Abstract:
Provided is a radio communication system using a relay transmission technique capable of optimizing use of radio resources in a radio base station and preventing reduction in capacity of the whole system. The relay frequency allocation method of the present invention has the steps of: radio base stations ( 10   a ) and ( 10   b ) transmitting downlink data to a relay station ( 30 ) using respective backhaul links established between the relay station ( 30 ) and the radio base stations ( 10   a ) and ( 10   b ); and the relay station ( 30 ) transmitting the downlink data received from the radio base stations ( 10   a ) and ( 10   b ), to a relay terminal ( 20   b ) by using an access link established between the relay station ( 30 ) and the relay terminal ( 20   b ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a relay transmission method, a relay station and a radio base station. 
       BACKGROUND ART 
       [0002]    In 3GPP (3 rd  Generation Partnership Project), standardization of LTE-Advanced (LTE (Long Term Evolution)-A) has been fostered as the 4 th  generation mobile communication system to realize further higher-speed and larger-capacity communications than LTE which is development standard in the 3 rd  generation mobile communication system. LTE-A has important issues to improve throughputs of cell-edge users as well as to realize higher-speed and larger-capacity communications, and as a way of this, study has been made of a relay transmission technique for relaying radio communications between a radio base station and a mobile terminal by a relay station. With use of this relay transmission technique, it is expected to extend the coverage effectively. 
         [0003]    The relay transmission technique includes layer 1 relay, layer 2 relay and layer 3 relay. The layer 1 relay is an AF (Amplifier and Forward) type relay technique of performing power amplification of downlink reception RF signals from a radio base station and transmitting the signals to a mobile terminal. This technique is called booster or repeater. In this technique, uplink reception RF signals from the mobile terminal are also power-amplified in the same manner and transmitted to the radio base station. The layer 2 relay is a DF (Decode and Forward) type relay technique of performing demodulation and decoding of downlink reception RF signals from the radio base station, then performing coding and modulation of the signals again and transmitting the signals to the mobile terminal. The layer 3 relay is a relay technique of performing demodulating and decoding on downlink reception RF signals from the radio base station, then, reproducing user data, performing the processing for radio-transmitting user data again (cyphering, user data division and combining processing and so on), performing coding and modulating of the signals and then transmitting the signals to the mobile terminal. Now in 3GPP, in view of improvement of reception performance by noise cancellation and study of standard specification and easy implementation, standardization has been advanced of the layer 3 relay. 
         [0004]      FIG. 1  is a diagram illustrating an overview of the layer 3 relay. A relay station (RN) of the layer 3 relay is characterized by not only performing user data reproducing processing, modulation and demodulation, coding and decoding processing but also having a specific cell ID (PCI: Physical Cell ID) which is different from that of a radio base station (eNB). With this characteristic, a mobile terminal (UE) recognizes a cell B provided by the relay station as a cell different from the cell A provided by the radio base station. And, control signals of the physical layer such as CQI (Channel Quality Indicator) and HARQ (Hybrid Automatic Repeat reQuest) are terminated at the relay station. Therefore, the relay station is recognized as a radio base station seen from the mobile terminal. In view of this, mobile terminals having only LTE functions are also allowed to be connected to the relay station. 
         [0005]    And, it is considered that the backhaul link as a radio link between the radio base station and the relay station and the access link between the relay station and the mobile terminal are used at different frequencies or same frequencies. In the latter case, when the transmission processing and reception processing are performed simultaneously by the relay station, transmission signals loop around to the receiver of the relay station, which causes interference unless sufficient isolation is assured between transmission and reception circuits. Therefore, as illustrated in  FIG. 2 , when both the links are used at the same frequencies (f1), radio resources of the backhaul link and the access link (eNB transmission and relay transmission) are subjected to TDM (Time Division Multiplexing) and controlled in such a manner as to prevent transmission and reception from being performed simultaneously at the relay station (Non Patent Literature 1). In view of this, for example, on the downlink, the relay station is prevented from transmitting downlink signals to the mobile terminal while it receives downlink signals from the radio base station. 
       CITATION LIST 
     Non Patent Literature 
       [0000]    
       
         Non-Patent Literature 1: 3GPP, TS36.814 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    In the radio communication system using the relay transmission technique as described above, there are demands for optimizing use of radio resources in the radio base station and preventing the reduction in capacity of the entire system. 
