Patent Publication Number: US-2021176663-A1

Title: Apparatus for transmitting and receiving data to provide high-speed data communication and method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/401,293, filed Mar. 10, 2009, which is a continuation of Ser. No. 11/767,797 filed Jun. 25, 2007, and claims priority to International Application PCT/KR2005/000393 filed Feb. 11, 2005 and Korean Application No. 10-2004-0111065, filed on Dec. 23, 2004, the disclosures of all which are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for transmitting and receiving data in radio data communication. More specifically, the present invention relates to an apparatus compatible with a conventional wireless local area network communication system, for transmitting and receiving data in high-speed and a method thereof. In addition, the present invention relates to a wireless communication system for increasing data rates from 54 Mbps which has been a maximum data rate in the conventional wireless local area network communication system, to hundreds of Mbps. 
     2. Description of the Related Art 
     In the conventional IEEE 802.11a wireless local area network (LAN) system using an orthogonal frequency division multiplexing method, a 20 MHz bandwidth is divided into 64 subcarriers, and 52 subcarriers of the 64 subcarriers are used to transmit data and pilot symbols. That is, the data are transmitted at a maximum speed of 54 Mbps by using a single antenna and the 20 MHz bandwidth. 
     The present invention provides an apparatus for transmitting and receiving data while being compatible with the conventional IEEE 802.11a orthogonal frequency division multiplexing (OFDM) method. The apparatus uses multiple antennas and a plurality of 20 MHz bandwidths to achieve a high data rate. 
     In response to the demand for high-speed multimedia data transmission, various practical applications requesting more than 100 Mbps throughput have been being developed. However, even the wireless LAN system having the greatest throughput of the current wireless communication systems does not offer over 25 Mbps of throughputs. Therefore the present invention suggests a system offering a data rate which is four times as fast as the conventional IEEE 802.11a system, or more. 
     In detail, the present invention suggests a configuration in which a number of antennas and bandwidths are systematically controlled and a maximum data rate is controlled according to characteristics of a system. The present invention also suggests a method for providing compatibility with the conventional system. 
       FIG. 1  shows a block diagram for representing a system for transmitting and receiving data in the conventional wireless LAN. 
     In the conventional IEEE 802.11a system shown in  FIG. 1 , 20 MHz bandwidth is divided into 64 subcarriers. Among the 64 subcarriers, 48 subcarriers are used for data transmission 4 subcarriers are used for pilot symbol transmission, and a DC subcarrier and the other 11 subcarriers are not used. 
     A convolutional code having ½, ⅔, and ¾ code rates, binary phase shift keying (BPSK) modulation, quaternary phase shift keying (QPSK) modulation, 16 quadrature amplitude modulation (QAM) modulation, and 64 quadrature amplitude modulation (QAM) are used to transmit the data. 
     In the system shown in  FIG. 1 , when a source unit  101  generates binary data, the binary data are provided to a scrambler  102  for randomizing a permutation of the binary data. 
     A convolution encoder  103  performs channel encoding according to a code rate and a modulation determined by a desired data rate, and a mapper  105  performs modulation to map the previous data permutation on a complex symbol permutation. 
     An interleaver  104  provided between the convolution encoder  103  and the mapper  105  interleaves the data permutation according to a predetermined rule. The mapper  105  establishes the complex number permutation to be a group of 48, and a subcarrier allocator  107  forms 48 data components and 4 pilot components from pilot unit  106 . 
     A 64 inverse fast Fourier transform (64-IFFT) unit  108  performs an inverse fast Fourier transform on the 48 data and 4 pilot components to form an OFDM symbol. 
     A cyclic prefix adder  109  adds a cyclic prefix which is a guard interval to the OFDM symbol. 
     A radio frequency (RF) transmit unit  110  transmits a transmission frame formed by the above configuration on a carrier frequency. An RF receive unit  112  receives the transmission signal (the transmission frame transmitted on the carrier frequency) through a radio channel  111 . The radio channel  111  includes a multi-path fading channel and Gaussian noise added from a receive terminal. 
     The RF receive unit  112  of the receive terminal receives the distorted signal passing through the radio channel  111 , and down-converts the signal transmitted on the carrier frequency to a base band signal in an opposite manner executed by the RF transmit unit  110  of the transmit terminal. 
     A cyclic prefix eliminator  113  eliminates the cyclic prefix added in a transmitter. A 64 fast Fourier transform (64-FFT) unit  114  converts a received OFDM symbol into a signal of a frequency domain by performing an FFT operation. 
     A subcarrier extractor  115  transmits the 48 complex symbols corresponding to the data subcarrier among 64 outputs to an equalizing and tracking unit  117 , and transmits the 4 subcarriers corresponding to the pilot to an equalizing and tracking parameter estimator  116 . 
     The equalizing and tracking parameter estimator  116  estimates a phase change caused by frequency and time errors by using the known symbols, and transmits an estimation result to the equalizing and tracking unit  117 . 
     The equalizing and tracking unit  117  uses the above estimation result to perform a tracking operation. The equalizing and tracking unit  117  also performs a frequency domain channel equalization operation for equalizing channel distortion in the frequency domain in addition to the tracking process. 
     A demapper  118  performs a hard decision operation for converting the output complex number after the channel equalizing and tracking operation into the binary data, or performs a soft decision for converting the output complex number into a real number. A deinterleaver  119  deinterleaves the data in an inverse process of the interleaver  104 , and a Viterbi decoder  120  performs decoding of the convolution code to correct errors and restore the transmitted data. 
     A descrambler  121  randomizes the data transmitted from the source unit in a like manner of the scrambler  102  and transmits the received data to a sink unit  122 . 
     The conventional wireless LAN system shown in  FIG. 1  has limits of data rate and throughput, and therefore the system is difficult to apply to a service requiring a high data rate such as a high quality moving picture service. 
     Systems using multiple bandwidths and antennas to provide a high speed data rate have previously not been compatible with the conventional transmitting and receiving system. 
     Accordingly, the present invention provides an apparatus for transmitting and receiving for providing compatibility with the conventional wireless communication system, and the high speed data rate and a method thereof. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The present invention provides a data transmitting and receiving device to provide a high data rate and compatibility with the conventional wireless communication system, and a method thereof. 
     Technical Solution 
     The present invention provides a data transmitting and receiving device to provide a high data rate and compatibility with the conventional wireless communication system, and a method thereof. 
