Patent Publication Number: US-8126033-B2

Title: Apparatus and method for relaying in a wireless communication system

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Nov. 29, 2006 and assigned Serial No. 2006-0118733, the entire disclosure of which is hereby incorporated by reference. 
     JOINT RESEARCH AGREEMENT 
     The claimed invention was made by, on behalf of, and/or in connection with one or more of the following parties to a joint research agreement: Samsung Electronics Company, Inc. and Korea Advanced Institute of Science and Technology. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a wireless communication system supporting relaying. More particularly, the present invention relates to an apparatus and method for increasing a capacity gain without adding time resources in a wireless communication system. 
     2. Description of the Related Art 
     In a wireless communication system, Mobile Stations (MSs) located at a service area boundary or in a shadowing area may suffer from a poor channel condition. Accordingly, the wireless communication system may adopt a relay scheme using Relay Stations (RSs) to improve the link performances of such MSs. The relay scheme provides high-rate data channels to the MSs, thereby increasing overall system performance and expanding service coverage. 
       FIG. 1  illustrates the configuration of a conventional wireless communication system supporting relaying. 
     Referring to  FIG. 1 , an MS  110  within the service area  101  of a BS  100  is connected to the BS  100  by a direct link. An MS  120 , which is outside the service area  101  of the BS  100  and thus has a poor channel condition, is connected to the BS  100  by a relay link established with an RS  130 . 
     Thus, via the RS  130 , the BS  100  can communicate with MSs that otherwise have a poor channel condition because they are outside the service area  101  of the BS  100  or because they are in a shadowing area and suffering from a shielding effect due to buildings. 
     The wireless communication system supports relaying in three methods. 
     One of them is Time Division Duplexing (TDD) relaying as illustrated in  FIG. 2 . 
       FIG. 2  illustrates the configuration of a conventional broadband wireless communication system supporting TDD relaying. 
     Referring to  FIG. 2 , the wireless communication system supports relaying using four independent time resources. The time resources include two uplink time resources and two downlink time resources. For example, a BS  201  transmits a signal {tilde over (S)} 1  to an RS  203  in first time resources in step  211  and the RS  203  relays the received signal {tilde over (S)} 1  to an MS  205  in second time resources in step  213 . 
     After receiving the signal {tilde over (S)} 1  from the RS  203 , the MS  205  transmits a signal {tilde over (S)} 2  to the RS  203  in third time resources in step  215  and the RS  203  relays the received signal {tilde over (S)} 2  to the BS  201  in fourth time resources in step  217 . 
     Another relaying method is coded bi-directional relaying, as illustrated in  FIG. 3 . 
       FIG. 3  illustrates the configuration of a conventional wireless communication system supporting coded bi-directional relaying. 
     Referring to  FIG. 3 , the wireless communication system supports relaying using three independent time resources. For example, a BS  301  transmits a downlink signal {tilde over (S)} 1  to an RS  303  in first time resources in step  311  and an MS  305  transmits an uplink signal {tilde over (S)} 2  to the RS  303  in second time resources in step  313 . The downlink and uplink signals {tilde over (S)} 1  and {tilde over (S)} 2  from the BS  301  and the MS  305  are code symbols. 
     The RS  303  detects and decodes the code symbols {tilde over (S)} 1  and {tilde over (S)} 2 . In step  315 , the RS  303  performs an exclusive OR (XOR) operation of the symbols {tilde over (S)} 1  and {tilde over (S)} 2  and transmits the resulting symbol {tilde over (S)} 3 ={tilde over (S)} 1 ⊕{tilde over (S)} 2  to the BS  301  and the MS  305  in third time resources. 
     The BS  301  can detect the uplink signal {tilde over (S)} 1  from the signal {tilde over (S)} 3  received from the RS  303  by performing an XOR operation of {tilde over (S)} 3  with {tilde over (S)} 1 . The MS  305  can also detect the downlink signal {tilde over (S)} 1  from the signal {tilde over (S)} 3  received from the RS  303  by performing an XOR operation of {tilde over (S)} 3  with {tilde over (S)} 2 . 
     The other relaying method is multi-user and Spatial Division Multiple Access (SDMA) relaying, as illustrated in  FIG. 4 . 
       FIG. 4  illustrates the configuration of a conventional wireless communication system supporting multi-user SDMA relaying. 
     Referring to  FIG. 4 , the wireless communication system supports relaying using two independent time resources. For example, a BS  401  and an MS  405  transmit signals {tilde over (S)} 1  and {tilde over (S)} 2  to an RS  403  in first time resources in  411  and the RS  403  relays the signals {tilde over (S)} 1  and {tilde over (S)} 2  to the MS  405  and the BS  401 , respectively in second time resources in step  413 . That is, the RS  403  can detect the signals {tilde over (S)} 1  and {tilde over (S)} 2  by a multi-user detection scheme used for a multi-antenna system, considering that the BS  401  and the MS  405  are multi-users. The RS  403  can then transmit a signal {tilde over (S)} 3  which includes the signals {tilde over (S)} 1  and {tilde over (S)} 2  as given by 
                 S   ~     3     =         [           H   T             G         ]       -   1       ⁡     [             S   ~     2                 S   ~     1           ]             
to the MS  405  and the BS  401  in the same time resources.
 