         [0008]    The present invention was carried out in view of the foregoing and aims to provide a relay transmission method, a relay station and a radio base station which are all capable of, in a radio communication system using a relay transmission technique, optimizing use of radio resources in the ratio base station and preventing reduction in capacity of the entire system. 
       Solution to Problem 
       [0009]    A first aspect of the present invention is a relay transmission method comprising the steps of: transmitting, at each of a plurality of radio base stations, downlink data to a relay station by using a first radio link established between each of the plurality of radio base station and the relay station; and transmitting, at the relay station, the downlink data received from each of the plurality of radio base stations, to a mobile terminal by using a second radio link established between the relay station and the mobile terminal. 
         [0010]    A second aspect of the present invention is a relay station comprising: a receiving section configured to receive downlink data from each of a plurality of radio base stations by using a first radio link established between each of the plurality of radio base stations and the relay station; and a transmitting section configured to transmit downlink data received from each of the plurality of radio base stations to a mobile terminal by using a second radio link established between the relay station and the mobile terminal. 
         [0011]    A third aspect of the present invention is a radio base station comprising: a determining section configured to determine distribution of downlink data from a plurality of radio base stations to a relay station; and a transmitting section configured to transmit the downlink data distributed to the radio base station by the determining section, to the relay station by using a radio link established between the relay station and the radio base station. 
       Advantageous Effects of Invention 
       [0012]    According to the present invention, it is possible to provide a relay transmission method, a relay station and a radio base station that are all capable of, in a radio communication system using a relay transmission technique, optimizing use of radio resources in the ratio base station and preventing reduction in capacity of the entire system. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a diagram for explaining a relay transmission technique; 
           [0014]      FIG. 2  is a diagram for explaining radio resources of backhaul link and access link; 
           [0015]      FIG. 3  provides diagrams for explaining reduction in radio resources that can be allocated to a macro terminal; 
           [0016]      FIG. 4  is a diagram for explaining a relay transmission method according to the present invention; 
           [0017]      FIG. 5  is a diagram for explaining the relay transmission method according to the present invention; 
           [0018]      FIG. 6  is a block diagram illustrating a configuration of a radio base station according to an embodiment of the present invention; 
           [0019]      FIG. 7  is a block diagram illustrating a configuration of a relay station according to the embodiment of the present invention; and 
           [0020]      FIG. 8  is a block diagram illustrating a configuration of a macro terminal according to the embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    In the LTE-A system, as illustrated in  FIG. 3A , a radio base station  10   a  performs radio communication with a mobile terminal  20   a  by using an access link which is a radio link established between the radio base station  10   a  and the mobile terminal  20   a . Further, in the LTE-A system, as illustrated in  FIG. 3B , the radio base station  10   a  performs radio communication with the mobile terminal  20   a  by using an access link. In addition, for the purpose of improving the throughput of a cell-edge mobile terminal  20   b , the radio base station  10   a  performs relay communication with a mobile terminal  20   b  via a relay station by using a backhaul link that is a radio link established between the radio base station  10   a  and the relay station  30  and an access link that is a radio link established between the relay station  30  and the mobile terminal  20   b.    
         [0022]    In the following, for ease of explanation, the mobile terminal  20   a  performing direct radio communication with the radio base station  10   a  is referred to as a macro terminal  20   a  and the mobile terminal  20   b  performing relay communication with the radio base station  10   a  via the relay station  30  is referred to as a relay terminal  20   b . And, the macro terminal  20   a  and the relay terminal  20   b  have the same configurations and when they are described indiscriminately, they are collectively referred to as mobile terminals  20 . 
         [0023]    In this LTE-A system, the radio base station  10   a  illustrated in  FIG. 3B  needs to allocate not only radio resources for the access link between the radio base station  10   a  and the macro terminal  20   a  and but also radio resources for the backhaul link between the radio base station  10   a  and the relay station  30 . Accordingly, in the case illustrated in  FIG. 3B , the radio resources that can be allocated to the macro terminal  20   a  are sometimes reduced as compared with the case illustrated in  FIG. 3A , which may cause reduction in capacity of the entire system. 
         [0024]    The present inventors have noted that when the radio base station  10   a  performs direct radio communication with the macro terminal  20   a  and relay communication with the relay terminal  20   b  via the relay station  30  as described above, the radio resources that can be allocated to the macro terminal  20   a  are reduced as compared with the case where relay communication is not performed, and have finally completed the present invention. 
