Patent Publication Number: US-9407350-B2

Title: Radio communication system, base station, relay station, and radio communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 14/793,110, filed on Jul. 7, 2015, which is a continuation of U.S. application Ser. No. 13/753,176, filed on Jan. 29, 2013, which issued as U.S. Pat. No. 9,113,466 on Aug. 18, 2015, which is a continuation of International Patent Application number PCT/JP2010/063748 filed Aug. 13, 2010 in Japan, the entire contents of each are herein incorporated by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a radio communication system, a base station, a relay station, and a radio communication method. 
     BACKGROUND 
     At present, radio communication systems such as a cell-phone system and a radio MAN (Metropolitan Area Network) are widely used. For attaining a further speeding up and large capacity of radio communication, lively discussion is continuously performed about a next generation radio communication technology. For example, the 3GPP (3rd Generation Partnership Project), which is one of international standardization organizations, proposes a standard referred to as LTE (Long Term Evolution), and a standard referred to as LTE-A (Long Term Evolution-Advanced) that is an evolution of LTE (see, for example, 3rd Generation Partnership Project, “Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA)”, 3GPP TR 36.913 V8.0.1, 2009-03). 
     In a radio communication system including a base station and a mobile station, a relay station which relays radio communication may be provided between the base station and the mobile station. By providing a relay station, an area (dead spot) in which radio communication is difficult due to radio propagation blocking caused by buildings is covered, a range of a cell covered by a base station is expanded, and communication throughput is improved. 
     However, in a relay station, interference (which may be referred to as self-interference) may occur between a transmission signal of its own station and a reception signal. Suppose, for example, that a frequency band used between a base station and a relay station and a frequency band used between the relay station and a mobile station are overlapped with each other. In this case, a radio signal transmitted to the mobile station comes into a receiver of the relay station, and as a result a radio signal may not correctly be received from the base station. To cope with the problem, the relay station is proposed to be controlled in such a manner that reception of a radio signal from the base station and transmission of a radio signal to the mobile station are not performed at the same time (see, for example, section 9. 3 of 3rd Generation Partnership Project, “Feasibility study for Further advancements for E-UTRA”, 3GPP TR 36.912 V9.0.0, 2009-09). 
     Another radio communication system including a base station and a mobile station may provide a configuration in which a procedure of random access from the mobile station to the base station is specified. In the random access, the mobile station accesses the base station without being dedicatedly allocated a radio resource by the base station (see, for example, section 10. 1. 5 of 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description”, 3GPP TS 36.300 V9.3.0, 2010-03). 
     As a random access preamble (which may be referred to as Msg 1), for example, the mobile station transmits to the base station a signal sequence selected from among a plurality of candidates through a predetermined random access channel. The base station, having received the Msg 1, transmits as a response a random access response (which may be referred to as Msg 2). Note that at this time, the base station does not recognize the transmission source device of the Msg 1. The mobile station, having received the Msg 2, transmits to the base station a message (which may be referred to as Msg 3) including the identifier of its own station. The base station, having received the Msg 3, transmits to the mobile station a message (which may be referred to as Msg 4) as a response. 
     Here, an interval until the base station sends back the Msg 2 from reception of the Msg 1 and an interval until the base station sends back the Msg 4 from reception of the Msg 3 are not fixed, and preferably stay within a predetermined allowable range. Based on this flexibility, the base station may perform scheduling and efficiently transmit the Msg 2 and Msg 4. While the base station may transmit the Msg 2 or Msg 4, the mobile station monitors a radio signal from the base station and detects the Msg 2 or Msg 4 (see, for example, section 5. 1 of 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”, 3GPP TS 36.321 V9.3.0, 2010-06). 
     Think of a radio communication system which includes a base station, a relay station, and a mobile station, and in which the relay station performs random access to the base station. At this time, there arises a problem that how the base station and the relay station preferably perform a random access procedure. Specifically, in the conventional random access, timing at which a base station may send back a message varies. If the message is sent back at timing at which the relay station transmits a radio signal to the mobile station, the relay station may not correctly receive the message due to self-interference. 
     SUMMARY 
     According to an aspect, there is provided a radio communication system which includes: a base station; a relay station which performs wireless communications with the base station; and a mobile station which performs wireless communications with the base station or relay station, wherein: the relay station includes: a first radio communication unit which transmits a first message about random access to the base station and which receives from the base station a radio signal including a second message about the random access; and a first controller which limits reception of a radio signal from the base station at timing at which a radio signal is transmitted to the mobile station; and the base station includes: a second radio communication unit which receives the first message and transmits the second message; and a second controller which controls transmission timing of the second message based on whether a transmission source of the received first message is the relay station. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a radio communication system according to a first embodiment; 
         FIG. 2  illustrates a radio communication system according to a second embodiment; 
         FIG. 3  illustrates a structure example of a radio frame; 
         FIG. 4  illustrates a use example of a radio resource; 
         FIG. 5  is a sequence diagram illustrating a procedure example of random access; 
         FIG. 6  is a block diagram illustrating a base station; 
         FIG. 7  is a block diagram illustrating a relay station; 
         FIG. 8  is a block diagram illustrating a mobile station; 
         FIG. 9  is a flowchart illustrating a process of a base station according to a second embodiment; 
         FIG. 10  is a flowchart illustrating a process of a relay station according to a second embodiment; 
         FIG. 11  is a flowchart illustrating a process of a mobile station according to a second embodiment; 
         FIG. 12  illustrates a random access example according to a second embodiment; 
         FIG. 13  is a flowchart illustrating a process of a base station according to a third embodiment; 
         FIG. 14  is a flowchart illustrating a process of a relay station according to a third embodiment; 
         FIG. 15  is a flowchart illustrating a process of a mobile station according to a third embodiment; 
         FIG. 16  illustrates a random access example according to a third embodiment; 
         FIG. 17  illustrates another random access example according to a third embodiment; 
         FIG. 18  is a flowchart illustrating a process of a base station according to a fourth embodiment; 
         FIG. 19  is a flowchart illustrating a process of a relay station according to a fourth embodiment; 
         FIG. 20  illustrates a random access example according to a fourth embodiment; 
         FIG. 21  illustrates another random access example according to a fourth embodiment; 
         FIG. 22  is a flowchart illustrating a process of a base station according to a fifth embodiment; 
         FIG. 23  is a flowchart illustrating a process of a relay station according to a fifth embodiment; 
         FIG. 24  illustrates a random access example according to a fifth embodiment; 
         FIG. 25  is a flowchart illustrating a process of a base station according to a sixth embodiment; 
         FIG. 26  is a flowchart illustrating a process of a relay station according to a sixth embodiment; 
         FIG. 27  illustrates a random access example according to a sixth embodiment; 
         FIG. 28  is a flowchart illustrating a process of a base station according to a seventh embodiment; 
         FIG. 29  is a flowchart illustrating a process of a relay station according to a seventh embodiment; and 
         FIG. 30  illustrates a random access example according to a seventh embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail below with reference to the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     First Embodiment 
       FIG. 1  illustrates a radio communication system according to a first embodiment. The radio communication system according to the first embodiment includes a base station  10 , a relay station  20 , and a mobile station  30 . Examples of the mobile station  30  include a cellular phone and a personal digital assistant device. The relay station  20  may be a mobile radio relay station or a fixed radio relay station. The mobile station  30  performs wireless communications with the base station  10  or relay station  20 . The relay station  20  performs random access (RA) to the base station  10  and establishes a connection, thus relaying data communication between the base station  10  and the mobile station  30 . 
     The base station  10  includes a radio communication unit  11  and a controller  12 . The radio communication unit  11  receives a first message (message #1) about random access and transmits a second message (message #2) about the random access. Examples of the message #1 include a Msg 1 and a Msg 3, and examples of the message #2 include a Msg 2 and a Msg 4. The controller  12  determines whether a transmission source of the message #1 received by the radio communication unit  11  is the relay station  20 . Based on whether the transmission source is the relay station  20 , the controller  12  then controls timing at which the radio communication unit  11  transmits the message #2. 
     The relay station  20  includes a radio communication unit  21  and a controller  22 . The radio communication unit  21  performs wireless communications with the base station  10 . During the random access, the radio communication unit  21  transmits the message #1 to the base station  10 , and receives the message #2 from the base station  10 . The controller  22  controls timing of radio communication so that self-interference will not occur between a radio signal received from the base station  10  and a radio signal transmitted to the mobile station  30 . Concretely, at timing at which a radio signal is transmitted to the mobile station  30 , the radio communication unit  21  limits reception of a radio signal from the base station  10  (e.g., stops a receiving circuit). In the radio signal transmitted to the mobile station  30  by the relay station  20 , an RS (Reference Signal) used for a measurement of communication quality through the mobile station  30  is included. 