     The present invention discloses a data transmitting device including a bandwidth distributor, an encoder, a mapper, an antenna distributor, a subcarrier allocator, an inverse Fourier transform unit, a preamble generator, and a frame generator. 
     The bandwidth distributor distributes data generated in a source unit to at least one bandwidth. The encoder performs encoding of the distributed data in order to perform error correction of the data. The mapper performs mapping of the encoded data into a complex number symbol. The antenna distributor distributes the complex number symbol to at least one antenna. The subcarrier allocator allocates a subcarrier for orthogonal frequency division multiplexing to the distributed complex number symbol. The inverse Fourier transform unit performs an inverse Fourier transform of the OFDM signal to which the subcarrier is allocated. The preamble generator generates a short preamble, a first long preamble, and a second long preamble of the subcarrier. The frame generator generates frames in an order of the short preamble, the first long preamble, a signal symbol, the second long preamble, and a data field. At this time, one of the first long preambles of a second antenna may be used for the second long preamble in order to perform a channel estimation of a subcarrier which is not used by a first antenna when two or more antennas are used. 
     The signal symbol generated by the frame generator comprises a transmit mode identifier for determining whether a transmit mode is a single antenna transmit mode or a multiple-input/multiple-output (MIMO) mode. 
     The transmit mode identifier uses an R 4  bit of the signal symbols in a frame of IEEE 802.11a. 
     A reserved bit of the signal symbol is used as a bit for determining whether the transmit mode uses a spatial division multiplexing (SDM) method or a space-time block code (STBC) method. 
     The data transmitting device according to the exemplary embodiment of the present invention further includes a scrambler, an interleaver, a cyclic prefix adder, and an RF transmit unit. 
     The scrambler is coupled between the bandwidth distributor and the encoder and performs a scrambling operation. The interleaver is coupled between the encoder and the mapper and performs an interleaving operation. The cyclic prefix adder adds a cyclic prefix to an inverse-Fourier-transformed orthogonal frequency division multiplexing (OFDM) signal. The RF transmit unit transmits the frame through a radio channel. The antenna distributor distributes the mapped symbols to antennas or encodes STBC. 
     The present invention discloses a data receiving device including an RF receiving unit, a channel mixer, an initial synchronizer, a Fourier transforming unit, a signal symbol demodulator, a channel estimator, and a detector. 
     The RF receiving unit receives a frame through a radio channel. The channel mixer performs a channel mixing operation in order to extract a 20 MHz short preamble and a 20 MHz first long preamble from the received frame. The initial synchronizer performs an initial synchronizing operation by using the extracted short preamble and first long preamble. The Fourier transforming unit performs a Fourier transforming operation of the frame. The signal symbol demodulator demodulates a signal symbol and demodulates information on a transmit mode. The channel estimator performs a first channel estimation by using the first long preamble, and performs a second channel estimation by using a second long preamble transmitting after the signal symbol when the information on the transmit mode is a MIMO-OFDM transmit mode. The detector detects a complex number symbol corresponding to the data with reference to the estimated channel and demodulated signal symbol. We detect a transmit mode identifier established in the signal symbol, and determine whether the transmit mode is a single antenna transmit mode or a MIMO-OFDM transmit mode. 
     The channel estimator uses the second long preamble to perform the second channel estimation of a subcarrier which is not used by a first antenna. 
     The data receiving device further includes a cyclic prefix eliminator, a subcarrier extractor, a demapper, a deinterleaver, and an error correction decoder. 
     The cyclic prefix eliminator eliminates a cyclic prefix of the signal received from the RF receiving unit. The subcarrier extractor extracts subcarriers from the Fourier-transformed signal and combines the subcarriers. The demapper performs demapping of the signal demodulated to the complex number signal into a binary data signal. The deinterleaver performs deinterleaving of the demapped signal. The error correction decoder performs an error correction decoding operation on the deinterleaved signal. The detector is a SDM detector or a STBC decoder. 
     Advantageous Effect 
     According to the present invention, an increased data rate is provided by using multiple bandwidths and antennas in a wireless communication system. 
     Because of compatibility with the conventional system, the increased data rate is provided without modifying the existing device and design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram for representing a conventional transmitting and receiving system in the wireless LAN. 
         FIG. 2  shows a block diagram for representing a configuration of a transmitter according to an exemplary embodiment of the present invention. 
         FIG. 3  shows a block diagram for representing a configuration of a receiver according to the exemplary embodiment of the present invention. 
         FIG. 4  shows an OFDM subcarrier allocation method supporting a single bandwidth and an OFDM subcarrier allocation method for supporting multiplex bandwidths. 
         FIG. 5  shows a diagram for representing the IEEE 802.11a frame configuration. 
         FIG. 6  shows a diagram for representing the frame configuration according to an exemplary embodiment of the present invention. 
         FIG. 7  shows a block diagram for representing a configuration for initial synchronization of the receiver according to an exemplary embodiment of the present invention. 
         FIG. 8  shows a flow chart for representing a method for transmitting the data according to an exemplary embodiment of the present invention. 
         FIG. 9  shows a flow chart for representing a method for receiving the data according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. To clarify the present invention, parts which are not described in the specification are omitted, and parts for which same descriptions are provided have the same reference numerals. 
     While this invention is described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
       FIG. 2  shows a block diagram for representing a configuration of a transmitter according to an exemplary embodiment of the present invention. 
     The transmitter includes a source unit  201 , a bandwidth distributor  202 , scrambler/convolution encoders  2031  to  203 L, an interleaver  204 , a mapper  205 , a pilot unit  206 , an antenna distributor  207 , subcarrier allocators  2081  to  208 M, IFFT units  2091  to  209 M, cyclic prefix adders  2101  to  210 M, preamble generators  2301  to  230 M, frame generators  2311  to  231 M, and RF transmit units  2111  to  211 M. 
     When binary data generated in the source unit  201  are transmitted to the bandwidth distributor  202 , the bandwidth distributor  202  distributes the binary data to L bandwidths according to a number (L) of 20 MHz bandwidths to be used in the band distributor  202 . 
     The scrambler/convolution encoders  2031  to  203 L perform scrambling and convolutional code encoding operations for the respective bandwidths. 
