Abstract:
An apparatus and method for transmitting data on a plurality of frequency sub-channels for a plurality of successive symbol periods through a plurality of transmit antennas in an OFDM system. In the data transmitting apparatus, an S/P converter converts information symbols received from a data source to an information symbol vector, a coder generates at least one code symbol vector using the information symbol vector in at least one symbol period, an IFFT unit generates at least as many transmission signal vectors as twice the number of code symbol vectors generated in the coder for the one symbol period, a plurality of P/S converters each convert the transmission signal vectors to a transmission signal stream, and a guard interval inserter inserts a guard interval into each of the transmission signal streams received from the P/S converters.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “Data Transmission Apparatus and Method in an OFDM Communication System” filed in the Korean Intellectual Property Office on Dec. 24, 2003 and assigned Serial No. 2003-96811, 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 an Orthogonal Frequency Division Multiplexing (OFDM) communication system, and in particular, to an apparatus and method for transmitting data in an OFDM communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     OFDM and space-time coding have recently received a great deal of interest as fundamental technologies for supporting high data rates required for future-generation wireless communication service. OFDM is a transmission scheme in which one serial data stream is divided into N c  parallel data streams and simultaneously transmitted on N c  sub-carriers. Given a sufficient N c  value and a sufficient guard interval, each sub-channel experiences frequency flat fading, making it possible to use a modulation scheme with a relatively high modulation order. Due to the advantages of high bandwidth efficiency and robustness under a multi-path channel environment, OFDM was adopted as the standard of a wireless LAN (Local Area Network) system such as IEEE (Institute of Electrical and Electronics Engineers) 802.11a or ETSI (European Telecommunications Standards Institute) HIPERLAN (High PERformance LAN) type2, and a broadcasting system such as DAB (Digital Audio Broadcasting) or DVB-T (Digital Video Broadcasting-Terrestrial).  
         [0006]     Space-time coding provides spatial diversity through a plurality of transmit antennas under a fading channel environment. The results of many studies on space-time trellis codes and space-time block codes under a frequency flat fading channel environment have recently been reported. Specifically, Alamouti&#39;s space-time block code offers a full diversity gain at a full rate in a system using two transmit antennas and a low decoding complexity. Therefore, it has been adopted as a standard for  3   rd  generation (3G) mobile communication systems, such as WCDMA (Wideband Code Division Multiple Access) and CDMA2000.  
         [0007]     OFDM systems using space-time block coding and space-frequency block coding based on the Alamouti&#39;s code have been proposed in the recent years. Assuming that a channel has not changed over two successive OFDM symbol periods, the Alamouti&#39;s code can be applied to the two OFDM symbols. This is called Alamouti&#39;s-Space-Time Block Code-Orthogonal Frequency Division Multiplexing (A-STBC-OFDM). If a channel has not changed with respect to adjacent sub-carriers, the Alamouti&#39;s code can be applied to the adjacent sub-carriers. This is called Alamouti&#39;s-Space-Frequency Block Code-Orthogonal Frequency Division Multiplexing (A-SFBC-OFDM).  
         [0008]      FIG. 1  is a block diagram of a conventional transmitter in an A-STBC-OFDM/A-SFBC-OFDM system using Alamouti&#39;s code. Referring to  FIG. 1 , in the conventional transmitter, a serial-to-parallel (S/P) converter  102  converts N c  information symbols received from a data source  100  to a symbol vector D s  of length N c , as is shown below in Equation (1).  
               D   s     ⁢     =   Δ     ⁢     [         D   s     ⁡     [   0   ]       ,       D   s     ⁡     [   1   ]       ,           ⁢   …   ⁢           ,       D   s     ⁡     [       N   c     -   1     ]         ]             (   1   )               
         [0009]     N c  is assumed to be equal to an IFFT (Inverse Discrete Fourier Transform) length. It is a power of 2.  
         [0010]     Using two successive symbol vectors D s  and D s+1 , an A-STBC-OFDM coder  104  generates four space-time code symbol vectors X 1,s , X 2,s , X 1,s+1  and X 2,s+1  to be transmitted in sth and (s+1)th OFDM symbol periods. The space-time code symbol vector X 1,s  can be generalized as in Equation (2),  
               X     l   ,   m       ⁢     =   Δ     ⁢     [         X     l   ,   m       ⁡     [   0   ]       ,       X     l   ,   m       ⁡     [   1   ]       ,           ⁢   …   ⁢           ,       X     l   ,   m       ⁡     [       N   c     -   1     ]         ]             (   2   )             
 
 where 1=1, 2 and m=s, s+1. X l,m [k] represents a space-time code symbol transmitted on a kth sub-carrier in an mth OFDM symbol period through an 1th transmit antenna. 
 
