Patent Publication Number: US-2012046043-A1

Title: Wireless communication system, 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. 2010-184404 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 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. 2003-198442, Japanese Laid-open Patent Publication No. 2007-214974, Japanese Laid-open Patent Publication No. 2008-527795, and Japanese Laid-open Patent Publication No. 2008-503907. 
     SUMMARY 
     According to an aspect of the invention, a wireless communication system includes a transmitter station and a receiver station capable of performing wireless communication via a relay station is disclosed. The transmitter station includes a first transmitter that transmits a same signal at least twice repeatedly. The relay station includes a receiver that receives the signals transmitted by the transmitter, a signal processor that performs predetermined signal processing on the signals received by the receiver, and eliminates the later one of two same signals received by the receiver, and a second transmitter that transmits the earlier one of the two same signals to the receiver station after the signal processing performed by the signal processor. 
     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 received by a receiver station in the first embodiment; 
         FIG. 3  illustrates examples of signals transmitted/received by a relay station in the first embodiment; 
         FIG. 4  illustrates examples of signals received by the receiver station in the first embodiment; 
         FIG. 5  is a block diagram illustrating an example of configuration of the transmitter station in the first embodiment; 
         FIG. 6  is a block diagram illustrating an example of configuration of the relay station in the first embodiment; 
         FIG. 7  is a block diagram illustrating an example of configuration of the receiver station in the first embodiment; 
         FIG. 8  is a sequence diagram illustrating a procedure of processing performed by the wireless communication system according to the first embodiment; 
         FIG. 9  illustrates examples of signals transmitted/received by the relay station in the first embodiment; 
         FIG. 10  illustrates examples of signals received by the receiver station in the first embodiment; 
         FIG. 11  illustrates examples of signals transmitted/received by the relay station in the first embodiment; 
         FIG. 12  illustrates examples of signals received by the receiver station in the first embodiment; and 
         FIG. 13  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. 13 .  FIG. 13  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. 13 , it is assumed that Orthogonal Frequency Division Multiplexing (OFDM) is used as a transmission scheme. The upper stage in  FIG. 13  illustrates signal components that the receiver station receives from the transmitter station and the lower stage in  FIG. 13  illustrates signal components that the receiver station receives from the relay station. Although  FIG. 13  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. 13 , 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. 13  indicates the amount of time taken for the signal processing performed by the relay station. Time “t 92 ” illustrated in  FIG. 13  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. 13 , 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 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 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 . The transmitter station  100  transmits the same signal at least twice repeatedly. The relay station  200  and the receiver station  300  receive the signal transmitted from the transmitter station  100 . 
     The relay station  200  in the first embodiment relays the signal, received from the transmitter station  100 , to the receiver station  300 . The relay station  200  performs, for example, signal processing for eliminating diffraction waves with respect to the signal received from the transmitter station  100 . The relay station  200  in the first embodiment eliminates the later one of two same signals received from the transmitter station  100  and relays the earlier one thereof to the receiver station  300 . 
     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). Upon receiving multiple same signals from the transmitter station  100  and the relay station  200 , the receiver station  300  in the first embodiment combines the same signals. 
     Signals received by the relay station  300  will now be described with reference to  FIG. 2 .  FIG. 2  illustrates examples of signals received by the receiver station  300  in the first embodiment. The upper stage in  FIG. 2  illustrates signal components that the receiver station  300  receives from the transmitter station  100  and the lower stage in  FIG. 2  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. 2 , signal components that are simultaneously received by the receiver station  300  are spatially multiplexed in practice. 
     As illustrated in the upper stage in  FIG. 2 , the transmitter station  100  transmits an OFDM symbol  10 - 1   a  containing a CP and a data signal D 10 - 1  and an OFDM symbol  10 - 2   a  containing a CP and a data signal D 10 - 2 . The term “data signal” as used herein refers to, for example, a control signal containing control data or a user-data signal containing user data. 
     In the following description, numeral “m” of “m-n” given to each OFDM symbol represents information for specifying an OFDM symbol. For example, the OFDM symbols  10 - 1   a  and  10 - 2   a  that are given the same m mean that they are the same OFDM symbol. Also, “n” of “m-n” given to each OFDM symbol represents the number of times the same OFDM symbol is transmitted by the transmitter station  100 . For example, the OFDM symbol  10 - 1   a  is a first OFDM symbol transmitted by the transmitter station  100  and the OFDM symbol  10 - 2   a  is a second OFDM symbol transmitted by the transmitter station  100 . 
