Abstract:
A system and a method for spreading/despreading in MC-CDMA are disclosed. Simple spreading and despreading procedures are provided to allow both frequency and time spreading/despreading. The system and method for spreading/despreading in MC-CDMA produce a high processing gain, frequency diversity, and multipath time diversity through two-dimensional time/frequency spreading/despreading. Moreover, the spreading/despreading system and method support a variable transmission rate by simply changing the spreading factor without changing the spreading/despreading procedure and structure.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is related to a system and method for spreading/despreading in multi-carrier code division multiple access (MC-CDMA), and more particularly, to a system and method for spreading/despreading in MC-CDMA for easily carrying out two-dimensional time/frequency spreading/despreading of direct spreading in time domain and frequency spreading over multiple subcarriers, and easily supporting a variable spreading factor.  
           [0003]    2. Background of the Related Art  
           [0004]    [0004]FIG. 1 illustrates the configuration of a transmitter of a conventional MC-CDMA system. Referring to FIG. 1, the transmitter of the conventional MC-CDMA includes a serial/parallel converter  11 , a modulation symbol mapping unit  12 , a symbol copying unit  13 , a multiplier  14 , an inverse Fourier transform unit  15 , and a guard time insertion unit  16 .  
           [0005]    The serial/parallel converter  11  divides a bit sequence to be transmitted into J parallel bit streams through serial-to-parallel conversion or demultiplexing. The modulation symbol mapping unit  12  converts each of the bit streams divided by the serial/parallel converter  11  into a complex symbol sequence according to a modulation method such as M-ary PSK or M-ary QAM.  
           [0006]    The symbol copying unit  13  copies each modulated symbol to produce as many parallel symbol sequences as there are subcarriers with which the symbols are to be spread and transmitted, i.e. the spreading factor (SF F ). Then, the multiplier  14  multiplies each of the copied symbols by a code chip C n  assigned to the corresponding subcarrier. Here, a Walsh code can be used as the spreading code (C=&lt;C 0 , C 1 , . . . , C SF     F     −1 &gt;) for orthogonality among signals simultaneously transmitted.  
           [0007]    In general, Walsh codes are used together with pseudo-noise scrambling, in order to discriminate among different signal sources such as base stations or mobile stations.  
           [0008]    The inverse Fourier transform unit  15  transforms the J×SF F  spread chips into a sample sequence transmitted over J×SF F  subcarriers. Here, the inverse Fourier transform unit  15  uses Inverse Discrete Fourier Transform (IDFT) for frequency transform or Inverse Fast Fourier Transform (IFFT).  
           [0009]    The number of subcarriers used for frequency transform of each symbol sequence corresponds to SF F , which is the spreading factor. For all the J symbol sequences, the number of subcarriers is equal to J×SF F . A transmission symbol generated by the inverse Fourier transform unit  15  includes J×SF F  samples.  
           [0010]    The guard time insertion unit  16  adds a guard time to the head of each transmission symbol in order to avoid inter-symbol interference that is generated due to multi-path propagation in channels. Finally, the transmission symbol with guard time is converted into RF signal to be transmitted.  
           [0011]    [0011]FIG. 2 illustrates the configuration of the spreading system corresponding to one modulation symbol sequence in FIG. 1, and FIG. 3 illustrates the configuration of a spreading system of MC-CDMA having a spreading factor reduced P times.  
           [0012]    As shown in FIG. 2, the spreading procedure is carried out in such a manner that the symbol copying unit  13  copies each modulated symbol S K  to produce as many parallel symbols as there are subcarriers, SF F , over which the symbol is to be spread and transmitted, and the multiplier  14  multiplies each copied symbol by one code chip (C n (n=0, 1, . . . , SF F −1)) allocated to a corresponding subcarrier, among the spectrum spreading code C. The parallel chip sequences generated by the multiplier  14  are fed to the inverse Fourier transform unit  15  for multi-carrier transmission.  
           [0013]    The CDMA system may employ a method of varying the spreading factor for each symbol in order to provide a variable transmission rate for appropriately controlling a data transmission rate according to a variation in the data generation rate or a variation in the channel state, as in the WCDMA technology of a third-generation mobile communication system.  
           [0014]    In the case where the variable spreading factor is applied to the spreading procedure in the MC-CDMA system of FIG. 2, in order to spread and transmit the modulated symbols with a spreading factor SF F /P for a P-time increased transmission rate, the spreading structure must be changed into one as shown in FIG. 3, which includes a serial/parallel converter  21 , a symbol copying unit  22 , a multiplier  23 , and an inverse Fourier transform unit  24 .  