         [0011]     Because the A-STBC-OFDM coder  104  is based on Alamouti&#39;s space-time block code, in Equation (3),  
               [             X     1   ,   s       ⁡     [   k   ]               X     1   ,     s   +   1         ⁡     [   k   ]                   X     2   ,   s       ⁡     [   k   ]               X     2   ,     s   +   1         ⁡     [   k   ]             ]     ⁢     =   Δ     ⁢     [             D   s     ⁡     [   k   ]               D     s   +   1     *     ⁡     [   k   ]                   D     s   +   1       ⁡     [   k   ]             -       D   s   *     ⁡     [   k   ]               ]             (   3   )             
 
 where x* is the complex conjugate of x. Further, in Equations (4a), (4b), (4c), and (4d): 
 
 X   1,s   =[D   s [0],  D   s [1 ], . . . , D   s   [N   c −1]]  (4a) 
 
 X   2,s   =[D   s+1 [0 ],D   s+1 [1 ], . . . , D   s+1   [N   c −1]]  (4b) 
 
 X   1,s+1   =[D*   s+1 [0 ], D*   s+1 [1 ], . . . , D*   s+1   [N   c −1]]  (4c) 
 
 X   1,s   =[−D*   s [0 ],−D*   s [1 ], . . . , −D*   s   [N   c −1]]  (4d) 
 
         [0012]     Two IFFTs  106  and  108  inverse-discrete-Fourier-transform the space-time code symbol vectors X l,m  and outputs four signal vectors x l,m , as shown below in Equation (5):  
               x     l   ,   m       ⁢     =   Δ     ⁢     [         x     l   ,   m       ⁡     [   0   ]       ,       x     l   ,   m       ⁡     [   1   ]       ,           ⁢   …   ⁢           ,       x     l   ,   m       ⁡     [       N   c     -   1     ]         ]             (   5   )             
 
 where x l,m [n] is an nth sample of an OFDM modulation symbol to be transmitted in an mth OFDM symbol period through an 1th transmit antenna. x l,m [n] is expressed in Equation (6),  
                 x     l   ,   m       ⁡     [   n   ]       ⁢     =   Δ     ⁢       1     N   c       ⁢       ∑     k   =   0         N   c     -   1       ⁢         X     l   ,   m       ⁡     [   k   ]       ⁢           ⁢     W   n     -   nk                     (   6   )             
 
 where n=0, 1, . . . , N c−1  and  
         W   Nc   m     ⁢     =   Δ     ⁢       ⅇ       -   j     ⁢       2   ⁢           ⁢   π   ⁢           ⁢   m       N   c           .         
 
         [0013]     Parallel-to-serial (P/S) converters  110  and  112  convert the samples x l,m [n] to serial data streams. CP (Cyclic Prefix) inserters  114  and  116  insert CPs into the serial data streams and transmit them through transmit antennas  118  and  120 , respectively.  
         [0014]     As described above, the A-STBC-OFDM transmitter performs four IFFT operations for two successive OFDM symbol periods and the IFFTs  106  and  108  are required for the individual transmit antennas  118  and  120 .  
         [0015]     Because an A-SFBC-OFDM transmitter is the same in structure as the A-STBC-OFDM transmitter, except for using an A-SFBC-OFDM coder rather than an A-STBC-OFDM coder, the A-SFBC-OFDM transmitter will be described herein below using  FIG. 1 . However, in this description, the A-STBC-OFDM coder  104  will be replace with an A-SFBC-OFDM coder  104 .  
         [0016]     As in the A-STBC-OFDM transmitter, in an A-SFBC-OFDM transmitter, the S/P converter  102  converts N c  information symbols received from the data source  100  to the symbol vector D s  of length N c  expressed in Equation (1).  
         [0017]     Using the symbol vector D s , an A-SFBC-OFDM coder  104  generates two space-frequency code symbol vectors X 1,s  and X 2,s  to be transmitted in the sth OFDM symbol period. The space-frequency code symbol vector X 1,s  is generalized in Equation (7),  
               X     l   ,   s       ⁢     =   Δ     ⁢     [         X     l   ,   s       ⁡     [   0   ]       ,       X     l   ,   s       ⁡     [   1   ]       ,           ⁢   …   ⁢           ,       X     l   ,   s       ⁡     [       N   c     -   1     ]         ]             (   7   )             
 
 where 1=1, 2 and X l,s [k] represents a space-frequency code symbol transmitted on a kth sub-carrier in the sth OFDM symbol period through an 1th transmit antenna. 
 
         [0018]     Because the A-SFBC-OFDM coder  104  is based on Alamouti&#39;s space-time block code, in Equation (8),  
               [             X     1   ,   s       ⁡     [     2   ⁢   v     ]               X     1   ,   s       ⁡     [       2   ⁢   v     +   1     ]                   X     2   ,   s       ⁡     [     2   ⁢   v     ]               X     2   ,   s       ⁡     [       2   ⁢   v     +   1     ]             ]     ⁢     =   Δ     ⁢     [             D   s     ⁡     [     2   ⁢   v     ]               D   s     ⁡     [       2   ⁢   v     +   1     ]                 -       D   s   *     ⁡     [       2   ⁢   v     +   1     ]                 D   s   *     ⁡     [     2   ⁢   v     ]             ]             (   8   )             
 
 where k=2v,2v+1,v=0,1, . . . 
           N   c     2     -   1.       
 