         [0025]    In the relay transmission method according to the present invention, as illustrated in  FIG. 4 , the backhaul links (first radio links) are established between the plural radio base stations  10   a  and  10   b  and the relay station  30  and the access link (second radio link) is established between the relay station  30  and the relay terminal  20   b . The radio base stations  10   a  and  10   b  transmit downlink data to the relay station  30  by using the backhaul links established respectively. The relay station  30  transmits the downlink data received from the radio base stations  10   a  and  10   b , to the relay terminal  20   b  by using the access link. 
         [0026]    According to this relay transmission method, the plural radio base stations  10   a  and  10   b  transmit the downlink data to the relay station  30  by using the backhaul links, respectively. Accordingly, the radio resources required for the backhaul link in each of the radio base stations  10   a  and  10   b  can be reduced as compared with the radio resources required for the backhaul link in the radio base station  10   a  in  FIG. 3B . In this way, in each of the radio base stations  10   a  and  10   b , the radio resources required for the backhaul link can be reduced, thereby increasing radio resources that can be allocated to the macro terminal  20   a  and preventing reduction in capacity of the entire system. 
         [0027]    And, in the relay transmission system according to the present invention, the plural radio base stations  10   a  and  10   b  respectively transmit downlink data in mutually different subframes. Specifically, as illustrated in  FIG. 5 , the radio base station  10   a  allocates certain subframes fixedly or semi-fixedly as radio resources for the backhaul link with the relay station  30 . Further, the radio base station  10   b  allocates subframes different from those allocated by the radio base station  10   a , fixedly or semi-fixedly as radio resources for the backhaul link between the radio base station  10   b  and the relay station  30 . Here, the subframes allocated as the radio resources for the backhaul link in each of the radio base stations  10   a  and  10   b  may be determined in advance or determined to be different from each other by signaling between the radio base stations  10   a  and  10   b.    
         [0028]    In such relay transmission, as the downlink data from the radio base stations  10   a  and  10   b  to the relay station  30  are time division multiplexed in mutually different subframes and transmitted, the relay station  30  can receive the downlink data from the radio base stations  10   a  and  10   b  properly. 
         [0029]    Further, in the relay transmission method according to the present invention, one radio base station  10   a  of the radio base stations  10   a  and  10   b  illustrated in  FIG. 4  may transmit a control signal (for example, R-PDCCH) used for the relay station  30  to receive the downlink data (for example, R-PDSCH). In this case, the downlink data may be transmit only from the base station  10   b  to the relay station  30  or from both of the radio base stations  10   a  and  10   b  to the relay station  30 . 
         [0030]    Or, in the relay transmission method according to the present invention, both of the radio base stations  10   a  and  10   b  illustrated in  FIG. 4  may transmit the downlink data (R-PDSCH) as well as the control signals (R-PDCCH) for the relay station  30  to receive the downlink data to the relay station  30 . 
         [0031]    Further, in the relay transmission method according to the present invention, as illustrated in  FIG. 3B , the radio base station  10   a  that independently transmits the downlink data to the relay station  30  may determine whether or not to transmit the downlink data from both of the radio base stations  10   a  and  10   b  to the relay station  30  as illustrated in  FIG. 4 , based on applying determination information described later. When it is determined that the downlink data should be transmitted from both of the radio base stations  10   a  and  10   b  to the relay station  30  as illustrated in  FIG. 4  based on the applying determination information described later, the radio base station  10   a  requires the radio base station  10   b  to transmit the downlink data to the relay station  30 , and transmission to the relay station  30  is started from both of the radio base stations  10   a  and  10   b . Further, in this case, the radio base station  10   a  may determine distribution of the downlink data to transmit and provide the radio base station  10   b  with instructions of the downlink data to transmit. 
         [0032]    Here, the above-mentioned applying determination information includes the number of relay terminals  20   b  connected to the relay station  30 , reception quality of signals from the radio base station  10   a  in the relay station  30 , data request information of the relay terminal  20   b  connected to the relay station  30 , the number of relay stations  30  in a cell of the radio base station  10   a , the number of macro terminals  20   a  connected to the radio base station  10   a , reception quality of downlink signals from the radio base station  10   a  in the macro terminal  20   a  and data request information of the macro terminal  20   a , which may be used alone or in combination. 
