Abstract:
A sub-carrier diversity method on an MB-OFDM (multi-band orthogonal-frequency-division-multiplexing) system repeatedly transmitting an identical frequency band of an identical symbol, including: setting a value of a TDS (time domain spreading) of sub-carriers depending on a transmission rate; and shifting the positions of the sub-carriers in a predetermined unit so that the sub-carriers diverge from one band to another.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2005-130800 filed Dec. 27, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An aspect of the present invention relates to a sub-carrier diversity method on a multi-band (MB)-orthogonal-frequency-division-multiplexing (OFDM) symbol, and more particularly, to a sub-carrier diversity method on an MB-OFDM symbol by which a time domain spreading (TDS) is set with respect to sub-carriers according to a frequency hopping pattern and positions of the sub-carriers are shifted in a predetermined unit within each time domain so as to allow the sub-carriers to diverge in an MB-OFDM system in which an identical frequency band of an identical symbol is repeated. 
     2. Description of the Related Art 
     In general, OFDM systems transform symbols input in series into parallel symbols having predetermined sizes, multiplex the parallel symbols into orthogonal different sub-carrier frequencies, and transmit the orthogonal different sub-carrier frequencies. 
     In an MB-OFDM method, a plurality of frequency bands of an OFDM symbol in the unit of symbol hop in order to transmit a signal. For example, the MB-OFDM method is a modulation technology used in a specific wireless communication system such as an ultra wide band (UWB) system. OFDM modulation technology and frequency hopping technology are combined into MB-OFDM modulation technology. 
     An MB-OFDM system divides a predetermined frequency band into a plurality of sub-bands. The MB-OFDM system can transmit data (a symbol) using the plurality of sub-bands so as to transmit or receive a large amount of data per unit time. A UWB system selects one of the pluralities of sub-bands and uses the selected sub-band according to set regulations so as to improve security of data. 
     [Table 1] below shows a method of transmitting payloads according to a transmission rate in the MB-OFDM system. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Transmission 
                 Modulation 
                 Encoding 
                   
                   
                 Spreading 
               
               
                 Rate 
                 Method 
                 Rate 
                 Conjugate 
                 TSF 
                 Gain 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 53.3 
                 QPSK 
                 1/3 
                 ∘ 
                 2 
                 4 
               
               
                 80 
                 QPSK 
                 1/2 
                 ∘ 
                 2 
                 4 
               
               
                 106.67 
                 QPSK 
                 1/3 
                 x 
                 2 
                 2 
               
               
                 160 
                 QPSK 
                 1/2 
                 x 
                 2 
                 2 
               
               
                 200 
                 QPSK 
                 5/8 
                 x 
                 2 
                 2 
               
               
                 320 
                 DCM 
                 1/2 
                 x 
                 1 
                 1 
               
               
                 400 
                 DCM 
                 5/8 
                 x 
                 1 
                 1 
               
               
                 480 
                 DCM 
                 3/4 
                 x 
                 1 
                 1 
               
               
                   
               
             
          
         
       
     
     In a case where the transmission rate is between 53.3 Mbps and 200 Mbps, the MB-OFDM system uses a quadrature phase shift keying (QPSK) method. In a case where the transmission rate is between 320 Mbps and 480 Mbps, the MB-OFDM system uses a dual carrier modulation (DCM) method. 
     In a case where the transmission rate is between 53.3 Mbps and 80 Mbps, the MB-OFDM system transmits a conjugate symbol. Thus, the spreading gain is “4.” In other words, in a case where the transmission rate is between 53.3 Mbps and 80 Mbps, a time spreading factor (TSF) is “2.” Thus, one symbol is transmitted four times, including conjugate symbols. 
     [Table 2] below shows an example of transmitting a symbol in an MB-OFDM system having a transmission rate between 53.3 Mbps and 80 Mbps. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Data 
                 Mapping Data 
               
               
                   
                   
               
             
             
               
                   
                 D0 
                 C0 
               
               
                   
                 D1 
                 C1 
               
               
                   
                 . . . 
                 . . . 
               
               
                   
                 D49 
                 C49 
               
               
                   
                 D49* 
                 C50 
               
               
                   
                 . . . 
                 . . . 
               