 Further, in Equations (9a) and (9b), 
 
 X   1,s   =[D   s [0],  D   s [1 ], D   s   [N   c −2 ],D   s   [N   c −1]]  (9a) 
 
 X   2,s   =└−D*   s *[1 ], D*   s *[0 ], . . . , −D*   s   [N   c −1 ], D*   s   [N   c −2]┘  (9b) 
 
         [0019]     The two IFFTs  106  and  108  inverse-discrete-Fourier-transform the space-frequency code symbol vectors X l,s  and outputs two signal vectors x l,s , as shown below in Equation (10):  
               x     l   ,   s       ⁢     =   Δ     ⁢     [         x     l   ,   s       ⁡     [   0   ]       ,       x     l   ,   s       ⁡     [   1   ]       ,           ⁢   …   ⁢           ,       x     l   ,   s       ⁡     [       N   c     -   1     ]         ]             (   10   )             
 
 where x l,s [n] is an nth sample of an OFDM modulation symbol to be transmitted in the sth OFDM symbol period through the 1th transmit antenna. x l,s [n] is expressed in Equation (11).  
                 x     l   ,   s       ⁡     [   n   ]       ⁢     =   Δ     ⁢       1     N   c       ⁢           ⁢       ∑     k   =   0         N   c     -   1       ⁢         X     l   ,   s       ⁡     [   k   ]       ⁢           ⁢     W   N     -   nk                     (   11   )             
 
