Abstract:
A transmitting/receiving apparatus and method for packet retransmission in a mobile communication system. Upon request for a retransmission from a receiver, a transmitter generates first coded bits by inverting initially transmitted coded bits, generates second coded bits by separating the initially transmitted coded bits into a first bit group having a relatively high priority and a second bit group having a relatively low priority and exchanging the first bit group with the second bit group, and generates third coded bits by inverting the exchanged coded bits. The transmitter selects one of the first coded bits, the second coded bits according to the sequence number of a retransmission request received from the receiver, and the third coded bits and maps the selected coded bits to modulation symbols. The transmitter then transmits the modulation symbols to the receiver.

Description:
PRIORITY 
   This application claims priority to an application entitled “Transmitting/Receiving Apparatus and Method for Packet Retransmission in a Mobile Communication System” filed in the Korean Industrial Property Office on Oct. 31, 2001 and assigned Ser. No. 2001-67694, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a W-CDMA (Wide-band Code Division Multiple Access) mobile communication system, and in particular, to a transmitting/receiving apparatus and method for reducing a transmission error rate and thus increasing decoding performance at retransmission. 
   2. Description of the Related Art 
   Adverse influences on high-speed, high-quality data service are attributed to a channel environment in a mobile communication system. The radio channel environment varies frequently because of signal power changes caused by white noise and fading, shadowing, the Doppler effect that occurs due to the movement and frequent velocity change of a terminal, and interference from other users and multi-path signals. Therefore, aside from conventional technologies in the second or third generation mobile communication system, an advanced technique is required to support wireless high-speed data packet service. In this context, the 3GPP (3 rd  Generation Partnership Project) and the 3GPP2 commonly addressed the techniques of AMCS (Adaptive Modulation &amp; Coding Scheme) and HARQ (Hybrid Automatic Repeat Request). 
   The AMCS adjusts a modulation order and a code rate according to changes in downlink channel condition. The downlink channel quality is usually obtained by measuring the SNR (Signal-to-Noise Ratio) of a received signal at a UE (User Equipment). The UE transmits the channel quality information to a BS (Base Station) on an uplink. Then the BS estimates the downlink channel condition based on the channel quality information and determines an appropriate modulation scheme and code rate according to the estimated downlink channel condition. 
   QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary PSK), and 16QAM (16-ary Quadrature Amplitude Modulation) and code rates of ½ and ¼ are considered in the current high-speed wireless data packet communication system. In AMCS, a BS applies a high-order modulation (e.g., 16QAM and 64QAM) and a high code rate of ¾ to a UE having good channel quality such as its adjacent UEs, and a low-order modulation (e.g., 8PSK and QPSK) and a low code rate of ½ to a UE having bad channel quality such as a UE at a cell boundary. The AMCS reduces interference signals remarkably and improves system performance, as compared to the conventional method relying on high-speed power control. 
   HARQ is a retransmission control technique to correct errors in initially transmitted data packets. Schemes for implementing HARQ include chase combining (CC), full incremental redundancy (FIR), and partial incremental redundancy (PIR). 
   With CC, the entire initial transmission packet including systematic bits and parity bits is retransmitted. A receiver combines the retransmission packet with the initial transmission packet stored in a reception buffer. The resulting increase of the transmission reliability of coded bits input to a decoder brings the performance gain of the overall mobile communication system. An approximate 3-dB performance gain is effected on average since combining of the same two packets is equivalent to repeated coding of the packet. 
   In FIR, a packet having only parity bits, different from an initial transmission packet, is retransmitted to thereby increase a decoding gain. A decoder decodes data using the new parity bits as well as initially transmitted systematic and parity bits. As a result, decoding performance is improved. It is well known in coding theory that a higher performance gain is yielded at a low code rate than by repeated coding. Therefore, FIR is superior to CC in terms of performance gain. 
   As compared to FIR, PIR is a retransmission scheme in which a packet having systematic bits and new parity bits is retransmitted. A receiver combines the retransmitted systematic bits with initially transmitted systematic bits for decoding, achieving similar effects to those of CC. PIR is also similar to FIR in that the new parity bits are used for decoding. Since PIR is implemented at a relatively high code rate than FIR, PIR is in the middle of FIR and CC in performance. 
   A combined use of the independent techniques of increasing adaptability to varying channel condition, AMCS and HARQ can improve system performance significantly. 
     FIG. 1  is a block diagram of a transmitter in a typical high-speed wireless data packet communication system. Referring to  FIG. 1 , the transmitter includes a channel encoder  110 , a rate matching controller  120 , an interleaver  130 , a modulator  140 , and a controller  150 . 
   Upon input of information bits in transport blocks of size N, the channel encoder  110  encodes the information bits at a code rate R (=n/k, n and k are prime), for example, ½ or ¾. With the code rate R, the channel encoder  110  outputs n coded bits for the input of k information bits. The channel encoder  110  can support a plurality of code rates using a mother code rate of ⅙ or ⅕ through symbol puncturing or symbol repetition. The controller  150  controls the code rate. 
   The future mobile communication system adopts turbo coding considered a more robust channel coding technique for high-speed reliable transmission of multimedia data. It is known that turbo coding has the nearest Shannon Limit performance in BER (Bit Error Rate) at a low SNR. Turbo coding is also adopted in the 1×EV-DV (Evolution in Data and Voice) standards which are under discussion in the 3GPP and 3GPP2. 
   The output of the channel encoder  110  being a turbo encoder includes systematic bits and parity bits. The systematic bits are information bits to be transmitted and the parity bits are error correction bits added to the information bits for a receiver to correct errors generated during transmission of the information bits at decoding. 
   The rate matching controller  120  generally matches the data rate of the coded bits generally by transport channel-multiplexing, or by repetition and puncturing if the number of the coded bits is different from that of bits transmitted in the air. To minimize data loss caused by burst errors, the interleaver  130  interleaves the rate-matched bits. Interleaving distributes damaged bits in a fading environment. Therefore, the interleaving allows adjacent bits to be randomly influenced by fading and thus prevents burst errors, increasing channel encoding performance. The modulator  140  maps the interleaved bits to symbols in a modulation scheme determined by the controller  150 . 
