lowered by inter-cell interference. Further, handovers for switching the cells to which the terminals connect will be frequently generated, increasing the process delay due to transmission and reception of control signals, synchronization with a new cell and the like, and lowering communication efficiency. 
     As a technology for decreasing inter-cell interference, fractional frequency reuse (FFR) is known. According to the FFR technology, the transmission power of the base station is varied on a frequency by frequency basis and controlled such that frequencies with high transmission power are not overlapped between base stations, thus decreasing interference. FFR is discussed in JP-2012-500524-W, for example. 
     As another technology for decreasing inter-cell interference, coordinated multi point operation (CoMP) is known, which is a technology for coordinated transmission and reception between base stations. 
     According to FFR or CoMP, the inter-cell interference can be decreased. However, handovers do occur due to movements between cells. Thus, the technologies cannot solve the problem of the decrease in communication efficiency due to the occurrence of frequent handovers. Further, in order to perform CoMP, the terminals need to be adapted for the signal transmission and reception with a plurality of cells, which may increase the complexity of the terminals for supporting this function. Thus, whether CoMP can be applied depends on the support function of the terminals, and CoMP may not be applicable to all of the terminals. 
     As a technology for achieving a decrease in handovers in a small cell, dual connectivity is being considered. Dual connectivity is contemplated for application in a network configuration in which a number of small cells are disposed in a macrocell in an overlapping manner. Such network configuration may be referred to as a heterogeneous network (HetNet). The macrocell and the small cells use different frequency carriers. In dual connectivity, the macrocell ensures coverage, while the small cells are responsible for increasing wireless capacity. The terminals perform communications using both the macrocell and the small cells. When a terminal moves into a different small cell, the small cells are switched while connection with the macrocell is maintained. Thus, even when the small cell is modified, handover can be decreased because the connection with the macrocell is maintained. 
     However, in the case of dual connectivity, the terminals also need to support the function for communication with a plurality of cells, and dual connectivity may not be applicable to all of the terminals, as in the case of CoMP. There is also the possibility that dual connectivity cannot be applied due to the absence of a macrocell coverage at the location of a small cell. 
     The present invention was made in view of the above, and is directed to enabling an improvement in communication quality and a decrease in handover in a wireless communication system. Particularly, the present invention is directed to a wireless communication system including a number of small cells, where communication quality is increased by a decrease in inter-cell interference and a decrease in handover even when a number of terminals are distributed near a cell boundary, or when a terminal that does not support CoMP or dual connectivity is present. 
     The outline of a representative aspect of the present invention disclosed herein is as follows. 
     A base station includes a plurality of radio units that communicate with a terminal; and a control device connected to the plurality of radio units. When there are a large number of terminals positioned at a boundary of communication areas of a first radio unit and a second radio unit among the plurality of radio units, the control device makes a first cell ID of the first radio unit and a second cell ID of the second radio unit being identical. 
     According to the present invention, communication quality can be increased and handover can be decreased in a wireless communication system. Particularly, in a wireless communication system including a number of small cells, when a number of terminals are distributed near a cell boundary, or when there is a terminal that does not support CoMP or dual connectivity, an increase in communication quality and a decrease in handover can be achieved by a decrease in inter-cell interference. 
     Other objects, configurations and effects of the invention will become apparent from the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a system configuration of the present invention; 
         FIG. 2  is a conceptual diagram of a first embodiment of the present invention; 
         FIG. 3  illustrates an example of a CSI-RS setting method; 
         FIG. 4  illustrates an example of a device configuration according to the first embodiment of the present invention; 
         FIG. 5  illustrates an example of a downlink configuration of a switch; 
         FIG. 6  illustrates an example of an uplink configuration of the switch; 
         FIG. 7  illustrates an example of an operation sequence for allocating an identical cell ID to a plurality of RUs in the first embodiment of the present invention; 
         FIG. 8  illustrates the correspondence between RU, cell ID, and L2/L3 processor; 
         FIGS. 9A and 9B  illustrate examples of the number of terminals positioned at the center and boundary of the communication area of RU; 
         FIG. 10  illustrates an example of changes in communication area when cell ID is modified; 
         FIG. 11  illustrates an example of an operation sequence for cell ID allocation; 
         FIG. 12  illustrates an example of an operation sequence in a case where an identical cell ID is allocated to a plurality of RUs in the first embodiment of the present invention; 
         FIG. 13  is a first conceptual diagram of a second embodiment of the present invention; 
         FIG. 14  is a second conceptual diagram of the second embodiment of the present invention; 
         FIG. 15  is a third conceptual diagram of the second embodiment of the present invention; 
         FIG. 16  illustrates an example of a device configuration according to the second embodiment of the present invention; 
         FIG. 17  illustrates an example of an operation sequence up to the allocation of an identical cell ID to a plurality of RUs according to the second embodiment of the present invention; 
         FIG. 18  illustrates a first example of an operation sequence in a case where an identical cell ID is allocated to a plurality of RUs according to the second embodiment of the present invention; 
         FIG. 19  illustrates a second example of the operation sequence in the case where an identical cell ID is allocated a plurality of RUs according to the second embodiment of the present invention; and 
         FIG. 20  illustrates an example of the device configuration according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the drawings. 
     While the following description of embodiments may be divided into a plurality of sections or embodiments as needed for convenience, they are not mutually unrelated unless specifically noted otherwise, and are in a relationship such that one may be a part of the other or a modification, a detail, a supplementary description or the like of the whole, for example. The embodiments may be implemented individually or in combination. 
     Further, in the following embodiments, when references are made to the number of elements and the like (including the number of items, numerical values, amounts, and ranges), the embodiments are not limited to the specific numbers, and more or less than the specific number may be used unless specifically noted otherwise, or unless the embodiments are obviously limited to the specific number in principle. 
     It should be obvious that in the following embodiments, the constituent elements and the like (including element steps and the like) may not necessarily be required unless specifically noted otherwise or unless obviously considered indispensable in principle. 
     Similarly, in the following embodiments, when references. are made to the shape of constituent elements, their positional relationship and the like, the shape and the like may include substantially approximate or similar shapes unless specifically noted otherwise or unless the shapes are obviously not the case in principle. This also applies to the numerical values or ranges. 
       FIG. 1  illustrates an example of a wireless communication system to which the present invention is directed. A macro base station  101  forms a macrocell  102 , and small cell base stations  103 - 1  to  103 - 4  form small cells  104 . The small cell base stations  103 - 1  to  103 - 4  are connected to a control device  105 . The control device  105  and the small cell base stations  103 - 1  to  103 - 4  may be connected via wired or wireless lines. In the following, unless distinctions are specifically required, the small cell base stations  103 - 1  to  103 - 4  will be simply referred to as a small cell base station  103 . The same applies to the others. While in the example of  FIG. 1 , the small cells  104  are overlapping the macrocell  102 , the area of the macrocell  102  may not be present. In some cases, the small cells  104  may be positioned near the boundary of the areas of a plurality of macrocells  102 . While in the following, the present invention will be described with reference to the small cells  104  and the small cell base stations  103  by way of example, the present invention is not limited to the small cells  104  or the small cell base stations  103  and may be applied to the macrocell  102  or the macro base station  101 . 
     Referring to  FIG. 1 , the small cell base stations  103  may include a remote radio head (RRH) with only a wireless function or with only some of base station functionality. Similarly, the control device  105  may be only provided with a control function, some of the base station functionality, or all of the base station functionality except for the wireless function. 
     Without loss of generality in description, in the following, the small cell base stations  103  will be denoted as a remote unit (RU)  103 , and the control device  105  will be denoted as a center unit (CU)  105 . The system including RU  103  and CU  105  may be referred to as a cloud-radio access network or centralized-RAN (C-RAN) system, or a C-RAN base station. 
     In such a small cell environment, a large number of terminals may be distributed near the cell boundary, such as of the small cells  104  formed by RUs  103 - 2  and  103 - 3  in  FIG. 1 , at a certain location or at a certain time. As a result, the signals transmitted from the respective RUs  103  may interfere with each other, causing a decrease in communication quality. Further, frequent handovers may occur between the small cells formed by RU  103 - 3  and RU  103 - 2 , decreasing the communication efficiency. 
     The present invention provides a wireless communication system, a base station, and a base station control method by which the above problems can be solved. 
     1. First Embodiment 
     In a first embodiment, it is contemplated that each RU  103  uses a single frequency carrier. 
       FIG. 2  is a conceptual diagram of the first embodiment of the present invention. RU  103 - 1  to RU  103 - 4  form different small cells  104 - 1  to  104 - 4 , respectively. RU  103 - 1  to  103 - 4  transmit signals using respectively different cell IDs (#1 to #4). The cell ID is an identifier for distinguishing the signal of each cell, and is referred to as a physical layer cell identity (PCI) according to LTE, for example. A terminal (or a group of terminals)  106 - 2  is positioned near the boundary of the communication areas of RUs  103 - 1  and  103 - 2 , where the communication quality is decreased by inter-cell interference. There is also the possibility of repeated handovers occurring between the cells  104 - 1  and  104 - 2 . 
     CU  105  collects terminal distribution information on the basis of information reported from the terminals  106  communicating with each RU  103 , and detects the area in which a number of terminals are distributed near the cell boundary, and RU  103  covering the area. In the example of  FIG. 2 , a number of terminals  106 - 2  are distributed at the boundary of the communication areas of RU  103 - 1  and RU  103 - 2 . CU  105  allocates an identical cell ID to the plurality of RUs  103  covering the area. In the example of  FIG. 2 , the cell ID of RU  103 - 2  is modified from #2 to #1, thus providing RUs  103 - 1  and  103 - 2  with an identical cell ID. As described above, the synchronization signal, a unique reference signal of each cell (Cell Specific Reference Signal: CRS) and the like become identical for RUs  103 - 1  and  103 - 2 . As a result, the communication areas  104 - 1  and  104 - 2  of RUs  103 - 1  and  103 - 2  equivalently constitute a single larger cell. Accordingly, handover ceases to occur at the boundary of the communication areas of RUs  103 - 1  and  103 - 2 . Further, because the area in which the terminal group  106 - 2  is positioned ceases to be a cell boundary, the problem of a decrease in communication quality in the area can be solved. Specifically, when a signal is transmitted to a terminal positioned near the boundary of the communication area of RU  103 , the same signal is transmitted from the plurality of RUs  103 . As a result, the effect of soft combining can be obtained, thus enabling an increase in communication quality as by CoMP. The signals transmitted from the plurality of RUs  103  with the identical cell ID allocated thereto are, to the terminal  106 , equivalent to a multipath with different channels. Thus, in order to receive the signals transmitted from the plurality of RUs  103  to which the identical cell ID is allocated, the function for receiving signals from a plurality of cell, as in CoMP, is unnecessary. 