         [0020]     The P/S converters  110  and  112  convert the samples x l,s [n] to serial data streams. The CP inserters  114  and  116  insert CPs into the serial data streams and transmit them through the transmit antennas  118  and  120 , respectively.  
         [0021]     As described above, the A-SFBC-OFDM transmitter performs two IFFT operations for one OFDM symbol period and the IFFTs  106  and  108  are required for the individual transmit antennas  118  and  120 .  
         [0022]      FIG. 2  is a block diagram of a typical transmitter in a conventional A-STBC-OFDMIA-SFBC-OFDM system. It is noted from  FIG. 2  that the number of IFFT operations increases in proportion of the number of transmit antennas.  
         [0023]     In the above-described conventional A-STBC-OFDMI A-SFBC-OFDM transmitter, an IFFT operation is performed for each transmit antenna to generate a transmission signal. Therefore, computation complexity is high and power consumption is increased.  
         [0024]     Aside from Alamouti&#39;s code-based OFDM systems, OFDM systems using space-time/space-frequency block coding based on space-time block codes require more transmit antennas perform IFFT operations in proportion to the number of transmit antennas. Consequently, the implementation complexity of transmitters is considerably increased. Therefore, there is a need for a method of reducing transmitter implementation complexity in an OFDM system based on space-time/space-frequency block coding.  
       SUMMARY OF THE INVENTION  
       [0025]     An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide a transmitting apparatus and method for reducing a number of IFFT operations required to generate transmission data in an OFDM system in which data is transmitted on a plurality of sub-carriers through a plurality of antennas.  
         [0026]     Another object of the present invention is to provide a transmitting apparatus and method for decreasing system implementation complexity by reducing a number of IFFT operations required to generate transmission data in an OFDM system in which data is transmitted on a plurality of sub-carriers through a plurality of antennas.  
         [0027]     The above and other objects are achieved by providing an apparatus and method for transmitting data on a plurality of frequency sub-channels for a plurality of successive symbol periods through a plurality of transmit antennas in an OFDM system.  
         [0028]     In the data transmitting apparatus, an S/P converter converts information symbols received from a data source to an information symbol vector, a coder generates at least one code symbol vector using the information symbol vector in at least one symbol period, an IFFT unit generates at least as many transmission signal vectors as twice the number of code symbol vectors generated in the coder for the one symbol period, a plurality of P/S converters each convert the transmission signal vectors to a transmission signal stream, and a guard interval inserter inserts a guard interval into each of the transmission signal streams received from the P/S converters and transmits the resulting signals through the transmit antennas.  
         [0029]     In the data transmitting method, information symbols received from a data source are converted to an information symbol vector. At least one code symbol vector is generated using the information symbol vector in at least one symbol period. At least as many transmission signal vectors as twice the number of code symbol vectors generated for the one symbol period are generated and each of the transmission signal vectors is converted to a transmission signal stream. A guard interval is inserted into each of the transmission signal streams and the guard interval-having transmission signal streams are transmitted through the transmit antennas. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     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:  
         [0031]      FIG. 1  is a block diagram of a transmitter in a conventional A-STBC-OFDMIA-SFBC-OFDM system using Alamouti&#39;s code;  
         [0032]      FIG. 2  is a block diagram of a typical transmitter in a conventional A-STBC-OFDM/A-SFBC-OFDM system;  
         [0033]      FIG. 3  is a block diagram of a transmitting apparatus according to an embodiment of the present invention;  
         [0034]      FIG. 4  is a detailed block diagram of an auxiliary converter in the transmitting apparatus illustrated in  FIG. 3 ;  
         [0035]      FIG. 5  is a block diagram of a transmitting apparatus according to another embodiment of the present invention;  
         [0036]      FIG. 6  is a block diagram of a transmitting apparatus according to a third embodiment of the present invention;  
         [0037]      FIG. 7  is a detailed block diagram of an IFFT (Inverse Fast Fourier Transformer) unit and an auxiliary converter in the transmitting apparatus illustrated in  FIG. 6 ; and  
         [0038]      FIG. 8  is a block diagram of a transmitting apparatus according to a fourth embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]     Preferred embodiments of the present invention will be described in detail 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.  
         [0040]      FIG. 3  is a block diagram of a transmitting apparatus according to an embodiment of the present invention. Referring to  FIG. 3 , the transmitting apparatus includes a data source  300 , an S/P converter  302  for converting N c  information symbols received from the data source  300  to an information symbol vector D s  (it is assumed herein that N c  is a power of 2, equal to an IFFT length used in the transmitting apparatus), an A-STBC-OFDM coder  304  for generating four space-time code symbol vectors X 1,s , X 2,s , X 1,s+1 , and X 2,s+1  for the input of two information symbol vectors D s  and D s+1  from the S/P converter  302 , a pair of IFFT units  306  and  308  for inverse-fast-Fourier-transforming the space-time code symbol vectors X 1,s  and X 2,s  to be transmitted in parallel in an sth OFDM symbol period and outputting transmission signal vectors x 1,s  and x 2,s , and auxiliary converters  310  and  312  for generating transmission signal vectors x 1,s+1  and x 2,s+1  to be transmitted in an (s+1)th OFDM symbol period, from the transmission signal vectors x 1,s  and x 2,s . A pair of P/S converters  318  and  320  selectively receive x 1,s  and x 2,s  or x 1,s+1  and x 2,s+1  and convert them to transmission signal streams x 1,s [n]/x 1,s+1 [n] and x 2,s [n]/x 2,s+1 [n] (n=0,1, . . . ,N x ). A pair of switches  314  and  316  switch x 1,s  and x 2,s  or x 1,s+1  and x 2,s+1  to the P/S converters  318  and  320 , and a pair of CP inserters  322  and  324  insert CPs into x 1,s [n]/x 1,s+1 [n] and x 2,s [n]/x 2,s+1 [n].  
         [0041]     The first switch  314  switches the output port of the first IFFT  306  to the input port of the first P/S converter  318  in the sth OFDM symbol period, and the output port of the second auxiliary converter  312  to the input port of the first P/S converter  318  in the (s+1)th OFDM symbol period. The second switch  316  switches the output port of the second IFFT  308  to the input port of the second P/S converter  320  in the sth OFDM symbol period, and the output port of the first auxiliary converter  310  to the input port of the second P/S converter  320  in the (s+1)th OFDM symbol period.  
         [0042]      FIG. 4  is a detailed block diagram of the auxiliary converters  310  and  312  illustrated in  FIG. 3 . Referring to  FIG. 4 , the auxiliary converter  310  (or  312 ) at the output end of the IFFT unit  306  (or  308 ) includes a bypass module  402  for outputting the transmission signal vector x 1,s  (or x 2,s ) received from the IFFT unit  306  (or  308 ), a negation module  400  for negating x 1,s  (or x 2,s ), a selection module  404  for selecting one of the outputs of the bypass module  400  and the negation module  402 , a conjugation module  406  for calculating the complex conjugate of the selected signal, and a rearrangement module  408  for rearranging the complex conjugate.  
         [0043]     The first auxiliary converter  310  selects the negated value of x 1,s  output from the negation module  402 , complex-conjugates the negated value of x 1,s , rearranges the complex conjugate, and outputs the transmission signal x 2,s+1  for the (s+1)th OFDM symbol period. The second auxiliary converter  312  selects the negated value of x 2,s  output from the negation module  402 , complex-conjugates the negated value of x 2,s , rearranges the complex conjugate, and outputs the transmission signal x 1,s+1 , for the (s+1)th OFDM symbol period.  
         [0044]     The space-time code symbol vectors X 1,s , X 2,s , X 1,s+1  and X 2,s+1  generated in the A-STBC-OFDM coder  304  are mutually correlated in the relation shown in Equations (12a) and (12b) below.  
                 X     1   ,     s   +   1         ⁡     [   k   ]       =       X     2   ,   s     *     ⁡     [   k   ]               (     12   ⁢   a     )                   X     2   ,     s   +   1         ⁡     [   k   ]       =     -       X     1   ,   s     *     ⁡     [   k   ]                 (     12   ⁢   b     )             
 
         [0045]     FFT is symmetrical to IFFT. Therefore, in Equation (13),  
                         
 
 where ((n)) N  denotes n modulo N. From Equation (6), Equation (12a), and Equation (12b), Equations (14a) and (14b) are:  
                 x     1   ,     s   +   1         ⁡     [   n   ]       =       x     2   ,   s     *     ⁡     [       (     (     -   n     )     )     N     ]               (     14   ⁢   a     )             and                             x     2   ,     s   +   1         ⁡     [   n   ]       =     -       x     1   ,   s     *     ⁡     [       (     (     -   n     )     )     N     ]                 (     14   ⁢   b     )             
 
 where n=0,1, . . . , N c −1. 
 
