Abstract:
A method and apparatus for transmitting and receiving a preamble having sequence information for an OFDM system is provided. The preamble transmission method includes generating a preamble block including at least one frame having a header with a known sequence and a code block containing control information and transmitting the preamble block mapped to Orthogonal Frequency Division Multiplexing (OFDM) cells by repeating in frequency axis direction.

Description:
PRIORITY 
       [0001]    This application claims priority to an application entitled “PREAMBLE TRANSMISSION AND RECEPTION METHOD AND APPARATUS FOR OFDM SYSTEM” filed in the Korean Intellectual Property Office on Oct. 20, 2008 and assigned Serial No. 10-2008-0102501, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) system and, in particular, to a method and apparatus for transmitting and receiving a preamble having sequence information for an OFDM system. 
         [0004]    2. Description of the Related Art 
         [0005]    Orthogonal Frequency Division Multiplexing (OFDM) is a bandwidth efficient digital modulation technique that has been adopted for a variety of advanced broadcast standards such as Digital Video Broadcasting-Terrestrial (DVB-T). Digital Video Broadcasting-Cable 2 (DVB-C2), a new standard for broadcast transmission over cable, has also selected OFDM as its modulation technique to provide a high degree of efficiency and flexibility. 
         [0006]    The DVB-C2 system supports the feature of flexible and dynamic bandwidth allocation by combining various adjacent channels to a single wideband channel. In the case of a single channel implementation, the bandwidth of the DVB-C2 becomes 8 MHz, equal to that of the DVB-C. Accordingly, when N channels are combined, the bandwidth of the DVB-C2 system increases N times compared to the DVB-C. This means that the DVB-C2 has the bandwidth of N×8 MHz. 
         [0007]    A comparison will now be made between the channel combinations in the DVB-C and the DVB-C2. In the DVB-C system the number of guard bands increases in proportion to the number of channels combined due to the guard bands required to ensure separation between channels. In an exemplary case of a combination of 4 channels, the total bandwidth available for the data transfer is equal to a value obtained by subtracting 5 guard bands from the entire 24 MHz bandwidth (4×8 MHz). 
         [0008]    In contrast, the DVB-C2 system transmits signals using the OFDM which does not require guard bands between channels except for the guard bands at the edges. Accordingly, when 4 channels are combined, the total bandwidth available for the data transfer is equal to a value obtained by subtracting 2 guard bands from the entire 24 MHz bandwidth (4×8 MHz). From the comparison, it is obvious that the DVB-C2 system is superior to the DVB-C system in spectral efficiency. 
         [0009]      FIG. 1  is a diagram illustrating a frame format of a conventional DVB-C2 system with a fixed tuning window. 
         [0010]    In  FIG. 1 , two channels  101  and  102  occupy two different frequency bands, and each channel is composed of different broadcast data (i.e. PLP 1   105  and PLP  2   106  for the first channel  101  and PLP 3   107 , PLP 4   108 , and PLP 5   109  for second channel  102 ). The frame starts with preambles  103  and  104  containing control information for the respective channels  101  and  102 . In this case, the receiver is tuned to channel  1   101  or channel  2   102  to receive the broadcast data transmitted on the corresponding channel. 
         [0011]    In order to improve the resource allocation efficiency, the broadcast data is scheduled over entire channel bandwidth.  FIG. 2  is a diagram illustrating a frame format of another conventional DVB-C2 system. In  FIG. 2 , the bandwidth of each preamble block is identical to a reception bandwidth of 8 MHz as denoted by reference numeral  220 . It is noted that the broadcast data  213  is allocated regardless of a boundary of the channels. In this case, the same control information is transmitted within the two preambles  202  and  203 . The receiver can align its tuning window  220  to receive the target broadcast data without a need to be aligned with the preambles. Here, the maximum frequency bandwidth of each broadcast data cannot be greater than the minimum reception bandwidth of the receiver (in  FIG. 2 , 8 MHz). 
         [0012]    It can be considered that a preamble block is allocated a bandwidth narrower than the reception bandwidth of the receiver unlike the exemplary case of  FIG. 2  in which the bandwidth of each preamble block is identical to the reception bandwidth of the receiver. In this case, the preamble can be received within the reception bandwidth (8 MHz) without being segmented. 
