Patent Publication Number: US-7586846-B2

Title: System and method for relaying signal in a communication system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a communication system, and in particular, to a signal relay system and method for increasing throughput in a communication system. 
     2. Description of the Related Art 
       FIG. 1  is a diagram illustrating a configuration of a signal relay system in a general communication system. 
     Referring to  FIG. 1 , the signal relay system includes a source node S, a destination node D, and a relay node R. The source node S transmits a signal to the destination node D, and the relay node R performs a relay operation for signal transmission from the source node S to the destination node D. 
     The signal transmitted by the source node S may be received at both the relay node R and the destination node D due to the characteristics of the wireless transmission medium. However, a signal-to-Noise Ratio (SNR) γ SR  between the source node S and the relay node R can be higher than an SNR γ SD  between the source node S and the destination node D, and this indicates that the signal transmission from the source node S to the relay node R is superior to the signal transmission from the source node S to the destination node D in terms of the throughput. In particular, there is a possible case where the signal transmitted by the source node S is safely received at the relay node R, but is never received at the destination node D. In this case, the signal transmission from the source node S to the relay node R is surely superior to the signal transmission from the source node S to the destination node D in terms of the throughput, because the signal transmission from the source node S to the destination node D is completely impossible. 
     Generally, in a multi-hop mode, the signal transmitted by the source node S is safely received at the relay node R, but is never received at the destination node D. In the multi-hop mode, the relay node R must relay all signals received from the source node S to the destination node D. In order to increase throughput of the communication system in this way, it is important to efficiently perform signal transmission between the source node S and the relay node R and increase throughput between the source node S and the destination node D. Accordingly, there is a need for a signal relay scheme for increasing the throughput of the communication system. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a signal relay system and method for increasing throughput in a communication system. 
     According to one aspect of the present invention, there is provided a system for relaying a signal in a communication system, comprising a source node; a destination node; and a relay node. The source node comprises calculating a data rate of an Adaptive Modulation and Coding (AMC) mode, which should be used to maximize a data rate on each of a first link between the source node and the destination node, a second link between the source node and the relay node, and a third link between the relay node and the destination node; calculating a first data rate for the case where a multi-hop mode is used for signal transmission between the source node and the destination node; calculating a first optimal data rate for the case where transmission modes available for signal transmission between the source node and the destination node include a direct mode and the multi-hop mode; selecting an AMC mode to be used for the first link and determining a second data rate for the first link when the selected AMC mode is used; calculating an available signal-to-noise ratio (SNR) needed to decode a superposed message after a basic message is decoded, when a signal transmitted from the source node to the destination node includes the basic message and the superposed message; determining a third data rate at which the superposed message is transmitted with reliability; calculating a second optimal data rate for the case where a transmission mode available for signal transmission between the source node and the destination node is the superposition mode; determining whether the second optimal data rate exceeds the first optimal data rate; and determining to use the superposition mode for signal transmission from the source node to the destination node, if the second optimal data rate exceeds the first optimal data rate. 
     According to another aspect of the present invention, there is provided a method for relaying a signal in a source node of a communication system. The method comprises the steps of: calculating a data rate of an Adaptive Modulation and Coding (AMC) mode, which should be used to maximize a data rate on each of a first link between the source node and a destination node, a second link between the source node and a relay node, and a third link between the relay node and the destination node; calculating a first data rate for the case where a multi-hop mode is used for signal transmission between the source node and the destination node; calculating a first optimal data rate for the case where transmission modes available for signal transmission between the source node and the destination node include a direct mode and the multi-hop mode; selecting an AMC mode to be used for the first link and determining a second data rate for the