   The controller  150  selects the code rate and the modulation scheme according to the radio downlink channel condition. To selectively use QPSK, 8PSK, 16QAM, and 64QAM according to the radio environment, the controller  150  supports AMCS. Though not shown, a UE spreads the modulated data with a plurality of Walsh codes to identify transport channels and with a PN (Pseudorandom Noise) code to identify a BS. 
   As stated before, the modulator  140  supports various modulation schemes including QPSK, 8PSK, 16QAM and 64QAM with respect to the interleaved bits. As a modulation order increases, the number of bits in one modulation symbol increases. Particularly in a higher-order modulation scheme greater than 8PSK, one modulation symbol includes three or more bits. In this case, bits mapped to one modulation symbol have different transmission reliabilities according to their positions. 
   With regard to transmission reliability, two bits of a modulation symbol representing a macro region defined by left/right and up/down have a relatively high reliability in an I (In Phase)-Q (Quadrature Phase) signal constellation. The other bits representing a micro region within the macro region have a relatively low reliability. 
     FIG. 2  illustrates an exemplary signal constellation in 16QAM. Referring to  FIG. 2 , one 16QAM modulation symbol contains 4 bits [i 1 , q 1 , i 2 , q 2 ] in a reliability pattern [H, H, L, L] (H denotes high reliability and L denotes low reliability). That is, the two upper bits [i 1 , q 1 ] have a relatively high reliability and the two lower bits [i 2 , q 2 ], a relatively low reliability. One 64QAM modulation symbol contains 6 bits [i 1 , q 1 , i 2 , q 2 , i 3 , q 3 ] in a reliability pattern [H, H, M, M, L, L] (M denotes medium reliability). Similarly, an 8PSK modulation symbol contains 3 bits. One of them has a lower reliability than the other two bits. Thus, a reliability pattern is [H, H, L]. 
   Considering the above reliability patterns, it is preferable to map coded bits output from the channel encoder  110  to regions having different reliabilities according to their significance levels. As stated before, the coded bits are divided into systematic bits and parity bits having different priority levels. In other words, if errors are generated at different rates in a transport channel according to the reliabilities, a receiver can recover original bits more accurately by decoding when the parity bits have errors than when the systematic bits have errors because the systematic bits are actual information and the parity bits are error correction bits. 
   In this context, SMP (Symbol Mapping method based on Priority) has been proposed in which systematic bits are mapped to a high reliability region and parity bits are mapped to a low reliability region, so that the error rate of the relatively significant systematic bits can be decreased. 
   Aside from the different reliabilities of coded bits, each modulation symbol is transmitted with a different error rate on a radio channel in a modulation scheme having a modulation order equal to higher than 16QAM. For example, in the signal constellation for 16QAM, 4 coded bits form one modulation symbol and are mapped to one of 16 signal points. The 16 signal points are classified into three regions according to their error rates. As a modulation symbol is farther along a real or imaginary number axis, it has a lower error rate, which means that the receiver identifies the modulation symbol more easily. 
     FIG. 3  illustrates graphs showing the error probabilities of the regions in a simulation under an AWGN (Additive White Gaussian Noise) environment. As shown in  FIG. 2 , the  16  modulation symbols are classified into region  1  having a high error probability, region  2  having a medium error probability, and region  3  having a low error probability. For example, modulation symbols  6 ,  7 ,  10  and  11  in region  1  have a relatively high error probability. 
   In packet data retransmission by HARQ, therefore, retransmission with the same reliability and/or error probability as that of initial transmission does not increase retransmission efficiency. Retransmission of specific bits with a consistently low reliability and/or high error probability deteriorates decoding performance since a channel decoder being a turbo decoder has good decoding performance when the LLRs (Log Likelihood Ratios) of input bits are homogeneous. Therefore, there is a need for exploring a novel retransmission technique that improves transmission performance at retransmission. 
   Techniques for improving transmission performance at retransmission include SRRC (Shifted Retransmission for Reliability Compensation) and BIR (Bit Inverted Retransmission). In the SSRC, the coded bits of a modulation symbol are shifted by a predetermined number of bits, for example, two bits and thus mapped to different reliability parts at a retransmission from those at their initial transmission. In the BIR, the coded bits are inverted and thus mapped to different error probability parts at a retransmission from those at the initial transmission. Those techniques commonly comprise the LLRs of bits input to a turbo decoder and thus improve decoding performance. 
   To describe the SRRC in more detail, an M-ary modulation symbol includes log 2 M bits having different reliabilities. For example, four coded bits form one modulation symbol with the two upper bits mapped to a high reliability and the two lower bits mapped to a low reliability in 16QAM, as illustrated in  FIG. 2 . Two-bit cyclic shifting of the coded bits of each modulation symbol at a retransmission effects averaging the transmission reliabilities of the coded bits, thereby improving decoding performance. 
   With regard to the BIR, 16 modulation symbols each having 4 coded bits are classified into region  1  having a relatively high error probability, region  3  having a relatively low error probability, and region  2  having a medium probability in 16QAM, as illustrated in  FIG. 2 . Inversion of the coded bits of each modulation symbol prior to symbol mapping at a retransmission also effects averaging the error probabilities of the coded bits and thus improves system performance at decoding. 
   Despite the advantage of improved system performance, however, a simple combined use of the above techniques is not effective in their application to systems. Therefore, the techniques need to be combined effectively so that optimum transmission efficiency can be achieved in a CDMA mobile communication system. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide in a wireless communication system a transmitting/receiving apparatus and method in which packet retransmission is carried out with system performance increased. 
   It is another object of the present invention to provide in a wireless communication system a transmitting/receiving apparatus and method that increase the reliabilities of bits at a packet retransmission. 
   It is also another object of the present invention to provide in a wireless communication system a transmitting/receiving apparatus and method for enabling a receiver to receive bits with a higher reception probability. 
   It is a further object of the present invention to provide a wireless communication system supporting HARQ a transmitting/receiving apparatus and method for more efficient packet retransmission. 
   It is still another object of the present invention to provide an apparatus and method for efficiently combining an initial transmission technique with a retransmission technique. 