     As described above, relaying in the wireless communication systems improves the link performances of MSs at a cell boundary and expands coverage. 
     However, the TDD relaying consumes twice as many resources as the multi-user SDMA relaying. The coded bi-directional relaying does not maximize an overall system capacity gain due to the use of three independent time resources. The multi-user SDMA relaying also has a shortcoming in that, since an RS treats a BS and an MS as multi-users and thus the BS and the MS share spatial resources of the RS, the number of streams exchanged between the BS and the MS is limited by the number of RS antennas. The multi-user SDMA relaying also increases the complexity of the RS because the RS uses the multi-user detection scheme of the multi-antenna system. Moreover, if the BS, the MS and the RS each have a single antenna, the multi-user SDMA relaying is not viable. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and method for supporting relaying without adding time resources in a wireless communication system. 
     Another aspect of the present invention is to provide an apparatus and method for relaying to improve a system capacity gain without adding time resources in a wireless communication system. 
     In accordance with an aspect of the present invention, a method for supporting relaying in a transmitting apparatus in a wireless communication system is provided. The method includes acquiring channel information between the transmitting apparatus and a Relay Station (RS), pre-encoding a transmission signal to be transmitted to the RS with the channel information, transmitting the pre-coded signal to the RS, and acquiring, upon receipt of a signal from the RS, a signal transmitted by a receiving apparatus by eliminating the transmission signal transmitted to the RS from the received signal. 
     In accordance with another aspect of the present invention, a method for supporting relaying in a Relay Station (RS) in a wireless communication system is provided. The method includes monitoring reception of signals from a Base Station (BS) and a Mobile Station (MS) in the same time resources, and transmitting, upon receipt of the signals from the BS and the MS, the sum of the received signals to the BS and the MS in the same time resources. 
     In accordance with a further aspect of the present invention, a transmitting apparatus for supporting relaying in a wireless communication system is provided. The apparatus includes a transmitter for pre-encoding a transmission signal to be transmitted to a Relay Station (RS) with channel information between the transmitting apparatus and an RS and for transmitting the pre-coded signal to the RS, and a receiver for detecting a signal transmitted by a receiving apparatus by eliminating the transmission signal transmitted to the RS from a signal received from the RS. 
     In accordance with still another aspect of the present invention, an apparatus of a Relay Station (RS) for supporting relaying in a wireless communication system is provided. The apparatus includes a receiver for receiving signals from a Base Station (BS) and a Mobile Station (MS) in the same time resources, and a transmitter for transmitting the sum of the received signals to the BS and the MS in the same time resources. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates the configuration of a conventional wireless communication system supporting relaying; 
         FIG. 2  illustrates the configuration of a conventional wireless communication system supporting TDD relaying; 
         FIG. 3  illustrates the configuration of a conventional wireless communication system supporting coded bi-directional relaying; 
         FIG. 4  illustrates the configuration of a conventional wireless communication system supporting multi-user SDMA relaying; 
         FIG. 5  illustrates the configuration of a wireless communication system supporting relaying according to an exemplary embodiment of the present invention; 
         FIG. 6  is a block diagram of a BS transmitter in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention; 
         FIG. 7  is a block diagram of an RS in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention; 
         FIG. 8  is a block diagram of a BS receiver in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention; 
         FIG. 9  is a flowchart of an operation of a BS in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention; 
         FIG. 10  is a flowchart of an operation of an RS in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention; 
         FIG. 11  is a flowchart of an operation of an MS in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention; 
         FIGS. 12A and 12B  are mapping graphs in a wireless communication system supporting relaying according to exemplary embodiments of the present invention; and 
         FIG. 13  is a performance change graph according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     The present invention pertains to a technique for improving a system capacity gain without additional time resources in a wireless communication system supporting relaying. The following description is made under the assumption that Mobile Stations (MSs) are immune to interference through independent resources distribution. 
     While the present invention will be described in the context of a Time Division Duplexing-Orthogonal Frequency Division Multiple Access (TDD-OFDMA) wireless communication system, the same description applies to other multiple access wireless communication systems. Due to TDD, the downlink and the uplink have the same channel information in the wireless communication system. 
     As illustrated in  FIG. 5 , an exemplary wireless communication system supports relaying. 
       FIG. 5  illustrates a configuration of a wireless communication system supporting relaying according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , a wireless communication system supports relaying using two time resources. For example, a Base Station (BS)  501  and a Mobile Station (MS)  505  transmit pre-coded signals to an RS  503  in first time resources in step  511 . The pre-coded signals are given in equation (1) as
 