               
                   
                 D1* 
                 C98 
               
               
                   
                 D0* 
                 C99 
               
               
                   
                   
               
             
          
         
       
     
     Referring to Table 2, one piece of data is transmitted two times, including conjugate data. In other words, a transmitter transmits data D 0  through D 49  together with conjugate data D 0 * through D 49 *. Also, if the QPSK method is used, the transmitter divides one piece of data into real and imaginary components and transmits the real and imaginary components. 
     In a case where a data transmission rate is between 53.3 Mbps and 200 Mbps in an MB-OFDM-based UWB system as described above, a value of a TDS is “2” as shown in  FIG. 1 . Thus, the MB-OFDM-based UWB system transmits a frequency band of a sub-carrier including a symbol and a conjugate symbol only two times. As a result, in a case where errors occur in transmitted frequency bands Band 1  and Band 2 , a bit error rate (BER) is increased. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present general inventive concept has been made to solve the above-mentioned and/or other problems, and an aspect of the present general inventive concept is to provide a sub-carrier diversity method on an MB-OFDM symbol by which a TDS is set with respect to sub-carriers according to a frequency hopping pattern, and positions of the sub-carriers are shifted in a predetermined unit within each time domain so as to allow the sub-carriers to diverge in an MB-OFDM system in which an identical frequency band of an identical symbol is repeated. 
     According to an aspect of the present invention, there is provided a sub-carrier diversity method on an MB-OFDM (multi-band orthogonal-frequency-division-multiplexing) system repeatedly transmitting an identical frequency band of an identical symbol, including setting a value of a TDS (time domain spreading) of sub-carriers depending on a transmission rate and shifting positions of the sub-carriers in a predetermined unit so that the sub-carriers diverge. 
     According to another aspect of the present invention, if the value of the TDS is set to 2 or a multiple of 2, the sub-carriers may be cyclically shifted in a predetermined unit in a frequency band transmitted at each stage so as to diverge. 
     According to another aspect of the present invention, if the value of the TDS is “4,” the sub-carriers may diverge in a unit of 50 in frequency bands transmitted at first through third stages and be cyclically shifted in a unit of 25 so as to diverge in a frequency band transmitted at a fourth stage. 
     According to another aspect of the present invention, 25 sub-carriers from  26  to  50  may be shifted to positions of 25 sub-carriers from  1  to  25  so as to diverge in the frequency band transmitted at the fourth stage S 4 , and 25 sub-carriers from  25 ′ to  1 ′ may be shifted to positions of 25 sub-carriers from  50 ′ to  26 ′ so as to diverge in a symmetric frequency band. 
     According to another aspect of the present invention, if the value of the TDS is set to 2 or a multiple of 2, the sub-carriers may be reversed in a predetermined unit so as to diverge in a frequency band transmitted at each stage. 
     According to another aspect of the present invention, if the value of the TDS is “4,” the sub-carriers may diverge in a unit of 50 in frequency bands transmitted at first through third stages and in a reverse order in a unit of 50 in a frequency band transmitted at a fourth stage. 
     According to another aspect of the present invention, sub-carriers in a reverse order from  50  to  1  may diverge in the frequency band transmitted at the fourth stage and sub-carriers in a reverse order from  1 ′ to  50 ′ may diverge in a symmetric frequency band. 
     According to another aspect of the present invention, if the value of the TDS is “3,” 50 sub-carriers and 50 symmetric sub-carriers may diverge in a frequency band transmitted at each stage. 
     According to another aspect of the present invention, if the value of the TDS is set to 2 or a multiple of 2, sub-carriers cyclically shifted in a predetermined unit and sub-carriers reversed in a predetermined unit may diverge in the frequency band transmitted at each stage. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent, and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a view illustrating frequency bands of sub-carriers having a TDS of “2” according to the prior art; 
