Abstract:
A method for selecting a relay mode depending on channel status of relay links in a multihop relay broadband wireless communication system and a Relay Station (RS) apparatus for supporting the method. When signals are received from a Base Station (BS) and a Mobile Station (MS), channel status values (e.g., eigenvalue, mutual information, and probability error) of relay links (BS-RS link and RS-MS link) are estimated using the received signals. The estimated channel status values are compared with a preset reference value. According to a result of the comparison, the relay mode for relaying the received signals is selected. Accordingly, the reliability of the relayed signal can be enhanced and the capacity of the signal link can be increased.

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on Jan. 24, 2006 and assigned Serial No. 2006-7069, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a multihop relay broadband wireless communication system, and in particular, to a method for relaying a signal by selecting a relay mode depending on a channel status of links between a Base Station, a Relay Station, and a Mobile Station in a multihop relay broadband wireless communication system, and a Relay Station apparatus for supporting the method. 
     2. Description of the Related Art 
     In fourth-generation (4G) mobile communication systems, cells having a very small radius are located to enable rapid communications and accommodate more traffic. However, it may be impossible to achieve a centralized design using current wireless network design schemes. Wireless networks should be controlled and deployed in a distributed manner, and actively adapt to environment changes, such as a joining of a new Base Station. To these ends, 4G mobile communication systems should be configured as autonomous adaptive wireless networks. 
     Techniques applied to an ad-hoc network are typically adopted by a mobile communication system for substantial implementation of an autonomous adaptive wireless network by a 4G mobile communication system. A representative example is a multihop relay broadband wireless communication system, in which a multihop relay scheme applied to an ad-hoc network is introduced to a broadband wireless communication system configured with a fixed Base Station. In the broadband wireless communication system, since communications are conducted through one direct link between a Base Station and a Mobile Station, it is easy to establish a highly reliable radio communication link between the Base Station and the Mobile Station. 
     However, since the wireless network configuration of the broadband wireless communication system has low flexibility because of the fixed Base Station, it is hard to provide efficient services in a radio environment, which is subject to severe change in traffic distribution or traffic. To overcome this shortcoming, it is possible to apply a relay scheme which delivers data in a multihop manner by use of neighboring Mobile Stations or Relay Stations. A multihop relay scheme can rapidly reconfigure the network under the environment change. Also, a multihop relay scheme can provide a Mobile Station with a radio channel of better channel status by building a multihop relay path by way of a repeater which is placed between the Base Station and the Mobile Station. Furthermore, a high speed data channel can be provided to Mobile Stations which cannot communicate with the Base Station in a shadow area, by means of the multihop relay path, to thereby expand the cell area. 
       FIG. 1  depicts a multilink configuration of a general multihop relay broadband wireless communication system. 
     As shown in  FIG. 1 , a Mobile Station (MS)  110  in a coverage  101  of a Base Station (BS)  100  is connected to BS  100  through a direct link. In contrast, a MS  120  with poor channel status, which resides outside the coverage  101  of BS  100 , is connected to a relay link via a Relay Station (RS)  130 . 
     When MSs  110  and  120  suffer poor channel status because they are outside the coverage  101  of BS  100  or in a shadow area under the severe shielding by buildings, BS  100  is able to provide better radio channels to MSs  110  and  120  by means of RS  130 . Accordingly, by adopting the multihop relay scheme, BS  100  can provide a high speed data channel in the boundary area of the poor channel status and expand the cell service area. To transmit uplink (UL) and downlink (DL) communications between the BS  100 , the RS  130 , and the second MS  120 , a BS-RS link between the BS  100  and the RS  130 , an RS-MS link between the RS  130  and the second MS  120 , and a BS-MS link between the BS  100  and the first MS  110  are established. Each link is divided to the UL or the DL according to the data transmission path. The respective links (BS-RS link, RS-MS link, and BS-MS link) are established independently from one another. 
     To relay signals between the BS and the MS, the RS adopts an Amplify and Forward (AF) scheme or a Decode and Forward (DF) relay scheme. The AF scheme and the DF scheme work using different Open System Interconnection (OSI) layers, as shown in  FIG. 2 . 
       FIG. 2  depicts the OSI layer structure of a general RS for performing relay modes. The OSI layers employ the OSI layers of Institute of Electrical and Electronics Engineers (IEEE) 802.11a. 