X BS =nH −1 {tilde over (S)} 1  
 
X MS =mG −1 {tilde over (S)} 2   (1)
 
     where X BS  denotes the pre-coded signal transmitted from the BS  501  to the RS  503 , X MS  denotes the pre-coded signal transmitted from the MS  505  to the RS  503 , {tilde over (S)} 1  denotes a transmission signal from the BS  501 , {tilde over (S)} 2  denotes a transmission signal from the MS  505 , n and m respectively denote the power control constants of the transmission signals {tilde over (S)} 1  and {tilde over (S)} 2 , H denotes channel information about a BS-RS link, and G denotes channel information about an RS-MS link. 
     As noted from equation (1), the BS  501  multiplies the transmission signal {tilde over (S)} 1  by the reciprocal of the channel information H of the BS-RS link, prior to transmission, and the MS  505  also multiplies the transmission signal {tilde over (S)} 2  by the reciprocal of the channel information G of the RS-MS link, prior to transmission. 
     The RS  503  receives the sum of the signals transmitted from the BS  501  and the MS  505  and detects it ({tilde over (S)} 3 =n{tilde over (S)} 1 +m{tilde over (S)} 2 ). For instance, the sum of the transmitted signals is expressed by equation (2) as
 
 y   RS   =n{tilde over (S)}   1   +m{tilde over (S)}   2   +n   RS   (2)
 
     where y RS  denotes the sum of the transmitted signals from the BS  501  and the MS  505 , received at the RS  503 , {tilde over (S)} 1  denotes the transmission signal of the BS  501 , {tilde over (S)} 2  denotes the transmission signal of the MS  505 , n and m respectively denote the power control constants of the signals {tilde over (S)} 1  and {tilde over (S)} 2 , and n RS  denotes the thermal noise of the RS  503 . 
     The RS  503  performs signal detection by eliminating the noise from the received signal given as equation (2). For example, the RS  503  detects the received signal by eliminating the noise through symbol constellation extension as expressed in equation (3)
 
 ŷ   RS   ≈n{tilde over (S)}   1   +m{tilde over (S)}   2   (3)
 
     where ŷ RS  denotes the received signal from which the noise is eliminated, {tilde over (S)} 1  denotes the transmission signal of the BS  501 , {tilde over (S)} 2  denotes the transmission signal of the MS  505 , and n and m respectively denote the power control constants of the signals {tilde over (S)} 1  and {tilde over (S)} 2 . 
     In step  513 , if the RS  503  uses an Amplify and Forward (AF) strategy, it amplifies the power of the detected signal expressed as equation (3) and transmits the amplified signal {tilde over (S)} 3  to the BS  501  and the MS  505  in second time resources. 
     Alternatively in step  513 , if the RS  503  uses a Decode and Forward (DF) strategy, it corrects errors in the detected signal by decoding and transmits the decoded signal {tilde over (S)} 3  to the BS  501  and the MS  505  in second time resources. 
     The signal received from the RS  503  in the BS  501  is shown by equation (4)
 
 y   BS   =H   T   {tilde over (X)}   RS   +n   BS   (4)
 
     where y BS  denotes the signal received from the RS  503 , H denotes the channel information of the BS-RS link, {tilde over (X)} RS  denotes the signal transmitted by the RS  503 , expressed as equation (3), and n BS  denotes the thermal noise of the BS  501 . 
     With knowledge of the signal {tilde over (S)} 1  transmitted to the RS  503  in the first time resources, the BS  501  can detect the signal {tilde over (S)} 2  transmitted by the MS  505  using equation (5)
 
