Abstract:
A transmission method and apparatus in a mobile communication system, which modulates input data with a specific size into an OFDM symbol before transmission. In the transmission apparatus, a controller determines to transmit replica data instead of the input data, if the input data is retransmission data. A replica generator generates replica data by cyclically-circulating the input data under the control of the controller. An IFFT block generates an OFDM symbol by IFFT-transforming the replica data.

Description:
PRIORITY  
         [0001]    This application claims priority to an application entitled “ARQ Apparatus and Method Using Frequency Diversity in an OFDM Mobile Communication System” filed in the Korean Industrial Property Office on Nov. 10, 2001 and assigned Serial No. 2001-69995, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to an OFDM (Orthogonal Frequency Division Multiplexing) mobile communication system, and in particular, to an apparatus and method for retransmitting data using frequency diversity.  
           [0004]    2. Description of the Related Art  
           [0005]    During downlink data communication in a mobile communication system, a UE (User Equipment) is assigned a downlink channel such as a dedicated channel (DCH) from a UTRAN (UMTS Terrestrial Radio Access Network), e.g., a Node B, and receives data over the assigned downlink channel. The mobile communication system includes a satellite system, an ISDN (Integrated Services Digital Network) system, a digital cellular system, a W-CDMA (Wideband-Code Division Multiple Access) system, a UMTS (Universal Mobile Telecommunications System) system, and an IMT-2000 (International Mobile Telecommunication-2000) system. The UE, if having correctly received packet data, transmits the received packet data to an upper layer. However, if defective packet data is received, the UE transmits a retransmission request for the defective packet data using an ARQ (Automatic Repeat Request) technique. The ARQ is a technique for sending a retransmission request for received packet data upon detecting an error in the received packet data.  
           [0006]    A brief description of the ARQ technique will be made herein below.  
           [0007]    The UE first receives initial packet data over a dedicated channel established by the Node B, and then determines whether an error occurs in the received initial packet data. If it is determined that an error occurs in the received initial packet data, the UE transmits a NACK signal, or a retransmission request signal for the initial packet data, to the Node B. The retransmission request signal NACK includes packet identification information, and the packet identification information includes a version number and a sequence number for the packet data. Thus, the Node B can identify information on the packet data to be retransmitted as soon as it receives the retransmission request. Upon receiving a retransmission request signal NACK transmitted by the UE, the Node B retransmits retransmission packet data corresponding to the retransmission request signal NACK to the UE over the same dedicated channel as the dedicated channel used for transmitting the initial packet data. However, if normal packet data is received, i.e., an error-free packet data is received, the UE transmits an acknowledgement signal ACK with packet identification information to the Node B. That is, the UE repeats the retransmission until it transmits an ACK signal after normal decoding, or repeats the retransmission as many times as a predetermined number of retransmissions. Here, the number of retransmissions is previously set in the system, and the UE can retransmit the defective packet data as many times as the preset number of retransmissions.  
           [0008]    Conventionally, the ARQ is performed in a MAC (Medium Access Control) layer of the mobile communication system based on time diversity. However, this method has a limitation in increasing data transmission efficiency. Recently, therefore, research has been conducted on a method of enabling a physical layer to perform the ARQ so that a transmission side, or a Node B, can immediately recognize whether a reception side, or a UE, has correctly received packet data. In addition, a method of reducing a data rate is combined with the ARQ in order to improve error correction capability for transmission packet data, thereby contributing to a decrease in transmission error of the packet data. Since such a retransmission technique for improving error correction capability has higher retransmission efficiency compared with the existing ARQ, the ARQ has recently been applied to the physical layer in order to transmit high-speed data. However, since the ARQ used for improving the error correction capability inevitably causes a reduction in a data rate, there is a limitation in improving the overall data rate of the system.  
           [0009]    Accordingly, there is a demand for a method of improving efficiency of the ARQ without causing a reduction in the overall data rate of the system. Such a method is required particularly in a future mobile communication system, which transmits a large amount of high-speed data. In addition, an OFDM (Orthogonal Frequency Division Multiplexing) technique using multiple carriers has recently been applied to a mobile communication system in order to transmit a large amount of high-speed data. A mobile communication system using the OFDM technique (hereinafter, referred to as an “OFDM mobile communication system”), to which the ARQ is applied, will be described with reference to FIG. 1.  