     Here, in the case where the message #1 is the Msg 1, examples of the method for determining a transmission source of the message #1 through the controller  12  include a method based on a signal sequence included in the message #1 and a method based on timing at which the message #1 is received. In the former method, a signal sequence for the relay station and a signal sequence for the mobile station are prepared and the radio communication unit  21  generates the message #1 by using the signal sequence for the relay station. In the case where another relay station is present in the radio communication system, the relay station  20  may use the signal sequence for the relay station shared with the another relay station. In the latter method, a RACH (random access channel) for the relay station and a RACH for the mobile station are configured separately and the radio communication unit  21  transmits the message #1 through the RACH for the relay station. 
     In the case where a transmission source of the message #1 is the relay station  20  (or another relay station) or the mobile station  30  (or another mobile station), the controller  12  then changes an algorithm for determining transmission timing of the message #2. In the former case, the controller  12  selects timing at which the relay station  20  does not limit reception of a radio signal. About the timing at which data is capable of being transmitted from the base station  10  to the relay station  20 , for example, when some agreements have been reached between both the stations, the controller  12  selects the timing according to the agreements. On the other hand, in the latter case, the controller  12  performs scheduling and selects arbitrary timing within an allowable period. 
     According to the proposed radio communication system of the first embodiment, the relay station  20  limits reception of a radio signal from the base station  10  at the timing at which a radio signal is transmitted to the mobile station  30 . The relay station  20  further transmits the message #1 to the base station  10 . The base station  10  determines whether a transmission source of the message #1 is the relay station  20 , and transmits the message #2 at timing determined according to determination results. The relay station  20  receives the message #2 from the base station  10 . 
     As a result, the radio communication system smoothly performs random access from the relay station  20  to the base station  10 . Specifically, the relay station  20  limits the reception so that self-interference will not occur between a radio signal received from the base station  10  and a radio signal transmitted to the mobile station  30 . When a transmission source of the message #1 is not the mobile station  30  but the relay station  20 , the base station  10  determines transmission timing of the message #2 in consideration of the limitation of the reception timing of the relay station  20 . Therefore, the radio communication system suppresses the possibility that the relay station  20  cannot receive the message #2 in a normal way. 
     In second to seventh embodiments described below, there is included an example of a radio communication system including a base station, a relay station, and a mobile station in conformity to the LTE or LTE-A. 
     Second Embodiment 
       FIG. 2  illustrates a radio communication system according to a second embodiment. The radio communication system according to the second embodiment includes a base station  100 , relay stations  200  and  200   a , and mobile stations  300  and  300   a . In the following description, suppose mainly that the mobile station  300  performs data communication with the base station  100  via the relay station  200  and the mobile station  300   a  directly performs data communication with the base station  100 . 
     The base station  100  is a radio communication apparatus which performs wireless communications with the relay stations  200  and  200   a , and the mobile station  300   a . The base station  100  is connected to a host station (not illustrated) via a wired line. The base station  100  receives data from the host station, and transfers it to the relay stations  200  and  200   a , and the mobile station  300   a  through a downlink (DL). On the other hand, through an uplink (UL), the base station  100  receives data from the relay stations  200  and  200   a , and the mobile station  300   a , and transfers it to the host station. 
     The relay station  200  is a radio communication apparatus which relays data communication between the base station  100  and the mobile station  300 . Through the DL, the relay station  200  receives data from the base station  100  and transfers it to the mobile station  300 . On the other hand, through the UL, the relay station  200  receives data from the mobile station  300  and transfers it to the base station  100 . In a similar fashion, the relay station  200   a  also relays data communication. The relay stations  200  and  200   a  may be mobile radio relay stations or fixed radio relay stations. 
     Here, the relay stations  200  and  200   a  correspond to a so-called Type 1 of relay station. Specifically, the relay stations  200  and  200   a  perform protocol processing up to a layer  3 , and behave to the mobile stations  300  and  300   a  in the same manner as in the base station  100 . From the mobile stations  300  and  300   a , a cell apart from that provided by the base station  100  is viewed to be provided by the relay stations  200  and  200   a . A frequency band used for radio communication between the base station and the relay station is at least partially overlapped with a frequency band used for radio communication between the relay station and the mobile station. 
     The mobile stations  300  and  300   a  are radio terminal apparatus which communicate with the base station  100 . The mobile stations  300  and  300   a  communicate with the base station  100  via the relay stations  200  and  200   a . Examples of the mobile stations  300  and  300   a  include a cellular phone and a personal digital assistant device. Through the DL, the mobile station  300  receives data from the relay station  200 . Through the UL, on the other hand, the mobile station  300  transmits data to the relay station  200 . 
     For radio communication through the DL, an OFDMA (Orthogonal Frequency Division Multiple Access) is used, and for radio communication through the UL, an SC-FDMA (Single Carrier Frequency Division Multiple Access) is used. Further, a base station may be called a BS (Base Station), a relay station may be called an RN (Relay Node) or an RS (Relay Station), and a mobile station may be called an MS (Mobile Station) or a UE (User Equipment). 
       FIG. 3  illustrates a structure example of a radio frame. Through each of the DL and UL, the radio frame as illustrated in  FIG. 3  is transmitted and received between a base station and a relay station as well as between a relay station and a mobile station. In the second embodiment, an FDD (Frequency Division Duplex) is used as a duplex system. Note that a TDD (Time Division Duplex) may be used. 
     A radio frame with a width of 10 msec includes ten subframes (subframes #0 to #9) with a width of 1 msec. Each subframe includes two slots (a first-half slot and a second-half slot) with a width of 0.5 msec. Scheduling of data or control signals is performed in units of subframes. A radio resource of the subframe is segmentalized in the frequency direction or in the time direction for management. A minimum unit in the frequency direction is a sub-carrier and a minimum unit in the time direction is a symbol. The number of symbols included in the subframe may be different depending on a type of subframes. 
       FIG. 4  illustrates an example of radio resource usage. A communication channel is provided on a DL subframe and an UL subframe transmitted and received by the relay stations  200  and  200   a.    
     In the DL subframe, several symbols (one to three symbols) of the head are an area for control and remaining symbols are an area for data. On the area for control, a PDCCH (Physical Downlink Control Channel) for transmitting a physical control signal is provided. On the area for data, a PBCH (Physical Broadcast Channel) for transmitting broadcast information and an R-PDCCH (Relay Physical Downlink Control Channel) for transmitting control data on a relay are provided. On the UL subframe, a PRACH (Physical Random Access Channel) for transmitting a random access preamble is provided. 
     On the DL subframe, a reference signal being a pilot signal is transmitted in both of the areas for control and data. While DL data communication is not performed, a reference signal is transmitted. The mobile station  300  measures a reception power level or radio quality by using a reference signal. 
     Here, the relay stations  200  and  200   a  receive data from the base station  100  and configure a subframe (DL backhaul) which does not carry data to a subordinate mobile station. In signals in which transmission is stopped, a reference signal is also included. Note that in the area for control, the relay stations  200  and  200   a  are allowed to receive data from the base station  100  and transmit data to the subordinate mobile station at the same time. In other words, the relay stations  200  and  200   a  may stop transmitting data in both of the areas for control and data, or only in the area for data. 
     For example, the DL backhaul is configured so that the DL subframe of the relay stations  200  and  200   a  may be an MBSFN (Multimedia Broadcast multicast service Single Frequency Network) subframe. The MBSFN subframe is a subframe in which the base station  100  normally performs MBSFN transmission. The MBSFN transmission is used in the case where a plurality of transmitting stations concertedly transmit data of the same content at the same timing by using the same frequency and modulation scheme. In the second embodiment, in the radio communication system including the relay stations  200  and  200   a , the MBSFN subframe is used for configuring a DL backhaul subframe of the relay stations  200  and  200   a . Through the process, as illustrated in  FIG. 4 , for example, since the relay stations  200  and  200   a  do not transmit a reference signal in the area for data of the MBSFN subframe, self-interference does not occur even if the relay stations  200  and  200   a  receive data in the area. 
     The relay stations  200  and  200   a  further transmit data to the base station  100  and configure a subframe (UL backhaul) in which data is not received from a subordinate mobile station. Timing for the DL backhaul and timing for the UL backhaul may be the same or different from each other. 
     The relay stations  200  and  200   a  configured with the UL backhaul do not give data transmission permissions (UL grant) before a predetermined time (e.g., four subframes) from the UL backhaul. As a result, the subordinate mobile stations do not transmit data in the UL backhaul. Further, the relay stations  200  and  200   a  do not transmit data to the subordinate mobile stations before a predetermined time from the UL backhaul. As a result, the subordinate mobile stations do not transmit an ACK (Acknowledgement)/NACK (Negative Acknowledgement) in the UL backhaul. 
