Patent Publication Number: US-2012045986-A1

Title: Wireless communication system, relay station, receiver station, and wireless communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2010-184391 filed on Aug. 19, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a wireless communication system, a relay station, a receiver station, and a wireless communication method. 
     BACKGROUND 
     A wireless communication system includes, for example, a transmitter station, such as a base station, and a receiver station, such as a mobile terminal device. When in a communication area covered by the transmitter station, the receiver station performs wireless communication with the transmitter station. 
     In recent years, in the wireless communication system, a relay station for relaying signals transmitted/received between the transmitter station and the receiver station may be installed in order to expand the communication area. An amplify-and-forward (AF) scheme is available as a relay scheme for the relay station. The relay station that performs relay processing based on the AF scheme amplifies a signal received from the transmitter station and transmits the amplified signal having the same frequency as the signal received from the transmitter station. In a wireless communication system employing such an AF scheme, the same signals transmitted from both of the transmitter station and the relay station may arrive at the receiver station in a spatially multiplexed manner. As a result, in the wireless communication system employing the AF scheme, the quality of the signals received by the receiver station may be improved. Technologies related to the wireless communication system that performs wireless communication using a relay station are disclosed in, for example, Japanese Laid-open Patent Publication No. 2007-295569, Japanese Laid-open Patent Publication No. 2007-500482, Japanese Laid-open Patent Publication No. 2003-198442, and Japanese Laid-open Patent Publication No. 2008-17487. 
     SUMMARY 
     According to an aspect of the invention, a wireless communication system in which a transmitter station and a receiver station are capable of performing wireless communication via a relay station is disclosed. The transmitter station includes a first processor that generates a first signal in which known data is assigned to a first region determined by a combination of a frequency domain and a time domain, and a first transmitter that transmits the first signal generated by the first processor. The relay station includes a second processor that generates a second signal in which the known data is assigned to a second region that is different from the first region in the first signal transmitted by the first transmitter, and a second transmitter that transmits the second signal generated by the second processor. The receiver station includes a receiver that receives the first and second signals, and a third processor that separates the first and second signals received by the receiver, based on the known data assigned to the first region and the known data assigned to the second region. 
     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 example of and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of configuration of a wireless communication system according to a first embodiment; 
         FIG. 2  illustrates examples of signals transmitted/received by a relay station in the first embodiment; 
         FIG. 3  illustrates examples of signals received by a receiver station in the first embodiment; 
         FIG. 4  is a block diagram illustrating an example of configuration of the transmitter station in the first embodiment; 
         FIG. 5  is a block diagram illustrating an example of configuration of the relay station in the first embodiment; 
         FIG. 6  is a block diagram illustrating an example of configuration of the receiver station in the first embodiment; 
         FIG. 7  is a sequence diagram illustrating a procedure of processing performed by the wireless communication system according to the first embodiment; 
         FIG. 8  illustrates examples of signals transmitted/received by the relay station in the first embodiment; 
         FIG. 9  illustrates examples of signals received by the receiver station in the first embodiment; and 
         FIG. 10  illustrates examples of signals that a receiver station of related art receives from a transmitter station and a relay station. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the related art described above, there are cases in which the quality of the signals received by the receiver station deteriorates. More specifically, in the wireless communication system including the relay station, the receiver station may receive a signal resulting from interference between a signal transmitted from the transmitter station and a signal transmitted from the relay station. 
     A reason why the receiver station receives an interfered signal will now be described. The relay station performs predetermined signal processing on the signal received from the transmitter station. For example, the relay station performs signal processing, such as processing for amplifying the received signal, demodulation processing, and modulation processing. There are also cases in which the relay station receives the signal, transmitted to the receiver station, via a transmitter-station-oriented antenna for transmitting/receiving a signal to/from the transmitter station. Such a signal is called “diffraction waves,” which may cause the internal circuitry of the relay station to oscillate. Thus, in order to prevent the oscillation, the relay station performs digital signal processing to eliminate the diffraction waves. 
     Since the relay station performs various types of signal processing as described above, the relay station transmits a signal to the receiver station when a period of time taken for the signal processing passes after the reception of the signal transmitted by the transmitter station. When the delay time caused by the signal processing performed by the relay station is larger than a predetermined value, there are cases in which different signals transmitted by the transmitter and the relay station arrive at the receiver station at the same time. That is, there are cases in which the receiver station receives a signal in which the different signals transmitted by the transmitter station and the relay station are spatially multiplexed. Such a signal may cause interference, which results in a problem in that the quality of the signal received by the receiver station deteriorates. 
     The problem will now be described with reference to  FIG. 10 .  FIG. 10  illustrates examples of signals that a receiver station of the related art receives from a transmitter station and a relay station. In the examples illustrated in  FIG. 10 , it is assumed that orthogonal frequency division multiplexing (OFDM) is used as a transmission scheme. The upper stage in  FIG. 10  illustrates signal components that the receiver station receives from the transmitter station and the lower stage in  FIG. 10  illustrates signal components that the receiver station receives from the relay station. Although  FIG. 10  illustrates an example in which the signal received by the receiver station is divided into signal components, signal components that are simultaneously received by the receiver station are spatially multiplexed in practice. 
     In the example illustrated in  FIG. 10 , the transmitter station transmits an OFDM symbol  90 - 1   a  containing a cyclic prefix (CP) and a data signal D 91 , an OFDM symbol  90 - 2   a  containing a CP and a data signal D 92 , and an OFDM symbol  90 - 3   a  containing a CP and a data signal D 93 . The relay station of the related art performs signal processing on the OFDM symbols  90 - 1   a  to  90 - 3   a  received from the transmitter station and then transmits signal-processed OFDM symbols  90 - 1   b  to  90 - 3   b.  The OFDM symbol  90 - 1   b  is an OFDM symbol obtained by performing the signal processing on the OFDM symbol  90 - 1   a,  the OFDM symbol  90 - 2   b  is an OFDM symbol obtained by performing the signal processing on the OFDM symbol  90 - 2   a,  and the OFDM symbol  90 - 3   b  is an OFDM symbol obtained by performing the signal processing on the OFDM symbol  90 - 3   a.    
     Time “t 91 ” illustrated in  FIG. 10  indicates the amount of time taken for the signal processing performed by the relay station. Time “t 92 ” illustrated in  FIG. 10  indicates a propagation delay difference that occurs since the path from the transmitter station to the receiver station and the path from the relay station to the receiver station are different from each other. That is, the signal transmitted from the transmitter station arrives at the relay station with a delay corresponding to a time “t 93 =t 91 +t 92 ” relative to the signal transmitted from the transmitter station. 
     As illustrated in  FIG. 10 , when the amount of delay time “t 93 ” is larger than the duration of the CP, different OFDM symbols in the signals transmitted from the transmitter station and the relay station are spatially multiplexed to thereby cause inter-OFDM-symbol interference. More specifically, the OFDM symbol  90 - 1   b  transmitted from the relay station is spatially multiplexed with both the OFDM symbols  90 - 1   a  and  90 - 2   a  transmitted from the transmitter station and the OFDM symbol  90 - 2   b  transmitted from the relay station is spatially multiplexed with both the OFDM symbols  90 - 2   a  and  90 - 3   a  transmitted from the transmitter station. Thus, in the period of time “t 94 ”, the receiver station receives a signal resulting from interference between the different OFDM symbols  90 - 2   a  and  90 - 1   b,  and in the period of time “t 95 ”, the receiver station receives a signal resulting from interference between the OFDM symbols  90 - 3   a  and  90 - 2   b.  For such a reason, in the wireless communication system including the relay station, when the amount of delay caused by the signal processing performed by the transmitter station is larger than the duration of the CP, the quality of the signals received by the receiver station may deteriorate. 