           [0010]    [0010]FIG. 1 schematically illustrates a structure of an OFDM mobile communication system supporting the ARQ. Referring to FIG. 1, the OFDM mobile communication system includes a transmitter and a receiver. The transmitter is comprised of a physical layer ARQ controller  111 , an IFFT (Inverse Fast Fourier Transform) block  113 , and an RF (Radio Frequency) processor  115 , and the receiver is comprised of a physical layer ARQ controller  117 , an FFT (Fast Fourier Transform) block  119  and an RF processor  121 . The physical layer ARQ controller  111  controls the overall transmission operation of the transmitter. If there is data to be retransmitted in the OFDM mobile communication system, the physical layer ARQ controller  111  controls retransmission of the corresponding data based on the ARQ. The IFFT block  113  IFFT-transforms output signals of the physical layer ARQ controller  111 , for frequency division multiplexing, and provides its output to the RF processor  115 . The RF processor  115  converts an output signal of the IFFT block  113  into an RF signal, and transmits the RF signal over the air.  
           [0011]    The signal transmitted over the air by the transmitter is applied to the RF processor  121 . The RF processor  121  RF-processes the received signal, and provides its output to the FFT block  119 . The FFT block  119  FFT-transforms an output signal of the RF processor  121 , and provides its output to the physical layer ARQ controller  117 . The physical layer ARQ controller  117  checks whether the received signal has an error. If the received signal has an error, the physical layer ARQ controller  117  transmits a retransmission request signal NACK for requesting retransmission of the received signal, to the physical layer ARQ controller  111  over a feedback channel  123 . However, if the received signal has no error, the physical layer ARQ controller  117  transmits an acknowledgement signal ACK indicating that the received data is error-free, to the physical layer ARQ controller  111  over the feedback channel  12 .  
           [0012]    Upon receiving the retransmission request signal NACK transmitted from the physical layer ARQ controller  117  over the feedback channel  123 , the physical layer ARQ controller  111  performs retransmission on the retransmission-requested signal.  
           [0013]    However, even in the OFDM mobile communication system supporting the ARQ, an increase in retransmission efficiency unavoidably causes a decrease in the overall data rate. Accordingly, there is demand for a new retransmission technique for preventing a reduction in the overall data rate of the system while increasing the retransmission efficiency.  
         SUMMARY OF THE INVENTION  
         [0014]    It is, therefore, an object of the present invention to provide an apparatus and method for retransmitting data using frequency diversity in an OFDM mobile communication system.  
           [0015]    It is another object of the present invention to provide a data retransmission apparatus and method for maintaining an overall system data rate while maintaining retransmission efficiency in an OFDM mobile communication system.  
           [0016]    To achieve the above and other objects, there is provided a transmission apparatus in a mobile communication system, which modulates input data with a specific size into an OFDM symbol before transmission. In the transmission apparatus, a controller determines whether to transmit replica data instead of the input data, if the input data is retransmission data. A replica generator generates replica data by cyclically-circulating the input data under the control of the controller. An IFFT block generates an OFDM symbol by IFFT-transforming the replica data.  
           [0017]    To achieve the above and other objects, there is provided a reception apparatus for receiving a signal in a mobile communication system, which modulates input data with a specific size into an OFDM symbol before transmission. In the reception apparatus, an FFT block generates an OFDM symbol by FFT-transforming the received signal. A controller determines whether the OFDM symbol is retransmission data, and if the OFDM symbol is retransmission data, the controller determines whether the retransmission data is replica data. Further, if the transmission data is replica data, the controller modulates the replica data by a frequency diversity technique. A frequency diversity combiner modulates the input data by inversely cyclically-circulating the replica data under the control of the controller.  
           [0018]    To achieve the above and other objects, there is provided a transmission method in a mobile communication system, which modulates input data with a specific size into an OFDM symbol before transmission. The method comprises determining whether to transmit replica data instead of the input data, if the input data is retransmission data; generating replica data by cyclically-circulating the input data after the determination; and generating an OFDM symbol by IFFT-transforming the replica data.  