     Note that the relay stations  200  and  200   a  control timing at which the subordinate mobile stations transmit data as described above. Therefore, even in every subframe except the predetermined UL backhaul, the relay stations  200  and  200   a  can stop receiving data from the subordinate mobile station and transmit data to the base station  100  through scheduling. 
     In addition, timing for the backhaul has been agreed between the base station  100  and the relay stations  200  and  200   a . The relay stations  200  and  200   a  may determine the timing and notify the base station  100  of the timing. Or, alternatively, the base station  100  may determine the timing and notify the relay stations  200  and  200   a  of the timing. The timing for the backhaul may be different between the relay stations  200  and  200   a . The relay stations  200  and  200   a  broadcast information indicating the timing for the backhaul. The mobile station  300  recognizes the timing based on the broadcast information received from the relay station  200 . 
       FIG. 5  is a sequence diagram illustrating a procedure example of the random access. Here, the mobile station  300   a  performs random access to the base station  100 . The procedure of the random access illustrated in  FIG. 5  includes the following steps: 
     (Step S 1 ) The mobile station  300   a  selects one signal sequence from among candidates of a plurality of signal sequences and transmits it as a random access preamble (Msg 1) through the PRACH. Hereinafter, the random access preamble may be referred to simply as a preamble. 
     (Step S 2 ) After detecting Msg 1, the base station  100  transmits a random access response (Msg 2). The Msg 2 is transmitted within a predetermined period. More specifically, the base station  100  transmits the Msg 2 within a period of A pieces of subframes counted after three subframes from the subframe in which the Msg 1 is received. A value of A is previously set to any of 2, 3, 4, 5, 6, 7, 8, and 10. 
     (Step S 3 ) Within the period in which the Msg 2 may be transmitted, the mobile station  300   a  monitors a received signal from the base station  100 . After detecting the Msg 2, the mobile station  300   a  transmits a message (Msg 3) called a scheduled transmission to the base station  100 . In the Msg 3, an identifier of the mobile station  300   a  is included. 
     (Step S 4 ) The base station  100  receives the Msg 3 from the mobile station  300   a  and transmits a message (Msg 4) called Contention Resolution to the mobile station  300   a . The Msg 4 is transmitted within a predetermined period. More specifically, the base station  100  transmits the Msg 4 within B pieces of subframes counted from the subframe in which the Msg 3 is received. A value of B is previously set to any of 8, 16, 24, 32, 40, 48, 56, and 64. Within the period in which the Msg 4 may be transmitted, the mobile station  300   a  monitors a received signal from the base signal  100 . 
     Incidentally, a clue (RA trigger) in which random access is performed includes the following. 
     (1) Abnormality is detected in a connection between the base station  100  and the mobile station  300   a . The abnormality in the connection includes a case where a timer T310 is time out as described in 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”, 3GPP TS 36.331 V9.2.0, 2010-03 and a case of failing in retransmission control of data in an RLC (Radio Link Control) layer. 
     (2) The mobile station  300   a  receives an instruction of starting a handover from a base station as a handover source, and accesses a base station as a handover destination. 
     (3) The mobile station  300   a  fails in access to the base station  100  by using a scheduling request scheme. The scheduling request scheme is an access scheme in which the mobile station  300   a  transmits a scheduling request to the base station  100  by using a radio resource for control data and receives an allocation of a radio resource for data transmission. 
     (4) The mobile station  300   a  fails in data transmission to the base station  100  by using a contention based uplink transmission method. The contention based uplink transmission method is a method in which the mobile station  300   a  transmits data to the base station  100  by using a radio resource shared by a plurality of mobile stations. Failure in the data transmission may be caused by the contention. The contention based uplink transmission method is described, for example, in a collection of writing about 3GPP (R2-093812, “Contention based uplink transmission”). 
     (5) The mobile station  300   a  fails in security authentication. 
     (6) The mobile station  300   a  fails in reconfiguration of a radio resource, namely, reconfiguration of RRC (Radio Resource Control) connection. Reconfiguration of the RRC connection is described, for example, in a section 5. 3. 5. 5 of 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”, 3GPP TS 36.331 V9.2.0, 2010-03. 
     Even at the time of the random access from the relay stations  200  and  200   a  to the base station  100 , the Msg 1 to Msg 4 are transmitted in the same manner as in the random access from the mobile station  300   a  to the base station  100 . Note that in the case of the relay stations  200  and  200   a , transmission timing of the Msg 2 and Msg 4 is different from that of the mobile station  300   a.    
       FIG. 6  is a block diagram illustrating the base station. The base station  100  includes a radio communication unit  110 , a wired communication unit  120 , a data processing unit  130 , and a controller  140 . 
     The radio communication unit  110  is a radio interface which performs wireless communications with the relay stations  200  and  200   a , and the mobile station  300   a . The radio communication unit  110  performs signal processing including demodulation and decoding to the received radio signal, and extracts data and a control signal. The radio communication unit  110  further detects a preamble transmitted through the PRACH. The radio communication unit  110  supplies data to be transferred to a host station to the data processing unit  130 . On the other hand, the radio communication unit  110  obtains data from the data processing unit  130  and generates a control signal based on an instruction from the controller  140 . The radio communication unit  110  then performs signal processing including coding and modulation to data and a control signal, and outputs a radio signal. 
     The wired communication unit  120  is a communication interface which performs wired communication with the host station. The wired communication unit  120  receives data addressed to the mobile stations  300  and  300   a  from the host station, and supplies it to the data processing unit  130 . On the other hand, the wired communication unit  120  converts the data obtained from the data processing unit  130  into a packet form of a wired network and transmits it to the host station. 
     The data processing unit  130  obtains data to be transferred to the host station from the radio communication unit  110  and supplies it to the wired communication unit  120 . On the other hand, the data processing unit  130  obtains data addressed to the mobile stations  300  and  300   a  from the wired communication unit  120  and maps data to a radio frame under the control of the controller  140 , thus supplying it to the radio communication unit  110 . 
     The controller  140  controls processes of the radio communication unit  110 , wired communication unit  120 , and data processing unit  130 . The controller  140  has a data plane unit  150  and a control plane unit  160 . The data plane unit  150  controls transmission and reception of data between its own station and any of the relay stations  200  and  200   a  and the mobile station  300   a . The control plane unit  160  controls transmission and reception of a control signal between its own station and any of the relay stations  200  and  200   a  and the mobile station  300   a.    
     Namely, the control plane unit  160  obtains the control signal extracted by the radio communication unit  110  and performs communication control according to the control signal. The control plane unit  160  further notifies the radio communication unit  110  of the transmitted control signal. The control plane unit  160  has a preamble management unit  161 , an RA slot management unit  162 , and a backhaul controller  163 . 
     The preamble management unit  161  manages candidates of the preambles used for random access. When a preamble for the relay station and a preamble for the mobile station are distinguished, the preamble management unit  161  determines whether the preamble detected by the radio communication unit  110  is one for the relay station or one for the mobile station. 
     The RA slot management unit  162  manages a slot (RA slot) in which the PRACH is configured. When an RA slot for the relay station and an RA slot for the mobile station are distinguished, the RA slot management unit  162  determines whether a slot in which a preamble is detected by the radio communication unit  110  is one for the relay station or one for the mobile station. Further, the RA slot management unit  162  may dynamically configure an RA slot according to occurrence conditions of an RA trigger or configuration status of a DL backhaul of the relay stations  200  and  200   a.    
     The backhaul controller  163  manages backhauls of the relay stations  200  and  200   a , and controls timing of transmission and reception of a radio signal. When a random access source is the relay station, the backhaul controller  163  refers to configuration status of the DL backhaul of the relay stations  200  and  200   a , and determines timing for transmitting the Msg 2 or Msg 4. 
       FIG. 7  is a block diagram illustrating the relay station. The relay station  200  includes radio communication units  210  and  220 , a scheduler  230 , and a controller  240 . The relay station  200   a  is also deployed through the same block structure as that of the relay station  200 . 
     The radio communication unit  210  is a radio interface which performs wireless communications with the base station  100 . The radio communication unit  210  performs signal processing including demodulation and decoding to the received radio signal, and extracts data or a control signal. The radio communication unit  210  supplies data addressed to the mobile station  300  to the scheduler  230 . On the other hand, the radio communication unit  210  obtains data from the scheduler  230  and generates a control signal or a preamble based on the instruction from the controller  240 . The radio communication unit  210  then performs signal processing including coding and modulation and outputs a radio signal. 
     The radio communication unit  220  is a radio interface which performs wireless communications with the mobile station  300 . The radio communication unit  220  performs signal processing including demodulation and decoding to the received radio signal and extracts data or a control signal. The radio communication unit  220  supplies data to be transferred to the base station  100  to the scheduler  230 . On the other hand, the radio communication unit  220  obtains data from the scheduler  230 , and generates a control signal or a reference signal based on the instruction from the controller  240 . The radio communication unit  220  then performs signal processing including coding and modulation, and outputs a radio signal. 