first link when the selected AMC mode is used; calculating an available signal-to-noise ratio (SNR) needed to decode a superposed message after a basic message is decoded, when a signal transmitted from the source node to the destination node includes the basic message and the superposed message; determining a third data rate at which the superposed message is transmitted with reliability; calculating a second optimal data rate for the case where a transmission mode available for signal transmission between the source node and the destination node is the superposition mode; determining whether the second optimal data rate exceeds the first optimal data rate; and determining to use the superposition mode for signal transmission from the source node to the destination node, if the second optimal data rate exceeds the first optimal data rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagram illustrating a configuration of a signal relay system in a general communication system; 
         FIG. 2  is a diagram illustrating a symbol constellation for general asymmetric QPSK (Binary Phase Shift Keying (BPSK) superposed to BPSK); 
         FIG. 3  is a diagram illustrating an example where an 8 PSK constellation is superposed to a QPSK constellation to acquire a non-uniform 32 QAM constellation; and 
         FIGS. 4A and 4B  are flowcharts illustrating a signal relay process performed by a source node S in a communication system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted for clarity and conciseness. 
     The present invention proposes a signal relay system and method for increasing throughput in a communication system. Although not separately illustrated in the following description, the signal relay scheme proposed in the present invention can be applied to the configuration of the communication system described with reference to  FIG. 1 . 
     Before a description of the present invention is given, a description will now be made of a superposition coding scheme and an Adaptive Modulation and Coding (AMC) scheme. 
     (1) Superposition Coding Scheme 
     The superposition coding scheme, which is a kind of an Unequal Error Protection (UEP) scheme, has recently attracted much attention in relation to the wireless systems. The UEP scheme differently protects different signals, for example, different data bits, using a modulation/coding scheme. The typical case for transmitting a signal using the UEP scheme includes the case where a transmission signal can be expressed with a basic message and a superposed message, like the case where the transmission signal includes basic multimedia contents and multimedia information including detailed information on the basic multimedia contents. The superposed message is more tolerable than the basic message in terms of the loss rate. 
     For example, assume that there is a packet including 10 bits of b1, b2, . . . , b10, and of the 10 bits, 5 bits of b1, b2, . . . , b5 should be better protected than the other 5 bits of b6, b7, . . . , b10. To transmit the bits b1, b2, . . . , b10, 5 non-uniform Quadrature Phase Shift Keying (QPSK) modulation symbols should be used, and a description thereof will be made with reference  FIG. 2 . 
       FIG. 2  is a diagram illustrating a symbol constellation for general asymmetric QPSK (Binary Phase Shift Keying (BPSK) superposed to BPSK). 
     Referring to  FIG. 2 , to generate a first modulation symbol, the bit b1 is mapped to the I component and the bit b 6  is mapped to the Q component. In this manner, the other 4 modulation symbols of second through fifth modulation symbols are generated. It will be assumed that the packet is divided into two sub-packets, and of the two sub-packets, a first sub-packet is denoted by b 1 =(b 1 , b 2 , . . . b 5 ) and a second sub-packet is denoted by b 2 =(b 6 , b 7 , . . . b 10 ). 
     In this case, in a poor channel state, i.e., in a low-SNR channel state, the first sub-packet b 1 =(b 1 , b 2 , . . . b 5 ) will be received with higher reliability than the second sub-packet b 2 =(b 6 , b 7 , . . . b 10 ). On the contrary, in an excellent channel state, i.e., a high-SNR channel state, there is a high probability that the second sub-packet b 2 =(b 6 , b 7 , . . . b 10 ) as well as the first sub-packet b 1 =(b 1 , b 2 , . . . b 5 ) will be received with high reliability. Therefore, the present invention proposes a scheme for relaying signals based on the superposition coding scheme. 
     The superposition coding scheme for the case where a BPSK-modulated signal is superposed to a stronger BPSK-modulated signal has been described so far with reference to  FIG. 2 . Next, with reference to  FIG. 3 , a description will be made of the superposition coding scheme for the case where 8-ary Phase Shift Keying (8 PSK) is superposed to QPSK. 
       FIG. 3  is a diagram illustrating an example where an 8 PSK constellation is superposed to a QPSK constellation to acquire a non-uniform 32-ary Quadrature Amplitude Modulation (32 QAM) constellation. 
     Referring to  FIG. 3 , it can be noted that the 3 bits that determine an 8 PSK component of the modulation symbol are less protected than the 2 bits that determine a QPSK component. Assuming that h denotes a channel gain between a transmitter and a receiver of the communication system, a signal r received at the receiver can be represented by
 
