Abstract:
A wireless communication system, using wireless base stations, and other devices, such as a relay node, interoperate with using spectrum aggregation and MIMO. Traffic usage is detected and based on channel utilization relative to capacity, spectrum aggregation is chosen over MIMO under certain conditions. On the other hand, under higher channel utilization system components switch to MIMO modes of operation to reduce demand on channel use, while providing good throughput for communications stations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/394,568, filed Mar. 7, 2012, which is a National Stage of PCT/JP10/04087, filed Jun. 8, 2010, and claims the benefit of priority under 35 U.S.C. §119 of Japanese Application No. 2009-220484, filed Sep. 25, 2009. The entire contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a communication system, a communication method, a base station, and a communication device. 
       BACKGROUND ART 
       [0003]    A relay technique is standardized in IEEE (Institute of Electrical and Electronics Engineers) 802.16j. Further, in 3GPP (3rd Generation Partnership Project) LTE-A (Long Term Evolution-Advanced) also, a technique using a relay node (RN) is studied actively in order to improve the throughput of a user equipment (UE) located at the cell edge. 
         [0004]    Further, in LTE, base stations are operated using a frequency band with a bandwidth of 1 MHz to 20 MHz with respect to a certain center frequency. It is thus not assumed that a communication device uses discrete channels. On the other hand, it is under consideration in LTE-A that the user equipment reserves a band of 20 MHz or more by spectrum aggregation that makes simultaneous use of discrete or sequential channels to achieve higher throughput. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [PTL 1] Japanese Unexamined Patent Application Publication No. 2006-148388 
       
     
         [0006]    Japanese Unexamined Patent Application Publication No. 2006-148388 (JP 2006-148388), attributable to the present inventor, and incorporated herein by reference in its entirety, discloses a radio communication device that includes a plurality of antennas, and uses some antennas for reception processing, for example, as a first reception/transmission process and uses other antennas for transmission processing, for example, as a second reception/transmission process. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    In order to handle the dispersive channels by one receiver “branch” (an antenna, an analog processing unit etc., and sometimes referred to as a receiver “channel”), a filter or FFT compatible with a high bandwidth is necessary. In view of this, it is possible to simplify the configuration of each branch by applying the radio communication device disclosed in the above-described JP 2006-148388 to spectrum aggregation and making different channels (communication channels) correspond to the respective branches (receiver branches). 
         [0008]    However, as recognized by the present inventor, the use of spectrum aggregation causes an increase in the number of channels allocated for communication with one communication device, which puts a severe strain on resources compared to multiple-input, multiple output (MIMO) communication. 
         [0009]    In light of the foregoing, it is desirable to provide a communication system, a base station and a communication device which are novel and improved, and which enable switching between spectrum aggregation mode and MIMO mode in accordance with observed traffic volume for a given channel capacity. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is an explanatory view showing a configuration of a communication system according to an embodiment of the present invention. 
           [0011]      FIG. 2  is an explanatory view showing an example of resource allocation in the case of using the same frequency in UL and DL. 
           [0012]      FIG. 3  is an explanatory view showing an example of resource allocation in the case of using different frequencies in UL and DL. 
           [0013]      FIG. 4  is an explanatory view showing an example of a format of DL radio frame. 
           [0014]      FIG. 5  is an explanatory view showing an example of a format of UL radio frame. 
           [0015]      FIG. 6  is an explanatory view showing a connection processing sequence. 
           [0016]      FIG. 7  is an explanatory view showing an illustrative example of MBSFN transmission/reception processing. 
           [0017]      FIG. 8  is an explanatory view showing an example of frequency allocation in each cell. 
           [0018]      FIG. 9  is a functional block diagram showing a configuration of a user equipment according to an embodiment of the present invention. 
           [0019]      FIG. 10  is an explanatory view showing an illustrative example of grouping of channels. 
           [0020]      FIG. 11  is a functional block diagram showing a configuration of a base station according to an embodiment of the present invention. 
           [0021]      FIG. 12  is an explanatory view showing an example of the degree of congestion of a channel group. 
           [0022]      FIG. 13  is a sequence chart showing a flow for switching of transmission/reception mode. 
           [0023]      FIG. 14  is an explanatory view showing an illustrative example of multi-link connection of a relay node. 
           [0024]      FIG. 15  is an explanatory view showing an illustrative example of multi-link connection of user equipment. 
           [0025]      FIG. 16  is an explanatory view showing an example of a combination of Comp and spectrum aggregation. 
           [0026]      FIG. 17  is an explanatory view showing relay transmission by a relay node. 
           [0027]      FIG. 18  is an explanatory view showing an example of a combination of Comp and spectrum aggregation. 
           [0028]      FIG. 19  is an explanatory view showing relay transmission by a relay node. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    Hereinafter, embodiments of the present invention will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
         [0030]    Further, in this specification and the drawings, each of a plurality of structural elements having substantially the same function is distinguished by affixing a different alphabetical letter to the same reference numeral in some cases. For example, a plurality of structural elements having substantially the same function are distinguished like user equipments  20 A,  20 B and  20 C where necessary. However, when there is no particular need to distinguish between a plurality of structural elements having the same function, they are denoted by the same reference numeral. For example, when there is no particular need to distinguish between the user equipments  20 A,  20 B and  20 C, they are referred to simply as the user equipment  20 . 
         [0031]    Embodiments of the present invention will be described hereinafter in the following order. 
         [0032]    1. Basic Configuration of Communication System 
         [0033]    (Example of Resource Allocation to Each Link) 
         [0034]    (Example of Format of Radio Frame) 
         [0035]    (Connection Processing Sequence) 
         [0036]    (MBSFN) 
         [0037]    (Example of Frequency Allocation to Each Cell) 
         [0038]    2. Illustrative Configuration of Communication System 
         [0039]    2-1. Switching Between Spectrum Aggregation Mode and MIMO Mode 
         [0040]    2-2. Multi-Link Connection 
         [0041]    2-3. Combination of Comp and Spectrum Aggregation 
         [0042]    &lt;1. Basic Configuration of Communication System&gt; 
         [0043]    A basic configuration of a communication system  1  according to an embodiment of the present invention is described hereinafter with reference to  FIGS. 1 to 8 .  FIG. 1  is an explanatory view showing a configuration of the communication system  1  according to an embodiment of the present invention. Referring to  FIG. 1 , the communication system  1  according to the embodiment of the present invention includes base stations  10 A and  10 B, a backbone network  12 , user equipments  20 A,  20 B and  20 X, and relay nodes  30 A and  30 B. The term “node” describes stations, devices, apparatuses, and equipment that relays a wireless signal from one device to another. 
         [0044]    The base station  10  manages communication between the relay node  30  and the user equipment  20  located inside a cell formed by the base station  10 . For example, the base station  10 A manages scheduling information for communication with the user equipment  20 X located inside the cell, and communicates with the user equipment  20 X according to the scheduling information. Further, the base station  10 A manages scheduling information for communication with the relay node  30 A located inside the cell and scheduling information for communication between the relay node  30 A and the user equipment  20 A. 
         [0045]    Note that management of the scheduling information may be performed in cooperation by the base station  10  and the relay node  30 , may be performed in cooperation by the base station  10 , the relay node  30  and the user equipment  20 , or may be performed by the relay node  30 . 
         [0046]    The relay node  30  relays communication between the base station  10  and the user equipment  20  according to the scheduling information managed by the base station  10 . Specifically, the relay node  30  receives a signal transmitted from the base station  10  and transmits the amplified signal to the user equipment  20  by using frequency/time according to the scheduling information in the downlink. With such a relay in the relay node  30 , a signal-to-noise ratio is higher compared to directly transmitting a signal from the base station  10  to the user equipment  20  near the cell edge. A more detailed explanation of the relay node and how it interoperates with a base station and user equipment is described in JP Patent application 2010-040224, filed in the Japanese Patent Office on Feb. 25, 2010, and in JP 2010-040227, filed in the Japanese Patent Office on Feb. 25, 2010, the entire contents of both of which being incorporated herein by reference. 
         [0047]    Likewise, in the uplink also, the relay node  30  relays a signal transmitted from the user equipment  20  to the base station  10  according to the scheduling information managed by the base station  10 , thereby maintaining a high signal-to-noise ratio. Although the case where only the relay node  30 A exists in the cell formed by the base station  10 A, a plurality of relay nodes  30  may exist in the cell formed by the base station  10 A. 
         [0048]    Proposed as the types of the relay nodes  30  are Type 1 and Type 2. The relay node  30  of Type 1 has an individual cell ID and is allowed to manage its own cell. Thus, the relay node  30  of Type 1 operates in such a way that it is recognized as the base station  10  by the user equipment  20 . However, the relay node  30  of Type 1 does not completely operate autonomously, and the relay node  30  performs relay communication within the range of resources allocated by the base station  10 . 
         [0049]    On the other hand, the relay node  30  of Type 2, differently from Type 1, does not have an individual cell ID and supports direct communication between the base station  10  and the user equipment  20 . For example, a relay transmission technique using cooperative relay or network coding is being studied. The following table 1 shows characteristics of Type 1 and Type 2 under study. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Item 
                 Type 1 
                 Type 2 
               