     The scheduler  230  schedules transfer of data from the base station  100  to the mobile station  300  and transfer of data from the mobile station  300  to the base station  100 . Namely, the scheduler  230  maps data addressed to the mobile station  300  obtained from the radio communication unit  210  to a DL radio frame, and supplies it to the radio communication unit  220 . On the other hand, the scheduler  230  maps data addressed to the base station  100  obtained from the radio communication unit  220  to an UL radio frame, and supplies it to the radio communication unit  210 . 
     The controller  240  controls processes of the radio communication units  210  and  220  and the scheduler  230 . The controller  240  has a data plane unit  250  and a control plane unit  260 . 
     The data plane unit  250  controls transmission and reception of data between its own station and any of the base station  100  and the mobile station  300 . The data plane unit  250  has a scheduler controller  251 . The scheduler controller  251  manages a method for scheduling the scheduler  230 . 
     The control plane unit  260  controls transmission and reception of a control signal between its own station and any of the base station  100  and the mobile station  300 . Specifically, the control plane unit  260  obtains the control signal extracted by the radio communication units  210  and  220 , and performs communication control according to the control signal. The control plane unit  260  further notifies the radio communication units  210  and  220  of the transmitted control signal. The control plane unit  260  has a preamble management unit  261 , an RA transmission management unit  262 , and a backhaul controller  263 . 
     The preamble management unit  261  manages candidates of the preambles used in the random access, and selects a preamble used in the Msg 1 during the random access to the base station  100 . When the preamble for the relay station and the preamble for the mobile station are distinguished, the preamble management unit  261  selects the preamble for the relay station. 
     The RA transmission management unit  262  grasps an RA slot and selects an RA slot used to transmit the Msg 1 during the random access to the base station  100 . When the RA slot for the relay station and the RA slot for the mobile station are distinguished, the RA transmission management unit  262  selects the RA slot for the relay station. 
     The backhaul controller  263  manages a backhaul of its own station and controls timing of transmission and reception of a radio signal. Particularly, at least in the area for data of the DL backhaul, the backhaul controller  263  does not transmit data or a reference signal to the mobile station  300 . In addition to the backhaul used in normal data communication, the backhaul controller  263  may configure a temporary backhaul during the random access. 
       FIG. 8  is a block diagram illustrating the mobile station. The mobile station  300  includes a radio communication unit  310 , a data processing unit  320 , and a controller  330 . The mobile station  300   a  is also deployed by using the same block structure as that of the mobile station  300 . 
     The radio communication unit  310  is a radio interface which performs wireless communications with the relay station  200  or base station  100 . The radio communication unit  310  performs signal processing including demodulation and decoding to the received radio signal, and extracts data or a control signal. The radio communication unit  310  supplies the extracted data to the data processing unit  320 . On the other hand, the radio communication unit  310  obtains data from the data processing unit  320  and generates a control signal based on the instruction from the controller  330 . The radio communication unit  310  then performs signal processing including coding and modulation, and outputs a radio signal. 
     The data processing unit  320  generates data to be transmitted to the relay station  200  or base station  100  and supplies it to the radio communication unit  310 . On the other hand, the data processing unit  320  obtains data addressed to its own station from the radio communication unit  310  and performs a process according to a type of data. 
     The controller  330  controls processes of the radio communication unit  310  and the data processing unit  320 . The controller  330  has a data plane unit  340  and a control plane unit  350 . The data plane unit  340  controls transmission and reception of radio data. The control plane unit  350  controls transmission and reception of a radio control signal. The control plane unit  350  has a preamble management unit  351  and an RA slot management unit  352 . 
     The preamble management unit  351  manages candidates of the preambles used in the random access, and selects the preamble used in the Msg 1 during the random access to the relay station  200  or base station  100 . When the preamble for the relay station and the preamble for the mobile station are distinguished, the preamble management unit  351  selects the preamble for the mobile station. 
     The RA slot management unit  352  grasps an RA slot, and selects an RA slot used to transmit the Msg 1 during the random access to the relay station  200  or base station  100 . When the RA slot for the relay station and the RA slot for the mobile station are distinguished, the RA slot management unit  352  selects the RA slot for the mobile station. 
     With regard to the random access, processes performed by the base station  100 , relay station  200  and mobile station  300  (or mobile station  300   a ) will be described below. In the second embodiment, the base station  100  determines based on the used preamble whether a transmission source of the Msg 1 is the relay station or mobile station. 
       FIG. 9  is a flowchart illustrating a process of the base station according to the second embodiment. The process illustrated in  FIG. 9  includes the following steps: 
     (Step S 111 ) The radio communication unit  110  receives the preamble (Msg 1) through the PRACH. At this time, a transmission source of the Msg 1 is not concretely identified. 
     (Step S 112 ) The preamble management unit  161  determines whether the received preamble is a preamble for the relay station. If so, the process advances to step S 113 . If not, the process proceeds to step S 114 . 
     (Step S 113 ) The backhaul controller  163  selects a DL backhaul of each of the relay stations  200  and  200   a  within the period in which the Msg 2 is transmitted (namely, the period within A pieces of subframes after three subframes from the subframe in which the Msg 1 is received). The process then proceeds to step S 115 . 
     (Step S 114 ) The control plane unit  160  performs scheduling and selects an arbitrary subframe within the period in which the Msg 2 is transmitted. 
     (Step S 115 ) The radio communication unit  110  transmits the Msg 2 through the PDCCH or R-PDCCH of the subframe selected at step S 113  or S 114 . In the case where a plurality of subframes are selected at step S 113  when timing of the DL backhauls is different between the relay stations  200  and  200   a , the radio communication unit  110  transmits the Msg 2 in each subframe. The reason is that whether at the time of receiving the Msg 1, any of the relay stations  200  and  200   a  transmit the Msg 1 is not identified. 
     (Step S 116 ) The radio communication unit  110  receives the Msg 3. 
     (Step S 117 ) The control plane unit  160  determines a transmission source of the Msg 3. The transmission source of the Msg 3 is determined by using an identifier included in the Msg 3. If the transmission source is the relay station, the process advances to step S 118 . If the transmission source is not the relay station, the process proceeds to step S 119 . Here, the transmission source is supposed to be the relay station  200 . 
     (Step S 118 ) The backhaul controller  163  selects a DL backhaul of the relay station  200  within the period (namely, the period within B pieces of subframes counted from the subframe in which the Msg 3 is received) in which the Msg 4 is transmitted. The process then proceeds to step S 120 . Since the transmission source of the Msg 3 is concretely identified at the time of receiving the Msg 3, the backhaul controller  163  need not select a DL backhaul of the relay station  200   a.    
     (Step S 119 ) The control plane unit  160  performs scheduling and selects an arbitrary subframe within the period in which the Msg 4 is transmitted. 
     (Step S 120 ) The radio communication unit  110  transmits the Msg 4 through the PDCCH or R-PDCCH of the subframe selected at step S 118  or S 119 . 
       FIG. 10  is a flowchart illustrating a process of the relay station according to the second embodiment. The process illustrated in  FIG. 10  includes the following steps: 
     (Step S 121 ) The radio communication unit  210  receives broadcast information through the PBCH from the base station  100 . In the broadcast information, information on timing of the RA slot is included. The RA slot may be shared by the relay station and the mobile station. The RA transmission management unit  262  selects the RA slot of the UL backhaul. Note that when reception of radio signals from the mobile station  300  may be stopped, the RA transmission management unit  262  may select RA slot except RA slot in the UL backhaul. 
     (Step S 122 ) From among candidates of the preambles, the preamble management unit  261  selects a preamble for the relay station. The preamble for the relay station may be shared by the relay stations  200  and  200   a . The radio communication unit  210  transmits the selected preamble by using the RA slot selected at step S 121 . 
     (Step S 123 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  through the DL backhaul. When the Msg 2 is transmitted through the PDCCH, the backhaul controller  263  stops transmitting a signal in both of the areas for control and data. When the Msg 2 is transmitted through the R-PDCCH, the backhaul controller  263  stops transmitting a signal at least in the area for data. 
     (Step S 124 ) The radio communication unit  210  receives the Msg 2 from the base station  100  through the PDCCH or R-PDCCH of the DL backhaul. 
     (Step S 125 ) The radio communication unit  210  transmits the Msg 3 to the base station  100 . Preferably, the radio communication unit  210  transmits the Msg 3 to the base station  100  through the UL backhaul. 
     (Step S 126 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  through the DL backhaul. When the Msg 4 is transmitted through the PDCCH, the backhaul controller  263  stops transmitting a signal in both of the areas for control and data. When the Msg 4 is transmitted through the R-PDCCH, the backhaul controller  263  stops transmitting a signal at least in the area for data. 