         FIG. 2  is a view illustrating frequency bands having a TDS value of “3” in a sub-carrier diversity method on an MB-OFDM symbol according to an embodiment of the present invention; 
         FIG. 3  is a view illustrating frequency bands having a TDS value of “4” in a sub-carrier diversity method on an MB-OFDM symbol according to an embodiment of the present invention; 
         FIG. 4  is a view illustrating disadvantages occurring when a TDS value is “4”; 
         FIG. 5  is a view illustrating frequency bands of sub-carriers diverging according to a cyclic shift method when a TDS value is “4”; 
         FIG. 6  is a view illustrating frequency bands of sub-carriers diverging according to a reversal method when a TDS value is “4”; 
         FIG. 7  is a view illustrating frequency bands of sub-carriers diverging according to a cyclic shift method when a TDS value is 2 or a multiple of 2; and 
         FIG. 8  is a view illustrating frequency bands of sub-carriers diverging according to a reversal method when a TDS value is 2 or a multiple of 2. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
       FIG. 2  is a view illustrating frequency bands having a TDS value of “3” in a sub-carrier diversity method on an MB-OFDM symbol according to an embodiment of the present invention. 
     As shown in  FIG. 2 , a TDS of a sub-carrier is set depending on a transmission rate. If the transmission rate is 35.53 Mbps, a value of the TDS is “3.” Here, a used frequency band of a transmitted symbol is 507.35 MHz. In other words, the frequency band of 507.35 MHz is transmitted three times. Referring to  FIG. 2 , a first frequency band Band 1  is transmitted at a first stage S 1 , a second frequency band Band  2  is transmitted at a second stage S 2 , and a third frequency band Band  3  is transmitted at a third stage S 3 . 
     Also, positions of sub-carriers are shifted in a predetermined unit so that sub-carriers diverge from each frequency band. In other words, 50 symbols from  1  to  50  and 50 symmetric symbols from  50 ′ to  1 ′ diverge in the first frequency band Band  1 . Here, one symbol includes conjugate symbols such as “1=b 1 +jb 2 ” and “50=b 99 +jb 100 .” Asymmetric symbol also includes conjugate symbols such as “50=b 99 −jb 100 ” and “1=b 1 −jb 2 .” 
     The frequency band Band  2  transmitted at the second stage S 2  has the same structure as the first frequency band Band  1 , and thus 50 symbols diverge from a sub-carrier in the second frequency band Band  2 . 
     The third frequency band Band  3  transmitted at the third stage S 3  has the same structure as the first and second frequency bands Band  1  and Band  2 , and thus 50 symbols diverge from a sub-carrier in the third frequency band Band  3 . 
       FIG. 3  is a view illustrating frequency bands having a TDS value of “4” in a sub-carrier diversity method on an MB-OFDM symbol according to an embodiment of the present invention. 
     Referring to  FIG. 3 , when a used frequency band is 507.35 MHz like the used frequency band shown in  FIG. 2  and a transmission rate is 26.65 Mbps, a value of a TDS is set to “4.” Thus, a frequency band to be transmitted is transmitted four times from a first stage S 1  through a fourth stage S 4 . 
     To transmit each frequency band, positions of sub-carriers are shifted in a predetermined unit in each frequency band so that the sub-carriers diverge in each frequency band and then the sub-carriers are transmitted. In other words, 50 symbols from  1  to  50  and 50 symmetric symbols from  50 ′ to  1 ′ diverge in a first frequency band Band  1  transmitted at a first stage S 1 . 
     50 symbols and 50 symmetric symbols diverge from a sub-carrier in a second frequency band Band  2  transmitted at a second stage S 2 . 
     50 symbols and 50 symmetric symbols diverge from a sub-carrier in each of third and fourth frequency bands Band  3  and Band  4  respectively transmitted at third and fourth stages S 3  and S 4 . 
     Thus, the first, second, third, and fourth frequency bands Band  1 , Band  2 , Band  3 , and Band  4  have the same diversity structure (S 1 =S 2 =S 3 =S 4 ). 
       FIG. 4  is a view illustrating disadvantages occurring when a value of a TDS is “4.” 
     As shown in  FIG. 4 , a used frequency band is transmitted four times depending on a value of a TDS. Here, a first frequency band Band  1  transmitted at a first stage S 1  is between 3168 MHz and 3595 MHz, and a second frequency band Band  2  transmitted at a second stage S 2  is between 3596 MHz and 4221 MHz. Also, a third frequency band Band  3  transmitted at a third stage S 3  is between 4224 MHz and 4752 MHz. 