     That is, in the example illustrated in  FIG. 2 , the transmitter station  100  transmits the same OFDM symbols  10 - 1   a  and  10 - 2   a . The relay station  200  relays the earlier OFDM symbol  10 - 1   a  of the same OFDM symbols  10 - 1   a  and  10 - 2   a  received from the transmitter station  100  and discards the later OFDM symbol  10 - 2   a  thereof without relaying it. Specifically, the relay station  200  performs predetermined signal processing on the OFDM symbol  10 - 1   a  and transmits a signal-processed OFDM symbol  10 - 1   b . That is, the receiver station  300  receives a signal in which the OFDM symbols  10 - 1   a  and  10 - 2   a  transmitted by the transmitter station  100  and the OFDM symbol  10 - 1   b  transmitted by the relay station  200  are spatially multiplexed as illustrated in  FIG. 2 . 
     Time “t 11 ” illustrated in  FIG. 2  is assumed to indicate a propagation delay difference due to a difference in the amount of time taken for the signal processing performed by the relay station  200  and a difference of the path. The propagation delay difference occurs since the path from the transmitter station  100  to the receiver station  300  and the path from the relay station  200  to the receiver station  300  are different from each other. Even when the amount of time “t 11 ” is larger than the duration of the CP, the receiver station  300  in the first embodiment receives the signal in which the same OFDM symbols  10 - 1   a ,  10 - 2   a , and  10 - 1   b  are spatially multiplexed as illustrated in  FIG. 2 . Thus, the receiver station  300  in the first embodiment may receive a signal that has no inter-OFDM-symbol interference. 
     For example, when the receiver station  300  performs processing on the signal illustrated in  FIG. 2  based on a known signal transmitted from the relay station  200 , the receiver station  300  obtains a signal in which the OFDM symbol  10 - 1   b  is spatially multiplexed with part of the OFDM symbols  10 - 1   a  and  10 - 2   a . That is, the receiver station  300  may obtain a high-quality signal having no inter-OFDM-symbol interference and resulting from combination of the same signals. The aforementioned known signal is also called 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. 
     Thus, the transmitter station  100  in the first embodiment transmits the same OFDM symbol at least twice repeatedly. The relay station  200  in the first embodiment relays the earlier one of the same OFDM symbols received from the transmitter station  100  and does not relay the later one of the same OFDM symbols. As a result, in the wireless communication system  1  according to the first embodiment, the receiver station  300  may receive a signal having no inter-OFDM-symbol interference, as in the example illustrated in  FIG. 2 . Thus, the wireless communication system  1  according to the first embodiment may improve the quality of the signal received by the receiver station  300 , even when the amount of processing delay caused by the relay station  200  is large. 
     Although  FIG. 2  illustrates an example in which the transmitter station  100  transmits the same OFDM symbol twice repeatedly, the transmitter station  100  may transmit the same OFDM symbols three or more times repeatedly. Upon receiving three or more same OFDM symbols from the transmitter station  100 , the relay station  200  may relay at least one of the OFDM symbols except the last OFDM symbol received. More specifically, upon receiving N same OFDM symbols, the relay station  200  may relay any of the OFDM symbols that are transmittable by the time the amount of time corresponding to N-times the OFDM symbol duration passes after the reception of the first one of the same OFDM symbols. 
     In order to ensure that the same OFDM symbols transmitted from the transmitter station  100  and the relay station  200  are to be received by the receiver station  300  at substantially the same timing, the relay station  200  may also transmit an OFDM symbol to be relayed, with a delay corresponding to a predetermined amount of time. More specifically, the relay station  200  may relay, of the same signals received from the transmitter station  100 , at least one of the same signals that are transmittable by the time the duration of the same signals passes after the start of the reception of the same signals. 
     Examples of a case in which the transmitter station  100  transmits the same OFDM symbol three or more times repeatedly and a case in which the relay station  200  performs delay processing will be described below with reference to  FIGS. 3 and 4 . 
       FIG. 3  illustrates examples of signals transmitted/received by the relay station  200  in the first embodiment. The upper stage in  FIG. 3  illustrates one example of a signal that the relay station  200  receives from the transmitter station  100 . The middle stage in  FIG. 3  illustrates an example of a signal transmitted by the relay station  200  when the relay station  200  is assumed to relay all signals. The lower stage in  FIG. 3  illustrates one example of a signal relayed by the relay station  200  in the first embodiment. 
     In the example illustrated in  FIG. 3 , the transmitter station  100  transmits the same OFDM symbol four times repeatedly. In the example illustrated in the upper stage in  FIG. 3 , the relay station  200  receives the same OFDM symbols  20 - 1   a  to  20 - 4   a  from the transmitter station  100 . Although not illustrated in  FIG. 3 , the relay station  200  receives OFDM symbols  30 - 2   a  to  30 - 4   a  that are the same as the OFDM symbol  30 - 1   a  transmitted from the transmitter station  100 . 