 r   BS   =y   BS   −H   T   n{tilde over (S)}   1   =mH   T   {tilde over (S)}   2   +n   BS   (5)
 
     where r BS  denotes the signal transmitted by the MS  505 , y BS  denotes the signal that the BS  501  has received from the RS  503 , H denotes the channel information of the BS-RS link, {tilde over (S)} 1  denotes the signal that the BS  501  transmitted to the RS  503  in the first time resources, {tilde over (S)} 1  denotes the signal that the MS  505  transmitted to the RS  503  in the first time resources, and n BS  denotes the thermal noise of the BS  501 . 
     The BS  501  has knowledge of the channel information of the BS-RS link and the signal transmitted to the RS  503  in the first time resources. Therefore, the BS  501  can detect and decode the signal transmitted by the MS  505  by eliminating the signal transmitted to the RS  503  from the received signal described as equation (4) according to equation (5). 
     Meanwhile, the signal received from the RS  503  in the MS  505  is as shown in equation (6)
 
 y   MS   =G   T   {tilde over (X)}   RS   +n   MS   (6)
 
     where y MS  denotes the signal received from the RS  503 , G denotes the channel information of the RS-MS link, {tilde over (X)} RS  denotes the signal transmitted by the RS  503 , expressed as equation (3), and n MS  denotes the thermal noise of the MS  505 . 
     With knowledge of the signal {tilde over (S)} 2  transmitted to the RS  503  in the first time resources, the MS  505  can detect the signal {tilde over (S)} 1  transmitted by the BS  501  using equation (7)
 
 r   MS   =y   MS   −G   T   m{tilde over (S)}   2   =nG   T   {tilde over (S)}   1   +n   MS   (7)
 
     where r MS  denotes the signal transmitted by the BS  501 , y MS  denotes the signal received from the RS  503 , G denotes the channel information of the RS-MS link, {tilde over (S)} 1  denotes the signal that the BS  501  transmitted to the RS  503  in the first time resources, {tilde over (S)} 2  denotes the signal that the MS  505  transmitted to the RS  503 , and n MS  denotes the thermal noise of the MS  505 . 
     The MS  505  has knowledge of the channel information of the RS-MS link and the signal transmitted to the RS  503  in the first time resources. Therefore, the MS  505  can detect and decode the signal transmitted by the BS  501  by eliminating the signal transmitted to the RS  503  from the received signal described as equation (6) according to equation (7). 
       FIG. 6  is a block diagram of a BS transmitter in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , the BS transmitter includes an encoder  601 , a modulator  603 , a precoder  605 , a channel estimator  607 , an OFDM modulator  609 , and an RF processor  611 . 
     The encoder  601  encodes a signal to be transmitted to the MS via the RS using a coding rate. The modulator  603  modulates the coded signal using a modulation scheme. The modulation scheme may be identical to a modulation scheme in which the MS transmits a signal to the RS. 
     The precoder  605  pre-encodes the modulated signal using channel information about the BS-RS link received from the channel estimator  607  according to equation (1). 
     The channel estimator  607  estimates the channel of the BS-RS link using a signal received from the RS and provides the estimated channel information to the precoder  605 . 
     The OFDM modulator  609  converts the pre-coded frequency signal received from the precoder  605  to a time signal by Inverse Fast Fourier Transform (IFFT). 
     The RF processor  611  upconverts the baseband time signal received from the OFDM modulator  609  to an RF signal and transmits the RF signal to the RS through an antenna. 
     Although not illustrated, an exemplary MS transmitter may be configured substantially similar to the BS transmitter illustrated in  FIG. 6 . That is, the blocks of an MS transmitter, except for a precoder and a channel estimator, operate in substantially the same manner as their counterparts illustrated in  FIG. 6 . However, in an MS transmitter, the channel estimator estimates the channel of the RS-MS link using a signal received from the RS. Also, the precoder of an MS transmitter pre-codes a transmission signal based on the channel information of the RS-MS link received from the channel estimator. 
       FIG. 7  is a block diagram of an RS in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention. The RS uses a DF strategy, by way of example only. 
     Referring to  FIG. 7 , an RS includes a receiver  700  and a transmitter  710 . 
     The receiver  700  is comprised of an RF processor  701 , an OFDM demodulator  703 , a demodulator  705 , and a decoder  707 . 
     The RF processor  701  downconverts an RF signal, for example an RF signal received from a BS or an MS, through an antenna to a baseband signal. 
     The OFDM demodulator  703  converts the time signal received from the RF processor  701  to a frequency signal by Fast Fourier Transform (FFT). 
     The demodulator  705  demodulates the frequency signal using a modulation scheme. In the received signal expressed as equation (2), which includes the transmission signals of the BS and the MS, signals {tilde over (S)} 1  and {tilde over (S)} 2  each have elements −1s or 1s. Thus, the demodulator  705  converts the signals {tilde over (S)} 1  and {tilde over (S)} 2  to signals each having elements 0s or 1s, {tilde over (X)} 1  and {tilde over (X)} 2  by scaling and shifting as illustrated in equation (8) 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     where y RS  denotes the signal received from the BS and the MS, {tilde over (S)} 1  denotes the transmission signal of the BS, {tilde over (S)} 2  denotes the transmission signal of the MS, n and m denote the power control constants of the signals {tilde over (S)} 1  and {tilde over (S)} 2 , and {tilde over (X)} 1  and {tilde over (X)} 2  denote the converted versions of {tilde over (S)} 1  and {tilde over (S)} 2 . 
     The decoder  707  decodes the demodulated signal received from the demodulator  705  using a coding rate, thus recovering the sum of the transmission signals of the BS and the MS. {tilde over (X)} 1  and {tilde over (X)} 2  of the signal r received from the demodulator  705  are linear codewords, but the signal r is the sum of the product of {tilde over (X)} 1  and one of {o, n, m, n+m} and the product of {tilde over (X)} 2  and one of {o, n, m, n+m}. Therefore, r cannot be a codeword used for generating {tilde over (X)} 1  and {tilde over (X)} 2  in the BS and the MS. That is, the decoder  707  cannot decode the signal r. Thus, the decoder  707  converts the signal r received from the demodulator  705  to a codeword by use of a codeword generator  721 . 
     The codeword generator  721  converts the signal r received from the demodulator  705  to 0s and 1s of a continuous function, as illustrated in  FIGS. 12A and 12B . 
       FIGS. 12A and 12B  are mapping graphs in a wireless communication system supporting relaying according to exemplary embodiments of the present invention. 
     When the power control constants of the BS and the MS, n and m, are different, the codeword generator  721  maps the signal r to a codeword with 0s or 1s by a linear function as illustrated in  FIG. 12A . 
     If the power control constants of the BS and the MS, n and m, are equal, the codeword generator  721  maps the signal r to a codeword with 0s or 1s by a linear function as illustrated in  FIG. 12B . 
     It can be further contemplated as another exemplary embodiment of the present invention that the codeword generator  721  converts the signal r to a codeword by using equation (9)
 
