Abstract:
An apparatus and method for canceling interference in an OFDM system using multiple antennas, where based on error estimation of a symbol received from a receive antenna, the error of a symbol received from another receive antenna is estimated. Prior to transmission, symbols to be transmitted through a plurality of transmit antennas are shifted by a predetermined number of bits without overlapping. Thus, the effects of an error of a symbol received from a receive antenna on error estimation of a symbol received from another receive antenna are reduced.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus and Method for Canceling Interference Signal in an Orthogonal Frequency Division Multiplexing System Using Multiple Antennas” filed in the Korean Intellectual Property Office on Nov. 5, 2003 and assigned Serial No. 2003-78133, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a MIMO (Multi-Input Multi-Output multiple antenna OFDM (Orthogonal Frequency Division Multiplexing) mobile communication system, and in particular, to an apparatus and method for improving the performance of an error correction code for correcting errors resulting from the effects of error propagation.  
         [0004]     2. Description of the Related Art  
         [0005]     A signal transmitted on a radio signal experiences multipath interference due to a variety of obstacles between a transmitter and a receiver. The characteristics of the multipath radio channel are determined by a maximum delay spread and signal transmission period. If the transmission period is longer than the maximum delay spread, no interference occurs between successive signals and the radio channel is characterized in the frequency domain as a frequency non-selective fading channel. However, the transmission period is shorter than the maximum delay spread at wideband high-speed transmission. As a result, interference occurs between successive signals and a received signal is subject to intersymbol interference (ISI). The radio channel is characterized in the frequency domain as a frequency selective fading channel. In the case of single carrier transmission using coherent modulation, an equalizer is required to cancel the ISI. Also, as data rate increases, ISI-incurred distortion increases and the complexity of the equalizer in turn increases. To solve the equalization problem in the single carrier transmission scheme, OFDM was proposed.  
         [0006]     In general, OFDM is defined as a two-dimensional access scheme of time division access and frequency division access in combination. An OFDM symbol is distributedly transmitted over subcarriers in a predetermined number of subchannels.  
         [0007]     In OFDM, the spectrums of subchannels orthogonally overlap with each other, having a positive effect on spectral efficiency. Also, implementation of OFDM modulation/demodulation by IFFT (Inverse Fast Fourier Transform) and FFT (Fast Fourier Transform) allows efficient digital realization of a modulator/demodulator. OFDM is robust against frequency selective fading or narrow band interference, which renders OFDM effective as a transmission scheme for European digital broadcasting and for high-speed data transmission adopted as the standards of large-volume wireless communication systems such as IEEE 802.11a, IEEE 802.16a and IEEE 802.16b.  
         [0008]     OFDM is a special case of MCM (Multi-Carrier Modulation) in which an input serial symbol sequence is converted to parallel symbol sequences and modulated to multiple orthogonal sub-carriers, prior to transmission.  
         [0009]     The first MCM systems appeared in the late 1950&#39;s for military high frequency (HF) radio communication, and OFDM with overlapping orthogonal sub-carriers was initially developed in the 1970&#39;s. In view of orthogonal modulation between multiple carriers, OFDM has limitations in actual implementation for systems. In 1971, Weinstein, et. al. proposed an OFDM scheme that applies DFT (Discrete Fourier Transform) to parallel data transmission as an efficient modulation/demodulation process, which was a driving force behind the development of OFDM. Also, the introduction of a guard interval and a cyclic prefix as the guard interval further mitigates adverse effects of multi-path propagation and delay spread on systems. As a result, OFDM has widely been exploited for digital data communications such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). Although hardware complexity was an obstacle to the wide use of OFDM, recent advances in digital signal processing technology including FFT and IFFT enable OFDM to be implemented. OFDM, similar to FDM (Frequency Division Multiplexing), boasts of optimum transmission efficiency in high-speed data transmission because it transmits data on sub-carriers, maintaining orthogonality among them. The optimum transmission efficiency is further attributed to good frequency useleading to efficiency and robustness against multipath fading in OFDM. In particular, overlapping frequency spectrums lead to efficient frequency use and robustness against frequency selective fading and multipath fading. OFDM reduces the effects of ISI by use of guard intervals and facilitates the design of a simple equalizer hardware structure. Furthermore, since OFDM is robust against impulse noise, it is increasingly popular in communication systems.  
