Abstract:
Provided is a method for retransmitting data based on antenna scheduling in a MIMO system to which a spatial multiplexing technique is applied. The method includes the steps of: (a) at a transmitter side, modulating an input packet into transmittable data to transmit to a receiver side; (b) at the receiver side, estimating channel values from the packet transmitted from the transmitter side, and selecting transmitting and receiving antennas for transmitting the next packet from the estimated channel values; (c) detecting whether an error is present in the packet or not, and transmitting a feedback signal to the transmitter side, the feedback signal including information on whether or not to transmit the packet and a list of the selected transmitting antennas; and (d) at the transmitter side, retransmitting the transmitted packet or transmitting the next packet through the transmitting antennas designated by the receiver side depending on the feedback signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application 10-2007-0034314 filed with the Korea Intellectual Property Office on Apr. 6, 2007, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for retransmitting data based on antenna scheduling in a MIMO (Multiple Input Multiple Output) system. 
     2. Description of the Related Art 
     Recently, as the mobile communication technology rapidly develops, a technique for increasing transfer speed of data to provide a service with a more enhanced quality is considered to be important. IMT-2000, which is the third-generation wireless communication, requires transfer speed of more than 10 Mbps during stop mode and more than 384 Kbps during moving mode. Further, the post-fourth-generation wireless communication requires transfer speed of more than 155 Mbps during stop mode and more than 2 Mbps during moving mode. 
     To satisfy such requirements, a MIMO system has been proposed which uses a plurality of antennas provided in both transmitter and receiver sides. The MIMO technique is divided into a spatial multiplexing scheme and a spatial diversity scheme. In the spatial multiplexing scheme, a transmitter and a receiver respectively have a plurality of antennas provided therein so as to simultaneously transmit data different from each other. Without increasing the bandwidth of the system, the data can be transmitted at high speed. In the spatial diversity scheme, identical data is transmitted through a plurality of transmitting antennas so as to obtain a transmission diversity gain. 
     The MIMO system can significantly enhance a communication capacity and transmission/reception performance and can provide a high transmission rate, without additional frequency allocation or increase in power. Researches on the MIMO system are actively carried out. The MIMO system is considered as a core technology of the next generation communication. However, the MIMO system is vulnerable to ISI (InterSymbol interference) generated during high-speed transfer of signals and frequency selective fading of frequency caused by multiple paths. Therefore, to overcome such a disadvantage, an OFDM (Orthogonal Frequency Division Multiplexing) scheme is used together in the MIMO system. 
     The OFDM scheme attracts attention as a method which can satisfy high speed, a high quality, and large-volume communication required by the fourth-generation communication system. Since an OFDM signal has a plurality of sub-carriers in a frequency region and data is transmitted in parallel, the overall transfer speed is maintained as it is, and the transfer speed per sub-carrier can be reduced. Further, when a high-speed data stream is transmitted using a low-speed parallel carrier wave, a symbol interval is increased, so that the ISI is reduced. In particular, as a guard interval (GI) is used, the ISI can be almost perfectly removed. Further, as a plurality of carrier waves are used in the OFDM signal, the OFDM signal is not affected by the frequency selective fading. As two of the systems are combined, the advantage of the MIMO system is used as it is, and the disadvantage of the MIMO system can be removed by the OFDM system. This system is referred to as a MIMO-OFDM system. 
     Meanwhile, as an error control means for overcoming a packet transmission error which frequently occurs in a poor wireless channel when data is transmitted and received through the MIMO system, an ARQ (Automatic Repeat reQuest) protocol has been proposed. Recently, as a method for maximizing the efficiency, a hybrid ARQ (HARQ) protocol is being adopted as the standard protocol. In the HARQ protocol in which forward error correction (FEC) and the ARQ protocol are combined, error correction is performed by a receiver side through the FEC. If the error correction is failed, retransmission is carried out. 
     The HARQ protocol can be roughly divided into two schemes. The first scheme is referred to as a chase combining scheme. In the chase combining scheme, when an error is present in a received packet, a request for retransmission of the packet is delivered to a transmitter side. Then, the retransmitted packet and the packet having the error are combined so as to judge whether an error is present in the packet. The second scheme is referred to as an incremental redundancy (IR) scheme. In the IR scheme, a packet in which an error occurred is not retransmitted, but only additional redundancy bits are retransmitted so as to be combined with the packet where an error occurred. In this case, different redundancy bits are retransmitted at every transmission. 
     As the HARQ scheme is applied to the MIMO-OFDM system, an error of a packet can be effectively corrected in comparison with an existing MIMO-OFDM system, thereby enhancing the reliability of the system. In the HARQ scheme of the conventional MIMO-OFDM system, however, the packet where the error occurred is retransmitted through the same antenna. Therefore, when an error occurs in a packet transmitted through a specific antenna, an error may again occur in the packet retransmitted through the same antenna, thereby reducing the yield rate of the entire system. Further, when the retransmission is continuously requested due to some poor communication links, the entire system may be paralyzed in some cases. 
     SUMMARY OF THE INVENTION 
     An advantage of the present invention is that it provides a method and apparatus for retransmitting data based on antenna scheduling in a MIMO (Multiple Input Multiple Output) system, which retransmits a packet through antennas with a high channel gain by scheduling the antennas on the basis of a transmitting and receiving antenna selection scheme in which a favorable communication link is adaptively selected depending on the channel state of the MIMO system, thereby enhancing the reliability of the system. 
     Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     According to an aspect of the invention, a method for retransmitting data based on antenna scheduling in a MIMO system, to which a spatial multiplexing technique is applied, comprises the steps of: (a) at a transmitter side, modulating an input packet into transmittable data to transmit to a receiver side; (b) at the receiver side, estimating channel values from the packet transmitted from the transmitter side, and selecting transmitting and receiving antennas for transmitting the next packet from the estimated channel values; (c) detecting whether an error is present in the packet or not, and transmitting a feedback signal to the transmitter side, the feedback signal including information on whether or not to transmit the packet and a list of the selected transmitting antennas; and (d) at the transmitter side, retransmitting the transmitted packet or transmitting the next packet through the transmitting antennas designated by the receiver side depending on the feedback signal. 
     According to another aspect of the invention, an apparatus for retransmitting data based on antenna scheduling in a MIMO system, to which a spatial multiplexing technique is applied, comprises a transmitter side including: a packet modulating unit that converts an input packet into transmittable data; a scheduler that determines whether or not to retransmit the packet and an encoding rate of the packet from a feedback signal transmitted from a receiver side, and delivers a transmitting antenna list, received from the receiver side, to a transmitting antenna designating unit; and the transmitting antenna designating unit that designates transmitting antennas for transmitting the packet, depending on the transmitting antenna list; and the receiver side including: a channel estimator that estimates channel values from the packet transmitted from the transmitter side; an antenna selecting unit that selects transmitting and receiving antennas for transmitting the next packet from the estimated channel values; a packet demodulating unit that detects whether an error occurs in the packet or not; and a feedback transmission unit that transmits a feedback signal to the transmitter side, the feedback signal including information on whether or not to retransmit the packet and the transmitting antenna list. 
     According to a further aspect of the invention, a method for retransmitting data based on antenna scheduling in a MIMO system, to which a spatial diversity technique is applied, comprises the steps of: (a) at a transmitter side, modulating an input packet into transmittable data to transmit to a receiver side; (b) at the receiver side, estimating channel values from the packet transmitted from the transmitter side, and selecting transmitting and receiving antennas for transmitting the next packet from the estimated channel values; (c) detecting whether an error is present in the packet or not, and transmitting a feedback signal to the transmitter side, the feedback signal including information on whether or not to retransmit the packet, a list of the selected transmitting antennas, and channel-state values; and (d) at the transmitter side, retransmitting the transmitted packet or transmitting the next packet through the transmitting antennas designated by the receiver side depending on the feedback signal. 
     According to a still further aspect of the invention, an apparatus for retransmitting data based on antenna scheduling in a MIMO system, to which a spatial diversity technique is applied, comprises a transmitter side including: a packet modulating unit that converts an input packet into transmittable data; a scheduler that determines whether or not to retransmit the packet and an encoding rate of the packet from a feedback signal transmitted from a receiver side, and delivers a transmitting antenna list, received from the receiver side, to a transmitting antenna designating unit; and the transmitting antenna designating unit that designates transmitting antennas for transmitting the packet depending on the transmitting antenna list; and the receiver side including: a channel estimator that estimates channel values from the packet transmitted from the transmitter side; an antenna selecting unit that selects transmitting and receiving antennas for transmitting the next packet from the estimated channel values; a packet demodulating unit that detects whether an error occurs in the packet or not; and a feedback transmission unit that transmits a feedback signal to the transmitter side, the feedback signal including information on whether or not to retransmit the packet, the transmitting antenna list, and channel state values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a diagram showing the transmitter-side configuration of a MIMO system, to which a spatial multiplexing technique is applied, according to an embodiment of the invention; 
         FIG. 2  is a diagram showing the receiver-side configuration of the MIMO system, to which the spatial multiplexing technique is applied, according to an embodiment of the invention; 
         FIG. 3  is a flow chart showing an operation in the receiver side of the MIMO system, to which the spatial multiplexing technique is applied, according to an embodiment of the invention; 
         FIG. 4  is a flow chart showing an operation in the transmitter side of the MIMO system, to which the spatial multiplexing technique is applied, according to an embodiment of the invention; 
         FIG. 5  is a diagram showing the transmitter-side configuration of a MIMO system, to which the spatial diversity scheme is applied, according to another embodiment of the invention; 
         FIG. 6  is a diagram showing the receiver-side configuration of the MIMO system, to which the spatial diversity scheme is applied, according to another embodiment of the invention; 
         FIG. 7  is a flow chart showing an operation in the receiver side of the MIMO system, to which the spatial diversity technique is applied, according to another embodiment; and 
         FIG. 8  is a flow chart showing an operation in the transmitter side of the MIMO system, to which the spatial diversity technique is applied, according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a diagram showing the transmitter-side configuration of a MIMO system, to which a spatial multiplexing technique is applied, according to an embodiment of the invention. 
     As shown in  FIG. 1 , the transmitter side  100  of the MIMO system includes a multiplexer  101  for multiplexing one input data sequence into a plurality of data having the same length, a plurality of CRC (Cyclic Redundancy Check) encoders  102  which add CRC codes for error detection to the data, a plurality of channel encoders  103  for correcting errors through channel fading, a plurality of QAM (Quadrature Amplitude Modulation) mappers  104 , a plurality of OFDM modulators  105 , a transmission antenna designating unit  106  for selecting transmitting antennas with an excellent communication link, a plurality of antennas  107 , and a scheduler  108  which adjusts a channel encoding rate and schedules antennas, through which a packet is to be transmitted, by using a feedback signal. 
     In  FIG. 1 , b j  represents an information bit stream, {tilde over (b)} j  represents a CRC-encoded information bit stream, c j  represents a channel-encoded bit stream, s j  represents a QAM-mapped symbol, and {tilde over (s)} j  represents an OFDM symbol (j=1, 2, . . . , N T ). 
     First, the multiplexer  101  multiplexes a packet input to the transmitter side  100  into a plurality of data having the same length, and the CRC encoders  102  add CRC codes for error detection to the data. A generator polynomial of the CRC codes is expressed by Equation 1.
 