     (Step S 127 ) The radio communication unit  210  receives the Msg 4 from the base station  100  through the PDCCH or R-PDCCH of the DL backhaul. Through the above steps, the radio communication unit  210  completes a procedure of random access from the relay station  200  to the base station  100 . 
       FIG. 11  is a flowchart illustrating a process of the mobile station according to the second embodiment. Suppose here that the mobile station  300  performs the random access to the relay station  200  or base station  100 . The process illustrated in  FIG. 11  includes the following steps: 
     (Step S 131 ) The radio communication unit  310  receives broadcast information through the PBCH from the relay station  200  or base station  100 . In the broadcast information, information on the timing of the RA slot is included. The RA slot management unit  352  selects one RA slot. 
     (Step S 132 ) The preamble management unit  351  selects one preamble for the mobile station from among candidates of the preambles. The radio communication unit  310  transmits the selected preamble through the RA slot selected at step S 131 . 
     (Step S 133 ) The radio communication unit  310  monitors the PDCCH within the period (namely, the period of A pieces of subframes after three subframes from the subframe in which the Msg 1 is transmitted) in which the Msg 2 is transmitted, and receives the Msg 2 from the relay station  200  or base station  100 . Note that the radio communication unit  310  receives the Msg 2 in subframe except the subframe in DL backhaul. 
     (Step S 134 ) The radio communication unit  310  transmits the Msg 3 to the relay station  200  or base station  100 . The radio communication unit  310  transmits the Msg 3 to the relay station  200  in subframe except the subframe in UL backhaul. 
     (Step S 135 ) The radio communication unit  310  monitors the PDCCH within the period (namely, the period of B pieces of subframes counted from the subframe in which the Msg 3 is transmitted) in which the Msg 4 is transmitted, and receives the Msg 4 from the relay station  200  or base station  100 . Note that the radio communication unit  310  receives the Msg 4 from the relay station  200  in subframe except the subframe in DL backhaul. 
     (Step S 136 ) Through the process up to step S 135 , the radio communication unit  310  completes a procedure of the random access from its own station to the relay station  200  or base station  100 . Subsequently, the radio communication unit  310  performs data communication between its own station and any of the relay station  200  and the base station  100 . Note that when connected to the relay station  200 , the mobile station  300  performs data communication in subframe except the subframe in backhaul. 
       FIG. 12  illustrates a random access example according to the second embodiment. The message flow illustrated in  FIG. 12  includes the following steps: 
     (Step S 11 ) The relay station  200  transmits the Msg 1 to the base station  100  through the UL backhaul. The preamble to be transmitted as the Msg 1 is a preamble for the relay station. Based on the preamble, the base station  100  recognizes that a transmission source of the Msg 1 is the relay station  200 . 
     (Step S 12 ) The base station  100  transmits the Msg 2 through the R-PDCCH of the DL backhaul of the relay station  200  within the period in which the Msg 2 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 2. 
     (Step S 13 ) The base station  100  transmits the Msg 2 through the DL backhaul of the relay station  200   a  within the period in which the Msg 2 is transmitted. Note that since the relay station  200   a  did not transmit Msg 1, it ignores the Msg 2 received from the base station  100 . 
     (Step S 14 ) The relay station  200  transmits the Msg 3 to the base station  100  through the UL backhaul. Based on an identifier included in the Msg 3, the base station  100  recognizes that a transmission source of the Msg 3 is the relay station  200 . 
     (Step S 15 ) The base station  100  transmits the Msg 4 through the R-PDCCH of the DL backhaul of the relay station  200  within the period in which the Msg 4 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 4. 
     In the proposed radio communication system of the second embodiment, the base station  100  is configured to determine based on the preamble received as the Msg 1 whether a transmission source of the Msg 1 is the relay station or mobile station. In the case where the transmission source is the relay station, the base station  100  then controls transmission timing so that the transmission source may receive the Msg 2 through an existing DL backhaul. In the case where a transmission source of the Msg 3 is the relay station  200 , the base station  100  further controls transmission timing so that the relay station  200  may receive the Msg 4 through an existing DL backhaul. According to the second embodiment, the radio communication system suppresses self-interference of the relay station  200  and smoothly performs random access from the relay station  200  to the base station  100 . 
     Third Embodiment 
     A third embodiment will be described below. Since the third embodiment shares some elements with the foregoing second embodiment, the following discussion will focus on their distinctive points, omitting explanations of similar elements. The third embodiment differs from the second embodiment in a method for determining whether a transmission source of the Msg 1 is the relay station or mobile station. 
     A radio communication system according to the third embodiment is deployed by using the same apparatus configuration as that of the second embodiment illustrated in  FIG. 2 . Further, a base station, a relay station, and a mobile station according to the third embodiment are deployed by using the same block configurations as those of the second embodiment illustrated in  FIGS. 6 to 8 . Hereinafter, the third embodiment will be described with reference to the same reference numerals as those illustrated in  FIGS. 6 to 8 . 
       FIG. 13  is a flowchart illustrating a process of the base station according to the third embodiment. The process illustrated in  FIG. 13  includes the following steps: 
     (Step S 211 ) The RA slot management unit  162  configures separate RA slots for the relay station and the mobile station. Through the PBCH, the radio communication unit  110  transmits information indicating timing of the RA slots for the mobile station and the relay station as broadcast information. Note that the radio communication unit  110  may transmit information indicating timing of the RA slot for the relay station as individual control data to the relay stations  200  and  200   a.    
     As described in 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation”, 3GPP TS 36.211 V9.1.0, 2010-03, normal information indicating timing of the RA slot is transmitted as a parameter (PRACH Configuration Index) in the broadcast information. Suppose, for example, that the information indicating timing of the RA slot for the relay station is inserted as a new parameter into the broadcast information for transmission. The RA slot for the relay station may be set to a low frequency such as one time per 40 msec. 
     (Step S 212 ) The radio communication unit  110  receives a preamble through the PRACH. 
     (Step S 213 ) The RA slot management unit  162  determines whether the preamble is received through the RA slot for the relay station or the mobile station. If the preamble is received through the RA slot for the relay station, the process advances to step S 214 . If the preamble is received through the RA slot for the mobile station, the process proceeds to step S 215 . 
     (Step S 214 ) The backhaul controller  163  selects each DL backhaul of the relay stations  200  and  200   a  within the period in which the Msg 2 is transmitted. The process then proceeds to step S 216 . 
     (Step S 215 ) The control plane unit  160  performs scheduling and selects an arbitrary subframe within the period in which the Msg 2 is transmitted. 
     (Step S 216 ) The radio communication unit  110  transmits the Msg 2 through the PDCCH or R-PDCCH of the subframe selected at step S 214  or S 215 . When the timing of the DL backhauls is different between the relay stations  200  and  200   a , the radio communication unit  110  transmits the Msg 2 through each DL backhaul. 
     (Step S 217 ) The radio communication unit  110  receives the Msg 3. 
     (Step S 218 ) The control plane unit  160  determines a transmission source of the Msg 3. If the transmission source is the relay station, the process advances to step S 219 . If the transmission source is not the relay station, the process proceeds to step S 220 . Here, the transmission source is supposed to be the relay station  200 . 
     (Step S 219 ) The backhaul controller  163  selects the DL backhaul of the relay station  200  within the period in which the Msg 4 is transmitted. The process then proceeds to step S 221 . The backhaul controller  163  need not select the DL backhaul of the relay station  200   a.    
     (Step S 220 ) The control plane unit  160  performs scheduling and selects an arbitrary subframe within the period in which the Msg 4 is transmitted. 
     (Step S 221 ) The radio communication unit  110  transmits the Msg 4 through the PDCCH or R-PDCCH of the subframe selected at step S 219  or S 220 . 
       FIG. 14  is a flowchart illustrating a process of the relay station according to the third embodiment. The process illustrated in  FIG. 14  includes the following steps: 
     (Step S 231 ) Through the PBCH, the radio communication unit  210  receives broadcast information from the base station  100 . In the broadcast information, information indicating timing of the RA slot for the relay station is included. 
     (Step S 232 ) The RA transmission management unit  262  selects the RA slot for the relay station. The RA transmission management unit  262  preferably selects the RA slot for the relay station provided on the UL backhaul. 
     (Step S 233 ) The preamble management unit  261  selects one preamble from among the candidates of the preambles. The candidates of the preambles are shared by the relay stations  200  and  200   a , or by the mobile stations  300  and  300   a . The radio communication unit  210  transmits the selected preamble through the RA slot selected at step S 232 . 
     (Step S 234 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  through the DL backhaul. 
     (Step S 235 ) The radio communication unit  210  receives the Msg 2 from the base station  100  through the PDCCH or R-PDCCH of the subframe in DL backhaul. 