   It is yet another object of the present invention to provide an apparatus and method for simultaneously supporting the BIR with the SRRC. 
   To achieve the above and other objects, according to one aspect of the present invention, upon request for a retransmission from a receiver, a transmitter generates first coded bits by inverting initially transmitted coded bits, generates second coded bits by separating the initially transmitted coded bits into a first bit group having a relatively high priority and a second bit group having a relatively low priority and exchanging the first bit group with the second bit group, and generates third coded bits by inverting the exchanged coded bits. The transmitter selects one of the first coded bits, the second coded bits (according to the sequence number of a retransmission request received from the receiver), and the third coded bits, and maps the selected coded bits to modulation symbols. The transmitter then transmits the modulation symbols to the receiver. 
   According to another aspect of the present invention, upon request for a retransmission from a receiver, a transmitter generates first coded bits by inverting initially transmitted coded bits, generates second coded bits by cyclically shifting the initially transmitted coded bits by a predetermined number of bits, and generates third coded bits by inverting the shifted coded bits. The transmitter selects one of the first coded bits, the second coded bits (according to the sequence number of a retransmission request received from the receiver), and the third coded bits, and maps the selected coded bits to modulation symbols. The transmitter then transmits the modulation symbols to the receiver. 

   
     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 block diagram of a transmitter in a typical CDMA mobile communication system; 
       FIG. 2  illustrates an example of a signal constellation in 16QAM in the CDMA mobile communication system; 
       FIG. 3  illustrates the error probabilities of regions in the signal constellation of 16QAM; 
       FIG. 4  is a block diagram of a transmitter in a CDMA mobile communication system according to an embodiment of the present invention; 
       FIG. 5  is a detailed block diagram of a channel encoder illustrated in FIG.  4 ; 
       FIG. 6  is a flowchart illustrating the operation of the transmitter in the CDMA mobile communication system according to an embodiment of the present invention; 
       FIG. 7  is a block diagram of a receiver for receiving signals from the transmitter illustrated in  FIG. 4  in the CDMA mobile communication system according to the embodiment of the present invention; 
       FIG. 8  is a flowchart illustrating the operation of the receiver in the CDMA mobile communication system according to an embodiment of the present invention; 
       FIG. 9  illustrates bit inversion in the transmitter according to an embodiment of the present invention; 
       FIG. 10  is a block diagram of a transmitter in a CDMA mobile communication system according to a second embodiment of the present invention; 
       FIG. 11  is a flowchart illustrating the operation of the transmitter in the CDMA mobile communication system according to the second embodiment of the present invention; 
       FIG. 12  is a block diagram of a receiver for receiving signals from the transmitter illustrated in  FIG. 10  in the CDMA mobile communication system according to the second embodiment of the present invention; 
       FIG. 13  is a flowchart illustrating the operation of the receiver in the CDMA mobile communication system according to the second embodiment of the present invention; and 
       FIG. 14  illustrates a comparison between frame error rates at retransmissions according to an embodiment of the present invention and at a retransmission according to a conventional method under an AWGN environment. 
   

   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. 
   HARQ, to which the present invention, is applied is a link controlling technique for correcting packet errors by retransmission. As is applied from its name, retransmission is one more transmission of initially transmitted but failed packet data. Therefore, new data is not transmitted at a retransmission. 
   As described before, HARQ techniques are divided into HARQ type II and HARQ type III depending on whether systematic bits are retransmitted or not. The major HARQ type II is FIR, and HARQ type III includes CC and PIR which are discriminated according to whether the same parity bits are retransmitted. 
   The present invention as described below is applied to all of the above HARQ techniques. In the CC, a retransmission packet has the same bits as an initial transmission packet, and in the FIR and PIR a retransmission packet and an initial transmission packet have different bits. Since the present invention pertains to a method of increasing the transmission efficiency of a retransmission packet, it is obviously applicable to the case where an initial transmission packet is different from its retransmission packet. Yet, the following description is made in the context of the CC by way of example. 
   The present invention can be implemented in two embodiments. In a first embodiment, SMP (Symbol Mapping method based on Priority) is combined with the BIR, and in a second embodiment, the SRRC is combined with the BIR. 
   First Embodiment: SMP+BIR 
     FIG. 4  is a block diagram of a transmitter in a CDMA mobile communication system according to an embodiment of the present invention. Referring to  FIG. 4 , the transmitter includes a CRC (Cyclic Redundancy Check) adder  210 , a channel encoder  220 , a rate matching controller  230 , a distributor  240 , an interleaver unit  250 , an exchange  260 , a parallel-to-serial converter (PSC)  270 , a bit inverter  280 , a modulator  290 , and a transmission controller  200 . 
   The transmitter exchanges systematic bits with parity bits at a retransmission when necessary. Therefore, the exchange  260  is optional. 
   Referring to  FIG. 4 , the CRC adder  210  adds CRC bits to input information bits for an error check on a packet data basis. The channel encoder  220  encodes the packet data with the CRC bits at a predetermined code rate by predetermined coding. 
   The packet data is coded to systematic bits and parity bits being error control bits for the systematic bits. Turbo coding or convolutional coding can be used. 
   The code rate determines the ratio of the parity bits to the systematic bits. With a code rate of ½, for example, the channel encoder  220  outputs one systematic bit and one parity bit for the input of one information bit. With a code rate of ¾, the channel encoder  220  outputs three systematic bits and one parity bit for the input of three information bits. In the embodiment of the present invention, other code rates can also be applied aside from ½ and ¾. 
   The rate matching controller  230  matches the data rate of the coded bits by repetition and/or puncturing. The distributor  240  separates the rate-matched bits into systematic bits and parity bits and feeds the systematic bits to a first interleaver  252  and the parity bits to a second interleaver  254 . With a symmetrical code rate such as ½, the first and second interleavers  252  and  254  receive the same number of bits. On the other hand, with an asymmetrical code rate such as ¾, systematic bits are first fed to the first interleaver  252  and the remaining systematic bits and the parity bits are then fed to the second interleaver  254 . 