         [0013]      FIG. 3  is a diagram illustrating a frame format of another conventional DVB-C2 system. In  FIG. 3 , the bandwidth of a preamble block is narrower than the reception bandwidth of the receiver. When the bandwidth of a preamble is narrower than the reception bandwidth of the receiver, the receiver can receive a non-segmented preamble block  302  within its reception bandwidth. In this case, however, a specific preamble block  303  is received due to the tuning to the bandwidth of the channel carrying the preamble block  302 . This is because the frequency band outside the frequency band  303  is another system bandwidth, e.g. a frequency bandwidth allocated for a communication system other than the DVB-C2. 
         [0014]      FIG. 4  is a diagram illustrating a principle of operation of the receiver in the conventional DVB-C2 system when segmented preambles are received in the reception bandwidth. When the reception bandwidth as denoted by reference numeral  420  is aligned to receive the broadcast data (PLP 2 ) as shown in  FIG. 4 , the preamble information as denoted by reference numeral  401  is received within the reception bandwidth. In this case, since the preamble information is received within two preamble blocks  410  and  411 , the information carried by the preamble blocks  410  and  411  must be reordered to obtain the complete information as denoted by reference numeral  430 . 
         [0015]    Accordingly, in order to obtain the complete control information in the above described conventional DVB-C2 system, the receiver must estimate a boundary frequency  425  and reorder information carried by the preambles based on the boundary frequency  425 , thereby processing latency. Particularly, when the boundary frequency is misestimated due to the frequency offset, the receiver is likely to fail to obtain complete control information from the preamble, resulting in reception failure of the data within the entire frame. 
       SUMMARY OF THE INVENTION 
       [0016]    In order to overcome at least the problems of the prior art, the present invention provides a method and apparatus for transmitting and receiving a preamble robust to frequency offset in an OFDM-based broadcast system. 
         [0017]    The present invention provides a method and apparatus for transmitting and receiving a preamble in an OFDM-based broadcast system that is capable of removing unnecessary dummy cells in a preamble block and improving reception performance by repeating control information. 
         [0018]    The present invention provides a method and apparatus for transmitting and receiving a preamble in an OFDM-based broadcast system that is capable of improving reception performance of a receiver by negating the reordering process cause by reception of segmented preambles. 
         [0019]    In accordance with an embodiment of the present invention, a preamble transmission method includes generating a preamble block including at least one frame having a header with a known sequence and a code block containing control information; and transmitting the preamble block mapped to Orthogonal Frequency Division Multiplexing (OFDM) cells by repeating in the frequency axis direction. 
         [0020]    In accordance with another embodiment of the present invention, a preamble reception method includes detecting a sequence from a signal received through a reception bandwidth; calculating a start position and a length of a code block using the detected sequence; extracting control information from the code block based on the start position and length of the code block; and decoding received data signal based on the control information. 
         [0021]    In accordance with another embodiment of the present invention, a preamble transmitter includes an encoder which encodes control information into a code block; a sequence generator which generates a sequence and inserts the sequence into a header of the code block; and a symbol mapper which maps a frame having the header and the code block to Orthogonal Frequency Division Multiplexing (OFDM) cells of a preamble block, repeats the frame in the frequency axis direction, and transmits the repeated frames. 
         [0022]    In accordance with still another embodiment of the present invention, a preamble receiver includes a sequence detector which detects a sequence from a signal received through a reception bandwidth and calculates a start position and a length of a code block using the detected sequence; a control information extractor which extracts control information from the code block base on the start position and length of the code block; and a data extractor which decodes received data signal based on the control information. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description in conjunction with the accompanying drawings, in which: 
           [0024]      FIG. 1  is a diagram illustrating a frame format of a conventional DVB-C2 system with a fixed tuning window; 
           [0025]      FIG. 2  is a diagram illustrating a frame format of another conventional DVB-C2 system; 
           [0026]      FIG. 3  is a diagram illustrating a frame format of another conventional DVB-C2 system; 
           [0027]      FIG. 4  is a diagram illustrating a principle of operation of the receiver in the conventional DVB-C2 system when segmented preambles are received in the reception bandwidth; 
           [0028]      FIG. 5  is a diagram illustrating a structure of a preamble block used in the conventional DVB-C2 system; 
           [0029]      FIG. 6  is a diagram illustrating a format of a preamble block for used in a preamble transmission and reception method according to an embodiment of the present invention; 
           [0030]      FIG. 7  is a diagram illustrating a principle of a preamble transmission and reception method according to an embodiment of the present invention; 
           [0031]      FIG. 8  is a diagram illustrating a principle of a preamble transmission and reception method according to another embodiment of the present invention; 
           [0032]      FIG. 9  is a block diagram illustrating a configuration of the conventional transmitter; 
           [0033]      FIG. 10  is a block diagram illustrating a configuration of a transmitter according to an embodiment of the present invention; 
           [0034]      FIG. 11  is a block diagram illustrating a configuration of a transmitter according to another embodiment of the present invention; 
           [0035]      FIG. 12  is a flowchart illustrating a method for generating a preamble having a known sequence is inserted in front of a FEC block as described with reference to  FIG. 7 ; 
           [0036]      FIG. 13  is a flowchart illustrating a method for generating a preamble block having a known sequence inserted in the middle of a FEC block as described with reference to  FIG. 8 ; 
           [0037]      FIG. 14  is a block diagram illustrating a configuration of a receiver according to an embodiment of the present invention; and 
           [0038]      FIG. 15  is a flowchart illustrating a method for receiving a preamble having a known sequence according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0039]    Embodiments of the present invention are described with reference to the accompanying drawings in detail. The same reference numbers are used throughout the drawings to refer to the same or like parts. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. 