 r=h (√{square root over ( E   1 )} x   1 +√{square root over ( E   2 )} x   2 )+ z   (1)
 
     In Equation (1), x 1  and x 2  are complex numbers with |x 1 |=|x 2 |=1, defined such that x 1  and x 2  determine the transmitted phases of the QPSK and 8 PSK components, respectively. A complex noise component is denoted by z and has a variance σ 2 . 
     Therefore, the QPSK component will be referred to as a “basic component” of a superposition code, while the 8 PSK component will be referred to as a “superposed component” of the superposition code. In addition, a message carried by the QPSK component will be referred to as a “basic message,” while a message carried by the 8 PSK component will be referred to as a “superposed message.” As a result, the total energy E total  present in a transmitted symbol is E total =E 1 +E 2 , and an SNR at the destination, i.e., at the receiver, is given by 
     
       
         
           
             
               
                 
                   γ 
                   = 
                   
                     
                       
                         
                            
                           h 
                            
                         
                         2 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             E 
                             1 
                           
                           + 
                           
                             E 
                             2 
                           
                         
                         ) 
                       
                     
                     
                       σ 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     It is assumed that each component of the superposed signal carries an independent message. This indicates that if N modulation symbols in the constellation of  FIG. 3  are sent, then the basic message contains 2 N bits, while the additional message, i.e., the superposed message, contains 3 N bits. A description will now be made of a scheme for decoding the coded signal using the superposition coding scheme. 
     First, the basic message can be detected by considering that the component h√{square root over (E 2 )}x 2  in the received signal r shown in Equation (1) is noise. In this case, the basic message becomes h√{square root over (E 2 )}x 2 +z. Therefore, an 8 SPK signal is demodulated taking only the basic message h√{square root over (E 2 )}x 2 +z into account. Next, amplitude of the superposed message can affect reception reliability of the basic message. That is, the error probability for the received QPSK signal depends on the actual value of the 8 PSK signal. In the worst case, assuming that an unfavorable 8 PSK signal is being transmitted, the unfavorable 8 PSK signal used for deciding the received QPSK signal is denoted by
 
 r   1   =h (√{square root over ( E   1 )}−√{square root over ( E   2 )}) x   1   +z   (3)
 
     In addition, a Bit Error Probability (BEP) for the basic message is upper-bounded by the BEP for transmission of a non-superposed QPSK signal with effective energy E eff =(√{square root over (E 1 )}−√{square root over (E 2 )}) 2 . That is, the SNR γ b  used for detection of the basic message is approximated by 
     
       
         
           
             
               
                 
                   
                     γ 
                     b 
                   
                   = 
                   
                     
                       
                         
                            
                           h 
                            
                         
                         2 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 E 
                                 1 
                               
                             
                             - 
                             
                               
                                 E 
                                 2 
                               
                             
                           
                           ) 
                         
                         2 
                       
                     
                     
                       σ 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Provided that the basic message has been decoded correctly, the SNR γ s  used for decoding the superposed message is 
     
       
         
           
             
               
                 
                   
                     γ 
                     s 
                   
                   = 
                   
                     
                       
                         
                            
                           h 
                            
                         
                         2 
                       
                       ⁢ 
                       
                         E 
                         2 
                       
                     
                     
                       σ 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     (2) Adaptive Modulation and Coding (AMC) Scheme 
     With the use of the AMC scheme, the link adapts to the actual transmitting conditions and attempts to maximize the attained spectral efficiency. Assuming that there are M AMC modes used in the communication system, an M th  AMC mode has a data rate R m , where R 1 &lt;R 2 &lt;. . . &lt;R M . Herein, the AMC scheme includes a plurality of Modulation and Coding Scheme (MCS) levels, and if the number of the MCS levels is M, there are M AMC modes. 
     In addition, assume that PER m (γ) denote a Packet Error Rate (PER) when the transmitter uses an m th  AMC mode and the SNR at the receiver is equal to γ. In this case, for given γ, PER 1 (γ)&lt;PER 2 (γ)&lt;. . . &lt;PER M (γ). When the transmitter uses the m th  AMC mode and the SNR in the receiver is γ, the throughput is given by
 
R m (1−PER m (γ))  (6)
 
     If an AMC mode selected for the SNR γ is denoted by m*, the AMC mode m* is selected based on the criterion given by 
     
       
         
           
             
               
                 
                   
                     m 
                     * 
                   
                   = 
                   
                     
                       