         [0046]     According to the correlation between transmission signal vectors as represented by Equation (14a) and Equation (14b), x 1,s [n] and x 2,s [n] are generated through two IFFT operations for the first OFDM symbol period, whereas x 1,s+1 [n] and x 2,s+1 [n] are generated for the second OFDM symbol period by negating, complex-conjugating, and rearranging x l,s [n] and X 2,s [n].  
         [0047]     More specifically, x l,s+1  to be transmitted through a first antenna  326  in the (s+1)th OFDM symbol period is produced by allowing x 2,s , which will be transmitted through a second antenna  328  in the sth OFDM symbol period, to bypass to the selection module  404  by the bypass module  400 , selecting x 2,s  by the selection module  404 , complex-conjugating x 2,s  by the conjugation module  406 , and rearranging the complex conjugate by the rearrangement module  408 . x 1,s+1  is transmitted to the first antenna  326  by switching the output port of the second auxiliary converter  312  to the input port of the first P/S converter  318  in the first switch  314  in the (s+1)th OFDM symbol period.  
         [0048]     x 2,s+1  to be transmitted through the second antenna  328  in the (s+1)th OFDM symbol period is produced by negating x 1,s , which will be transmitted through the first antenna  326  in the sth OFDM symbol period, by the negation module  402 , selecting the negated value of x 1,s  by the selection module  404 , complex-conjugating x 1,s  by the conjugation module  406 , and rearranging the complex conjugate by the rearrangement module  408 . x 2,s+1  is transmitted to the second antenna  328  by switching the output port of the first auxiliary converter  410  to the input port of the second P/S converter  320  in the second switch  316  in the (s+1)th OFDM symbol period.  
         [0049]      FIG. 5  is a block diagram of a transmitting apparatus according to another embodiment of the present invention. N t  denotes the number of transmit antennas and N x  denotes the number of successive OFDM symbols.  
         [0050]     Referring to  FIG. 5 , the transmitting apparatus includes a data source  500 , an S/P converter  502  for converting information symbols received from the data source  500  to an information symbol vector, an A-STBC-OFDM coder  504  for generating space-time code symbol vectors for the input of the information symbol vector, a plurality of IFFT units  506  to  508  for inverse-fast-Fourier-transforming the space-time code symbol vectors and outputting transmission signal vectors, an auxiliary converter  510  for generating transmission signal vectors to be transmitted in the next OFDM symbol period, from the transmission signal vectors received from the IFFT units  506  to  508 , P/S converters  516  to  518  for selectively receiving the transmission signal vectors from the IFFT units  508  and  508  and the auxiliary converter  510  and converting them to transmission signal streams, a plurality of switches  512  to  514  for switching the transmission signal vectors from the IFFY units  508  and  508  and the auxiliary converter  510  to the P/S converters  516  to  518  according to OFDM symbol periods, and a plurality of CP inserters  520  to  522  for inserting CPs into the transmission signal streams received from the P/S converters  516  to  518 .  
         [0051]     The components of the transmitting apparatus according to the second embodiment of the present invention operate in a similar manner to those of the transmitting apparatus according to the first embodiment of the present invention, except that the operation is performed with respect to N t  transmit antennas and N x  successive OFDM symbol periods.  
         [0052]     The space-time code symbol vectors output from the A-STBC-OFDM coder  504  are mutually correlated in the relation shown in Equation (15a) and (15b).  
                 X       l   2     ,     m   2         =     ±     X       l   1     ,   s           ,       m   2     ∈     {       s   +   1     ,     s   +   2     ,   ⋯   ⁢           ,     s   +     N   x     -   1       }       ,           (     15   ⁢   a     )                       ⁢         l   1     ∈     {     1   ,   2   ,     ⋯   ⁢           ⁢     N   t         }       ,       l   2     ∈     {     1   ,   2   ,     ⋯   ⁢           ⁢     N   t         }                                     X       l   2     ,     m   2         =     ±     X       l   1     ,   s     *         ,       m   2     ∈     {       s   +   1     ,     s   +   2     ,   ⋯   ⁢           ,     s   +     N   x     -   1       }       ,           (     15   ⁢   b     )                       ⁢         l   1     ∈     {     1   ,   2   ,     ⋯   ⁢           ⁢     N   t         }       ,       l   2     ∈     {     1   ,   2   ,     ⋯   ⁢           ⁢     N   t         }                               
 