     Thus, CU  105  detects that a large number of terminals are distributed near the boundary of the communication areas of certain RUs  103 , and allocates an identical cell ID to a plurality of RUs  103  covering the area. In this way, the communication quality of an area having the problem of a decrease in communication quality due to inter-cell interference or frequent handovers can be increased, and handovers can be decreased. 
     However, when a plurality of RUs  103  are given with an identical cell ID, if signals are transmitted to all of the terminals  106  using the multiple RUs  103  to which the terminals are connected, the number of terminals that can communicate simultaneously, i.e., the amount of wireless resources available per terminal, may be decreased, resulting in a decrease in throughput compared with when different cell IDs are allocated. For example, because the influence of inter-cell interference is small on the terminals  106  positioned at the center of the communication area of RU  103  (such as terminals  106 - 1  and  106 - 3  in  FIG. 2 ), the effect of communication quality improvement obtained by transmitting signals using a plurality of RUs  103  (RU  103 - 1  and RU  103 - 2  in  FIG. 2 ) is believed to be small. Thus, by transmitting data to such terminals  106  using a single RU  103 , the number of simultaneously communicating terminals, or the amount of wireless resources available per terminal may be increased, whereby throughput may be increased. 
     A method of solving the above problem will be described below. The LTE standard includes a transmission mode using the above-described CRS as a reference signal (referred to as Transmission Mode), and a transmission mode using a reference signal for demodulation (referred to as demodulation RS (DMRS) or UE specific RS). CRS includes a sequence unique to the cell ID, and cannot be transmitted from only some of the plurality of RUs  103  having an identical cell ID. This is because CRS is used not only for demodulation of data but also for reception power measurement, control channel demodulation, and demodulation of broadcast signal or paging, and is therefore received by terminals other than the data receiving terminal. Namely, it may be said that the size of the cell is determined by the area in which CRS can be received. Thus, for CRS, the same signal is transmitted from a plurality of RUs  103  having the identical cell ID. Accordingly, if data is transmitted only from a single RU  103 , the channel of the data and the channel estimated from CRS would not correspond to each other, thus lowering reception performance. 
     On the other hand, DMRS is a dedicated reference signal for data demodulation, and may be transmitted from only some of the plurality of RUs  103  having an identical cell ID. Thus, to the terminals  106  positioned at the center of the communication area of RU  103 , such as terminals  106 - 1  and  106 - 3  in  FIG. 2 , signals can be transmitted from the single RU  103  using the same wireless resources. However, when the cell ID is different, rules for mapping to the data resource and the like are different. Thus, the transmitting RU  103  needs to be RU  103  that uses the identical cell ID to the cell to which the terminal  106  connects. Also, DMRSs transmitted from different RUs  103  need to be different in logical antenna port number or the DMRS signal sequence. For example, in transmission mode 7 (TM7) in LTE, logical antenna port 5 is used, and the signal sequence of DMRS is dependent on an identifier of the terminal  106  referred to as radio network temporary identifier (RNTI). Generally, different RNTIs are allocated to the terminals  106  connecting to the same cell, so that TM7 satisfies the above condition. Meanwhile, in TM8 or TM9, antenna ports 7-14 are used, and the signal sequence of DMRS is determined by the cell ID and the signal sequence of DMRS called an SCID, or by parameters for determining a scramble sequence. SCID takes the value of 0 or 1. The SCID and the antenna port number used are notified from the base station to the terminals by physical downlink control channel (PDCCH) for scheduling information notification. Thus, when TM8 or 9 is used, it is necessary to use different antenna ports or different SCIDs between terminals to which signals are transmitted from different RUs  103  having the identical cell ID. Further, in TM 10, in addition to the antenna port and SCID, a virtual cell ID may be set in the terminal. The virtual cell ID is a parameter for determining the scramble sequence of DMRS, as in SCID. Thus, in TM 10, the antenna port, SCID, or the virtual cell ID is varied between the terminals. 
     Based on the foregoing, the operation in the case where an identical cell ID is allocated to a plurality of RUs  103  is as follows. 
     In the terminals  106  positioned at the center of the communication area of RUs  103  to which an identical cell ID is allocated, a transmission mode using DMRS (such as TM7, 8, 9, or 10) is set, and signals are transmitted using a single RU  103 . 
     In the terminals  106  positioned near the boundary of the communication areas of RUs  103  to which an identical cell ID is allocated, a transmission mode using CRS or a transmission mode using DMRS is set, and signals are transmitted using a plurality of RUs  103 . When signals of different terminals  106  are transmitted from different RUs  103  having an identical cell ID, different DMRS antenna port numbers or signal sequences (scramble sequences) are used. 
     The transmission using a plurality of RUs  103  and the transmission using a single RU  103  may be handled as an example of multi-user multiple input multiple output (MIMO) or beamforming, rather than the CoMP function. In multi-user MIMO, simultaneous communications with a plurality of terminals are performed using a plurality of antennas and the same wireless resources. On the other hand, in single user MIMO, communication with a single terminal is performed using a plurality of antennas. Multi-user MIMO is a MIMO precoding control method for preventing mutual interference between signals from simultaneously communicating terminals by the use of a directional beam. Meanwhile, the method where communications with different terminals are performed using one of the plurality of RUs  103  having an identical cell ID may be considered to be a multi-user MIMO in that simultaneous communications are performed with a plurality of terminals using different RUs  103  such that mutual interference can be decreased by radio wave attenuation. Also, in the present embodiment, the method of communicating with a single terminal using a plurality of RUs  103  may be considered to be a single user MIMO. 
     The terminals  106  also measure the communication quality and channel information of the connected cell, and feedback a measurement result to the base station. The feedback information is referred to as channel state information (CSI). The CSI includes, for example, a channel quality indicator (CQI) indicating communication quality, a precoding matrix indicator (PMI) indicating MIMO precoding information, and a rank indicator (RI) indicating the number of layers that can be transmitted by MIMO. In the transmission mode using DMRS, CSI measurement is performed using CRS, CSI-RS or the like. When CSI is measured using CRS, the measured CSI is a combination of CRS&#39; transmitted from a plurality of RUs  103 . The signals between RUs  103  having the identical cell ID are not included in interference. Namely, the CSI fed back by the terminal  106  is the CSI in the case of transmission using a plurality of RUs  103  having the identical cell ID. Thus, the CSI in the case of signal transmission using a single RU  103  may differ from the information fed back by the terminal  106 . Accordingly, it is necessary to correct or estimate in CU  105  the CSI in the case of the signal transmission from the single RU  103 . For example, a method may be contemplated where CU  105  estimates the uplink channel on the basis of an uplink reference signal, and uses that information for downlink CSI. This will be particularly effective in a time division duplex (TDD) system. Alternatively, outer loop link adaptation (OLLA) that corrects CQI in accordance with the ACK information of data may be used. In this case, OLLA may be applied independently (distinguishing the number of times of ACK) between when a plurality of RUs  103  are used and when a single RU  103  is used. Alternatively, OLLA may be applied only when a single RU  103  is used. 
     When the CSI is measured using CSI-RS, the same CSI-RS may be transmitted from a plurality of RUs  103  having an identical cell ID, as in the case of CRS. In this case, correction of the CSI may be performed by the same method as for CRS. Alternatively, different CSI-RS may be transmitted from each RU  103 , as in  FIG. 3 . The “different CSI-RS” means that the signal is different in any of the timing of transmission of CSI-RS, the time and frequency resource, and the sequence of CSI-RS. In  FIG. 3 , RUs  103 - 1  and  103 - 2  transmit different CSI-RS (CSI-RS1 and CSI-RS2). CU  105  detects whether the terminal  106  is positioned in the communication area of RU  103 - 1  or  103 - 2 . When the area in which the terminal is positioned is changed, the corresponding CSI-RS may be reset. Alternatively, a separate CSI-RS (such as CSI-RS3) may be transmitted from both RUs  103 - 1  and  103 - 2 , and CSI-RS3 may be set when the terminal  106  is positioned near the area boundary. 
       FIG. 4  illustrates an example of the configuration of CU  201  and RU  203  according to the present embodiment. The device illustrated in  FIG. 4  may be realized by a memory, a digital signal processor (DSP), a field programmable gate array (FPGA), a central processing unit (CPU), a micro-processing unit (MPU) and the like. 
     CU  201  and RU  203  are connected by a wired line such as an optical fiber line, or a wireless line. RU  203  is also connected to an antenna  202 . However, RF function and the antenna  202  may be incorporated into RU  203 . 
     The antenna  202  transmits a downlink radio frequency (RF) signal input from RU  203 . The antenna  202  also receives an uplink RF signal transmitted from the terminal. A plurality of antennas may be connected to one RU  203 . 
     RU  203  is provided with an RF function. RU  203  converts a downlink base band IQ signal input from CU  201  into an RF signal which is transmitted via the antenna  202 . RU  203  also converts an uplink RF signal input from the antenna  202  into a base band IQ signal which is input to CU  201 . RU  203  includes an electric power amplifier. RU  203  is provided with an interface between RU  203  and CU  201 . For example, when RU  203  and CU  201  are connected via an optical fiber, RU  203  may include an electrical/optical converter and an optical/electrical converter. RU  203  is further provided with a signal transmission/reception function based on a common public radio interface (CPRI), and may perform signal transmission and reception with CU  201  using a plurality of antennas or a plurality of frequencies. 
     CU  201  includes a switch  204 , a base band unit (BBU)  205 , an L2/L3 processor  206 , a control unit  207 , and a network interface (I/F)  208 . 
     The switch  204  connects BBU  205  and RU  203 . The correspondence between BBU  205  and RU  203  is notified by the control unit  207 . The connection between BBU  205  and RU  203  may be one-to-one or one-to-many. For example, when each RU  203  has a different cell ID, one RU  203  is connected to one BBU  205 . When an identical cell ID is allocated to a plurality of RUs  203 , the plurality of RUs  203  are connected to one BBU  205 . The connection may include an adjustment of signal power (amplitude) or a weighted average. The details of such connecting operations in the switch may be implemented in the form of a matrix operation illustrated in  FIGS. 5 and 6 , for example. 
     BBU  205  includes the function of outputting signals corresponding to a plurality of RUs  203 . Each output from BBU  205  may correspond to one signal process device (such as DSP), for example. When the signal input from BBU  205  to the switch  204  is expressed by a vector D DL  of 2*number of BBUs rows and one column, the output from the switch  204  (input to RU  203 ) is expressed by a vector S DL  of the number of RUs rows and one column, and the connection in the switch is expressed by a matrix W DL  of the number of RUs rows and 2*number of BBUs columns, the relationship between D DL , S DL , and W DL  can be expressed by Mathematical Formula 1. 
         S   DL   =W   DL   D   DL   [Mathematical Formula 1]
 
     In the example of  FIG. 5 , D DL =[d DL, 1, 1  d DL, 1, 2  d DL, 2, 1  d DL, 2, 2  d DL, 3, 1  d DL, 3, 2  d DL, 4, 1  d DL, 4, 2 ] T , and S DL =[s DL, 1  s DL, 2  s DL, 3  s DL, 4 ] T , where d DL, i, j  is a j-th output signal of BBU  205 - i , and s DL, i  is an output signal from the switch  204  to RU  203 - i . For example, when a different cell ID is allocated to each RU  203 , i.e., when BBU  205  and RU  203  are connected one-to-one, the downlink connection matrix W DL  can be expressed according to Mathematical Formula 2. 
     
       
         
           
             
               
                 
                   
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     Mathematical Formula 2 indicates that only the first output from each BBU  205  is output to RU  203 . Alternatively, when BBU  205  and RU  203  are connected one-to-one, the downlink connection matrix W DL  according to Mathematical Formula 3 may be used. 
     
       
         
           
             
               
                 
                   
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     Mathematical Formula 3 indicates that a sum of signals output from BBU  205  is transmitted to RU  203 . In this case, each output from BBU  205  is, for example, a layer-by-layer signal or a user-by-user signal. 
     When an identical cell ID is allocated to RUs  203 - 1  and  203 - 2 , and RUs  203 - 1  and  203 - 2  are connected to BBU  205 - 1 , the downlink connection matrix W DL  can be expressed by Mathematical Formula 4. 
     
       
         
           
             
               
                 
                   
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                           0 
                         
                         
                           0 
                         
                         
                           1 
                         
                         
                           0 
                         
                         
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                           0 
                         
                         
                           0 
                         
                         
                           0 
                         
                         
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                     ] 
                   
                 
               
               
                 
                   [ 
                   
                     Mathematical 
                      
                     
                         
                     
                      
                     Formula 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Mathematical Formula 4 indicates that a second output from BBU  205 - 1  is output to RU  203 - 2 , as illustrated in  FIG. 5 . By using a value other than 1 as the coefficient in Mathematical Formula 2, the power can be adjusted. The connection matrix W DL  is provided from a control unit  207 . 
     Similarly, in uplink, when the signal input from RU  203  to the switch  204  is expressed by a vector D UL  of the number of RUs rows and one column, the output from the switch  204  (input to BBU  205 ) by a vector S UL  of a 2*number of BBUs rows and one column, and the connection in the switch by a matrix W UL  of 2*number of BBUs rows and number of RUs columns, the relationship between D UL , S UL , and W UL  can be expressed by Mathematical Formula 5. 
         S   UL   =W   UL   D   UL   [Mathematical Formula 5]
 
     In the example of  FIG. 6 , the uplink connection matrix W UL  in the case where an identical cell ID is allocated to RUs  203 - 1  and  203 - 2  can be expressed by Mathematical Formula 6. 
     
       
         
           
             
               
                 
                   
                     W 
                     UL 
                   
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     Alternatively, when BBU  205  and RU  203  are connected one-to-one, the same signal may be input to a plurality of inputs of BBU  205  as according to Mathematical Formula 7. 
     
       
         
           
             
               
                 
                   
                     W 
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     The connection between BBU  205  and RU  203  in the switch  204  may be realized by another method. For example, for downlink, signals input from BBU  205  to the switch may include information of destination RU  203 , and the signals may be sorted in the switch  204 . For uplink, signals input from a plurality of RUs  203  may be weighted and summed, and the weighted sum may be output to one BBU  205 . 
     BBU  205  mainly performs a signal process in the physical layer (L1, Layer 1). For example, BBU  205  performs a physical layer signal process of a downlink physical data channel (which may be referred to as physical downlink shared channel (PDSCH)) or a physical control channel (which may be referred to as physical downlink control channel (PDCCH), Enhanced PDCCH, physical hybridARQ indicator channel (PHICH), or physical control format indicator channel (PCFICH)) of each terminal that is input from the L2/L3 processor  206 , or generates a physical layer control channel. BBU  205  also performs a physical layer signal process of an uplink data channel (physical uplink shared channel (PUSCH)) and a control channel (physical uplink control channel (PUCCH)) and the like that is input from RU  203  via the switch  204 . Specifically, the downlink signal process may include error-correcting coding of a data signal and a control signal, rate matching, modulation, a MIMO signal process such as layer mapping or precoding, mapping to a wireless resource (which may be referred to as a resource element (RE)), or inverse fast Fourier transform (IFFT). BBU  205  also performs generation of reference signals (such as CRS, CSI-RS, and DMRS) for channel estimation for demodulation, CSI measurement, or reception power measurement by the terminals, and insertion of the reference signals in the wireless resource. BBU  205  also performs generation of synchronization signals and a physical layer broadcast channel (physical broadcast channel (PBCH)) and their insertion in RE. As illustrated in  FIG. 5  or  FIG. 6 , BBU  205  has the function of performing a signal process corresponding to a plurality of RUs  203 . BBU  205  may also include the function of switching between a signal process corresponding to a plurality of RUs  203  and a signal process corresponding to a single RU  203  under the control of the control unit  207  or the L2/L3 processor  206 . A base band signal generated by the signal processes is transmitted via the switch  204  to RU  203 . The uplink signal process includes FFT, RE demapping, a MIMO signal process such as multiplication of a MIMO reception weight or layer demapping, demodulation, and error-correcting demodulation for the signals input via the switch  204  from RU  203 . BBU  205  also performs channel estimation and reception power measurement using an uplink RS (such as DMRS or Sounding RS (SRS), uplink CSI measurement and the like. The decoded data channel or control channel, various measurement results and the like are transmitted to the L2/L3 processor  206 . The result of uplink reception power measurement may be reported to the control unit  207 . 
     The L2/L3 processor  206  is a processor that performs Layer 2 and Layer 3 processes of the base station. The L2/L3 processor  206  stores respective terminal data transmitted from the core network via the network I/F  208 , and control signals received from another base station or a mobility management entity (MME) in a buffer. The L2/L3 processor  206  also performs, e.g., scheduling for determining a terminal for communication or the time and frequency resources allocated to the terminal, HARQ management, packet processing, a wireless line hiding process, and generation of a control signal of an upper layer to a terminal. The L2/L3 processor  206  also determines, based on a measurement report reported from the terminal  106  or uplink reception power information, whether the terminal  106  is positioned at the center of the communication area of each RU  203  or at the boundary of communication areas. The L2/L3 processor  206  also notifies the control unit  207  of information about the terminal positioned near the boundary of communication areas, a measurement result of uplink reception power and the like. Depending on the result of determination of the terminal positioned area, the L2/L3 processor  206  also makes a determination of RU  203  to which the signal from each terminal is transmitted, a transmission mode setting and the like. The L2/L3 processor  206  includes the function of switching the signal process corresponding to a plurality of RUs  203  and a process corresponding to a single RU  203  under the control of the control unit  207 . 
     The control unit  207 , based on the uplink reception power information of each terminal notified from the L2/L3 processor  206  or BBU  205 , the information of the terminal positioned in the communication area of each RU  203  and the like, detects a set of RUs  203  of which a number of terminals are distributed near the boundary of communication areas. Then, the control unit  207  allocates an identical cell ID to the detected RUs  203 , modifies the connection in the switch  204 , and notifies connection information to the switch  204 . The connection information may also be notified to BBU  205  and the L2/L3 processor  206 . 
     The network I/F  208  provides an interface for connecting CU  201  and the core network through a backhaul line. The network I/F  208  transfers to the core network data and control information of each terminal input from the L2/L3 processor, control information or the like to another base station, or the mobility management entity. The network I/F  208  also transfers data or control information of each terminal and control information and the like for the L2/L3 processor  206 , which are input from the core network, to the corresponding L2/L3 processor  206 . The network I/F  208  may include the function of managing movements, such as a handover, that takes place in CU  201 . Namely, the network I/F may include the function of a local gateway in CU  201 , or as a mobility management entity. 
       FIG. 7  illustrates an example of an operation sequence up to cell ID modification according to the first embodiment of the present invention. While in  FIG. 7 , two RUs (RU #1 and RU #2), and two BBUs and two L2/L3 processors (BBU #1 and BBU #2, L2/L3 processor #1 and L2/L3 processor #2) are described, other RUs and BBUs that are not described may be present. The number of terminals is three, with terminal #1 positioned near the boundary of the communication areas of RU #1 and RU #2 (corresponding to the terminal  106 - 2  in  FIG. 2 ). In the illustrated example, communication with BBU #1 is conducted using RU #1. Terminal #2 is positioned at the center of the communication area of RU #1 (corresponding to the terminal  106 - 1  in  FIG. 2 ). Terminal #3 is positioned at the center of the communication area of RU #2 (corresponding to the terminal  106 - 3  in  FIG. 2 ). In the initial state, it is assumed that the switch connects RU #1 and BBU #1, and RU #2 and BBU #2 (S 101 ). 
     The L2/L3 processors #1 and #2 make reception power information measurement and report settings (measurement configuration) in the respective terminals #1 to #3 to determine whether the terminals are at the boundary of the communication areas of RU #1 and RU #2 (S 102 ). The measurement configuration set here is referred to as an event A3, for example. The event is generated when the reception power of an adjacent cell becomes greater than the sum of the reception power of a connected cell (referred to as a “serving cell”) and a predetermined offset value. For example, when the offset value is set at −3 dB, the terminals #1 to #3 start reporting of the reception power of the cell when the difference between the reception power of the connected cell and the reception power of the adjacent cell is 3 dB or less. The report is referred to as a measurement report. The terminal making the measurement report can be determined to be a terminal positioned at the boundary of the communication areas of RU #1 and RU #2. The report may be caused (by setting of On/Off of parameter reportOnLeave) to be made when the cell ceased to satisfy the above condition. The measurement configuration that is set herein may be common with the one set for a handover, and an offset value larger than the handover may be set. In the example of  FIG. 7 , it is assumed that the terminal #1 satisfies the present condition, and reports the cell IDs of the connected cell and the adjacent cell, reception power and the like (S 103 ). The L2/L3 processor #1 determines that the terminal #1 that has made the measurement report is positioned at the boundary of the communication areas of RU #1 and RU #2 (S 104 ). The L2/L3 processors #1 and #2 count, of the connected terminals, the number of terminals that satisfy the condition set in S 102 , and the number of terminals that do not (i.e., the terminals that do not make the measurement report). Then, the L2/L3 processors #1 and #2 periodically report information to the control unit, such as the number of the terminals satisfying the measurement configuration condition set in S 102 , the cell ID of the adjacent cell satisfying the condition, and the number of the terminals not satisfying the condition (S 105 ). 
     The control unit, based on the information about the terminals at the communication area boundary reported from each L2/L3 processor, computes the number of the terminals at the communication area boundary of each RU per unit time, and an average number of the terminals at the area boundary (S 106 ). For example, the control unit stores each RU number, the allocated cell ID, and the L2/L3 processor number in association with one another in a RU/cell ID mapping table illustrated in  FIG. 8 . The control unit then refers to  FIG. 8  and computes the RU number corresponding to the cell ID reported in S 104 . Herein, the RU corresponding to the connected cell is referred to as a connected RU, and the RU corresponding to the adjacent cell is referred to as an adjacent RU. Then, the reported number of terminals is saved in a format as shown in  FIG. 9A . The diagonal component u i-i  in  FIG. 9A  indicates the number of the terminals positioned at the center of the communication area of RU #i, and the non-diagonal component u i-j  indicates the number of the terminals positioned at the boundary of the communication areas of RU #i and RU #j, with RU #i having greater reception power (RU #i providing the connected RU). The control unit also stores the average number of the terminals at the area boundary in a format as shown in  FIG. 9B . The control unit updates the information about the average number of terminals periodically (such as each time the information of the number of terminals is reported from the L2/L3 processor). The averaging may be performed by computing a forgetting average according to Mathematical Formula 8, for example. 
         U   i-j =(1−α) U   i-j   +αu   i-j   [Mathematical Formula 8]
 
     wherein α is a forgetting coefficient. 
     Then, the control unit, based on the computed number of terminals at the communication area boundary, makes a cell ID allocation determination for each RU (S 107 ). A concrete method for the allocation determination will be described later. Herein, it is assumed that the control unit has determined to allocate an identical cell ID to RU #1 and RU #2. 
     Thereafter, switch connection control is performed from S 108  to S 114 . How the communication area of each RU is changed by the present operation is illustrated in  FIG. 10 . If the cell ID of the signal transmitted by each RU is instantaneously modified, the cell prior to the modification would cease to exist all of a sudden, creating the possibility of disconnecting the terminal that has been connected to the cell prior to the modification. The operation from S 108  to S 114  represents a method for minimizing the influence of the cell ID modification on the terminal. First, the control unit notifies the switch requesting a decrease in the transmission power of the other RU to which the identical cell ID is allocated (S 108 ). The switch, in accordance with the notification from the control unit, decreases the transmission power of RU #2 (S 109 ). The present operation is repeated a certain number of times at certain periods, for example. As a result, the communication area of RU #2 is reduced, as illustrated in  FIG. 10 . Thus, the terminals that have been connected to RU #2 are handed over to the cell of another RU (S 110 ). In  FIG. 7 , it is assumed that the handover occurs from the cell of RU #2 (cell ID #2) to the cell of RU #1 (cell ID #1). When the terminal connected to the RU prior to the modification (RU #2 in  FIG. 7  or  FIG. 10 ) ceases to be present, or when the transmission power is decreased by a certain amount, the control unit notifies the switch of the connection information of BBU and RU determined in S 107  (S 111 ). The establishment of connections with a plurality of RUs is also notified to the L2/L3 processor #1 or BBU #1. The switch, based on the connection information from the control unit, makes a connection modification (S 112 ). In  FIG. 7 , both RU #1 and RU #2 are connected to BBU #1. As a result, RU #1 and RU #2 have the same cell ID. Then, the control unit, in order to return the reduced area of the RU back to the original area, notifies the switch requesting an increase in transmission power (S 113 ). The switch, in accordance with the notification from the control unit, increases the transmission power of RU #2 (S 114 ). The operation of S 113  and S 114  is also repeated a certain number of times at certain periods. As a result, as illustrated in  FIG. 10 , the communication area of RU #2 is returned to the original state. S 108 , S 111 , and S 113  may be implemented by notifying the switch of the notified downlink and uplink connection matrices W DL  and W UL . 
     As a result of the above operation, RU #1 and RU #2 are provided with the identical cell ID. However, BBU #1, immediately after connection modification, is not cognizant of whether each terminal is positioned at the center of the communication area of RU #1, the center of the communication area of RU #2, or at the boundary of the communication areas of RU #1 and RU #2. Thus, immediately after the connection control, transmission is performed to all of the terminals using a plurality of RUs (S 115 ). As to reception, BBU #1 may receive the signals from the plurality of RUs which may be combined at a maximum ratio, simply added up, or averaged, assuming that the number of antennas has been doubled (S 116 ). 
     However, the determination from S 102  to S 106  regarding the boundary of the communication area of RU may be implemented by other methods. For example, each terminal measures the position of the terminal using GPS and the like, and notifies the L2/L3 processor of the measured position. When the distance between the terminal and a plurality of RUs is within a certain threshold value, the L2/L3 processor may determine that the terminal is positioned at the boundary of the communication areas of the RUs. 
       FIG. 11  illustrates an example of the method for cell ID allocation determination in S 107  of  FIG. 7 . In the example method of  FIG. 11 , based on the number of the terminals positioned near the boundary of the communication areas of RU and the ratio of the terminals, it is determined whether an identical cell ID should be allocated based on a detection of a number of terminals positioned at the boundary of the communication areas. 
     When a set of RUs is referred to as an RU set, the possible number M of RU sets is N (N−1)/2, where N is the number of RUs (S 200 ). The control unit sorts RU sets #1 to #M in order of decreasing number of terminals positioned at the boundary of the communication areas of the RU set (S 201 ). However, in S 201 , the sorting may be in order of decreasing ratio of the terminals positioned at the boundary of the communication areas. First, a determination is made with respect to the first RU set (S 202 ). Here, the RUs of the i-th RU set #i are assumed to be RU #i1 and RU #i2 (i1&lt;i2) (S 203 ). The control unit checks to see if the cell ID identical to that of another RU (S 204 ) is allocated to RU #i1 or RU #i2. It should be noted, however, that the determination in S 204  is whether the cell ID has been allocated in S 207  which will be described below, and not whether the identical cell ID is currently actually allocated. If there is an RU in RU #i1 or #i2 to which the identical cell ID is already allocated (Yes), the process proceeds to the next RU set (S 209 ). If not (No), the control unit determines whether the ratio of the area boundary terminals of RU set #i exceeds a predetermined first threshold value to which a first offset is added (S 205 ). The number of terminals at the area boundary can be computed by adding up transposed elements of the non-diagonal components in  FIG. 9B . For example, the number of the terminals at the area boundary of RU #i and RU #j is U i-j +U j-i . The ratio of the terminals at the area boundary can be computed by (U i-j +U j-i )/(U i-j +U j-i +U i-i +U j-j ). If the ratio of the terminals at the area boundary of RU set #i does not exceed the value of the first threshold value to which the first offset is added (No), it is determined that the RU set has another cell ID, and the process proceeds to the next RU set. If the ratio of the terminals at the area boundary of RU set #i exceeds the value of the first threshold value to which the first offset is added (Yes), the control unit determines whether the number of terminals U i1 +U i2  at the area boundary of RU set #i exceeds the value of a predetermined second threshold value to which a second offset is added (S 206 ). If the number of terminals at the area boundary of RU set #i does not exceed the value of the second threshold value with the added second offset (No), it is determined that the RU set has another cell ID, and the process proceeds to the next RU set. If the number of terminals at the area boundary of RU set #i exceeds the value of the second threshold value with the added second offset (Yes), the control unit determines that an identical cell ID is to be allocated to the RU set (S 207 ). Then, it is checked to see if the determination has been made with respect to all of the RU sets. If “Yes”, the process ends; if “No”, the process proceeds to the next RU set (S 208 ). Thus, by allocating the identical cell ID to the RU where the number of the terminals positioned at the boundary of the communication areas of RU and its ratio are large, the communication quality can be improved in the area where the problem of degradation of communication quality due to inter-cell interference and a decrease in communication efficiency due to handover is the most pronounced, thus decreasing handover. 
     The present determination method may also be used when an RU to which an identical cell ID has been allocated is again allocated another cell ID, or when the cell ID identical to that of the other RU is allocated to the RU. The first offset and the second offset are offsets for varying the threshold value for determination depending on whether the identical cell ID has been already allocated to a plurality of RUs. For example, when the particular RU set is already operating as the same cell, the first or second offset may be given a negative value; otherwise, a positive value or 0 may be given. As a result, the RU set already operating as the identical cell ID is unlikely to assume a different cell ID, while the identical cell ID is more likely to be allocated to an RU set that is not operating as such. Alternatively, the opposite offsets may be set. 
     Only one of S 205  and S 206  may be implemented. Whether only one of the steps is to be implemented may be controlled by making the first threshold value or the second threshold value zero. 
     In S 205  or S 206 , other references may be used, or an additional reference may be used. For example, the determination may be made based on the amount of traffic of the terminal&#39;s at the area boundary, or a traffic ratio, instead of the number of the terminals at the area boundary. Namely, an identical cell ID may be allocated to the RU when the amount of traffic of the terminals at the area boundary, its ratio, or both exceed certain threshold values. On the other hand, when the amount of traffic of the terminals at the area boundary or its ratio is small, the communication speed (or the modulation scheme and coding rate) of the terminals may be lowered so as to increase the amount of allocated wireless resources, thus addressing the decrease in communication quality. The influence of the increase in the amount of allocated wireless resources on the terminals at the area center is small. Thus, when the amount of traffic of the terminals at the area boundary, or its ratio is small, the necessity of allocating an identical cell ID to a plurality of RUs is low. Accordingly, by performing the cell ID allocation by taking the amount of traffic of the terminals at the area boundary or its ratio into consideration, the influence of cell ID modification on the terminals at the area center can be minimized. It also becomes possible to increase the communication quality in the area where the problem of inter-cell interference is most pronounced, thus decreasing handover. Alternatively, the terminals supporting the CoMP function may not be counted as the terminals at the area boundary in consideration of the fact that the communication quality can be increased by CoMP. While in  FIG. 11 , the number of RUs to which the identical cell ID is allocated is two, the method of  FIG. 11  may be readily extended for more than two RUs, and may not be limited to the number two. 
       FIG. 12  shows an example of the operation sequence according to the first embodiment where a plurality of RUs have the same cell ID. In the RUs to which the identical cell ID is allocated, the same synchronization signal, reference signal (CRS), broadcast signal and the like are transmitted. Thus, the terminals cannot distinguish the RUs having the identical cell ID. Specifically, the reception power of each cell as reported by the terminals is a combination of the reference signals transmitted from the RUs having the identical cell ID. Thus, the CU detects, through the following procedure, the communication area of which RU each terminal is positioned in. In  FIG. 12 , it is assumed that RU #1 and RU #2 are allocated the same cell ID and are connected to the same BBU #1 (S 301 ). The L2/L3 processor #1 notifies the control unit of the setting information of uplink reference signals (which may be referred to as Sounding RS (SRS)) of the connected terminals. The L2/L3 processor #1 also receives from the control unit information of uplink reference signals of the terminals connected to the other BBU and L2/L3 processor (S 302 ). The information of uplink reference signals include, for example, the cell ID of the measured cell, the ID of the terminals whose uplink reference signals are transmitted, a transmission period and a transmission timing, and a frequency resource. Each terminal periodically transmits the uplink reference signal (S 303 ). The BBU #1 measures the reception power of the uplink reference signal transmitted from each terminal at RU #1 and RU #2 (S 304 ). The reception power is averaged by computing a forgetting average or a time average, for example. The measurement of the uplink reception power is also performed with regard to the uplink reference signals of the terminals connected to the other BBU and L2/L3 processor on the basis of the information notified in S 302 . The control unit is notified of the measured reception power, together with the ID of RU, the ID of the terminals, and the ID of the cell to which the terminals are connected (S 305 ). BBU #1 determines, based on the reception power of RU #1 and RU #2 measured in S 304 , whether each terminal is positioned at the boundary of the communication areas of RU #1 and RU #2, or at the center of the communication area of RU #1 or RU #2 (S 306 ). For example, as in the case of  FIG. 7 , when the difference in reception power between RU #1 and RU #2 is not greater than a certain threshold value, the L2/L3 processor #1 determines that the terminal is positioned at the boundary of the communication areas of RU #1 and RU #2. When the difference in reception power is greater than the threshold value, the L2/L3 processor determines that the terminal is positioned at the center of the communication area of the RU having the maximum reception power. Then, the L2/L3 processor, based on the area boundary information of each terminal determined in S 304 , controls the transmission mode (S 307 ) and performs scheduling (S 308 ), for example. 
     For example, the transmission mode is controlled (S 307 ) as follows. As described above, for the terminals positioned at the center of the communication area of RU #1 or RU #2 to which an identical cell ID is allocated (such as terminals #2 and #3 in  FIG. 12 ), it is believed that the available amount of wireless resources can be effectively increased by having the terminals communicate using a single RU. Thus, a transmission mode using DMRS (such as TM7, 8, 9, or 10) is set. On the other hand, for the terminals positioned near the boundary of the communication areas of RU #1 and RU #2, it is believed that the communication quality can be effectively increased by having the terminals communicate using a plurality of RUs. Thus, a transmission mode using CRS (such as TM 1 to 4) is set. 
     Also, with regard to the terminals positioned at the boundary of the communication areas of RU #1 and RU #2, the terminals of which the difference in reception power between RU #1 and RU #2 measured in S 304  is not greater than a threshold value A (e.g., 3 dB) have the possibility of a significant decrease in communication quality when transmitting with a single RU. Thus, assuming that a plurality of RUs will be used at all times, a transmission mode using CRS may be set. Meanwhile, for the terminals of which the difference in reception power is greater than the threshold value A but not greater than a threshold value B (e.g., 9 dB), transmission using a plurality of RUs and transmission using a single RU may be switched depending on the situation. Thus, a transmission mode using DMRS may be set. Further, the transmission mode may be controlled depending on the transmission mode support situation notified by the terminal. For example, a terminal not supporting the transmission mode using DMRS may set a transmission mode using CRS, while a terminal supporting the transmission mode using DMRS may set a transmission mode using DMRS. 
     In another method, the transmission mode may be controlled depending on the mobility situation of the terminal. For example, a terminal with high speed of movement may be set for a transmission mode using CRS, while a terminal with low speed of movement may be set for a transmission mode using DMRS. The terminal with high speed of movement, even if positioned at the center of the communication area of a certain RU, has the high probability of moving to the boundary of the communication area of the RU or to the center of the communication area of a different RU in a short time. Thus, it may become necessary to reconfigure the transmission mode frequently, or the modification of the transmitting RU may not be able to catch up with the movement of the terminal. Accordingly, by setting a transmission mode using CRS for the terminal with high speed of movement and performing transmission using a plurality of RUs, the frequent reconfiguration of the transmission mode or the modifying of the transmitting RU can be avoided. The transmission mode thus controlled is notified from the L2/L3 processor #1 to the respective terminals #1 to #3. 
     The scheduling (S 308 ) may be performed as follows, for example. First, a metric for scheduling in the case of transmission using a plurality of RUs is computed, and a terminal of which the metric becomes maximum is extracted. The metric in the case where the plurality of RUs are used is computed only for the terminals positioned at the area boundary of RU #1 and RU #2. It is assumed that the terminal extracted herein is u1-2, and the metric for scheduling is metric 1-2. As the metric for scheduling, a proportional fairness (PF) metric may be used, for example. The PF metric is an instantaneous throughput divided by an average throughput. The instantaneous throughput can be computed from the CQI reported by the terminal, or from the CQI estimated or corrected in CU. Similarly, the metric for scheduling in the case where a single RU, i.e., RU #1 or RU #2, is used for communication is computed. The metric in the case where the single RU is used for communication is computed for the terminal of which the reception power of RU #1 or RU #2 becomes maximum. In this case, the terminals of which the reception power of each RU becomes maximum may include only the terminals positioned at the area center of RU #1 or RU #2, and terminals positioned at the area boundary of RU #1 and RU #2. Then, the terminals of which the metric is maximized with respect to each RU are extracted. It is assumed that the extracted terminals are u1 and u2, and the metrics for scheduling are metric 1 and metric 2. When the following Mathematical Formula 9 is satisfied, u1-2 is scheduled. 
       Metric 1−2&gt;Metric 1+Metric 2  [Mathematical Formula 9]
 