     (Step S 236 ) The radio communication unit  210  transmits the Msg 3 to the base station  100 . Preferably, the radio communication unit  210  transmits the Msg 3 to the base station  100  through the UL backhaul. 
     (Step S 237 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  through the DL backhaul. 
     (Step S 238 ) The radio communication unit  210  receives the Msg 4 from the base station  100  through the PDCCH or R-PDCCH of the subframe in DL backhaul. 
       FIG. 15  is a flowchart illustrating a process of the mobile station according to the third embodiment. Here, the mobile station  300  is supposed to perform random access to the relay station  200  or base station  100 . The process illustrated in  FIG. 15  includes the following steps: 
     (Step S 241 ) The radio communication unit  310  receives broadcast information from the relay station  200  or base station  100  through the PBCH. In the broadcast information, information indicating timing of the RA slot for the mobile station is included. The RA slot management unit  352  selects the RA slot for the mobile station. 
     (Step S 242 ) The preamble management unit  351  selects one preamble from among the candidates of the preambles. The candidates of the preambles are shared by the relay stations  200  and  200   a  or by the mobile stations  300  and  300   a . The radio communication unit  310  transmits the selected preamble through the RA slot selected at step S 241 . 
     (Step S 243 ) The radio communication unit  310  monitors the PDCCH within the period in which the Msg 2 is transmitted, and receives the Msg 2 from the relay station  200  or base station  100 . Note that the radio communication unit  310  receives the Msg 2 from the relay station  200  through subframe except the subframe in DL backhaul. 
     (Step S 244 ) The radio communication unit  310  transmits the Msg 3 to the relay station  200  or base station  100 . The radio communication unit  310  transmits the Msg 3 to the relay station  200  through subframe except the subframe in UL backhaul. 
     (Step S 245 ) The radio communication unit  310  monitors the PDCCH within the period in which the Msg 4 is transmitted, and receives the Msg 4 from the relay station  200  or base station  100 . Note that the radio communication unit  310  receives the Msg 4 from the relay station  200  through subframe except the subframe in DL backhaul. 
     (Step S 246 ) The radio communication unit  310  performs data communication between its own station and any of the relay station  200  and the base station  100 . Note that when connected to the relay station  200 , the radio communication unit  310  performs data communication through subframe except subframe in the backhaul. 
       FIG. 16  illustrates a random access example according to the third embodiment. The message flow illustrated in  FIG. 16  includes the following steps: 
     (Step S 21 ) The relay station  200  transmits the Msg 1 to the base station  100  through the RA slot for the relay station of the UL backhaul subframe. The preamble transmitted as the Msg 1 may be shared by the relay station and the mobile station. Based on the reception timing of the preamble, the base station  100  recognizes that a transmission source of the Msg 1 is the relay station  200 . 
     (Step S 22 ) The base station  100  transmits the Msg 2 through the R-PDCCH of the DL backhaul of the relay station  200  within the period in which the Msg 2 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 2. 
     (Step S 23 ) The base station  100  transmits the Msg 2 through the DL backhaul of the relay station  200   a  within the period in which the Msg 2 is transmitted. 
     (Step S 24 ) The relay station  200  transmits the Msg 3 to the base station  100  through the UL backhaul. Based on the identifier included in the Msg 3, the base station  100  recognizes that a transmission source of the Msg 3 is the relay station  200 . 
     (Step S 25 ) The base station  100  transmits the Msg 4 through the R-PDCCH of the DL backhaul of the relay station  200  within the period in which the Msg 4 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 4. 
     Incidentally, the base station  100  need not always configure the RA slot for the relay station. Specifically, the base station  100  may configure the RA slot after detecting the RA trigger about the relay stations  200  and  200   a.    
       FIG. 17  illustrates another random access example according to the third embodiment. The base station  100  does not allocate a radio resource to the PRACH for the relay station in normal situation. On the other hand, when detecting an RA trigger and determining that any of the relay stations  200  and  200   a  may perform random access, the base station  100  allocates a radio resource to the PRACH for the relay station. At this time, by using a timer, the base station  100  may limit a period in which a radio resource is allocated. The relay stations  200  and  200   a  transmit the Msg 1 within the period. As can be seen from the above discussion, when the relay stations  200  and  200   a  do not perform random access, the base station  100  allocates a radio resource of the RA slot used for the relay station to arbitrary channel except the PRACH, thus making use of a radio resource efficient. 
     According to the third embodiment, the proposed radio communication system permits the base station  100  to determine, based on the timing at which the Msg 1 is received, whether a transmission source of the Msg 1 is the relay station or mobile station. When the transmission source is the relay station, the base station  100  then controls transmission timing so that the relay station may receive the Msg 2 through an existing DL backhaul. When the transmission source of the Msg 3 is the relay station  200 , the base station  100  further controls transmission timing so that the relay station  200  may receive the Msg 4 through an existing DL backhaul. According to the third embodiment, the radio communication system suppresses self-interference of the relay station  200  and smoothly performs random access from the relay station  200  to the base station  100 . 
     Fourth Embodiment 
     A fourth embodiment will be described below. Since the fourth embodiment shares some elements with the foregoing second and third embodiments, the following discussion will focus on their distinctive points, omitting explanations of similar elements. The fourth embodiment differs from the second and third embodiments in timing at which the Msg 2 or 4 is transmitted. 
     A radio communication system according to the fourth embodiment is deployed by using the same apparatus configuration as that of the second embodiment illustrated in  FIG. 2 . Further, a base station, a relay station, and a mobile station according to the fourth embodiment are deployed by using the same block configurations as those of the second embodiment illustrated in  FIGS. 6 to 8 . Hereinafter, the fourth embodiment will be described with reference to the same reference numerals as those illustrated in  FIGS. 6 to 8 . 
       FIG. 18  is a flowchart illustrating a process of the base station according to the fourth embodiment. Since processes of steps S 311 , S 312 , S 314  to S 317 , S 319 , and S 320  illustrated in  FIG. 18  are the same as those of steps S 111 , S 112 , S 114  to S 117 , S 119 , and S 120  illustrated in  FIG. 9 , a description will not be repeated. 
     (Step S 313 ) The backhaul controller  163  selects the DL default backhaul within the period in which the Msg 2 is transmitted. The default backhaul is a backhaul which is not effective in normal situation but automatically becomes effective at the time of the random access. The timing of the default backhaul is, for example, mutually configured between the relay stations  200  and  200   a . The timing of the default backhaul of the relay stations  200  and  200   a  is previously configured between their own stations and the base station  100 , and is configured, for example, by a telecommunications operator at the time of deploying the relay stations  200  and  200   a.    
     (Step S 318 ) The backhaul controller  163  selects the DL default backhaul within the period in which the Msg 4 is transmitted. 
       FIG. 19  is a flowchart illustrating a process of the relay station according to the fourth embodiment. The process illustrated in  FIG. 19  includes the following steps: 
     (Step S 321 ) The control plane unit  260  determines whether to detect the RA trigger. If so, the process proceeds to step S 322 . If not, the control plane unit  260  repeats a process of step S 321 . 
     (Step S 322 ) In addition to an existing backhaul, the backhaul controller  263  makes the default backhaul with predetermined timing effective. 
     (Step S 323 ) The RA transmission management unit  262  selects an RA slot of the UL default backhaul subframe. Note that when stopping receiving a radio signal from the mobile station  300 , the RA transmission management unit  262  may select RA slot except that of the UL default backhaul. 
     (Step S 324 ) From among the candidates of the preambles, the preamble management unit  261  selects the preamble for the relay station. The radio communication unit  210  transmits the selected preamble through the RA slot selected at step S 323 . 
     (Step S 325 ) In addition to the DL backhaul, the backhaul controller  263  limits transmission of a radio signal to the mobile station  300  through the DL default backhaul. 
     (Step S 326 ) The radio communication unit  210  receives the Msg 2 from the base station  100  through the PDCCH or R-PDCCH of the DL default backhaul. 
     (Step S 327 ) The radio communication unit  210  transmits the Msg 3 to the base station  100 . Preferably, the radio communication unit  210  transmits the Msg 3 to the base station  100  through the UL default backhaul. 
     (Step S 328 ) In addition to the DL backhaul, the backhaul controller  263  limits transmission of a radio signal to the mobile station  300  through the DL default backhaul. 
     (Step S 329 ) The radio communication unit  210  receives the Msg 4 from the base station  100  through the PDCCH or R-PDCCH of the DL default backhaul. 
       FIG. 20  illustrates a random access example according to the fourth embodiment. The message flow illustrated in  FIG. 20  includes the following steps: 
     (Step S 31 ) The relay station  200  transmits the Msg 1 to the base station  100  through the UL default backhaul. The preamble to be transmitted is a preamble for the relay station. Based on the preamble, the base station  100  recognizes that a transmission source of the Msg 1 is the relay station. 
