Abstract:
An STFBC coding apparatus for a transmitter with four Tx antennas is provided. In the transmitter, an encoder generates a code symbol vector by encoding an input symbol sequence in a predetermined coding method. A grouping block permutes the elements of the code symbol vector by multiplying the code symbol vector by a permutation antenna grouping pattern selected among predetermined permutation antenna grouping patterns according to a predetermined order and outputs the permuted code symbol vector as a grouping symbol vector. An Alamouti encoder encodes the grouping symbol vector in an Alamouti scheme and transmits Alamouti-coded symbols through the four transmit antennas.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus And Method For Transmitting And Receiving Data Using Space-Time Block Coding To Increase Performance” filed in the Korean Intellectual Property Office on Nov. 4, 2004 and assigned Serial No. 2004-89484 and “Apparatus And Method For Transmitting And Receiving Data Using Space-Time Block Coding To Increase Performance” filed in the Korean Intellectual Property Office on Mar. 9, 2005 and assigned Serial No. 2005-19848, 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 space-time-frequency block coding apparatus in a transmitter with four transmit (Tx) antennas, and in particular, to an apparatus and method for transmitting an input symbol sequence through four Tx antennas according to a predetermined method using feedback information received from a receiver or using a selected transmission matrix having regularities in order to improve the performance of a space-time-frequency block code (STFBC).  
         [0004]     2. Description of the Related Art  
         [0005]     The fundamental issue in communications is the efficiency and reliability with which data is transmitted on channels. As future-generation multimedia mobile communications require high-speed communication systems capable of transmitting a variety of information including video and wireless data beyond solely voice information, it is very important to increase system efficiency through the use of a suitable channel coding method.  
         [0006]     Generally, in the wireless channel environment of a mobile communication system, unlike that of a wired channel environment, a transmission signal inevitably experiences loss due to several factors such as multipath interference, shadowing, wave attenuation, time-variant noise and fading.  
         [0007]     The information loss causes a severe distortion to the transmission signal, degrading the overall system performance. In order to reduce the information loss and increase system reliability, many error control techniques are usually adopted. Typically, the use an error correction code is employed.  
         [0008]     Multipath fading is relieved by diversity techniques in the wireless communication system. The diversity techniques include time diversity, frequency diversity and antenna diversity.  
         [0009]     The antenna diversity uses multiple antennas. This diversity scheme is further sub-divided into receive (Rx) antenna diversity using a plurality of Rx antennas, Tx antenna diversity using a plurality of Tx antennas, and multiple-input multiple-output (MIMO) using a plurality of Tx antennas and a plurality of Rx antennas.  
         [0010]     The MIMO is a special case of space-time coding (STC) that extends coding of the time domain to the space domain by transmission of a signal encoded in a predetermined coding method through a plurality of Tx antennas, in order to achieve a lower error rate.  
         [0011]     V. Tarokh et al. proposed space-time block coding (STBC) for efficiently applying antenna diversity (see “Space-Time Block Coding from Orthogonal Designs”, IEEE Trans. On Info., Theory, Vol. 45, pp. 1456-1467, July 1999). The Tarokh STBC scheme is an extension of the transmit antenna diversity scheme of S. M. Alamouti (see, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1988), for two or more Tx antennas.  
         [0012]      FIG. 1  is a block diagram of a transmitter in a mobile communication system using the conventional Tarokh STBC scheme. The transmitter is comprised of a modulator  100 , a serial-to-parallel (S/P) converter  102 , an STBC coder  104  and four Tx antennas  106 ,  108 ,  110  and  112 .  
         [0013]     Referring to  FIG. 1 , the modulator  100  modulates input information data (or coded data) in a predetermined modulation scheme. The modulation scheme can be binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), pulse amplitude modulation (PAM) or phase shift keying (PSK).  
         [0014]     The S/P converter  102  converts serial modulation symbols received from the modulator  100 , s 1 , s 2 , s 3 , s 4  into parallel symbols. The STBC coder  104  creates eight symbol combinations by STBC-encoding the four modulation symbols, s 1 , s 2 , s 3 , s 4  and sequentially transmits them through the four Tx antennas  106  to  112 . A coding matrix used to generate the eight symbol combinations is expressed as Equation (1):  
               G   4     =     [           s   1           s   2           s   3           s   4               -     s   2             s   1           -     s   4             s   3               -     s   3             s   4           s   1           -     s   2                 -     s   4             -     s   3             s   2           s   1               s   1   *           s   2   *           s   3   *           s   4   *               -     s   2   *             s   1   *           -     s   4   *             s   3   *               -     s   3   *             s   4   *           s   1   *           -     s   2   *                 -     s   4   *             -     s   3   *             s   2   *           s   1   *           ]             (   1   )             
 
 where G 4  denotes the coding matrix for symbols transmitted through the four Tx antennas  106  to  112  and s 1 , s 2 , s 3 , s 4  denote the input four symbols to be transmitted. The columns of the coding matrix represent the Tx antennas and the rows represent time required to transmit the four symbols. Thus, the four symbols are transmitted through the four Tx antennas for eight time intervals. 
 