   The first interleaver  252  interleaves the systematic bits and the second interleaver  254  interleaves the parity bits in a predetermined interleaving method. While the first and second interleavers  252  and  254  are discriminated in hardware in  FIG. 4 , they can also be discriminated logically. This means that the interleaver unit  250  uses a single memory having a memory area for storing systematic bits and a memory area for storing parity bits. The thus-constituted interleaver unit  250  operates to map the systematic bits and the parity bits to different reliability parts. In other words, the SMP is achieved with the use of the distributor  240  and the interleaver unit  250 . 
   The interleaver outputs are stored in a buffer (not shown) for use at retransmission. Upon request of a receiver for a retransmission, the whole or part of the buffered bits are output under the control of the transmission controller  200 . 
   The coded bits, of which the sequences have been permuted by the first and second interleavers  252  and  254 , are exchanged in the exchange  260  under the control of the transmission controller  200 . At an initial transmission, the transmission controller  200  disables the exchange  260  so that the first interleaver output and the second interleaver output bypass the exchange  260 . At a retransmission, the transmission controller  200  determines whether to enable the exchange  260  according to the number of retransmission occurrences. For example, bit exchange occurs at each third or fourth retransmission, and no bit exchange occurs at each first or second retransmission. 
   The coded bits that have passed through the exchange  260  are converted to a serial bit stream in the PSC  270 . The bit inverter  280  inverts the bits of the serial bit stream under the control of the transmission controller  200 . The transmission controller  200  enables or disables the bit inverter  280  according to the sequence number of a retransmission. For example, the bit inverter  280  inverts the coded bits only at each odd-numbered retransmission. The bit inverter  280  is an inverter that inverts input bits 0 or 1. 
   When bit inversion is not needed, the input coded bits bypass the bit inverter  280 . This bit inverter  280  functions to map coded bits to a modulation symbol with a different error probability at a retransmission from that at an initial transmission, to thereby implement the BIR. 
   The modulator  290  modulates input coded bits in a predetermined modulation scheme. In 16QAM, the modulator  290  maps every four input coded bits to a modulation symbol having a bit reliability pattern [H, H, L, L]. H denotes a high reliability part and L denotes a low reliability part. 
   The transmission controller  200  provides overall control to the components of the transmitter in accordance with upper layer signaling. The transmission controller  200  determines the code rate of the channel encoder  220  and the modulation scheme of the modulator  290  according to the current radio channel condition. 
   The transmission controller  200  also controls the exchange  260  and the bit inverter  280  by a retransmission request from an upper layer in response for a retransmission request from a receiver. The retransmission request information from the upper layer indicates whether the receiver has requested a packet retransmission and how many times retransmission has been carried out so far. 
   Aside from the sequence number of a retransmission, the bit inverter  280  is enabled or disabled according to an SFN (System Frame Number). In this case, the transmitter can determine whether to perform bit inversion or not using the SFN only without the need for additional information such as the sequence number of a retransmission. This is because modulation without inversion at an initial transmission and inversion prior to modulation at a retransmission is equivalent to inversion prior to modulation at an initial transmission and modulation without inversion at a retransmission. That is, it does not matter whether bit inversion is performed at an initial transmission or at a retransmission in the present invention. 
     FIG. 5  is a detailed block diagram of the channel encoder  220  illustrated in  FIG. 4 . It is assumed that the channel encoder  220  uses a mother code rate of ⅙ adopted in the 3GPP (3 rd  Generation Partnership Project) standards. 
   Referring to  FIG. 5 , the channel encoder  220  simply outputs one data frame of size N as a systematic bit frame X (=x 1 , x 2 , . . . , x N ). Here, N is determined according to the code rate. A first constituent encoder  224  outputs two different parity bit frames Y 1  (=y 11 , y 12 , . . . , y 1N ) and Y 2  (=y 21 , y 22 , . . . , y 2N ) for the input of the data frame. 
   An internal interleaver  222  interleaves the data frame and outputs an interleaved systematic bit frame X′ (=x′ 1 , x′ 2 , . . . , x′ N ). A second constituent encoder  226  encodes the interleaved systematic bit frame X′ to two different parity bit frames Z 1  (=z 11 , z 12 , . . . , z 1N ) and Z 2  (=z 21 , z 22 , . . . , z 2N ). 
   A puncturer  228  generates intended systematic bits S and parity bits P by puncturing the systematic bit frame X, the interleaved systematic bit frame X′, and the parity bit frames Y 1 , Y 2 , Z 1  and Z 2  in a puncturing pattern received from the controller  270 . 
   The puncturing pattern is determined according to the code rate of the channel encoder  220  and an H-ARQ method used. For example, when the code rate is ½, puncturing patterns available in H-ARQ type III (CC and PIR) are as follows. 
                   P   1     =     [         1       1           1       0           0       0           0       0           0       0           0       1         ]             (   1   )                 P   2     =     [         1       1           1       0           0       0           0       0           0       1           0       0         ]             (   2   )               
where 1 indicates a transmission bit and 0 indicates a punctured bit. Input bits are punctured from the left column to the right column.
 
   One of the above puncturing patterns is used at an initial transmission and retransmissions in the CC, while they are alternately used at each transmission in the PIR. 
   In HARQ type II (FIR), systematic bits are punctured at retransmission. In this case, a puncturing pattern is “010010”, for example. 
   In the CC, if the puncturing pattern P 1  (i.e., “110000” and “100001”) is used, the puncturer  228  outputs bits X, Y 1 , X and Z 2  with the other bits punctured at each transmission. If the puncturing pattern P 2  (i.e., “110000” and “100010”) is used, the puncturer  228  outputs bits X, Y 1 , X and Z 1  with the other bits punctured at each transmission. 
   In the PIR, the puncturer  228  outputs bits X, Y 1 , X and Z 2  at an initial transmission and bits X, Y 1 , X and Z 1  at a retransmission. 
   Though not shown, a channel encoder using a mother code rate of ⅓ adopted in the 3GPP2 is realized using one constituent encoder and a puncturer. 
     FIG. 6  is a flowchart illustrating the operation of the transmitter according to the embodiment of the present invention. Referring to  FIG. 6 , the CRC adder  210  adds CRC bits to input data on a packet basis in step  300  and the channel encoder  220  encodes the packet data with the CRC bits at a code rate preset between the transmitter and the receiver in step  305 . 