           [0015]    In such conventional method, there is a problem that the number of branches generated in parallel in the serial/parallel conversion procedure, that is, the number of branches corresponding to the outputs of the serial/parallel converter  21  or the inputs of the inverse Fourier transform unit  24 , should be controlled according to P, and the connection between the components in the spreading system should be rearranged.  
           [0016]    [0016]FIG. 4 shows a despreading system of a conventional MC-CDMA receiver. Referring to FIG. 4, the conventional despreading system includes a Fourier transform unit  31 , a multiplier  32 , a weighting unit  33 , a combiner  34 , and a parallel/serial converter  35 .  
           [0017]    The Fourier transform unit  31  performs a Fourier transform of a received sample sequence and divides it into SF F  parallel chip sequences corresponding to each subcarrier. The multiplier  32  and weighting unit  33  multiply each of the symbols by a code chip C n  corresponding to each subcarrier and an appropriate weight W n , respectively. The combiner  34  combines as many chips as the spreading factor, SF F /P, and the parallel/serial converter  35  converts the combined chips into one symbol.  
           [0018]    A weight for each of the received chips is determined according to a chip combing method. The combining method includes ORC (Orthogonality Restoring Combining), EGC (Equal Gain Combining), MRC (Maximal Ratio Combining), MMSEC (Minimum Mean Square Error Combining), and the like. For example, the weighting unit  33  may simply perform the combining with the same weight for all received chips.  
           [0019]    Similar to the spreading procedure of the transmitter shown in FIG. 3, the despreading procedure of the receiver shown in FIG. 4 also has a problem in that the number of chips to be combined should be changed and the connection of branches to the combiner  34  should be rearranged according to the varying spreading factor, SF F /P.  
           [0020]    In the case where both the frequency spreading/despreading and the direct time spreading/despreading are simultaneously required, the aforementioned conventional spreading/despreading methods should add direct spreading and despreading procedures for each branch and appropriately arrange a code corresponding to each branch for carrying out two-dimensional frequency/time spreading and despreading.  
           [0021]    Furthermore, in the case where the variable spreading factor is applied to the two-dimensional frequency/time spreading and despreading, spreading and despreading structures should be rearranged depending on the varying spreading factor.  
         SUMMARY OF THE INVENTION  
         [0022]    Accordingly, the present invention is related to a system and a method for spreading/despreading in MC-CDMA that substantially solves one or more problems due to limitations and disadvantages of the related art.  
           [0023]    An object of the present invention is to provide a system and a method for spreading/despreading in MC-CDMA for easily carrying out two-dimensional hybrid time/frequency spreading and despreading and easily providing a variable spreading factor.  
           [0024]    To accomplish the object of the present invention, a spreading system in MC-CDMA is provided, which comprises a symbol repeating unit for repeating each symbol of a modulated symbol sequence for spreading, with the number of repetitions, R, which is determined by a variable spreading factor; a spreading unit for performing a direct spreading for the repeated symbols using a spreading code of the same chip rate as the repeated symbol rate, to obtain a spread chip sequence; a demultiplexer for dividing the spread chip sequence into as many parallel chip sequences as there are subcarriers, SF F , as a serial/parallel conversion; and an inverse Fourier transform unit for transforming the chips into a transmission symbol, which is a sample sequence including SF F  samples, for multi-carrier transmission.  
           [0025]    To accomplish the object of the present invention, a despreading system in MC-CDMA is provided, which comprises a Fourier transform unit for extracting parallel chip sequences from signal samples received over subcarriers using Discrete Fourier Transform or Fast Fourier Transform; a multiplexer for converting the parallel chip sequences into a single chip sequence; a despreading unit for performing a direct despreading for the single chip sequence using the same spreading code and chip rate as those used by the transmitting side; and a chip combining unit for sequentially combining the chip samples, corresponding to the number of chips to be combined, R, which is determined by a variable spreading factor SF, to output a received symbol sequence.  
           [0026]    To accomplish the object of the present invention, a spreading method in MC-CDMA is also provided, which comprises a step (a) of repeating each symbol of a modulated symbol sequence for spreading, by the number of repetitions, R, which is determined by a variable spreading factor; a step (b) of performing a direct spreading for the symbols repeated at the step (a) using a spreading code of the same chip rate as the repeated symbol rate, and then dividing the spread symbol into as many parallel chips as there are subcarriers, SF F , as a serial/parallel conversion; and a step (c) of transforming the chips into a sample sequence for multi-carrier transmission, using inverse Discrete Fourier Transform or Fast Fourier Transform.  