                         
                           max 
                         
                       
                       
                         
                           m 
                         
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           R 
                           m 
                         
                         ⁡ 
                         
                           ( 
                           
                             1 
                             - 
                             
                               
                                 PER 
                                 m 
                               
                               ⁡ 
                               
                                 ( 
                                 γ 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     The AMC mode m* selected for the SNR γ in accordance with Equation (7) is an AMC mode that maximizes the throughput, i.e., the data rate. 
     In order to have definite PER values, the packet size N should be fixed to 1080 (N=1080). Due to the monotonicity of PER m (γ), SNR thresholds for selecting a given AMC mode can be found by comparing two AMC modes at a certain time. If the lowest SNR threshold is denoted by τ 1 , it can be noted that for γ&lt;τ 1 , even the lowest AMC mode is unusable. Herein, the term “lowest AMC mode” refers to an AMC mode that uses the lowest MCS level. Assume that there are a total of M SNR thresholds τ m , where τ 1 &lt;τ 2 &lt;. . . &lt;τ M . In this case, for τ m &lt;τ m+1 , the m th  AMC mode is utilized and the SNR threshold τ m+1  is determined by
 
 R   m+1 ·(1−PER m+1 (τ m+1 ))= R   m ·(1−PER m (τ m ))  (8)
 
     The mapping determined by the AMC mode selection is defined as
 
 R=A (γ)  (9)
 
     In Equation (9), R denotes a data rate of an AMC mode that should be applied to maximize the throughput when the SNR over the link is γ. Table 1 below shows the SNR thresholds when uncoded M-ary QAM modulation is used with a packet size of N=1080 bits. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Mode m 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                 Modulation 
                 BPSK 
                 QPSK 
                 8PSK 
                 16QAM 
                 32QAM 
                 64QAM 
                 128QAM 
               
               
                 Rate r m  [bps/sym] 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                 Thresholds τ m  [dB] 
                 6.3281 
                 10.0481 
                 14.9881 
                 17.3881 
                 21.6181 
                 23.7281 
                 27.7881 
               
               
                   
               
            
           
         
       
     
     In Table 1, Mode denotes the AMC mode, Modulation denotes a modulation scheme, Rate denotes a data rate, and Threshold denotes an SNR threshold. 
     Table 2 below shows the SNR thresholds when coded M-ary QAM modulation is used with a packet size of N=1080 bits. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 Mode m 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                 Modulation 
                 BPSK 
                 QPSK 
                 8PSK 
                 16QAM 
                 32QAM 
                 64QAM 
               
               
                 Coding rate 
                 1/2 
                 1/2 
                 3/4 
                 9/16 
                 3/4 
                 3/4 
               
               
                 Rate 
                 0.5 
                 1 
                 1.5 
                 2.25 
                 3 
                 4.5 
               
               
                 r m  [bps/sym] 
               
               
                 Thresholds 
                 −1.5331 
                 1.7169 
                 4.9769 
                 8.7769 
                 11.5469 
                 17.1469 
               
               
                 τ m  [dB] 
               
               
                   
               
            
           
         
       
     
     In Table 2, Coding rate denotes a coding rate. 
     The superposition coding scheme, realized with the foregoing PSK modulation schemes, can be utilized to achieve rate adaptation with fine granularity. 
     A description will now be made of the general signal relay method. 
     It will be assumed that the source node S described with reference to  FIG. 1  transmits the signal given as
 
√{square root over (E 1 )}x 1 +√{square root over (E 2 )}x 2   (10)
 
     The signal received at the destination node D is given by
 
 r   D   =h   SD (√{square root over ( E   1 )} x   1 +√{square root over ( E   2 )} x   2 )+z D   (11)
 
     In Equation (11), r D  denotes a signal received at the destination node D, h SD  denotes a channel gain between the source node S and the destination node D, and z D  denotes a complex noise signal at the destination node D. 
     The signal received at the relay node R is given by
 
 r   R   =h   SR (√{square root over ( E   1 )} x   1 +√{square root over ( E   2 )} x   2 )+ z   R   (12)
 