         [0000]    (1) In a case where the applying determination information is the number of relay terminals  20   b    
         [0033]    In this case, the radio base station  10   a  determines whether or not to transmit the downlink data to the relay station  30  from both of the radio base stations  10   a  and  10   b , based on the number of relay terminals  20   b  connected to the relay station  30 . Note that the number of relay terminals  20   b  is calculated for example, in the relay station  30 , based on uplink signals from the relay terminals  20  connected to the relay station  30 . The number of relay terminals  20   b  is reported from the relay station  30  to the radio base station  10   a.    
         [0034]    For example, when the number of relay terminals  20   b  connected to the relay station  30  exceeds a predetermined value, the radio base station  10   a  illustrated in  FIG. 3B  determines that the downlink data to the relay station  30  should be transmitted from both of the radio base stations  10   a  and  10   b  and requests the radio base station  10   b  to transmit the downlink data to the relay station  30 . As a result, transmission to the relay station  30  from both of the radio base stations  10   a  and  10   b  as illustrated in  FIG. 4  is started and in the radio base station  10   a , it is possible to reduce the radio resources required for the backhaul link. Accordingly, it is possible to prevent shortage of radio resources that can be allocated to the macro terminal  20   a  connected to the radio base station  10   a , due to the increase in number of the relay terminals  20   b.    
         [0000]    (2) In a case where the applying determination information is reception quality of the relay station  30   
         [0035]    In this case, the radio base station  10   a  determines whether or not to transmit the downlink data to the relay station  30  from both of the radio base stations  10   a  and  10   b  based on the reception quality of the relay station  30 . Note that the reception quality of the relay station  30  is, for example, reception quality of downlink signals from the radio base station  10  measured in the relay station  30 , and is reported from the relay station  30  to the radio base station  10   a.    
         [0036]    For example, when the reception quality of the relay station  30  is reduced below a predetermined value, the radio base station  10   a  illustrated in  FIG. 3B  determines that the downlink data to the relay station  30  should be transmitted from both of the radio base stations  10   a  and  10   b  and requests the radio base station  10   b  to transmit the downlink data to the relay station  30 . As a result, transmission to the relay station  30  from both of the radio base stations  10   a  and  10   b  as illustrated in  FIG. 4  is started and in the radio base station  10   a , it is possible to reduce the radio resources required for the backhaul link. Consequently, it is possible to prevent shortage of radio resources that can be allocated to the macro terminal  20   a  connected to the radio base station  10   a  due to improvement of reception quality of the relay station  30 . 
         [0000]    (3) In a case where the applying determination information is data request information of relay terminal  20   b    
         [0037]    In this case, the radio base station  10   a  determines whether or not to transmit the downlink data to the relay station  30  from both of the radio base stations  10   a  and  10   b  based on the data request information of the relay terminal  20   b  connected to the relay station  30 . Note that the data request information of the relay terminal  20   b  is, for example, information indicating the type of data requested by the relay terminal  20   b , and shows, for example, the data is large-volume data such as video or small-volume data such as speech. The data request information is reported from the relay terminal  20   b  via the relay station  30  to the radio base station  10   a.    
         [0038]    For example, when the data request information of the relay terminal  20   b  connected to the relay station  30  indicates large-volume data, the radio base station  10   a  illustrated in  FIG. 3B  determines that the downlink data to the relay station  30  should be transmitted from both of the radio base stations  10   a  and  10   b  and requests the radio base station  10   b  to transmit the downlink data to the relay station  30 . As a result, transmission to the relay station  30  from both of the radio base stations  10   a  and  10   b  as illustrated in  FIG. 4  is started and in the radio base station  10   a , it is possible to reduce the radio resources required for the backhaul link. Consequently, it is possible to prevent shortage of radio resources that can be allocated to the macro terminal  20   a  connected to the radio base station  10   a  due to transmission of large-volume data to the relay terminal  20   b.    
         [0000]    (4) In a case where the applying determination information is the number of relay stations  30   
         [0039]    In this case, the radio base station  10   a  determines whether or not to transmit the downlink data to the relay station  30  from both of the radio base stations  10   a  and  10   b  based on the number of relay stations  30  in a cell of the radio base station  10   a . Note that the number of relay stations  30  in the cell of the radio base station  10  is calculated in the radio base station  10   a  based on uplink signals from the relay stations  30 . This is because each relay station  30  may be either of fixed type and moving type and the number of relay stations  30  varies by moving-type relay stations  30  moving into or out of the cell. 