           [0027]    To accomplish the object of the present invention, a despreading method in MC-CDMA is also provided, which comprises a step (a) of extracting parallel chip sequences received over subcarriers from a signal received from a transmitting side, using Discrete Fourier Transform or Fast Fourier Transform; a step (b) of converting the parallel chip sequences extracted at the step (a) into a single chip sequence, as a multiplexing, and performing a direct despreading for the single chip sequence using the same spreading code and chip rate as those used by the transmitting side; and a step (c) of sequentially combining the chip samples, despread at the step (b), corresponding to the number of chips to be combined, R, which is determined by a variable spreading factor SF, to output a received symbol sequence.  
           [0028]    To accomplish the object of the present invention, there is provided a spreading/despreading method in MC-CDMA, which comprises a spreading step of repeating each symbol of a modulated symbol sequence for spreading by the number of repetitions, R, which is determined by a variable spreading factor, performing a direct spreading for the repeated symbols using a spreading code, demultiplexing the spread chips into as many parallel chip sequences as there are subcarriers, SF F , and transforming the chips into a transmission symbol for multi-carrier transmission using inverse Discrete Fourier Transform or Fast Fourier Transform; and a despreading step of extracting parallel chip sequences received over subcarriers from a received signal using Discrete Fourier Transform or Fast Fourier Transform, multiplexing the parallel chip sequences into a single chip sequence, performing a direct despreading for the single chip sequence using the same spreading code and chip rate as those used by the transmitting side, and sequentially combining the despread chip samples corresponding to the number of chips to be combined, R, which is determined by the variable spreading factor SF, to output a received symbol sequence.  
           [0029]    The spreading and despreading steps may perform two-dimensional frequency/time spreading/despreading of frequency spreading over subcarriers and direct spreading in the time domain. In the steps of Fourier transform, demultiplexing, and multiplexing, the number of the parallel chip sequences is matched with the spreading factor over subcarriers in the frequency domain.  
           [0030]    It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings;  
         [0032]    [0032]FIG. 1 illustrates the configuration of a transmitter of a conventional MC-CDMA system;  
         [0033]    [0033]FIG. 2 illustrates the spreading procedure corresponding to one symbol sequence in FIG. 1;  
         [0034]    [0034]FIG. 3 illustrates the configuration of a spreading system of MC-CDMA having a spreading factor reduced P times;  
         [0035]    [0035]FIG. 4 illustrates a despreading system of a conventional MC-CDMA receiver;  
         [0036]    [0036]FIG. 5 illustrates the configuration of a spreading system in MC-CDMA according to a first embodiment of the present invention;  
         [0037]    [0037]FIG. 6 illustrates the configuration of a spreading system in MC-CDMA according to a second embodiment of the present invention;  
         [0038]    [0038]FIG. 7 illustrates the configuration of a spreading system in MC-CDMA according to a third embodiment of the present invention;  
         [0039]    [0039]FIG. 8 illustrates the configuration of a spreading system in MC-CDMA according to a fourth embodiment of the present invention; and  
         [0040]    [0040]FIG. 9 illustrates the configuration of a despreading system in MC-CDMA according to a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0042]    [0042]FIG. 5 illustrates the configuration of a spreading system in MC-CDMA according to a first preferred embodiment of the present invention. Referring to FIG. 5, the spreading system according to the first embodiment of the present invention includes a symbol repeating unit  51 , a spreading unit  52 , a demultiplexer  53 , and an inverse Fourier transform unit  54 .  
         [0043]    The symbol repeating unit  51  repeats each symbol of a modulated symbol sequence for spreading, by the number of repetitions R determined by a variable spreading factor. The spreading unit  52  performs a direct spreading for the repeated symbol using a spreading code of the same chip rate as the repeated symbol rate. The demultiplexer, which is a serial/parallel converter  53 , divides the spread chips generated by the spreading unit into as many parallel chip sequences as there are subcarriers, SF F .  
         [0044]    The inverse Fourier transform unit, transforms the parallel chips into a sample sequence for multicarrier transmission using inverse Discrete Fourier Transform or Fast Fourier Transform.  
         [0045]    The operation of the spreading system according to the first embodiment of the invention, constructed as above, is described below in more detail through a spreading method in MC-CDMA according to the present invention.  