         [0015]     Specifically, for a first time interval, s 1  is transmitted through the first Tx antenna  106 , s 2  through the second Tx antenna  108 , s 3  through the third Tx antenna  110  and s 4  through the fourth Tx antenna  112 . In this manner, − 4 *, −s 3 *, s 2 *, −s 1 * are transmitted through the first to fourth Tx antennas  106  to  112 , respectively, for an eighth time interval. That is, the STBC coder  104  sequentially provides the symbols of an i th  column in the coding matrix to an i th  Tx antenna.  
         [0016]     As described above, the STBC coder  104  generates the eight symbol sequences using the input four symbols and their conjugates and negatives and transmits them through the four Tx antennas  106  to  112  for eight time intervals. Since the symbol sequences for the respective Tx antennas are mutually orthogonal, the diversity gain achieved is as high as the diversity order.  
         [0017]      FIG. 2  is a block diagram of a receiver in the mobile communication system using the conventional STBC scheme. The receiver is the counterpart of the transmitter illustrated in  FIG. 1 .  
         [0018]     The receiver is comprised of a plurality of Rx antennas  200  to  202 , a channel estimator  204 , a signal combiner  206 , a detector  208 , a parallel-to-serial (P/S) converter  210  and a demodulator  212 .  
         [0019]     Referring to  FIG. 2 , the first to Pth Rx antennas  200  to  202  provide signals received from the four Tx antennas of the transmitter illustrated in  FIG. 1  to the channel estimator  204  and the signal combiner  206 .  
         [0020]     The channel estimator  204  estimates channel coefficients representing channel gains from the Tx antennas  106  to  112  to the Rx antennas  200  to  202  using the signals received from the first to P th  Rx antennas  200  to  202 .  
         [0021]     The signal combiner  206  combines the signals received from the 1 to P th  Rx antennas  200  to  202  with the channel coefficients in a predetermined method.  
         [0022]     The detector  208  generates hypothesis symbols by multiplying the combined symbols by the channel coefficients, calculates decision statistics for all possible transmitted symbols from the transmitter using the hypothesis symbols and detects the actual transmitted symbols through threshold detection.  
         [0023]     The P/S converter  210  converts the parallel symbols received from the detector  208  into serial symbols. The demodulator  212  demodulates the serial symbol sequence in a predetermined demodulation method, thereby recovering the original information bits.  
         [0024]     As stated earlier, the Alamouti STBC technique offers the benefit of achieving as high a diversity order as the number of Tx antennas, namely a full diversity order, without sacrificing data rate by transmitting complex symbols through only two Tx antennas.  
         [0025]     Meanwhile, the Tarokh STBC scheme achieves a full diversity order using an STBC in the form of a matrix with orthogonal columns, as described with reference to  FIGS. 1 and 2 . However, because four complex symbols are transmitted for eight time intervals, the Tarokh STBC scheme causes a half decrease in data rate. In addition, since it takes eight time intervals to completely transmit one block with four complex symbols, reception performance is reduced due to channel changes within the block over a fast fading channel. In other words, the transmission of complex symbols through four or more Tx antennas requires 2N time intervals for N symbols, causing a longer latency and a decrease in data rate.  
         [0026]     To achieve a full rate in a MIMO system that transmits a complex signal through three or more Tx antennas, the Giannakis group presented a full-diversity, full-rate (FDFR) STBC for four Tx antennas using constellation rotation over a complex field.  
         [0027]      FIG. 3  is a block diagram of a transmitter in a mobile communication system using the conventional Giannakis STBC scheme. The transmitter includes a modulator  300 , a pre-coder  302 , a space-time mapper  304 , and a plurality of Tx antennas  306 ,  308 ,  310  and  312 .  
         [0028]     Referring to  FIG. 3 , the modulator  300  modulates input information data (or coded data) in a predetermined modulation scheme such as BPSK, QPSK, QAM, PAM or PSK.  
         [0029]     The pre-coder  302  pre-encodes N t  modulation symbols received from the modulator  300 , d 1 , d 2 , d 3 , d 4  such that signal rotation occurs in a signal space, and outputs the resulting N t  symbols. For notational simplicity, four Tx antennas are assumed. The symbol d denotes a sequence of four modulation symbols from the modulator  300 . The pre-coder  302  generates a complex vector r by computing the modulation symbol sequence, d using Equation (2).  
             r   =       Θ   ⁢           ⁢   d     =         [         1         α   0   1           α   0   2           α   0   3             1         α   1   1           α   1   2           α   1   3             1         α   2   1           α   2   2           α   2   3             1         α   3   1           α   3   2           α   3   3           ]     ⁡     [           d   1               d   2               d   3               d   4           ]       =     [           r   1               r   2               r   3               r   4           ]                 (   2   )             
 
 where Θ denotes a pre-coding matrix. The Giannakis group uses a unitary Vandermonde matrix as the pre-coding matrix. In the pre-coding matrix, α i  is given as Equation (3): 
 