               
                   
               
             
             
               
                 Decision 
                 R1-091098 
                 R1-091632 
               
               
                 Type of Relay 
                 L2 and L3 Relay 
                 L2 
               
               
                 PHY Cell ID 
                 Own cell ID 
                 No cell ID 
               
               
                 Transparency 
                 Non transparent Relay 
                 Transparent Relay node to 
               
               
                   
                 node to UE 
                 UE 
               
               
                 New cell 
                 Create new cell (another 
                 Not create new cell 
               
               
                   
                 eNB) 
               
               
                 RF parameters 
                 Optimized parameters 
                 N/A 
               
               
                 HO 
                 Inter cell HO (generic 
                 HO transparently to UE 
               
               
                   
                 HO) 
               
               
                 Control Channel 
                 Generate synch. channel, 
                 Not generate its own 
               
               
                 Generation 
                 RS, H-ARQ channel and 
                 channel but decodes/ 
               
               
                   
                 scheduling information 
                 forwards donor eNB&#39;s 
               
               
                   
                 etc. 
                 signal to UE 
               
               
                 Backward 
                 Support (appear as a 
                 Support (able to relay also 
               
               
                 compatibility 
                 Rel-8 eNB to Rel-8 UE) 
                 to/from Rel-8 UE) 
               
               
                 LTE-A (Forward 
                 Support (it appear 
                 ?? 
               
               
                 compatibility) 
                 differently than Rel-8 
               
               
                   
                 eNB to LTE-A UE) 
               
               
                 Awareness to MS 
                 ?? (&gt;Rel-8 eNB to 
                 ?? 
               
               
                   
                 LTE-A UEs or Relay) 
               
               
                 Cooperation 
                 Inter cell cooperation 
                 Intra cell cooperation 
               
               
                 Backhaul 
                 Higher 
                 Lower 
               
               
                 utilization 
               
               
                 Usage model 
                 Coverage extension 
                 Throughput enhancement 
               
               
                   
                   
                 and coverage extension 
               
               
                 Cost 
                 Higher 
                 Lower 
               
               
                   
               
             
          
         
       
     