     The interleaver  204  receives the convolutionally encoded data. At this time, two types of interleavers  204  are available. One interleaver performs interleaving of each OFDM symbol of the respective bandwidths in a like manner of the scrambler/convolution encoders  2031  to  203 L, and the other interleaver performs interleaving of the L number of OFDM symbols in every bandwidth. The former interleaver is simple and easy to understand, and the latter interleaver is complex to be realized and it is expected to obtain performance gain due to diversity gain. 
     The mapper  205  converts the binary data into complex symbols. The converted complex symbols are distributed to M number of transmit antennas by the antenna distributor  207 . The subcarrier allocators  2081  to  208 M use pilot symbols from the pilot unit  206  and the distributed data complex symbols in order to allocate subcarriers for OFDM modulation. Allocation of the subcarriers will be described later. 
     Frequency domain OFDM symbols corresponding to the allocated M number of the transmit antennas are inverse-Fourier-transformed into time domain OFDM symbols by the (L*64)-IFFT units  2091  to  209 M. The cyclic prefix adders  2101  to  210 M add cyclic prefixes corresponding to the OFDM symbols of each path. 
     The frame generators  2311  to  231 M generate proper frames for a system shown in  FIG. 2 . Similar to the conventional IEEE 802.11a frame configuration, a frame configuration according to an exemplary embodiment of the present invention includes a short preamble, a first long preamble, a signal symbol, and data. In addition, the frame configuration includes a second long preamble in the preamble generators  2301  to  230 M. The second long preamble is a long preamble having been used in another antenna, and multiple-input/multiple-output (MIMO) channel estimation on the subcarriers is performed by the second long preamble. 
     The preamble generators  2301  to  230 M generate the short preamble, the first long preamble, and the second long preamble, and provide the same to the frame generators  2311  to  231 M. 
     The frame used in the exemplary embodiment of the present invention will be described later. 
       FIG. 3  shows a block diagram for representing a receiver according to the exemplary embodiment of the present invention. 
     The receiver shown in  FIG. 3  performs an inverse operation on the signal transmitted from the transmitter shown in  FIG. 2 . 
     The signal transmitted through the channel  212  from the transmitter is received by N number of receive antennas in N number of RF receive units  2131  to  213 N. The received signal is restored to a transmit signal while passing through cyclic prefix eliminators  2141  to  214 N, (L*64) FFT units  2151  to  215 N, subcarrier extractors  2161  to  216 N, a channel and tracking parameter estimation unit  217 , an MIMO detector  218 , a demapper  219 , a deinterleaver  220 , descrambler/Viterbi decoders  2211  to  221 L, and a bandwidth combining unit  222 , and data are transmitted to a sink unit  223 . 
     A demodulation process of the receiver shown in  FIG. 3  is similar to that of the receiver shown in  FIG. 1 . However, the channel estimation unit  217  in the receiver shown in  FIG. 3  estimates the MIMO channel, which is different from the system shown in  FIG. 1 . In addition, the equalizing unit  117  shown in  FIG. 1  is substituted to the MIMO detector  218  in the system shown in  FIG. 3 . A configuration of the deinterleaver has to be changed according to a varied configuration of the interleaver. 
     The bandwidth combining unit  222  added in the system shown in  FIG. 3  performs an inverse operation of the bandwidth distributor  202  of the transmitter shown in  FIG. 2 . 
     While the (L*64) IFFT and (L*64) FFT are used in  FIG. 2  and  FIG. 3 , L number of 64 FFTs and 64 IFFTs may be used, and one (L*64) IFFT and one (L*64) FFT may be also used. These modifications are apparent to those skilled in the art. 
       FIG. 3  shows a receiving and demodulating configuration in correspondence to the MIMO transmitter shown in  FIG. 2 , and a configuration of the receiver for performing initial synchronization and channel estimation will be described later. 
     In  FIG. 2 , a spatial division multiplexing (SDM) method for increasing the data rate by using the multiple transmit/receive antennas has been described. 
     The SDM method, one of the MIMO methods, increases the data rate by transmitting independent data via the respective transmit antennas. 
     When a system is designed for the purpose of broadening a service area and increasing a signal to noise ratio (SNR) rather than for increasing the data rate, a space-time block code (STBC) for achieving the diversity gain may be applied to the exemplary embodiment of the present invention. 
     When the STBC is applied in the exemplary embodiment of the present invention, the antenna distributor  207  is substituted for an STBC encoder, and the MIMO detector  218  is substituted for an STBC decoder. 
     For convenience of description, a system including two transmit antennas and two bandwidths will be exemplified to describe the frame configuration of the exemplary embodiment of the present invention. That is, L is 2 and M is 2 in the system shown in  FIG. 2 . The conventional frame configuration and OFDM symbol configuration are used in the exemplary embodiment of the present invention for the purpose of providing compatibility with the existing IEEE 802.11a system. 
     As to the OFDM symbol configuration, a 40 MHz bandwidth is divided into 128 subcarriers which are generated by combining two 20 MHz bandwidths each of which is divided into 64 subcarriers in the prior art in the exemplary embodiment of the present invention. Accordingly, 128-IFFT is used to perform the OFDM modulation in 20 MHZ and 40 MHz bandwidths. 
       FIG. 4  shows an OFDM subcarrier allocation method supporting a single bandwidth and an OFDM subcarrier allocation method for supporting multiplex bandwidths. 
     A subcarrier allocation configuration (a) is formed when a signal is transmitted by a single antenna and a single bandwidth in the conventional IEEE 802.11a. The configuration (b) according to the exemplary embodiment of the present invention corresponds to that of the conventional IEEE 802.11a when a signal fills a desired bandwidth, 0 fills other bandwidths, and the signal is transmitted through the single antenna. 
     That is, the data and pilot are allocated in 52 subcarriers between 0 and 63, and 0&#39;s are filled between −64 and −1 when one side bandwidth having a lower frequency is used in a signal configuration (b) using the two bandwidths of the subcarrier allocation configuration shown in  FIG. 4 . Accordingly, the system according to the exemplary embodiment of the present invention is compatible with the conventional IEEE 802.11a system because the conventional frame configuration is transmitted in the new system. 
     The frame configuration according to the exemplary embodiment of the present invention will be described. 
       FIG. 5  shows a diagram for representing the IEEE 802.11a frame configuration. 
     The IEEE 802.11a frame configuration shown in  FIG. 5  includes short preambles t 1  to t 10 , long preambles T 1  and T 2 , guard intervals G 1  and G 2 , a signal symbol SIGNAL, and data. The short preamble and the long preamble are symbols for synchronization and channel estimation in a case of demodulation. The signal symbol includes information on data rate, length, and parity. 
     The short preamble is a symbol generated by Fourier-transforming an OFDM frequency domain signal as given in Math Formula 1, and the long preamble is a symbol generated by Fourier-transforming an OFDM frequency domain signal as given in Math Formula 2. 
         S   −26,26 =√{square root over ((13/6))}·{0,0,1+ j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,−1− j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,0,0,0,0,−1− j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0}  [Math Formula 1]
 