α i =exp( j 2π( i+ 1/4)/4), i=0,1,2,3  (3) 
 
         [0030]     The Giannakis STBC scheme uses four Tx antennas and is easily extended to more than four Tx antennas, as well. The space-time mapper  304  STBC-encodes the pre-coded symbols in the following matrix of Equation (4):  
             S   =     [           r   1         0       0       0           0         r   2         0       0           0       0         r   3         0           0       0       0         r   4           ]             (   4   )             
 
 where S is a coding matrix for symbols transmitted through the four Tx antennas  306  to  312 . The number of columns of the coding matrix is equal to that of the Tx antennas and the number of rows corresponds to the time required to transmit the four symbols. That is, the four symbols are transmitted through the four Tx antennas for the four time intervals. 
 
         [0031]     Specifically, for a first time interval, r 1  is transmitted through the first Tx antenna  306 . For a second time interval, r 2  is transmitted through the second Tx antenna  308 . For a third time interval, r 3  is transmitted through the third Tx antenna  310 . For a fourth time interval, r 4  is transmitted through the fourth Tx antenna  312 .  
         [0032]     Upon receipt of the four symbols on a radio channel for the four time intervals, a receiver (not shown) recovers the modulation symbol sequence, d by maximum likelihood (ML) decoding.  
         [0033]     Tae-Jin Jung and Kyung-Whoon Cheun proposed a pre-coder and a concatenated code with an excellent coding gain in 2003, compared to the Giannakis STBC. They enhance the coding gain by concatenating Alamouti STBCs instead of using a diagonal matrix proposed by the Giannakis group. For convenience&#39; sake, their STBC is called “Alamouti FDFR STBC”.  
         [0034]     The Alamouti FDFR STBC will be described below.  FIG. 4  is a block diagram of a transmitter in a mobile communication system using the conventional Alamouti FDFR STBC for four Tx antennas. As illustrated in  FIG. 4 , the transmitter includes a pre-coder  400 , a mapper  402 , a delay  404 , two Alamouti coders  406  and  408  and four Tx antennas  410 ,  412 ,  414  and  416 .  
         [0035]     Referring to  FIG. 4 , the pre-coder  400  pre-encodes input four modulation symbols, d 1 , d 2 , d 3 , d 4  such that signal rotation occurs in a signal space. For the input of a sequence of the four modulation symbols, d, the pre-coder  400  generates a complex vector, r by computing according to Equation (5):  
             r   =       Θ   ⁢           ⁢   d     =         [         1         α   0   1           α   0   2           α   0   3             1         α   1   1           α   1   2           α   1   3             1         α   2   1           α   2   2           α   2   3             1         α   3   1           α   3   2           α   3   3           ]     ⁡     [           d   1               d   2               d   3               d   4           ]       =     [           r   1               r   2               r   3               r   4           ]                 (   5   )             
 
 where α i =exp(j2π(i+1/4)/4), i=0,1,2,3. 
 
         [0036]     The mapper  402  groups the four pre-coded symbols in pairs and outputs two vectors each including two elements, [r 1 , r 2 ] T  and [r 3 , r 4 ] T  to the Alamouti coder  406  and the delay  404 , respectively.  
         [0037]     The delay  404  delays the second vector [r 3 , r 4 ] T  for one time interval. Thus, the first vector [r 1 , r 2 ] T  is provided to the Alamouti coder  406  in a first time interval and the second vector [r 3 , r 4 ] T  is provided to the Alamouti coder  408  in a second time interval. The Alamouti coder refers to a coder that operates in the Alamouti STBC scheme.  
         [0038]     The Alamouti coder  406  encodes [r 1 , r 2 ] T  so that it is transmitted through the first and second Tx-antennas  410  and  412  for first and second time intervals. The Alamouti coder  408  encodes [r 3 , r 4 ] T  so that it is transmitted through the third and fourth Tx antennas  414  and  416  for third and fourth time intervals. The following is a coding matrix used to transmit the four symbols from the mapper  402  through the multiple antennas as set forth in Equation (6):  
             S   =     [           r   1           r   2         0       0             -     r   2   *             r   1   *         0       0           0       0         r   3           r   4             0       0         -     r   4   *             r   3   *           ]             (   6   )             
 