     As shown in  FIG. 2 , being processed in a Physical Medium Dependent (PMD) sublayer and a Packet Level Control Protocol (PLCP) sublayer of a physical layer, the AF scheme merely amplifies and forwards the received signal. 
     Being processed in the Medium Access Control (MAC) layer as well as the physical layer, the DF scheme decodes and forwards the received signal after encoding and modulating the received frame. Advantageously, the DF scheme can obtain an additional coding gain by encoding the received signal differently from the coding scheme of the received signal depending on the channel status of the BS-RS link and the RS-MS link. 
     As discussed above, in the multihop relay broadband wireless communication system, the respective links (BS-RS link, RS-MS link, and BS-MS link) are established independently from one another. The RS relays the signal using a preset relay scheme (e.g., AF scheme or DF scheme). 
     Using an AF relay scheme, when the channel status of the BS-RS link and the RS-MS link are good, the RS can relay the signal. However, under a poor channel status of the RS-MS link, the RS merely amplifies the signal of the same modulation and coding as in the BS-RS link of the good channel status, and forwards it. As a result, the MS is not able to detect the relayed signal. 
     In the event of a poor channel status of the BS-RS link, the RS merely amplifies and forwards the signal distorted in the BS-RS link to the MS, to thus amplify the noise as well. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an aspect of the present invention is to provide a method for relaying a signal by selecting a relay mode depending on channel status between a BS, an RS, and an MS in a multihop relay broadband wireless communication system, and an RS apparatus for supporting the method. 
     Another aspect of the present invention is to provide a method for relaying a signal by selecting a relay mode depending on eigenvalue or mutual information between a BS, an RS, and an MS in a multihop relay broadband wireless communication system, and an RS apparatus for supporting the method. 
     A further aspect of the present invention is to provide a method for relaying a signal by selecting a relay mode depending on an eigenvalue or mutual information between a BS, an RS, and an MS with respect to each antenna in a multihop relay broadband wireless communication system, and an RS apparatus for supporting the method. 
     The above aspects are achieved by providing a method for selecting a relay mode at an RS in a multihop relay broadband wireless communication system, which includes estimating channel status values of relay links using signals received from a BS and an MS, and selecting a relay mode for relaying the received signals by comparing the estimated channel status values with a reference value. 
     According to one aspect of the present invention, a method for selecting a relay mode in an RS of a multihop relay Multiple Input Multiple Output (MIMO) system, includes estimating channel status values of relay links for respective antennas using signals received from a BS and an MS, and selecting a relay mode for relaying the received signals for the respective antennas by comparing the estimated channel status values with a reference value. 
     According to another aspect of the present invention, an RS apparatus for selecting a relay mode in a multihop relay broadband wireless communication system, includes a receiver which receives signals from a BS and an MS; a channel estimator which estimates channel status values of relay links using the received signals; a relay mode selector which selects the relay mode by comparing the estimated channel status values of the relay links with a reference value; and a transmitter which relays the received signals depending on the selected relay mode. 
     According to a further aspect of the present invention, an RS apparatus for selecting a relay mode in a multihop relay wireless communication system, includes a receiver which receives signals from a BS and an MS using at least two antennas; a channel estimator which estimates channel status values of relay links for the respective antennas using the received signals; a relay mode selector which selects a relay mode for the respective antennas by comparing the estimated channel status values of the relay links with a reference value; and a transmitter which relays the received signals via the at least two antennas depending on the selected relay mode. 
    
    
     
       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  depicts a multilink configuration of a general multihop relay broadband wireless communication system; 
         FIG. 2  depicts an OSI layer structure for performing relay modes of a general RS; 
         FIG. 3  depicts signal link flows for selecting a relay mode at an RS according to the present invention; 
         FIG. 4  is a block diagram of an RS transceiver apparatus for selecting a relay mode of a relay link in a multihop relay Multiple Input Multiple Output (MIMO) system according to the present invention; 
         FIG. 5  is a block diagram of a channel information calculator in the multihop relay MIMO system according to the present invention; 
         FIG. 6  is a flowchart outlining a relay mode selecting procedure at the RS according to the present invention; and 
         FIG. 7  depicts a standard for selecting the relay mode at the RS according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
     Descriptions provide a technique for selecting a relay mode depending on channel status of relay links in a multihop relay broadband wireless communication system. The relay mode includes Amplify and Forward (AF), Decode and Forward (DF), Selection Decode and Forward (SDF), and Incremental Amplify and Forward (IAF). The AF scheme and the DF scheme are for understanding. The relay links refer to a link between a BS and an RS (BS-RS link) and a link between the RS and an MS (RS-MS link). 
     The following explanation relates to a Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system. The present invention is applicable to other multiple access schemes. 
       FIG. 3  depicts signal link flows for selecting a relay mode at an RS according to the present invention. 
     Referring to  FIG. 3 , a BS  301  sends a downlink (DL) signal to an MS  303  and an RS  305  in steps  311  and  312 . The MS  303  sends an uplink (UL) signal to the BS  301  and the RS  305  in steps  313  and  316 . 
     Upon receiving the DL signal from the BS  301 , the RS  305  relays the DL signal to the MS  303  in step  314 . Upon receiving the UL signal from the MS  303 , the RS  305  relays the UL signal to the BS  301  in step  315 . At this time, the RS  305  checks the channel status of the relay links (BS-RS link and RS-MS link) using the signals received from the BS  301  and the MS  303 , selects a relay mode (AF scheme or DF scheme) based on the channel status of each link, and then relays the signals. Since the RS  305  has the multiple antennas, the channel status of each link is determined according to an eigenvalue between antennas, mutual information based on the eigenvalue, or probability error. 
       FIG. 4  shows an RS transceiver apparatus for selecting a relay mode of a relay link in a multihop relay MIMO system according to the present invention. 
     As shown in  FIG. 4 , the RS includes a transmitter and a receiver. 
     The receiver includes a Radio Frequency (RF) processor  401 , an Analog/Digital Converter (ADC)  403 , a transmission mode processor  405 , a header extractor  407 , a symbol duplicator  409 , switches  411 ,  413 , and  421 , a Cyclic Prefix (CP) eliminator  415 , a Fast Fourier Transform (TTF) operator  417 , a decoder  419 , a channel information calculator  423 , and a relay mode selector  425 . 
     The RF processor  401  down-converts an RF signal, which is received at the MIMO antenna from the BS or the MS, to a baseband signal and then outputs the baseband signal. The ADC  403  digitizes the baseband analog signal fed from the RF processor  401 . 
     The transmission mode processor  405  reprocesses the digital signal fed from the ADC  403  based on a transmission mode of the signal sent from the BS or the MS and outputs the reprocessed signal to the header extractor  407  and the symbol duplicator  409 . The transmission mode includes diversity, multiplexing, and beam forming. 
     The header extractor  407  extracts a header from the signal provided from the transmission mode processor  405 . The receiver can reduce a channel prediction time by predicting a channel using a pilot signal contained in the header extracted by the head extractor  407 . 
     The symbol duplicator  409  duplicates the entire symbol of the signal provided from the transmission mode processor  405 . 
     The first switch  411  selectively forwards the signals of the header extractor  407  and the symbol duplicator  409  to the next stage under the control of the relay mode selector  425 . Specifically, to select a relay mode of the received signal, the first switch  411  channels the output signal of the header extractor  407  to the CP eliminator  415  under the control of the relay mode selector  425 . Next, when the relay mode of the received signal is determined, the first switch  411  forwards the entire receive symbol duplicated at the symbol duplicator  409  to the second switch  413  under the control of the relay mode selector  425 . 
     The second switch  413  sends the signal from the symbol duplicator  409  to the CP eliminator  415  or a power amplifier  427  according to the relay mode determined at the relay mode selector  425 . Specifically, when the determined relay mode is the AF relay mode, the second switch  413  sends the signal from the symbol duplicator  409  to the power amplifier  427 . When the determined relay mode is the DF relay mode, the second switch  413  sends the signal from the symbol duplicator  409  to the CP eliminator  415 . 
     The CP eliminator  415  removes a CP from the output signal of the header extractor  407 , which is fed from the switch  411 , or from the output signal of the symbol duplicator  409 , which is fed from the second switch  413 . 
     The FFT operator  417  transforms the time domain signal fed from the CP eliminator  415  to a frequency domain signal through the FFT. 
     The decoder  419  demodulates and decodes the frequency domain signal fed from the FFT operator  417  according to the corresponding modulation level (Modulation and Coding Scheme (MCS) level). 