         L   −26,26 ={1,1,−,1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}  [Math Formula 2]
 
     The signal symbol includes information on length of data sections (0 to 4,095 bytes), code rates (½, ⅔, and ¾), and mapping methods (BPSK, QPSK, 16-QAM, and 64-QAM). 
     For the purpose of providing compatibility with the IEEE 802.11a, frame configuration shown in  FIG. 5  is slightly modified for the characteristics of the multiple antennas when signals are transmitted according to the conventional OFDM mode (IEEE 802.11a) in the exemplary embodiment of the present invention. 
     When two transmit antennas are used, 52 subcarriers of preambles are equally divided by 26 subcarriers to be transmitted. A second long preamble is further provided after the signal symbol in order to estimate the channel of the subcarrier which is not used in the first long preamble. 
     The MIMO channel estimation of the subcarriers is performed by transmitting the first long preamble used as the second long preamble by another antenna. Accordingly, the length of the long preamble is increased by the number of the transmit antennas. 
     A frequency domain signal of the short preamble to be transmitted by the two antennas is given by Math  FIG. 3 . S(0)−26,26 is transmitted by the antenna 0, and S(1)−26,26 is transmitted by the antenna 1. 
     A frequency domain signal of the first long preamble provided before the signal symbol is given by Math Formula 4. L(0)−26,26 is transmitted by the antenna 0, and L(1)−26,26 is transmitted by the antenna 1. 
         S   −26,26   0 =√{square root over ((26/6))}·{0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0}
 