         [0040]    A structure of a frame used in the preamble transmission method and apparatus according to an embodiment of the present invention is described first. 
         [0041]      FIG. 5  is a diagram illustrating a structure of a preamble block used in the conventional DVB-C2 system, and  FIG. 6  is a diagram illustrating a format of a preamble block for used in a preamble transmission and reception method according to an embodiment of the present invention. 
         [0042]    As shown in  FIG. 5 , a preamble block  501  of the conventional C2 frame is composed of K OFDM cells and has a bandwidth less than or equal to a reception bandwidth of a receiver. 
         [0043]    The control information for a frame is encoded and modulated into one or more Forward Error Correction (FEC) blocks  511  to  513  and then inserted into a beginning part of the preamble block. In  FIG. 5 , it is assumed that an FEC block consists of M cells and the control information is carried by N FEC blocks. Accordingly, K−M×N cells are empty in the preamble block  501 . The length of the preamble block  501  has a fixed value determined previously, such that the empty part of the preamble block is filled with dummy cells. The number of dummy cells is variable according to the length of the control information and thus, when the length of the control information is short, the preamble block is mostly filled with dummy cells, resulting in a waste of resources. 
         [0044]    A preamble block according to an embodiment of the present invention is depicted in  FIG. 6  under the assumption in that the preamble block is composed of K cells, a preamble header is composed of A cells, a FEC block includes M cells, and the control information is carried by N FEC blocks. In this case, the preamble headers  611 ,  613 , and  615  are inserted in front of the corresponding FEC blocks  612 ,  614 , and  616 . Here, the control information is information relevant to all of the channels. That is, the first to N th  code blocks carry the data on all of the channels. 
         [0045]    In an embodiment of the present invention, a preamble header  611  (or  613  or  615 ) has a known sequence value. The sequence can be a Pseudo Noise sequence. Additionally, each preamble header can include an index of the FEC block and a length of the FEC block. In  FIG. 6 , the length of the header is predetermined with A cells, K−N*(M+a) cells are empty in the preamble block. In an embodiment of the present invention, the empty part of the preamble block is filled with the region copied from the beginning of the preamble block as much as the length of the empty part unlike the conventional preamble block in which the empty part is filled with the dummy cells. Accordingly, the empty part of the preamble block is filled with the preamble header  617  identical to the preamble header  611  and the partial FEC block  618  identically to the beginning part of the FEC block  612 . 
         [0046]    The partial FEC block  618  can be combined with the FEC block  612  at the receiver by means of a Maximal Ratio Combining method for improving the reception performance. 
         [0047]    How to insert the sequence into the preamble headers at the transmitter and how to recover the control information from the preamble headers at the receiver are described hereinafter.  FIGS. 7 and 8  are diagrams illustrating principles of preamble transmission and reception methods according to embodiments of the present invention. 
         [0048]    In  FIGS. 7 and 8 , it is assumed that all of the control information is transmitted within a single FEC block which is repeated in the frequency direction. 