           [0019]    To achieve the above and other objects, there is provided a reception method for receiving a signal in a mobile communication system, which modulates input data with a specific size into an OFDM symbol before transmission. The method comprises generating an OFDM symbol by FFT-transforming the received signal; determining whether the OFDM symbol is retransmission data; if the OFDM symbol is retransmission data, determining whether the retransmission data is replica data; if the transmission data is replica data, modulating the replica data by a frequency diversity technique; and modulating the input data by inversely cyclically-circulating the replica data according to the frequency diversity technique. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0021]    [0021]FIG. 1 schematically illustrates a structure of a conventional OFDM mobile communication system supporting the ARQ;  
         [0022]    [0022]FIG. 2 schematically illustrates a structure of an OFDM mobile communication system supporting the ARQ according to an embodiment of the present invention;  
         [0023]    [0023]FIG. 3 illustrates a detailed structure of the replica generator illustrated in FIG. 2;  
         [0024]    [0024]FIG. 4 is a flowchart illustrating an operation of a transmitter in the OFDM mobile communication system illustrated in FIG. 2;  
         [0025]    [0025]FIG. 5 is a flowchart illustrating an operation of a receiver in the OFDM mobile communication system illustrated in FIG. 2;  
         [0026]    [0026]FIG. 6 is a flowchart illustrating an operation of the replica generator illustrated in FIG. 2;  
         [0027]    [0027]FIG. 7 is a flowchart illustrating operations of the transmitter and the receiver in the OFDM mobile communication system illustrated in FIG. 2; and  
         [0028]    [0028]FIG. 8 is a block diagram illustrating a detailed structure of the frequency diversity combiner illustrated in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0030]    [0030]FIG. 2 schematically illustrates a structure of an OFDM mobile communication system supporting ARQ according to an embodiment of the present invention. Referring to FIG. 2, the OFDM mobile communication system includes a transmitter and a receiver. The transmitter is comprised of a physical layer ARQ (Automatic Repeat Request) controller  211 , a zero (0) generator  213 , a controller  215 , a first switch  217 , a buffer  219 , a replica generator  221 , a second switch  223 , an IFFT (Inverse Fast Fourier Transform) block  225 , a guard interval inserter  227 , and an RF (Radio Frequency) processor  229 . The receiver is comprised of a physical layer ARQ controller  251 , a controller  253 , a third switch  255 , a frequency diversity combiner  257 , a buffer  259 , a fourth switch  261 , a zero generator  263 , an FFT (Fast Fourier Transform) block  265 , a guard interval eliminator  267 , and an RF processor  269 .  
         [0031]    First, a structure of the transmitter will be described in detail. The physical layer ARQ controller  211  controls the overall transmission operation of the transmitter. If there is data to be retransmitted in the OFDM mobile communication system, the physical layer ARQ controller  211  determines whether it will retransmit the corresponding data based on the general ARQ or it will retransmit the corresponding data using a replica generated by cyclic circulation. The physical layer ARQ controller  211  provides the determined retransmission method to the controller  215  so that the corresponding data, i.e., the retransmission-requested data, can be transmitted in the determined retransmission method. The controller  215  controls switching operations of the first switch  217  and the second switch  223  according to the retransmission method determined by the physical layer ARQ controller  211 . The zero generator  213 , under the control of the controller  215 , generates as many 0&#39;s as a predetermined number d over a symbol by QAM (Quadrature Amplitude Modulation)/QPSK (Quadrature Phase Shift Keying) mapping, in order to prevent a transmission delay that occurs during data transmission using a replica generated by the cyclic circulation from being longer than a transmission delay that occurs during data transmission not using the replica generated by the cyclic circulation. The buffer  219  buffers an output signal of the first switch  217 . The replica generator  221  generates a replica by cyclically-circulating the symbols buffered in the buffer  219  at predetermined periods, i.e., at periods of one OFDM symbol, and provides the generated replica to the second switch  223 . A detailed operation of the replica generator  221  will be described later on with reference to FIG. 3.  
         [0032]    An output signal of the replica generator  221  is provided to the IFFT block  225  through the second switch  223 . The IFFT block  225  IFFT-transforms an output signal of the second switch  223 , for frequency division multiplexing, and provides its output to the guard interval inserter  227 . The guard interval inserter  227  inserts a guard interval into an output signal of the IFFT block  225 , and provides its output to the RF processor  229 . Here, the guard interval is inserted in order to minimize inter-symbol interference (ISI) between the OFDM symbols. The RF processor  229  converts an output signal of the guard interval inserter  227  into an RF signal, and transmits the RF signal over the air.  