         [0053]     In this case, the IFFT units  506  to  508  output a transmission signal vector x l     1     ,s  for the input of the space-time code symbol vector X l     1     ,s . x l     1     ,s  is converted to a serial signal stream in the P/S converters  516  to  518 , added with a CP in the CP inserters  520  to  522 , and transmitted through the transmit antennas  524  to  526 .  
         [0054]     According to Equations (15a) and (15b), the auxiliary converter  510  outputs a transmission signal vector x l     2     ,m     2    for the input of the signal vector x l     1     ,s  of length N c . X 1     2     ,m     2    is converted to a serial signal stream in the P/S converters  516  to  518 , added with a CP in the CP inserters  520  to  522 , and transmitted through the transmit antennas  524  to  526 .  
         [0055]      FIG. 6  is a block diagram of a transmitting apparatus according to a third embodiment of the present invention. The transmitting apparatus is characterized by transmitting two signals through one IFFT operation utilizing the symmetry between IFFT and IFFT.  
         [0056]     Referring to  FIG. 6 , the transmitting apparatus includes a data source  600 , an S/P converter  602  for converting N c  information symbols received from the data source  600  to an information symbol vector D s  (it is assumed herein that N c  is a power of 2, equal to an IFFT length used in the transmitting apparatus), an A-SFBC-OFDM coder  604  for generating two space-frequency code symbol vectors of length N c , X 1,s  and X 2,s  for the input of one information symbol vector D s  from the S/P converter  602 , an IFFT; unit  606  for inverse-fast-Fourier-transforming the space-frequency code symbol vectors X 1,s  and outputting a transmission signal vector x 1,s  for an sth OFDM symbol period, an auxiliary converter  608  for generating another transmission signal vector X 2,s  from the transmission signal vector x 1,S  a pair of P/S converters  610  and  612  for converting x 1,s  and x 2,s  to transmission signal streams x 1,s [n] and x 2,s [n] (n=0,1, . . . ,N c ), and a pair of CP inserters  614  and  616  for inserting CPs into x 1,s [n] and x 2,s [n] and transmitting the resulting signals through antennas  618  and  620 .  
         [0057]      FIG. 7  is a detailed block diagram of the IFFT unit  606  and the auxiliary converter  608  illustrated in  FIG. 6 . Referring to  FIG. 7 , the IFFT unit  606  includes a separator  701  for separating the space-frequency code symbol vector X 1,s  received from the A-SFBC-OFDM coder  604  into an odd-numbered element and an even-numbered element, a pair of IFFTs  702   a  and  702   b  for generating  
         x     2   ,   s       (   e   )       ⁢           ⁢   and   ⁢           ⁢     x     2   ,   s       (   o   )             
 by performing IFFT on the signals of length N c /2 received from the separator  701 , a first multiplier  705  for multiplying  
       x     2   ,   s       (   o   )           
 by  
             (     -   1     )       r   ⁡     (     n   /   Nc     )         ⁢     W   N       -     (     (   n   )     )       ⁢     N   /   2           ,         
 and a first adder  703  for adding  
       x     2   ,   s       (   e   )           
 to the product received from the first multiplier  705  and providing the sum to the P/S converter  610 . 
 
         [0058]     The auxiliary converter  608  includes a bypass module  704  for outputting the output of the first IFFT  702   a , a first conjugation module  708  for calculating the complex conjugate of the output of the bypass module  704 , a first rearrangement module  712  for rearranging the output of the first conjugation module  708  and outputting the resulting signal  
         x     2   ,   s       (   e   )       ,       
 
 a negation module  706  for negating the output of the second IFFT  702   b , a second conjugation module  710  for calculating the complex conjugate of the negated value, a second rearrangement module  714  for rearranging the complex conjugate and outputting the resulting signal  
         x     2   ,   s       (   o   )       ,       
 
 a second multiplier  716  for multiplying  
       x     2   ,   s       (   o   )         
 
 by  
             (     -   1     )       r   ⁡     (     n   /   Nc     )         ⁢     W   N       -     (     (   n   )     )       ⁢     N   /   2           ,       
 
 and a second adder  718  for adding the outputs of the first rearrangement module  712  and the second multiplier  716 , and outputting the sum x 2,s  to the second P/S converter  612 . 
 
         [0059]     As indicated above, the auxiliary converter  608  separates the space-frequency code symbol vector X l,s , l=1,2 into an even-numbered element and an odd-numbered element. Therefore, the transmission signal stream x l,s [n],l=1,2,n==0,1, . . . ,N c −1 is represented in Equation (16),  
                 x     l   ,   s       ⁡     [   n   ]       =         1     N   c       ⁢       ∑     k   =   0         N   c     -   1       ⁢         X     l   ,   s       ⁡     [   k   ]       ⁢     W   Nc     -   nk             ⁢     
     ⁢           =         1     N   c       ⁢       ∑     v   =   0           N   c     2     -   1       ⁢       (         X     l   ,   s       ⁡     [     2   ⁢   v     ]       +       W   Nc     -   n       ⁢       X     l   ,   s       ⁡     [       2   ⁢   v     +   1     ]           )     ⁢     W       N   c     /   2       -   nv             ⁢     
     ⁢           =       1   2     ⁢     (         x     l   ,   s       (   e   )       ⁡     [   n   ]       +       W     N   c       -   n       ⁢       x     l   ,   s       (   o   )       ⁡     [   n   ]           )                   (   16   )             
 