     Time “t 21 ” illustrated in  FIG. 3  indicates the amount of time taken for the signal processing performed by the relay station  200 . In this case, when the relay station  200  is assumed to relay all of the OFDM symbols  20 - 1   a  to  20 - 4   a , the OFDM symbols  20 - 3   a  and  20 - 4   a  arrive at the receiver station  300  at the same timing as the timing of the OFDM symbol  30 - 1   a  and so on, as illustrated in the middle stage in  FIG. 3 . This means that the OFDM symbols  20 - 3   a  and  20 - 4   a  interfere with the other OFDM symbols  30 - 1   a  and so on, and thus the quality of the signals received by the receiver station  300  deteriorates. 
     Accordingly, for relaying the OFDM symbols  20 - 1   a  to  20 - 4   a , the relay station  200  relays at least one of the OFDM symbols  20 - 1   a  and  20 - 2   a , as illustrated in the lower stage in  FIG. 3 . More specifically, the relay station  200  relays the OFDM symbol(s) that are transmittable by the time an OFDM symbol duration “t 20 ” of the OFDM symbols  20 - 1   a  to  20 - 4   a  passes after the reception of the first OFDM symbol  20 - 1   a . In other words, the relay station  200  relays at least one of the OFDM symbols  20 - 1   a  and  20 - 2   a  that are transmittable within a time “t 22 ” obtained by subtracting the signal processing time “t 21 ” from the time “t 20 ”. 
     In the example illustrated in  FIG. 3 , the relay station  200  relays both the OFDM symbols  20 - 1   a  and  20 - 2   a . Specifically, the relay station  200  performs signal processing on the OFDM symbols  20 - 1   a  to  20 - 4   a  and transmits signal-processed OFDM symbols  20 - 1   b  and  20 - 2   b . The OFDM symbol  20 - 1   b  is an OFDM symbol obtained by performing the signal processing on the OFDM symbol  20 - 1   a  and the OFDM symbol  20 - 2   b  is an OFDM symbol obtained by performing the signal processing on the OFDM symbol  20 - 2   a.    
     Since the symbol duration “t 20 ” of the OFDM symbols  20 - 1   a  to  20 - 4   a  is the duration of four OFDM symbols, it is known to the relay station  200 . It is also assumed that the signal processing time “t 21 ” is, for example, an amount of time measured during manufacture of the relay station  200  and is known to the relay station  200 . For example, the relay station  200  stores, in a predetermined storage unit, the signal processing time “t 21 ” measured during the manufacture. Thus, using the known time “t 20 ” and the signal processing time “t 21 ”, the relay station  200  may determine the time “t 22 ”. 
     The relay station  200  performs delay processing so that the difference between the time at which the OFDM symbol transmitted from the transmitter station  100  arrives at the receiver station  300  and the time at which the OFDM symbol transmitted by the relay station  200  arrives at the receiver station  300  is smaller than or equal to the CP duration. More specifically, the relay station  200  performs relay processing so that the difference between the time at which the OFDM symbols  20 - 1   b  and  20 - 2   b  arrive at the receiver station  300  and the time at which any of the OFDM symbols  20 - 1   a  to  20 - 4   a  arrives at the receiver station  300  is smaller than or equal to the CP duration. In the example illustrated in  FIG. 3 , after completing the signal processing, the relay station  200  transmits the OFDM symbol  20 - 1   b  with a delay corresponding to a time “t 23 ”. Similarly, after completing the signal processing, the relay station  200  transmits the OFDM symbol  20 - 2   b  with a delay corresponding to the time “t 23 ”. 
     The relay station  200  may determine the delay time “t 23 ” by subtracting the signal processing time “t 21 ” from an integer multiple of the duration of the OFDM symbol. For example, when the signal processing time “t 21 ” is larger than the duration of one OFDM symbol and is smaller than the duration of two OFDM symbols, as illustrated in  FIG. 3 , the relay station  200  determines the delay time “t 23 ” by subtracting the signal processing time “t 21 ” from twice the duration of the OFDM symbol. That is, when a signal processing time “X” is in the range of Y times the duration of the OFDM symbol to Z times the duration of the OFDM symbol, the relay station  200  determines the delay time by subtracting the signal processing time “X” from Z times the duration of the OFDM symbol. Y and Z are successive integers. 
     Next, signals received by the receiver station  300  when the signal illustrated in the lower stage in  FIG. 3  is relayed by the relay station  200  will be described with reference to  FIG. 4 .  FIG. 4  illustrates examples of signals received by the receiver station  300  in the first embodiment. The upper stage in  FIG. 4  illustrates signal components that the receiver station  300  receives from the transmitter station  100  and the lower stage in  FIG. 4  illustrates signal components that the receiver station  300  receives from the relay station  200 . Time “t 24 ” illustrated in  FIG. 4  indicates a propagation delay difference that occurs since the path from the transmitter station  100  to the receiver station  300  and the path from the relay station  200  to the receiver station  300  are different from each other. 