         [0046]    The symbol repeating unit  51  repeats each symbol of the modulated symbol sequence by the number of repetitions R that is matched with the spreading factor SF F . Thus, the repeated symbol becomes a symbol sequence, S K =&lt;S K,0 , S K,1 , . . . , S K,SF     F     −1 &gt;, which is obtained in a manner such that the corresponding symbol S K  is repeated R=SF F  times.  
         [0047]    The spreading unit  52  perform a direct spreading for the repeated symbol using a spreading code (C=&lt;C 0 , C 1 , . . . , C SF     F     −1 &gt;), as represented in the following expression (1).  
           C·S   K   =&lt;C   0   S   K   , . . . , C   SF     F     −1   S   K &gt;  (1)  
         [0048]    In expressions of embodiments of the present invention, A·B represents that components of two vectors A and B at the same location are multiplied by each other.  
         [0049]    Here, the spreading code can be assigned a unique code for the discrimination of the desired signal, and a code obtained by combining a Walsh code for orthogonality and a pseudo-noise scrambling code can be used as the spreading code.  
         [0050]    The symbol rate of the symbol repeated by the symbol repeating unit  51  is matched with the chip rate of the spreading code. The chip sequence spread by the spreading unit  52  is divided into SF F  parallel chip sequences by the serial/parallel converting unit  53 , and the parallel chips is transformed into a transmission symbol for multicarrier transmission by the inverse Fourier-transform unit  54 .  
         [0051]    The spreading procedure according to the first embodiment of the invention, shown in FIG. 5, brings the same results as that of the spreading procedure described above with reference to FIG. 2, so as to spread and transmit a modulation symbol over subcarriers.  
         [0052]    [0052]FIG. 6 illustrates the configuration of a spreading system in MC-CDMA according to a second preferred embodiment of the present invention.  
         [0053]    In the spreading procedure according to the first embodiment of the invention, as shown in FIG. 6, increasing the symbol (or data) transmission rate P times and spreading with a variable spreading factor SF F /P can be easily performed by simply reducing the number of symbol repetition R to be SF F /P. FIG. 6 shows a symbol spreading procedure in the case of P=2, which leads to the same result as that of the spreading procedure described in FIG. 3. Modulated symbols S K  and S K+1  are inputted into the symbol repeating unit  51  at the symbol rate 2R s , which is twice the symbol rate of the first embodiment. Then, the symbol repeating unit  51  repeats each symbol R=SF F /2 times, which is a half of the repetition number of the first embodiment, as represented by the following expression (2).  
         &lt; S   K   ,S   K+1   &gt;=&lt;S   K   ,S   K   , . . . , S   K   ,S   K+1   , S   K+1   , . . . , S   K+1 &gt;  (2)  
         [0054]    Here, the symbol rate of the symbols repeated by the symbol repeating unit  51  is matched with that of the first embodiment, and it is always matched with the system chip rate.  
         [0055]    The repeated symbols are spread by the spreading unit  52  according to the spreading code C, as represented in the following expression (3). At this time, the symbol S K  is spread by a subcode C 0  including SF F /2 chips in the front of the spreading code C, and the symbol S K+1  is spread by a subcode C 1  including SF F /2 chips in the rear of the spreading code C, which are shown in the expressions (4) and (5).  
         &lt; C   0   ·S   K   ,C   1   ·S   K+1   &gt;=&lt;C   0   S   K   ,C   1   S   K   , . . . , C   SF     F     /2−1   S   K   ,C   SF     F     /2   S   K+1   ,C   SF     F     /2+1   S   K+1   , . . . ,C   SF     F     −1   S   K+1 &gt;  (3)  
           C   0   =&lt;C   0   ,C   1   , . . . , C   SF     F     /2−1 &gt;  (4)  
           C   1   =&lt;C   SF     F     /2   ,C   SF     F     /2+1   , . . . , C   SF     F     −1 &gt;  (5)  
         [0056]    The spread chip sequence &lt;C 0 ·S K , C 1 ·S K+1 &gt; is divided into SF F  parallel chips by the serial/parallel converter  53 , and the chips are then transformed into a sequence for multi-carrier transmission by the inverse Fourier transform unit  54 , as in the first embodiment of FIG. 5.  
         [0057]    While the symbol S K  is spread over SF F  subcarriers and transmitted in the first embodiment of the invention shown in FIG. 5, the symbols S K  and S K+1  are respectively spread over SF F /2 subcarriers and transmitted in the second embodiment of the invention shown in FIG. 6, which means that the symbols are transmitted at the spreading rate reduced by half.  