         [0033]    Next, a structure of the receiver will be described in detail.  
         [0034]    The signal transmitted over the air by the transmitter is applied to the RF processor  269 . The RF processor  269  RF-processes the received signal, and provides its output to the guard interval eliminator  267 . The guard interval eliminator  267  receives an output signal of the RF processor  269 , eliminates a guard interval included therein, and provides its output to the FFT block  265 . The FFT block  265  FFT-transforms an output signal of the guard interval eliminator  267 , and provides its output to the third switch  255  and the fourth switch  261 . If an output signal of the FFT block  265  is an initially transmitted signal, the fourth switch  261  is switched on to provide the output signal of the FFT block  265  to the buffer  259 . The buffer  259  then buffers the received signal provided from the switch  261 . The physical layer ARQ controller  251  receives the signal stored in the buffer  259  at predetermined periods, and determines whether the received signal has an error. If the received signal has an error, the physical layer ARQ controller  251  transmits a retransmission request signal NACK for requesting retransmission of the received signal, to the physical layer ARQ controller  211  over a feedback channel  271 . The third switch  255  and the fourth switch  261  are controlled by the controller  253 . Further, the physical layer ARQ controller  251  can determine whether the received signal is an initially transmitted signal or a retransmitted signal.  
         [0035]    However, if the received signal provided from the FFT block  265  is not an initially transmitted signal but a retransmitted signal, the physical layer ARQ controller  251  provides the controller  253  with a control signal for inserting 0&#39;s into the received retransmitted signal in order to prevent an output signal of the FFT block  265 , if it is not a cyclic circulation-based retransmission signal, from being delayed against the cyclic circulation-based retransmission signal. The controller  253  then generates the output data of the FFT block  265  by controlling the fourth switch  261 , and enables the zero generator  263  for a predetermined number d of symbols, where d is previously determined to prevent the transmission delay. Of course, if the output signal of the FFT block  265  is a cyclic circulation-based retransmission signal, the controller  253  disables the zero generator  263 . The transmitted signal is provided to the frequency diversity combiner  257  after being buffered by the buffer  259 . Further, in order to apply frequency diversity to the retransmission signal, the physical layer ARQ controller  251  provides the controller  253  with a control signal for enabling the frequency diversity combiner  257 . The controller  253  then applies frequency diversity to the retransmission signal output from the buffer  259 , and provides the signal to the physical layer ARQ controller  251 . A detailed structure of the frequency diversity combiner  257  will be described in detail later with reference to FIG. 8. The physical layer ARQ controller  251  combines the retransmission data with the initial transmission data previously buffered in the buffer  259 , and finally decodes the combined data. The physical layer ARQ controller  251  determines whether to perform the retransmission operation again, according to whether the combined data has an error.  
         [0036]    Next, an internal structure of the replica generator  221  will be described with reference to FIG. 3.  
         [0037]    [0037]FIG. 3 illustrates a detailed structure of the replica generator  221  illustrated in FIG. 2. Referring to FIG. 3, a signal output from the buffer  219  at predetermined periods, i.e., at periods of one OFDM symbol is applied to the replica generator  221  as described in conjunction with FIG. 2. The output signal of the buffer  219  is a signal that has undergone QAM/QPSK mapping and scrambling. The replica generator  221  is comprised of a cyclic circulator  311 , a counter  313 , and a cyclic circulation distance determiner  315 . The cyclic circulation distance determiner  315  determines a cyclic circulation distance “d” (or an amount of cyclic circulation) of an OFDM symbol output from the buffer  219 . The counter  313  counts the cyclic circulation distance d determined by the cyclic circulation distance determiner  315 . The cyclic circulator  311  cyclically-circulates an OFDM symbol stored in the buffer  219  based on the cyclic circulation distance d determined by the cyclic circulation distance determiner  315 . That is, the cyclic circulator  311  cyclically-circulates an OFDM symbol output from the buffer  219  by the cyclic circulation distance d output from the counter  313 .  
         [0038]    Now, a process of cyclically-circulating an OFDM symbol “s” output from the buffer  219  by the cyclic circulator  311  will be described herein below.  
         [0039]    The OFDM symbol s is represented by 
           s=[s (0) . . .  s ( N −1)] T   Equation (1) 
         [0040]    In Equation (1), N denotes the total number of subcarriers used in the OFDM mobile communication system and T is a transpose.  