         S   −26,26   (1) =√{square root over ((26/6))}·{0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0}  [Math Formula 3]
 
         L   −26,26   (0) =√{square root over ( 2 )}·{1,0,−1,0,1,0,−1,0,−1,0,1,0,1,0,1,0,−1,0,1,0,1,0,1,0,1,0,0,0,−1,0,1,0,−1,0,−1,0,−1,0,−1,0,−1,0,1,0,−1,0,−1,0,−1,0,1,0,1}
 
         L   26,26   (1) =√{square root over (2)}·{0,1,0,−1,0,1,0,1,0,1,0,1,0,1,0,−1,0,1,0,−1,0,−1,0,1,0,1,0,1,0,−1,0,1,0,1,0,1,0,−1,0,−1,0,1,0,−1,0,1,0,1,0,1,0,1,0}  [Math Formula 4]
 
     In a case of the second long preamble following the signal symbol, a location of the first long preamble is changed so that L(1)−26,26 is transmitted by the antenna 0 and L(0)−26,26 is transmitted by the antenna 1. 
       FIG. 6  shows a diagram for representing the frame configuration according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 6 , the frame transmitted by the first antenna (antenna 0) uses the even subcarriers to transmit the frame, and the frame is formed by using the first long preamble of the odd subcarriers as the second long preamble. 
     The configuration of the preamble and the signal symbol are repeatedly connected to support multiple bandwidths. For example, the short preamble and long preamble for the conventional mode (dual band IEEE 802.11a) using two bandwidths are represented by Math  FIG. 5  and Math Formula 6 when the two bandwidths are used. 
         S   −58,58 =√{square root over ((13/6))}·{0,0,1+ j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,−1− j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,0,0,0,0,−1− j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1+ j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,−1− j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,0,0,0,0,−1− j, 0,0,0,−1− j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0,0,1+ j, 0,0}  [Math Formula 5]
 
         L   −58,58 ={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,}  [Math Formula 6]
 
     When the two bandwidths and two antennas are used, the short preamble and the long preamble transmitted by the respective antennas are given by Math Formula 7 and Math Formula 8. 
         S   −58,58   (0) =√{square root over ((26/6))}·{0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,−− j, 0,0,0,0,0,0,0,0,0,0,0,−1− j, 0,00,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0}
 
         S   −58,58   (1) =√{square root over ((26/6))}·={0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,0,0,0,0,−1− j, 0,0,0,0,0,0,0,1+ j, 0,0,0,0,0,0,0,1+ j, 0,0}  [Math Formula 7]
 
         L   −58,58   (0) √{square root over (2)}·{1,0,−1,0,1,0,−1,0,−1,0,1,0,1,0,1,0,−1,0,1,0,1,0,1,0,1,0,0,0,−1,0,1,0,−1,0,−1,0,−1,0,−1,0,−1,0,1,0,−1,0,−1,0,−1,0,1,0,1,0,0,0,0,0,0,0,0,0,0,0,1,0,−1,0,1,0,−1,0,−1,0,1,0,1,0,1,0,−1,0,1,0,1,0,1,0,1,0,0,0,−1,0,1,0,−1,0,−1,0,−1,0,−1,0,−1,0,1,0,−1,0,−1,0,−1,0,1,0,1}
 
         L   −58,58   (1) =√{square root over (2)}·{0,1,0,−1,0,1,0,1,0,1,0,1,0,1,0,−1,0,1,0,−1,0,−1,0,1,0,1,0,1,0,−1,0,1,0,1,0,1,0,−1,0,−1,0,1,0,−1,0,1,0,1,0,1,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,−1,0,1,0,1,0,1,0,0,0,1,0,−1,0,1,0,−1,0,−1,0,1,0,1,0,1,0,−1,0,1,0,1,0,1,0,−1,0,−1,0,1,0,−1,0,1,0,1,0,1,0,1,0}  [Math Formula 8]
 