 Z= mod 2 ( {tilde over (X)}   1   +{tilde over (X)}   2 )= XOR ( {tilde over (X)}   1   , {tilde over (X)}   2 )  (9)
 
     where {tilde over (X)} 1  and {tilde over (X)} 2  denote the linear codewords for {tilde over (S)} 1  and {tilde over (S)} 2 . 
     According to equation (9), the codeword generator  721  converts the signal r to a codeword by modulo-operating the combination of {tilde over (X)} 1  and {tilde over (X)} 2  being linear codewords. The modulo-operation of the combination of {tilde over (X)} 1  and {tilde over (X)} 2  is equivalent to XOR-operation of {tilde over (X)} 1  and {tilde over (X)} 2 . 
     After the decoder  707  converts the signal r to a codeword, a decoding portion  723  of the decoder  707  decodes the codeword received from the codeword generator  721 , thus correcting errors. Here, the decoder  723  considers that it receives XOR({tilde over (X)} 1 ,{tilde over (X)} 2 ) rather than n{tilde over (X)} 1 +m{tilde over (X)} 2 . As illustrated in equation (10), because n{tilde over (X)} 1 +m{tilde over (X)} 2  and XOR({tilde over (X)} 1 , {tilde over (X)} 2 ) should have the same soft-decision value (i.e. Log Likelihood Ratio (LLR), 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                                         ( 
                                         
                                           
                                             r 
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                                         ) 
                                       
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                                   / 
                                   
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                                     0 
                                   
                                 
                               
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                                     - 
                                     
                                       
                                         ( 
                                         
                                           
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                                     - 
                                     
                                       
                                         ( 
                                         
                                           
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                                     - 
                                     
                                       
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                                         ) 
                                       
                                       2 
                                     
                                   
                                   / 
                                   
                                     N 
                                     0 
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           z 
                           i 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 N 
                                 o 
                               
                               2 
                             
                             ⁢ 
                             ln 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 
                                   ⅇ 
                                   