     As illustrated in  FIG. 4 , the receiver station  300  receives a signal in which the OFDM symbols  20 - 1   a  to  20 - 4   a  and so on transmitted by the transmitter station  100  and the OFDM symbols  20 - 1   b  and  20 - 2   b  transmitted by the relay station  200  are spatially multiplexed. More specifically, the OFDM symbol  20 - 1   b  transmitted by the relay station  200  is spatially multiplexed with the OFDM symbol  20 - 3   a  and the CP in the OFDM symbol  20 - 4   a , the symbols  20 - 3   a  and  20 - 4   a  being transmitted by the transmitter station  100 . The OFDM symbol  20 - 2   b  transmitted by the relay station  200  is also spatially multiplexed with the OFDM symbol  20 - 4   a  and the CP in the OFDM symbol  30 - 1   a , the symbols  20 - 4   a  and  30 - 1   a  being transmitted by the transmitter station  100 . 
     The OFDM symbol  20 - 2   b  and the OFDM symbol  30 - 1   a , which are OFDM symbols that are different from each other, do not interfere with each other since the OFDM symbol  20 - 2   b  and the OFDM symbol  30 - 1   a  are spatially multiplexed within the range of the CP duration. 
     In such a manner, the receiver station  300  receives, from the transmitter station  100  and the relay station  200 , a signal in which the same OFDM symbols are spatially multiplexed. Thus, the receiver station  300  in the first embodiment may receive a signal having no inter-OFDM-symbol interference. 
     In addition, since the relay station  200  transmits the OFDM symbol  20 - 1   b  with a delay corresponding to the time “t 23 ”, the propagation delay difference between the OFDM symbol  20 - 3   a  and the OFDM symbol  20 - 1   b  is within the CP duration. Similarly, the propagation delay difference between the OFDM symbol  20 - 4   a  and the OFDM symbol  20 - 2   b  is within the CP duration. 
     As a result, the receiver station  300  may extract a signal for each OFDM symbol even when using any of known signals transmitted by the transmitter station  100  or the relay station  200 . For example, in the example illustrated in  FIG. 4 , when using a known signal transmitted from the transmitter station  100 , the receiver station  300  may obtain the signal in which the OFDM symbols  20 - 3   a  and  20 - 1   b  are spatially multiplexed. For example, when using a known signal transmitted from the relay station  200 , the receiver station  300  may obtain the signal in which the OFDM symbols  20 - 3   a  and  20 - 1   b  are spatially multiplexed. Similarly, even when using a known signal transmitted from the transmitter station  100  or the relay station  200 , the receiver station  300  may obtain a signal in which the OFDM symbols  20 - 4   a  and  20 - 2   b  are spatially multiplexed. 
     Upon receiving the OFDM symbols illustrated in  FIG. 4 , the receiver station  300  combines the same OFDM symbols of the received OFDM symbols. More specifically, upon receiving the OFDM symbol  20 - 1   a , the receiver station  300  stores the OFDM symbol  20 - 1   a  in a predetermined buffer. Similarly, the receiver station  300  stores the OFDM symbol  20 - 2   a  in the buffer. The receiver station  300  also stores, in the buffer, the OFDM symbol in which the OFDM symbols  20 - 3   a  and  20 - 1   b  are spatially multiplexed and the OFDM symbol in which the OFDM symbols  20 - 4   a  and  20 - 2   b  are spatially multiplexed. The receiver station  300  then combines the OFDM symbols stored in the buffer. The receiver station  300  performs, for example, log-likelihood ratio (LLR) combining processing for combining likelihood information of the same data contained in the OFDM symbols. 
     As described above, the receiver station  300  in the first embodiment may receive a signal having no inter-OFDM-symbol interference and may also improve the reception characteristics by combining the OFDM symbols. 
     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 perform processing for repeatedly transmitting the same signal to the specific receiver station and does not necessarily have to perform processing for repeatedly transmitting the same signal 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. 5 .  FIG. 5  is a block diagram of an example of configuration of the transmitter station  100  in the first embodiment. As illustrated in  FIG. 5 , 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. 5  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 on the signal received by the antenna  101  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. 5 , 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  performs processing for assigning control data to be transmitted to the receiver station  300  to resources and processing for assigning user data to be transmitted to the receiver station  300  to resources. The processing performed by the scheduler unit  130  will be described below as separately as the processing for assigning control data to resources and the processing for assigning user data to resources. 
     The processing for assigning control data to resources will be described first. The scheduler unit  130  outputs, to the error-correction encoder  141 , control data containing, such as resource assignment information regarding resources to which user data and so on are assigned. 