     The third switch  421  sends the decoded signal from the decoder  419  to either the channel information calculator  423  or the transmitter under the control of the relay mode selector  425 . Under the control of the relay mode selector  425 , the third switch  421  sends the header information decoded at the decoder  419  to the channel information calculator  423  to determine the relay mode. When the DF relay mode is selected, the third switch  421  sends the signal decoded at the decoder  419  to the transmitter under the control of the relay mode selector  425 . 
     The channel information calculator  423  estimates channel information using the decoded header information fed from the third switch  421 . Next, the channel information calculator  423  calculates eigenvalues for the respective antennas of the relay links (BS-RS link and RS-MS link) using the estimated channel information. Using the estimated channel information, the channel information calculator  423  issues control signals to control the transmission mode processor  405  and the power amplifier  427  of the transmitter. 
     The relay mode selector  425  selects the relay mode of the received signal by comparing the link eigenvalues fed from the channel information calculator  423  with a predefined reference value. For instance, the relay mode selector  425  selects the relay mode using the link eigenvalues as shown in  FIG. 7 . 
       FIG. 7  depicts a standard for selecting the relay mode at the RS according to the present invention. 
     As shown in  FIG. 7 , the relay mode selector  425  selects the AF scheme  703  as the relay mode when the calculated link eigenvalues (λ BS-RS  and λ MS-RS ) are greater than or equal to the reference value. 
     When at least one of the link eigenvalues is less than the reference value, the relay mode selector  425  selects the DF scheme  701  as the relay mode. 
     The relay mode selector  425  issues control information for controlling the switches  411 ,  413 ,  421 ,  429 , and  431  of the transmitter and the receiver. Specifically, the relay mode selector  425  issues control signals to control the first switch  411  and the third switch  421  to select the relay mode of the received signal. Next, when the relay mode is determined, the relay mode selector  425  issues control signals to control the first switch  411  and the second switch  413  according to the determined relay mode. Also, to map the signals to be relayed to the respective antennas depending on the AF scheme or the DF scheme at the transmitter, the relay mode selector  425  issues control signals to control the fourth switch  429  and the fifth switch  431 . 
     Now, the transmitter includes the power amplifier  427 , the switches  429  and  431 , a Digital/Analog Converter (DAC)  433 , an RF processor  435 , an encoder  437 , an Inverse Fast Fourier Transform (IFFT) operator  439 , a CP inserter  441 , and a transmission mode processor  443 . 
     When the relay mode is determined to the AF scheme, the power amplifier  427  amplifies the received signals for the respective antennas, which are fed from the second switch  413 , under control of the channel information calculator  423  and then outputs the amplified signals. 
     The fourth switch  429  maps the signals amplified at the power amplifier  427  to the respective antenna paths under control of the relay mode selector  425 . 
     When the relay mode is determined to the DF scheme, the encoder  437  encodes and modulates the encoded receive signals for the respective antennas, which are fed from the third switch  421 , according to the corresponding modulation level (MCS level), and then outputs the encoded and modulated signals. 
     The IFFT operator  439  transforms the frequency domain signal fed from the encoder  437  to a time domain signal through the IFFT, and then outputs the time domain signal. 
     The CP inserter  441  inserts a CP to the data fed from the IFFT operator  439  to eliminate intersymbol interference which occurs due to the multipath fading of the radio channel, and outputs the CP-inserted data. 
     The transmission mode processor  443  converts and outputs the signal fed from the CP inserter  441  according to the corresponding transmission mode. The transmission mode includes diversity, multiplexing, and beam forming. 
     The fifth switch  431  maps the signal from the transmission mode processor  443  to the corresponding antenna path under control of the relay mode selector  425 . 
     The DAC  433  receives the digital signals for the respective antennas from the fourth switch  429  or the fifth switch  431  and converts the digital signals to analog signals. 
     The RF processor  435  up-converts the baseband signals for the respective antennas, which are fed from the DAC  433 , to RF signals and transmits the RF signals to the MS or the BS via the MIMO antenna. 
     According to the present invention, the RS selects the relay mode by calculating the eigenvalue. Alternatively, the channel information calculator  423  can include modules which calculate mutual information and probability error as well as the eigenvalue, and the RS can select the relay mode using the mutual information or the probability error as shown in  FIG. 5 . 
     Referring to  FIG. 5 , the channel information calculator  423  includes an eigenvalue calculator  501 , a mutual information calculator  503 , and a probability error calculator  505 . 
     The eigenvalue calculator  501  calculates the eigenvalue from the estimated channel information using the decoded header information. 
     The mutual information calculator  503  calculates the mutual information using the acquired eigenvalue. The mutual information is expressed as Equation (1).
 
 I   M =log 2   det ( I   M     R     +E/M   T   N   0   HH   H )bps/Hz,  M   T   ,M   R =1,2,3  (1)
 
     In Equation (1), I M  denotes the mutual information, I M     R    denotes a unit matrix, E/M T N 0  denotes a Signal to Noise Ratio (SNR) of the received signal, and M T  denotes the number of Tx antennas. 
     When the BS is aware of the channel status, HH H  in Equation (1) can be expressed as the eigenvalue in Equation (2). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     In Equation (2), I M  denotes the mutual information, E/M T N 0  denotes the SNR of the received signal, and M T  denotes the number of Tx antennas. |λ m | denotes an eigenvalue of the m-th largest HH H  and n denotes a rank with respect to H. 
     The mutual information can be acquired using the eigenvalue based on Equation (2). 
     The probability error calculator  505  calculates a probability error value from the decoded header which is fed from the third switch  421 . 
       FIG. 6  shows a relay mode selecting procedure at the RS according to the present invention. The following description explains that the relay mode is selected using the eigenvalue of the relay link by way of example. 
     Referring to  FIG. 6 , the RS checks whether signals are received from the BS and the MS in step  601 . 
     Upon receiving the signals from the BS and the MS, the RS extracts headers from the received signals in step  603 . In step  605 , the RS calculates eigenvalues of the relay links (BS-RS link and RS-MS link) using pilot signals contained in the extracted headers of the received signals. 
     In step  607 , the RS compares the acquired relay link eigenvalues with a preset reference value. 
     When each eigenvalue is greater than or equal to the reference value (λ BS-RS , λ MS-RS ≧reference value), the RS selects the AF mode as the relay mode in step  609 . For instance, when both eigenvalues of the BS-RS link and the MS-RS link are greater than or equal to the preset reference value, the RS selects the AF mode  703  as the relay mode, as shown in  FIG. 7 . 
     Upon selecting the AF mode as the relay mode, in step  611 , the RS amplifies the power of the signal received from the BS or the MS according to the channel status value of each link which is estimated in step  605 . 
     Next, in step  613 , the RS transmits the power-amplified signal to the BS or the MS. That is, the RS amplifies the power of the signal received from the BS and then sends the amplified signal to the MS. Also, the RS amplifies the power of the signal received from the MS and sends the amplified signal to the BS. Next, the RS terminates the process. 
     By contrast, when each link eigenvalue is less than the reference value (λ BS-RS , λ MS-RS &lt;reference value), the RS selects the DF mode as the relay mode in step  615 . For instance, when one of the eigenvalues of the BS-RS link and the MS-RS link is less than the preset reference value, the RS selects the DF mode  701  as the relay mode, as show in  FIG. 7 . 
     After selecting the DF mode as the relay mode, the RS demodulates and decodes the signals received in the respective links according to the corresponding modulation level (MCS level) in step  617 . 
     After decoding the received signals, in step  619 , the RS encodes the signals according to the channel status of the link in which the decoded signal is to be relayed. That is, the signal received from the BS is encoded and modulated according to the modulation level which is determined based on the channel status of the RS-MS link. The signal received from the MS is encoded and modulated according to the modulation level which is determined based on the channel status of the BS-RS link. 
     In step  613 , the RS forwards the signal from the BS to the MS. Also, the RS forwards the signal from the MS to the BS. Next, the RS terminates the process. 
     It has been shown that the relay mode is selected by calculating the eigenvalue by way of example. Additionally, the relay mode can be selected using the mutual information or the probability error, which is acquired from the eigenvalue. 
     While the signals are relayed by selecting the relay mode for the respective antennas at the RS having the MIMO antenna, the RS having the single antenna can relay the signal by selecting the relay mode based on the channel status in the similar manner. 
     In a multihop relay broadband wireless communication system, an RS relays signals by selecting a relay mode based on channel status (e.g., eigenvalue, mutual information and probability error) of a BS-RS link and an RS-MS link. Thus, reliability of the relayed signal can be enhanced and capacity of the signal link can be increased. Furthermore, since the channel is estimated using merely the header of the received signal, it is possible to reduce a time taken to process the channel estimation. 
     While the invention has been shown and described with reference to certain preferred embodiments 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.