         [0050]    The user equipment  20  communicates with the base station  10  directly or through the relay node  30  according to the scheduling information managed by the base station  10 . Data transmitted or received by the user equipment  20  may be voice data, music data such as music, lectures or radio programs static image data such as photographs, documents, pictures or charts, or video data such as movies, television programs, video programs, game images or the like. Further, the user equipment  20  may be an information processing device having a radio communication function such as a mobile phone or a personal computer (PC). 
         [0051]    A management server  16  is connected to each base station  10  through the backbone network  12 . The management server  16  functions as a mobile management entity (MME). Further, the management server  16  may function as a serving gateway. The management server  16  receives management information indicating the status of cell formed by each base station  10  from the respective base stations  10  and controls communication in the cell formed by each base station  10  based on the management information. The function of the management server  16  may be incorporated into a plurality of physically separated structures in a distributed manner. 
         [0052]    (Example of Resource Allocation to Each Link) 
         [0053]    Resource allocation to each link is described hereinafter. In the following description, a communication path between the base station  10  and the relay node  30  is referred to as a relay link, a communication path between the relay node  30  and the user equipment  20  is referred to as an access link, and a direct communication path between the base station  10  and the user equipment  20  is referred to as a direct link. Further, a communication path toward the base station  10  is referred to as UL (uplink), and a communication path toward the user equipment  20  is referred to as DL (downlink). Communication in each link is based on OFDMA. 
         [0054]    The relay node  30  separates the relay link and the access link by frequency or time in order to avoid interference between the relay link and the access link. For example, the relay node  30  may separate the relay link and the access link in the same direction by TDD (Time Division Duplexing) with use of a common frequency. 
         [0055]      FIG. 2  is an explanatory view showing an example of resource allocation in the case of using the same frequency in UL and DL. Referring to  FIG. 2 , one radio frame is made up of subframes 0 to 9. Further, in the example shown in  FIG. 2 , the relay node  30  recognizes the subframes 8 and 9 as resources for DL of the access link according to a direction from the base station  10  and therefore relays a signal transmitted from the base station  10  to the user equipment  20  with use of the subframes 8 and 9. 
         [0056]    Note that PSC (Primary Synchronization Channel) and SSC (Secondary Synchronization Channel), which are synchronous signals of the downlink, or PBCH (Physical Broadcast CHannel) is allocated to the subframes 0 and 5. Further, a paging channel is allocated to the subframes 1 and 6. 
         [0057]      FIG. 3  is an explanatory view showing an example of resource allocation in the case of using different frequencies in UL and DL. Referring to  FIG. 3 , a frequency f0 is used for DL, and a frequency f1 is used for UL. Further, in the example shown in  FIG. 3 , the relay node  30  recognizes the subframes 6 to 8 of the frequency f0 as resources for DL of the access link according to a direction from the base station  10  and therefore relays a signal transmitted from the base station  10  to the user equipment  20  with use of the subframes 6 to 8 of the frequency f0. 
         [0058]    Note that PSC and SSC, which are synchronous signals of the downlink, are allocated to the subframes 0 and 5 of the frequency f0 (for DL), and a paging channel is allocated to the subframes 4 and 9. 
         [0059]    (Example of Format of Radio Frame) 
         [0060]    Detailed examples of the frame format of DL radio frame and UL radio frame are described hereinafter with reference to  FIGS. 4 and 5 . 
         [0061]      FIG. 4  is an explanatory view showing an example of the format of DL radio frame. The DL radio frame is made up of subframes 0 to 9, each subframe is made up of two 0.5 ms slots, and each 0.5 ms slot is made up of seven OFDM (Orthogonal Frequency Division Multiplexing) symbols. 
         [0062]    As shown in  FIG. 4 , a control channel such as PCFICH (Physical Control Format Indicator CHannel), PHICH (Physical Hybrid ARQ Indicator CHannel) or PDCCH (Physical Downlink Control CHannel) is present in the first to third OFDM symbols at the head of each subframe. 
         [0063]    Each of the above channels contains the following information as an example. 
         [0064]    PCFICH: The number of symbols of PDCCH related to Layer 1 and Layer 2 
         [0065]    PHICH: ACK/NACK for PUSCH 
         [0066]    PDCCH: Downlink control information. Scheduling information (format such as modulation scheme or coding rate) of PDSCH/PUSCH 
         [0067]    Further, one resource block (1RB), which is a minimum unit of resource allocation, is made up of six or seven OFDM symbols and 12 subcarriers. A demodulation reference (reference signal) is present in a part of the resource block. 
         [0068]    Further, SSC, PBCH and PSC are present in the subframes 0 and 5. A free space in the radio frame shown in  FIG. 4  is used as PDSCH (Physical Downlink Shared CHannel). 
         [0069]      FIG. 5  is an explanatory view showing an example of the format of UL radio frame. Like the DL radio frame, the UL radio frame is made up of subframes 0 to 9, each subframe is made up of two 0.5 ms slots, and each 0.5 ms slot is made up of seven OFDM symbols. 
         [0070]    As shown in  FIG. 5 , a demodulation reference (reference signal) is present in each of the 0.5 ms slots, and a CQI measurement reference is present in a distributed manner. The base station  10  or the relay node  30  at the receiving end performs channel estimation by using the demodulation reference and demodulates a received signal according to the channel estimation result. Further, the base station  10  or the relay node  30  at the receiving end measures the CQI measurement reference and thereby acquires CQI with the relay node  30  or the user equipment  20  at the transmitting end. 
         [0071]    Further, a free space in the radio frame shown in  FIG. 5  is used as PUSCH (Physical Uplink Shared CHannel). Note that, upon receiving a request for CQI report, the user equipment  20  or the relay node  30  transmits the CQI report by using PUSCH. 
         [0072]    (Connection Processing Sequence) 
         [0073]    A connection processing sequence between the relay node  30  or the user equipment  20  and the base station  10  is described hereinafter with reference to  FIG. 6 . 
         [0074]      FIG. 6  is an explanatory view showing a connection processing sequence. Referring to  FIG. 6 , the relay node  30  or the user equipment  20  transmits RACH (Random Access CHannel) preamble to the base station  10  (S 62 ). Receiving the RACH preamble, the base station  10  acquires TA (Timing Advance) information and transmits the TA information together with allocated resource information to the relay node  30  or the user equipment  20  (S 64 ). For example, in the case where the transmission timing of the RACH preamble is known, the base station  10  may acquire a difference between the transmission timing and the reception timing of the RACH preamble as the TA information. 
         [0075]    After that, the relay node  30  or the user equipment  20  transmits RRC connection request to the base station  10  by using resources indicated by the allocated resource information (S 66 ). Receiving the RRC connection request, the base station  10  transmits RRC connection resolution indicating a transmission source of the RRC connection request (S 68 ). The relay node  30  or the user equipment  20  can thereby confirm whether the base station  10  has received the RRC connection request. 
         [0076]    Then, the base station  10  transmits connection request indicating that the relay node  30  or the user equipment  20  is making a request for service to the management server  16  that functions as MME (S 70 ). Receiving the connection request, the management server  16  transmits information to be set to the relay node  30  or the user equipment  20  as connection setup (S 72 ). 
         [0077]    Then, the base station  10  transmits RRC connection setup to the relay node  30  or the user equipment  20  based on the connection setup from the management server  16  (S 74 ), and the relay node  30  or the user equipment  20  makes connection setting. After that, the relay node  30  or the user equipment  20  transmits RRC connection complete indicating completion of connection setting to the base station  10  (S 76 ). 