         [0010]      FIG. 1  is a block diagram of a typical OFDM mobile communication system. Referring to  FIG. 1 , an encoder  100  encodes binary input bits and outputs coded bit streams. An interleaver  102  interleaves the serial coded bit streams and a modulator  104  maps the interleaved bit streams to symbols on a signal constellation. QPSK (Quadrature Phase Shift Keying), 8PSK (8ary Phase Shift Keying), 16QAM (16ary Quadrature Amplitude Modulation) or 64QAM (64ary QAM) has been adopted as a modulation scheme in the modulator  104 . The number of bits in one symbol is determined in correspondence with the modulation scheme used. A QPSK modulation symbol includes 2 bits, an 8PSK modulation symbol 3 bits, a 16QAM modulation symbol 4 bits, and a 64QAM modulation scheme 6 bits. An IFFT  106  processor IFFT-processes the modulated symbols and transmits the IFFT signal through a transmit antenna  108 .  
         [0011]     A receive antenna  110  receives the symbols from the transmit antenna  108 . An FFT processor  112  FFT-processes the received signal and a demodulator  114 , having the same signal constellation as used in the modulator  104 , converts despread symbols to binary symbols in a demodulation scheme. The demodulation scheme is determined in correspondence with the modulation scheme. A deinterleaver  116  deinterleaves the demodulated binary bit streams in a deinterleaving method corresponding to the interleaving method of the interleaver  102 . A decoder  118  decodes the interleaved binary bit streams.  
         [0012]      FIG. 2  is a block diagram of an OFDM mobile communication system using multiple transmit/receive antennas for data transmission/reception. Referring to  FIG. 2 , an encoder  200  encodes binary input bits and outputs a coded bit stream. A serial-to-parallel (S/P) converter  202  converts the serial coded bit stream into parallel coded bit streams, which will be described later with reference to  FIG. 4 . The parallel bit streams are provided to interleavers  204  to  206 . The interleavers  204  to  206 , modulators  208  to  210 , IFFTs  212  to  214 , and transmit antennas  216  to  218  operate in the same manner as their respective counterparts  102 ,  104 ,  106  and  108  illustrated in  FIG. 1 , except that due to the use of multiple transmit antennas, the number of subcarriers assigned to each IFFT is less than the number of subcarriers assigned to the IFFT  106  illustrated in  FIG. 1 .  
         [0013]     Receive antennas  220  to  222  receive symbols from the transmit antennas  216  to  218 . FFTs  224  to  226  FFT-process the received signal and output FFT signals to a successive interference cancellation (SIC) receiver  228 . The operation of the SIC receiver  228  will be described with reference to  FIG. 3 . The output of the SIC receiver  228  is applied to a de-orderer  230 . The SIC receiver  228  first detects a stream in a good reception state and then detects another stream using the detected stream. Because the SIC receiver  228  determines which stream is in a better reception state, a detection order is different from the order of transmitted signals. Therefore, the de-orderer  230  de-orders the transmitted signals according to their reception states. Demodulators  232  to  234  and deinterleavers  236  to  238  process the de-ordered symbols in the same manner as the demodulator  114  and the deinterleaver  116  illustrated in  FIG. 1 . A parallel-to-serial (P/S) converter  240  converts the parallel deinterleaved bit streams to a serial binary bit stream, which will be described with reference to  FIG. 4 . A decoder  242  decodes the binary bit stream.  
         [0014]     Signals transmitted from the different transmit antennas are received linearly overlapped at the receive antennas in the multiple antenna system. Hence, as the number of the transmit/receive antennas increases, decoding complexity increases. The SIC receiver uses low-computation linear receivers repeatedly to reduce the decoding complexity. The SIC receiver achieves gradually improved performance by canceling interference in a previous decoded signal. Yet, the SIC scheme has a distinctive shortcoming in that errors generated in the previous determined signal are increased in the current stage. Referring to  FIG. 3 , the structure of the SIC receiver will be described. The SIC receiver receives signals through two receive antennas by way of example. In  FIG. 3 , the signals received through the two receive antennas are y 1  and y 2 , as set forth in Equation (1): 
 