     In this case, 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. 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”. 
     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 , the number “N” of times the same signal is to be repeatedly transmitted. The number of times the transmitter station  100  repeatedly transmits the same signal may hereinafter be referred to as the “number of repeated transmissions”. 
     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  assigns control information to resources so that the same control data is repeatedly transmitted to the receiver station  300 . In this case, the scheduler unit  130  assigns the control data to the resources so that the same control data is transmitted to the receiver station  300  according to the number “N” of repeated transmissions. 
     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 , relay-station user information indicating that the receiver station  300  is not the relay-station user. 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 the number “N” of repeated transmissions which indicates “1” to the error-correction encoder  141 . 
     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  assigns the control data to the resources so that the same control data is transmitted to the receiver station  300  only once. 
     The processing for assigning data information to resources will be described next. 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  assigns user data to resources so that the same user data is repeatedly transmitted to the receiver station  300 . In this case, the scheduler unit  130  assigns the user data to the resources so that the same user data is transmitted to the receiver station  300  according to the number “N” of repeated transmissions. 
     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  assigns the user data to the resources so that the same user data is transmitted to the receiver station  300  only once. 
     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 various signals mapped onto subcarriers. The physical-channel multiplexer  170  frequency-multiplexes the control signal output from the control-information modulator  151 , the user-data signal output from the data-information modulator  152 , and the known signal output from the known-signal generator  160 . 
     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 . 
     An RF processing unit  1 A that includes the reception RF unit  111  and the transmission RF unit  183  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. 
     [Configuration of Relay Station in First Embodiment] 
     The relay station  200  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 relay station  200  in the first embodiment. As illustrated in  FIG. 6 , 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. 6 , 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. 5 , 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. 6 , 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, the number of repeated transmissions, and so on from the control signal transmitted by the transmitter station  100 . The control-signal demodulator  240  outputs the resource assignment information, the relay-station user information, the number of repeated transmissions, 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, of the signals output from the FFT unit  214 , the signal 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  relays, of the same signals contained in the signals destined for the receiver station  300 , at least one of the signals that are transmittable by the time the duration of the same signals passes. More specifically, the mapping controller  250  substitutes “0” for the signals that are not to be relayed. 
     As illustrated in  FIG. 6 , 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 delay time obtained by subtracting a predetermined signal processing time from the integer multiple of the duration of the OFDM symbol, the delay unit  270  outputs the OFDM symbols, output from the CP adding unit  262 , to the transmission RF unit  280 . More specifically, when a signal processing time “X” is in the range of Y times the duration of the OFDM symbol to Z times the duration of the OFDM symbol, the delay unit  270  waits for a period of delay time obtained by subtracting the signal processing time “X” from Z times the duration of the OFDM symbol, as described above. The signal processing time “X” 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. 
     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. 5 , 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. 
     [Configuration of Receiver Station in First Embodiment] 
     The receiver station  300  in the first embodiment will be described next with reference to  FIG. 7 .  FIG. 7  is a block diagram illustrating an example of configuration of the receiver station  300  in the first embodiment. As illustrated in  FIG. 7 , 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. 5 , 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. 7 . 
     As illustrated in  FIG. 7 , the receiver station  300  includes a reception RF unit  321 , a CP remover  322 , an FFT unit  323 , and a physical channel separator  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. 5 , 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 physical-channel separator  330  receives, from the FFT unit  323 , the signal in which physical channels are frequency-multiplexed, and separates the frequency-multiplexed signal into a known signal, a control signal, and a data signal. The physical-channel separator  330  receives the control information from the error-correction decoder  392  and performs physical-channel separating processing based on the resource assignment information contained in the control information. 
     As illustrated in  FIG. 7 , the receiver station  300  further includes a channel estimator  340 , a compensator  350 , a data-signal demodulator  361 , a control-signal demodulator  362 , an LLR combination controller  370 , a combining unit  380 , and error-correction decoders  391  and  392 . 
     The channel estimator  340  performs channel estimation processing based on the known signal extracted by the physical-channel separator  330 . More specifically, the channel estimator  340  estimates a wireless channel state by determining the correlation between the known signal transmitted from the transmitter station  100  and the signal known to the receiver station  300 . 
     The compensator  350  includes channel compensators  351  and  352 . Based on a result of the channel estimation processing performed by the channel estimator  340 , the channel estimator  351  performs channel compensation on the data signal extracted by the physical-channel separator  330 . Based on a result of the channel estimation processing performed by the channel estimator  340 , the channel compensator  352  performs channel compensation on the control signal extracted by the physical-channel separator  330 . 
     The data-signal demodulator  361  performs demodulation processing on the data signal channel-compensated by the channel compensator  351 . The control-signal demodulator  362  performs demodulation processing on the control signal channel-compensated by the channel compensator  352 . 