         [0078]    Connection between the relay node  30  or the user equipment  20  and the base station  10  is thereby completed, and communication becomes available. The above-described connection processing sequence is just by way of illustration, and the relay node  30  or the user equipment  20  and the base station  10  may be connected by another sequence. 
         [0079]    (MBSFN) 
         [0080]    Hereinafter, MBSFN (Multimedia Broadcasting Single Frequency Network) transmission that is performed by the base station  10  and an exemplary operation of the relay node  30  in response to the MBSFN transmission are described. 
         [0081]    MBSFN is the mode where a plurality of base stations  10  simultaneously transmits data in a broadcast manner at the same frequency. Therefore, in MBSFN, the relay node  30  of Type 1 that virtually operates as a base station transmits a control channel for DL or the like by using the same frequency as that of the base station  10 . A specific flow of MBSFN transmission/reception processing is described hereinafter with reference to  FIG. 7 . 
         [0082]      FIG. 7  is an explanatory view showing an illustrative example of MBSFN transmission/reception processing. First, as shown in  FIG. 7 , the base station  10  and the relay node  30  simultaneously transmit PDCCH. The base station  10  transmits, after PDCCH, PDSCH for the user equipment  20  and R-PDCCH for controlling a relay. After R-PDCCH, the base station  10  transmits PDSCH for the relay node  30  (relay target data). A non-transmission period comes after PDSCH for the relay node  30 . 
         [0083]    The relay node  30  receives, after transmitting PDCCH, PDSCH (relay target data) from the base station  10  subsequent to a switching period to reception processing. The relay node  30  then switches reception processing to transmission processing in the non-transmission period that comes after PDSCH (relay target data) from the base station  10 . Further, in the next step, the relay node  30  adds PDCCH to decoded PDSCH (relay target data) and then transmits the data to the user equipment  20 . 
         [0084]    The existing user equipment that does not assume the existence of the relay node  30  can thereby make an advantage of the relay by the relay node  30  without confusion. 
         [0085]    (Example of Frequency Allocation to Each Cell) 
         [0086]    An example of frequency allocation to each cell in the case where a plurality of cells are adjacent is described hereinafter. 
         [0087]      FIG. 8  is an explanatory view showing an example of frequency allocation in each cell. In the case where each cell is made up of three sectors, frequencies f1 to f3 are allocated to the respective sectors as shown in  FIG. 8 , thereby suppressing the interference of frequencies at the cell boundary. Such allocation is particularly effective in a densely populated area with heavy traffic. 
         [0088]    In LTE-A, in order to achieve end-to-end high-throughput, various novel techniques such as spectrum aggregation, network MIMO, uplink multi-user MIMO and relay technique are being studied. Therefore, with the advent of high-throughput novel mobile applications, there is a possibility that exhaustion of frequency resources appears as an issue in a suburban area also. Further, in the introduction of LTE-A, it is highly possible that the installation of the relay node  30  will be activated for the purpose of achieving infrastructure development at low costs. 
         [0089]    &lt;2. Illustrative Configuration of Communication System&gt; 
         [0090]    The basic configuration of the communication system  1  according to the embodiment is described above with reference to  FIGS. 1 to 8 . Hereinafter, an illustrative configuration of the communication system  1  according to the embodiment is described. 
         [0091]    (2-1. Switching Between Spectrum Aggregation Mode and MIMO Mode) 
         [0092]    Recently, it has been studied that a communication device (the relay node  30  or the user equipment  20 ) reserves a band of 20 MHz or more by spectrum aggregation that makes simultaneous use of discrete or sequential channels. However, in order to handle the dispersive channels by one branch (transmission/reception resources such as an antenna and an analog processing unit), a filter or FFT compatible with a high bandwidth is necessary. Further, the use of spectrum aggregation causes an increase in the number of channels allocated for communication with one communication device, which would raise a concern that a severe strain is placed on resources compared to MIMO communication. 
         [0093]    Against the above background, the communication system  1  according to an embodiment has been invented. According to the embodiment, it is possible to simplify the configuration of each branch of a communication device and selectively use spectrum aggregation mode and MIMO mode in accordance with a traffic volume. Hereinafter, the user equipment  20  and the base station  10  that constitute the communication system  1  according to the embodiment are described in detail. Moreover, more detailed explanations of MIMO operation are provided in PCT International Publication WO 2004/030238, and U.S. Pat. No. 6,862,271, the entire contents of both of which being incorporated herein by reference. 
         [0094]      FIG. 9  is a functional block diagram showing the configuration of the user equipment  20  according to the embodiment of the present invention. Referring to  FIG. 9 , the user equipment  20  includes an analog processing unit  210 , a digital processing unit  230 , a control unit  242 , and a channel selection unit  244 . The analog processing unit  210  is made up of a plurality of branches A, B and C. 
         [0095]    Each branch includes an antenna  220  and a signal processing unit such as a BPF (Band-Pass Filter)  222 , an AGC (Automatic Gain Control)  224 , a DC (Down Converter)/UC(Up Converter)  226  and an AD/DA  228 . The respective branches (transmission/reception resources) may include not only elements in the analog processing unit  210  but also elements in the digital processing unit  230  such as FFT and IFFT. 
         [0096]    The antenna  220  receives a radio signal from the base station  10  or the relay node  30  and acquires an electrical high-frequency received signal. Further, the antenna  220  transmits a radio signal to the base station  10  or the relay node  30  based on a high-frequency transmission signal supplied from the BPF  222 . 
         [0097]    The BPF  222  passes certain frequency components of a high-frequency received signal acquired by the antenna  220 . Further, the BPF  222  passes certain frequency components of a high-frequency transmission signal supplied from the AGC  224 . The AGC  224  makes automatic gain control of a high-frequency received signal supplied from the BPF  222  and a high-frequency transmission signal supplied from the DC/UC  226 . 
         [0098]    The DC/UC  226  down-converts a high-frequency received signal supplied from the AGC  224  to a baseband received signal. Further, the DC/UC  226  up-converts a baseband transmission signal supplied from the AD/DA  228  to a high-frequency transmission signal. 
         [0099]    The AD/DA  228  converts a baseband received signal supplied from the DC/UC  226  from analog to digital. Further, the AD/DA  228  converts a baseband transmission signal supplied from the digital processing unit  230  from digital to analog. 
         [0100]    The digital processing unit  230  includes an FFT that performs fast Fourier transform on a baseband received signal supplied from each branch, a P(Parallel)/S(Serial), a demodulator, a decoder and so on as the elements for reception. Further, the digital processing unit  230  includes an encoder, a modulator, an S/P, an IFFT and so on as the elements for transmission, and supplies a baseband transmission signal on which a subcarrier signal is superimposed, for example, to the AD/DA  228 . Further, the digital processing unit  230  has MIMO processing function that enables MIMO communication. 
         [0101]    The channel selection unit  244  selects channels (or channel group) to be used for communication in spectrum aggregation mode. Inappropriate selection can cause a problem in the following cases:
       When a channel group to be processed by a certain branch exceeds the limits of the capacity of the branch; i.e., when channels in the channel group to be processed are too dispersed.   When there is a large difference in propagation path characteristics between channels in a channel group to be processed by a certain branch, and the expected improvement in throughput is not achieved.       
 