         [0058]    [0058]FIG. 7 illustrates the configuration of a spreading system in MC-CDMA according to a third preferred embodiment of the present invention. The MC-CDMA system shown in FIG. 7 simultaneously uses both frequency spreading over subcarriers and direct time spreading as in the DS-CDMA system to obtain processing gain and diversity gain according to an increase in the spreading factor. The third embodiment of the invention has the same configuration as that of the first embodiment shown in FIG. 5.  
         [0059]    In the third embodiment of the present invention, the spreading factor (SF) is represented by SF=SF F ×SF T  because of two-dimensional hybrid time/frequency spreading. Here, SF F  represents the spreading factor in the frequency domain, and SF T  means the spreading factor in the time domain.  
         [0060]    In the case where the modulated symbol is spread with this two-dimensional spreading factor, the number of parallel branches corresponding to the outputs of the serial/parallel converter and the inputs of the inverse Fourier transform unit is matched with the frequency spreading factor SF F , as shown in FIG. 7. For instance, in FIG. 7 the modulated symbol S K  is two-dimensionally spread with the spreading factor, SF=SF F ×SF T =4×4=16.  
         [0061]    [0061]FIG. 8 illustrates the configuration of a spreading system in MC-CDMA according to a fourth preferred embodiment of the present invention. FIG. 8 shows a procedure in which spreading is carried out by simply controlling the symbol repetition number to R=SF/P without changing the configuration of the spreading system as in the aforementioned embodiments in the case where the variable spreading factor, SF/P, is applied to the two-dimensional time/frequency spreading.  
         [0062]    In case of SF=16, to reduce the spreading factor by a half for the purpose of increasing the symbol transmission rate by two times, i.e., P=2, the symbol S K  is spread by the code C 0  corresponding to the first eight chips of the sixteen chips of the spreading code, and the symbol S K+1  is spread by the code C 1  corresponding to the remaining eight chips of the sixteen chips of the spreading code.  
         [0063]    [0063]FIG. 9 illustrates the configuration of a despreading system in MC-CDMA according to a preferred embodiment of the present invention. Referring to FIG. 9, the despreading system according to the preferred embodiment of the invention includes a Fourier transform unit  61 , a multiplexer  62 , a despreader  63 , a weighting unit  64 , and a chip combiner  65 .  
         [0064]    The Fourier transform unit  61  extracts parallel chip sequences received over subcarriers from a received signal by using DFT or FFT. The multiplexer, which is a parallel/serial converter  62 , converts the parallel chip sequences into a single chip sequence.  
         [0065]    The despreader  63  directly despreads the single chip sequence outputted from the parallel/serial converter  62  using the same spreading code as used by the transmitter, at the same chip rate. The weighting unit  64  multiplies each chip by a weight which is used for chip combining.  
         [0066]    The chip combiner  65  sequentially combines R contiguous chips, where R is determined by the variable spreading factor SF, to output a received modulated symbol sequence.  
         [0067]    In the despreading procedure at the receiving end, the variable spreading factor can be applied to the two-dimensional hybrid spreading as well as the one-dimensional spreading in the frequency domain as in the spreading procedure described with reference to FIGS.  5  to  8 .  
         [0068]    It is apparent that the one-dimensional spreading/despreading can be seen as a part of the two-dimensional hybrid spreading/despreading. Therefore, the two-dimensional hybrid despreading is only described.  
         [0069]    The operation of the two-dimensional hybrid despreading system in MC-CDMA according to the preferred embodiment of the invention is as follows.  
         [0070]    At the receiving side, the Fourier transform unit  61  transforms every SF F  samples of a received signal into SF F  parallel samples over SF F  subcarriers, and the parallel/serial converter  62  converts a serial sample sequence including SF F  samples, as represented by the following expression (6).  
           Z   0   =&lt;Z   0   ,Z   1   , . . . , Z   SF     F     −1 &gt;  (6)  
         [0071]    A received chip sequence including chip samples corresponding to the spreading factor SF can be obtained by carrying out the Fourier transform and parallel/serial conversion procedures sequentially SF T  times, and it can be represented by the following expression (7).  