     However, a first frequency band Band  1  is transmitted at a fourth stage S 4  that is a last stage of the TDS. Thus, as shown in  FIG. 4 , a frequency band A  410  transmitted at a first stage S 1  and a frequency band A  420  transmitted at a fourth stage S 4  exist in the same band and have the same information as in a first frequency band Band  1 . In other words, an overlapping phenomenon occurs. If a value of a TDS is increased to a multiple equal to or more than 4, frequency bands B in a second frequency band Band  2  shown in  FIG. 4  exist at the same band and have the same information. In other words, an overlapping phenomenon occurs. 
       FIG. 5  is a view illustrating frequency bands of sub-carriers diverging according to a cyclic shift method when a value of a TDS is “4.” 
     As shown in  FIG. 5 , to solve the disadvantages described with reference to  FIG. 4 , sub-carriers are cyclically shifted in units of 25 so as to diverge in a first frequency band Band  1  transmitted at a fourth stage S 4 , the first frequency band Band  1  having the same structure and the same information as a frequency band Band  1  transmitted at a first stage S 1 . In other words, as shown in  FIG. 5 , 25 sub-carriers from  26  to  50  are shifted to the positions of sub-carriers  1  to  25  so as to diverge in the first frequency band Band  1  transmitted at the fourth stage S 4 . Also, 25 sub-carriers from  25 ′ to  1 ′ are shifted to the positions of 25 sub-carriers  50 ′ to  26 ′ so as to diverge in a symmetric frequency band. 
     Thus, the first frequency band Band  1  transmitted at the first stage S 1  has a different structure from the frequency band Band  1  transmitted at the fourth stage S 4 . The first, second, and third frequency bands Band  1 , Band  2 , and Band  3  respectively transmitted at the first, second, and third stages S 1 , S 2 , and S 3  have the same structure and the same information. 
       FIG. 6  is a view illustrating frequency bands of sub-carriers diverging according to a reversal method when a value of a TDS is “4.” 
     As shown in  FIG. 6 , to solve the disadvantages described with reference to  FIG. 4 , sub-carriers diverge in a reverse order in a unit of 50 in a first frequency band Band  1  of 507.35 MHz transmitted at a fourth stage S 4 , the first frequency band Band  1  having the same structure and the same information as a first frequency band Band  1  transmitted at a first stage S 1 . In other words, 50 sub-carriers from  50  to  1  diverge in the first frequency band Band  1  transmitted at the fourth stage S 4  and 50 sub-carriers from  1 ′ to  50 ′ diverge in a symmetric frequency band according to a reversal method. 
     Thus, the first frequency band Band  1  transmitted at the first stage S 1  has a different structure from the first frequency band Band  1  transmitted at the fourth stage S 4 . The first frequency band Band  1  at the first stage S 1  and second and third frequency bands Band  2  and Band  3  transmitted at second and third stages S 2  and S 3  have the same structure and the same information. 
       FIG. 7  is a view illustrating frequency bands of sub-carriers diverging according to a cyclic shift method when a value of a TDS is 2 or a multiple of 2. 
     In a case where the value of the TDS is increased to 2 or a multiple of 2, such as 4, 6, or 8, disadvantages as described with reference to  FIG. 4  occur. Thus, in the present embodiment, in a case where the value of the TDS is increased to 2 or a multiple of 2, sub-carriers diverge according to a cyclic shift method. 
       FIG. 7  shows a case where a value of a TDS is increased to a number multiple of 2, i.e., to 8. Here, each frequency band transmitted eight times is 507.35 MHz, and a transmission rate is 13.3 Mbps as shown in  FIG. 7 . 
     Referring to  FIG. 7 , a frequency band Band  1  including 50 sub-carriers and 50 symmetric sub-carriers is transmitted at a first stage S 1 , a second frequency band Band  2  having the same structure as the first frequency Band  1  is transmitted at a second stage S 2 , and a third frequency band Band  3  having the same structure as the first and second frequency bands Band  1  and Band  2  is transmitted at a third stage S 3 . 
     A first frequency band Band  1  is transmitted at a fourth stage S 4 , and is different from the first frequency band Band  1  transmitted at the first stage S 1 , since some sub-carriers are cyclically shifted so as to diverge from the first frequency band Band  1  transmitted at the fourth stage S 4 . In other words, 17 sub-carriers from positions  34  to  50  are shifted to the positions of sub-carrier  1  to  33  so as to diverge from the first frequency band Band  1  transmitted at the fourth stage S 4 . Also, 33 sub-carriers from positions  33 ′ to  1 ′ are shifted to the positions of sub-carriers  50 ′ to  34 ′ so as to diverge in a symmetric frequency band. Thus, the first frequency band Band  1  transmitted at the stage S 4  has a different structure from the first frequency band Band  1  transmitted at the first stage S 1 . 
     A second frequency band Band  2  is transmitted at a fifth stage S 5 , and is different from the second frequency band Band  2  transmitted at the second stage S 2 , since some sub-carriers are cyclically shifted so as to diverge in the second frequency band Band  2  transmitted at the fifth stage S 5 . In other words, 17 sub-carriers from positions  34  to  50  are shifted to positions of 33 sub-carriers from  1  to  33  so as to diverge from the second frequency band Band  2  transmitted at the fifth stage S 5 . Also, 33 sub-carriers from  33 ′ to  1 ′ are shifted to the positions of 17 sub-carriers from  50 ′ to  34 ′ so as to diverge in a symmetric frequency. Thus, the second frequency band Band  2  transmitted at the fifth stage S 5  has a different structure from the second frequency band Band  2  transmitted at the second stage S 2 . 
     A third frequency band Band  3  is transmitted at a sixth stage S 6 , and is different from the third frequency band transmitted at a third stage S 3 , since some sub-carriers are cyclically shifted so as to diverge in the third frequency band Band  3  transmitted at the sixth stage S 6 . In other words, 17 sub-carriers from  34  to  50  are shifted to previous positions of 33 sub-carriers from  1  to  33  so as to diverge from the third frequency band Band  3  transmitted at the third stage S 6 . Also, 33 sub-carriers from  33 ′ to  1 ′ are shifted to the positions of 17 sub-carriers from  50 ′ to  34 ′ so as to diverge in a symmetric frequency band. Thus, the third frequency band Band  3  transmitted at the sixth stage S 6  has a different structure from the third frequency band Band  3  transmitted at the third stage S 3 . 
     A first frequency band Band  1  is transmitted at a seventh stage S 7 , and is different from the first frequency band Band  1  transmitted at a fourth stage S 4 , since some sub-carriers are cyclically shifted so as to diverge in the first frequency band Band  1  transmitted at the seventh stage S 7 . In other words, 33 sub-carriers from positions  18  to  50  are shifted to the positions of 17 sub-carriers  1  to  17  so as to diverge in the first frequency band Band  1  transmitted at the seventh stage S 7 . Also, 17 sub-carriers from  17 ′ to  1 ′ are shifted to the positions of 33 sub-carriers from  50 ′ to  18 ′ so as to diverge in a symmetric frequency band. Thus, the first frequency band Band  1  transmitted at the seventh stage S 7  has a different structure from the first frequency band Band  1  transmitted at the first or fourth stage S 1  or S 4 . 
     A second frequency band Band  2  is transmitted at an eighth stage S 8  and is different from the second frequency band Band  2  transmitted at the second or fifth stage S 2  or S 5 , since some sub-carriers are cyclically shifted so as to diverge from the second frequency band Band  2  transmitted at the eighth stage S 8 . In other words, 33 sub-carriers from  18  to  50  are shifted to the positions of 17 sub-carriers from  1  to  17  so as to diverge in the second frequency band Band  2  transmitted at the eighth stage S 8 . Also, 17 sub-carries from  17 ′ to  1 ′ are shifted to the positions of 33 sub-carriers from  50 ′ to  18 ′ so as to diverge in a symmetric frequency band. Thus, the second frequency band Band  2  transmitted at the eighth stage S 8  has a different structure from the second frequency band Band  2  transmitted at the second or fifth stage S 2  or S 5 . 
     Accordingly, sub-carriers in frequency bands transmitted at first, second, and third stages S 1 , S 2 , and S 3  have the same structure. Sub-carriers in frequency bands transmitted at fourth, fifth, and sixth stages S 4 , S 5 , and S 6  also have the same structure. Sub-carriers in frequency bands transmitted at the seventh and eighth stages S 7  and S 8  have the same structure. 
     However, the sub-carriers in the frequency bands transmitted at the first, second, and third stages S 1 , S 2 , and S 3 , the sub-carriers in the frequency bands transmitted at the fourth, fifth, and sixth stages S 4 , S 5 , and S 6 , and the sub-carriers in the frequency bands transmitted at the seventh and eighth stages S 7  and S 8  have different structures. 
       FIG. 8  is a view illustrating frequency bands of sub-carriers diverging according to a reversal method when a value of a TDS is a multiple of 2. 
       FIG. 8  illustrates a case where a value of a TDS is increased to a multiple of 2, i.e., to 8. Here, each of the frequency bands transmitted eighth times is 507.35 MHz, and a transmission rate is 13.3 Mbps. 
     As shown in  FIG. 8 , a first frequency band Band  1  including 50 sub-carriers and 50 symmetric sub-carriers is transmitted at a first stage S 1 , a second frequency band Band  2  having the same structure as the first frequency band Band  1  is transmitted at a second stage S 2 , and a third frequency band Band  3  having the same structure as the first and second frequency bands Band  1  and Band  2  is transmitted at a third stage S 3 . 
     A first frequency band Band  1  is transmitted at a fourth stage S 4 , and is different from the first frequency band Band  1  transmitted at the first stage S 1 , since the sub-carriers diverge in a reverse order from  50  to  1  from the first frequency band Band  1  transmitted at the fourth stage S 4 . 
     A second frequency band Band  2  is transmitted at a fifth stage S 5 , and is different from the second frequency band Band  2  transmitted at the second stage S 2 , since the sub-carriers diverge in a reverse order from  50  to  1  from the second frequency band Band  2  transmitted at the fifth stage S 5 . 
     A third frequency band Band  3  is transmitted at a sixth stage S 6 , and is different from the third frequency band Band  3  transmitted at the third stage S 3 , since the sub-carriers diverge in a reverse order from  50  to  1  from the third frequency band Band  3  transmitted in the sixth stage S 6 . 
     A first frequency band Band  1  is transmitted at a seventh stage S 7 , and is different from the first frequency band Band  1  transmitted at the first or fourth stage S 1  or S 4 , since some sub-carriers diverge in a reverse order from the first frequency band Band  1  transmitted at the seventh stage S 7 . In other words, sub-carriers in a reverse order from  25  to  1  and sub-carriers in a reverse order from  50  to  26  diverge in the first frequency band Band  1  transmitted at the seventh stage S 7 . Also, sub-carriers in a reverse order from  26 ′ to  50 ′ and sub-carriers in a reverse order from  1 ′ to  25 ′ diverge in a symmetric frequency band. Thus, the first frequency band Band  1  transmitted at the seventh stage S 7  has a different structure from the first frequency band Band  1  transmitted at the first or fourth stage S 1  or S 4 . 
     A second frequency band Band  2  is transmitted at an eighth stage S 8 , and is different from the second frequency band Band  2  transmitted at the second or fifth stage S 2  or S 5 , since the sub-carriers in a reverse order from  25  to  1  and sub-carriers in a reverse order from  50  to  26  diverge from the second frequency band Band  2  transmitted at the eighth stage S 8 . Also, sub-carriers in a reverse order from  26 ′ to  50 ′ and sub-carriers in a reverse order from  1 ′ to  25 ′ diverge in a symmetric frequency band. Thus, the second frequency band Band  2  transmitted at the eighth stage S 8  has a different structure from the second frequency band Band  2  transmitted at the second or fifth stage S 2  or S 5 . 
     Accordingly, sub-carriers in frequency bands transmitted at first, second, and third stages S 1 , S 2 , and S 3  have the same structure. Sub-carriers in frequency bands transmitted at fourth, fifth, and sixth stages S 4 , S 5 , and S 6  also have the same structure. Sub-carriers in frequency bands transmitted at seventh and eighth stages S 7  and S 8  have the same structure. 
     However, the sub-carriers in the frequency bands transmitted at the first, second, and third stages S 1 , S 2 , and S 3 , the sub-carriers in the frequency bands transmitted at the fourth, fifth, and sixth stages S 4 , S 5 , and S 6 , and the sub-carriers in the frequency bands transmitted at the seventh and eighth stages S 7  and S 8  have different structures. 
     As described above, according to an embodiment of the present invention, a frequency band in which sub-carriers diverge can be transmitted a plurality of times during the transmission of an MB-OFDM symbol. Thus, stable frequency links can be secured. Also, a BER can be reduced. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.