     Based on the relay-station-user information and the number of repeated transmissions, the information and the number being output 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 output from the error-correction decoder  392 . When the local station is the relay-station user, the LLR combination controller  370  outputs the number of repeated transmissions to the combining unit  380  and controls the combining unit  380  so that it performs the combination processing. On the other hand, 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 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 signal, input from the data-signal demodulator  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 data-signal demodulator  361 . In this case, the same user-data signal is repeatedly input from the data-signal demodulator  361  to the LLR combining unit  381  according to the number “N” of repeated transmissions which is output from the LLR combination controller  370 . In this case, the LLR combining unit  381  stores, in a predetermined buffer, the same user-data signals input from the data-signal demodulator  361 . For example, after storing 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 signal, input from the control-signal demodulator  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 control-signal demodulator  362  and performs the LLR combination processing on the same control signals. The LLR combining unit  382  then outputs the control signal, obtained by the LLR combination processing, to the error-correction decoder  392 . 
     The error-correction decoder  391  performs error-correction decoding processing on the data signal output from the LLR combining unit  381 . As a result, the error-correction decoder  391  obtains the user data from the user-data signal. 
     The error-correction decoder  392  performs error-correction decoding processing on the control signal output from the LLR combining unit  382 . As a result, the error-correction decoder  392  obtains, from the control signal, the control information containing the resource-assignment information, the relay-station-user information, the number of repeated transmissions, and so on. The error-correction decoder  392  outputs the various types of information contained in the control information to the LLR combination controller  370  and the physical-channel separator  330 . 
     The reception RF unit  321  and the transmission RF unit  313  are included in 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 physical channel separator  330 , the channel estimator  340 , the compensator  350 , the data-signal demodulator  361 , the control-signal demodulator  362 , the LLR combination controller  370 , the combining unit  380 , and the error-correction decoders  391  and  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. 
     [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. 8 .  FIG. 8  is a sequence diagram illustrating a procedure of processing performed by the wireless communication system  1  according to the first embodiment.  FIG. 8  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. 8 , 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. 8 , 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 and the number “N” of repeated transmissions. For example, the transmitter station  100  transmits, to the relay station  200  and the receiver station  300 , a control signal containing resource-assignment information, the relay-station-user information, and the number “N” of repeated transmissions. 
     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. When the receiver station  300  is the relay-station user, the relay station  200  and the receiver station  300  may also detect the number “N” of times the same signal is transmitted from the transmitter station  100 . 
     In operation S 15 , during transmission of a control signal and a data signal, the transmitter station  100  transmits the same OFDM symbol N times repeatedly. The relay station  200  and the receiver station  300  receive the OFDM symbols transmitted by the transmitter station  100 . 
     When the relay station  200  receives the OFDM symbols transmitted by the transmitter station  100 , the relay station  200  performs predetermined reception processing in operation S 16 . Examples of the predetermined reception processing include frequency conversion processing, orthogonal demodulation processing, A/D conversion processing, and diffraction-wave elimination processing. 
     In operation S 17 , the relay station  200  relays, of the same OFDM symbols contained in the signals received from the transmitter station  100 , at least one of the OFDM symbols that are transmittable by the time the duration of the multiple OFDM symbols passes. In this case, the relay station  200  relays the at least one OFDM symbol with a delay corresponding to the amount of delay time obtained by subtracting a predetermined signal processing time from the integer multiple of the duration of the OFDM symbol. 
     In operation S 18 , the receiver station  300  combines the signals transmitted by the transmitter station  100  and the relay station  200 . More specifically, the receiver station  300  combines the OFDM symbols transmitted by the transmitter station  100  and the OFDM symbols transmitted by the transmitter station  100  and the relay station  200  and spatially multiplexed. 
     [Other Examples of Signals Transmitted/Received] 
     The OFDM symbols transmitted by the transmitter station  100  and the relay station  200  and the OFDM symbols received by the receiver station  300  will be described next in conjunction with specific examples other than the examples illustrated in  FIGS. 2 to 4 . 
     First, a description will be given in conjunction with examples illustrated in  FIGS. 9 and 10 .  FIG. 9  illustrates examples of signals transmitted/received by the relay station  200  in the first embodiment. The upper stage in  FIG. 9  illustrates one example of a signal that the relay station  200  receives from the transmitter station  100 . The lower stage in  FIG. 9  illustrates one example of a signal relayed by the relay station  200 . 
     In the example illustrated in  FIG. 9 , the transmitter station  100  transmits the same OFDM symbol twice repeatedly. In the example illustrated in the upper stage in  FIG. 9 , the relay station  200  receives the same OFDM symbols  40 - 1   a  and  40 - 2   a  and the same OFDM symbols  50 - 1   a  and  50 - 2   a  from the transmitter station  100 . Time “t 31 ” illustrated in  FIG. 9  indicates the amount of time taken for the signal processing performed by the relay station  200 . 
     The relay station  200  relays the first OFDM symbol  40 - 1   a  of the same OFDM symbols  40 - 1   a  and  40 - 2   a  received from the transmitter station  100  and does not relay the second OFDM symbol  40 - 2   a  of the received OFDM symbols  40 - 1   a  and  40 - 2   a . That is, the relay station  200  transmits an OFDM symbol  40 - 1   b  that is the same as the OFDM symbol  40 - 1   a  received from the transmitter station  100 . In this case, the relay station  200  transmits the OFDM symbol  40 - 1   b  with a delay corresponding to a time “t 32 ” obtained by subtracting the signal processing time “t 31 ” from the duration of one OFDM symbol. 
     Similarly, the relay station  200  relays the first OFDM symbol  50 - 1   a  of the same OFDM symbols  50 - 1   a  and  50 - 2   a  received from the transmitter station  100  and does not relay the second OFDM symbol  50 - 2   a  of the received OFDM symbols  50 - 1   a  and  50 - 2   a . That is, the relay station  200  transmits an OFDM symbol  50 - 1   b  that is the same as the OFDM symbol  50 - 1   a  received from the transmitter station  100 , with a delay corresponding to the time “t 32 ”. 
     Next, signals received by the receiver station  300  when the signal illustrated in  FIG. 9  is relayed by the relay station  200  will be described with reference to  FIG. 10 .  FIG. 10  illustrates examples of signals received by the receiver station  300  in the first embodiment. The upper stage in  FIG. 10  illustrates signal components that the receiver station  300  receives from the transmitter station  100  and the lower stage in  FIG. 10  illustrates signal components that the receiver station  300  receives from the relay station  200 . Times “t 33 ” and “t 34 ” illustrated in  FIG. 10  indicate propagation delay differences that occur since the path from the transmitter station  100  to the receiver station  300  and the path from the relay station  200  to the receiver station  300  are different from each other. 
     As illustrated in  FIG. 10 , the receiver station  300  receives a signal in which the OFDM symbols  40 - 1   a ,  40 - 2   a ,  50 - 1   a , and  50 - 2   a  transmitted by the transmitter station  100  and the OFDM symbols  40 - 1   b  and  50 - 1   b  transmitted by the relay station  200  are spatially multiplexed. 
     Upon receiving the signals illustrated in  FIG. 10 , the receiver station  300  may extract the OFDM symbols by using the known signals transmitted by the transmitter station  100  or the relay station  200 . For example, the receiver station  300  may obtain the OFDM symbol  40 - 1   a  and the OFDM symbol in which the OFDM symbol  40 - 2   a  and  40 - 1   b  are spatially multiplexed. Similarly, the receiver station  300  may obtain the OFDM symbol  50 - 1   a  and the OFDM symbol in which the OFDM symbols  50 - 2   a  and  50 - 1   b  are spatially multiplexed. 
     The receiver station  300  then combines the thus-obtained OFDM symbols. More specifically, the receiver station  300  combines the OFDM symbol  40 - 1   a  with the OFDM symbol in which the OFDM symbols  40 - 2   a  and  40 - 1   b  are spatially multiplexed and also combines the OFDM symbol  50 - 1   a  with the OFDM symbols  50 - 2   a  and  50 - 1   b.    
     Next, a description will be given in conjunction with examples illustrated in  FIGS. 11 and 12 .  FIG. 11  illustrates examples of signals transmitted/received by the relay station  200  in the first embodiment. The upper stage in  FIG. 11  illustrates one example of a signal that the relay station  200  receives from the transmitter station  100 . The lower stage in  FIG. 11  illustrates one example of a signal relayed by the relay station  200 . 
     In the example illustrated in  FIG. 11 , the transmitter station  100  transmits the same OFDM symbol three times repeatedly. In the example illustrated in the upper stage in  FIG. 11 , the relay station  200  receives the same OFDM symbols  60 - 1   a  to  60 - 3   a  and the same OFDM symbols  70 - 1   a  to  70 - 3   a  from the transmitter station  100 . Time “t 41 ” illustrated in  FIG. 11  indicates the amount of time taken for the signal processing performed by the relay station  200 . 
     The relay station  200  may relay the OFDM symbols  60 - 1   a  and  60 - 2   a  of the same OFDM symbols  60 - 1   a  to  60 - 3   a  received from the transmitter station  100 . However, in the example illustrated in  FIG. 11 , the relay station  200  relays the first received OFDM symbol  60 - 1   a  and does not relay the second and subsequently received OFDM symbols  60 - 2   a  and  60 - 3   a . That is, the relay station  200  transmits an OFDM symbol  60 - 1   b  that is the same as the OFDM symbol  60 - 1   a  received from the transmitter station  100 . In this case, the relay station  200  transmits the OFDM symbol  60 - 1   b  with a delay corresponding to a time “t 42 ” obtained by subtracting the signal processing time “t 41 ” from the duration of one OFDM symbol. 