 G   CRC     24   ( D )= D   24   +D   23   +D   6   +D   5   +D+ 1  [Equation 1]
 
     The CRC generator polynomial is determined by the MAC (Medium Access Control) layer of the system. If necessary, another polynomial between the transmitter and receiver sides may be defined and used. 
     The channel encoders  103  encode the data, to which the CRC codes are added, by using ACK (Acknowledgement) values received from the scheduler  108 . At this time, when the encoding is performed, a chase combining scheme is applied to the data of which the ACK value is 0, and an incremental redundancy (IR) scheme is applied to the data of which the ACK value is 1. 
     The scheduler  108  determines whether or not to retransmit the packet by using the ACK values received from a receiver side, schedules a channel encoding rate and the antennas  107  through which the packet is to be retransmitted, and selects the chase combining scheme or the IR scheme depending on the ACK values received from the receiver side so as to retransmit the encoded data to the selected transmission antennas  107 . 
       FIG. 2  is a diagram showing the receiver-side configuration of the MIMO system, to which the spatial multiplexing technique is applied, according to an embodiment of the invention. 
     As shown in  FIG. 2 , the receiver side  100  of the MIMO system includes a plurality of antennas  201 , a channel estimator  202  which estimates channel values from the packet transmitted from the transmitter side, a channel measurer  203  which calculates channel sums or channel norms from the channel values, a transmitting/receiving antenna selector  204  which selects antennas by using the channel sums or channel norms, a MIMO detector  205 , an OFDM demodulator  206 , a QAM de-mapper  207 , a plurality of channel decoders  208 , a plurality of CRC decoders  209  for checking whether an error occurs in the received packet or not, and a feedback transmission unit (not shown) which transmits a feedback signal to the transmitter side, the feedback signal including an ACK value of the packet for each transmitting antenna and a list of transmitting antennas. 
     The channel measurer  203  calculates channel sums or channel norms by using the channel values estimated by the channel estimator  202 . The channel sums for each transmitting antenna and each receiving antenna are calculated by Equations 2 and 3, respectively. The channel norms for each transmitting antenna and each receiving antenna are calculated by Equations 4 and 5, respectively. 
     