         [0040]    For example, when the number of relay stations  30  in the cell of the radio base station  10   a  exceeds a predetermined value, the radio base station  10   a  illustrated in  FIG. 3B  determines that the downlink data to the relay station  30  should be transmitted from both of the radio base stations  10   a  and  10   b  and requests the radio base station  10   b  to transmit the downlink data to the relay station  30 . As a result, transmission to the relay station  30  from both of the radio base stations  10   a  and  10   b  as illustrated in  FIG. 4  is started and in the radio base station  10   a , it is possible to reduce the radio resources required for the backhaul link. Consequently, it is possible to prevent shortage of radio resources that can be allocated to the macro terminal  20   a  connected to the radio base station  10   a  due to increase in number of relay stations  30 . 
         [0000]    (5) In a case where the applying determination information is the number of macro terminals  20   a    
         [0041]    In this case, the radio base station  10   a  determines whether or not to transmit the downlink data to the relay station  30  from both of the radio base stations  10   a  and  10   b  based on the number of macro terminals to be connected to the radio base station  10 . Note that the number of macro terminals  20   a  is, for example, calculated in the radio base station  10   a  based on uplink signals from the respective macro terminals  20   a  to be connected to the radio base station  10   a.    
         [0042]    For example, when the number of macro terminals  20   a  to be connected to the radio base station  10   a  exceeds a predetermined value, the radio base station  10   a  illustrated in  FIG. 3B  determines that the downlink data to the relay station  30  should be transmitted from both of the radio base stations  10   a  and  10   b  and requests the radio base station  10   b  to transmit the downlink data to the relay station  30 . As a result, transmission to the relay station  30  from both of the radio base stations  10   a  and  10   b  as illustrated in  FIG. 4  is started and in the radio base station  10   a , it is possible to reduce the radio resources required for the backhaul link. Consequently, it is possible to allocate more radio resources to the macro terminals  20   a  as far as it can tolerate the increase in number of macro terminals  20   a.    
         [0000]    (6) In a case where the applying determination information is reception quality of the macro terminal  20   a    
         [0043]    In this case, the radio base station  10   a  determines whether or not to transmit the downlink data to the relay station  30  from both of the radio base stations  10   a  and  10   b  based on the reception quality of the macro terminal  20   a  connected to the radio base station  10   a . Note that the reception quality of the macro terminal  20   a  is, for example, reception quality of downlink signals from the radio base station  10   a  measured in the macro terminal  20   a , and is reported from the macro terminal  20   a  to the radio base station  10   a.    
         [0044]    For example, when the reception quality of the macro terminal  20   a  connected to the radio base station  10   a  is reduced below a predetermined value, the radio base station  10   a  illustrated in  FIG. 3B  determines that the downlink data to the relay station  30  should be transmitted from both of the radio base stations  10   a  and  10   b  and requests the radio base station  10   b  to transmit the downlink data to the relay station  30 . As a result, transmission to the relay station  30  from both of the radio base stations  10   a  and  10   b  as illustrated in  FIG. 4  is started and in the radio base station  10   a , it is possible to reduce the radio resources required for the backhaul link. Consequently, it is possible to allocate more radio resources to the macro terminal  20   a  for improvement of the reception quality of the macro terminal  20   a.    
         [0000]    (7) In a case where the applying determination information is data request information of the macro terminal  20   a    
         [0045]    In this case, the radio base station  10   a  determines whether or not to transmit the downlink data to the relay station  30  from both of the radio base stations  10   a  and  10   b  based on the data request information of the macro terminal  20   a  connected to the radio base station  10   a . Note that the data request information of the macro terminal  20   a  is information indicating the type of data requested to be transmitted to the macro terminal  20   a  by the radio base station  10   a , and shows, for example, the data is large-volume data such as video and small-volume data such as speech. The data request information is reported from the macro terminal  20   a  to the radio base station  10 . 
         [0046]    For example, when the data request information of the macro terminal  20   a  connected to the radio base station  10   a  indicates large-volume data, the radio base station  10   a  illustrated in  FIG. 3B  determines that the downlink data to the relay station  30  should be transmitted from both of the radio base stations  10   a  and  10   b  and requests the radio base station  10   b  to transmit the downlink data to the relay station  30 . As a result, transmission to the relay station  30  from both of the radio base stations  10   a  and  10   b  as illustrated in  FIG. 4  is started and in the radio base station  10   a , it is possible to reduce the radio resources required for the backhaul link. Consequently, it is possible to allocate more radio resources to the macro terminal  20   a  so as to transmit the large volume data to the macro terminal  20   a.    