 where  
           x     l   ,   s       (   e   )       ⁡     [   n   ]       ,       x     l   ,   s       (   o   )       ⁡     [   n   ]       ,     l   =   1     ,   2   ,     n   =   0     ,   1   ,   ⋯   ⁢           ,       N   c     -   1         
 
 is defined as shown in Equation (17).  
                   x     l   ,   s       (   e   )       ⁡     [   n   ]       =       2     N   c       ⁢       ∑     v   =   0           N   c     2     -   1       ⁢         X     l   ,   s       ⁡     [     2   ⁢   v     ]       ⁢     W       N   c     /   2       -   nv               ,     
     ⁢         x     l   ,   s       (   o   )       ⁡     [   n   ]       =       2     N   c       ⁢       ∑     v   =   0           N   c     2     -   1       ⁢         X     l   ,   s       ⁡     [       2   ⁢   v     +   1     ]       ⁢     W       N   c     /   2       -   nv                       (   17   )             
 
         [0060]     Because  
           x     l   ,   s       (   e   )       ⁡     [   n   ]       ,       x     l   ,   s       (   o   )       ⁡     [   n   ]           
 
 has a period of N c /2 for n, it can be replaced by  
           x     l   ,   s       (   e   )       ⁡     [       (     (   n   )     )       N   /   2       ]       ,       x     l   ,   s       (   o   )       ⁡     [       (     (   n   )     )       N   /   2       ]           
 
 and the relationship in Equation (18) is established:  
                     W     N   e       -   n       =       ⁢     W     N   e       -     (         (     (   n   )     )         N   e     /   2       +         N   e     2     ⁢     r   ⁡     (     n     N   e       )           )                     =       ⁢         W     N   e       -       (     (   n   )     )         N   e     /   2           ⁡     (     W     N   e         -     N   e       /   2       )         r   ⁡     (     n     N   e       )                     =       ⁢         (     -   1     )       r   ⁡     (     n     N   e       )         ⁢           ⁢     W     N   e       -       (     (   n   )     )         N   e     /   2                           (   18   )             
 
 where r(x) is a rounded-off number. Therefore, x l,s [n] of Equation (16) can be expressed as shown in equation (19).  
                 x     l   ,   s       ⁡     [   n   ]       =       1   2     ⁢     {         x     l   ,   s       (   e   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       +         (     -   1     )       r   ⁡     (     n     N   e       )         ⁢     W     N   e       -       (     (   n   )     )         N   e     /   2           ⁢       x     l   ,   s       (   o   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]           }               (   19   )             
 
         [0061]     Meanwhile, from Equation (8), in Equations (20a) and (20b)  
                 X     2   ,   s       ⁡     [     2   ⁢   v     ]       =     -       X     1   ,   s     *     ⁡     [       2   ⁢   v     +   1     ]                 (     20   ⁢   a     )             and                             X     2   ,   s       ⁡     [       2   ⁢   v     +   1     ]       =       X     1   ,   s     *     ⁡     [     2   ⁢   v     ]               (     20   ⁢   b     )                   where   ⁢           ⁢   v     =   0     ,   1   ,   ⋯   ⁢           ,         N   c     2     -   1.                           
 
         [0062]     From Equation (13), Equation (17), Equation (20a) and (20b), Equations (21a) and (21b) are:  
                 x     2   ,   s       (   e   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       =     -       (       x     1   ,   s       (   o   )       ⁡     [       (     (     -   n     )     )         N   e     /   2       ]       )     *               (     21   ⁢   a     )             and                             x     2   ,   s       (   o   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       =         (       x     1   ,   s       (   3   )       ⁡     [       (     (     -   n     )     )         N   e     /   2       ]       )     *     .             (     21   ⁢   b     )             
 
         [0063]     It is noted from the above equations that  
           x     2   ,   s       (   e   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       ,       x     2   ,   s       (   o   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]           
 
 are easily achieved by negating, complex-conjugating, and rearranging  
           x     1   ,   s       (   e   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       ⁢           ⁢   and   ⁢           ⁢         x     1   ,   s       (   o   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       .         
 
 Therefore, x 2,s [n] is obtained from Equation (22).  
                       x     2   ,   s       ⁡     [   n   ]       =       ⁢       1   2     ⁢     {       -       (       x     1   ,   s       (   o   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       )     *       +                         ⁢         (     -   1     )       r   ⁡     (     n     N   e       )         ⁢         W   n     -       (     (   n   )     )         N   e     /   2           ⁡     (       x     1   ,   s       (   e   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       )       *       }                 (   22   )             
 
         [0064]     Because x 2,s [n] is achieved from  
           x     1   ,   s       (   e   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]       ,       x     1   ,   s       (   o   )       ⁡     [       (     (   n   )     )         N   e     /   2       ]           
 
 involved in calculating x l,s [n], there is no need to perform an additional IFFT operation. However, N c /2 complex multiplications and N c  complex additions are additionally required. 
 