         [0104]    Therefore, the channel selection unit  244  makes channel selection by the following procedure: 
         [0105]    (1) Acquire information (use channel information) such as the center frequency and the bandwidth of the respective channels used by the connected base station  10   
         [0106]    (2) Determine resources (data rate) to be reserved for the user equipment  20   
         [0107]    (3) Classify a plurality of channels used by the connected base station  10  as a channel group 
         [0108]    (4) Determine a combination of a channel group and a branch. 
         [0109]    Specifically, in the above (3), the channel selection unit  244  classifies one or more than one channels which can be simultaneously processed by each branch as a group according to a settable center frequency, filter size, FFT size or the like. For example, the channel selection unit  244  classifies channels into groups in such a way that the bandwidth of each group does not exceed the bandwidth that can be handled by the FFT. Grouping of channels is specifically described hereinafter with reference to  FIG. 10 . 
         [0110]      FIG. 10  is an explanatory view showing an illustrative example of grouping of channels. In the example shown in  FIG. 10 , use channels of the base station  10  are O, P, Q, R, S, T, U and so on. In this case, the channel selection unit  244  classifies the channels O, P and Q as a channel group #1, classifies the channels R, S and T as a channel group #2, and classifies the channel U as a channel group #3, for example. 
         [0111]    Further, regarding the above (4), each branch performs signal processing on known signals (e.g. reference signals) of all channels transmitted from the base station  10 . Then, the channel selection unit  244  averages out the communication quality such as reception level or SINR of channels constituting each channel group and thereby acquires the communication quality of each channel group with respect to each branch. For example, the channel selection unit  244  averages out the communication quality of the channels O, P and Q by the branch A and thereby acquires the communication quality of the channel group #1. 
         [0112]    Further, the channel selection unit  244  combines each branch and a channel group with the highest communication quality in each branch. For example, the channel selection unit  244  combines the branch A and the channel group #1, combines the branch B and the channel group #2, and combines the branch C and the channel group #3. Note that if one channel group has the highest communication quality in different branches, the channel selection unit  244  may combine the branch in which the communication quality of the channel group is higher and the channel group. Further, the channel selection unit  244  may combine the other branch and the channel group with the second highest communication quality in the other branch. 
         [0113]    Referring back to  FIG. 9 , the configuration of the user equipment  20  is further described hereinafter. The control unit  242  of the user equipment  20  controls the overall operation in the user equipment  20 , such as transmission processing, reception processing, and connection processing with the relay node  30  or the base station  10 . For example, the control unit  242  performs TPC (Transmit Power Control), CQI report transmission control or the like. 
         [0114]    Further, the control unit  242  requests the base station  10  to use the channel group selected by the channel selection unit  244  (i.e. the channel group combined with the branch). Although a method of the request is not particularly limited, exemplary methods are as follows
       Acquire channels for use by autonomously making a connection request by a given slot (RACH: Random Access CHannel) with respect to each selected channels. The connection request may be made from the branch combined with the channel.   Notify the selected channels by using one channel, not with respect to each selected channels. One channel may be any one of the selected channels or another channel. Further, one channel may be transmitted from the base station  10  through a broadcast channel such as PBCH or operating parameter information of an adjacent base station for handover. Further, the notification may be contained in any transmission signal (e.g. RACH) in a connection processing sequence (Call set up).       
 