                                     
                                       - 
                                       
                                         
                                           ( 
                                           
                                             
                                               r 
                                               i 
                                             
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                                           ) 
                                         
                                         2 
                                       
                                     
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                                       0 
                                     
                                   
                                 
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                                   ⅇ 
                                   
                                     
                                       - 
                                       
                                         
                                           ( 
                                           
                                             
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                                               i 
                                             
                                             - 
                                             n 
                                           
                                           ) 
                                         
                                         2 
                                       
                                     
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                                       0 
                                     
                                   
                                 
                               
                               
                                 
                                   ⅇ 
                                   
                                     
                                       - 
                                       
                                         
                                           ( 
                                           
                                             
                                               r 
                                               i 
                                             
                                             - 
                                             0 
                                           
                                           ) 
                                         
                                         2 
                                       
                                     
                                     / 
                                     
                                       N 
                                       0 
                                     
                                   
                                 
                                 + 
                                 
                                   ⅇ 
                                   
                                     
                                       - 
                                       
                                         
                                           ( 
                                           
                                             
                                               r 
                                               i 
                                             
                                             - 
                                             m 
                                             - 
                                             n 
                                           
                                           ) 
                                         
                                         2 
                                       
                                     
                                     / 
                                     
                                       N 
                                       0 
                                     
                                   
                                 
                               
                             
                           
                           + 
                           
                             1 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     where n and m denote the power control constants of the signals {tilde over (S)} 1  and {tilde over (S)} 2  and {tilde over (X)} 1  and {tilde over (X)} 2  denote the converted versions of {tilde over (S)} 1  and {tilde over (S)} 2 . 
     Since the probability of z being 1 should be equal to the probability of n or m being 1, equation (10) is substantiated. Equation (10) is an optimal solution for achieving z in the codeword generator  721 . The simple linear mapping graphs of  FIGS. 12A and 12B  can be drawn based on equation (10). 
     The transmitter  710  includes an encoder  711 , a modulator  713 , an OFDM modulator  715 , and an RF processor  717 . 
     The encoder  711  encodes the signal decoded by the decoder  707  of the receiver  700  using a coding rate. Specifically, an encoding portion  725  of the encoder  711  encodes the decoded signal that is not the decoded version of the signal r provided from the demodulator  705  to the decoder  707  but the decoded version of the codeword z generated from the codeword generator  721  of the decoder  707 . 
     Hence, a demapper  727  of the encoder  711  reconstructs the coded signal received from the encoding portion  725  to a signal before the codeword generation of the codeword generator  721 . The reconstructed signal is a signal resulting from correcting errors in the signal r. 
     The demapper  727  reconstructs the signal before the codeword generation in the codeword generator  721  according to equation (11) using the method illustrated in  FIGS. 12A and 12B : 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             i 
                             ) 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           m 
                         
                         ≠ 
                         n 
                       
                     
                     
                       
                           
                       
                     
                   
                   
                     
                       
                         
                           
                             if 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               z 
                               ~ 
                             
                           
                           = 
                           0 
                         
                         , 
                       
                     
                     
                       
                         
                           
                             if 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               z 
                               ~ 
                             
                           
                           = 
                           1 
                         
                         , 
                       
                     
                   
                   
                     
                       
                         
                           r 
                           ^ 
                         
                         = 
                         
                           { 
                           
                             
                               
                                 0 
                               
                               
                                 
                                   
                                      
                                     
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                                       - 
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                                   ≤ 
                                   
                                      
                                     
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                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     where {tilde over (z)} denotes the signal encoded by the encoding portion  725  and n and m respectively denote the power control constants of the transmission signals of the BS and the MS. 
     According to equation (11), if n and m are different, the demapper  727  maps {tilde over (z)} to 0 or n+m depending on the value of r, when {tilde over (z)} is 0. When {tilde over (z)} is 1, the demapper  727  maps {tilde over (z)} to min {m,n} or max {m,n} depending on the value of r. 
     If n and m are equal, the demapper  727  maps {tilde over (z)} to 0 or 2m depending on the value of r, when {tilde over (z)} is 0. When {tilde over (z)} is 1, the demapper  727  maps {tilde over (z)} to m. 
     The modulator  713  modulates the coded signal received from the encoder  711  using a modulation scheme. 
     The OFDM modulator  715  converts the frequency signal received from the modulator  713  to a time signal by IFFT. 
     The RF processor  717  upconverts the baseband signal received from the OFDM modulator  715  to an RF signal and transmits the RF signal to the BS and the MS through an antenna. 
     The transmitted signal is given as equation (12) 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             X 
                             ~ 
                           