         [0049]    In the embodiment of  FIG. 7 , the known sequence is inserted in front of the FEC block. Referring to  FIG. 7 , an FEC frame including a header is cyclically repeated until the complete preamble block is filled, such that the preamble block  710  includes the header  701 , the first FEC block  702 , a repeated header  703 , and a repeated FEC block  704 . After placing the header  710  and the FEC block  702 , the FEC frame is copied from its starting point for as much as remained space of the preamble block  710  to fill the empty part of the preamble block  710 . 
         [0050]    The second preamble block  720  is identical to the first preamble block  710  in structure and data. That is, the second preamble block  720  is a duplicate of the first preamble block  710 . 
         [0051]    When the tuning window (reception bandwidth) of the receiver is aligned as denoted by reference numeral  730 , three headers  703 ,  705  and  707  are received within the tuning window, each header having a sequence. Accordingly, a sequence detector of the receiver can obtain peak values a, b, and c of the three headers  703 ,  705 , and  707  using a correlation method. That is, the receiver performs correlation operation with the known sequences to obtain the peak values through auto-correlation. 
         [0052]    The receiver can estimate the length of the control information based on the interval between the peak values and determines a start point “b” and length of the complete control information  740 . Also, the receiver can recognize the partial control information  750 , such that the data reliability can be improved by combining the complete control information  740  and the partial control information  750 . 
         [0053]    Unlike the conventional preamble reception method in which the control information is recovered by finding the boundary frequency between the preamble blocks, i.e. the start point of the header  705 , within the tuning window and reordering the control information based on the boundary frequency, the preamble reception method according to the present invention enables the receiver to recovery the complete control information using a simple sequence detector. Accordingly, even when a frequency offset occurs unexpectedly, it is possible to acquire the complete control information with reliable preamble block detection. 
         [0054]    Although the known sequence is put in front of the FEC block in the above description, the position of the known sequence can be changed. In another embodiment of the present invention, the known sequence is put in the middle of an FEC block. 
         [0055]    In the embodiment of  FIG. 8 , it is assumed that a systematic code such as Low Density Parity Check (LDPC) code is used for encoding the control information. The systematic code means that the FEC block consists of the input control information bit string and a parity bits string. In this case, the known sequence can be inserted between the information bits string and the parity bits string so as to detect the length of the control information. 
         [0056]    Referring to  FIG. 8 , an FEC block  801  is composed of an information bit string  10 , a header  20  having the known sequence, and a parity bit string  30  in series. The FEC block  801  is repeated until a complete preamble block is filled. Here, the first preamble block  810  and the second preamble block  820  are identical to each other and have the known sequences  803  and  805  respectively. 
         [0057]    If the receiver is configured with the tuning window (reception band), as denoted by reference numeral  830 , equal to or greater than the size of a preamble block, at least one header is received within the tuning window. Accordingly, the receiver can detect a peak value  870  by means of a sequence detector. 
         [0058]    Once the peak value  870  is detected, the receiver can calculate the position of the boundary frequency  850  between the first and second preamble blocks  810  and  820  base on position of the peak value  870  and then the information bit string length  860  using the positions of the peak value  870  and the boundary frequency  850 . In order to calculate the length of the information bit string in this manner, the receiver should have the tuning information. 
         [0059]    A structure of a conventional transmitter and a transmitter for transmitting the preamble block generated with the control information having the known sequence will now be compared. 
         [0060]      FIG. 9  is a block diagram illustrating a configuration of the conventional transmitter. Referring to  FIG. 9 , the conventional transmitter includes a FEC encoder  902 , a symbol mapper  904 , and a dummy cell inserter  906 . 
         [0061]    The FEC encoder  902  encodes and modulates the input control information and outputs the FEC blocks. The symbol mapper  904  maps the FEC blocks to a required number of OFDM cells regardless of the length of the preamble and outputs the OFDM cells. The dummy cell inserter  906  inserts dummy cells into the remaining capacity of a preamble block and outputs a preamble block having a predetermined length. The preamble block formed in this manner is repeated for all the channels. 
         [0062]    In an embodiment of the present invention, the transmitter does not insert dummy cells into the remaining capacity of the preamble block. The structure of the transmitter according to an embodiment of the present invention is described hereinafter. 
         [0063]      FIG. 10  is a block diagram illustrating a configuration of a transmitter according to an embodiment of the present invention. The transmitter depicted in  FIG. 10  is configured to insert the known sequence in front of the FEC block as described with reference to  FIG. 7 . 