         [0117]    Based on the above request, the base station  10  allocates resources in the channels to the user equipment  20 , and the base station  10  and the user equipment  20  can thereby communicate in the spectrum aggregation mode. For example, the base station  10  can transmit radio signals by using the channel groups #1 to #3, the user equipment  20  performs reception processing of the radio signal using the channel group #1 by the branch A, performs reception processing of the radio signal using the channel group #2 by the branch B, and performs reception processing of the radio signal using the channel group #3 by the branch C. 
         [0118]    Note that the channel selection unit  244  and the control unit  242  may execute the selection of the channel group and the use request to the base station  10  as described above according to a command from the base station  10 . Further, the configuration of the user equipment  20  is applicable also to the relay node  30 . Specifically, the relay node  30  may include a plurality of branches which respectively transmit and receive signals using different channels to thereby realize spectrum aggregation. At this time, the relay node  30  may select a channel group with the suitable communication quality and request the base station  10  to use the selected channel group by the above-described method. 
         [0119]    Hereinafter, the configuration of the base station  10  is described with reference to  FIG. 11 . 
         [0120]      FIG. 11  is a functional block diagram showing the configuration of the base station  10  according to the embodiment of the present invention. Referring to  FIG. 11 , the base station  10  includes an analog processing unit  110 , a digital processing unit  130  and a control unit  142 . Further, the analog processing unit  110  is made up of a plurality of branches A, B and C. 
         [0121]    Each branch includes an antenna  120  and a signal processing unit such as a BPF  122 , an AGC  124 , a DC/UC  126  and an AD/DA  128 . The respective branches may include not only elements in the analog processing unit  110  but also elements in the digital processing unit  130  such as FFT and IFFT. Further, although the base station  10  includes three branches in the example shown in  FIG. 11 , the number of branches in the base station  10  is not particularly limited. 
         [0122]    The antenna  120  receives a radio signal from the user equipment  20  or the relay node  30  and acquires an electrical high-frequency received signal. Further, the antenna  120  transmits a radio signal to the user equipment  20  or the relay node  30  based on a high-frequency transmission signal supplied from the BPF  122 . 
         [0123]    The BPF  122  passes certain frequency components of a high-frequency received signal acquired by the antenna  120 . Further, the BPF  122  passes certain frequency components of a high-frequency transmission signal supplied from the AGC  124 . The AGC  124  makes automatic gain control of a high-frequency received signal supplied from the BPF  122  and a high-frequency transmission signal supplied from the DC/UC  126 . 
         [0124]    The DC/UC  126  down-converts a high-frequency received signal supplied from the AGC  124  to a baseband received signal. Further, the DC/UC  126  up-converts a baseband transmission signal supplied from the AD/DA  128  to a high-frequency transmission signal. 
         [0125]    The AD/DA  128  converts a baseband received signal supplied from the DC/UC  126  from analog to digital. Further, the AD/DA  128  converts a baseband transmission signal supplied from the digital processing unit  130  from digital to analog. 
         [0126]    The digital processing unit  130  includes an FFT that performs fast Fourier transform on a baseband received signal supplied from each branch, a P/S, a demodulator, a decoder and so on as the elements for reception. Further, the digital processing unit  130  includes an encoder, a modulator, an S/P, an IFFT and so on as the elements for transmission, and supplies a baseband transmission signal on which a subcarrier signal is superimposed, for example, to the AD/DA  128 . Further, the digital processing unit  130  has MIMO processing function that enables MIMO communication. 
         [0127]    The control unit  142  controls the overall communication in the cell formed by the base station  10 , such as transmission processing, reception processing, connection processing with the relay node  30  or the user equipment  20 , and management of scheduling information. For example, when use (connection) of a plurality of channels is requested from the relay node  30  or the user equipment  20 , the control unit  142  may execute a connection processing sequence with the relay node  30  or the user equipment  20  and make scheduling of a resource block in the requested channels to the relay node  30  or the user equipment  20 . The base station  10  can thereby realize spectrum aggregation that uses a plurality of channels requested from the relay node  30  or the user equipment  20 . Note that, in spectrum aggregation, the control unit  142  may associate each of the requested channels to any branch of the base station  10  and communicate with the relay node  30  or the user equipment  20  by using the associated branch. 
         [0128]    Further, the control unit  142  functions as a mode control unit that switches the spectrum aggregation mode to MIMO mode in accordance with the degree of congestion (traffic volume) of channels being used for spectrum aggregation. The mode switching is described herein below. More generally, when the channel usage (e.g., traffic volume, SNIR level, % of channel capacity, error rate, spectral occupancy, number of users, reserved etc.) is detected to be above a certain level the control unit  142  switches to the MIMO mode of operation. 
         [0129]    As a communication technique for improving throughput, MIMO is used besides spectrum aggregation. MIMO is a technique that transmits a plurality of signal sequences in parallel from a plurality of transmission antennas, receives them with a plurality of receptions antennas, and separates the plurality of signal sequences by using the independence of propagation path characteristics between the plurality of transmission antennas and the plurality of receptions antennas. 
         [0130]    However, in MIMO, there is a case where the independence of propagation path characteristics between the plurality of transmission antennas and the plurality of receptions antennas is low, and high throughput is unachievable in this case. On the other hand, in spectrum aggregation, throughput increases with the number of channels. Therefore, high throughput is more reliably achieved by spectrum aggregation than MIMO. 
         [0131]    In view of the above, the control unit  142  gives a higher priority to the operation in the spectrum aggregation mode. This ensures higher throughput. On the other hand, when the traffic volume increases and the degree of congestion (or usage) becomes higher, it is desirable to reduce the occupied bandwidth per user. Therefore, the control unit  142  switches the spectrum aggregation mode to the MIMO mode according to an increase in the traffic volume. 
         [0132]    For example, in the case where the channel groups #1 to #3 are used for spectrum aggregation communication with the relay node  30  or the user equipment  20  and when the degree of congestion of the channel groups #1 and #2 exceeds a threshold as shown in  FIG. 12 , the control unit  142  switches the spectrum aggregation mode to the MIMO mode. The control unit  142  may use the channel group #3 with the degree of congestion below the threshold in the MIMO mode. Note that the degree of congestion may be the absolute traffic volume in each channel group or the resource usage rate in each channel group. Further, the threshold of the degree of congestion may be different among channel groups. 
         [0133]    Further, when the degree of congestion of a certain channel group exceeds the threshold but the degree of congestion of a plurality of channel groups remains below the threshold, the control unit  142  may continue to operate in the spectrum aggregation mode by excluding the certain channel group with the degree of congestion exceeding the threshold. For example, when the degree of congestion of only the channel group #1 exceeds the threshold, the control unit  142  may continue to perform spectrum aggregation by using the channel groups #2 and #3. 