     Embodiments of a wireless communication system, a relay station, a receiver station, and a wireless communication method disclosed herein will be described below in detail with reference to the accompanying drawings. The embodiments, however, are not intended to limit the wireless communication system, the relay station, the receiver station, and the wireless communication method disclosed herein. A wireless communication system that uses Orthogonal Frequency Division Multiplexing (OFDM) as one example of a transmission scheme will be described in the following embodiments by way of example. The wireless communication system disclosed herein, however, is also applicable to a wireless communication system that uses another transmission scheme, such as Orthogonal Frequency Division Multiple Access (OFDMA). 
     First Embodiment 
     Configuration of Wireless Communication System of First Embodiment 
     First, a wireless communication system according to a first embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating an example of configuration of a wireless communication system according to a first embodiment. As illustrated in  FIG. 1 , a wireless communication system  1  according to a first embodiment includes a transmitter station  100 , a relay station  200 , and a receiver station  300 . 
     The transmitter station  100  is, for example, a base station and transmits a signal to the receiver station  300 . Since the wireless communication system  1  according to the first embodiment employs OFDM as a transmission scheme, the signal transmitted by the transmitter station  100  is frequency-multiplexed and is represented by frequency domain and time domain. That is, the transmitter station  100  assigns control data and user data to each resource region determined by a combination of a predetermined frequency domain and a predetermined time domain, to thereby generate a transmission signal. 
     The transmitter station  100  in the first embodiment assigns a known signal to a first resource region determined by a combination of a predetermined frequency domain and a predetermined time domain and also assigns predetermined data to a second resource region that is different from the first resource region. The expression “predetermined data” includes, for example, null data and data containing a null symbol with a transmission power of zero. That is, the transmitter station  100  does not use the second resource region to transmit control data and user data. The known signal is also referred to as a “pilot signal”, a “reference signal”, or the like, and is used when the receiver station  300  or the like performs channel estimation (also called “propagation-path estimation”) and so on. A signal generated by the transmitter station  100  in the first embodiment may be referred to as a “transmitter-station signal”. Hereinafter, the predetermined data, such as a null symbol, may be referred to as “null data”. 
     Upon receiving a transmitter-station signal from the transmitter station  100 , the relay station  200  in the first embodiment relays the transmitter-station signal to the receiver station  300 . The relay station  200  performs, for example, signal processing for eliminating diffraction waves from the transmitter-station signal. The relay station  200  in the first embodiment interchanges the mapping positions of the known signal and the null data assigned in the transmitter-station signal. More specifically, the relay station  200  assigns the known signal, assigned to the first resource region in the transmitter-station signal, to the second resource region and sets the first resource region as a reserved region. For example, the relay station  200  assigns null data to the first resource region. A signal generated by the relay station  200  in the first embodiment may be referred to as a “relay signal” hereinafter. 
     The relay station  200  transmits the thus-generated relay signal with a delay corresponding to a predetermined amount of time. More specifically, the relay station  200  transmits the generated relay signal with a delay corresponding to the amount of time obtained by subtracting the time taken for the signal processing from the duration of the relay signal. 
     The receiver station  300  may be a mobile terminal device, such as a mobile phone, a personal handy-phone system (PHS), or a personal digital assistant (PDA). The receiver station  300  in the first embodiment receives a signal in which the transmitter-station signal transmitted by the transmitter station  100  and the relay signal transmitted by the relay station  200  are spatially multiplexed. 
     The first resource regions in the signal received by the receiver station  300  are assigned known signals transmitted by the transmitter station  100 . The first resource regions are assigned null data by the relay station  200 . The second resource regions in the signal received by the receiver station  300  are assigned known signals transmitted by the relay station  200 . The second resource regions are assigned null data by the transmitter station  100 . That is, the first resource regions in the signal received by the receiver station  300  are assigned only the known signals transmitted by the transmitter station  100  and the second resource regions are assigned only the known signals transmitted by the relay station  200 . 
     Thus, upon receiving the spatially multiplexed signal from the transmitter station  100  and the relay station  200 , the receiver station  300  may extract the known signals assigned by the transmitter station  100  from the first resource regions in the received signal. In addition, the receiver station  300  may extract the known signals, assigned by the relay station  200 , from the second resource regions in the received signal. 
     Assigning the known signals of the signals transmitted by the transmitter station  100  to the first resource regions and assigning the null data to the second resource regions may be predetermined for the system. For example, based on the number of known-signal resource blocks (RBs) contained in the resource-assignment information, the receiver station  300  may determine the frequency band of the known signals of the signals transmitted by the transmitter station  100 . When no signals are assigned to the frequency band of the known signals transmitted by the transmitter station  100 , the receiver station  300  may determine that known signals transmitted by the relay station  200  are contained in the frequency band to which null data, such as null symbols, are assigned. 
     With this arrangement, the receiver station  300  may independently perform channel estimation processing on the transmitter-station signal directly received from the transmitter station  100  and the relay signal received from the relay station  200 . By performing such independent channel estimation processing, the receiver station  300  separates the spatially multiplexed signal by using a channel separation algorithm for Multiple-Input Multiple-Output (MIMO). More specifically, upon receiving a spatially multiplexed signal from the transmitter station  100  and the relay station  200 , the receiver station  300  separates the signal into a transmitter-station signal and a relay signal. 
     Signals transmitted/received by the relay station  200  will now be described with reference to  FIG. 2 .  FIG. 2  illustrates examples of signals transmitted/received by the relay station  200  in the first embodiment. The upper stage in  FIG. 2  illustrates one example of a transmitter-station signal that the relay station  200  receives from the transmitter station  100 . The lower stage in  FIG. 2  illustrates one example of a relay signal transmitted by the relay station  200 . 
     In the example illustrated in  FIG. 2 , the transmitter station  100  transmits OFDM symbols  10   a,    20   a,  and  30   a.  More specifically, as illustrated in the upper stage in  FIG. 2 , the transmitter station  100  transmits an OFDM symbol  10   a  in which a frequency domain “f 0 ” is assigned a known signal R 10  and a frequency domain “f 1 ” is assigned null data, an OFDM symbol  20   a  in which a frequency domain “f 0 ” is assigned a known signal R 20  and a frequency domain “f 1 ” is assigned null data, and an OFDM symbol  30   a  in which a frequency domain “f 0 ” is assigned a known signal R 30  and a frequency domain “f 1 ” is assigned null data. 
     When the relay station  200  receives the transmitter-station signal illustrated in the upper stage in  FIG. 2 , it interchanges the mapping positions of the known signals and the null data contained in the transmitter-station signal, to thereby generate a relay signal, as illustrated in the lower stage in  FIG. 2 . 
     More specifically, the relay station  200  assigns the known signal R 10 , assigned to the frequency domain “f 0 ” in the OFDM symbol  10   a,  to the frequency domain “f 1 ” and assigns the null data to the frequency domain “f 0 ”, to thereby generate an OFDM symbol  10   b.  In this case, the relay station  200  does not change the mapping positions of data signals D 11  to D 13  assigned to frequency domains other than the frequency domains “f 0 ” and “f 1 ” in the OFDM symbol  10   a.  The term “data signals” as used herein refer to, for example, control signals containing control data and user-data signals containing user data. 
     Similarly, the relay station  200  assigns the known signal R 20 , assigned to the frequency domain “f 0 ” in the OFDM symbol  20   a,  to the frequency domain “f 1 ” and assigns the null data to the frequency domain “f 0 ”, to thereby generate an OFDM symbol  20   b.  The relay station  200  also assigns the known signal R 30 , assigned to the frequency domain “f 0 ” in the OFDM symbol  30   a,  to the frequency domain “f 1 ” and assigns the null data to the frequency domain “f 0 ”, to thereby generate an OFDM symbol  30   b.    