         [0039]     Unlike the coding matrix illustrated in Equation (4), the above coding matrix is designed to be an Alamouti STBC rather than a diagonal matrix. The use of the Alamouti STBC scheme increases coding gain. An i th  row represents an i th  time interval and a j th  column represents a j th  Tx antenna.  
         [0040]     Thus, r 1  and r 2  are transmitted through the first and second Tx antennas  410  and  412 , respectively, for a first time interval and −r 2 * and r 1 * are transmitted through the first and second Tx antennas  410  and  412 , respectively, for a second time interval. r 3  and r 4  are transmitted through the third and fourth Tx antennas  414  and  416 , respectively, for a third time interval and −r 4 *; and r 3 * are transmitted through the third and fourth Tx antennas  414  and  416 , respectively, for a fourth time interval.  
         [0041]     This Alamouti FDFR STBC, however, has the distinctive shortcoming of increased coding complexity because the transmitter must perform pre-coding computations between all elements of the pre-coding matrix and an input vector. For example, since 0 is not included in the elements of the pre-coding matrix, computation must be carried out on 16 elements for four Tx antennas. Also, the receiver must perform ML decoding with a large volume of computation in order to decode the signal, d transmitted by the transmitter. To reduce such high complexity, Chan-Byoung Chae et al. of Samsung Electronics proposed the following matrix of Equation (7):.  
             Θ   =     [         1         α   0   1         ⋯         α   0         N   t     /   2     -   1           0       ⋯       0           0       0       ⋯       0       1       ⋯         α   1         N   t     /   2     -   1               ⋮       ⋮       ⋰       ⋯       ⋯       ⋰       ⋮           1         α       N   t     -   2     1         ⋯         α       N   t     -   2           N   t     /   2     -   1           0       ⋯       0           0       0       ⋯       0       1       ⋯         α       N   t     -   1           N   t     /   2     -   1             ]             (   7   )             
 
 where Θ is a pre-coding matrix for an arbitrary even number of Tx antennas. The subsequent operations are performed in the same manner as performed in Cheun; however, compared to the FDFR Alamouti STBC scheme, Chae&#39;s scheme remarkably reduces ML decoding complexity at the receiver through a series of puncturing and shifting operations. 
 
         [0042]     All of the above approaches suffer from high decoding complexity relative to the Alamouti scheme that allows linear decoding of transmitted symbols. Thus, continual efforts have been made to further decrease the decoding complexity. In this context, Professor Sundar Rajan&#39;s group from India (hereinafter referred to as Sundar Rajan group) presented an FDFR STBC that allows linear decoding.  
         [0043]     In this STBC, every value r i  of the coding matrix illustrated in Equation (6) is multiplied by e jθ  (i.e., rotation on a complex plane), and the real and imaginary parts of the resulting new value x i +jy i  are reconstructed. The resulting coding matrix is expressed as the following in Equation (8):  
             S   =     [             x   1     +     j   ⁢           ⁢     y   3                 x   2     +     jy   4           0       0             -       (       x   2     +     jy   4       )     *               (       x   1     +     j   ⁢           ⁢     y   3         )     *         0       0           0       0           x   3     +     j   ⁢           ⁢     y   1                 x   4     +     j   ⁢           ⁢     y   2                 0       0         -       (       x   4     +     jy   2       )     *               (       x   3     +     j   ⁢           ⁢     y   1         )     *           ]             (   8   )             
 
         [0044]     The use of Equation (8) allows linear decoding at the receiver, thus decreasing decoding complexity. The Sundar Rajan group uses a fixed phase rotation angle θ. Here, θ=(1/2)a tan 2.  
         [0045]     A mobile communication system using the Sundar Rajan group STBC scheme adopts a transmitter having the configuration illustrated in  FIG. 5 . Information symbols s 1 , s 2 , s 3 , s 4  are multiplied by exp(jθ) in a pre-coder and then reconstructed in a mapper.  
         [0046]     Specifically, the mapper reconstructs pre-coded symbols c i =x i +jy i  to c 1 ′=x 1 +jy 3 , c 2 ′=x 2 +jy 4 , c 3 ′=x 3 +jy 1 , and c 4 ′=x 4 +jy 2 , and groups the reconstructed symbols in pairs to vectors [c 2 ′c 1 ′] and [c 4 ′c 3 ′]. The vectors [c 2 ′c 1 ′] and [c 4 ′c 3 ′] are transmitted through their corresponding Alamouti coders.  
         [0047]     However, the above-described coding methods commonly increase receiver complexity in implementing an FDFR system with four Tx antennas.  
         [0048]     Accordingly, a system capable of improving performance without increasing receiver complexity is required. Thus an IEEE 802.16 system uses an STC described as an identity matrix in such a pre-coder as illustrated in  FIG. 4 . In this case, although a diversity gain is no more than 2 in a system with four Tx antennas, an existing Alamouti receiver can still be used.  
         [0049]     Yet, this system needs further improvement in performance for more accurate communications. Hence, a need exists for an apparatus and method for improving the bit error rate (BER)/frame error rate (FER) performance of a communication system using an STC represented as an identity matrix for four Tx antennas.  
       SUMMARY OF THE INVENTION  
       [0050]     An object of the present invention is to provide a transmitting apparatus and method using an STBC scheme for improving BER/FER performance in a mobile communication system with four Tx antennas.  
         [0051]     Another object of the present invention is to provide a transmitting apparatus and method using an STBC scheme for improving BER/FER performance by selecting an antenna grouping pattern based on feedback channel information from a receiver, multiplying a symbol vector by the antenna grouping pattern, and transmitting the resulting grouping symbol vector through four Tx antennas in a mobile communication system with four Tx antennas.  
         [0052]     A further object of the present invention is to provide an STBC coding apparatus and method for improving BER/FER performance by multiplying a symbol vector by a predetermined permutation antenna grouping pattern and transmitting the resulting grouping symbol vector through four Tx antennas in a mobile communication system with a plurality of Tx antennas.  
         [0053]     The above objects are achieved by providing an apparatus and method for transmitting and receiving a signal using an STBC scheme.  
         [0054]     According to one aspect of the present invention, in a transmitter with four transmit antennas in a communication system, an encoder generates a code symbol vector by encoding an input symbol sequence in a predetermined coding method. A grouping block permutes the elements of the code symbol vector by multiplying the code symbol vector by a permutation antenna grouping pattern selected among predetermined permutation antenna grouping patterns according to a predetermined order and outputs the permuted code symbol vector as a grouping symbol vector. An Alamouti encoder encodes the grouping symbol vector in an Alamouti scheme and transmits Alamouti-coded symbols through the four transmit antennas.  
         [0055]     According to another aspect of the present invention, in a transmission method for four transmit antennas in a communication system, a code symbol vector is generated by encoding an input symbol sequence in a predetermined coding method. One of predetermined permutation antenna grouping patterns is selected according to a predetermined order, the elements of the code symbol vector are permuted by multiplying the code symbol vector by the selected permutation antenna grouping pattern, and the permuted code symbol vector is output as a grouping symbol vector. The grouping symbol vector is encoded in an Alamouti scheme and transmitted through the four transmit antennas. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0056]     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:  
         [0057]      FIG. 1  is a block diagram of a transmitter in a mobile communication system using a conventional STBC scheme;  
         [0058]      FIG. 2  is a block diagram of a receiver in the mobile communication system using the conventional STBC scheme;  
         [0059]      FIG. 3  is a block diagram of a transmitter in a mobile communication system using a conventional Giannakis STBC scheme;  
         [0060]      FIG. 4  is a block diagram of a transmitter in a mobile communication system using a conventional Alamouti FDFR STBC scheme with four Tx antennas;  
         [0061]      FIG. 5  is a block diagram of a transmitter in a mobile communication system using an STBC scheme according to an embodiment of the present invention;  
         [0062]      FIG. 6  is a block diagram of a transmitter in a mobile communication system using an STFBC scheme according to another embodiment of the present invention;  
         [0063]      FIG. 7  is a block diagram of a receiver in the mobile communication system using the STBC scheme according to the present invention;  
         [0064]      FIG. 8  is a flowchart illustrating a transmission operation in the mobile communication system using the STBC scheme according to the present invention;  
         [0065]      FIG. 9  is a flowchart illustrating a reception operation of the receiver in the mobile communication system using the STBC scheme according to the present invention;  
         [0066]      FIG. 10  is a graph illustrating the uncoded BER performance of the mobile communication system using the STBC scheme according to the present invention; and  
         [0067]      FIG. 11  is a graph illustrating the coded BER/FER performance of the mobile communication system using the STBC scheme according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0068]     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.  
         [0069]     The present invention is intended to provide a technique of improving performance by grouping Tx antennas using an identity matrix intended for reducing receiver complexity or other matrices derived from the identity matrix with respect to an STC described as the following matrix A in Equation (9) in a communication system and illustrated in  FIGS. 5 and 6  in a transmitter:  
             A   =     [           s   1           -     s   2   *           0       0             s   2           s   1   *         0       0           0       0         s   3           -     s   4   *               0       0         s   4           s   3   *           ]             (   9   )             
 
 where the columns of the matrix A represent time and the rows represent Tx antennas. 
 
         [0070]      FIG. 5  is a block diagram of a transmitter in a mobile communication system using an STBC scheme according to an embodiment of the present invention. The transmitter is shown as having four Tx antennas by way of example.  
         [0071]     Referring to  FIG. 5 , a matrix A encoder  510  resides before a grouping block  520 , for generating an STC represented as the matrix A. The grouping block  520  receives the STC symbol sequence from the matrix A encoder  510  and CQI (Channel Quality Information) or a grouping index fed back from a receiver. The grouping index indicates a grouping pattern by which particular antennas are grouped to be mapped to an Alamouti encoder. The receiver selects one of an identity matrix AG 1  and other matrices AG 2  and AG 3  according to Equation (11). These matrices AG 1 , AG 2  and AG 3  represent antenna grouping patterns as illustrated in  FIGS. 5 and 6 . In the case where the transmitter receives the feedback CQI, it selects one of the matrices AG 1 , AG 2  and AG 3  by computing Equation (11).  
         [0072]     The grouping block  520  selects one of the matrices AG 1 , AG 2  and AG 3  based on the CQI or the grouping index, multiplies the matrix A by the selected matrix and maps the symbols of the resulting matrix to four Tx antennas. For instance, if a feedback grouping index indicates grouping of the first and second Tx antennas to be mapped to a first Alamouti encoder and grouping of the third and fourth Tx antennas to be mapped to a second Alamouti encoder, some input symbols are transmitted at times t 1  and t 2  through the first and second Tx antennas and the other input symbols are transmitted at times t 3  and t 4  through the third and fourth Tx antennas, whereas the columns represent time and the rows of the matrix A represent the Tx antennas.  
         [0073]     In  FIG. 5 , upon receipt of feedback CQI or a feedback grouping index from the receiver, the grouping block  520  multiplies the matrix A by one of the antenna grouping matrices AG 1 , AG 2  and AG 3  and Alamouti encoders  530  and  540  encode symbols received from the grouping block  520 . The Alamouti code symbols are expressed as one of matrices A 1 , A 2  and A 3 , which will be described later.  
         [0074]      FIG. 6  is a block diagram of a transmitter in a mobile communication system using an STFBC scheme according to another embodiment of the present invention. A matrix A encoder  610  resides before a grouping block  620 . The rows of the matrix A expressed as Equation (9) represent Tx antennas and the columns represent time and frequencies. The data of the first two columns is transmitted at frequency f 1 , and the data of the last two columns is transmitted at frequency f 2 . The data of the first column in each pair is transmitted at time t 1  and the data of the second column at time t 2 . This matrix can be used for an Orthogonal Frequency Division Multiplexing (OFDM) system.  
         [0075]     The grouping block  620  maps input information symbols to four Tx antennas based on CQI or a grouping index received from the receiver. For instance, if the feedback grouping index indicates grouping of the first and second Tx antennas to be mapped to a first Alamouti encoder and grouping of the third and fourth Tx antennas to be mapped to a second Alamouti encoder, the input symbols are transmitted according to Equation (9). That is, the first two columns are mapped to f 1  and transmitted at time t 1  and t 2  through the first and second Tx antennas, whereas the last two columns are mapped to frequency f 2  and transmitted at time t 1  and t 2  through the third and fourth Tx antennas.  
         [0076]     In  FIG. 6 , antenna grouping is applied to an STFBC and the subsequent processes are performed in the same manner as in the transmitter illustrated in  FIG. 5 .  
         [0077]      FIG. 7  is a block diagram of a receiver in the mobile communication system using the STBC scheme according to the present invention. For notational simplicity, the receiver is assumed to have a single Rx antenna.  
         [0078]     Referring to  FIG. 7 , a channel estimator  702  in the receiver performs channel estimation on a signal received through an Rx antenna  700  and outputs the resulting channel coefficients as CQI. The received signal is then decoded after processing in a detector  704 , a parallel-to-serial (P/S) converter  706  and a demodulator  708 . Meanwhile, a feedback transmitter  710  transmits the channel coefficients as CQI, or a grouping index indicating an antenna grouping pattern to the grouping block of the transmitter.  
         [0079]     The receiver transmits the CQI resulting from channel estimation or a grouping index indicating an antenna grouping pattern to the transmitter, as described above.  
         [0080]     (1) Feedback of COI  
         [0081]     Upon receipt of CQI (i.e. channel coefficients or channel values) from the receiver, the grouping block of the transmitter computes Equation (10): 
 
 arg  min|ρ 1 −ρ 2 |  (10) 
 
 where ρ 1 =|h i | 2 +|h j | 2  and ρ 2 =|h m | 2 +|h n | 2  (i, j, m, n range from 1 to 4). The grouping block receives the feedback CQI of the channels h 1 , h 2 , h 3  and h 4  between the Tx antennas and the Rx antenna and detects (i, j) and (m, n) pairs that satisfy Equation (10), thereby selecting an antenna grouping pattern. The grouping block multiplies the matrix A described as Equation (9) by the selected one of antenna grouping patterns AG 1 , AG 2  and AG 3 . The resulting matrix is one of the following matrices A 1 , A 2  and A 3  of Equation (11):  
                 A   1     =     [           s   1           -     s   2   *           0       0             s   2           s   1   *         0       0           0       0         s   3           -     s   4   *               0       0         s   4           s   3   *           ]       ⁢     
     ⁢       A   2     =     [           s   1           -     s   2   *           0       0           0       0         s   3           -     s   4   *                 s   2           s   1   *         0       0           0       0         s   4           s   3   *           ]       ⁢     
     ⁢       A   3     =     [           s   1           -     s   2   *           0       0           0       0         s   3           -     s   4   *               0       0         s   4           s   3   *               s   2           s   1   *         0       0         ]               (   11   )             
 
         [0082]     For two or more Rx antennas, the following operation is first performed. Given two Rx antennas, eight channels are defined between the four Tx antennas and the two Rx antennas. These channels are generalized to h i =(|h 1i | 2 +|h 2i | 2 )/2 where h 1i  and h 2i  denote channel values between Tx antenna i and Rx antenna 1 and between Tx antenna i and Rx antenna 2, respectively. Thus, h 11  and h 21  denote channel values between Tx antenna 1 and Rx antenna 1 and between Tx antenna 1 and Rx antenna 2, respectively, and h 1 =(|h 11 | 2 +|h 21 | 2 )/2. In the same manner, h 1  to h 4  are computed and an antenna grouping pattern is obtained by computing Equation (10) using h 1  to h 4 .  
         [0083]     (2) Feedback of Grouping Index  
         [0084]     From the perspective of system implementation, many limitations are imposed on transmission of the CQI of all channels received at the receiver to the transmitter. Hence, the receiver calculates a grouping index by Equation (10) and feeds back the grouping index to the transmitter so that the grouping block of the transmitter groups Tx antennas based on an antenna grouping pattern indicated by the grouping index. The grouping index occupies two bits to represent the antenna grouping patterns AG 1 , AG 2  and AG 3  illustrated in  FIGS. 5 and 6 .  
         [0085]      FIG. 8  is a flowchart illustrating a transmission operation in the mobile communication system using the STBC scheme according to the present invention. Upon receipt of a transmission data stream (i.e. the matrix A) in step  802 , the transmitter calculates an antenna grouping pattern by Equation (10) using CQI received from the receiver in step  806  or selects the antenna grouping pattern according to a grouping index received from the receiver in step  816 . That is, the receiver feeds back the CQI or the grouping index to the transmitter in accordance with the present invention. In step  808 , the transmitter multiplies the antenna grouping pattern by the data stream (the matrix A) and generates two symbol vectors each having two symbols. The transmitter then maps the two vectors to the Tx antennas in the space-time-frequency plane through Alamouti coding in step  810  and transmits the mapped signals through the corresponding Tx antennas in step  812 .  
         [0086]      FIG. 9  is a flowchart illustrating a reception operation of the receiver in the mobile communication system using the STBC scheme according to the present invention. Upon receipt of a transmission data stream in step  902 , the receiver performs a channel estimation on the received signal in step  904  and feeds back the resulting CQI to the transmitter in step  914 . In this case, the transmitter calculates an antenna grouping pattern based on the CQI by Equation (9). Alternatively, when agreed between the transmitter and the receiver, the receiver calculates an antenna grouping pattern by Equation (10) without feeding back the CQI and transmits a grouping index indicating the antenna grouping pattern to the transmitter. Particularly, in the case where the transmitter itself calculates the antenna grouping pattern, the transmitter notifies the receiver of the calculated antenna grouping pattern to increase the accuracy of communications. That is, when the antenna grouping pattern calculated in the transmitter is different from that obtained in the receiver, the transmitter transmits a grouping index indicating the antenna grouping pattern to the receiver on a common channel, thereby improving data transmission accuracy. The receiver then detects the received signal based on the channel coefficients resulting from the channel estimation in step  906 , converts the detected signal to a serial signal in step  908 , and demodulates the serial signal in step  910 .  
         [0087]      FIG. 10  is a graph illustrating the uncoded BER performance of the mobile communication system using the STBC scheme according to the present invention. As shown in  FIG. 10 , the present invention provides a 3 dB or above gain at a BER of 10 −3 , compared to the conventional method using only the matrix A without antenna grouping. In  FIG. 10 , w denotes with and wo denotes without. The performance curves shown in  FIG. 10  were simulated under a Rayleigh flat fading channel-QPSK environment.  
         [0088]      FIG. 11  is a graph illustrating the coded BER/FER performance of the mobile communication system using the STBC scheme according to the present invention. It is noted from  FIG. 11  that the present invention outperforms the conventional method using only the matrix A without antenna grouping. The performance curves shown in  FIG. 11  were simulated in an IEEE 802.16 system with QPSK and rate 1/2 convolutional Turbo coding. Subchannel structures, band AMC and Full Usage of SubChannel (FUSC) are defined for the IEEE 802.16a system. In the simulation, the band AMC was used.  
         [0089]     In application of the present invention to the IEEE 802.16 system being an OFDM system, the average channel values of subchannels each having N subcarriers are fed back to reduce the amount of feedback information. In this case, the transmitter calculates an antenna grouping pattern based on the average channel values and notifies the receiver of the calculated antenna grouping pattern, thereby communicating bi-directionally with accuracy.  
         [0090]     Alternatively, the receiver feeds back a grouping index to the transmitter and the transmitter selects a STBC coder corresponding to the grouping index.  
         [0091]     For example, as illustrated in Table 1 below, upon receipt of “0b  110001 ” on a CQI Channel (CQICH) from the receiver, the transmitter transmits A 1  described in Equation (11). When “0b110010” is received on the CQICH from the receiver, the transmitter transmits A 2 , whereas when “0b110011” is received on the CQICH from the receiver, the transmitter transmits A 3 .  
                   TABLE 1                       Value   Description                   0b110000   Closed-loop Adaptive Rate SM and adjacent subcarrier           permutation       0b110001   Antenna Group A1 for rate 1           For 3-antenna BS, See 8.4.8.3.4           For 4-antenna BS, See 8.4.8.3.5       0b110010   Antenna Group A2 for rate 1       0b110011   Antenna Group A3 for rate 1       0b110100   Antenna Group B1 for rate 2           For 3-antenna BS, See 8.4.8.3.4           For 4-antenna BS, See 8.4.8.3.5       0b110101   Antenna Group B2 for rate 2       0b110110   Antenna Group B3 for rate 2       0b110111   Antenna Group B4 for rate 2 (only for 4-antenna BS)       0b111000   Antenna Group B5 for rate 2 (only for 4-antenna BS)       0b111001   Antenna Group B6 for rate 2 (only for 4-antenna BS)       0b111010   Antenna Group C1 for rate 3 (only for 4-antenna BS)           See 8.4.8.3.5       0b111011   Antenna Group C2 for rate 3 (only for 4-antenna BS)       0b111100   Antenna Group C3 for rate 3 (only for 4-antenna BS)       0b111101   Antenna Group C4 for rate 3 (only for 4-antenna BS)       0b111110   Closed-loop Precoding and adjacent subcarrier           permutation       0b110001   Reserved       0b111111   Reserved                  
 
         [0092]     As described above, the receiver feeds back CQI or a grouping index to the transmitter.  
         [0093]     Without the feedback information from the receiver (i.e. a subscriber station), the subject matter of the present invention can also be achieved. In an open loop without feedback information from the receiver, the same performance improvement is achieved by using the following antenna grouping patterns in a predetermined order in the grouping block of the transmitter (i.e. a base station) so that grouping symbol vectors can be permuted as shown in Equation (12):  
               A   =     [       A   1     |     A   2     |     A   3       ]       ⁢     
     ⁢       A   1     =     [           s   1           -     s   2   *           0       0             s   2           s   1   *         0       0           0       0         s   3           -     s   4   *               0       0         s   4           s   3   *           ]       ⁢     
     ⁢       A   2     =     [           s   1           -     s   2   *           0       0           0       0         s   3           -     s   4   *                 s   2           s   1   *         0       0           0       0         s   4           s   3   *           ]       ⁢     
     ⁢       A   3     =     [           s   1           -     s   2   *           0       0           0       0         s   3           -     s   4   *               0       0         s   4           s   3   *               s   2           s   1   *         0       0         ]               (   12   )             
 
         [0094]     Permutation of the sequence of antenna grouping patterns in time leads to the increase of system performance without channel feedback. The antenna grouping patterns may be used in the sequential order of A 1 , A 2  and A 3  or in any other order.  
         [0095]     In the OFDMA communication system, the permutation order for subcarriers is determined by Equation (13): 
 
 A   k   : k =mod(floor( Nc− 1)/2,3)+1  (13) 
 
 where Nc denotes the number of a logical data subcarrier, Nc={1, 2, 3, . . . , total number of subcarriers}. The logical data subcarrier number corresponds to a subcarrier number in OFDM FFT. In Equation 13, A 1  applies to logical data subcarriers #1 and #2, A 2  applies to logical data subcarriers #3 and #4, and A 3  applies to logical data subcarriers #5 and #6. Antenna grouping patterns for the other subcarriers are decided also by Equation (13). 
 
         [0096]     As described above, the present invention provides an STFBC coding apparatus for a transmitter with four Tx antennas. An input symbol sequence is transmitted through the four Tx antennas in a predetermined method based on feedback information received from a receiver or a selected matrix with regularities. Therefore, the performance of an STFBC is improved.  
         [0097]     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.