   Specifically, the input packet data is simply output as a systematic bit frame X in the channel encoder  220 . The first constituent channel encoder  224  encodes the systematic bit frame X at a predetermined code rate and outputs different parity bit frames Y 1  and Y 2 . 
   The internal interleaver  222  interleaves the packet data and outputs another systematic bit frame X′. The second constituent channel encoder  226  encodes the systematic bit frame X′ and outputs two different parity bit frames Z 1  and Z 2 . 
   The puncturer  228  punctures the systematic bit frames X and X′ and the parity bit frames Y 1 , Y 2 , Z 1  and Z 2  according to a desired code rate in a predetermined puncturing pattern. 
   As described before, the same puncturing pattern is used at an initial transmission and retransmissions in the CC. The puncturing pattern is stored in the puncturer  228  or received from the transmission controller  200 . In  FIG. 5 , the puncturing pattern is illustrated to be externally received. 
   In step  310 , the rate matching controller  230  matches the rate of the coded bits by repetition and puncturing. The rate matching controller  230  operates for transport channel multiplexing, or when the number of encoder output bits is different from the number of bits in a transmission frame. 
   In step  315 , the distributor  240  separates the rate-matched bits into systematic bits and parity bits. If the number of the systematic bits are equal to that of the parity bits, the systematic bits and the parity bits are fed to the first and second interleavers  252  and  254 , respectively. On the other hand, if they are different, the first interleaver  252  first receives systematic bits. The first and second interleavers  252  and  254  interleave the input coded bits in step  320 . 
   The transmission controller  200  determines in step  325  whether a retransmission request command received from the upper layer indicates the initial transmission of a new packet or a retransmission of a previous packet. In the case of the initial transmission of the new packet, the procedure goes to step  340 . 
   In the case of a retransmission of the same packet, the transmission controller  200  calculates MOD (the sequence number of the retransmission, log 2 M) in step  330 . MOD denotes a modulo operation and M indicates the modulation order used in the modulator  290 . If the solution is less than 2, the procedure jumps to step  340 . On the other hand, if the solution is equal to or greater than 2, the transmission controller  200  enables the exchange  260 . The exchange  260  then exchanges in step  335  the outputs of the first and second interleavers  252  and  254 . As a result, the systematic bits are fed to the second interleaver  254 , and the parity bits to the first interleaver  252 . 
   In step  340 , the PSC  270  converts the coded bits received in two paths to a serial bit stream. The transmission controller  200  in step  345  calculates MOD (the sequence number of the retransmission, 2) to determine whether to invert the bits of the serial bit stream. If the solution is 0, this indicates an even-numbered retransmission and if the solution is not 0, this indicates an odd-numbered retransmission. In the former, the transmission controller  200  disables the bit inverter  280 , and in the latter, it enables the bit inverter  280 . When enabled, the bit inverter  280  inverts in step  350  the bits of the serial bit stream. On the contrary, when the bit inverter is disabled, the serial bit stream is directly fed to the modulator  290  without bit inversion. 
   The modulator  290  maps the input bits to symbols in step  355 . In 16QAM, every four coded bits are mapped to a modulation symbol having a reliability pattern [H, H, L, L]. The modulation symbols are spread with a predetermined spreading code and transmitted to the receiver in step  360 . 
     FIG. 7  is a block diagram of a receiver being the counterpart of the transmitter illustrated in  FIG. 4  according to an embodiment of the present invention. Referring to  FIG. 7 , the receiver includes a demodulator  410 , a bit inverter  420 , a serial-to-parallel converter (SPC)  430 , an exchange  440 , a deinterleaver unit  450 , a combiner  460 , a buffer  470 , a channel decoder  480 , a CRC checker  490 , and a reception controller  400 . 
   In operation, the demodulator  410  demodulates data received from the transmitter in a demodulation method corresponding to the modulation scheme used in the modulator  290 . The bit inverter  420  inverts the bits of the demodulated symbols under the control of the reception controller  400 . The reception controller  400  enables the bit inverter  420  only at each odd-numbered retransmission. 
   The bit inverter  420  is a multiplier that selectively multiplies −1 by input bits because demodulated bits output from the demodulator  410  have soft values −1 and 1. That is, the multiplier converts 1 to −1 and −1 to 1 by sign inversion. Specifically, the multiplier multiplies −1 by input bits at each odd-numbered retransmission of the same packet under the control of the reception controller  400 . Thus, the multiplier performs the same function as the inverter illustrated in  FIG. 4 . If the demodulator  410  outputs coded bits expressed in hard values 0 and 1, the multiplier must be replaced with an inverter. 
   The SPC  430  converts the coded bits received from the bit inverter  420  to two parallel bit streams under the control of the reception controller  400 . If the solution of MOD (the sequence number of a retransmission, log 2 M) is less than 2, the reception controller  400  disables the exchange  440 . Then the two parallel coded bit streams are directly fed to the deinterleaver. If the solution of MOD (the sequence number of a retransmission, log 2 M) is equal to or greater than 2, the reception controller  400  enables the exchange  400  and the exchange  440  exchanges the two parallel coded bit streams with each other. 
   One of the parallel coded bit streams is fed to a first deinterleaver  452  and the other coded bit stream, to a second deinterleaver  454 . The first and second deinterleavers  452  and  454  deinterleave the input coded bits in a deinterleaving rule corresponding to the interleaving rule used in the first and second interleavers  252  and  254  of the transmitter. 
   The combiner  460  combines the current received coded bits of a packet with the coded bits of the same packet accumulated in the buffer  470 . If there are no coded bits of the same packet in the buffer  470 , that is, in the case of initial transmission, the combiner  460  simply outputs the current received coded bits and simultaneously stores them in the buffer  470 . 
   The channel decoder  480  recovers the coded bits received from the combiner  460  by decoding them in a predetermined decoding method, turbo decoding here corresponding to the coding method in the channel encoder  220  of the transmitter. 