     In the manner described above, the relay station  200  generates a relay signal from a transmitter-station signal received from the transmitter station  100 . The relay station  200  then transmits the relay signal with a delay corresponding to the amount of time obtained by subtracting the time taken for the signal processing from the OFDM symbol duration. For example, time “t 11 ” illustrated in  FIG. 2  is assumed to indicate the amount of time taken for the signal processing performed by the relay station  200 . In this case, the relay station  200  transmits the relay signal, such as the OFDM symbols  10   b,    20   b,  and  30   b,  with a delay corresponding to a time “t 12 ” obtained by subtracting the signal processing time “t 11 ” from the OFDM symbol duration “t 10 ”. 
     Next, signals received by the receiver station  300  when the relay signal illustrated in the lower stage in  FIG. 2  is transmitted by the relay station  200  will be described with reference to  FIG. 3 .  FIG. 3  illustrates examples of signals received by the receiver station  300  in the first embodiment. The upper stage in  FIG. 3  illustrates signal components that the receiver station  300  receives from the transmitter station  100  and the lower stage in  FIG. 3  illustrates signal components that the receiver station  300  receives from the relay station  200 . Although the signal components transmitted from the transmitter station  100  and the signal components transmitted from the relay station  200  are separately illustrated in  FIG. 3 , signal components that are simultaneously received by the receiver station  300  are spatially multiplexed in practice. Time “t 13 ” illustrated in  FIG. 3  indicates a propagation delay difference that occurs since a path of a transmitter-station signal and a path of a relay signal are different from each other. 
     As illustrated in  FIG. 3 , the receiver station  300  receives a signal in which the OFDM symbols  10   a,    20   a,  and  30   a  transmitted by the transmitter station  100  and the OFDM symbols  10   b,    20   b,  and  30   b  transmitted by the relay station  200  are spatially multiplexed. More specifically, the OFDM symbols  20   a  and  30   a  transmitted by the transmitter station  100  and the OFDM symbols  10   b  and  20   b  transmitted by the relay station  200  are spatially multiplexed. 
     In the example illustrated in  FIG. 3 , since the OFDM symbol  10   a  is not spatially multiplexed with another OFDM symbol, the receiver station  300  may extract the known signal R 10  from the OFDM symbol  10   a.  Based on the known signal R 10 , the receiver station  300  extracts the data signals D 11  to D 13  from the OFDM symbol  10   a.    
     In the OFDM symbol in which the OFDM symbols  20   a  and  10   b  are spatially multiplexed, the frequency domain “f 0 ” is assigned only the known signal R 20  and the frequency domain “f 1 ” is assigned only the known signal R 10 . Thus, the receiver station  300  may extract, from the OFDM symbol in which the OFDM symbols  20   a  and  10   b  are spatially multiplexed, the known signal R 20  transmitted by the transmitter station  100  and the known signal R 10  transmitted by the relay station  200 . 
     The receiver station  300  uses the extracted known signals R 20  and R 10  to perform independent channel estimation processing on the path of the transmitter-station signal and the path of the relay signal. As a result, the receiver station  300  separates the OFDM symbol in which the OFDM symbols  20   a  and  10   b  are spatially multiplexed into the OFDM symbol  20   a  and the OFDM symbol  10   b.  The receiver station  300  then extracts the data signals D 21  to D 23  from the separated OFDM symbol  20   a  and also extracts the data signals D 11  to D 13  from the separated OFDM symbol  10   b.    
     Similarly, the receiver station  300  separates the OFDM symbol in which the OFDM symbols  30   a  and  20   b  are spatially multiplexed into the OFDM symbol  30   a  and the OFDM symbol  20   b.  The receiver station  300  then extracts data signals D 31  to D 33  from the separated OFDM symbol  30   a  and also extracts data signals D 21  to D 23  from the separated OFDM symbol  20   b.  The receiver station  300  also extracts data signals D 31  to D 33  from the OFDM symbol  30   b.    
     The receiver station  300  then combines the same data signals of the data signals extracted from the OFDM symbols  10   a,    20   a,    30   a,    10   b,    20   b,  and  30   b.  More specifically, the receiver station  300  stores, in a predetermined buffer, the data D 11  to D 13  extracted from the OFDM symbol  10   a.  The receiver station  300  then combines the data signal D 11  extracted from the OFDM symbol  10   b  and the data D 11  stored in the buffer. In the same manner, the receiver station  300  performs combination with respect to the data D 12  and D 13 . The receiver station  300  performs log-likelihood ratio (LLR) combining processing for combining likelihood information of the same data contained in the OFDM symbols. 
     As described above, the transmitter station  100  in the first embodiment transmits a transmitter-station signal in which known signals and null data are assigned. Upon receiving the transmitter-station signal, the relay station  200  in the first embodiment transmits a relay signal in which the mapping positions of the known signals and the null data are interchanged. Even when receiving the spatially multiplexed signal from the transmitter station  100  and the relay station  200 , the receiver station  300  may perform independent channel estimation processing on the path of the transmitter-station signal and the path of the relay signal. Thus, upon receiving the spatially multiplexed signal from the transmitter station  100  and the relay station  200 , the receiver station  300  in the first embodiment may perform reception processing that is similar to reception processing for a MIMO-compliant transmitter station. Hence, the wireless communication system  1  according to the first embodiment may improve the quality of the signals received by the receiver station  300 . 
     Although  FIG. 2  illustrates an example in which the transmitter station  100  assigns known signals to the frequency domains “f 0 ” and assigns null data to the frequency domains “f 1 ”, the resource regions to which the transmitter station  100  assigns the known signals and null data are not limited to the example illustrated in  FIG. 2 . Specifically, the transmitter station  100  may assign the known signals and null data to different resource regions. For example, in the example illustrated in  FIG. 2 , the transmitter station  100  may assign the known signals to the frequency domains “f 1 ” and assign the null data to the frequency domains “f 2 ”. 
     Although no description has been given above, the relay station  200  in the wireless communication system  1  in the first embodiment may relay the signal to a specific receiver station and does not need to relay the signal to a receiver station other than the specific receiver station. The transmitter station  100  may transmit the transmitter-station signal (illustrated in the upper stage in  FIG. 2 ) to the specific receiver station and does not need to transmit, to a receiver station other than the specific receiver station. 
     Configuration of Transmitter Station in First Embodiment 
     The transmitter station  100  in the first embodiment will be described next with reference to  FIG. 4 .  FIG. 4  is a block diagram of an example of configuration of the transmitter station  100  in the first embodiment. As illustrated in  FIG. 4 , the transmitter station  100  includes antennas  101  and  102 , a reception radio-frequency (RF) unit  111 , a control-signal demodulator  112 , and a relay-station-user selector  120 . 
     The antenna  101  receives a signal transmitted from an external apparatus (not illustrated). The antenna  101  receives, for example, an uplink signal transmitted from the receiver station  300 . The antenna  102  transmits a signal to an external apparatus (not illustrated). For example, the antenna  102  transmits a downlink signal to the relay station  200  and the receiver station  300 . Although  FIG. 4  illustrates an example in which the transmitter station  100  has both the receive antenna  101  and the transmit antenna  102 , the transmitter station  100  may have a shared antenna via which transmission and reception are possible, instead of the receive antenna  101  and the transmit antenna  102 . 
     The reception RF unit  111  performs various types of processing on the signal received by the antenna  101 . Examples of the processing that the reception RF unit  111  performs include frequency conversion processing for converting a radio frequency band into a baseband, orthogonal demodulation processing, and analog-to-digital (A/D) conversion processing. 