     (Step S 32 ) The base station  100  transmits the Msg 2 through the R-PDCCH of the DL default backhaul within the period in which the Msg 2 is transmitted. The base station  100  need not transmit the Msg 2 through each DL backhaul of the relay stations  200  and  200   a . When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 2. When the default backhaul is validated, the relay station  200   a  receives the Msg 2, whereas when the default backhaul is not validated, the relay station  200   a  does not receive the Msg 2. 
     (Step S 33 ) The relay station  200  transmits the Msg 3 to the base station  100  through the UL default backhaul. Based on the identifier included in the Msg 3, the base station  100  recognizes that a transmission source of the Msg 3 is the relay station  200 . 
     (Step S 34 ) The base station  100  transmits the Msg 4 through the R-PDCCH of the DL default backhaul within the period in which the Msg 4 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg  4 . 
     In the foregoing description, whether a transmission source of the Msg 1 is the relay station is determined by using a method according to the second embodiment. Further, determination may be performed by using a method according to the third embodiment. 
       FIG. 21  illustrates another random access example according to the fourth embodiment. The message flow illustrated in  FIG. 21  includes the following steps: 
     (Step S 35 ) The relay station  200  transmits the Msg 1 to the base station  100  through the RA slot for the relay station of the UL default backhaul subframe. Based on the reception timing of the preamble, the base station  100  recognizes that a transmission source of the Msg 1 is the relay station. After detecting the RA trigger, the base station  100  may make the RA slot for the relay station validated. 
     (Step S 36 ) The base station  100  transmits the Msg 2 through the R-PDCCH of the DL default backhaul within the period in which the Msg 2 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 2. 
     (Step S 37 ) The relay station  200  transmits the Msg 3 to the base station  100  through the UL default backhaul. Based on an identifier included in the Msg 3, the base station  100  recognizes that a transmission source of the Msg 3 is the relay station  200 . 
     (Step S 38 ) The base station  100  transmits the Msg 4 through the R-PDCCH of the DL default backhaul within the period in which the Msg 4 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 4. 
     In the fourth embodiment, in the same manner as in the second and third embodiments, the proposed radio communication system smoothly performs random access from the relay station  200  to the base station  100 . Further, in the fourth embodiment, the radio communication system makes common transmission timing of the Msg 2 and Msg 4, and simplifies control of random access. A point that the transmission timing is made to be common is particularly validated in the case of random access involved in a handover. 
     Fifth Embodiment 
     A fifth embodiment will be described below. Since the fifth embodiment shares some elements with the foregoing second and third embodiments, the following discussion will focus on their distinctive points, omitting explanations of similar elements. The fifth embodiment differs from the second and third embodiments in a subframe in which an RA slot is configured. 
     A radio communication system according to the fifth embodiment is deployed by using the same apparatus configuration as that of the second embodiment illustrated in  FIG. 2 . Further, a base station, a relay station, and a mobile station according to the fifth embodiment are deployed by using the same block configurations as those of the second embodiment illustrated in  FIGS. 6 to 8 . Hereinafter, the fifth embodiment will be described with reference to the same reference numerals as those illustrated in  FIGS. 6 to 8 . 
       FIG. 22  is a flowchart illustrating a process of the base station according to the fifth embodiment. Since processes of steps S 412 , S 413 , and S 415  to S 421  illustrated in  FIG. 22  are the same as those of steps S 212 , S 213 , and S 215  to S 221  illustrated in  FIG. 13 , a description will not be repeated. 
     (Step S 411 ) The RA slot management unit  162  configures an RA slot for the relay station in a subframe before k pieces of subframes (e.g., three subframes) from the DL backhaul of the relay stations  200  and  200   a . When timing of the DL backhauls is different between the relay stations  200  and  200   a , the RA slot management unit  162  configures the RA slot for the relay station before k pieces of subframes from each DL backhaul. 
     (Step S 414 ) The backhaul controller  163  selects a DL backhaul after k pieces of subframes from the subframe in which the Msg 1 is received. Note that when there are circumstances that the Msg 2 cannot be transmitted from a subframe after k pieces of subframes from the above subframe, the backhaul controller  163  may select another DL backhaul after k pieces of subframes or later from the above subframe within the period in which the Msg 2 is transmitted. 
       FIG. 23  is a flowchart illustrating a process of the relay station according to the fifth embodiment. Since processes of steps S 432 , S 433 , and S 435  to S 437  illustrated in  FIG. 23  are the same as those of steps S 233 , S 234 , and S 236  to S 238  illustrated in  FIG. 14 , a description will not be repeated. 
     (Step S 431 ) The RA transmission management unit  262  selects an RA slot of a subframe before k pieces of subframes (e.g., three subframes) from the DL backhaul as the RA slot for the relay station. When a subframe of the selected RA slot is not an UL backhaul subframe, the backhaul controller  263  controls that data not to be received from the mobile station  300  through the subframe. 
     (Step S 434 ) The radio communication unit  210  starts monitoring the PDCCH or R-PDCCH from the DL backhaul subframe after k pieces of subframes from the subframe in which the Msg 1 is transmitted. The Msg 2 is expected to be received through the DL backhaul. Note that when the Msg 2 is not received through the DL backhaul, the radio communication unit  210  monitors the PDCCH or R-PDCCH from the DL backhaul subframe after the above subframes or later. 
       FIG. 24  illustrates a random access example according to the fifth embodiment. The message flow illustrated in  FIG. 24  includes the following steps: 
     (Step S 41 ) The relay station  200  transmits the Msg 1 to the base station  100  through the RA slot of a subframe before three subframes from the DL backhaul subframe. Based on the reception timing of the preamble, the base station  100  recognizes that a transmission source of the Msg 1 is the relay station. 
     (Step S 42 ) The base station  100  transmits the Msg 2 through the R-PDCCH of the DL backhaul subframe after three subframes from the subframe in which the Msg 1 is received. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 2. 
     (Step S 43 ) The relay station  200  transmits the Msg 3 to the base station  100  through the UL backhaul. Based on the identifier included in the Msg 3, the base station  100  recognizes that a transmission source of the Msg 3 is the relay station  200 . 
     (Step S 44 ) The base station  100  transmits the Msg 4 through the R-PDCCH of the DL backhaul of the relay station  200  within the period in which the Msg 4 is transmitted. When a signal such as a reference signal is not transmitted in the area for data, the relay station  200  receives the Msg 4. 
     In the foregoing description, whether a transmission source of the Msg 1 is the relay station is determined by using a method according to the third embodiment; further, may be determined by using a method according to the second embodiment. 
     In the fifth embodiment, in the same manner as in the second and third embodiments, the proposed radio communication system smoothly performs random access from the relay station  200  to the base station  100 . Further, in the fifth embodiment, the relay station  200  smoothly starts random access even if an RA slot is not provided on the UL backhaul. In addition, the relay station  200  preferably monitors a reception signal after predetermined time after transmitting the Msg 1, thus simplifying control. 
     Sixth Embodiment 
     A sixth embodiment will be described below. Since the sixth embodiment shares some elements with the foregoing second and third embodiments, the following discussion will focus on their distinctive points, omitting explanations of similar elements. In the sixth embodiment, the Msg 2 and Msg 4 are transmitted also through subframe except the subframe in DL backhaul of the relay station. 
     A radio communication system according to the sixth embodiment is deployed by using the same apparatus configuration as that of the second embodiment illustrated in  FIG. 2 . Further, a base station, a relay station, and a mobile station according to the sixth embodiment are deployed by using the same block configurations as those of the second embodiment illustrated in  FIGS. 6 to 8 . Hereinafter, the sixth embodiment will be described with reference to the same reference numerals as those illustrated in  FIGS. 6 to 8 . 
       FIG. 25  is a flowchart illustrating a process of the base station according to the sixth embodiment. Since processes of steps S 511 , S 512 , S 514  to S 517 , S 519 , and S 520  illustrated in  FIG. 25  are the same as those of steps S 111 , S 112 , S 114  to S 117 , S 119 , and S 120  illustrated in  FIG. 9 , a description will not be repeated. 
     (Step S 513 ) The backhaul controller  163  selects a subframe after m pieces of subframes from the subframe in which the Msg 1 is received. A character m indicates a predetermined value of the base station  100  and the relay stations  200  and  200   a , and is set, for example, to be equal to three. 
     (Step S 518 ) The backhaul controller  163  selects a subframe after n pieces of subframes from the subframe in which the Msg 3 is received. A character n indicates a predetermined value of the base station  100  and the relay stations  200  and  200   a , and is set, for example, to be equal to eight. 
       FIG. 26  is a flowchart illustrating a process of the relay station according to the sixth embodiment. Since processes of steps S 521 , S 522 , and S 525  illustrated in  FIG. 26  are the same as those of steps S 121 , S 122 , and S 125  illustrated in  FIG. 10 , a description will not be repeated. 