     When the Mathematical Formula 9 is not satisfied, both u1 and u2 are scheduled. u1-2 and either u1 or u2 may be the same terminal. The scheduling using such metrics may be performed on a unit time (Subframe) basis, on a minimum unit wireless resource (Resource Block (RB)) basis, or for each sub-band including a plurality of RBs. 
     In the same terminal, whether the terminal is the object for transmission using a plurality of RUs or a single RU may be controlled depending on the type of data transmitted. For example, with regard to traffic for control purpose (control plane traffic: C-plane traffic) for which communication stability is important, a plurality of RUs may be used for transmission regardless of whether the terminal is at the area boundary or center. On the other hand, with regard to data traffic for which wireless capacity is important (user-plane traffic: U-plane traffic), the terminal may be the object of transmission using a single RU as long as the terminal is positioned at the center of the communication area of RU. Alternatively, with regard to real-time traffic among the data traffic for which stability is important, such as audio and video, a plurality of RUs may be used for transmission regardless of whether the terminal is at the area boundary or center. Meanwhile, in the case of data traffic, for best effort traffic for which wireless capacity is important, such as for Web browsing, the terminal may be the object for transmission using a single RU as long as the terminal is positioned at the center of the communication area of RU. 
     The L2/L3 processor #1 and BBU #1, based on the scheduling result in S 307  as described above, performs data transmission and reception using various methods. An example will be described with reference to S 309  to S 312  in  FIG. 12 . When the scheduling result indicates downlink data for the terminal #1 positioned at the area boundary of RU #1 and RU #2, BBU #1 performs transmission using a plurality of RUs, namely RU #1 and RU #2 (S 309 ). At this time, the same signal is transmitted from RU #1 and RU #2. The transmission mode may be one using CRS or DMRS. When the scheduling result indicates downlink data of the terminal #2 positioned at the area center of RU #1 and the terminal #3 positioned at the area center of RU #2, BBU #1 performs a single RU transmission (S 310 ). At this time, from RU #1, the data of the terminal #2 is transmitted, and from RU #2 the data of the terminal #3 is transmitted. At this time, the terminal #2 and the terminal #3 use DMRS. Further, in the terminal #2 and the terminal #3, different DMRS antenna ports or scramble sequences (SCID) are used. However, at this time, CRS is transmitted using a plurality of RUs (RU #1 and RU #2). Physical control channels demodulated using CRS, such as PDCCH, are also transmitted using a plurality of RUs (RU #1 and RU #2). When the scheduling result indicates the uplink data of the terminal #1 positioned at the area boundary of RU #1 and RU #2, BBU #1 performs multiple RU reception using RU #1 and RU #2 (S 311 ). At this time, BBU #1 demodulates the signal received by each RU by maximum ratio combining. When the scheduling result indicates the uplink data of the terminal #2 positioned at the area center of RU #1 and the terminal #3 positioned at the area center of RU #2, BBU #1 performs multiple RU reception (S 312 ). At this time, BBU #1 may perform demodulation by multi-user MIMO reception. For example, demodulation may be performed using an interference canceller that cancels out the interference of the signal of the terminal #3 to the signal of the terminal #2 in RU #1, and the interference of the signal of the terminal #2 to the signal of the terminal #3 in RU #2. 
     The control unit, based on the reception power of the uplink reference signal of each terminal at each RU that has been notified from the L2/L3 processor #1 and other L2/L3 processors which are not shown in S 305 , determines whether the terminal is at the boundary of the communication area of each RU, and counts the number of the terminals at the area boundary (S 313 ). For the present operation, an inverse operation to the measurement configuration set in the terminal in S 102  may be performed by the control unit. Namely, when the RU where the uplink reference signal reception power is at a maximum is the connected RU, and the other RU is an adjacent RU, if the reception power of the adjacent RU is within a certain offset value from the maximum reception power, the control unit determines that the terminal is positioned near the boundary of the communication areas of the connected RU and the adjacent RU. Otherwise, the control unit determines that the terminal is at the center of the communication area of the connected RU. The control unit performs the present determination on the unit time basis, and, based on the result, counts the number of terminals at the area boundary of the connected RU and the adjacent RU and the number of terminals at the area center per unit time, as in  FIG. 9A . Also, as in  FIG. 9B , the control unit computes an average number of terminals, and determines the cell ID allocated to each RU (S 314 ). The method of cell ID allocation determination may be similar to the method in S 107  of  FIG. 7  or in  FIG. 11 . In accordance with the result of cell ID allocation in S 314 , the control unit re-allocates different cell IDs to the plurality of RUs to which an identical cell ID has been allocated, and switches the connection of BBU and RU. Alternatively, the control unit may newly allocate an identical cell ID to the RUs to which currently different cell IDs are allocated. Alternatively, the control unit may allocate, to the RUs to which currently an identical cell ID is allocated, the identical cell ID to that of another RU. Specifically, when the number of terminals at the communication area boundary of RUs having an identical cell ID or the ratio of such terminals is decreased to a value smaller than the value with the added threshold value in  FIG. 11 , the RUs are re-allocated different cell IDs. Alternatively, when the number of the terminals at the communication area boundary of one of RUs having an identical cell ID and another RU, or the ratio of such terminals is greater than the RUs currently having the identical cell ID and greater than the threshold value in  FIG. 11 , an identical cell ID to the other RU is allocated to the one RU. The operation sequence for modifying the cell ID of RU is similar to the sequence of S 108  to S 114  of  FIG. 7 . 
     The measurement of uplink reception power in S 303  and S 304  may be implemented using other signals. For example, a random access channel that is used when a terminal newly makes an access or performs uplink synchronization may be used. 
     2. Second Embodiment 
     In the second embodiment, it is contemplated that each RU uses a plurality of frequency carriers. 
       FIGS. 13 to 15  are conceptual diagrams for the second embodiment of the present invention. While the basic configuration is similar to  FIG. 2 , the second embodiment differs in that CU  105  and RU  103  include a transmission and reception function for a plurality of frequencies. Thus, the communication area of each RU is formed at a plurality of frequencies, as indicated by  104 - 1  to  104 - 4  and  107 - 1  to  107 - 4  in  FIGS. 13 to 15 . In the second embodiment, as in  FIG. 2 , RUs in which a number of terminals are distributed near the boundary of the communication areas are detected, and an identical cell ID is allocated to the RUs. However, the control for allocating an identical cell ID is limited to some of the frequency carriers. The frequency carrier that implements cell ID allocation control is denoted as a first frequency, and the other frequency carriers are denoted as a second frequency. In this case, in the communication area of the RU to which an identical cell ID is allocated at the first frequency, the first frequency has a large cell size, and a high quality area is formed also at the boundary of a center area of RU. On the other hand, at the second frequency, while the cell size is small and communication quality is lowered at the RU communication area boundary, there is formed an area in which the wireless capacity increasing effect, i.e., the conventional effect of reducing the size of the cell, is maintained. Namely, the first frequency can provide the role of a conventional macrocell, while the second frequency provides the original role of a small cell. The second embodiment of the present invention aims to enable efficient communication by forming a plurality of frequencies with such different characteristics and selectively using them. 
       FIG. 13  illustrates an example of the selective use of frequencies depending on the position of the terminal in a case where the terminal can communicate only using a single frequency. The terminal capable of communication only using a single frequency is referred to as a carrier aggregation (CA) incapable terminal. Whether the terminal is adapted to CA is notified from the terminal to the base station (L2/L3 processor). In  FIG. 13 , at the first frequency, an identical cell ID is allocated to RU  103 - 1  and RU  103 - 2 . At this time, CU  105  preferentially connects the terminal  106 - 2  positioned at the boundary of the communication area of RU  103 - 1  or RU  103 - 2  to the first frequency. Meanwhile, the terminals  106 - 1  and  106 - 3  positioned at the center of the communication areas of RU  103 - 1  and RU  103 - 2  are preferentially connected to the second frequency. This can be realized by setting certain offset values to the reception power of the first frequency or the second frequency. For example, an offset value is set such that the terminal is connected to the first frequency when the reception power of the first frequency becomes greater than the value of the reception power of the second frequency to which the offset value is added. The effect of increasing the reception power by using the same cell ID is large at the boundary of the communication area of RU but small at the center of the communication area. Thus, at the boundary of the communication area of RU, the reception power of the first frequency becomes greater than the value of the reception power of the second frequency to which the offset value is added, making connection to the first frequency easier. On the other hand, at the center of the communication area of RU, the reception power of the first frequency becomes smaller than the value of the reception power of the second frequency to which the offset value is added, making connection to the second frequency easier. As a result, the terminals  106 - 1  to  106 - 3  can perform communication at the frequency with high communication quality. With regard to RU  103 - 3  and  103 - 4  to which an identical cell ID is not allocated, the priority for connection may be the same between the frequencies. 