 y   1   =x   1   h   11   +x   2   h   12   +z   1  
 
 y   2   =x   1   h   21   +x   2   h   22   +x   2  
 
         [0015]     As noted from Equation (1), two transmit antennas transmit signals. In Equation (1), x 1  and x 2  are signals transmitted from first and second transmit antennas, respectively, h 11  and h 12  are a channel coefficient between the first transmit antenna and a first receive antenna and a channel coefficient between the second transmit antenna and the first receive antenna, respectively, h 21  and h 22  are a channel coefficient between the first transmit antenna and a second receive antenna and a channel coefficient between the second transmit antenna and the second receive antenna, respectively, and z 1  and Z 2  are noise on radio channels.  
         [0016]     An MMSE (Minimum Mean Square Error) receiver  300  estimates x 1  and x 2  from y 1  and y 2 . As described earlier, the SIC receiver  228  estimates the signals transmitted from the transmit antennas in a plurality of stages. The SIC receiver first estimates a signal transmitted from one transmit antenna (the first transmit antenna) and then a signal transmitted from the other transmit antenna (the second transmit antenna) using the estimated signal. In the case of three transmit antennas, the SIC receiver further estimates a signal transmitted from a third transmit antenna using the estimates of the transmitted signals from the first and second transmit antennas. The signals received at the MMSE receiver from the first and second receive antennas are shown in Equation (2): 
 
 y   1   =x   1   h   11   +z   3  
 
 y   2   =x   1   h   21   +z   4   (2) 
 
         [0017]     As noted from Equation (2), the MMSE receiver  300  estimates the signal transmitted from the second antenna as noise. By Equation (1) and Equation (2), Equation (3) is derived as follows: 
 
 z   3   =x   2   h   12   +z   1  
 
 z   4   =x   2   h   22   +z   2   (3) 
 
         [0018]     While the transmitted signal from the second transmit antenna is estimated as noise in Equation (2), the transmitted signal from the first transmit antenna can be estimated as noise, instead. In this case, as shown in Equation (4), 
 
 y   1   =x   2   h   12   +z   6  
 
 y   2   =x   2   h   22   +z   6   (4) 
 
         [0019]     The MMSE receiver  300  estimates the transmitted signal x 1  using Equation (2) according to Equation (5): 
 
 E=|A   y   −x   1 | 2   (5) 
 
 where y is the sum of y 1  and y 2 . Using Equation (5), x 1  having a minimum E is achieved. Therefore, the estimate {tilde over (x)} 1  of x 1  is calculated according to equation (6): 
 
{tilde over (x)} 1 =Ay  (6) 
 
 In the same manner, x 2  can be estimated. A stream orderer  302  prioritizes the estimates of x 1  and x 2  according to their MMSE values. That is, it determines a received signal having minimum errors on a radio channel based on the MMSE values. In the case illustrated in  FIG. 3 , x 1  has less errors than x 2 . 
 
         [0020]     The stream orderer  302  provides {tilde over (x)} 1  to the de-orderer illustrated in FIG.  2  and a decider  304 . The decider  304  decides the values of the estimated bits. Because the MMSE receiver  300  estimates the transmitted signals simply based on mathematical calculation, the estimates may be values that cannot be available for transmission. Therefore, the decider  304  decides an available value for transmission in the transmitter using the received estimate, and outputs the value to an inserter  306 . If no errors occur on the radio channel, the estimate is identical to the decided value. The inserter  306  provides the decided {tilde over (x)} 1  to calculators  308  and  310 . The calculators  308  and  310  estimate the received signals y 1  and y 2  according to Equation (7): 
 
 {overscore (y)}   1   ={tilde over (x)}   1   h   11   +x   2   h   12   +z   1  
 
 {overscore (y)}   2   ={tilde over (x)}   1   h   21   +x   2   h   22   +z   2   (7) 
 
         [0021]     An MMSE receiver  312  estimates the signal transmitted from the second transmit antenna using the estimated received signals according to Equation (8): 
 
 E=|B{overscore (y)}−x   2 | 2   (8) 
 
 where {tilde over (y)} is the sum of {tilde over (y)} 1  and {tilde over (y)} 2 . By Equation (8), x 2  resulting in a minimum E is achieved. Thus, an estimate {tilde over (x)} 2  of x 2  is calculated according to Equation (9): 
 
{overscore (x)} 2 =B{overscore (y)}  (9) 
 
 and x 2  is provided to the de-orderer  230  illustrated in  FIG. 2 . 
 