     The control-signal demodulator  112  performs demodulation processing and the like on, of the signals output from the reception RF unit  111 , the control signal transmitted by the receiver station  300 . The control signal transmitted by the receiver station  300  contains position information indicating the location of the receiver station  300 . Upon receiving the control signal containing the position information from the receiver station  300 , the control-signal demodulator  112  extracts the receiver station  300  position information from the control signal. 
     Based on the receiver station  300  position information output from the control-signal demodulator  112 , the relay-station-user selector  120  determines whether or not the receiver station  300  is to be set as a receiver station for receiving a signal relayed by the relay station  200 . The receiver station for receiving the signal relayed by the relay station  200  may be referred to as a “relay-station user” hereinafter. 
     More specifically, when the distance between the receiver station  300  and the relay station  200  is smaller than a predetermined threshold, the relay-station-user selector  120  determines that the receiver station  300  is to be set as the relay-station user. On the other hand, when the distance between the receiver station  300  and the relay station  200  is larger than or equal to the predetermined threshold, the relay-station-user selector  120  determines that the receiver station  300  is not to be set as the relay-station user. This is because, when the receiver station  300  and the relay station  200  are not located a short distance from each other, there are, for example, a case in which the receiver station  300  is not located within the communication area of the relay station  200  and a case in which the receiver station  300  may not receive the signal, relayed by the relay station  200 , with a high quality. 
     The transmitter station  100  also receives a data signal containing user data and so on and performs reception processing on the data signal. A description of the reception processing performed on the data signal including user data and so on is omitted in  FIG. 4 . 
     As illustrated in  FIG. 4 , the transmitter station  100  further includes a scheduler unit  130 , error-correction encoders  141  and  142 , a control-information modulator  151 , a data-information modulator  152 , a known-signal generator  160 , and a physical-channel multiplexer  170 . The transmitter station  100  further includes an inverse fast Fourier transform (IFFT) unit  181 , a cyclic prefix (CP) adding unit  182 , and a transmission RF unit  183 . 
     The scheduler unit  130  assigns control data, user data, and so on to be transmitted to the receiver station  300  to resources. More specifically, the scheduler unit  130  outputs, to the error-correction encoder  141 , resource-assignment information regarding the resources assigned the user data and so on and control data containing, for example, information indicating that the receiver station  300  is the relay-station user. The scheduler unit  130  outputs, to the error-correction encoder  142 , the user data assigned to the resources. 
     When the relay-station-user selector  120  determines that the receiver station  300  is to be set as the relay-station user, the scheduler unit  130  outputs, to the error-correction encoder  141 , information indicating that the receiver station  300  is the relay-station user. On the other hand, when the relay-station-user selector  120  determines that the receiver station  300  is not to be set as the relay-station user, the scheduler unit  130  outputs, to the error-correction encoder  141 , information indicating that the receiver station  300  is not the relay-station user. The information indicating whether or not the receiver station  300  is the relay-station user may hereinafter be referred to as “relay-station-user information”. 
     The error-correction encoder  141  performs error-correction encoding processing on the control data assigned to the resources by the scheduler unit  130 . The error-correction encoder  142  performs error-correction encoding processing on the user data assigned to the resources by the scheduler unit  130 . 
     The control-information modulator  151  generates a control signal by performing modulation processing on the control data on which the error-correction encoding processing was performed by the error-correction encoder  141 . The data-information modulator  152  generates a user-data signal by performing modulation processing on the user data on which the error-correction encoding processing was performed by the error-correction encoder  142 . 
     The known-signal generator  160  generates a known signal that is known to the receiver station  300 . The known signal generated by the known-signal generator  160  is also called a “pilot signal” or “reference signal” and is used when the receiver station  300  performs channel estimation processing and so on. 
     The physical-channel multiplexer  170  frequency-multiplexes the control signal output from the control-information modulator  151 , the user data output from the data-information modulator  152 , and the known signal output from the known-signal generator  160 . 
     For frequency-multiplexing the known signal, the physical-channel multiplexer  170  in the first embodiment assigns null data to a predetermined frequency domain. For example, as in the example illustrated in the upper stage in  FIG. 2 , for each OFDM symbol, the physical-channel multiplexer  170  assigns a known signal to a predetermined frequency domain “f 0 ” and assigns null data to a frequency domain “f 1 ” that is different from the frequency domain “f 0 ”. 
     The IFFT unit  181  generates a time-domain signal by performing IFFT processing on the frequency-domain signal frequency-multiplexed by the physical-channel multiplexer  170 . The CP adding unit  182  divides the signal, generated by the IFFT unit  181 , into signals according to an OFDM symbol duration, and adds a CP to each of the signals having the OFDM symbol duration. 
     The transmission RF unit  183  performs various types of processing on the signal output from the CP adding unit  182 . Examples of the processing that the transmission RF unit  183  performs on the signal output from the CP adding unit  182  include digital-to-analog (D/A) conversion processing, orthogonal modulation processing, and frequency conversion processing for converting a baseband into a radio frequency band. The transmission RF unit  183  outputs a signal, obtained by the various types of processing, via the antenna  102 . 
     The reception RF unit  111  and the transmission RF unit  183  are included in an RF processor  1 A, which may be realized by hardware, for example, an integrated circuit, such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA). The control-signal demodulator  112 , the relay-station-user selector  120 , the scheduler unit  130 , the error-correction encoders  141  and  142 , the control-information modulator  151 , the data-information modulator  152 , the known-signal generator  160 , the physical-channel multiplexer  170 , the IFFT unit  181 , and the CP adding unit  182  are included in a baseband processor  1 B, which may be realized by, for example, hardware, such as a central processing unit (CPU) or a micro processing unit (MPU). That is, the RF processor  1 A and the baseband processor  1 B may be realized by pieces of hardware that are different from each other. The baseband processor  1 B is one example of a first processor. 
     Configuration of Relay Station in First Embodiment 
     The relay station  200  in the first embodiment will be described next with reference to  FIG. 5 .  FIG. 5  is a block diagram illustrating an example of configuration of the relay station  200  in the first embodiment. As illustrated in  FIG. 5 , the relay station  200  includes antennas  201  and  202 , a reception RF unit  211 , a diffraction-wave eliminator  212 , a CP remover  213 , and a fast Fourier transform (FFT) unit  214 . 
     The antenna  201  receives a signal transmitted from an external apparatus (not illustrated). The antenna  201  receives, for example, a signal transmitted from the transmitter station  100 . The antenna  202  transmits a signal to an external apparatus (not illustrated). The antenna  202  transmits a signal to, for example, the receiver station  300 . The relay station  200  may have a shared antenna via which transmission and reception are possible, instead of the antennas  201  and  202 . 
     In the example illustrated in  FIG. 5 , the signal transmitted from the transmit antenna  202  may be received, as diffraction waves, by the receive antenna  201 . When received by the receive antenna  201 , such diffraction waves may cause internal circuitry of the relay station  200  to oscillate. 
     The reception RF unit  211  performs various types of processing on the signal received by the antenna  201 . For example, similarly to the reception RF unit  111  illustrated in  FIG. 4 , the reception RF unit  211  performs frequency conversion processing, orthogonal demodulation processing, A/D conversion processing, and so on. 
     By using the signal output from a delay unit  270  (described below), the diffraction-wave eliminator  212  eliminates diffraction waves from the signal input from the reception RF unit  211 . With this arrangement, even when the antenna  201  receives diffraction waves, the diffraction-wave eliminator  212  may prevent the internal circuitry of the relay station  200  from oscillating. 
     The CP remover  213  removes the CP from the signal output from the diffraction-wave eliminator  212 . The FFT unit  214  performs FFT processing on a signal, output from the CP remover  213 , to generate a frequency-domain signal. 