         [0047]    Here, determination based on the applying determination information described above may be performed by a higher apparatus above the radio base stations  10   a  and  10   b . In this case, the higher apparatus requests the radio base stations  10   a  and  10   b  to transmit the downlink data to the relay station  30  and transmission to the relay station  30  is started from both of the radio base stations  10   a  and  10   b.    
         [0048]    In the relay transmission method according to the present invention described above, the radio base station  10   a  may be any of Node B, eNode B, BDE (Base station Digital Equipment) and so on. And, the radio base station  10   b  may be any radio base station having equivalent functions to the radio base station  10   a , such as, Node B, eNode B, or BDE (Base station Digital Equipment). Or, the radio base station  10   b  may be a radio base station acting as a slave of the radio base station  10   a  such as, for example, RRE (Remote Radio Equipment) connected to the BDE by an optical fiber. In the following description, the radio base stations  10   a  and  10   b  are collectively referred to as radio base stations  10  if they are treated indiscriminately. And, the number of radio base stations  10  is not limited to two illustrated in  FIG. 4  and the above-mentioned relay transmission method may be applied as appropriate to three or more radio base stations  10 . 
         [0049]    Further, in the relay transmission method according to the present invention described above, the downlink data to the relay station  30  from the plural radio base stations  10  is time division multiplexed in mutually different subframes and transmitted. However, the downlink data to the relay station  30  from the radio base stations  10  may be frequency division multiplexed or code division multiplexed in the same subframes and transmitted. 
         [0050]    Further, in the relay transmission method according to the present invention described above, the distribution of the downlink data for the relay station  30  to the plural radio base stations  10  may be determined by one radio base station  10  (for example, radio base station  10   a ) or by a higher apparatus above the plural radio base stations  10 . If the distribution is determined by one radio base station  10 , the data distribution information determined by the radio base station  10  (for example, radio base station  10   a ) may be transmitted to other radio base stations  10  via inter-base station interfaces. Or, if the distribution is determined by the higher apparatus, data transmission from each of the radio base stations  10  to the relay station  30  is performed in accordance with the data distribution information transmitted from the higher apparatus to the radio base stations  10 . 
         [0051]    With reference to the accompanying drawings, an embodiment of the present invention will be described in detail below. 
       First Embodiment 
       [0052]    The first embodiment is described on the assumption that determination based on the above-mentioned applying determination information and determination of the distribution of downlink data to the radio base stations  10  are performed by a radio base station  10 . 
         [0053]      FIG. 6  is a block diagram illustrating a configuration of the radio base station according to the first embodiment. The radio base station  10  illustrated in  FIG. 6  has a transmitting section for downlink signals and a receiving section for uplink signals. Here, description is made principally about the configuration of the transmitting section for downlink signals. 
         [0054]    As illustrated in  FIG. 6 , the radio base station  10  has an applying determining section  101  (determining section), a data distribution determining section  102 , an inter-base station IF (InterFace)  103 , a downlink signal generating section  104 , a channel coding section  105 , a modulating section  106 , a mapping section  107 , a reference signal generating section  108 , an IFFT section  109  and a CP inserting section  110 . 
         [0055]    The applying determining section  101  determines whether or not to transmit the downlink data to the relay station  30  from each of the radio base stations  10 . Concretely, the applying determining section  101  determines whether or not to transmit the downlink data to the relay station  30  from each of the radio base stations  10  based on applying determination information as described above. When it determines that the downlink data should be transmitted from each of the radio base stations  10  to the relay station  30 , the applying determining section  101  outputs a control signal of the determination result to the data distribution determining section  102 . 
         [0056]    Here, as described above, the applying determination information includes the number of relay terminals  20   b  connected to the relay station  30 , reception quality of signals from the radio base station  10   a  in the relay station  30 , data request information of the relay terminal  20   b  connected to the relay station  30 , the number of relay stations  30  in a cell of the radio base station  10   a , the number of macro terminals  20   a  connected to the radio base station  10   a , reception quality of signals from the radio base station  10   a  in the macro terminal  20   a  and data request information of the macro terminal  20   a , which may be used alone or in combination. 