   The CRC checker  490  extracts CRC bits from the decoded information bits on a packet basis and determines whether the packet has errors using the extracted CRC bits. An upper layer processes the packet if the packet has no errors and an ACK (Acknowledgement) signal for the packet is transmitted to the transmitter. On the contrary, if the packet has errors, an NACK (Non-Acknowledgement) signal for the packet is transmitted to the transmitter, requesting a retransmission of the packet. 
   If the ACK signal is transmitted to the transmitter, the buffer  470  is initialized with the coded bits of the corresponding packet deleted. If the NACK signal is transmitted to the transmitter, the coded bits of the packet remain in the buffer  470 . The reception controller  400  counts transmissions of the NACK signal to determine the sequence number of the next retransmission and control the bit inverter  420  and the exchange  440  correspondingly. 
     FIG. 8  is a flowchart illustrating the operation of the receiver according to an embodiment of the present invention. Referring to  FIG. 8 , upon receipt of data on a radio transport channel in step  500 , the demodulator  410  recovers coded bits by demodulating the received data on a modulation symbol basis according to a modulation scheme preset between the receiver and the transmitter in step  505 . In step  510 , the reception controller  400  determines whether the coded bits are an initial transmission packet or a retransmission packet. 
   In the case of retransmission, the reception controller  400  calculates MOD (the sequence number of the retransmission, 2) in step  515 . If the solution is not 0, that is, if the retransmission is an odd-numbered one, the reception controller  400  enables the bit inverter  420 . The bit inverter  420  then inverts the coded bits in step  520 . On the other hand, in the case of initial transmission, the reception controller  400  disables the bit inverter  420  and the coded bits bypass the bit inverter  420 . 
   Bit inversion will be described in more detail with reference to  FIG. 9 .  FIG. 9  illustrates a 12-bit frame with a modulation order of 16. Here, one modulation symbol has 4 bits. Referring to  FIG. 9 , the first, second and third modulation symbols are [0000], [1100], and [0111], respectively. When an NACK signal is received and thus a retransmission is requested, the original bits are inverted. Thus, [0000], [1100] and [0111] are converted [1111], [0011] and [1000], respectively. 
   In connection with the signal constellation of  FIG. 2 , the initial transmission modulation symbol [0000] in region  1  is retransmitted as [1111] in region  3 . From the graphs of  FIG. 3 , it is noted that the error probability of region  1  is much higher than that of region  3 . Transmission of a specific symbol consistently in a region with a high error probability adversely influences system performance. However, retransmission of a symbol in a different transmission region leads to averaging the error probabilities of bits and thus increases decoding performance according to the present invention. 
   Returning again to  FIG. 8 , coded bits that have passed through or bypassed the bit inverter  420  are separated into two parallel bit streams in the SPC  430  in step  525 . The reception controller  400  calculates MOD (the sequence number of the retransmission, log 2 M) in step  530 . If the solution is less than 2, the reception controller  400  disables the exchange  440  and the parallel coded bit streams are directly fed to the deinterleaver  450 . On the other hand, if the solution is equal to or greater than 2, the reception controller  400  enables the exchange  440  and the exchange  535  exchanges the two parallel coded bit streams with each other in step  440 . The first and second deinterleavers  452  and  454  deinterleave the coded bit streams in two paths in step  540 . 
   The combiner  460  in step  545  combines the deinterleaved coded bits with coded bits of the same packet accumulated in the buffer  470 . In step  550 , the channel decoder  480  decodes the combined bits in a decoding method preset between the transmitter and the receiver and outputs the original information bits. 
   In step  555 , the CRC checker  490  determines whether the packet has errors by a CRC check on the decoded information bits on a packet basis. If the packet has no errors, the buffer  470  is initialized and an ACK signal is transmitted to the transmitter in step  560 . Then the packet is provided to the upper layer. 
   On the contrary, if the packet has errors, the coded bits stored in the buffer  470  are preserved and an NACK signal requesting a retransmission of the packet is transmitted to the transmitter in step  565 . 
   Packet retransmission with 16QAM used as a modulation scheme according to the embodiment of the present invention can be generalized as follows: 
   (1) coded bits are initially transmitted; 
   (2) the coded bits are inverted for modulation at a first retransmission; 
   (3) systematic bits are exchanged with parity bits prior to modulation at a second retransmission; 
   (4) the systematic bits are exchanged with the parity bits and then the coded bits are inverted prior to modulation at a third retransmission; 
   (5) the coded bits are modulated without modification in the same manner as at the initial transmission at a fourth retransmission; and 
   (6) steps (1) to (5) are repeated upon request for the next retransmissions. 
   Second Embodiment: SRRC+BIR 
     FIG. 10  is a block diagram of a transmitter in a CDMA mobile communication system according to another embodiment of the present invention. Referring to  FIG. 10 , the transmitter includes a CRC adder  610 , a channel encoder  620 , a rate matching controller  630 , an interleaver  640 , a bit rearranger  650 , a bit inverter  660 , a modulator  670 , and a transmission controller  600 . The transmitter shifts retransmission bits by a predetermined number of bits and inverts the shifted bits according to the sequence number of a retransmission. 
   Referring to  FIG. 10 , the CRC adder  610  adds CRC bits to input information bits for an error check on a packet data basis. The channel encoder  620  encodes the packet data with the CRC bits at a predetermined code rate by predetermined coding. 
   The packet data is coded to systematic bits and parity bits being error control bits for the systematic bits. Turbo coding or convolutional coding can be used. The detailed structure of the channel encoder  620  is illustrated in  FIG. 5 . 
   The code rate determines the ratio of the parity bits to the systematic bits. With a code rate of ½, for example, the channel encoder  620  outputs one systematic bit and one parity bit for the input of one information bit. With a code rate of ¾, the channel encoder  620  outputs three systematic bits and one parity bit for the input of three information bits. In the embodiment of the present invention, other code rates can also be applied aside from ½ and ¾. 
   The rate matching controller  630  matches the data rate of the coded bits by repetition or puncturing. The interleaver  640  interleaves the rate-matched bits and the interleaver output is stored in a buffer (not shown) for use at retransmission. Upon request of a receiver for a retransmission, the whole or part of the buffered bits are output under the control of the transmission controller  600 . 