       
         
           
             
               
                 
                   
                     
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                         T 
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                     ⁢ 
                     
                         
                     
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                         R 
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     Here, CHSum T     j    represents the channel sum of a j th  transmitting antenna, CHSsum R     i    represents the channel sum of an i th  receiving antenna, CHNorm T     j    represents the channel norm of a j th  transmitting antenna, CHNorm R     i    represents the channel norm of an i th  receiving antenna, H represents a channel value estimated from the received packet, N T  represents the number of transmitting antennas, and N R  represents the number of receiving antennas. 
     By using the channel sums or channel norms calculated by Equations 2 to 5, the transmitting/receiving antenna selector  204  selects a predetermined number (L T ×L R ) of transmitting and receiving antennas of which the channel sum or channel norm is large, as expressed by Equation 6. 
     
       
         
           
             
               
                 
                   
                     
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     The CRC decoder  209  determines an ACK value of the packet by checking through the CRC generator polynomial whether an error occurs in the received packet or not. The CRC equation is expressed by Equation 7.
 
 P ( x )= Q ( x )× G ( x )+ R ( x )  [Equation 7]
 
     Here, P(x) represents an information polynomial, G(x) represents a CRC generator polynomial, Q(x) represents a quotient, and R(x) represents a remainder. When R(x) is 0, it indicates that an error did not occur in the received packet. 
     The ACK value is determined by Equation 8. 
     
       
         
           
             
               
                 
                   
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     After the CRC decoding is completed, the receiver side  200  feeds back to the transmitter side  100  the indexes of transmitting antennas through which the ACK values and the packet are to be transmitted. 
       FIG. 3  is a flow chart showing an operation in the receiver side of the MIMO system, to which the spatial multiplexing technique is applied, according to an embodiment of the invention. 
     First, when data are received from the transmitter side (step S 101 ), channel values for the received data are estimated (step S 102 ), and channel sums or channel norms are calculated using the estimated channel values (step S 103 ). Then, a predetermined number (L T ×L R ) of antennas having a high channel gain are selected using the calculated channel sums or channel norms (step S 104 ). 
     Next, it is checked whether the received packet is a new packet or not (step S 105 ). When the packet is a retransmitted packet, the packet is combined with a previous packet stored in a buffer (step S 106 ). When the packet is a new packet, step S 106  is not performed, but step S 107  which will be described below is performed. 
     In step S 107  where the CRC decoding is performed, CRC codes are extracted from the received packet. Then, it is detected through the CRC code whether an error occurs in the received packet or not (step S 108 ). When an error occurred in the received packet, 1 is allocated as an ACK value (step S 109   a ). When an error did not occur in the received packet, 0 is allocated as an ACK value (step S 109   b ). 
     Finally, the ACK values determined in step S 109   a  or S 109   b  and the list of transmitting antennas determined in step S 104  are fed back to the transmitter side (step S 110 ). 
       FIG. 4  is a flow chart showing an operation in the transmitter side of the MIMO system, to which the spatial multiplexing technique is applied, according to an embodiment of the invention. 
     First, when the feedback signal is received from the receiver side (step S 201 ), the fed-back ACK values are checked so as to determine whether or not to retransmit the packet (step S 202 ). At this time, when any one of the ACK values is 1, it is judged that the retransmission is necessary, and the process proceeds to step S 203  which will be described below. When the ACK values are all 0, new data is transmitted to the receiver side through the transmitting antennas designated by the receiver side in accordance with the feedback signal (step S 208 ). 
     When it is judged in step S 202  that the retransmission is necessary, it is checked whether the ACK values of all the transmitting antennas are 1 or not (step S 203 ). When the ACK values of all the transmitting antennas are 1, it means that errors occur in the signals transmitted through the all the transmitting antennas. In this case, the IR scheme for increasing the encoding rate of the channel encoders is applied to the packet which is to be retransmitted (step S 204 ). Then, new data is transmitted to the receiver side through the transmitting antennas designated by the receiver side (step S 208 ). 
     Meanwhile, when it is judged in step S 203  that the ACK values of all the transmitting antennas are not 1, that is, when some of the ACK values are 1 and the others are 0, the ACK value of each transmitted data is discriminated (step S 206 ). The IR scheme is applied to the data which is transmitted through the antenna of which the ACK value is 1 (step S 207   a ), and the chase combining scheme is scheme is applied to the data which is transmitted through the antenna of which the ACK value is 0 (step S 207   b ). Then, new data is transmitted to the receiver side through the transmitting antenna designated by the receiver side. 
       FIG. 5  is a diagram showing the transmitter-side configuration of a MIMO system, to which the spatial diversity scheme is applied, according to another embodiment of the invention. 
     As shown in  FIG. 5 , the transmitter side  300  includes a CRC encoder  301  which adds a CRC code for error detection to data, a channel encoder  302  which corrects an error through channel fading, a QAM mapper  302 , an OFDM modulator  304 , an OFDM antenna designating unit  305  which transmits OFDM-modulated signals through a plurality of antennas, a transmitting antenna designating unit  306 , a plurality of transmitting antennas  307 , and a scheduler  308  which adjusts a channel encoding rate by using a feedback signal and schedules antennas through which a packet is to be transmitted. 
     In  FIG. 5 , b represents a CRC-encoded information bit stream, c represents a channel-encoded bit stream, s represents a QAM-mapped symbol, {tilde over (s)} represents an OFDM symbol, {tilde over (s)} j  represents an OFDM symbol designated to each transmitting antenna by the OFDM antenna designating unit  305  (j=1, 2, . . . , N T ). 
     First, as shown in  FIG. 5 , the CRC encoder  301  adds a CRC code for error detection to a packet input to the receiver side  300 . A generator polynomial of the CRC code is expressed by Equation 1 which has been described above. 
     The channel encoder  302  encodes the data, to which the CRC code is added, by using an ACK value received from the scheduler  308 . In this case, the chase combining scheme is applied to the data of which the ACK is 0, and the IR scheme is applied to the data of which the ACK is 1. 
     When receiving the ACK value and a channel norm or channel sum for each antenna from the receiver side, the scheduler  308  arranges the transmitting antennas in magnitude order of the channel norms or channel sums. Then, retransmission is performed through the transmitting antenna having the greatest channel norm or channel sum, among the transmitting antennas of which the ACK value is 0. If an antenna of which the ACK value is 0 is not present, the channel encoding rate is increased (using the IR scheme) so as to retransmit the packet. 
       FIG. 6  is a diagram showing the receiver-side configuration of the MIMO system, to which the spatial diversity scheme is applied, according to another embodiment of the invention. 