         [0065]      FIG. 8  is a block diagram of a transmitting apparatus according to a fourth embodiment of the present invention. N t  denotes the number of transmit antennas and N s  denotes the number of contiguous subcarriers, which is generalized by a power of  2  and less than N c .  
         [0066]     Referring to  FIG. 8 , the transmitting apparatus includes a data source  800 , an S/P converter  802  for converting information symbols received from the data source  800  to an information symbol vector D s , an A-SFBC-OFDM coder  804  for generating space-frequency code symbol vectors X 1,s , X 2,s , . . . , X Nt,s  for the input of D s , an IFFT  806  for inverse-fast-Fourier-transforming the space-frequency code symbol vectors and outputting a transmission signal vector, a plurality of auxiliary converters  808  to  810 , each for generating another transmission signal vector from the transmission signal vector received from the IFFT unit  806 , a plurality of P/S converters  812  to  816  for converting transmission signal vectors x 1,s , X 2,s , . . . ,x Nt,s  received from the IFFT unit  806  and auxiliary converters  812  to  816  to transmission signal streams x 1,s [n], x 2,s [n], x Nt,s [n], n=0,1, . . . , N c , and a plurality of CP inserters  818  to  822  for inserting CPs into the transmission signal streams received from the P/S converters  812  to  816  and transmitting the resulting signals through transmit antennas  824  to  828  at their respective output ends.  
         [0067]     The components of the transmitting apparatus according to the fourth embodiment of the present invention operate in a similar manner to those of the transmitting apparatus according to the third embodiment of the present invention, except that the operation is performed with respect to N t  transmit antennas and N s  successive sub-carriers.  
         [0068]     In the fourth embodiment of the present invention, the space-frequency code symbol vectors output from the A-SFBC-OFDM coder  804  are correlated in the relationship shown in Equations (23a) and (23b):  
                   X       l   2     ,   s       (     q   2     )       =     ±     X     1   ,   s       (     q   1     )           ,       q   1     ∈     {     0   ,   1   ,   2   ,   3   ⁢           ,           ⁢   …   ,           ⁢       N   s     -   1       }       ,     
     ⁢       q   2     ∈     {     1   ,   2   ,   3   ,           ⁢   …   ,           ⁢     N   s       }       ,       l   1     ∈     {     2   ,     …   ⁢           ⁢     N   t         }         ⁢     
     ⁢   and           (     23   ⁢   a     )                     X       l   2     ,   s       (     q   2     )       =     ±       {     X     1   ,   s       (     q   1     )       }     *         ,       q   1     ∈     {     0   ,   1   ,   2   ,   3   ,           ⁢   …   ,           ⁢       N   s     -   1       }       ,     
     ⁢       q   2     ∈     {     1   ,   2   ,   3   ,           ⁢   …   ,           ⁢     N   s       }       ,       l   1     ∈     {     2   ,     …   ⁢           ⁢     N   t         }         ⁢     
     ⁢       where   ⁢             ⁢             ⁢     X     l   ,   s       (   q   )         ⁢     =   Δ     ⁢     [         X     l   ,   s       ⁡     [       0   ·     N   s       +   q     ]       ,       X     l   ,   s       ⁡     [       1   ·     N   s       +   q     ]       ,           ⁢   …   ,           ⁢       X     l   ,   s       ⁡     [         (         N   c     /     N   s       -   1     )     ·     N   s       +   q     ]       ,     q   =   0     ,   1   ,           ⁢   …   ,           ⁢       N   s     -   1.                   (     23   ⁢   b     )             
 
         [0069]     In this case, the space-frequency code symbol vector X 1,s  is converted to the transmission signal vector x 1,s  by the IFFT unit  806 . x 1,s  is serialized by the P/S converter  812 , added with a CP by the CP inserter  818 , and transmitted through the transmit antenna  824 .  
         [0070]     According to Equation (23a) and Equation (23b), x l     2     ,s  is achieved by processing  
         X     1   ,   s       (     q   1     )       ,       
 
 which is obtained through IFFT operation of X 1,s , in the auxiliary converters  814  to  816 . The transmission signal vectors x 2,s [n] to x Nt,s [n] from the auxiliary converter  808  to  810  are serialized by the P/S converters  820  to  822 , added with CPs by the CP inserters  820  to  822 , and transmitted through the transmit antennas  826  to  828 . 
 
         [0071]     In accordance with the present invention as described above, an OFDM transmitting apparatus reduces the number of IFFT operations required for generation of transmission signals to be transmitted through a plurality of transmit antennas. Therefore, the implementation complexity of the transmitting apparatus is minimized and coding efficiency is increased.  
         [0072]     While the present 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 present invention as defined by the appended claims.