         [0022]     As described above, the SIC receiver  228  estimates another transmitted signal in a current stage using a transmitted signal estimated in the previous stage. If the transmitted signals are interleaved in the same interleaver prior to transmission and errors are generated in a particular bit during transmission, the receiver determines that its adjacent bits as well as the particular bit have errors. Now a description will be made of error generation in a signal received at a receiver from a transmitter with reference to  FIG. 4 .  
         [0023]     Referring to  FIG. 4 , reference character (A) denotes a binary bit stream including 20 bits to be transmitted. The bit stream is expressed bit by bit. Reference character (B) denotes two groups of bits separated from the 20-bit stream by the S/P converter. A first group includes odd-numbered bits and a second group includes even-numbered bits. Reference character (C) denotes the two groups of bits interleaved by the interleavers. Both groups are interleaved in the same interleaving method.  
         [0024]     Reference character (D) denotes bits having errors, #17, #7 and #3 in the first group during transmission, received at the receiver. Because the bits of the second group are estimated using estimates of the bits of the first group, the second group has errors in the same bit positions as in the first group. Thus, bits #18, #8 and #4 have errors in the second group.  
         [0025]     Reference character (E) denotes deinterleaving of the received signal by the deinterleavers. Reference character (F) denotes parallel-to-serial conversion of the deinterleaved bit streams. As indicated by (F), errors have occurred in adjacent bits. This implies that error correction performance is degraded in the receiver due to the nature of the SIC receiver. While using different interleaving patterns for different transmit antennas has been proposed as a way to overcome this problem, this scheme has drawbacks in that interleaving time is increased for each transmit antenna and the interleaving pattern of each transmit antenna must be known to the receiver. Thus, a method of solving the problem is discussed hereinbelow.  
       SUMMARY OF THE INVENTION  
       [0026]     An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide a bit-by-bit interleaving pattern that offers excellent error correction performance.  
         [0027]     Another object of the present invention is to provide, in a system that detects information in a current stage using information detected in a previous stage, an apparatus and method for reducing the effect of errors in the previous detected information on the current information detection.  
         [0028]     The above objects are achieved by providing an apparatus and method for transmitting/receiving signals through a plurality of transmit/receive antennas in a mobile communication system.  
         [0029]     According to one aspect of the present invention, in an apparatus having an encoder for encoding information bits to a coded bit stream, for transmitting signals through a plurality of transmit antennas in a mobile communication system, a serial-to-parallel converter converts the coded bit stream to a plurality of coded bit streams according to the number of the transmit antennas, an interleaver interleaves the coded bit streams, stream by stream, a plurality of modulators modulate the interleaved coded bit streams to a plurality of modulation symbol sequences, and a plurality of shifters shift the modulation symbol sequences in different patterns and transmit the shifted modulation symbol sequences through the respective transmit antennas.  
         [0030]     According to another aspect of the present invention, in an apparatus having a decoder for decoding a coded bit stream to information bits, for receiving signals through a plurality of receive antennas in a mobile communication system, a plurality of shifters shift modulation symbol sequences received through the receive antennas in the same patterns as used in a transmitting apparatus, a plurality of demodulators demodulate the shifted modulation symbol sequences to a plurality of coded bit streams, a plurality of deinterleavers deinterleave the coded bit streams, respectively, and a parallel-to-serial converter converts the deinterleaved coded bit streams to one coded bit stream.  
         [0031]     According to a further aspect of the present invention, in a method of transmitting signals through a plurality of transmit antennas in a mobile communication system having an encoder for encoding information bits to a coded bit stream, the coded bit stream is converted to a plurality of coded bit streams according to the number of the transmit antennas. The coded bit streams are interleaved, stream by stream and modulated to a plurality of modulation symbol sequences. The modulation symbol sequences are shifted in different patterns and transmitted through the respective transmit antennas.  
         [0032]     According to still another aspect of the present invention, in a method of receiving signals through a plurality of receive antennas in a mobile communication system having a decoder for decoding a coded bit stream to information bits, modulation symbol sequences received through the receive antennas are shifted in the same patterns as used in a transmitting apparatus, demodulated to a plurality of coded bit streams, deinterleaved, and converted to one coded bit stream. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     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:  
         [0034]      FIG. 1  is a block diagram of a typical OFDM mobile communication system;  
         [0035]      FIG. 2  is a block diagram of a typical multiple antenna OFDM mobile communication system;  
         [0036]      FIG. 3  is a block diagram of an SIC receiver illustrated in  FIG. 2 ;  
         [0037]      FIG. 4  sequentially illustrates data transmission and reception in the typical multiple antenna OFDM mobile communication system;  
         [0038]      FIG. 5  is a block diagram of a transmitter in a multiple antenna OFDM mobile communication system according to the present invention;  
         [0039]      FIG. 6  is a block diagram of a receiver in the multiple antenna OFDM mobile communication system according to the present invention;  
         [0040]      FIG. 7  sequentially illustrates data transmission and reception in the multiple antenna OFDM mobile communication system according to the present invention;  
         [0041]      FIG. 8  is a graph comparing the present invention with a conventional method; and  
         [0042]      FIG. 9  is another graph comparing the present invention with the conventional method. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]     A preferred embodiment 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.  
         [0044]      FIG. 5  is a block diagram of a transmitter in a multiple antenna OFDM mobile communication system according to the present invention. Referring to  FIG. 5 , an encoder  500  encodes input binary bits and outputs a coded bit stream. An S/P converter  502  divides the serial coded bit stream into as many parallel coded bit streams as the number of transmit antennas  520  to  522 . Interleavers  504  to  506  interleave the parallel coded bit streams and modulators  508  to  510  map the interleaved coded bits to modulation symbols of QPSK, 8PSK, 16QAM or 64QAM. The number of bits per symbol is determined according to the modulation scheme used. A QPSK modulation symbol has 2 bits, an 8PSK modulation symbol 3 bits, a 16QAM modulation symbol 4 bits, and a 64QAM modulation symbol 6 bits.  
         [0045]     Shifters  512  to  514  shift the modulation symbols in different patterns according to the present invention in order to prevent burst errors in the modulation symbols. The shifting of the modulation symbols will be described in connection with the number of the multiple antennas, taken as an example. Given two transmit antennas, one shifter provides a receiver modulation symbol to an IFFT without shifting, and another shifter exchanges an odd-numbered bit with an even-numbered bit in position and transmits the position-changed bits to an IFFT. Table 1 below demonstrates bit shifting in four shifters when four transmit antennas are used.  
                   TABLE 1                           Input bits   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...       First shifter (output bits)   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...       Second shifter (output bits)   2 3 4 1 6 7 8 5 10 11 12 9 14 15 16 13 ...       Third shifter (output bits)   3 4 1 2 7 8 5 6 11 12 9 10 15 16 13 14 ...       Fourth shifter (output bits)   4 1 2 3 8 5 6 7 12 9 10 11 16 13 14 15 ...                  
 