       FIG. 14  illustrates an example of the selective use of the frequency depending on the position of the terminal capable of communication using a plurality of frequencies. The terminal capable of communication using a plurality of frequencies is referred to as a CA-capable terminal. The CA-capable terminal can perform communication using the first frequency and the second frequency simultaneously or using only one of the frequencies. At this time, the terminal  106 - 2  positioned at the boundary of the communication areas of RU  103 - 1  and RU  103 - 2  have high communication quality at the first frequency and low communication quality at the second frequency. Meanwhile, the terminals  106 - 1  and  106 - 3  positioned at the center of the communication areas of RU  103 - 1  and RU  103 - 2  have a small difference in communication quality between the first frequency and the second frequency. Thus, CU  105  preferentially schedules the terminal  106 - 2  positioned at the boundary of the communication areas of RU  103 - 1  and RU  103 - 2  at the first frequency, while preferentially scheduling the terminals  106 - 1  and  106 - 3  positioned at the center of the communication area at the second frequency. However, because the terminals  106 - 1  to  106 - 3  are capable of communication at both the first frequency and the second frequency, the terminal  106 - 2  may perform communication using the second frequency, depending on the result of scheduling. In  FIG. 14 , this corresponds to the signal (dashed line) being transmitted from RU  103 - 1  (cell ID #A) to the terminal  106 - 2  and the signal (dot-dash line) being transmitted from RU  103 - 2  (cell ID #B) to the terminal  106 - 2 . These signals are different signals. The terminals  106 - 1  and  106 - 3  may perform communication using the first frequency. In  FIG. 14 , this corresponds to the signal (dashed line) being transmitted from RU  103 - 1  to the terminal  106 - 1  and the signal (dot-dash line) being transmitted from RU  103 - 2  to the terminal  106 - 3  at the first frequency. These signals are different signals. With regard to RU  103 - 3  and  103 - 4  to which an identical cell ID is not allocated, the scheduling priority may be the same between the frequencies. 
       FIG. 15  illustrates an example of the selective use of the frequency depending on the type of traffic and the terminal position. The present operation is directed to the CA-capable terminal. The boundary of the communication areas of RU  103 - 1  and RU  103 - 2  is an area in which a number of terminals are distributed and in which the communication quality of the first frequency is increased by the allocation of the same cell ID. Thus, for the C-plane traffic which is a terminal control traffic for which communication stability is important, the first frequency is used for transmission. On the other hand, for the U-plane traffic which is a terminal data traffic for which wireless capacity is important, the second frequency is used for transmission. However, with regard to the terminals positioned at the boundary of the communication areas of RU, because the communication quality at the second frequency is low, the first frequency may also be preferentially used for transmission for the U-plane. Similarly, for real-time traffic for which communication stability and small delay are important, the first frequency is used for transmission. Meanwhile, for best effort traffic for which wireless capacity is important, the second frequency is used for transmission. However, with regard to the terminals positioned at the boundary of the communication areas of RU, the first frequency may be preferentially used for transmission even for the best effort traffic. 
     According to CA, the frequency at which a terminal establishes connection is referred to as a primary cell (PCell), while the frequency used as an additional wireless resource is referred to as a secondary cell (SCell). Information about terminal security, information between a terminal and a mobility management entity (which may be referred to as Non Access Stratum information or NAS) and the like are exchanged using PCell. While modification of PCell requires a handover, modification of SCell can be performed by modifying the wireless resource setting and does not require a handover. Thus, when CA is implemented, the generation of a handover as a result of the movement between RUs having an identical cell ID can be decreased by operating the first frequency as PCell and the second frequency as SCell. For this purpose, the offset value set for each cell or frequency for cell selection may be set such that the cell at the first frequency is more easily connectable than the cell at the second frequency. Further, in this case, in consideration of the ease of handover and the like, the first frequency may be used for PCell while the second frequency may also be used for SCell in RUs  103 - 3  and  103 - 4  to which an identical cell ID is not allocated. 
     In another method for the selective use of the frequency, the frequency may be selected depending on the speed of movement of the terminal. For example, the terminal with high speed of movement is preferentially connected to the first frequency, and the terminal with low speed of movement is preferentially connected to the second frequency. Alternatively, when CA is implemented, the terminal with high speed of movement is preferentially scheduled at the first frequency while the terminal with low speed of movement is preferentially scheduled at the second frequency. As in the case of transmission mode control according to the first embodiment, the terminal with high speed of movement, even if positioned at the center of the communication area of a certain RU, has a high probability of moving to the boundary of the communication area of the RU or to the center of the communication area of a different RU in a short time. Thus, by preferentially connecting the terminal with high speed of movement to the first frequency, the handover as a result of the movement between the RUs can be avoided. Similarly, by preferentially scheduling the terminal with high speed of movement at the first frequency, the problem of the modification of RU (Namely, modification of SCell) failing to catching up with the movement of the terminal can be avoided. 
       FIG. 16  illustrates an example of the configuration of RU  303  and CU  301  according to the second embodiment where a plurality of frequencies are used. While the basic configuration is similar to  FIG. 4 , the example differs in that RU  303 , BBU  305 , and the L2/L3 processor  306  include a multiple frequencies function. The switch  304  connects BBU  305  and RU  303  one-to-one with respect to the second frequency, while connecting BBU  305  and RU  303  one-to-many with respect to the first frequency in accordance with control from the control unit. However, at the first frequency, when an identical cell ID is allocated, the corresponding L2/L3 processors  306  need to operate in a coordinated manner when CA is used. For example, in  FIG. 16 , coordinated scheduling and the like is performed by sharing the buffer information of the connected terminals, and communication quality information and the like between the L2/L3 processor  306 - 1  and the L2/L3 processor  306 - 2 . 
       FIG. 17  illustrates an example of the operation sequence up to the allocation of the identical cell ID to a plurality of RUs at the first frequency according to the second embodiment of the present invention. Here, for the sake of description, the functions in RU #1 and RU #2 corresponding to the first frequency and the second frequency are respectively denoted by RU #1-1, RU #1-2 and RU #2-1, RU #2-2. Similarly, the functions in BBU #1 and BBU #2 corresponding to the first frequency and the second frequency are respectively denoted by BBU #1-1, BBU #1-2 and BBU #2-1, BBU #2-2. The functions of L2/L3 processor #1 and L2/L3 processor #2 corresponding to the first frequency and the second frequency are respectively denoted by L2/L3 processor #1-1, L2/L3 processor #1-2 and L2/L3 processor #2-1, L2/L3 processor #2-2. However, when the distinction of the frequency is not required, the notations RU #1 and RU #2, BBU #1 and BBU #2, and L2/L3 processor #1 and L2/L3 processor #2 will be used. 