         [0057]    When it is determined by the applying determining section  101  that the downlink data should be transmitted from each of the radio base stations  10  to the relay station  30 , the data distribution determining section  102  determines the distribution of downlink data to the radio base station and other radio base stations  10 . The data distribution determining section  102  outputs the data distribution information indicating downlink data distributed to the other base stations  10 , to the inter-base station interface (IF)  103 , and outputs the data distribution information indicating downlink data distributed to the base station, to the downlink signal generating section  104 . Note that the other radio base stations  10  may be determined in advance or reported dynamically based on load information from the higher apparatus above the radio base station. 
         [0058]    The inter-base station interface (IF)  103  performs transmission and reception of signals with the other radio base stations  10 . Specifically, when applying of the distribution transmission is determined by the applying determining section  101 , the inter-base station interface  103  transmits the data distribution information received as input from the data distribution determining section  102 , to the other radio base stations  10 . 
         [0059]    The downlink signal generating section  104  generates downlink signals. The downlink signals include downlink data such as PDSCH for the macro terminal  20   a  and R-PDSCH for the relay terminal  20   b  and control signals such as PDCCH for the macro terminal  20   a  and R-PDCCH for the relay terminal  20   b . The downlink signal generating section  104  outputs the generated downlink signals to the channel coding section  105 . 
         [0060]    Particularly, when it is determined by the applying determining section  101  that the downlink data to the relay station  30  should be transmitted from each of plural radio base stations  10 , the downlink signal generating section  104  generates the downlink data (R-PDSCH) based on the data distribution information received as input from the data distribution determining section  102 . And, the downlink signal generating section  104  generates control signals (R-PDCCH) for the relay station  30  to receive the downlink data (R-PDSCH). 
         [0061]    The channel coding section  105  performs channel coding on the downlink signals received as input from the downlink signal generating section  104 . The channel coding section  105  outputs the channel-coded downlink signals to the modulating section  106 . The modulating section  106  modulates the channel-coded downlink signals. The modulating section  106  outputs the modulated downlink signals to the mapping section  107 . 
         [0062]    The mapping section  107  maps the downlink signals received as input from the modulating section  106 , to subcarriers based on the resource allocation information. The mapping section  107  outputs the mapped downlink signals to the IFFT section  109 . Note that the resource allocation information is information indicating radio resources allocated to the input downlink signals. The downlink signals for the relay station  30  are allocated to subframes prepared fixedly or semi-fixedly for the backhaul link, as described above. 
         [0063]    The signal generating section  108  generates reference signals and outputs the reference signals to the IFFT section  109 . The IFFT section  109  performs IFFT on the downlink signals received as input from the mapping section  107  and the reference signals received as input from the reference signal generating section  108  and converts them into time domain signals. The IFFT section  109  outputs the signals having been subjected to IFFT, to the CP inserting section  110 . The CP inserting section  110  inserts CPs to the signals having been subjected to IFFT. Note that the signals to which CPs are inserted are transmitted to the relay station  30  or to the macro terminal  20   a.    
         [0064]      FIG. 7  is a block diagram illustrating a configuration of the relay station according to the first embodiment. The relay station  30  illustrated in  FIG. 7  has a receiving section for receiving downlink signals from the radio base station  10  and receiving uplink signals from the relay terminal  20   b , and a transmitting section for transmitting downlink signals to the relay terminal  20   b  and transmitting uplink signals to the radio base station  10 . Note that description is made principally about the configuration of the receiving section for receiving the downlink signals from the radio base station  10  and the transmitting section for transmitting the downlink signals to the relay terminal  20   b.    
         [0065]    As illustrated in  FIG. 7 , the relay station  30  has a CP removing section  301 , an FFT (Fast Fourier Transform) section  302 , a demapping section  303 , a demodulating section  304 , a channel decoding section  305 , a downlink signal generating section  306 , a channel coding section  307 , a modulating section  308 , a mapping section  309 , a reference signal generating section  310 , an IFFT section  311 , a CP inserting section  312  and a feedback information generating section  313 . 
         [0066]    The CP removing section  301  removes CPs added to reception signals from the radio base station  10 . The CP removing section  301  outputs the CP-removed signals to the FFT section  302 . The FFT section  302  performs FFT processing on the CP-removed signals. The FFT section  302  outputs the signals having been subjected to FFT, to the demapping section  303 . The demapping section  303  demaps the signals having been subjected to FFT and outputs the demapped signals to the demodulating section  304 . The channel decoding section  305  performs channel decoding on the downlink data demodulated by the demodulating section  304 . The channel decoding section  305  outputs the channel-decoded downlink data to the downlink signal generating section  306 . 