   The coded bits, of which the sequence has been permuted by the interleaver  640 , are shifted in the bit rearranger  650  under the control of the transmission controller  600 . The bit rearranger  650  includes a shifter for cyclically shifting input coded bits by a predetermined number of bits. The transmission controller  600  determines whether to rearrange coded bits at the bit rearranger  650  according to the sequence number of a retransmission and the bit rearranger  650  rearranges the coded bits when the transmission controller  600  commands bit rearrangement. The bit rearranger  650  implements the SRRC. 
   For example, the transmission controller  600  disables the bit rearranger  650  at each first or second retransmission, and enables the bit rearranger  650  at each third or fourth retransmission. In the former case, the coded bits bypass the bit rearranger  650 , and in the latter case, the bit rearranger  650  cyclically shifts the coded bits by a predetermined number of, for example, two bits. 
   As described before, pairs of coded bits are mapped to different reliability parts in 16QAM or 64QAM. Hence the bit rearranger  650  cyclically shifts the coded bits of each modulation symbol by two bits so that the coded bits can be mapped to different reliability parts at a retransmission from those at an initial transmission. 
   If coded bits for initial transmission are [a, b, c, d] in 16QAM, the two upper bits [a, b] are mapped to a high reliability part and the two lower bits [c, d], to a low reliability part. At a retransmission, the coded bits [a, b, c, d] are converted to [c, d, a, b] by two-bit cyclic shifting. The two upper bits [c, d] are mapped to have a high reliability, and the two lower bits [a, b], to have a low reliability. 
   The bit inverter  660  inverts the coded bits that have passed through or bypassed the bit rearranger  650  under the control of the transmission controller  600 . The transmission controller  600  enables or disables the bit inverter  660  according to the sequence number of a retransmission. For example, the bit inverter  660  inverts the coded bits only at each odd-numbered retransmission. The bit inverter  280  is an inverter that inverts input bits 0 or 1. 
   When bit inversion is not needed, the input coded bits bypass the bit inverter  660 . This bit inverter  660  functions to map coded bits to a modulation symbol with a different error probability at a retransmission from that at an initial transmission. 
   The modulator  670  modulates input coded bits in a predetermined modulation scheme. In 16QAM, the modulator  670  maps every four input coded bits to a modulation symbol having a bit reliability pattern [H, H, L, L]. 
   The transmission controller  600  provides overall control to the components of the transmitter according to the second embodiment of the present invention. The transmission controller  600  determines the code rate of the channel encoder  620  and the modulation scheme of the modulator  670  according to the current radio channel condition. The transmission controller  600  also processes a retransmission request from an upper layer that has received a retransmission request from a receiver and controls the bit rearranger  650  and the bit inverter  660  correspondingly. 
   The retransmission request information from the upper layer indicates whether the receiver has requested a packet retransmission and how many times retransmission has been carried out so far. At a retransmission of the same packet, the bit rearranger  650  is enabled only if MOD (the sequence number of the retransmission, log 2 M) is equal to or greater than 2, and the bit inverter  660  is enabled only if MOD (the sequence number of the retransmission, 2) is 1. 
     FIG. 11  is a flowchart illustrating the operation of the transmitter according to the second embodiment of the present invention. Referring to  FIG. 11 , the CRC adder  610  adds CRC bits to input data on a packet basis in step  700  and the channel encoder  620  encodes the packet data with the CRC bits in step  705 . In step  710 , the rate matching controller  630  matches the rate of the coded bits by repetition or puncturing. The interleaver  640  interleaves the rate-matched bits in step  715 . 
   In step  720 , the transmission controller  600  determines whether a retransmission request command received from the upper layer indicates the initial transmission of a new packet or a retransmission of a previous packet. In the case of the initial transmission of the new packet, the procedure goes to step  745 . 
   In the case of a retransmission of the same packet, the transmission controller  600  calculates MOD (the sequence number of the retransmission, log 2 M) in step  725 . If the solution is equal to or greater than 2, the procedure jumps to step  735 . On the other hand, if the solution is less than 2, the transmission controller  600  enables the bit rearranger  650 . The bit rearranger  650  then rearranges the interleaver output by two bit-cyclic shifting in step  730 . 
   In step  735 , the transmission controller  600  calculates MOD (the sequence number of the retransmission, 2) to determine whether to enable the bit inverter  660 . If the solution is 0, this indicates an even-numbered retransmission and if the solution is not 0, this indicates an odd-numbered retransmission. In the former, the transmission controller  600  disables the bit inverter  660  and in the latter, it enables the bit inverter  660 . When enabled, the bit inverter  660  inverts the coded bits in step  740 . On the contrary, when the bit inverter  660  is disabled, the coded bits are directly fed to the modulator  670  without bit inversion. 
   The modulator  670  maps the input bits to symbols in step  745 . In 16QAM, every four coded bits are mapped to a modulation symbol having a reliability pattern [H, H, L, L]. The modulation symbols are spread with a predetermined spreading code and transmitted to the receiver in step  750 . 
     FIG. 12  is a block diagram of a receiver being the counterpart of the transmitter illustrated in  FIG. 10  according to the second embodiment of the present invention. Referring to  FIG. 12 , the receiver includes a demodulator  810 , a bit inverter  820 , a bit rearranger  830 , a deinterleaver unit  840 , a combiner  650 , a buffer  860 , a channel decoder  870 , a CRC checker  880 , and a reception controller  800 . 
   In operation, the demodulator  810  demodulates data received from the transmitter in a demodulation method corresponding to the modulation scheme used in the modulator  670 . The bit inverter  820  inverts the bits of the demodulated symbols under the control of the reception controller  800 . The reception controller  800  enables the bit inverter  820  only at each odd-numbered retransmission. 
   The bit inverter  820  is a multiplier that multiplies −1 by input bits selectively. Specifically, the multiplier multiplies −1 by input bits at each odd-numbered retransmission of the same packet under the control of the reception controller  800 . Thus, the multiplier performs the same function as the inverter illustrated in  FIG. 10 . If the demodulator  810  outputs coded bits expressed in hard values 0 and 1, the multiplier is replaced with an inverter. 