     The basic operation of  FIG. 17  is similar to  FIG. 7 . It is assumed that in the initial state, the connection between RU and BBU is such that RU #1-1 and BBU #1-1, RU #2-1 and BBU #2-1, RU #1-2 and BBU #1-2, and RU #2-2 and BBU #2-2 are respectively connected (S 400 ). The positions of the terminals #1 to #3 are the same as in  FIG. 7  or  12 . However, the terminals #1 to #3 are CA-capable terminals, with the terminal #1 and the terminal #2 implementing CA using cell ID #1 (BBU #1-1, L2/L3 processor #1-1) for PCell, and cell ID #A (BBU #1-2, L2/L3 processor #1-2) (S 401 - 1 , S 401 - 2 ) for SCell. The terminal #3 implements CA using cell ID #2 (BBU #2-1, L2/L3 processor #2-1) for PCell and cell ID #B (BBU #1-2, L2/L3 processor #1-2) for SCell (S 401 - 3 ). The L2/L3 processors #1 and #2, as in S 102  in  FIG. 7 , perform measurement configuration in each terminal for determining whether the terminal is positioned at the boundary of the communication area of RU (S 402 ). The terminal #1 positioned at the boundary of the communication areas of RU #1 and RU #2 makes a measurement report corresponding to the set measurement configuration (S 403 ). The L2/L3 processor, based on the report from the terminal #1, determines that the terminal #1 is positioned at the boundary of the communication areas of RU #1 and RU #2 (S 404 ). As in the case of  FIG. 7 , the L2/L3 processors #1 and #2 count, among the connected terminals, the number of the terminals satisfying the condition set in S 402 , the cell IDs satisfying the condition, and the number of the terminals not satisfying the condition. The number of the terminals herein counted may be distinguished on a frequency by frequency basis; alternatively, a total value for a plurality of frequencies may be calculated. Then, as in S 105 , the information is combined with the frequency number and reported to the control unit (S 405 ). The control unit, as in S 106  of  FIG. 7 , computes the number of the terminals at the communication area boundary of each RU per unit time, and an average number of terminals at the area boundary (S 406 ). For the number of the terminals at the area boundary, the control unit computes a total value for a plurality of frequencies. Then, as in S 107 , cell ID allocation determination is performed. It is now assumed that the control unit determines that an identical cell ID should be allocated to RU #1 and RU #2. While the operation from S 408  to S 414  is similar to S 108  to S 114 , the operation is performed only with respect to the first frequency. As a result, a handover of the terminal #3 occurs from the cell of RU #1-2 (cell ID #2) to the cell of RU #1-1 and 1-2 (cell ID #1). Namely, the cell ID of PCell is modified from cell ID #2 to cell ID #1. However, with respect to the second frequency, the cell ID is not modified, so that the terminal #3 can still utilize the cell of cell ID #B formed by RU #2-2 even after the handover. 
       FIG. 18  illustrates an example of the operation sequence according to the second embodiment where a plurality of RUs have the same cell ID. As the basic operation is similar to FIG.  12 , only differences will be described. In S 502 , the L2/L3 processor notifies the control unit of information of the uplink reference signals of all frequencies, and is notified by the control unit of information of the uplink reference signals of all frequencies. Even when the same cell ID is allocated to RU #1 and RU #2 at the first frequency, the cell IDs are different at the second frequency. Thus, there is an RU (RU #2-2) to be connected for BBU #2-2 and L2/L3 processor #2-2. Thus, the control unit also exchanges the information of the uplink reference signals with the L2/L3 processor #2. In S 503  and S 504 , the uplink reference signals are transmitted at both the first frequency and the second frequency, and the respectively corresponding L2/L3 processors measure the uplink reception power. In S 505 , the measured reception power is notified to the control unit, together with information about the RU number, the terminal ID, the corresponding cell ID, the frequency number and the like. In S 506 , the L2/L3 processor #1 determines, based on the reception power of each RU at the first frequency, whether each terminal is positioned at the area boundary of RU #1 and RU #2. Alternatively, the information of uplink reception power may be exchanged between the L2/L3 processor #1 and the L2/L3 processor #2, and whether the terminal is at the area boundary may be determined by averaging the reception power of a plurality of frequencies. When the terminal is determined to be at the area boundary at either one of the frequencies, the terminal may be determined to be an area boundary terminal. The transmission mode control in S 507  is performed at the first frequency. In S 508 , in accordance with the various methods for selective use of frequencies described with reference to  FIGS. 13 to 15 , scheduling is performed in the L2/L3 processors #1 and #2 in a coordinated manner. For example, the terminal #1 positioned at the boundary of the communication areas of RU #1 and RU #2 performs multiple RU transmission at the first frequency (i.e., PCell) (S 509 ). The terminal #2 and the terminal #3 positioned at the center of the communication areas of RU #1 and RU #2 perform transmission each using a single RU at the second frequency (i.e., SCell) (S 510 ). 
       FIG. 19  illustrates another example of the operation sequence of the second embodiment where a plurality of RUs have the same cell ID. The example differs from  FIG. 18  in that the RU area boundary determination is performed using the measurement report reported from the terminal. When a plurality of frequencies are used, an identical cell ID is allocated to only some of the frequencies (the first frequency). Thus, at the other frequencies (the second frequency), each RU has a different cell ID. Accordingly, by using the reception power of the downlink reference signal at the frequency, it can be determined whether each terminal is positioned at the boundary or the center of the communication area of each RU. 
     In S 602 , a measurement configuration for the measurement and reporting of the reception power of each RU at the second frequency is performed. For the measurement configuration that is set herein, event A6 as defined by LTE may be used. According to event A6, a determination similar to the one for the above-described event A3 is performed for an adjacent cell having the same frequency as SCell and, when a set condition is satisfied, the terminal reports a measurement report (S 603 ). This method may be used when each terminal is using CA. The L2/L3 processor #1 may then determine that the terminal that has reported the measurement report set in S 603  is positioned at the boundary of the communication areas of the RU corresponding to SCell and the RU satisfying the condition (S 604 ). Also, the terminal not satisfying the present condition may be determined to be a terminal positioned at the center of the communication area of the RU corresponding to SCell. Meanwhile, the terminal not using or supporting CA may be set to report the reception power of each cell at the second frequency periodically rather than on an event driven basis. Then, in the L2/L3 processor, it is determined whether the condition of event A6 is satisfied. The L2/L3 processor notifies the control unit of the information about the number of the terminals that has been determined by the above method as being at the area boundary, as in S 405  of  FIG. 17  (S 605 ). The operation from S 606  to S 609  is similar to S 507  to S 510 . The operations for computing the number of the terminals at the communication area boundary of each RU per unit time and the average number of terminals at the area boundary in S 612  are similar to S 406  in  FIG. 17 . For the cell ID allocation determination in S 613 , a method similar to  FIG. 11  may be used. 
     3. Third Embodiment 
       FIG. 20  illustrates an example of the device configuration according to a third embodiment of the present invention. In the third embodiment, a switch  404  is disposed between the L2/L3 processor  406  and BBU  405 . While the example of  FIG. 20  is based on the use of a plurality of frequencies, a single frequency may be used. Further, in the configuration according to the third embodiment, BBU  405  may be disposed toward RU  403 . The functions of an antenna  402 , RU  403 , and a network I/F  408  are similar to  FIG. 16 . A L2/L3 processor  406  notifies BBU  405  of information required by BBU  405  to perform a signal process. For example, the information includes the cell ID used by each BBU  405 , the type of the physical control channel (PDCCH, PHICH, PCFICH) or the content of its information, data of a terminal for which PDSCH is scheduled, the terminal ID or wireless resource allocation information, a precoding matrix, a modulation system, and a coding system. The information may be in accordance with the Femto Application Platform Interface (FAPI) standard, for example. The L2/L3 processor  406  to which a plurality of BBUs  405  are connected outputs, to each BBU  405 , the address of the destination BBU  405  or its number, and information required by the destination BBU  405  to perform a signal process. When the same signal is transmitted from a plurality of RUs  403 , the L2/L3 processor  406  notifies BBU  405  of the same information. The information may include the physical control channel such as CRS, PDCCH, PHICH, or PCFICH, cell system information, a broadcast signal, a synchronization signal, data addressed to a terminal at the boundary of the communication area of RU and its scheduling information. When different signals are transmitted from each of the RUs  403 , the L2/L3 processor  406  notifies the different BBUs  405  of different information. The information mainly includes data addressed to a terminal positioned at the center of the communication area of each RU  403  and corresponding scheduling information. 
     The switch  404  transfers the input information to the destination BBU  405  designated by each L2/L3 processor  406 . Generally, the transmission rate of the information notified from the L2/L3 processor  406  to BBU  405  is lower than the transmission rate of the base band signal output from BBU  405 . Thus, by using the present configuration, the required performance of delay due to the process in the switch  404  can be mitigated. 
     BBU  405  performs a physical layer signal process based on the information notified from the L2/L3 processor  406  via the switch. 
     Alternatively, the L2/L3 processor  406  may attach a flag to each information of which BBU  405  is notified, indicating whether there is a plurality of destinations. When there is a plurality of the destinations, the switch  404  reproduces the information and transfers it to a plurality of BBUs  405 . When the destination is a single BBU  405 , the L2/L3 processor  406  may output the information including the destination BBU  405  number to the switch  404 , and the information may be sorted in the switch  404  in accordance with the destination. 
     In the uplink, each BBU  405  performs demodulation and decode processes, and notifies the L2/L3 processor  406  of the result. The L2/L3 processor  406 , based on the uplink reception result from each BBU  405 , selectively receives only correctly decoded data, for example. 
     When the present configuration is used, while the switch process can be simplified for the downlink, the processes for maximum ratio combining or interference cancellation in the uplink may become difficult. Thus, the configuration of  FIG. 20  may be adopted for the downlink, while the configuration of  FIG. 16  may be adopted for the uplink.