     (Step S 523 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  through a subframe after m pieces of subframes from the subframe in which the Msg 1 is transmitted. When the Msg 2 is transmitted through the PDCCH, the backhaul controller  263  stops transmitting a signal in both of the areas for control and data. When the Msg 2 is transmitted through the R-PDCCH, the backhaul controller  263  stops transmitting a signal at least in the area for data. 
     (Step S 524 ) The radio communication unit  210  receives the Msg 2 from the base station  100  through the PDCCH or R-PDCCH of a subframe after the m pieces of subframes from the above subframe. 
     (Step S 526 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  through a subframe after n pieces of subframes from the subframe in which the Msg 3 is transmitted. When the Msg 4 is transmitted through the PDCCH, the backhaul controller  263  stops transmitting a signal in both of the areas for control and data. When the Msg 4 is transmitted through the R-PDCCH, the backhaul controller  263  stops transmitting a signal at least in the area for data. 
     (Step S 527 ) The radio communication unit  210  receives the Msg 4 from the base station  100  through the PDCCH or R-PDCCH of a subframe after the n pieces of subframes from the above subframe. 
       FIG. 27  illustrates a random access example according to the sixth embodiment. The message flow illustrated in  FIG. 27  includes the following steps: 
     (Step S 51 ) The relay station  200  transmits the Msg 1 to the base station  100  by using a preamble for the relay station. Based on the preamble, the base station  100  recognizes that a transmission source of the Msg 1 is the relay station. 
     (Step S 52 ) The base station  100  transmits the Msg 2 through the R-PDCCH of a subframe after three subframes from the subframe in which the Msg 1 is received. Through a subframe after three subframes of the subframe in which the Msg 1 is transmitted, the relay station  200  controls a signal such as a reference signal not to be transmitted in the area for data, thus receiving the Msg 2. 
     (Step S 53 ) The relay station  200  transmits the Msg 3 to the base station  100 . Based on the identifier included in the Msg 3, the base station  100  recognizes that a transmission source of the Msg 3 is the relay station  200 . 
     (Step S 54 ) Through the R-PDCCH of a subframe after eight subframes from the subframe in which the Msg 3 is received, the base station  100  transmits the Msg 4. Through a subframe after eight subframes from the subframe in which the Msg 3 is transmitted, the relay station  200  controls a signal such as a reference signal not to be transmitted in the area for data, thus receiving the Msg 4. 
     In the foregoing description, whether a transmission source of the Msg 1 is the relay station is determined by using a method according to the second embodiment; further, may be determined by using a method according to the third embodiment. 
     In the sixth embodiment, in the same manner as in the second and third embodiments, the proposed radio communication system smoothly performs random access from the relay station  200  to the base station  100 . In the sixth embodiment, since an interval between the Msg 1 and Msg 2 as well as the interval between the Msg 3 and Msg 4 is fixed, the radio communication system simplifies control of random access in the relay station  200 . On the other hand, about the random access from the mobile station  300   a  to the base station  100 , since the intervals are not fixed, the radio communication system secures flexibility of scheduling. 
     Seventh Embodiment 
     A seventh embodiment will be described below. Since the seventh embodiment shares some elements with the foregoing second embodiment, the following discussion will focus on their distinctive points, omitting explanations of similar elements. In the foregoing second to sixth embodiments, transmission timing of the Msg 2 or Msg 4 is changed based on the fact that a transmission source of the Msg 1 or Msg 3 is the relay station or mobile station. As compared with the above, in the seventh embodiment, even if the base station does not distinguish that a transmission source is the relay station or the mobile station, the radio communication system performs random access. 
     A radio communication system according to the seventh embodiment is deployed by using the same apparatus configuration as that of the second embodiment illustrated in  FIG. 2 . Further, a base station, a relay station, and a mobile station according to the seventh embodiment are deployed by using the same block configurations as those of the second embodiment illustrated in  FIGS. 6 to 8 . Hereinafter, the seventh embodiment will be described with reference to the same reference numerals as those illustrated in  FIGS. 6 to 8 . 
       FIG. 28  is a flowchart illustrating a process of the base station according to the seventh embodiment. The process illustrated in  FIG. 28  includes the following steps: 
     (Step S 611 ) The radio communication unit  110  receives a preamble (Msg 1) through the PRACH. At this time, a transmission source of the Msg 1 is not concretely identified. 
     (Step S 612 ) The control plane unit  160  performs scheduling, and selects an arbitrary subframe within the period in which the Msg 2 is transmitted. 
     (Step S 613 ) The radio communication unit  110  transmits the Msg 2 through the PDCCH or R-PDCCH of the subframe selected at step S 612 . 
     (Step S 614 ) The radio communication unit  110  receives the Msg 3. Here, the transmission source is supposed to be the relay station  200 . 
     (Step S 615 ) The control plane unit  160  performs scheduling and selects an arbitrary subframe within the period in which the Msg 4 is transmitted. 
     (Step S 616 ) The radio communication unit  110  transmits the Msg 4 through the PDCCH or R-PDCCH of the subframe selected at step S 615 . 
       FIG. 29  is a flowchart illustrating a process of the relay station according to the seventh embodiment. The process illustrated in  FIG. 29  includes the following steps: 
     (Step S 621 ) The radio communication unit  210  receives broadcast information from the base station  100  through the PBCH. In the broadcast information, information on the timing of the RA slot is included. The RA transmission management unit  262  selects one RA slot. The backhaul controller  263  controls data not to be received from the mobile station  300  through the subframe on which the RA slot is provided. 
     (Step S 622 ) The preamble management unit  261  selects one preamble from among candidates of the preambles. The radio communication unit  210  transmits the selected preamble through the RA slot selected at step S 621 . 
     (Step S 623 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  until receiving the Msg 2 from a head of the period in which the Msg 2 may be received. When the Msg 2 is transmitted through the PDCCH, the backhaul controller  263  stops transmitting a signal in both of the areas for control and data. When the Msg 2 is transmitted through the R-PDCCH, the backhaul controller  263  stops transmitting a signal at least in the area for data. 
     (Step S 624 ) The radio communication unit  210  receives the Msg 2 from the base station  100  through the PDCCH or R-PDCCH. 
     (Step S 625 ) The backhaul controller  263  restarts transmission of a radio signal (including a reference signal) to the mobile station  300 . 
     (Step S 626 ) The radio communication unit  210  transmits the Msg 3 to the base station  100 . 
     (Step S 627 ) The backhaul controller  263  limits transmission of a radio signal (including a reference signal) to the mobile station  300  until receiving the Msg 4 from a head of the period in which the Msg 4 may be received. When the Msg 2 is transmitted through the PDCCH, the backhaul controller  263  stops transmitting a signal in both of the areas for control and data. When the Msg 2 is transmitted through the R-PDCCH, the backhaul controller  263  stops transmitting a signal at least in the area for data. 
     (Step S 628 ) The radio communication unit  210  receives the Msg 2 from the base station  100  through the PDCCH or R-PDCCH. 
     (Step S 629 ) The backhaul controller  263  restarts transmission of a radio signal (including a reference signal) to the mobile station  300 . 
       FIG. 30  illustrates a random access example according to the seventh embodiment. The message flow illustrated in  FIG. 30  includes the following steps: 
     (Step S 61 ) The relay station  200  transmits the Msg 1 to the base station  100 . 
     (Step S 62 ) The base station  100  performs scheduling, and transmits the Msg 2 through the R-PDCCH of any subframe within the period in which the Msg 2 is transmitted. Through a subframe after three subframes or later from the subframe in which the Msg 1 is transmitted, the relay station  200  controls a signal such as a reference signal not to be transmitted in the area for data, thus receiving the Msg 2. 
     (Step S 63 ) The relay station  200  transmits the Msg 3 to the base station  100 . Based on the identifier included in the Msg 3, the base station  100  recognizes that a transmission source of the Msg 3 is the relay station  200 . 
     (Step S 64 ) The base station  100  performs scheduling, and transmits the Msg 4 through the R-PDCCH of any subframe within the period in which the Msg 4 is transmitted. Through the subframe or later in which the Msg 3 is transmitted, the relay station  200  controls a signal such as a reference signal not to be transmitted in the area for data, thus receiving the Msg 4. 
     In the seventh embodiment, the proposed radio communication system suppresses self-interference of the relay station  200  and smoothly performs random access from the relay station  200  to the base station  100 . Further, the base station  100  does not distinguish random access through the relay station  200  and random access through the mobile station  300   a , and therefore preferably performs a common procedure. As a result, the radio communication system simplifies control of the random access. 
     According to the above-described radio communication system, base station, relay station, and radio communication method, random access through the relay station is smoothly performed. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.