         [0046]     The shifting patterns for the input symbols can be changed according to user selection, i.e. in a different manner from Table 1. IFFTs  516  to  518  IFFT-process the shifted symbols and transmit them through the transmit antennas  520  to  522 .  
         [0047]      FIG. 6  is a block diagram of a receiver in the multiple antenna OFDM mobile communication system according to the present invention. Referring to  FIG. 6 , symbols transmitted from transmit antennas are received at receive antennas  600  to  602 . FFTs  604  to  606  FFT-process the received symbols. An SIC receiver  608  operates on the FFT symbols in the manner described before. A de-orderer  610  de-orders the output of the SIC receiver  608 . Shifters  612  to  614  shift the de-ordered symbols in the following way.  
         [0048]     The shifters  612  to  614  rearrange the bits shifted by the shifters illustrated in  FIG. 5  in the original order. Table 2 below illustrates an operation of the shifters  612  to  614  in correspondence with the shifting illustrated in Table 1.  
                       TABLE 2                       Input bits   shifters   Output bits                   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...   First shifter   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...       1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...   Second shifter   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...       3 4 1 2 7 8 5 6 11 12 9 10 15 16 13 14 ...   Third shifter   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...       4 1 2 3 8 5 6 7 12 9 10 11 16 13 14 15 ...   Fourth shifter   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ...                  
 
