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 . fig3 is a block diagram of a transmitting apparatus according to an embodiment of the present invention . referring to fig3 , 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 ]. 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 . fig4 is a detailed block diagram of the auxiliary converters 310 and 312 illustrated in fig3 . referring to fig4 , 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 . 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 . 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 ) 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 ) 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 ]. 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 . 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 . fig5 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 . referring to fig5 , 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 . 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 . 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 } 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 . 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 . fig6 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 . referring to fig6 , 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 . fig7 is a detailed block diagram of the ifft unit 606 and the auxiliary converter 608 illustrated in fig6 . referring to fig7 , 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 ( - 1 ) r ⁡ ( n / nc ) ⁢ w n - ( ( n ) ) ⁢ n / 2 , to the product received from the first multiplier 705 and providing the sum to the p / s converter 610 . 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 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 ( - 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 . 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 ) x l , s ( e ) ⁡ [ n ] , x l , s ( o ) ⁡ [ n ] , l = 1 , 2 , n = 0 , 1 , ⋯ ⁢ , n c - 1 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 ) 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 ] 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 ) 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 . 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 ) x 2 , s ( e ) ⁡ [ ( ( n ) ) n e / 2 ] , x 2 , s ( o ) ⁡ [ ( ( n ) ) n e / 2 ] x 1 , s ( e ) ⁡ [ ( ( n ) ) n e / 2 ] ⁢ ⁢ and ⁢ ⁢ x 1 , s ( o ) ⁡ [ ( ( n ) ) n e / 2 ] . 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 ) 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 . fig8 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 . referring to fig8 , 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 . 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 . 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 ) 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 . according to equation ( 23a ) and equation ( 23b ), x l 2 , s is achieved by processing 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 . 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 . 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 .