     As illustrated in  FIG. 5 , the relay station  200  includes a known-signal extractor  221 , a control-signal extractor  222 , a channel estimator  230 , a control-signal demodulator  240 , and a mapping controller  250 . 
     The known-signal extractor  221  extracts the known signal from the frequency-domain signal generated by the FFT unit  214 . The control-signal extractor  222  extracts the control signal from the frequency-domain signal generated by the FFT unit  214 . 
     The channel estimator  230  performs channel estimation processing based on the known signal extracted by the known-signal extractor  221 . The control-signal demodulator  240  performs, for example, channel-compensation processing, demodulation processing, and error-correction decoding processing on the control signal extracted by the control-signal extractor  222 . As a result, the control-signal demodulator  240  extracts the resource-assignment information, the relay-station-user information, and so on from the control signal transmitted by the transmitter station  100 . The control-signal demodulator  240  then outputs the resource-assignment information, the relay-station-user information, and so on to the mapping controller  250 . 
     Based on the multiple types of information output from the control-signal demodulator  240 , the mapping controller  250  performs processing for adjusting the mapping positions of subcarriers with respect to the frequency-domain signal output from the FFT unit  214 . 
     More specifically, the mapping controller  250  determines whether or not the receiver station  300  is the relay-station user, based on the relay-station-user information output from the control-signal demodulator  240 . When the receiver station  300  is not the relay-station user, the mapping controller  250  substitutes “0” for the signal included in the signals output from the FFT unit  214  and destined for the receiver station  300 . This is because, when the receiver station  300  is not the relay-station user, the relay station  200  does not relay the signal, received from the transmitter station  100  and destined for the receiver station  300 , to the receiver station  300 . 
     On the other hand, when the receiver station  300  is the relay-station user, the mapping controller  250  interchanges the mapping positions of the known signals and the null data assigned to the signals destined for the receiver station  300 . That is, the mapping controller  250  assigns, of the signals destined for the receiver station  300 , the known signals to the resource regions to which the null data has been assigned and also assigns the null data to the resource regions to which the known signal has been assigned. 
     As illustrated in  FIG. 5 , the relay station  200  further includes an IFFT unit  261 , a CP adding unit  262 , a delay unit  270 , and a transmission RF unit  280 . The IFFT unit  261  performs IFFT processing on a signal, output from the mapping controller  250 , to generate a time-domain signal. The CP adding unit  262  divides the signal, generated by the IFFT unit  261 , into signals according to an OFDM symbol duration, and adds a CP to each of the signals having the OFDM symbol duration. 
     After waiting for a period of time obtained by subtracting the time taken for the signal processing from the duration of the relay signal transmitted by the relay station  200 , the delay unit  270  outputs the signal, input from the CP adding unit  262 , to the transmission RF unit  280 . The time taken for the signal processing corresponds to, for example, the time from when the antenna  201  receives the signal until the CP adding unit  262  completes the CP addition processing. 
     For example, when the transmitter station  100  assigns a known signal to each of N OFDM symbols, the delay unit  270  waits for a period of time obtained by subtracting the time taken for the signal processing from N-times the OFDM symbol duration. In the example illustrated in  FIG. 3 , the transmitter station  100  assigns a known signal to each OFDM symbol. In such a case, the delay unit  270  waits for a period of time obtained by subtracting the time taken for the signal processing from the duration of one OFDM symbol. 
     The transmission RF unit  280  performs various types of processing on the signal output from the delay unit  270 . For example, similarly to the transmission RF unit  183  illustrated in  FIG. 4 , the transmission RF unit  280  performs D/A conversion processing, orthogonal modulation processing, frequency conversion processing, and so on. 
     The reception RF unit  211  and the transmission RF unit  280  are included in an RF processor  2 A, which may be realized by hardware, for example, an integrated circuit, such as an ASIC or FPGA. The diffraction-wave eliminator  212 , the CP remover  213 , the FFT unit  214 , the known-signal extractor  221 , the control-signal extractor  222 , the channel estimator  230 , the control-signal demodulator  240 , the mapping controller  250 , the IFFT unit  261 , the CP adding unit  262 , and the delay unit  270  are included in a baseband processor  2 B, which may be realized by hardware, such as a CPU or MPU. That is, the RF processor  2 A and the baseband processor  2 B may be realized by pieces of hardware that are different from each other. The baseband processor  2 B is one example of a second processor. 
     Configuration of Receiver Station in First Embodiment 
     The receiver station  300  in the first embodiment will be described next with reference to  FIG. 6 .  FIG. 6  is a block diagram illustrating an example of configuration of the receiver station  300  in the first embodiment. As illustrated in  FIG. 6 , the receiver station  300  includes antennas  301  and  302 , a position-information detector  311 , a control-signal generator  312 , and a transmission RF unit  313 . 
     The antenna  301  receives a signal transmitted from an external apparatus (not illustrated). For example, the antenna  301  receives a downlink signal transmitted from the transmitter station  100  and the relay station  200 . The antenna  302  transmits a signal to an external apparatus (not illustrated). For example, the antenna  302  transmits an uplink signal to the transmitter station  100 . The receiver station  300  may have a shared antenna via which transmission and reception are possible, instead of the antennas  301  and  302 . 
     The position-information detector  311  detects the location of the receiver station  300 . For example, the position-information detector  311  detects the location of the receiver station  300 , for example, by receiving signals transmitted from global positioning system (GPS) satellites. The position-information detector  311  then outputs position information indicating the location of the receiver station  300  to the control-signal generator  312 . 
     The control-signal generator  312  in the first embodiment generates a control signal. More specifically, the control-signal generator  312  generates a control signal containing the receiver station  300  position information detected by the position-information detector  311 . 
     The transmission RF unit  313  performs various types of processing on the control signal generated by the control-signal generator  312 . For example, similarly to the transmission RF unit  183  illustrated in  FIG. 4 , the transmission RF unit  313  performs D/A conversion processing, orthogonal modulation processing, frequency conversion processing, and so on. The transmission RF unit  313  transmits the control signal, obtained by the frequency conversion processing, to the transmitter station  100  via the antenna  302 . 
     The receiver station  300  also performs processing for generating a data signal containing user data and so on and transmitting the data signal containing the user data and so on. A description of the processing for transmitting the data signal containing the user data and so on is omitted in  FIG. 6 . 
     As illustrated in  FIG. 6 , the receiver station  300  includes a reception RF unit  321 , a CP remover  322 , an FFT unit  323 , and a reception-mode switching unit  330 . The reception RF unit  321  performs various types of processing on the signal received by the antenna  301 . For example, similarly to the reception RF unit  111  illustrated in  FIG. 4 , the reception RF unit  321  performs frequency conversion processing, orthogonal demodulation processing, A/D conversion processing, and so on. 
     The CP remover  322  removes the CP from the signal output from the reception RF unit  321 . The FFT unit  323  performs FFT processing on a signal, output from the CP remover  322 , to generate a frequency-domain signal. 
     The reception-mode switching unit  330  receives control data from an error-correction decoder  392  (described below). Based on relay-station-user information contained in the control data, the reception-mode switching unit  330  determines whether or not the receiver station  300 , which is the local station, is the relay-station user. When the local station is not the relay-station user, the reception-mode switching unit  330  outputs the signal, input from the FFT unit  323 , to a non-multiplexed-signal processor  340  (described below). On the other hand, when the local station is the relay-station user, the reception-mode switching unit  330  outputs the signal, input from the FFT unit  323 , to a multiplexed-signal processor  350  (described below). 
     As illustrated in  FIG. 6 , the receiver station  300  includes, in addition to the non-multiplexed-signal processor  340  and the multiplexed-signal processor  350 , a switching unit  360 , an LLR combination controller  370 , a combining unit  380 , and error-correction decoders  391  and  392 . 