         [0134]    Further, upon switching to the MIMO mode, the control unit  142  may transmit trigger information for notifying (prompting) the switching to the relay node  30  or the user equipment  20  which is the other end of communication by spectrum aggregation. For example, the control unit  142  may transmit the trigger information with use of PDCCH or PDSCH. Further, the trigger information may contain channel information (center frequency, bandwidth etc.) used for MIMO communication or information indicating switching timing. 
         [0135]    The control unit  242  of the relay node  30  or the user equipment  20  can thereby switch the transmission/reception mode of the analog processing unit  210  and/or the digital processing unit  230  to the MIMO mode based on the trigger information. Note that the analog processing unit  210  and/or the digital processing unit  230  waits for receiving a MIMO preamble upon switching to the MIMO mode. 
         [0136]    The configurations of the user equipment  20  and the base station  10  are described above. Hereinafter, a series of flow for switching of the transmission/reception mode is described with reference to  FIG. 13 . 
         [0137]      FIG. 13  is a sequence chart showing a flow for switching of the transmission/reception mode. First, the user equipment  20  acquires information of a plurality of channels used by the base station  10  in response to a command from the base station  10 , for example (S 404 ). After that, the user equipment  20  classifies the plurality of channels used by the base station  10  into channel groups (S 408 ). Specifically, the user equipment  20  classifies one or more than one channels which can be simultaneously processed by each branch as one channel group. 
         [0138]    Then, the user equipment  20  determines a combination of each branch and a channel group with the highest communication quality in each branch (S 412 ), and requests the base station  10  to use the channel group whose combination with the branch is determined (S 416 ). After that, the base station  10  performs connection processing with the user equipment  20  and allocates a resource block in the requested channels to the user equipment  20 , and the base station  10  and the user equipment  20  can thereby perform data communication by spectrum aggregation (S 420 ). 
         [0139]    After that, when the degree of congestion of the traffic in the cell exceeds a certain criterion (YES in S 424 ), the base station  10  transmits trigger information indicating switching from the spectrum aggregation mode to the MIMO mode to the user equipment  20  (S 428 ). Based on the trigger information, the user equipment  20  switches the transmission/reception mode to the MIMO mode and then performs data communication with the base station  10  by MIMO (S 432 ). Note that, when the congestion of the traffic in the cell is reduced, the base station  10  may give a command to execute the processing from S 404  to the user equipment  20  for switching to the spectrum aggregation mode. 
         [0140]    As described above, when operating in the spectrum aggregation mode, the base station  10  switches the transmission/reception mode to the MIMO mode according to an increase in the traffic volume. It is thus possible to ensure high throughput by the spectrum aggregation mode when the traffic volume is low, and reduce the occupied bandwidth per user by the MIMO mode when the traffic volume increases. 
         [0141]    (2-2. Multi-Link Connection) 
         [0142]    When there are more available resources in another base station  10  than in the connected base station  10 , the relay node  30  can switch the relay link to that base station  10  to thereby make effective use of the resources. 
         [0143]    However, if a connection processing sequence (Call set up) with another base station  10  is performed each time switching the relay link, a switching delay due to the multi-procedure occurs. In light of this, the relay node  30  according to the embodiment creates multi-link connection with a plurality of base stations  10  with use of a plurality of branches to thereby reduce the switching delay. This is described in detail below. 
         [0144]    First, the relay node  30  acquires use channel information (center frequency, bandwidth etc.) of a plurality of connectable base stations  10 . Then, the relay node  30  makes Call set up with the plurality of connectable base stations  10  and completes the procedure up to RRC connection complete. The relay node  30  is thereby in multi-link connection with the plurality of base stations  10 . 
         [0145]      FIG. 14  is an explanatory view showing an illustrative example of multi-link connection of the relay node  30 . In the example shown in  FIG. 14 , the relay node  30  is in multi-link connection with the base station  10 A and the base station  10 B. Note that the relay node  30  can make Call set up in parallel with use of a plurality of branches (or links). For example, as shown in  FIG. 14 , the relay node  30  may simultaneously make Call set up with the base station  10 A with use of the branch A (or link A) and Call set up with the base station  10 B with use of the branch B (or link B). 
         [0146]    After that, the relay node  30  uses the relay link with the highest gain among the relay links with the plurality of base stations  10 . For example, if the gain of the relay link with the base station  10 A is higher than the gain of the relay link with the base station  10 B in the example shown in  FIG. 14 , the relay node  30  selects use of the relay link with the base station  10 A. Specifically, the relay node  30  relays communication related to the user equipment  20  by using the relay link with the base station  10 A, and sets the relay link with the base station  10 B as a standby link. 
         [0147]    The base station  10  may add a specifier that specifies whether the relay link with the connected relay node  30  is a standby link or not to an interface (S1-MME IF) between the management server  16  (MME) and the base station  10 . For example, in the example shown in  FIG. 14 , the base station  10 B may add the specifier that specifies that the relay link with the relay node  30  is a standby link to the interface with the management server  16 . The management server  16  or the base station  10  can thereby perform processing according to whether each relay link is a standby link or not. For example, the management server  16  or the base station  10  may give a higher priority to scheduling of the relay link which is not a standby link, and may approve a request related to a standby link if there are available resources. Further, the relay node  30  may use different branches for different links. For example, the relay node  30  may use the branch A for the relay link with the base station  10 A, use the branch B for the relay link with the base station  10 B, and use the branch N (or link N) for the access link with the user equipment  20 . 
         [0148]    After that, when the necessity for reducing the traffic or allocating resources or the like arises, the relay node  30  selects use of the standby link. For example, the relay node  30  may switch the relay link for use from the relay link with the base station  10 A to the relay link with the base station  10 B. Note that the relay node  30  may take necessary steps for obtaining resources related to the relay link with the base station  10 B while relaying communication related to the user equipment  20  by using the relay link with the base station  10 A. For example, the relay node  30  may make advance contact with the management server  16  through the base station  10  about resources intended to be obtained (resources likely to be switched). It is thereby expected to instantaneously respond to a request for obtaining resources from the relay node  30 . 
         [0149]    By the above manner, it is possible to reduce the delay time from the rise of the necessity to switch the relay link to the switching of the relay link. The same procedure is applicable to the access link also. Specifically, the user equipment  20  can reduce the switching delay of the access link by creating multi-link connection with a plurality of connectable relay nodes  30 . 
         [0150]      FIG. 15  is an explanatory view showing an illustrative example of multi-link connection of the user equipment  20 . In the example shown in  FIG. 15 , the relay node  30 A is connected with the base station  10 A, the relay node  30 B is connected with the base station  10 B, and the user equipment  20  is in multi-link connection with the relay node  30 A and the relay node  30 B. In this case, the user equipment  20  can switch the access link for use between the access link with the relay node  30 A and the access link with the relay node  30 B according to need. 