         [0064]    Referring to  FIG. 10 , the transmitter according to an embodiment of the present invention includes a FEC encoder  1002 , a sequence generator  1005 , a repeater  1003 , a symbol mapper  1004 , and a controller  1010 . 
         [0065]    The FEC encoder  1002  encodes and modulates the input control information. The sequence generator  1005  generates and outputs a header having the known sequence. The header output by the sequence generator  1005  is added in front of the FEC block output by the FEC encoder  1002  such that an FEC frame is input to the repeater  1003 . 
         [0066]    The repeater  1003  repeats the FEC frame cyclically until a complete preamble block is filled. As described with reference to  FIG. 6 , the FEC frame can be broken to fit into the remaining part of the preamble block. 
         [0067]    Next, the symbol mapper  1004  maps the FEC blocks to the OFDM cells so as to output a preamble block. In this case, there is no empty space within the preamble block, such that the dummy cell insertion process is negated. 
         [0068]    The control unit  1010  outputs a seed value to the sequence generator  1005  in order for the sequence generator  1005  to generate sequences for identifying the FEC blocks and informs the repeater  1003  of a number of cells for filling the preamble block. 
         [0069]      FIG. 11  is a block diagram illustrating a configuration of a transmitter according to another embodiment of the present invention. The transmitter depicted in  FIG. 11  is configured to insert the know sequence in the middle of the FEC block as described with reference to  FIG. 8 . 
         [0070]    Referring to  FIG. 11 , the transmitter according to another embodiment of the present invention includes an FEC encoder  1102 , a sequence generator  1105 , a repeater  1103 , a symbol mapper  1104 , and a controller  1110 . 
         [0071]    The sequence generator  1105  generates the known sequence and outputs a header in which the known sequence is inserted. The FEC encoder  1102  inserts the header output by the sequence generator  1105  into the input control information bit string and encodes and modulates the header-inserted bit string to output an FEC block. 
         [0072]    Accordingly, the header is inserted in the middle of the FEC block, and the FEC block is input to the repeater  1103 . The repeater  1103  repeats the FEC block cyclically until a complete preamble block is filled. 
         [0073]    Next, the symbol mapper  1104  maps the FEC blocks to the OFDM cells so as to output a preamble block. In this case, there is no empty space within the preamble block, such that the dummy cell insertion process is negated. 
         [0074]    The control unit  1110  outputs a seed value to the sequence generator  1105  in order for the sequence generator  1105  to generate sequences for identifying the FEC blocks and informs the repeater  1103  of a number of cells for filling the preamble block. 
         [0075]    The structures of the transmitters according to embodiments of the present invention have been described hereinabove. The preamble block generation methods according to an embodiment of the present invention are described hereinafter with reference to  FIGS. 12 and 13 . 
         [0076]      FIG. 12  is a flowchart illustrating a method for generating a preamble having a known sequence is inserted in front of a FEC block as described with reference to  FIG. 7 . 
         [0077]    Referring to  FIG. 12 , the transmitter first generates control information corresponding to an FEC block in step  1201 . Next, the transmitter encodes and modulates the control information into a FEC block in step  1203  and inserts a known sequence in front of the FEC block to output a complete FEC block in step  1205 . Once an FEC block is generated, the transmitter determines whether there is further control information to be transmitted in step  1207 . If there is further control information to be transmitted, the procedure goes to step  1201 . Otherwise, if there is no further control information to be transmitted, the transmitter repeats the FEC block cyclically until a preamble block is filled in step  1209 . Next, the transmitter maps the FEC blocks to OFDM cells to generate the preamble block in step  1211 . Finally, the transmitter copies the preamble block to be applied within the entire system in step  1213 . 
         [0078]      FIG. 13  is a flowchart illustrating a method for generating a preamble block having a known sequence inserted in the middle of a FEC block as described with reference to  FIG. 8 . 
         [0079]    The preamble block generation procedure depicted in  FIG. 13  is similar to that of  FIG. 12  except that the encoding step  1203  and the sequence insertion step  1205  are changed each other. 
         [0080]    Referring to  FIG. 13 , the transmitter generates control information corresponding to an FEC block in step  1301 . Next, the transmitter inserts a known sequence into the control information bit string in step  1303  and then encodes and modulates the known sequence-inserted control information bit string to output an FEC block in step  1305 . Once an FEC block is generated, the transmitter determines whether there is further control information to be transmitted in step  1307 . If there is further control information to be transmitted, the procedure goes to step  1301 . Otherwise, if there is no further control information to be transmitted, the transmitter repeats the FEC block cyclically until a preamble block is filled in step  1309 . Next, the transmitter maps the FEC blocks to OFDM cells to generate the preamble block in step  1311 . Finally, the transmitter copies the preamble block to be applied within the entire system in step  1313 . 