     When the local station is not the relay-station user, the non-multiplexed-signal processor  340  performs various types of processing on the frequency-domain signal input from the reception-mode switching unit  330 . The non-multiplexed-signal processor  340  includes a known-signal extractor  341 , a control-signal extractor  342 , a data-signal extractor  343 , a channel estimator  344 , a control-signal demodulator  345 , and a data-signal demodulator  346 . 
     Based on the resource-assignment information input from the error-correction decoder  392 , the known-signal extractor  341  extracts the known signal from the signal input from the reception-mode switching unit  330 . Based on the resource-assignment information input from the error-correction decoder  392 , the control-signal extractor  342  extracts the control signal from the signal input from the reception-mode switching unit  330 . Based on the resource-assignment information input from the error-correction decoder  392 , the data-signal extractor  343  extracts the user-data signal from the signal input from the reception-mode switching unit  330 . 
     The channel estimator  344  performs channel estimation processing based on the known signal extracted by the known-signal extractor  341 . More specifically, the channel estimator  344  estimates a wireless channel state by determining the correlation between the known signal extracted by the known-signal extractor  341  and the signal known to the receiver station  300 . 
     Based on a result of the channel estimation processing performed by the channel estimator  344 , the control-signal demodulator  345  performs channel compensation and demodulation processing on the control signal extracted by the control-signal extractor  342 . Based on a result of the channel estimation processing performed by the channel estimator  344 , the data-signal demodulator  346  performs channel compensation and demodulation processing on the user-data signal extracted by the data-signal extractor  343 . 
     When the local station is the relay-station user, the multiplexed-signal processor  350  performs various types of processing on the frequency-domain signal input from the reception-mode switching unit  330 . The multiplexed-signal processor  350  includes a known-signal extractor  351 , a data-control-signal extractor  352 , a channel estimator  353 , and a channel separator  354 . 
     Based on the resource-assignment information input from the error-correction decoder  392 , the known-signal extractor  351  extracts the known signal from the signal input from the reception-mode switching unit  330 . As described above with reference to  FIGS. 2 and 3 , the relay station  200  assigns the null data to the resource region to which the known signal has been assigned by the transmitter station  100  and the relay station  200  also assigns the known signal to the resource region to which the null data has been assigned by the transmitter station  100 . Thus, the known-signal extractor  351  may extract, from the signal in which the transmitter-station signal and the relay signal are spatially multiplexed, the known signals transmitted by the transmitter station  100  and the known signals transmitted by the relay station  200 . 
     Based on the resource-assignment information input from the error-correction decoder  392 , the data-control-signal extractor  352  extracts the control signal and the user-data signal from the signal input from the reception-mode switching unit  330 . When the local station is the relay-station user, the receiver station  300  may receive a signal in which control signals and user-data signals are spatially multiplexed. For example, in the example illustrated in  FIG. 3 , when the data signals D 11  to D 13  are user-data signals and the data signals D 21  to D 23  are control signals, the receiver station  300  receives a signal in which the control signals and the user-data signals are multiplexed. Thus, the data-control-signal extractor  352  may extract, from the signals input from the reception-mode switching unit  330 , only control signals, only user-data signals, or a signal in which control signals and user-data signals are spatially multiplexed. 
     The channel estimator  353  performs channel estimation processing based on the known signals extracted by the known-signal extractor  351 . As described above, the known-signal extractor  351  extracts, from the signal in which the transmitter-station signal and the relay signal are spatially multiplexed, the known signals transmitted by the transmitter station  100  and the known signals transmitted by the relay station  200 . Thus, upon receiving the signal in which the transmitter-station signal and the relay signal are multiplexed, the channel estimator  353  may perform independent channel estimation processing on both the path of the transmitter-station signal and the path of the relay signal. 
     Based on the result of the channel estimation processing performed by the channel estimator  353 , the channel separator  354  separates the signal, extracted by the data-control-signal extractor  352 , into the transmitter-station signal and the relay signal. More specifically, based on the channel estimation processing that the channel estimator  353  individually performed on the path of the transmitter-station signal and the path of the relay signal, the channel separator  354  separates the signal in which the transmitter-station signal and the relay signal are spatially multiplexed into the transmitter-station signal and the relay signal. For example, the channel separator  354  uses a MIMO channel separation algorithm, such as MMSE (minimum means square error) equalization, to separate the spatially multiplexed signal into the transmitter-station signal and the relay signal. The channel separator  354  further extracts the control signals and the user-data signals from the separated transmitter-station signal and also extracts the control signals and the user-data signals from the separated relay signal. The channel separator  354  then outputs the extracted user-data signals to a reception-mode switching unit  361  and outputs the extracted control signals to a reception-mode switching unit  362 . 
     The switching unit  360  includes the reception-mode switching unit  361  and the reception-mode switching unit  362 . The reception-mode switching unit  361  determines whether or not the receiver station  300  that is the local station is the relay-station user, based on the relay-station-user information input from the error-correction decoder  392 . When the local station is not the relay-station user, the reception-mode switching unit  361  outputs the user-data signals, input from the data-signal demodulator  346 , to an LLR combining unit  381  (described below). On the other hand, when the local station is the relay-station user, the reception-mode switching unit  361  outputs the user-data signals, input from the channel separator  354 , to the LLR combining unit  381 . 
     The reception-mode switching unit  362  determines whether or not the receiver station  300  that is the local station is the relay-station user, based on the relay-station-user information input from the error-correction decoder  392 . When the local station is not the relay-station user, the reception-mode switching unit  362  outputs the control signals, input from the control-signal demodulator  345 , to an LLR combining unit  382 . On the other hand, when the local station is the relay-station user, the reception-mode switching unit  362  outputs the control signals, input from the channel separator  354 , to the LLR combining unit  382 . 
     Based on the relay-station-user information input from the error-correction decoder  392 , the LLR combination controller  370  controls the combination processing performed by the combining unit  380 . More specifically, the LLR combination controller  370  determines whether or not the receiver station  300  that is the local station is the relay-station user, based on the relay-station-user information input from the error-correction decoder  392 . When the local station is the relay-station user, the LLR combination controller  370  controls the combining unit  380  so that it performs the combination processing. When the local station is not the relay-station user, the LLR combination controller  370  controls the combining unit  380  so that it does not perform the combination processing. 
     The combining unit  380  includes the LLR combining units  381  and  382 . When the LLR combining unit  381  is controlled by the LLR combination controller  370  so as not to perform the combination processing, the LLR combining unit  381  outputs the user-data signals, input from the reception-mode switching unit  361 , to the error-correction decoder  391 . 
     On the other hand, when the LLR combining unit  381  is controlled by the LLR combination controller  370  so as to perform the combination processing, the LLR combining unit  381  combines the user-data signals input from the reception-mode switching unit  361 . In this case, the LLR combining unit  381  stores, in a predetermined buffer, the same user-data signals input from the reception-mode switching unit  361 . For example, after holding all the same user-data signals in the buffer, the LLR combining unit  381  performs LLR combination processing on the user-data signals in the buffer. The LLR combining unit  381  then outputs the user-data signals, obtained by the LLR combination processing, to the error-correction decoder  391 . 
     When the LLR combining unit  382  is controlled by the LLR combination controller  370  so as not to perform the combination processing, the LLR combining unit  382  outputs the control signals, input from the reception-mode switching unit  362 , to the error-correction decoder  392 . On the other hand, when the LLR combining unit  382  is controlled by the LLR combination controller  370  so as to perform the combination processing, the LLR combining unit  382  receives the same control signals from the reception-mode switching unit  362  and performs the LLR combination processing on the same control signals. The LLR combining unit  382  then outputs the control signals, obtained by the LLR combination processing, to the error-correction decoder  392 . 