         [0151]    (2-3. Combination of Comp and Spectrum Aggregation) 
         [0152]    Recently, CoMP (Coordinated Multipoint Transmission) has been studied as a technique of improving the link tolerance with respect to user equipment existing at the cell edge. Comp is a technique in which a plurality of adjacent base stations simultaneously transmit the same signal with use of the same channel. An embodiment that combines Comp and spectrum aggregation is described hereinafter. 
       Example 1 
       [0153]    In this example, when the traffic of a certain base station  10  is congested and a plurality of channels are not allocable to one user equipment  20 , the base station  10  transmits a signal to the user equipment  20  by using one channel and, simultaneously, a nearby base station  10  transmits a signal to the user equipment  20  by using a different channel. Then, the relay node  30  receives the signals that are transmitted from a plurality of base stations  10  using different channels and transmits them to the user equipment  20 . It is thereby possible to improve the throughput of communication related to the user equipment  20 . This is specifically described hereinafter with reference to  FIGS. 16 and 17 . 
         [0154]      FIG. 16  is an explanatory view showing an example of a combination of Comp and spectrum aggregation. As shown in  FIG. 16 , the base station  10 A transmits a signal to the user equipment  20  by using f1 and, simultaneously, the base station  10 B and the base station  10 C transmit signals to the user equipment  20  by using f5 and f9, respectively. 
         [0155]    Then, the relay node  30  receives the signals transmitted from the respective base stations  10  and transmits them to the user equipment  20 . The relay node  30  may perform communication with the respective base stations  10  by using different branches. For example, the relay node  30  may communicate with the base station  10 A by using the branch A, communicate with the base station  10 B by using the branch B, and communicate with the base station  10 C by using the branch C. 
         [0156]    Further, although the relay node  30  receives signals to the user equipment  20  from the respective base stations  10  with discrete channels in the frequency domain, the relay node  30  relays the signals to the user equipment  20  by using less dispersed channels. For example, when the relay node  30  receives signals with channels f1, f5 and f9 which are discrete in the frequency domain as shown in  FIG. 17 , the relay node  30  may relay the signals to the user equipment  20  by using channels f4, f5 and f6 which are consecutive in the frequency domain. Because the user equipment  20  can thereby receive the signals with the channels f4, f5 and f6 which are consecutive in the frequency domain, it is possible to reduce the processing load of the user equipment  20 . 
         [0157]    Although the case where the number of channels for transmission is the same as the number of channels for reception is shown in  FIG. 17 , the number of channels for transmission may be smaller than the number of channels for reception. For example, the number of channels for reception may be three, and the number of channels for transmission may be two. In this case, a coding rate in the channels for transmission may be set higher than a coding rate in the channels for reception. Further, the number of channels for transmission may be one. 
         [0158]    Further, a method of selecting a channel to be used for transmission is not particularly limited. For example, a channel to be used for transmission may be selected from channels close to the frequency band in which high SINR is obtained with the user equipment  20 . 
       Example 2 
       [0159]    In this example, the base station  10  transmits signals to the user equipment  20  belonging thereto with a plurality of channels by spectrum aggregation. Then, a nearby base station  10  also transmits a signal in the channel with a large attenuation among the signals in the plurality of channels transmitted from the base station  10 , and the relay node  30  relays the signal to the user equipment  20 . It is thereby possible to strengthen the signal transmitted from the base station  10 . This is specifically described hereinafter with reference to  FIGS. 18 and 19 . 
         [0160]      FIG. 18  is an explanatory view showing an example of a combination of Comp and spectrum aggregation. As shown in  FIG. 18 , the base station  10 A transmits signals to the user equipment  20  by using f1, f3 and f6. If the attenuation of f3 and f6 is large (when needed SNIR becomes not satisfied), the base station  10 B and the base station  10 C also transmit the signals that are transmitted by the base station  10 A by using f3 and f6. 
         [0161]    For example, the signal transmitted by the base station  10 A with use of f3 is transmitted also by the base station  10 B with use of f2, and the signal transmitted by the base station  10 A with use of f6 is also transmitted by the base station  10 C with use of f7. Then, as shown in  FIG. 19 , the relay node  30  transmits the signal received from the base station  10 B with f2 to the user equipment  20  with use of f3 and transmits the signal received from the base station  10 C with f7 to the user equipment  20  with use of f6. In this configuration, it is possible to strengthen the signals transmitted from the base station  10 A by using f3 and f6. Although the case where the base stations  10 B and  10 C strengthen the signals by using frequencies different from that of the base station  10 A is described above, the base stations  10 B and  10 C may strengthen the signals by using the same frequency as that of the base station  10 A. For example, the base station  10 B may use f3, and the base station  10 C may use f6. 
         [0162]    Further, in both the example 1 and the example 2, the management server  16  (MME/Serving Gateway) that monitors the respective links among the base station  10 , the relay node  30  and the user equipment  20  serves an important role. Further, in the example 1, information for making spectrum aggregation between a plurality of adjacent base stations  10  in cooperation with each other is transmitted and received by the X2 interface between the base stations  10  and the S1 interface between the base station  10  and the management server  16 . The information may be channel measurement report list with respect to each channel used for spectrum aggregation, position information and capability (bandwidth that can be transmitted and received at a time etc.) of the relay node  30  or the user equipment  20 , extra resource information of each base station  10  or the like. In the example 2 also, information for strengthening spectrum aggregation channels is transmitted and received by the X2 interface and the S1 interface. 
         [0163]    Although preferred embodiments of the present invention are described in detail above with reference to the appended drawings, the present invention is not limited thereto. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 
         [0164]    For example, it is not always necessary to execute the respective steps in the processing of the communication system  1  in this specification in chronological order according to the sequence shown in the sequence charts. For example, the respective steps in the processing of the communication system  1  may be executed in the sequence different from the sequence shown in the sequence charts or may be executed in parallel. 
         [0165]    Furthermore, it is possible to create a computer program that causes hardware such as a CPU, ROM and RAM incorporated in the base station  10 , the user equipment  20  and the relay node  30  to function equally to the respective elements of the base station  10 , the user equipment  20  and the relay node  30  described above. Further, a memory medium that stores such a computer program may be provided. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               10 A,  10 A,  10 B,  10 C Base station 
               20  User equipment 
               30 ,  30 A,  30 B Relay node 
               110 ,  210  Analog processing unit 
               130 ,  230  Digital processing unit 
               142 ,  242  Control unit 
               244  Channel selection unit