                           RS 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 - 
                                 2 
                               
                               ⁢ 
                               
                                 r 
                                 ^ 
                               
                             
                             + 
                             n 
                             + 
                             m 
                           
                           ≈ 
                           
                             
                               
                                 - 
                                 2 
                               
                               ⁢ 
                               n 
                               ⁢ 
                               
                                 
                                   X 
                                   ~ 
                                 
                                 1 
                               
                             
                             + 
                             n 
                             - 
                             
                               2 
                               ⁢ 
                               m 
                               ⁢ 
                               
                                 
                                   X 
                                   ~ 
                                 
                                 2 
                               
                             
                             + 
                             m 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             n 
                             ⁢ 
                             
                               
                                 S 
                                 ~ 
                               
                               1 
                             
                           
                           + 
                           
                             m 
                             ⁢ 
                             
                               
                                 S 
                                 ~ 
                               
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     where {tilde over (X)} RS  denotes the transmitted signal, {circumflex over (r)} denotes the coded signal from the encoder  711 , {tilde over (S)} 1  denotes the modulated signal transmitted by the BS, {tilde over (S)} 2  denotes the modulated signal transmitted by the MS, n and m respectively denote the power control constants of the signals transmitted by the BS and the MS, and {tilde over (X)} 1  and {tilde over (X)} 2  denote the demodulated signals, i.e. codewords of {tilde over (S)} 1  and {tilde over (S)} 2 . 
       FIG. 8  is a block diagram of a BS receiver in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , a BS receiver includes an RF processor  801 , an OFDM demodulator  803 , a demodulator  805 , a signal detector  807 , and a decoder  809 . 
     The RF processor  801  downconverts an RF signal received through an antenna to a baseband signal. The received signal is given as equation (4). 
     The OFDM demodulator  803  converts the time signal received from the RF processor  801  to a frequency signal by FFT. 
     The demodulator  805  demodulates the frequency signal in accordance with a modulation scheme. 
     The signal detector  807 , which has knowledge of a signal transmitted to the RS, eliminates the transmitted signal from the demodulated signal according to equation (5). 
     The decoder  809  decodes the signal received from the signal detector  807 , thereby recovering the signal transmitted by the MS. 
     Although not illustrated, an exemplary MS receiver may be configured substantially similar to the BS transmitter illustrated in  FIG. 8 . That is, the blocks of an MS receiver, except for a demodulator, operate in substantially the same manner as their counterparts of the BS receiver illustrated in  FIG. 8 . However, an MS receiver has knowledge of a signal transmitted to the RS and thus the demodulator of the MS receiver can recover the signal transmitted by the BS by eliminating the signal transmitted to the RS from a signal received from the RS. 
     Now a description will be made of exemplary operations of a BS, an RS and an MS for signal relaying in a wireless communication system. 
       FIG. 9  is a flowchart of an operation of a BS in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 9 , a BS estimates a channel on a BS-RS link in step  901 . For example, the BS estimates the BS-RS link channel using an uplink signal received from the RS. It can be further contemplated as another exemplary embodiment of the present invention that the BS acquires channel information about the BS-RS link from a feedback signal received from the RS. 
     In step  903 , the BS encodes and modulates a signal to be transmitted to the MS via the RS using a Modulation and Coding Scheme (MCS) level. 
     The BS pre-encodes the modulated signal using the BS-RS link channel information in step  905 . To be more specific, the BS multiplies the modulated transmission signal by the reciprocal of the BS-RS link channel information according to equation (1). The BS transmits the pre-coded signal to the RS in step  907 . 
     In step  909 , the BS monitors reception of a signal from the RS. Upon receipt of the signal from the RS, the BS eliminates the signal transmitted to the RS from the received signal in step  911 . For instance, the BS receives a signal expressed as equation (4) from the RS and eliminates the signal {tilde over (S)} 1  transmitted to the RS from the received signal by equation (5). 