     As described above, S(0)−58,58 is transmitted by the antenna 0 and S(1)−58,58 is transmitted by the antenna 1. L(0)−58,58 is transmitted by the antenna 0, and L(1)−58,58 is transmitted by the antenna 1. However, the second long preamble following the signal symbol is transmitted in an inverse order. 
     According to the above-described configuration, the receive terminal performs the channel estimation of the subcarriers by further performing the channel estimation using the second long preamble without determining which antenna transmits the signal in the system using the multiple bandwidths and the multiple antennas. 
     Accordingly, the long preamble is generated in the like manner of generating the long preamble by the preamble generators  2301  to  230 M shown in  FIG. 2 , and the preamble generators  2301  to  230 M additionally inserts the second long preamble after the signal symbol to generate the frame. 
     The frame generator modifies the signal symbol in order to provide compatibility with the conventional system. 
     A bit which has not been used as a reserved bit in the conventional symbol configuration is redefined as an antenna bit A, and the bit is used for discerning between the SDM and the STBC. 
     An R 4  bit of four RATE bits is used for distinguishing between the conventional IEEE 802.11a mode and the multiple antenna OFDM mode. Accordingly, the frame generator allocates the RATE bits R 1  to R 4  and the antenna bit A as shown in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 RATE, 
                   
                   
                   
                   
               
               
                 ANTENNA bit allocation. 
                 Data 
                 Mapping 
                 Code 
                 Transmit 
               
               
                 (R1-R4, A) 
                 rate 
                 method 
                 rate 
                 mode 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1101X 
                 6 
                 BPSK 
                 1/2 
                 IEEE802.11a 
               
               
                 1111X 
                 9 
                 BPSK 
                 3/4 
                 IEEE802.11a 
               
               
                 0101X 
                 12 
                 QPSK 
                 1/2 
                 IEEE802.11a 
               
               
                 0111X 
                 18 
                 QPSK 
                 3/4 
                 IEEE802.11a 
               
               
                 1001X 
                 24 
                 16QAM 
                 1/2 
                 IEEE802.11a 
               
               
                 1011X 
                 36 
                 16QAM 
                 3/4 
                 IEEE802.11a 
               
               
                 0001X 
                 48 
                 64QAM 
                 2/3 
                 IEEE802.11a 
               
               
                 0011X 
                 54 
                 64QAM 
                 3/4 
                 IEEE802.11a 
               
               
                 11000 
                 6 
                 BPSK 
                 1/2 
                 STBC-OFDM 
               
               
                 11100 
                 9 
                 BPSK 
                 3/4 
                 STBC-OFDM 
               
               
                 01000 
                 12 
                 QPSK 
                 1/2 
                 STBC-OFDM 
               
               
                 01100 
                 18 
                 QPSK 
                 3/4 
                 STBC-OFDM 
               
               
                 10000 
                 24 
                 16QAM 
                 1/2 
                 STBC-OFDM 
               
               
                 10100 
                 36 
                 16QAM 
                 3/4 
                 STBC-OFDM 
               
               
                 00000 
                 48 
                 64QAM 
                 2/3 
                 STBC-OFDM 
               
               
                 00100 
                 54 
                 64QAM 
                 3/4 
                 STBC-OFDM 
               
               
                 11001 
                 12 
                 BPSK 
                 1/2 
                 SDM-OFDM 
               
               
                 11101 
                 18 
                 BPSK 
                 3/4 
                 SDM-OFDM 
               
               
                 01001 
                 24 
                 QPSK 
                 1/2 
                 SDM-OFDM 
               
               
                 01101 
                 36 
                 QPSK 
                 3/4 
                 SDM-OFDM 
               
               
                 10001 
                 48 
                 16QAM 
                 1/2 
                 SDM-OFDM 
               
               
                 10101 
                 72 
                 16QAM 
                 3/4 
                 SDM-OFDM 
               
               
                 00001 
                 96 
                 64QAM 
                 2/3 
                 SDM-OFDM 
               
               
                 00101 
                 108 
                 64QAM 
                 3/4 
                 SDM-OFDM 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, when the R 4  bit is established to be 1, the data is received in the IEEE 802.11a method. Because the transmission mode is the IEEE 802.11a mode when the R 4  bit is 1, a value of the antenna bit A has no effect, and the configuration of the signal symbol corresponds to that of the IEEE 802.11a. 
     However, when the R 4  bit is established to be 0, the system is the MIMO system. At this time, it is determined whether the transmit mode is the SDM mode or the STBC mode with reference to the antenna bit A. 
     The R 1  to R 3  bits respectively correspond to information on eight data rates, mapping methods, and code rates. 
     Accordingly, the signal symbol is configured by combining 24 bits in alike manner of the conventional signal symbol. The 24 bits include length of 12 bits, parity of 1 bit, and tail of 6 bits. The data is transmitted on the 64 or repeated 128 (64+64) subcarriers in the conventional IEEE 802.11a mode, and the data is separately transmitted on the even subcarriers and the odd subcarriers in the multiple antenna mode as shown in Math  FIG. 4  and Math  FIG. 8 . 
     In terms of the output of the transmit antenna, predetermined preamble and signal symbol configurations are formed regardless of the number of the transmit antennas and bandwidths. 
     In the above frame configuration, a process for maintaining the compatibility by the conventional system and the system according to the exemplary embodiment in the receive terminal will be described. 
     When the data is transmitted in the conventional IEEE 802.11a system, the conventional receiver may perform a demodulation of the short preamble, the first long preamble, and the signal symbol field. However, when the signal symbol is interpreted, the data following the signal symbol is demodulated because the frame corresponds to the conventional frame when the R 4  bit of the RATE bits is 1, the data demodulation is not performed until the frame ends because the frame is not demodulated by the conventional demodulator when the R 4  bit is 0. Accordingly, the compatibility is provided in a network formed by combing the conventional system and the system according to the exemplary embodiment of the present invention. 
     The receiver of the system according to the exemplary embodiment of the present invention starts to perform demodulating of the data following the signal symbol after the receiver acknowledges that the frame is the IEEE 802.11a frame when R 4  of the signal symbols is 1. When the R 4  is 0, however, the receiver performs the channel estimation by using the second long preamble following the signal symbol, searches the antenna bit A, determines whether the transmit mode is the SDM-OFDM or the STBC-OFDM, and restores the transmit data after a proper demodulation process according to the determined mode. 
     Accordingly, the system according to the exemplary embodiment of the present invention is allowed to be compatible with the conventional IEEE 802.11a system. 
       FIG. 7  shows a block diagram for representing a configuration for an initial synchronization of the receiver according to the exemplary embodiment of the present invention. 
     In  FIG. 7 , the receiver includes DC-offset compensators  300   a  and  300   b , and inphase and quadrature (I/Q) compensators  310   a  and  310   b  for compensating I/Q mismatch, for a path of the respective antennas. The DC-offset compensators  300   a  and  300   b  eliminate a DC-offset on the path of the respective antennas which may be generated in an analog and RF circuits. The I/Q compensators  310   a  and  310   b  compensate the I/Q mismatch which may be generated in the analog and RF circuits. 
     The data before the signal symbol, which are the short preamble part and the first long preamble part, is input to a channel mixer  400 . In the channel mixer  400 , the frequency is shifted by +10 MHz and by −10 MHz in order to respectively divide two bandwidth 40 MHz signals into channel 0 of 20 MHZ and channel 1 of 20 MHz. Accordingly, two outputs are generated from the respective antenna paths. The signals pass through a low pass filter (LPF)  410  and the signals are decimated by ½ in order to convert the signals to 20 MHz bandwidth signals. The initial synchronization is performed by using the short preamble and first long preamble of 20 MHz. 
     A carrier frequency offset (CFO) estimator  430  estimates a carrier frequency offset by using an auto-correlation of the short preamble and the first long preamble. A carrier offset (CFO) compensator  320   a ,  320   b  compensates the carrier frequency offset based on the estimated value output from the CFO estimator  430 . 
     A frame synchronizer  420  performs frame synchronization by using across correlation of the short preamble and the first long preamble. A bandwidth detector  440  performs bandwidth detection for determining the operational bandwidth by using the auto correlation of the first long preamble. 