   The bit rearranger  830  rearranges the coded bits received from the bit inverter  820  under the control of the reception controller  800 . If the solution of MOD (the sequence number of a retransmission, log 2 M) is less than 2, the reception controller  800  disables the bit rearranger  830 . Then the coded bit streams are directly fed to the deinterleaver  840 . If the solution of MOD (the sequence number of a retransmission, log 2 M) is equal to or greater than 2, the reception controller  800  enables the bit rearranger  830  and the bit rearranger  830  rearranges the coded bits by reverse cyclic shifting in correspondence to the bit rearrangement in the transmitter. 
   The deinterleaver  840  deinterleaves the input coded bits in a deinterleaving rule corresponding to the interleaving rule used in the interleaver  640  of the transmitter. The combiner  850  combines the current received coded bits of a packet with the coded bits of the same packet accumulated in the buffer  860 . If there are no coded bits of the same packet in the buffer  860 , that is, in the case of initial transmission, the combiner  850  simply outputs the current received coded bits and simultaneously stores them in the buffer  860 . 
   The channel decoder  870  recovers the coded bits received from the combiner  850  by decoding them in a predetermined decoding method corresponding to the coding method in the channel encoder  620  of the transmitter. By decoding, systematic bits are decoded for the input of the systematic bits and parity bits. 
   The CRC checker  880  extracts CRC bits from the decoded information bits on a packet basis and determines whether the packet has errors using the extracted CRC bits. If the packet has no errors, an ACK signal for the packet is transmitted to the transmitter. On the contrary, if the packet has errors, an NACK (Non-Acknowledgement) signal for the packet is transmitted to the transmitter, requesting a retransmission of the packet. 
   If the ACK signal is transmitted to the transmitter, the buffer  860  is initialized with the coded bits of the corresponding packet deleted. If the NACK signal is transmitted to the transmitter, the coded bits of the packet remain in the buffer  870 . The reception controller  800  counts transmissions of the NACK signal to determine the sequence number of the next retransmission and control the bit inverter  820  and the bit rearranger  830  correspondingly. 
     FIG. 13  is a flowchart illustrating the operation of the receiver according to the second embodiment of the present invention. Referring to  FIG. 13 , upon receipt of data on a radio transport channel in step  900 , the demodulator  810  recovers coded bits by demodulating the received data on a modulation symbol basis according to a modulation scheme preset between the receiver and the transmitter in step  905 . In step  910 , the reception controller  800  determines whether the coded bits are an initial transmission packet or a retransmission packet. In the case of initial transmission, the reception controller  800  disables the bit inverter  820  and the coded bits bypass the bit inverter  820 . 
   In the case of retransmission, the reception controller  800  calculates MOD (the sequence number of the retransmission, 2) in step  915 . If the solution is not 0, that is, if the retransmission is an odd-numbered one, the reception controller  800  enables the bit inverter  820 . The bit inverter  820  then inverts the coded bits in step  920 . 
   In step  925 , the reception controller  800  calculates MOD (the sequence number of the retransmission, log 2 M). If the solution is less than 2, the reception controller  800  disables the bit rearranger  830  and the coded bits are directly fed to the deinterleaver  840 . On the other hand, if the solution is equal to or greater than 2, the reception controller  800  enables the bit rearranger  830  and the bit rearranger  830  rearranges the coded bits by reverse cyclic shifting in correspondence to the bit rearrangement in the bit rearranger  650  of the transmitter in step  930 . 
   The deinterleaver  840  deinterleaves the input coded bits in a deinterleaving method corresponding to the interleaving in the interleaver  640  in step  935 , and the combiner  850  combines the deinterleaved coded bits with coded bits of the same packet accumulated in the buffer  860  in step  940 . In step  945 , the channel decoder  870  decodes the combined bits in a decoding method preset between the transmitter and the receiver and outputs the original information bits. 
   In step  950 , the CRC checker  880  determines whether the packet has errors by a CRC check on the decoded information bits on a packet basis. If the packet has no errors, the buffer  860  is initialized and an ACK signal is transmitted to the transmitter in step  955 . Then the packet is provided to the upper layer. On the contrary, if the packet has errors, the coded bits stored in the buffer  860  are preserved and an NACK signal requesting a retransmission of the packet is transmitted to the transmitter in step  960 . 
   Packet retransmission with 16QAM used as a modulation scheme according to the second embodiment of the present invention can be generalized as follows: 
   (1) coded bits are initially transmitted; 
   (2) the coded bits are inverted for modulation at a first retransmission; 
   (3) the coded bits are shifted by two bits prior to modulation at a second retransmission; 
   (4) the coded bits are shifted by two bits and then inverted prior to modulation at a third retransmission; 
   (5) the coded bits are modulated without modification in the same manner as at the initial transmission at a fourth retransmission; and 
   (6) steps (1) to (5) are repeated upon request for the next retransmissions. 
     FIG. 14  illustrates graphs comparing throughputs of retransmissions according to the present invention and a conventional method in terms of frame error rates under an AWGN environment. Referring to  FIG. 14 , PRIOR ART denotes a retransmission according to the conventional method, BIR+SMP denotes a retransmission according to the first embodiment of the present invention, and BIR+SRRC denotes a retransmission according to the second embodiment of the present invention. As noted from  FIG. 14 , BIR+SRRC brings a 0.5 to 1 dB error rate decrease and BIR+SMP brings an up to 2.5 dB error rate decrease, as compared to the conventional method. 
   In accordance with the present invention as described above, a combined use of BIR and SMP or BIR and SRRC effects a remarkable performance improvement without modifying the conventional packet retransmission method. Therefore, the reliabilities and error probabilities of transmitted bits are averaged at retransmission, decoding performance is improved, and transmission efficiency is increased. 
   The present invention is applicable to all transmitters irrespective of wireless or wired communication, and it can be expected that the overall system performance will be significantly improved without an increase in system complexity. That is, a decrease in BER from the existing systems leads to an increase in transmission throughput. By application of the present invention, retransmission techniques are effectively combined, not to speak of an effective combination of an initial transmission technique and a retransmission technique, creating a synergy of benefits. 
   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.