         [0049]     The shifted symbols are applied to demodulators  616  to  618 . The demodulators  616  to  618  convert the despread symbols to binary bits by the same signal constellation as used in the modulators of  FIG. 5 . The demodulation method depends on the modulation scheme used in the transmitter. Deinterleavers  620  to  622  deinterleave the demodulated binary bit streams in a deinterleaving method in correspondence with the interleaving method used in the interleavers of  FIG. 5 . A P/S converter  624  converts the deinterleaved bit streams to a serial bit stream. A decoder  626  decodes the binary bit stream and outputs binary information bits.  
         [0050]      FIG. 7  illustrates data symbol processing in each component of the transmitter and the receiver according to the present invention. The present invention will be compared with the conventional processing illustrated in  FIG. 4 .  
         [0051]     Referring to  FIG. 7 , reference (F) denotes a binary bit stream including 20 bits, like the transmission data illustrated in  FIG. 4 . Reference character (G) denotes two groups of bits separated from the 20-bit stream by the S/P converter. A first group includes odd-numbered bits and a second group includes even-numbered bits. Reference character (H) denotes interleaving of the two groups of bits by the interleavers. Both groups are interleaved in the same interleaving method.  
         [0052]     Reference character (I) denotes shifting of the bit symbols of either of the two groups. Since data is transmitted through two transmit antennas in the illustrated case, the transmission data is divided into two groups. As described above, the other group is not subject to shifting. As indicated by (I), the shifting is performed on a two-bit basis. That is, the first bit is exchanged with the second bit and the third bit with the fourth bit, in position. The other bits are shifted in the same manner. Since shifting indicated by (I) is given just for illustrative purposes, the shifting can be performed in a different manner.  
         [0053]     Reference character (J) denotes bits having errors, #17, #7 and #3 in the first group in the receiver. Because the bits of the second group are estimated using estimates of the bits of the first group, the second group has errors in the same bit positions as in the first group. Thus, bits #6, #10 and #14 have errors in the second group.  
         [0054]     Reference character (K) denotes shifting in the shifters of the receiver. The shifting is performed in the reverse order to the shifting done in the transmitter, to thereby rearrange received symbols in the original order before the shifting in the transmitter. That is, the symbols are returned to the original positions by shifting twice, nullifying the effect of the shifting in the transmitter.  
         [0055]     Reference character (L) denotes deinterleaving of the received signal by the deinterleavers. Reference character (M) denotes parallel-to-serial conversion of the deinterleaved bit streams. As indicated by (M), errors in adjacent bits are remarkably reduced, compared to (E) illustrated in  FIG. 4 .  
         [0056]      FIGS. 8 and 9  illustrate the effects of the present invention. Specifically,  FIG. 8  illustrates the effects of the present invention in the case where QPSK modulation symbols transmitted through two transmit antennas are received through two receive antennas and  FIG. 9  illustrates the effects of the present invention in the case where 64QAM modulation symbols transmitted through two transmit antennas are received through two receive antennas. The graphs illustrated in  FIGS. 8 and 9  demonstrate that the present invention offers far better performance than the conventional method.  
         [0057]     In accordance with the present invention as described above, the influence of errors and interference during data transmission is reduced by minimizing the effects of errors generated in a previous stage on data processing in a current stage. Also, since a plurality of interleavers/deinterleavers interleave/deinterleave in the same manner, a time delay involved in interleaving/deinterleaving is minimized.  
         [0058]     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.