         [0081]    The preamble generation methods according to the embodiments of the present invention are described hereinabove. Operations of a receiver for receiving the preamble generated as described above are described hereinafter.  FIG. 14  is a block diagram illustrating a configuration of a receiver according to an embodiment of the present invention. 
         [0082]    Referring to  FIG. 14 , the receiver according to an embodiment of the present invention includes a tuner  1402 , a tuning controller  1403 , a preamble detector  1404 , a sequence detector  1405 , a control information extractor  1406 , and a data extractor  1407 . 
         [0083]    The tuning controller  1403  determines a tuning window (reception bandwidth) over a system bandwidth. That is, the tuning controller  1403  controls such that the tuning window is positioned to receive target broadcast data transmitted over a specific broadcast bandwidth. For instance, the tuning window can be aligned as denoted by reference numeral  730  in  FIG. 7  or reference numeral  830  in  FIG. 8 . 
         [0084]    The tuner  1402  is tuned to receive the broadcast data through the tuning window under the control of the tuning controller  1403 . The preamble detector  1404  detects the preamble from the broadcast signal received through the tuning window. The sequence detector  1405  searches the preamble for the sequence of a first FEC block and calculates the length and position of the control information based on the found sequence. Calculating the length and position of the control information has been described with reference to  FIGS. 7 and 8 . 
         [0085]    The control information extractor  1406  extracts the control information based on the length and position of the control information. As aforementioned, the FEC block is repeated within a preamble block such that a partial FEC block placed at the last part of the preamble block can be combined with the previous FEC block to acquire the complete control information. The data extractor  1407  decodes the data received on the channel base on the parameters contained in the extracted control information. 
         [0086]    The structure of a receiver for receiving the preamble has been described hereinabove. A method for receiving the preamble according to an embodiment of the present invention is described hereinafter.  FIG. 15  is a flowchart illustrating a method for receiving a preamble having a known sequence according to an embodiment of the present invention. 
         [0087]    Referring to  FIG. 15 , the receiver receives a broadcast signal through a predetermined tuning window (i.e. reception bandwidth) in step  1501 . While receiving the broadcast signal, the receiver acquires frame synchronization on the time axis and detects a preamble in the time direction in step  1503 . Next, the receiver detects a position of a sequence of the first FEC block within the preamble in the frequency direction in step  1505  and then calculates the length and position of the control information base on the position of the sequence in step  1507 . 
         [0088]    In an embodiment of the present invention, the FEC block is repeated cyclically to fill the preamble block such that the partial F EC blocks can be combined to the complete control information. Once the length and position of the control information are calculated, the receiver determines whether to combine the FEC blocks received within the preamble block in step  1509 . It is determined to combine the FEC blocks, the receiver combines the repeated FEC blocks in step  1511 . 
         [0089]    Referring to the exemplary case of  FIG. 7 , the header  703  followed by the FEC block  704  are identical with the header  705  followed the corresponding part of the FEC block  706  such that the header  703  followed by the FEC block  704  can be combined with the header  705  followed by the FEC  706  by synchronizing their start points. 
         [0090]    Referring to the exemplary case of  FIG. 8 , since the parts of blocks  809  and  804  and block  805  received within the tuning window  803  correspond to parts of the blocks  806  and  807  and block  808 , the parts of the blocks  809  and  804  and the block  805  can be combined with the FEC block including the blocks  806 ,  807 , and  808 . 
         [0091]    After combining the FEC blocks, the receiver demodulates and extracts the control information base on the length and position of the control information in step  1513 . Finally, the receiver decodes the data base on the parameters contained in the control information in step  1515 . If it is determined not to combine the FEC blocks, step  1511  is skipped. 
         [0092]    As described above, the preamble transmission and reception method and apparatus for an OFDM system operate with an improved preamble designed to simplify the structure of a receiver and use combining diversity, resulting in improvement of the reception performance. 
         [0093]    Also, the preamble transmission and reception method and apparatus for an OFDM system is robust to the frequency offset error by using an improved preamble having a known sequence, resulting in improvement of reception reliability. 
         [0094]    Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.