           Z=&lt;Z   0 ,Z 1 , . . . , Z SF     T     −1   &gt;   
         [0072]    =&lt;Z 0 ,Z 1   , . . . , Z   SF     F     −1   ,Z   SF     F     ,Z   SF     F     +1 , . . . , Z 2SF     F     −1   , . . . , Z   (SF     T     −1   )SF     F     ,Z   (SF     T     −1)SF     F     +1   , . . . , Z   SF     T     SF     F     −1   &gt;=&lt;Z   0   ,Z   1   , . . . , Z   SF−1 &gt;  (7) 
         [0073]    The despreader  63  performs a direct despreading for the chip sequence using the same code C as the spreading code used by the transmitter side, as represented in the following expression (8).  
         
       C·Z 
       0 
       =&lt;C 
       0 
       ·Z 
       0 
       ,C 
       1 
       ·Z 
       1 
       , . . . , C 
       SF 
       
         T 
       
       −1 
       ·Z 
       SF 
       
         T 
       
       −1 
       &gt; 
     
         [0074]    =&lt;C 0   Z   0   ,C   1   Z   1   , . . . , C   SF     F     −1   Z   SF     F     −1   ,C   SF     F     ,C   2SF     F     −1   Z   SF     F     +1   , . . . , C   2SF     F     −1   Z   2SF     F     −1   , . . . , C   (SF     T     −1)SF     F     Z   (SF     T     −1)SF     F     ,C   (SF     T     −1)SF     F     +1   Z   (SF     T     −1)SF     F     +1   , . . . , C   SF     T     SF     F     −1   Z   SF     T     SF     F     −1   &gt;=&lt;C   0   Z   0   ,C   1   Z   1   , . . . , C   SF−1   Z   SF−1 &gt;  (8) 
         [0075]    The weighting unit  64  multiplies each of the chips of the expression (8) by a weight w, as represented in the following expression (9). Here, the weight values used for chip combining can be determined by a method of ORC, EGC, MRC, MMSEC, or the like.  
           X=W·C·Z=&lt;W   0   C   0   ·Z   0   ,W   1   ·C   1   ·Z   1   , . . . , W   SF     T     −1   ·C   SF     T     −1   ·Z   SF     T     −1   &gt;=&lt;W   0   C   0   Z   0   ,W   1   C   1   Z   1   , . . . ,W   SR−1   C   SF−1   Z   SF−1   &gt;=&lt;X   0   ,X   1   , . . . ,X   SF−1 &gt;  (9)  
         [0076]    The chip combiner  65  combines R=SF/P contiguous chip samples as, to obtain a received modulated symbol, Y m , as represented by the following expression (10).  
                 Y   m     =       ∑     n   =   0       R   -   1            X     mR   +   n           ,                m   =   0     ,   1   ,   …              ,     P   -   1             (   10   )                               
 
         [0077]    This chip-combining procedure is sequentially repeated P times so as to obtain P received symbols &lt;Y 0 , Y 1 , . . . , Y P−1 &gt;, which is corresponding to the despreding by the whole spreading code C.  
         [0078]    Though spreading and despreading procedures for one symbol sequence were explained in the aforementioned embodiments, the embodiments of the invention can be applied to spreading and despreading for each symbol sequence in a MC-CDMA system that transmits more than one symbol sequences in parallel.  
         [0079]    Furthermore, the embodiments of the present invention can be employed irrespective of a modulation type, code type, and Fourier transform procedure.  
         [0080]    In the MC-CDMA system, it can be required to simultaneously use both frequency spreading over subcarriers and direct spreading in time domain as in DS-CDMA systems. This two-dimensional frequency/time spreading provides a spreading factor larger than the number of subcarriers used in the system so that it is employed when a large processing gain is needed.  
         [0081]    In the case where a signal transmitted by the two-dimensional spreading method is received through a multipath fading channel, it is possible to obtain frequency diversity according to spreading over subcarriers, and multipath time diversity by the direct spreading and Rake receiver in addition to a high processing gain.  
         [0082]    The forgoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present invention can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.  
         [0083]    The spreading/despreading system and method in MC-CDMA according to the present invention provides simple spreading and despreading procedures so that frequency spreading/despreading over subcarriers and time direct spreading/despreading can be simultaneously carried out. Furthermore, the present invention may easily realize a variable spreading factor for supporting a variable transmission rate. Therefore, the spreading/despreading system and method in MC-CDMA according to the present invention can obtain frequency diversity and multipath time diversity in addition to a high processing gain through two-dimensional time/frequency spreading/despreading, and can easily provide a variable spreading factor through the same procedure as that of the prior art.