     Similarly, the relay station  200  relays the first OFDM symbol  70 - 1   a  of the same OFDM symbols  70 - 1   a  to  70 - 3   a  received from the transmitter station  100  and does not relay the second and subsequently received OFDM symbols  70 - 2   a  and  70 - 3   a . That is, the relay station  200  transmits an OFDM symbol  70 - 1   b  that is the same as the OFDM symbol  70 - 1   a  received from the transmitter station  100 , with a delay corresponding to the time “t 42 ”. 
     In the example illustrated in  FIG. 11 , the relay station  200  may relay both the OFDM symbols  60 - 1   a  and  60 - 2   a  or may relay only the OFDM symbol  60 - 2   a . Similarly, the relay station  200  may relay both the OFDM symbols  70 - 1   a  and  70 - 2   a  or may relay only the OFDM symbol  70 - 2   a.    
     Next, signals received by the receiver station  300  when the signal illustrated in  FIG. 11  is relayed by the relay station  200  will be described with reference to  FIG. 12 .  FIG. 12  illustrates examples of signals received by the receiver station  300  in the first embodiment. The upper stage in  FIG. 12  illustrates signal components that the receiver station  300  receives from the transmitter station  100  and the lower stage in  FIG. 12  illustrates signal components that the receiver station  300  receives from the relay station  200 . Times “t 43 ” and “t 44 ” illustrated in  FIG. 12  indicate propagation delay differences that occur since the path from the transmitter station  100  to the receiver station  300  and the path from the relay station  200  to the receiver station  300  are different from each other. 
     As illustrated in  FIG. 12 , the receiver station  300  receives a signal in which the OFDM symbols  60 - 1   a  to  60 - 3   a  and  70 - 1   a  to  70 - 3   a  transmitted by the transmitter station  100  and the OFDM symbols  60 - 1   b  and  70 - 1   b  transmitted by the relay station  200  are spatially multiplexed. 
     Upon receiving the signals illustrated in  FIG. 12 , the receiver station  300  combines the OFDM symbol  60 - 1   a , the OFDM symbol in which the OFDM symbols  60 - 2   a  and  60 - 1   b  are spatially multiplexed, and the OFDM symbol  60 - 3   a  and also combines the OFDM symbol  70 - 1   a , the OFDM symbol in which the OFDM symbols  70 - 2   a  and  70 - 1   b  are spatially multiplexed, and the OFDM symbol  70 - 3   a.    
     [Advantages of First Embodiment] 
     As described above, the transmitter station  100  in the first embodiment transmits the same OFDM symbol N times repeatedly. The relay station  200  in the first embodiment relays, of the same OFDM symbols received from the transmitter station  100 , any of the OFDM symbols that are transmittable by the time N-times the duration of the OFDM symbol passes after the reception of the first one of the OFDM symbols. With this arrangement, the receiver station  300  in the first embodiment may receive a signal that has no inter-OFDM-symbol interference. Thus, the wireless communication system  1  according to the first embodiment may improve the quality of the signal received by the receiver station  300 . 
     The relay station  200  in the first embodiment performs delay processing so that the difference between the time at which the OFDM symbol transmitted from the transmitter station  100  arrives at the receiver station  300  and the time at which the OFDM symbol transmitted by the relay station  200  arrives at the receiver station  300  is smaller than or equal to the CP duration. With this arrangement, the receiver station  300  may extract a signal for each OFDM symbol even when using any of the known signals transmitted by the transmitter station  100  or the relay station  200 . 
     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. This arrangement allows the transmitter station  100  to perform processing for repeatedly transmitting the same signal to only the receiver station  300  that is the relay-station user. Thus, it is possible to reduce the amount of processing load and it is also possible to make effective use of frequency resources. 
     Second Embodiment 
     The wireless communication system, 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 receiver station, and the wireless communication method disclosed herein. 
     [Delay Processing] 
     Although an example in which the relay station  200  performs the delay processing and so on by using the known signal processing time has been described in the first embodiment, the relay station  200  may also perform the delay processing and so on by using a result of dynamic measurement of the signal processing time. For example, the relay station  200  may measure the time from when a signal is received by the antenna  201  until the CP addition processing performed by the CP adding unit  262  is completed and may use the result of the measurement as the signal processing time. Such an arrangement allows the relay station  200  to perform the delay processing with high accuracy. 
     [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 loads, a use state, and so on. 
     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.