     The error-correction decoder  391  performs error-correction decoding processing on the user-data signals output from the LLR combining unit  381 . As a result, the error-correction decoder  391  obtains the user data from the user-data signals. 
     The error-correction decoder  392  performs error-correction decoding processing on the control signals output from the LLR combining unit  382 . As a result, the error-correction decoder  392  obtains, from the control signals, the control information containing the resource-assignment information, the relay-station-user information, and so on. The error-correction decoder  392  outputs the various types of information, contained in the control information, to the reception-mode switching unit  330 , the non-multiplexed-signal processor  340 , the multiplexed-signal processor  350 , the switching unit  360 , and the LLR combination controller  370 . 
     When the non-multiplexed-signal processor  340  and the multiplexed-signal processor  350  do not operate simultaneously, the control-signal extractor  342 , the data-signal extractor  343 , and the data-control-signal extractor  352  may be shared as a single unit. The channel estimator  344  and the channel estimator  353  may also be shared as a single unit. For example, for a system aimed for a reduction in the hardware size, the processors may be shared. For a system aimed for a reduction in processing time by using dedicated hardware, the processors do not necessarily have to be shared. 
     The reception RF unit  321  and the transmission RF unit  313  constitute an RF processor  3 A, which may be realized by hardware, for example, an integrated circuit, such as an ASIC or FPGA. The position-information detector  311 , the control-signal generator  312 , the CP remover  322 , the FFT unit  323 , the reception-mode switching unit  330 , the non-multiplexed-signal processor  340 , the multiplexed-signal processor  350 , the switching unit  360 , the LLR combination controller  370 , the combining unit  380 , the error-correction decoder  391 , and the error-correction decoder  392  are included in a baseband processor  3 B, which may be realized by hardware, such as a CPU or MPU. That is, the RF processor  3 A and the baseband processor  3 B may be realized by pieces of hardware that are different from each other. The baseband processor  3 B is one example of a third processor. 
     Sequence of Processing performed by Wireless Communication System of First Embodiment 
     Next, the sequence of processing performed by the wireless communication system  1  according to the first embodiment will be described with reference to  FIG. 7 .  FIG. 7  is a sequence diagram illustrating a procedure of processing performed by the wireless communication system  1  according to the first embodiment.  FIG. 7  illustrates a procedure of processing performed by the transmitter station  100 , the relay station  200 , and the receiver station  300  in the first embodiment. 
     As illustrated in  FIG. 7 , in operation S 11 , the position-information detector  311  in the receiver station  300  obtains position information indicating the location of the receiver station  300 . Subsequently, in operation S 12 , the receiver station  300  transmits the obtained position information to the transmitter station  100 . For example, the receiver station  300  transmits a control signal containing the position information to the transmitter station  100 . 
     Subsequently, in operation S 13 , based on the position information received from the receiver station  300 , the relay-station-user selector  120  in the transmitter station  100  determines whether or not the receiver station  300  is to be set as the relay-station user. For example, the relay-station-user selector  120  determines whether or not the receiver station  300  is the relay-station user, based on the distance between the receiver station  300  and the relay station  200 . In the example illustrated in  FIG. 7 , the relay-station-user selector  120  is assumed to set the receiver station  300  as the relay-station user. 
     In operation S 14 , the transmitter station  100  transmits, to the relay station  200  and the receiver station  300 , relay-station-user information indicating whether or not the receiver station  300  is the relay-station user. For example, the transmitter station  100  transmits, to the relay station  200  and the receiver station  300 , a control signal containing resource-assignment information and the relay-station-user information. As a result, the relay station  200  and the receiver station  300  may check whether or not the receiver station  300  is the relay-station user. 
     In operation S 15 , the transmitter station  100  generates a transmitter-station signal in which null data are assigned to the resource regions, the number thereof being equal to the number of resource regions to which known signals are assigned, and transmits the generated transmitter-station signal. The relay station  200  and the receiver station  300  receive the transmitter-station signal transmitted by the transmitter station  100 . 
     Upon receiving the transmitter-station signal transmitted by the transmitter station  100 , the relay station  200  performs predetermined reception processing in operation S 16 . The relay station  200  performs, for example, frequency conversion processing, orthogonal demodulation processing, A/D conversion processing, and diffraction-wave elimination processing. 
     In operation S 17 , the relay station  200  generates a relay signal by interchanging the mapping positions of the known signals and the null data assigned in the transmitter-station signal received from the transmitter station  100  and then transmits the generated relay signal. In this case, the relay station  200  outputs the generated relay signal with a delay corresponding to the amount of time obtained by subtracting the time taken for the signal processing from the duration of the relay signal. 
     In operation S 18 , the receiver station  300  combines the signals transmitted by the transmitter station  100  and the relay station  200 . More specifically, by using the known signals transmitted by the transmitter station  100  and the known signals transmitted by the relay station  200 , the receiver station  300  performs channel estimation on both the path of the transmitter-station signal and the path of the relay signal. The receiver station  300  then uses the MIMO channel separation algorithm to separate the spatially multiplexed signal into the transmitter-station signal and the relay signal and combines the same control signals and data signals contained in the separated transmitter-station signal and relay signal. 
     Other Examples of Transmitter-Station Signal and Relay Signal 
     Although an example in which the transmitter station  100  assigns a known signal and null data to each OFDM symbol has been described above, the transmitter station  100  may assign a known signal and null data to each set of two or more OFDM symbols. An example in which a known signal and null data are assigned to each set of OFDM symbols will be described below with reference to  FIGS. 8 and 9 . 
       FIG. 8  illustrates examples of signals transmitted/received by the relay station  200  in the first embodiment. The upper stage in  FIG. 8  illustrates one example of a transmitter-station signal that the relay station  200  receives from the transmitter station  100 . The lower stage in  FIG. 8  illustrates one example of a relay signal transmitted by the relay station  200 . Time “t 21 ” illustrated in  FIG. 8  indicates the amount of time taken for the signal processing performed by the relay station  200 . 
     In the example illustrated in  FIG. 8 , the transmitter station  100  transmits subframes  40   a,    50   a,  and  60   a.  Each subframe contains four OFDM symbols. For example, the subframe  40   a  contains OFDM symbols  41   a  to  44   a,  the subframe  50   a  contains OFDM symbols  51   a  to  54   a,  and the subframe  60   a  contains OFDM symbols  61   a  to  64   a.    
     In the example illustrated in  FIG. 8 , the transmitter station  100  assigns null data to, of the OFDM symbols in one subframe, the OFDM symbol adjacent to the OFDM symbol to which a known signal is assigned. For example, the transmitter station  100  assigns a known signal R 41  to a frequency domain “f 0 ” in the OFDM symbol  41   a  contained in the subframe  40   a  and assigns null data to a frequency domain “f 0 ” in the OFDM symbol  42   a.  For example, the transmitter station  100  assigns a known signal R 42  to a frequency domain “f 2 ” in the OFDM symbol  41   a  contained in the subframe  40   a  and assigns null data to a frequency domain “f 2 ” in the OFDM symbol  42   a.  Similarly, with respect to each of the subframes  50   a  and  60   a,  the transmitter station  100  assigns known signals and null data to respective different OFDM symbols in the same subframe. 
     Upon receiving the subframes  40   a,    50   a,  and  60   a  illustrated in the upper stage in  FIG. 8 , the relay station  200  interchanges the mapping positions of the known signals and the null data. That is, in the example illustrated in  FIG. 8 , the relay station  200  assigns the known signal R 41 , assigned to the frequency domain “f 0 ” in the OFDM symbol  41   a  contained in the subframe  40   a,  to the frequency domain “f 0 ” in the OFDM symbol  42   a  and also assigns the null data to the frequency domain “f 0 ” in the OFDM symbol  41   a.  The relay station  200  also assigns the known signal R 42 , assigned to the frequency domain “f 2 ” in the OFDM symbol  41   a,  to the frequency domain “f 2 ” in the OFDM symbol  42   a  and assigns the null data to the frequency domain “f 2 ” in the OFDM symbol  41   a.    