     The signal symbol including the first long preamble and the data part are input to FFT units  330   a  and  330   b  after the initial synchronization is performed. At this time, the channel is estimated and the signal symbol is demodulated by using an FFT output of the first long preamble. 
     The signal symbol is demodulated without having information on the transmit mode because a method for transmitting the signal symbol is always the same. After the signal symbol is demodulated, the information on the transmit mode, operational bandwidth, frame length, demodulation method, and code rate is provided. 
     As described above, when the R 4  is 1 (that is, when the transmit mode is the MIMO-OFDM mode), a channel estimator  450  further performs the channel estimation by using the second long preamble. 
     The data field is demodulated with reference to the information established in the signal symbol when the channel estimation is performed. 
     Phase compensators  340   a  and  340   b  estimate and compensate residual frequency and phase offsets by using the pilot subcarrier. 
     The signal is detected according to the transmit mode by the detector  300 , and the receiver combines the data passed through the demapper, the deinterleaver, the Viterbi decoder, and the descrambler and transmits the combined data to a media access control (MAC) layer. 
     Therefore, the system supporting the multiple antennas facilitates the channel estimation and provides the compatibility with the conventional system. 
       FIG. 8  shows a flowchart for representing a method for transmitting the data according to an exemplary embodiment of the present invention. 
     The binary data generated in the source unit are distributed to the plurality of bandwidths in step S 100 . The data rate may be increased as the binary data are distributed to the plurality of bandwidths. 
     The data distributed to the respective bandwidths are respectively encoded in step S 110  by exemplarily using the convolution code for increasing error correction of data. The scrambling operation may be further performed before the encoding operation. 
     The interleaving operation for preventing a burst transmit error is performed, and the binary data are mapped into a plurality of complex symbols in step S 120  when the data are encoded. The mapping method includes the BPSK, QPSK, 16QAM, and 64QAM modulations. 
     The data mapped into the complex number symbols are distributed to the antennas, and the subcarriers allocated to the respective antennas are allocated to the distributed complex symbols in step S 140 . The OFDM signals formed by allocating the subcarriers respectively perform the inverse fast Fourier transform, to transform the frequency domain signal to the time domain signal. 
     When the subcarriers are allocated, the signal fills the desired bandwidths, and 0 fills other bandwidths. The subcarriers may be also allocated such that a subcarrier used by an antenna may not be used by another antenna. 
     Not only the multiple bandwidths and antennas but also a single bandwidth and a single antenna may be also used in steps S 100  and S 130 . 
     When the single bandwidth and antenna are used, the data modulation process corresponds to that of the conventional IEEE 802.11a. 
     Accordingly, it is determined whether the OFDM signal is to be transmitted according to the MIMO transmit method using the multiple bandwidths and antennas in step S 150 . The information for determining the MIMO state is determined by searching the configuration and previous operation of the transmitter. 
     When the OFDM signal is to be transmitted according to the MIMO transmit method using the multiple antennas, the preambles for the respective subcarriers are generated in step S 160 . The preamble includes the long preamble of the operational antennas and subcarriers. The long preamble includes the first long preamble for the channel estimation of the operational subcarriers of the antenna and the second long preamble for the channel estimation of the subcarriers which are not used. 
     At this time, the first long preamble which has been used for a subcarrier by an antenna may be used for the second long preamble. 
     The signal symbol having information on the data demodulation is generated in step S 161 . The signal symbol is generated by mapping the information on the transmit mode, the data rate, the mapping method, and the code rate on the bits R 1  to R 4  and the antenna bit as shown in Table 1. 
     The data field and the frame for the MIMO antenna are generated by using the generated short preamble, the first long preamble, and the second long preamble in step S 162 . The frame is configured in an order of the short preamble, the first long preamble, the signal symbol, the second long preamble, and the data field. 
     When it is determined that the OFDM signal is not to be transmitted according to the MIMO transmit method, the frame for the single antenna is generated in step S 170  in a like manner of the conventional system. The frame for the single antenna also includes a short preamble, a long preamble, a signal symbol, and a data field. A description of the generation of the frame for the single antenna which has been described above will be omitted. 
     The frame generated by the above configuration is transmitted to the receiver through the RF transmit unit in step S 180 . 
       FIG. 9  shows a flowchart for representing a method for receiving the data according to an exemplary embodiment of the present invention. 
     In the method for receiving the data, the OFDM signal received through the radio channel is initially synchronized in step S 210 . In addition, the DC offset is eliminated by using a filter, and the I/Q discordance is compensated in step S 210 . The short preamble and the first long preamble before the signal symbol are used to perform the initial synchronization of the compensated signal. 
     The subcarrier frequency offset is estimated by using the auto-correlation of the short preamble and the first long preamble, and the frame synchronization is performed by using the cross-correlation of the short preamble and the first long preamble in step S 220 . 
     The bandwidth detection is performed for determining the operational bandwidth by using the auto correlation of the first long preamble in step S 230 . 
     A first channel estimation is performed by the fast Fourier transform of the first long preamble in step S 240 . Methods for the initial timing synchronization, frequency synchronization, and channel estimation are easily selected by those skilled in the art because a physical layer convergence procedure (PLCP) preamble which is a train signal for the synchronization has been defined in the IEEE 802.11a. 
     The receiver demodulates the signal symbol and determines the information on the signal symbol in step S 250 . The signal symbol includes information on transmit mode, data rate, mapping method, and code rate. 
     The receiver determines whether the demodulated signal symbol is transmitted from the MIMO system with reference to the transmit mode information in step S 260 . The transmit mode information is given based on an establishment value of the R 4  bit among the signal symbols. 
     When the transmit mode is the MIMO-OFDM mode, the channel estimation is performed by using the second long preamble transmitted after the signal symbol. The first long preamble of a subcarrier which is not used by another antenna is substituted for the second long preamble. Accordingly, the channel estimation on the MIMO-OFDM signal is finished when the second estimation is performed S 270 . 
     The phase offset is compensated by using the pilot subcarrier, and the data demodulation is performed according to the data rate, mapping method, and code rate in the signal symbol S 280 . The data demodulation has been described with reference to  FIG. 3 . 
     When the transmit mode is not the MIMO-OFDM mode in the previous step S 260 , the phase compensation and the data demodulation are performed without performing another channel estimation. 
     According to the exemplary embodiment of the present invention, the high-speed data rate is provided by the MIMO-OFDM system, and the compatibility with the conventional system is also provided because most of the frame configuration of the conventional single antenna OFDM system is maintained in the exemplary embodiment of the present invention. 
     While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.