         [0067]    The downlink signal generating section  306  generates downlink signals based on the downlink data decoded by the channel decoding section  305  and outputs the downlink signals to the channel coding section  307 . Notes that the downlink signals include downlink data (PDSCH) to the relay terminal  20   b  and control signals (PDCCH) for the relay terminal  20   b  to receive the downlink data. 
         [0068]    The channel coding section  307  performs channel coding on the downlink signals received as input from the downlink signal generating section  306  and outputs the downlink signals to the modulating section  308 . The modulating section  308  modulates the channel-coded data. The modulating section  308  outputs the data-modulated downlink signals to the mapping section  309 . 
         [0069]    The mapping section  309  maps the downlink signals received as input from the modulating section  308 , to subcarriers based on the resource allocation information. The mapping section  309  outputs the mapped downlink signals to the IFFT section  311 . The reference signal generating section  310  generates reference signals and outputs the reference signals to the IFFT section  311 . The IFFT section  311  performs IFFT on the downlink signals received as input from the mapping section  309  and the reference signals received as input from the reference signal generating section  310  and converts these signals into time domain signals. The IFFT section  311  outputs the signals having been subjected to IFFT, to the CP inserting section  312 . The CP inserting section  312  inserts CPs to the signals having been subjected to IFFT. The CP-inserted signals are transmitted to the relay terminal  20   b.    
         [0070]    The feedback information generating section  313  generates feedback information for the ratio base station  10 . Note that the feedback information includes reception quality of downlink signals which are received from the radio base station  10  and demodulated by the demodulating section  304 , the number of relay terminals  20   b  connected to the relay station  30 , data request information of the relay terminal  20   b  connected to the relay station  30 , and so on. This feedback information is reported to the radio base station  10  and used as the above-mentioned applying determination information in the radio base station  10 . 
         [0071]      FIG. 8  is a block diagram illustrating a configuration of the macro terminal according to the first embodiment. The macro terminal  20   a  illustrated in  FIG. 8  has a receiving section for receiving downlink signals and a transmitting section for transmitting uplink signals. Description here is made principally about the configuration of the receiving section for downlink signals. 
         [0072]    As illustrated in  FIG. 8 , the macro terminal  20   a  has a CP removing section  201 , an FFT (Fast Fourier Transform) section  202 , a demapping section  203 , a demodulating section  204  and a feedback information generating section  205 . 
         [0073]    The CP removing section  201 , the FFT section  202 , the demapping section  203  and the demodulating section  204  have the same functions as the CP removing section  301 , the FFT section  302 , the demapping section  303  and the demodulating section  304  described above, and their explanation is omitted here. 
         [0074]    The feedback information generating section  205  generates feedback information for the radio base station  10 . Note that the feedback information include reception quality of downlink signals which are received from the radio base station  10  and demodulated by the demodulating section  204 , data request information from the macro terminal  20   a  to the radio base station  10 , and so on. This feedback information is reported to the radio base station  10  and is used as the above-mentioned applying determination information in the radio base station  10 . 
         [0075]    In the thus-configured radio communication system performing relay transmission, the backhaul link (first radio link) is established between each of radio base stations  10  and the relay station  30  and the access link (second radio link) is established between the relay station  30  and the relay terminal  20   b . Each of the radio base stations  10  transmits the distributed downlink data to the relay station  30  by using the established backhaul link. The relay station  30  transmits the downlink data received from the radio base stations  10   a  and  10   b , to the relay terminal  20   b  by using the access link. 
         [0076]    In this way, according to the present invention, each of the plural radio base stations  10  transmits downlink data to the relay station  30  by using the backhaul link. Therefore, the radio resources required for the backhaul link in each of the radio base stations  10  can be reduced as compared with radio resources required for single transmission of one radio base station  10 . Therefore, in each of the radio base stations  10 , it is possible to reduce the radio resources required for the backhaul link, thereby increasing radio resources allocatable to the macro terminal  20   a  and preventing the reduction in capacity of the entire system. 
         [0077]    The embodiment described here has been given for illustrative purposes in all the points and is by no means limiting. The scope of the present invention is defined by the claims, but not by the above-described embodiment only. It should be understood that the scope of the present invention includes equivalences and all modifications to the claims. 
         [0078]    The disclosure of Japanese Patent Application No. 2010-181910, filed on Aug. 16, 2010, including the specification, drawings, and abstract, is incorporated herein by reference in its entirety.