     In Equation (12), r R  denotes a signal received at the relay node R, h SR  denotes a channel gain between the source node S and the relay node R, and z R  denotes a complex noise signal at the relay node R. 
     Herein, an SNR of the link between the source node S and the destination node D will be defined as γ SD , an SNR of the link between the source node S and the relay node R will be defined as γ SR , and an SNR of the link between the relay node R and the destination node D will be defined as γ RD . A data rate achievable for use of an AMC mode at the link between the source node S and the destination node D will be defined as R SD , a data rate achievable for use of an AMC mode at the link between the source node S and the relay node R will be defined as R SR , and a data rate achievable for use of an AMC mode at the link between the relay node R and the destination node D will be defined as R RD . The R SD , R SR , and R RD  are given by
 
 R   SD   =A (γ SD ) R   SR   =A (γ SR ) R   RD   =A (γ RD )  (13)
 
     A description will now be made of two transmission modes for transmitting signals from the source node S to the destination node D: a direct mode and a multi-hop mode. 
     In the direct mode, the relay node R is unused and the data rate is R SD . In the multi-hop mode, the signal is transmitted first from the source node S to the relay node R at the data rate R SR  and then transmitted from the relay node R to the destination node D at the data rate R RD . Herein, the time required to transmit N bits from the source node S to the relay node R at the data rate R SR  is 
             T   =       N     R   SR       .           
The time required to transmit N bits from the relay node R to the destination node D at the data rate R RD  is
 
                 R   SR       R   RD       ⁢     T   .           
Therefore, the data rate R mh  in the multi-hop mode is given by
 
     
       
         
           
             
               
                 
                   
                     R 
                     mh 
                   
                   = 
                   
                     
                       N 
                       
                         T 
                         + 
                         
                           
                             
                               R 
                               SR 
                             
                             
                               R 
                               RD 
                             
                           
                           ⁢ 
                           T 
                         
                       
                     
                     = 
                     
                       
                         
                           R 
                           SR 
                         
                         ⁢ 
                         
                           R 
                           RD 
                         
                       
                       
                         
                           R 
                           SR 
                         
                         + 
                         
                           R 
                           RD 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     Assume that the source node S is a base station (BS) and the BS is aware of all of the three rates R SD , R SR , and R RD . Then the BS can calculate the R mh  and compare it with R SD . If R mh &gt;R SD , then the BS selects the multi-hop mode to transmit the signal. Otherwise, if R mh ≦R SD , the BS selects the direct mode. Hence, the optimal data rate R conv  between the source node S and the destination node D is determined by Equation (15). Herein, the optimal data rate R conv  represents the optimal data rate for the case where both the direct mode and the multi-hop mode can be used for signal transmission between the source node S and the destination node D.
 
 R   conv =max { R   SD   ,R   mh }  (15)
 
     The present invention proposes a new transmission mode, i.e., a superposition mode, for transmitting a signal from the source node S to the destination node D. The superposition mode will now be described below. 
     Assume that γ SD  denotes an SNR for the case where a b th  AMC mode can be applied on the link between the source node S and the destination node D with the data rate R SD =R b &gt;0. The applicability of the b th  AMC mode on the link between the source node S and the destination node D implies that γ SD &gt;τ b . Then, Equation (2) can rewritten as 
     
       
         
           
             
               
                 
                   
                     γ 
                     SD 
                   
                   = 
                   
                     
                       
                         
                           
                              
                             
                               h 
                               SD 
                             
                              
                           
                           2 
                         
                         ⁢ 
                         
                           ( 
                           
                             
                               E 
                               1 
                             
                             + 
                             
                               E 
                               2 
                             
                           
                           ) 
                         
                       
                       
                         σ 
                         2 
                       
                     
                     = 
                     
                       
                         
                           
                              
                             
                               h 
                               SD 
                             
                              
                           
                           2 
                         
                         ⁢ 
                         
                           E 
                           total 
                         
                       
                       
                         σ 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     In Equation (16), the total transmission power E total  is fixed. In order to transmit the basic message at the data rate R b , Equation (4) should be used. Hence, it is sufficient to have 
     
       
         
           
             
               
                 
                   
                     τ 
                     b 
                   
                   = 
                   
                     
                       
                         
                            
                           
                             h 
                             SD 
                           
                            
                         
                         2 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 E 
                                 1 
                               
                             
                             - 
                             
                               
                                 E 
                                 2 
                               
                             
                           
                           ) 
                         
                         2 
                       
                     
                     
                       σ 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     For the fixed transmission power E total  and the channel gain |h SD |, the γ SD  and τ b  have fixed values and it can be found that 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   SD 
                                 
                                  
                               
                               2 
                             
                             
                               σ 
                               2 
                             
                           
                           ⁢ 
                           
                             E 
                             1 
                           
                         
                         = 
                         
                           
                             
                               ( 
                               
                                 
                                   
                                     
                                       2 
                                       ⁢ 
                                       
                                         γ 
                                         SD 
                                       
                                     
                                     - 
                                     
                                       τ 
                                       b 
                                     
                                   
                                 
                                 + 
                                 
                                   
                                     τ 
                                     b 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                           4 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             
                               
                                  
                                 
                                   h 
                                   SD 
                                 
                                  
                               
                               2 
                             
                             
                               σ 
                               2 
                             
                           
                           ⁢ 
                           
                             E 
                             2 
                           
                         
                         = 
                         
                           
                             
                               ( 
                               
                                 
                                   
                                     
                                       2 
                                       ⁢ 
                                       
                                         γ 
                                         SD 
                                       
                                     
                                     - 
                                     
                                       τ 
                                       b 
                                     
                                   
                                 
                                 + 
                                 
                                   
                                     τ 
                                     b 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                           4 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
     In Equation (18), for shorter notation, it is assumed that both γ SD  and τ b  are the real values, not expressed in [dB]. 
     Assume that the destination node D decodes only the basic message and does not even attempt to decode the superposed message. In this case, the received signal at the relay node R shown in Equation (12) will be taken into consideration. The natural assumption for the signal relay systems will be used, shown below.
 
| h   SR   |&gt;|h   SD |  (19)
 
     With the condition of Equation (19), the available SNR γ b,R  used for decoding the basic message transmitted from the source node S at the relay node R is given by 
     
       
         
           
             
               
                 
                   
                     γ 
                     
                       b 
                       , 
                       R 
                     
                   
                   = 
                   
                     
                       
                         
                           
                              
                             
                               h 
                               
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   S 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   R 
                                 
                               
                             
                              
                           
                           2 
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 
                                   E 
                                   1 
                                 
                               
                               - 
                               
                                 
                                   E 
                                   2 
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                       
                         σ 
                         2 
                       
                     
                     = 
                     
                       
                         
                           
                             
                                
                               
                                 h 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     S 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     R 
                                   
                                 
                               
                                
                             
                             2 
                           
                           
                             
                                
                               
                                 h 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     S 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     D 
                                   
                                 
                               
                                
                             
                             2 
                           
                         
                         ⁢ 
                         
                           τ 
                           b 
                         
                       
                       &gt; 
                       
                         τ 
                         b 
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
           
         
       
     
     The basic message is decoded at the relay node R with very high reliability, i.e., the available SNR is unnecessarily high for that AMC mode. Regarding the superposed message, after the basic message is decoded correctly, the available SNR γ s,R  necessary for decoding the superposed message is 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       γ 
                       
                         
                             
                         
                         ⁢ 
                         
                           s 
                           , 
                           R 
                         
                       
                     
                     ⁢ 
                     
                         
                     
                     = 
                     
                       
                         
                           
                               
                           
                           ⁢ 
                           
                             
                               
                                  
                                 
                                   h 
                                   
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       S 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       R 
                                     
                                   
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               E 
                               
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                         
                         
                           
                               
                           
                           ⁢ 
                           
                             σ 
                             2 
                           
                         
                       
                       = 
                       
                         
                           
                             
                                
                               
                                 h 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     S 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     R 
                                   
                                 
                               
                                
                             
                             2 
                           
                           
                             
                                
                               
                                 h 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     S 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     D 
                                   
                                 
                               
                                
                             
                             2 
                           
                         
                         ⁢ 
                         
                           
                             
                               ( 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       
                                         2 
                                         ⁢ 
                                         
                                           γ 
                                           
                                             S 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             D 
                                           
                                         
                                       
                                       - 
                                       
                                         τ 
                                         b 
                                       
                                     
                                   
                                 
                                 - 
                                 
                                   
                                     
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         τ 
                                         
                                           
                                               
                                           
                                           ⁢ 
                                           b 
                                         
                                       
                                     
                                   
                                   
                                       
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                           4 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   21 
                   ) 
                 
               
             
           
         
       
     
     In Equation (21), if γ s,R &gt;τ s , where s∈{1, 2, . . . M}, then the superposed message reliably carries information from the source node S to the relay node R at the data rate R s =A(γ s,R ). Hence, the total data rate carried from the source node S to the relay node R is R b +R s . 
     When the superposition mode is used to convey the signal from the source node S to the destination node D, after the signal transmission of the source node S, the relay node R retransmits the superposed message on the link between the relay node R and the destination node D at the data rate R RD . Hence, the optimal data rate R sup  between the source node S and the destination node D when the superposition mode is used can be calculated as follows. Herein, the term “optimal data rate R sup ” refers to the optimal data rate for the case where the transmission mode available for signal transmission between the source node S and the destination node D is the superposition mode. 
     Assume that during time T, the source node S transmits N b  bits of the basic message and N s  bits of the superposed message, such that 
               R   b     =       N   b     T           
and
 
               R   s     =         N   s     T     .           
The time required to retransmit N s  bits from the relay node R to the destination node D at the data rate R RD  is
 
                 N   s       R     R   ⁢           ⁢   D         ,         
so the optimal data rate R sup  can be written as
 
     
       
         
           
             
               
                 
                   
                     R 
                     sup 
                   
                   = 
                   
                     
                       
                         
                           N 
                           b 
                         
                         + 
                         
                           N 
                           s 
                         
                       
                       
                         T 
                         + 
                         
                           
                             N 
                             s 
                           
                           
                             R 
                             
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               D 
                             
                           
                         
                       
                     
                     = 
                     
                       
                         
                           R 
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             D 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               R 
                               b 
                             
                             + 
                             
                               R 
                               s 
                             
                           
                           ) 
                         
                       
                       
                         
                           R 
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             D 
                           
                         
                         + 
                         
                           R 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   22 
                   ) 
                 
               
             
           
         
       
     
     As a result, the source node S, i.e., the base station (BS), selects the transmission mode to maximize the throughput, i.e., to maximize the data rate, on the links between the source node S and the destination node D, between the source node S and the relay node R, and between the relay node R and the destination node D, for a given SNR. Hence, the optimal data rate R proposed  between the source node S and the destination node D is given by Equation (23) below. Herein, the term “optimal data rate R proposed ” refers to the optimal data rate for the case where the transmission mode available for signal transmission between the source node S and the destination node D is the direct mode, the multi-hop mode and the superposition mode.
 
 R   proposed =max{ R   SD   , R   mh   , R   sup }  (23)
 
     If the total energy E total  is fixed and the superposition mode is used, then the energy of the basic message and the superposed message is determined by 
     
       
         
           
               
             
               
                 
                   
                     
                       
                         
                           
                               
                           
                           ⁢ 
                           
                             
                                
                               
                                 h 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     S 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     D 
                                   
                                 
                               
                                
                             
                             2 
                           
                           ⁢ 
                           
                               
                           
                         
                         
                           
                               
                           
                           ⁢ 
                           
                             σ 
                             2 
                           
                         
                       
                       = 
                       
                         
                           γ 
                           
                             S 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             D 
                           
                         
                         
                           E 
                           total 
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         E 
                         1 
                       
                       = 
                       
                         
                           
                             E 
                             total 
                           
                           
                             γ 
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               D 
                             
                           
                         
                         ⁢ 
                         
                           
                             
                               ( 
                               
                                 
                                   
                                     
                                       2 
                                       ⁢ 
                                       
                                         γ 
                                         
                                           S 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           D 
                                         
                                       
                                     
                                     - 
                                     
                                       τ 
                                       b 
                                     
                                   
                                 
                                 + 
                                 
                                   
                                     τ 
                                     b 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                           4 
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         E 
                         2 
                       
                       = 
                       
                         
                           
                             E 
                             total 
                           
                           
                             γ 
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               D 
                             
                           
                         
                         ⁢ 
                         
                           
                             
                               ( 
                               
                                 
                                   
                                     
                                       2 
                                       ⁢ 
                                       
                                         γ 
                                         
                                           S 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           D 
                                         
                                       
                                     
                                     - 
                                     
                                       τ 
                                       b 
                                     
                                   
                                 
                                 - 
                                 
                                   
                                     τ 
                                     b 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                           4 
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     24 
                     ) 
                   
                 
               
             
           
         
       
     
     With reference to  FIGS. 4A and 4B , a description will now be made of a signal relay process in a communication system according to an embodiment of the present invention. 
       FIGS. 4A and 4B  are flowcharts illustrating a signal relay process performed by a source node S in a communication system according to an embodiment of the present invention. 
     Referring to  FIGS. 4A and 4B , in step  411 , the source node S calculates data rates of an AMC mode, which should be used to maximize a data rate on each of the links between the source node S and a destination node D, between the source node S and a relay node R, and between the relay node R and the destination node D. Herein, assuming that an SNR between the source node S and the destination node D is denoted by γ SD , an SNR between the source node S and the relay node R is denoted by γ SR , and an SNR between the relay node R and the destination node D is denoted by γ RD , the data rates of the AMC mode, which should be used to maximize a data rate on each of the links between the source node S and the destination node D, between the source node S and the relay node R, and between the relay node R and the destination node D are given by Equation (13). 
     In step  413 , the source node S calculates a data rate R mh  for the case where it uses a multi-hop mode for signal transmission between the source node S and the destination node D. The data rate R mh  is calculated using Equation (14). In step  415 , the source node S calculates an optimal data rate R conv  for the case where transmission modes available for signal transmission between the source node S and the destination node D include the direct mode and the multi-hop mode. The optimal data rate R conv  is calculated using Equation (15). 
     In step  417 , the source node S selects an AMC mode to be used for the link between the source node S and the destination node D, and determines a data rate R SD  for the link between the source node S and the destination node D when the selected AMC mode is used. If it is assumed that the source node S selects a b th  AMC mode as the AMC mode to be used for the link between the source node S and the destination node D, the data rate R SD  is R b  (R SD =R b ). In step  419 , the source node S calculates an available SNR γ s,R  necessary for decoding a superposed message, after a basic message is correctly decoded. The available SNR γ s,R  is calculated using Equation (21). 
     In step  421 , the source node S determines a data rate R s =A(γ s,R ) at which the superposed message is transmitted with reliability. In step  423 , the source node S calculates an optimal data rate R sup  for the case where a transmission mode available for signal transmission between the source node S and the destination node D is the superposition mode. The optimal data rate R sup  is calculated using Equation (22). In step  425 , the source node S determines whether the optimal data rate R sup  exceeds the optimal data rate R conv . If the optimal data rate R sup  exceeds the optimal data rate R conv , the source node S proceeds to step  427  where it determines to use the superposition mode for the signal transmission from the source node S to the destination node D, and then ends the signal relay process. The energy of the basic message and the superposed message for the case where the superposition mode is used for the signal transmission from the source node S to the destination node D is determined using Equation (24). 
     However, if it is determined in step  425  that the optimal data rate R sup  does not exceed the optimal data rate R conv , the source node S proceeds to step  429  where it sets energy of the basic message to 
               E   total       γ     S   ⁢           ⁢   D             
and sets energy of the superposed message to ‘0’. In step  431 , the source node S determines whether the data rate R mh  exceeds the data rate R SD . If the data rate R mh  exceeds the data rate R SD , the source node S proceeds to step  433  where it determines to use the multi-hop mode for the signal transmission from the source node S to the destination node D, and then ends the signal relay process. However, if it is determined in step  431  that the data rate R mh  does not exceed the data rate R SD , the source node S proceeds to step  435  where it determines to use the direct mode for the signal transmission from the source node S to the destination node D, and then ends the signal relay process.
 
     As can be understood from the foregoing description, the present invention determines the transmission mode used for signal transmission from the source node to the destination node in the form of increasing the throughput, i.e., in the form of increasing the data rate in the communication system, thereby contributing to an increase in the total efficiency of the communication system. 
     While the invention has been shown and described with reference to a certain preferred embodiment 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 invention as defined by the appended claims.