         [0041]    In the OFDM mobile communication system supporting the ARQ, in order to prevent a decrease in reliability of retransmission due to transmission over the same path, the present invention performs cyclic circulation on subcarriers, producing the diversity effect. To this end, in the OFDM mobile communication system, replicas must be transmitted over subcarriers having no correlation with one another. A symbol s′ generated by cyclically-circulating the OFDM symbol is expressed as 
           s′=[s ( N−d ) . . .  s ( N −1) s (0) . . .  s ( N−d −1)] T   Equation (2) 
         [0042]    In Equation (2), the cyclic circulation distance d is calculated by  
             d   =       ⌊     N   L     ⌋     ·     ⌊     L   2     ⌋               Equation                   (   3   )                                 
 
         [0043]    In Equation (3), L denotes the number of multiple paths of a selective frequency fading channel.  
         [0044]    Next, an operation of a transmitter in the OFDM mobile communication system supporting the ARQ will be described with reference to FIG. 4.  
         [0045]    [0045]FIG. 4 is a flowchart illustrating an operation of a transmitter in the OFDM mobile communication system illustrated in FIG. 2. Referring to FIG. 4, if there is transmission data, the physical layer ARQ controller  211  determines in step  411  whether the transmission data is retransmission data. If the transmission data is not retransmission data, but initial transmission data, the physical layer ARQ controller  211  proceeds to step  413 . In step  413 , the physical layer ARQ controller  211  encodes the transmission data by a predetermined coding technique, and then proceeds to step  415 . In step  415 , the physical layer ARQ controller  211  provides the controller  215  with a control signal for switching on the first switch  217  to store the coded transmission data in the buffer  219 , and switching on the second switch  223  to provide the transmission data to the IFFT block  225 .  
         [0046]    However, if it is determined in step  411  that the transmission data is retransmission data, the physical layer ARQ controller  211  determines in step  417  whether to use the replica generator  221  for the retransmission data. If the physical layer ARQ controller  211  determines not to use the replica generator  221  for the retransmission data, i.e., if the physical layer ARQ controller  211  determines to use the general ARQ, the physical layer ARQ controller  211  proceeds to step  419 . In step  419 , the physical layer ARQ controller  211  converts the data stored in the buffer  219 , i.e., the initially transmitted data, into retransmission data, and then proceeds to step  415 .  
         [0047]    Otherwise, if it is determined in step  417  that the physical layer ARQ controller  211  determines to use the replica generator  221 , the physical layer ARQ controller  211  proceeds to step  421 . In step  421 , the physical layer ARQ controller  211  provides the data stored in the buffer  219 , i.e., the initially transmitted data, to the replica generator  221 , and then proceeds to step  423 . In step  423 , the replica generator  221  generates a replica by cyclically-circulating output data of the buffer  219  by a cyclic circulation distance d, provides the generated replica to the IFFT block  225 , and then proceeds to step  425 . In step  425 , the IFFT block  225  retransmits the generated replica, and then ends the process.  
         [0048]    Next, an operation of a receiver in the OFDM mobile communication system supporting the ARQ will be described with reference to FIG. 5.  
         [0049]    [0049]FIG. 5 is a flowchart illustrating an operation of a receiver in the OFDM mobile communication system illustrated in FIG. 2. Referring to FIG. 5, if data is received, the physical layer ARQ controller  251  determines in step  511  whether the received data is retransmission data. As a result of the determination, if the received data is not retransmission data, the physical layer ARQ controller  251  proceeds to step  513 . In step  513 , the physical layer ARQ controller  251  stores the received data, i.e., the initial transmission data, in the buffer  259 , decodes the received data, and based on the decoding results, transmits a retransmission request signal NACK or an acknowledgement signal ACK to the physical layer ARQ controller  211 .  
         [0050]    However, if it is determined in step  511  that the received data is retransmission data, the physical layer ARQ controller  251  determines in step  515  whether the retransmission data is a replica generated by cyclic circulation. If the retransmission data is not a replica generated by cyclic circulation, the physical layer ARQ controller  251  proceeds to step  517 . In step  517 , the physical layer ARQ controller  251  stores the received retransmission data in the buffer  259 , combines the received data with the corresponding data previously stored in the buffer  259 , and based on the combining results, transmits a retransmission request signal NACK or an acknowledgement signal ACK to the physical layer ARQ controller  211 .  