     In step  913 , the BS decodes the resulting signal, thus recovering a signal transmitted by the MS via the RS. 
     Then, the BS ends the algorithm. 
       FIG. 10  is a flowchart of an operation of an RS in a wireless communication system supporting relaying according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 10 , an RS monitors reception of signals from the BS and the MS in step  1001 . 
     Upon receipt of the signals from the BS and the MS, the RS generates a codeword z by performing an XOR operation on the signal {tilde over (S)} 1  received from the BS and the signal {tilde over (S)} 2  received from the MS in step  1003 . For example, the RS generates the codeword by mapping the XOR-operation value in a mapping table as illustrated in  FIGS. 12A and 12B , or by modular operation or XOR-operation of {tilde over (S)} 1  and {tilde over (S)} 2  described as equation (9). 
     In step  1005 , the RS corrects errors in the codeword by decoding. 
     The RS encodes the decoded signal at a coding rate in step  1007 . The RS demaps the sum of the signals received from the BS and the MS from the coded signal in step  1009 . That is, the coded signal is not a signal resulting from correcting errors in the signal {tilde over (S)} 3  and then encoding the error-corrected signal, but a signal resulting from correcting errors in the codeword z of the signal {tilde over (S)} 3  and then encoding the error-corrected signal. Therefore, the RS recovers the signal {tilde over (S)} 3  by demapping the coded signal. The recovered signal {tilde over (S)} 3  is an error-corrected signal. 
     In step  1011 , the RS transmits the signal {tilde over (S)} 3  to the BS and the MS. For example, the RS transmits a signal expressed as equation (12) to the BS and the MS. 
     Then, the RS ends the algorithm. 
       FIG. 11  is a flowchart of an operation of an MS in the wireless communication system supporting relaying according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 11 , an MS estimates a channel on an RS-MS link in step  1101 . For example, the MS estimates the RS-MS link channel using a downlink signal received from the RS. 
     In step  1103 , the MS encodes and modulates a signal to be transmitted to the BS via the RS at an MCS level. 
     The MS pre-encodes the modulated signal using the RS-MS link channel information in step  1105 . To be more specific, the MS multiplies the modulated transmission signal by the reciprocal of the RS-MS link channel information according to equation (1). The MS transmits the pre-coded signal to the RS in step  1107 . 
     In step  1109 , the MS monitors reception of a signal from the RS. Upon receipt of the signal from the RS, the MS eliminates the signal transmitted to the RS from the received signal in step  1111 . For instance, the MS receives a signal expressed as equation (6) from the RS and eliminates the signal transmitted to the RS from the received signal by equation (7). 
     In step  1113 , the MS decodes the resulting signal, thus recovering a signal transmitted by the BS via the RS. 
     Then, the MS ends the algorithm. 
     In the above-described exemplary embodiment of the present invention, the RS recovers a codeword generated for decoding the sum of signals transmitted from the BS and the MS and transmits the sum of the signals to the BS and the MS. As another exemplary embodiment of the present invention, the RS may transmit the codeword to the BS and the RS without recovering it to the sum of the transmission signals of the BS and the MS. The BS recovers the signal transmitted by the MS by XOR-operating the received signal and the signal transmitted to the RS, and the MS recovers the signal transmitted by the BS by XOR-operating the received signal and the signal transmitted to the RS. 
     A description will be made of performance change when signals are relayed in the manner illustrated in  FIG. 5 . 
       FIG. 13  is a performance change graph according to an exemplary embodiment of the present invention. The horizontal axis represents average Signal-to-Noise Ratio (SNR) and the vertical axis represents Bit Error Rate (BER). 
     Referring to  FIG. 13 , the conventional technology supporting relaying using four independent time resources is compared with the present invention supporting relaying using two independent time resources. Another line indicates a performance graph of AF relaying. 
     As noted from  FIG. 13 , given the same coding rate, the conventional technology outperforms the present invention in BER. Nonetheless, the present invention offers twice the capacity gain using half the time resources, compared to the conventional technology. 
     For a coding rate of ⅔, exemplary embodiments of the present invention have a capacity gain of 
               77.78   ⁢   %   ⁢     (           (       2   /   3     ×   2     )     -     3   /   4         3   /   4       ×   100     )       ,         
despite the BER increase. For a coding rate of ½, exemplary embodiments of the present invention have a capacity gain of
 
               33.33   ⁢   %   ⁢     (           (       1   /   2     ×   2     )     -     3   /   4         3   /   4       ×   100     )       ,         
despite the BER increase.
 
     As is apparent from the above description, exemplary embodiments of the present invention advantageously increase an overall system capacity gain by multiplying a transmission signal by the reciprocal function of BS-RS link channel information or RS-MS link channel information in a BS and an MS, for relaying using two independent time resources in a multi-hop relay wireless communication system. 
     While the invention has been shown and described with reference to certain exemplary embodiments of the present invention thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents.