Abstract:
A method and apparatus for reducing other cell interference in a wireless communication system are provided, in which a symbol generator for generating at least one modulation symbols to be transmitted, a resource mapper for mapping and assigning the modulation symbols into a resource block common to a Base Station (BS) and at least one neighboring Base Station (BS), and a spreader for spreading the mapped modulation symbol with an BS-specific code allocated to the BS, the BS-specific codes of the BS being one of orthogonal and quasi-orthogonal to an BS-specific code allocated to the neighbor BS.

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119(a) to a Korean Patent Application filed in the Korean Intellectual Property Office on Feb. 3, 2006 and assigned Serial No. 2006-10387, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an wireless communications system. More particularly, the present invention relates to a method and apparatus for reducing other cell interference by using a Base Station (BS)-specific orthogonal code or quasi-orthogonal code. 
     2. Description of the Related Art 
     Beginning with analog cellular service called 1 st  Generation (1G), mobile communication systems are evolving toward 4 th  Generation (4G) that provides ultra high-speed multimedia service beyond 2 nd  Generation (2G) digital technology and 3 rd  Generation (3G) providing high-speed multimedia service in International Mobile Telecommunications-2000 (IMT-2000). The 4G mobile communication system is designed to support higher data rates, aiming at data transmission at or above 100 MBps. Under a multipath radio channel environment, the 4G mobile communication system compensates for multipath fading and ensures transmission of burst packet data that has rapidly increased in amount along with provisioning of packet service. 
     OFDMA is a promising candidate for radio access technology satisfying 4G mobile communication requirements. OFDMA is a special case of MultiCarrier Modulation (MCM) in which input data is converted to as many parallel data sequences as subcarriers are used and the input data is modulated in the subcarriers, prior to transmission. 
     Since the number of subcarriers changes according to a user-requested data rate, OFDMA enables efficient resource distribution and increases transmission efficiency. Due to its feasibility for multiple subcarriers (i.e. a large Fast Fourier Transform (FFT) size), OFDMA is efficient for a wireless communication system having cells with a relatively long time delay spread. 
       FIG. 1  is a block diagram of a typical OFDMA transmission structure. In the illustrated case of  FIG. 1 , transmitters  100 ,  120  and  140  are provided in a plurality of BSs controlling their respective cells, BS 0 , BS 1  and BS 2 . 
     Referring to  FIG. 1 , Forward Error Correction (FEC) encoders  102 ,  122  and  142  encode information sequences as source data to be transmitted from their BSs, i.e. BS 0 , BS 1  and BS 2 . Symbol mappers  104 ,  124  and  144  modulate the coded data in Quadrature Phase Shift Keying (QPSK)/16-ary Quadrature Amplitude Modulation (16QAM)/64-ary QAM (64QAM), thus producing modulation symbols s 0 (k), s 1 (k) and s 2 (k). Repeaters  106 ,  126 , and  146  repeat the modulation symbols s 0 (k), s 1 (k) and s 2 (k) according to repetition numbers set in their BSs (BS 0 , BS 1  and BS 2 ) and allocate the repeated symbols to a plurality of subcarriers. 
     Scramblers  108 ,  128  and  148  scramble the signal sequences S R,0 (k), S R,1 (k) and S R,2 (k) allocated to the subcarriers with BS-specific scrambling sequences sc 0 (k), sc 1 (k) and sc 2 (k). Inverse Fast Fourier Transform (IFFT) processors  110 ,  130  and  150  convert the scrambled signals to time-domain signals. The time-domain signals are sent to users within the cell areas of BS 0 , BS 1  and BS 2  via antennas  112 ,  132  and  152 . 
     The operation of transmitters  100 ,  120  and  140  is formulated as follows. 
     The outputs of symbol mappers  104 ,  124  and  144  are given as Equation (1),
 
 s ( m ),  m= 0 , . . . , M− 1  (1)
 
where m denotes a bit index and M denotes the length of a modulation symbol, i.e. the number of bits per modulation symbol.
 
     Repeaters  106 ,  126  and  146  repeat s(m) R times as expressed by Equation (2). Thus,
 
 s   R ( k )= s ( k  mod  M ),  k= 0, . . . , N− 1  (2)
 
where k denotes a subcarrier index and mod represents a modulo operation. N is the total length of a symbol which is repeated R times as expressed by Equation (3). Therefore,
 
 N=RM   (3)
 
     Let a scrambling sequence with 1s or −1s be denoted by sc(k). Then, the outputs of the scramblers  108 ,  128  and  148  are expressed as Equation (4),
 
 x ( k )= sc ( k ) s   R ( k ),  k= 0, . . . ,  N− 1  (4)
 
     A receiver of a Mobile Terminal (MT) receives interference signals from neighbor cells, i.e. neighbor cell interference signals as well as a signal from a serving BS. If BS 0  is a serving cell and BS 1  is a neighbor cell in  FIG. 1 , the received signal y(k) includes noise n(k), expressed as Equation (5),
 
 y ( k )= h   0 ( k ) x   0 ( k )+ h   1 ( k ) x   1 ( k )+ n ( k )  (5)
 
where x 0 (k) and x 1 (k) denote signals transmitted from BS 0  and BS 1 , h 0 (k) denotes a channel frequency response representing a k th  subchannel between BS 0  and the MT, and h 1 (k) denotes a channel frequency response between BS 1  and the MT.
 
       FIG. 2  is a block diagram of a typical OFDMA receiver. 
     Referring to  FIG. 2 , an FFT processor  202  converts a time-domain signal y(k) received through an antenna  200  to a frequency-domain signal by FFT and a descrambler  204  descrambles the FFT signals as modeled by Equation (6),
 
 z   p ( k )= sc   0 ( k ) y ( k )  (6)
 
where sc 0 (k) denotes a scrambling sequence allocated to the serving cell (e.g. BS 0 ). A channel compensator  208  compensates the descrambled signal using a channel frequency response estimated by a channel estimator  206  as expressed by Equation (7).
 
 z   R ( k )= z   p ( k )= sc   0 ( k ) y ( k )/ h   0 ( k )  (7)
 
where h 0 (k) denotes the channel frequency response estimated for the serving cell BS 0 .
 
     A combiner  210  accumulates the channel-compensated signal as many times as the repetition number R of the repeater  106  as given by Equation (8). 
                             z   ⁡     (   m   )       =       1             ⁢   R       ⁢       ∑     r   ⁢           =           ⁢   0               ⁢     R   ⁢           -           ⁢   1         ⁢       z             ⁢   R       ⁡     (     r   +   mR     )                       =         1             ⁢   R       ⁢       ∑     r   ⁢           =           ⁢   0               ⁢     R   ⁢           -           ⁢   1         ⁢       s             ⁢     R   ,           ⁢   0         ⁢     (     r   +   mR     )           +       1             ⁢   R       ⁢       ∑     r   ⁢           =           ⁢   0               ⁢     R   ⁢           -           ⁢   1         ⁢     i   ⁡     (     r   +   mR     )                     ⁢     
     ⁢       i   ⁢     (   k   )       =         sc             ⁢   0       ⁡     (   k   )       ⁢       (         h             ⁢   1       ⁢     (   k   )     ⁢     x             ⁢   1       ⁢     (   k   )       +     n   ⁢     (   k   )         )     /       h             ⁢   0       ⁡     (   k   )                     (   8   )               
where S R,0 (k) denotes a repeated transmission signal from BS 0  and I(k) denotes a noise and interference signal.
 
     Considering s R,0 (0+mR)=s R,0 (1+mR)= . . . =s R,0 (R−1+mR), then z (m) is given by Equation (9) as, 
                     z   ⁡     (   m   )       =         s   0     ⁡     (   m   )       +       1   R     ⁢       ∑     r   =   0       R   -   1       ⁢     i   ⁡     (     m   +   Mr     )                     (   9   )               
where s 0 (m) denotes a modulation symbol from BS 0 . Assuming that the noise and interference signal is Additive White Gaussian Noise (AWGN), an averaging effect reduces the noise and interference signal, thus improving the reception performance of the MT.
 
     Wireless Broadband (WiBro), which is an Institute of Electrical and Electronics Engineers (IEEE) 802.16-based OFDMA system, basically uses a frequency reuse factor of 1. With the frequency reuse factor of 1, the WiBro system is advantageous in terms of frequency efficiency but suffers from interference between BSs because all subcarriers used in one BS are overlapped with those of neighbor BSs. The neighbor BS interference signals degrade the reception performance of an MT at a cell boundary and often interrupt communications during handover. 
       FIG. 3  illustrates a typical MT located in an overlap region among three cells. Three BSs  312 ,  314  and  316  cover cell areas  302 ,  304  and  306 , respectively and an MT  310  is located equidistantly from BSs  312 ,  314  and  316 . In the case where the MT is located at any place within the overlap area as well as at the equidistant area, signals from BSs other than a serving BS selected by MT  310  interfere with a signal from the serving BS. To avert the interference problem at the cell boundary, the IEEE 802.16 standards provide that a low modulation order like QPSK, a low FEC coding rate, and a repetition number of up to 6 are used for a transmission signal from a BS. Despite these efforts, an existing MT receiver has a high outage probability in the vicinity of the cell boundary and experiences degradation of handover performance on a fading channel. To fundamentally solve this problem, the frequency reuse factor should be at least 3, but at the expense of decreasing frequency efficiency to ⅓ and cell planning complexity. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method and apparatus for preventing the degradation of the reception performance of an MT at a cell boundary, caused by neighboring cell interference signals in an OFDMA system. 
     Moreover, the present invention provides a method and apparatus for canceling neighboring cell interference at a receiver, in which BS-specific orthogonal codes or quasi-orthogonal codes of length R are appropriated to BSs, defining a Code Division Multiple Access (CDMA) slot by subcarriers allocated according to a repetition number R in an OFDMA system. 
     In accordance with an aspect of the present invention, there is provided a transmission method for reducing other cell interference in a wireless communication system, in which at least one modulation symbol to be transmitted is generated and the modulation symbol is mapped and assigned into a resource block common to a BS and at least one neighboring BS, and the mapped modulation symbol is spread with a BS-specific code allocated to the BS, the BS-specific codes of the BS being one of orthogonal and quasi-orthogonal to a BS-specific code allocated to the neighboring BS. 
     In accordance with another aspect of the present invention, there is provided a transmitter for reducing other cell interference in a wireless communication system, in which a symbol generator for generating at least one modulation symbols to be transmitted, a resource mapper for mapping and assigning the modulation symbols into a resource block common to a Base Station (BS) and at least one neighboring Base Station (BS), and a spreader for spreading the mapped modulation symbol with an BS-specific code allocated to the BS, the BS-specific codes of the BS being one of orthogonal and quasi-orthogonal to an BS-specific code allocated to the neighbor BS. 
     In accordance with the other aspect of the present invention, there is provided a receiving method for reducing other cell interference in a wireless communication system, in which a spread data is received through a resource block common to a Base Station (BS) and at least one neighboring Base Station (BS), the spread data is despreaded with an BS-specific code allocated to the BS and outputting at least one modulation symbol, the BS-specific codes of the BS being one of orthogonal and quasi-orthogonal to an BS-specific code allocated to the neighbor BS, and the modulated symbol is decoded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a typical OFDMA transmission structure; 
         FIG. 2  is a block diagram of a typical OFDMA receiver; 
         FIG. 3  illustrates a typical MT located in an overlap region among the cells of three BSs; 
         FIG. 4  is a block diagram of an OFDMA transmission structure according to the present invention; 
         FIG. 5  illustrates symbol repetition and code allocation on a frequency domain according to the present invention; 
         FIG. 6  illustrates symbol repetition and code allocation on a time domain according to the present invention; 
         FIG. 7  illustrates a variable CDMA slot structure according to the present invention; and 
         FIGS. 8 and 9  are graphs illustrating results of a simulation comparing the present invention with a conventional technology in terms of reception performance. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the preferred embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     The present invention provides a technique for preventing the reception performance of an MT at a cell boundary from being degraded due to neighboring cell interference signals in an OFDMA system such as WiBro, in which an OFDMA frame is divided into CDMA slots each having a time-frequency resource block of a predetermined size, and a plurality of BSs use orthogonal codes or less correlated codes (hereinafter, quasi-orthogonal codes) in each CDMA slot, that is, the BSs spread information symbols with BS-specific orthogonal codes rather than scramble the information symbols with scrambling sequences. 
     Referring to  FIG. 4 , transmitters  400 ,  420  and  440  are provided in a plurality of BSs i.e. BS 0 , BS 1  and BS 2  that control their own cells. FEC encoders  402 ,  422  and  442  encode information sequences as source data to be transmitted from their BSs i.e. BS 0 , BS 1  and BS 2 . Symbol mappers  404 ,  424  and  444  modulate the coded data in QPSK/16QAM/64QAM, thus producing modulation symbols s 0 (k), s 1 (k) and s 2 (k). Variable repeaters  406 ,  426 , and  446  operated as resource mappers, repeat the modulation symbols s 0 (k), s 1 (k) and s 2 (k) according to repetition numbers R set in their BSs (BS 0 , BS 1  and BS 2 ) and allocate the repeated symbols to a plurality of subcarriers. 
     R is variably set under the control of a higher-layer system. The repetition of the modulation symbols may occur in the time domain and/or the frequency domain. The frequency-domain repetition amounts to mapping the same modulation symbol to R subcarriers, and the time-domain repetition amounts to R occurrences of the same modulation symbol in R time intervals. 
     Spreaders  408 ,  428  and  448  spread the signal sequences S R,0 (k), S R,1 (k) and S R,2 (k) allocated to the subcarriers with BS-specific orthogonal codes oc 0 (k), oc 1 (k) and oc 2 (k). The orthogonal codes are of a length equal to the repetition number R, under control of the variable repeaters  406 ,  426  and  446 . IFFT processors  410 ,  430  and  450  convert the spread signals to time-domain signals. The IFFT signals are sent to users within the cell areas of BS 0 , BS 1  and BS 2  via antennas  412 ,  432  and  452 . 
     In a BS transmitter having the above configuration, information symbols occur R times in a CDMA slot of a length equal to the repetition number R. Neighbor BSs have the same CDMA slot structure at least and neighboring cell interference with an MT is minimized by use of spreading codes (i.e. orthogonal codes or quasi-orthogonal codes) of the same length as that of the CDMA slot. Preferred embodiments of the CDMA slot are illustrated in  FIGS. 5 and 6 . 
     Referring to  FIG. 5 , the horizontal axis represents the time domain and the vertical axis represents the frequency domain. For R=6, information symbols s( 0 ) and s( 1 ) occur six times on the frequency domain as they are multiplied by a spreading code of length 6 oc(r)={oc( 0 ), oc( 1 ), oc( 2 ), oc( 3 ), oc( 4 ), oc( 5 )} in a time interval t=0 (period=Ts). In each time interval, six subcarriers form a CDMA slot of length R. 
     From the perspective of a CDMA slot, conventionally, each BS scrambles information symbols with a pseudo-random Noise (PN) sequence, i.e. a scrambling sequence in each CDMA slot. On the other hand, in the present invention, each BS is allocated an orthogonal or quasi-orthogonal code of a length equal to the repetition number R and spreads information symbols with the orthogonal or quasi-orthogonal code, for transmission in each CDMA slot. Herein, it is said that orthogonal codes have orthogonality and quasi-orthogonal codes have quasi-orthogonality. The quasi-orthogonal codes are defined as codes having the least correlation between them, for a predetermined number of codes and a predetermined code length. 
     Referring to  FIG. 6 , the vertical axis represents the time domain and the vertical axis represents the frequency domain. Information symbols s( 0 ), s( 1 ) and s( 2 ) occur six times on the time domain as they each are multiplied by a spreading code of length 6 oc(r)={oc( 0 ), oc( 1 ), oc( 2 ), oc( 3 ), oc( 4 ), oc( 5 )}. Six time intervals t=0, Ts, . . . , 5 Ts form a CDMA slot on each subcarrier. 
     When a channel varies little within the CDMA slot having the above configuration, an MT can effectively eliminate neighboring cell signal components by despreading a received signal with the orthogonal code of its serving BS. The receiver of the MT has almost the same configuration as shown in  FIG. 2  and thus its detailed description is not provided herein. 
     The MT receiver of the present invention differs in operation from the receiver illustrated in  FIG. 2  in that the descrambler is replaced with a despreader that receives the BS-specific orthogonal code of the serving cell and information describing a CDMA slot structure from a higher-layer system or the BS and eliminates neighbor cell interference signals by despreading a signal received in a CDMA slot with the orthogonal code. Another difference is that a combiner accumulates the despread data as many times as a predetermined repetition number R. 
     A preferred embodiment of the present invention will be described referring to equations. It is assumed herein that a serving cell is BS 0  (i=0) and one interfering neighboring BS BS 1  (i=1) exists. An OFDM frame is so configured that information symbols are repeated over R subcarriers in the frequency domain, as illustrated in  FIG. 5 . That is, a CDMA slot is formed with R subcarriers in each time interval. 
     In accordance with the preferred embodiment of the present invention, instead of a scrambling sequence of period N used in the conventional technology, a spreading code of length R is allocated to a time-frequency resource block, i.e. a CDMA slot in which an information symbol is repeated. The time-frequency resource block is commonly allocated to the same logical channel in a plurality of BSs. The spreading code is an orthogonal code given as Equation (10), 
                       ∑     r   =   0       R   -   1       ⁢         oc   0     ⁡     (   r   )       ⁢       oc   1     ⁡     (   r   )           =   0           (   10   )               
where oc 0 (r) denotes an orthogonal code allocated to BS 0 , oc 1 (r) denotes an orthogonal code allocated to BS 1 , R denotes a repetition number (the length of a repeated area), the length of a CDMA slot, or the length of the orthogonal code. If R is an exponent of 2, for example, R=2, 4, or 8, the orthogonal codes are preferably Walsh codes. This property is different from that of scrambling sequences being PN sequences, expressed as
 
     
       
         
           
             
               
                 ∑ 
                 
                   r 
                   = 
                   0 
                 
                 
                   R 
                   - 
                   1 
                 
               
               ⁢ 
               
                 
                   
                     sc 
                     0 
                   
                   ⁡ 
                   
                     ( 
                     r 
                     ) 
                   
                 
                 ⁢ 
                 
                   
                     sc 
                     1 
                   
                   ⁡ 
                   
                     ( 
                     r 
                     ) 
                   
                 
               
             
             ≠ 
             0. 
           
         
       
     
     For R=6 and one neighbor BS, oc 0 (r)={1, 1, 1, 1, 1, 1} and oc 1 (r)={1, −1, 1, −1, 1, −1}, for example. As the orthogonal codes are used in each CDMA slot, signals from the BSs are orthogonal to each other in the CDMA slot. 
     Meanwhile, the receiver eliminates the repetition effect through the despreader and the combiner. The resulting signal z(m) is expressed by Equation (11) as,
 
 z ( m )= s   0 ( m )+ I ( m )  (11)
 
The interference and noise I(m) is given by Equation (12),
 
     
       
         
           
             
               
                 
                   
                     I 
                     ⁡ 
                     
                       ( 
                       m 
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       R 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           r 
                           = 
                           0 
                         
                         
                           R 
                           - 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 
                                   oc 
                                   1 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   r 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 
                                   h 
                                   1 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     r 
                                     + 
                                     mR 
                                   
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 
                                   s 
                                   
                                     R 
                                     , 
                                     1 
                                   
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     r 
                                     + 
                                     mR 
                                   
                                   ) 
                                 
                               
                             
                             + 
                             
                               n 
                               ⁡ 
                               
                                 ( 
                                 
                                   r 
                                   + 
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                                 ) 
                               
                             
                           
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                         ⁢ 
                         
                           
                             
                               oc 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               r 
                               ) 
                             
                           
                           / 
                           
                             
                               h 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               
                                 r 
                                 + 
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                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     When the length of the repeated area R is relatively small, the channel changes less within the repeated area. Therefore, the channel frequency characteristics h 0  and h 1  of BS 0  and BS 1  are approximated by Equation (13),
 
 h   0 ( mR )≈ h   0 ( mR+ 1)≈ . . . ≈ h   0 ( mR+R− 1)
 
 h   1 ( mR )≈ h   1 ( mR+ 1)≈ . . . ≈ h   1 ( mR+R− 1)  (13)
 
     Hence, I(m) is approximated by Equation (14), 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       ⁡ 
                       
                         ( 
                         m 
                         ) 
                       
                     
                     = 
                     
                       
                         
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                           ⁡ 
                           
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                         ⁢ 
                         
                           
                             ∑ 
                             
                               r 
                               = 
                               0 
                             
                             
                               R 
                               - 
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                           ⁢ 
                           
                             
                               
                                 oc 
                                 1 
                               
                               ⁡ 
                               
                                 ( 
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                             ⁢ 
                             
                               
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                         ⁢ 
                         
                           
                             ∑ 
                             
                               r 
                               = 
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                               - 
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                           ⁢ 
                           
                             
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                                 ⁡ 
                                 
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                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
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                           ( 
                           m 
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                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     Owing to the orthogonality represented as equation (10), neighboring cell interference signals are entirely cancelled. Thus, a final noise signal is given by Equation (15), 
     
       
         
           
             
               
                 
                   
                     I 
                     ⁡ 
                     
                       ( 
                       m 
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       R 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           r 
                           = 
                           0 
                         
                         
                           R 
                           - 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           n 
                           ⁡ 
                           
                             ( 
                             
                               r 
                               + 
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                             ) 
                           
                         
                         ⁢ 
                         
                           
                             
                               oc 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               
                                 r 
                                 + 
                                 mR 
                               
                               ) 
                             
                           
                           / 
                           
                             
                               h 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               
                                 r 
                                 + 
                                 mR 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     As stated before, the size and configuration of the CD slot can be changed by changing the repetition number R and modifying the orthogonal codes. Referring to  FIG. 7 , in an OFDMA frame defined by a plurality of subcarriers and a plurality of time intervals, CDMA slot  0  is composed of six subcarriers (k=0 to 5) in a time interval t=0, CDMA slot  1  is composed of four time intervals (t=Ts to 4 Ts) on a subcarrier k=3, CDMA slot  2  is composed of total time intervals (t=0 to 5 Ts) on a subcarrier k=8, CDMA slot  3  is composed of four time intervals (t=0 to 3 Ts) on a subcarrier k=9, and CDMA slot  4  is composed of eight subcarriers (k=0 to 7) in a time interval t=5 Ts. 
     In this way, CDMA slots of different sizes and different resource positions can be allocated to an OFDMA frame. Orthogonal codes are set for each of the CDMA slots and allocated to BSs for use in the CDMA slot. Thus, various orthogonal codes can be applied to the CDMA slots. 
     The transmission technique of the present invention for a WiBro system was simulated under the conditions that the total number of available subcarriers N is 1024, a frequency reuse factor N used  is 864, a repetition number R is 6, repetition occurs only on the frequency domain, and two neighboring BSs exist in addition to a serving BS. The following orthogonal codes were used, for example:
 
 oc   0 ( k )={1,1,1,1,1,1}
 
 oc   1 ( k )={1,−1,1,−1,1,−1}
 
 oc   0   k )={1,1,−1,−1,1,−1}
 
     A flat fading channel environment and a pedestrian B channel environment with a background noise level of −20 dB are assumed. 
     The results of the simulation in the above channel environment are illustrated in  FIGS. 8 and 9 . 
       FIG. 8  is a graph comparing use of orthogonal codes according to the present invention (Proposed Code Spreading) with conventional use of scrambling sequences (PN Code Spreading) under the flat fading channel environment, in terms of Bite Error rate (BER) versus Channel Impulse Response (CIR). As noted from the graph, since the two cases demonstrate the same channel characteristics over successive subcarriers with R=6 in the flat channel environment, code orthogonality (or low correlation coefficient) is maintained between the BSs. Therefore, interference from the two neighboring BSs is considerably reduced. 
       FIG. 9  is a graph comparing Proposed Code Spreading with PN Code Spreading under the pedestrian B channel environment, i.e. the frequency-selective channel environment in terms of BER versus CIR. 
     Referring to  FIG. 9 , the channel changes within a single CDMA slot, which changes the correlation property between orthogonal codes. Despite the resulting imperfect cancellation of other cell interference, Proposed Code Spreading outperforms PN Code Spreading by an about 5 dB gain at BER=10e−2. 
     As described above, the present invention advantageously increases the reception performance of an MT located at a cell boundary between BSs and further increases handover performance by canceling other cell interference in an OFDMA system. Specifically, a repeated area of an information symbol is defined as a CDMA slot and orthogonal codes or less-correlated codes are allocated to each CDMA slot, for spreading information symbols. Therefore, other cell interference is considerably reduced. 
     While the invention has been shown and described with reference to certain preferred embodiments of the present invention 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 further defined by the appended claims and their equivalents.