     As shown in  FIG. 6 , the receiver side  400  includes a plurality of receiving antennas  401 , a channel estimator  402  which estimates channel values from the packet transmitted from the transmitter side, a channel measurer  403  which calculates channel sums or channel norms from the channel values, a transmitting/receiving antenna selector  404  which selects antennas by using the channel sums or channel norms, a MIMO detector  405 , a data combining unit  406  which combines signals received from the plurality of antennas, an OFDM demodulator  407 , a QAM de-mapper  408 , a channel decoder  409 , a CRC encoder  410  for checking whether an error occurs in the received packet or not, and a feedback transmission unit which transmits a feedback signal to the transmitter side, the feedback signal including the ACK value of the packet, a list of transmitting antennas, and the channel sum or channel norm of each antenna. 
     The channel measurer  403  calculates channel sums or channel norms by using channel values estimated by the channel estimator  402 . The channel sums or channel norms are calculated through Equations 2 to 5. 
     By using the channel sums or channel norms calculated through Equations 2 to 5, the transmitting/receiving antenna selector  404  selects a predetermined number (L T ×L R ) of transmitting and receiving antennas of which the channel sum or channel norm is large, as expressed by Equation 6. 
     The CRC decoder  410  checks through the CRC generator polynomial whether an error occurs in the received packet or not, and then determines the ACK value of the packet. The CRC equation is expressed by Equation 7. 
     Further, the ACK value is determined by Equation 8. 
     After the CRC decoding is completed, the receiver side  400  feeds back to the transmitter side  300  the indexes of transmitting antennas through which the ACK value and the packet are to be transmitted. 
       FIG. 7  is a flow chart showing an operation in the receiver side of the MIMO system, to which the spatial diversity technique is applied, according to another embodiment. 
     First, when data are received from the transmitter side (step S 301 ), channel values for the received data are estimated (step S 302 ), and channel sums or channel norms are calculated using the estimated channel values (step S 303 ). Then, a predetermined number (L T ×L R ) of antennas having a large channel gain are selected using the calculated channel sums or channel norms (step S 304 ). 
     Next, signals received from the plurality of antennas are combined (step S 305 ), and it is checked whether the received packet is a new packet or not (step S 306 ). At this time, when the packet is a retransmitted packet, the packet is combined with a previous packet stored in a buffer (step S 307 ). When the packet is a new packet, step S 307  is not performed, and the process immediately proceeds to step S 308  which will be described below. 
     In step S 308  where CRC decoding is performed, CRC codes are extracted from the received packet. Then, it is detected through the CRC code whether an error is present in the received data or not (step S 309 ). When an error is present in the received data, 1 is allocated as an ACK value (step S 310   a ). When an error is not present in the received data, 0 is allocated as an ACK value (step S 310   b ). 
     Finally, the ACK values determined in step S 310   a  or S 310   b , the list of transmitting antennas determined in step S 304 , and the channel sum or channel norm of each transmitting antenna, which is calculated in step S 303 , are fed back to the transmitter side (step S 311 ). 
       FIG. 8  is a flow chart showing an operation in the transmitter side of the MIMO system, to which the spatial diversity technique is applied, according to another embodiment of the invention. 
     First, when the feedback signal is received from the receiver side (step S 401 ), the received ACK values are checked so as to check whether or not to retransmit the packet or not (step S 402 ). At this time, when any one of the ACK values is 1, it is judged that the retransmission is necessary, and the process proceeds to step S 403  which will be described below. When the ACK values are all 0, new data is transmitted to the receiver side through the transmitting antennas designated by the receiver side depending on the feedback signal (step S 410 ). 
     When it is judged in step S 402  that the retransmission is necessary, it is checked whether the ACK values of all the transmitting antennas are 1 or not (step S 403 ). When the ACK values of all the transmitting antennas are 1, the transmitting antennas are arranged in accordance with the fed-back channel norms or channel sums (step S 404 ). Among them, the transmitting antenna with the greatest channel norm or channel sum is selected (step S 405 ). Then, the IR scheme for increasing the encoding rate of the channel encoder is applied to the packet which is to be retransmitted (step S 406 ), and new data is transmitted to the receiver side through the transmitting antenna selected in the step S 405  (step S 410 ). 
     Meanwhile, when it is judged in step S 403  that the ACK values of all the transmitting antennas are not 1, that is, when some of the ACK values are 1 and the others are 0, the transmitting antennas are arranged in accordance with the fed-back channel norms or channel sums (step S 407 ). Among the antennas of which the ACK value is 0, the transmitting antenna with the greatest channel norm or channel sum is selected (step S 408 ). Then, the chase combining scheme is applied to the packet which is to be retransmitted (step S 409 ), and new data is transmitted to the receiver side through the transmitting antenna selected in the step S 408  (step S 410 ). 
     According to the present invention, antennas with a favorable communication link are always selected by the antenna selection scheme. When retransmission is requested, the antennas are scheduled adaptively to the communication link so as to retransmit a packet. Therefore, it is possible to provide a higher retransmission probability than in the conventional system. Further, the reliability of the system can be enhanced, and the number of retransmissions can be reduced in comparison with the conventional method. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.