         [0051]    Otherwise, if it is determined in step  515  that the retransmission data is a replica generated by cyclic circulation, the physical layer ARQ controller  251  proceeds to step  519 . In step  519 , the physical layer ARQ controller  251  provides the received replica and the corresponding data stored in the buffer  259  to the frequency diversity combiner  257  to perform a frequency diversity operation, and then proceeds to step  521 . In step  521 , the frequency diversity combiner  257  performs a frequency diversity operation on the received replica and the corresponding data stored in the buffer  259 , and then proceeds to step  523 . Here, the “frequency diversity operation” performed by the frequency diversity combiner  257  refers to a process of inversely cyclically-circulating the received retransmission data by the cyclic circulation distance d used by the replica generator  221  in the transmitter to generate replica data. That is, since the received retransmission data (or replica data) must be inversely cyclically-circulated by the cyclic circulation distance d in order to be restored to its original data, the frequency diversity combiner  257  performs the frequency diversity operation. In step  523 , the physical layer ARQ controller  251  decodes the data that underwent the frequency diversity operation, checks whether the decoded data has an error, and based on the error check results, transmits a retransmission request signal NACK or an acknowledgement signal ACK to the physical layer ARQ controller  211 .  
         [0052]    [0052]FIG. 6 is a flowchart illustrating an operation of the replica generator  221  illustrated in FIG. 2. Referring to FIG. 6, if output data of the buffer  219  is received, the replica generator  221  calculates, in step  611 , a cyclic circulation distance d to be applied to the output data of the buffer  219 , and then proceeds to step  613 . Here, the cyclic circulation distance d is calculated in accordance with Equation (3). In step  613 , the replica generator  221  cyclically-circulates the output data of the buffer  219 , i.e., one OFDM symbol, by the calculated cyclic circulation distance d, and then ends the process. The cyclically-circulated data is expressed as Equation (2).  
         [0053]    [0053]FIG. 7 is a flowchart illustrating operations of the transmitter and the receiver in the OFDM mobile communication system illustrated in FIG. 2. The operations of FIG. 7 are similar to the operations described in conjunction with FIGS. 5 and 6, except that the operation of the transmitter is separated according to whether a response signal received from the receiver is an acknowledgement signal ACK or a retransmission request signal NACK, and the operation of the receiver is separated according to whether data received from the transmitter is initial transmission data or retransmission data. Therefore, a detailed description of the operations will not be provided. In addition, FIG. 7 illustrates feedback channels established between the transmitter and the receiver, over which the acknowledgement signal ACK and the retransmission request signal NACK are transmitted.  
         [0054]    [0054]FIG. 8 is a block diagram illustrating a detailed structure of the frequency diversity combiner  257  illustrated in FIG. 2. Referring to FIG. 8, the frequency diversity combiner  257  is comprised of an inverse cyclic circulator  811 , a counter  813 , and a cyclic circulation distance determiner  815 . The cyclic circulation distance determiner  815  determines a cyclic circulation distance d of an OFDM symbol output from the buffer  259 . Here, the cyclic circulation distance d is identical to the cyclic circulation distance d used by the transmitter. The counter  813  counts the cyclic circulation distance d output from the cyclic circulation distance determiner  815 . The inverse cyclic circulator  811  inversely cyclically-circulates an OFDM symbol stored in the buffer  259  based on the cyclic circulation distance d determined by the cyclic circulation distance determiner  815 , and provides its output to the physical layer ARQ controller  251 . That is, the inverse cyclic circulator  811  inversely cyclically-circulates an OFDM symbol output from the buffer  259  by the cyclic circulation distance d output from the counter  813 , and provides its output to the physical layer ARQ controller  251 .  
         [0055]    As described above, in the OFDM mobile communication system, the present invention performs data retransmission with cyclic circulation-based replicas, thereby acquiring not only the time diversity effect through simple data retransmission by the conventional ARQ, but also the frequency diversity effect using the retransmitted data. As a result, the data retransmission efficiency is increased. In addition, it is possible to prevent a reduction in the overall data rate of the system by transmitting the cyclic circulation-based replicas during data retransmission.  
         [0056]    While the invention has been shown and described with reference to a certain preferred embodiment 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 invention as defined by the appended claims.