     As a result of the processing, the relay station  200  generates an OFDM symbol  41   b  from the OFDM symbol  41   a  and generates an OFDM symbol  42   b  from the OFDM symbol  42   a.  The relay station  200  then generates a subframe  40   b  containing the OFDM symbols  41   b  to  44   b.  Similarly, the relay station  200  generates a subframe  50   b  from the subframe  50   a.  The subframe  50   b  serves as a relay signal. 
     In the example illustrated in  FIG. 8 , the OFDM symbols  41   b  to  44   b  and the OFDM symbols  51   b  to  54   b  correspond to the OFDM symbols  41   a  to  44   a  and the OFDM symbols  51   a  to  54   a,  respectively. Although not illustrated in  FIG. 8 , the relay station  200  also performs processing for interchanging the mapping positions of the known signals and the null data in the subframe  60   a.    
     In this case, the relay station  200  transmits the subframes  40   b  and  50   b,  which serve as a relay signal, with a delay corresponding to a time “t 22 ” obtained by subtracting the signal processing time “t 21 ” from a subframe duration “t 20 ”. More specifically, in the example illustrated in  FIG. 8 , since the transmitter station  100  assigns the known signals to each set of four OFDM symbols, the receiver station  300  transmits the subframes  40   b  and  50   b  with a delay corresponding to the amount of time obtained by subtracting the time taken for the signal processing from the duration “t 20 ” that is four times the OFDM symbol duration. 
     Next, signals received by the receiver station  300  when the relay signal illustrated in the lower stage in  FIG. 8  is transmitted by the relay station  200  will be described with reference to  FIG. 9 .  FIG. 9  illustrates examples of signals received by the receiver station  300  in the first embodiment. The upper stage in  FIG. 9  illustrates signal components that the receiver station  300  receives from the transmitter station  100  and the lower stage in  FIG. 9  illustrates signal components that the receiver station  300  receives from the relay station  200 .  FIG. 9  illustrates only the subframes  50   a,    60   a,    40   b,  and  50   b  illustrated in  FIG. 8 . Time “t 23 ” illustrated in  FIG. 9  indicates a propagation delay difference that occurs since the path of the transmitter-station signal and the path of the relay signal are different from each other. 
     As illustrated in  FIG. 9 , the receiver station  300  receives the signal in which the subframes  50   a  and  60   a  transmitted by the transmitter station  100  and the subframes  40   b  and  50   b  transmitted by the relay station  200  are spatially multiplexed. 
     In the example illustrated in  FIG. 9 , in the OFDM symbol in which the OFDM symbols  51   a  and  41   b  are spatially multiplexed, the frequency domain “f 0 ” is assigned only the known signal R 51  and the frequency domain “f 2 ” is assigned only the known signal R 52 . Thus, the receiver station  300  may extract the known signals R 51  and R 52 , assigned by the transmitter station  100 , from the OFDM symbol in which the OFDM symbols  51   a  and  41   b  are spatially multiplexed. 
     In the OFDM symbol in which the OFDM symbol  52   a  and the OFDM symbol  42   b  are spatially multiplexed, only the known signal R 41  is assigned to the frequency domain “f 0 ” and only the known signal R 42  is assigned to the frequency domain “f 2 ”. Thus, the receiver station  300  may extract the known signals R 41  and R 42 , assigned by the relay station  200 , from the OFDM symbol in which the OFDM symbols  52   a  and  42   b  are spatially multiplexed. 
     By using the known signals R 51  and R 52  extracted as described above, the receiver station  300  performs channel estimation processing on the path of the transmitter-station signal. By using the known signals R 41  and R 42 , the receiver station  300  also performs channel estimation processing on the path of the relay signal. As a result, the receiver station  300  separates the signal in which the subframes  50   a  and  40   b  are spatially multiplexed into the transmitter-station signal and the relay signal. Similarly, the receiver station  300  separates the signal in which the subframes  60   a  and  50   b  are spatially multiplexed into the transmitter-station signal and the relay signal. The receiver station  300  then combines the same data signals of the data signals contained in the separated transmitter-station signal and relay signal. 
     In the manner described above, the transmitter station  100  in the first embodiment may generate, for each predetermined number of OFDM symbols, a transmitter-station signal in which a known signal is assigned to one OFDM symbol X of the OFDM symbols and null data is assigned to another OFDM symbol Y thereof, as illustrated in  FIGS. 8 and 9 . Upon receiving such a transmitter-station signal, the relay station  200  may generate a relay signal in which the known signal assigned to the OFDM symbol X is assigned to the other OFDM symbol Y and null data is assigned to the OFDM symbol X. 
     Advantages of First Embodiment 
     As described above, the transmitter station  100  in the first embodiment transmits a transmitter-station signal in which known signals and null data are assigned and, upon receiving the transmitter-station signal, the relay station  200  in the first embodiment transmits a relay signal in which the mapping positions of the known signals and the null data are interchanged. Thus, upon receiving the spatially multiplexed signal from the transmitter station  100  and the relay station  200 , the receiver station  300  may perform reception processing that is similar to reception processing for a MIMO-compliant transmitter station. Hence, the wireless communication system  1  according to the first embodiment may improve the quality of the signal received by the receiver station  300 . 
     In addition, based on the position information of the receiver station  300 , the transmitter station  100  in the first embodiment determines whether or not the receiver station  300  is to be set as the relay-station user. Thus, the transmitter station  100  may transmit, to only the receiver station  300  that is the relay-station user, the transmitter-station signal in which corresponding null data are assigned to known signals. As a result, the transmitter station  100  may reduce the processing load and may make effective use of the frequency resources. 
     Second Embodiment 
     The wireless communication system, the relay station, the receiver station, and the wireless communication method disclosed herein may also be implemented in various forms other than the above-described embodiment. Accordingly, a description will now be given of a second embodiment of the wireless communication system, the relay station, the receiver station, and the wireless communication method disclosed herein. 
     Mapping Position 
     The examples described above with reference to  FIGS. 2 and 3  have been directed to a case in which the transmitter station  100  maps a known signal and null data for each OFDM symbol. That is, in the example illustrated in  FIGS. 2 and 3 , the transmitter station  100  maps, for each same time domain, the known signal and the null data to respective different frequency domains. In contrast, in the examples illustrated in  FIGS. 8 and 9 , the transmitter station  100  maps, for each set of at least two or more OFDM symbols, the known signal and the null data to the same frequency domain of the different OFDM symbols. 
     In the example illustrated in  FIGS. 8 and 9 , the transmitter station  100  may map a known signal and null data to different frequency domains in different OFDM symbols. For example, the transmitter station  100  may assign a known signal to a frequency domain “f 0 ” in the OFDM symbol located at the beginning of a subframe and assign null data to a frequency domain “f 1 ” in another OFDM symbol in the same subframe. 
     System Configuration, Etc. 
     The elements of the illustrated apparatuses/devices are merely functionally conceptual and do not necessarily have to be physically configured as illustrated. That is, specific forms of separation/integration of the apparatuses/devices are not limited to those illustrated, and all or a portion thereof may be functionally or physically separated or integrated in an arbitrary manner, depending on various types of load, a use state, and so on. For example, the non-multiplexed-signal processor  340  and the multiplexed-signal processor  350  illustrated in  FIG. 6  may be integrated together. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.