Abstract:
An OFDM communication system is provided that can implement more accurate signal transmission. The guard interval in the symbol section is formed by copying a predetermined number of symbols at the end portion of the symbol section and adding them to the forefront portion of the symbol portion. In the subsidiary station, the clipping start position stored in its memory is referred. Then, a predetermined number of symbols are clipped from the clipping start position. The clipped signal includes the guard interval of the symbols. However, since the guard interval is the signal obtained by copying the rear portion of the symbol, the clipping signal is the signal in which the phase of the symbol is merely deviated. An accurate symbol is obtained by performing phase correction. Thus, the influence due to multipath is suppressed while the signal transmitted from the base station can be accurately decoded.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of Japanese Patent Application No. 2007-007388 filed on Jan. 16, 2007. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to an OFDM communication system that carries out communications according to the OFDM (Orthogonal Frequency Division Multiplexing) scheme and an OFDM receiver suitable for the OFDM communication system. 
         [0005]    2. Description of the Related Art 
         [0006]    Conventionally, the OFDM scheme, which is a sort of multicarrier communication scheme, has been used to perform high-speed signal transmission in radio LAN (Local Area Network), ground digital broadcast and the like. Example of such a scheme is found in Japanese Patent Publication No. 2005-191662. In the OFDM scheme, a guard interval is inserted between symbols to alleviate the influence due to a multipath. This allows the influence of the multipath to be relieved to some extent. However, when the delay time of waves delayed due to the multipath becomes longer than the guard interval length, it is impossible to alleviate the influence of the multipath. 
         [0007]      FIG. 6  is the diagram illustrating a conventional OFDM communication system. Referring to  FIG. 6 , three base stations  601  to  603 , which are deployed at predetermined intervals, are inter-linked through the signal cable  607 . The base stations  601  to  603  operate synchronously to each other and transmit the same signals with the same timing. The base stations  601  to  603  have predetermined communication areas, respectively, and use different frequencies f 1 , f 2  and f 3  for communication to avoid mutual interference, respectively. A subsidiary station  604  selects the frequency f 1 , f 2  or f 3  through the scanning operation to carry out communications at the corresponding frequency. When the subsidiary station  604  is located in the communication area  606  of the base station  603 , the subsidiary station  604  (represented with  604   b ) performs OFDM communication with the base station  603  at the frequency f 3 . When the subsidiary station  604  moves from the communication area  606  of the base station  603  to the communication area  605  of the base station  601 , or becomes a hand-off mode, the subsidiary station  604  (represented with  604   a ) scans the communication frequencies and thus performs OFDM communication with the base station  601  at the frequency f 1 , used by the base station  601 . 
         [0008]    In this manner, the subsidiary station  604 , which is in the communication area of any one of the base station  601  to  603 , is handed off between the communication areas of the base stations  601  to  603  and can communicate with the base station  601 ,  602 , or  603  in the communication area using any one of the frequencies f 1  to f 3 . 
         [0009]    In the conventional OFDM system, as shown in  FIG. 7 , the packet signal transmitted from the base station includes a guard interval (GI)  701 . A predetermined number of symbols in the rear portion of the symbol portion  702 , that is, effective symbols in the OFDM (SC-OFDM) using the frequency domain equalization technique are copied and the copied portion is added as a guard interval (G 1 )  701  to the forefront portion of the symbol portion. In the subsidiary station  604 , when the signal received from the base station is decoded, all the guard interval  701  in the forefront portion of a single symbol  702 , which vanishes multipath, are removed. In the next symbol, all guard intervals  703  are removed. 
         [0010]    In this manner, even if the interference due to the multipath as shown in  FIG. 7(   b ) occurs to the original signal shown in  FIG. 7(   a ), the interference to the symbol portion  702  due to the multipath in the previous symbol portion can be removed and only the information regarding the symbol portion  702  can be extracted. However, the clip timing of the above described symbol is extracted by determining the clip timing based on the information on the symbol length included in the preamble portion and the unique word portion (UW) through repeating “1010101” in the packet signal at the time the subsidiary station  604  begins receiving. The clip timing may fluctuate back and forth due to the multipath. Back or forth variation of the timing depends on the multipath environment. 
         [0011]    If a delay due to deviation of symbol clip timing occurs as shown in  FIG. 7(   c ), the position to be recognized for the corresponding symbol will draw behind. In such a case, the problem is that the delay of the corresponding symbol causes an interference between the corresponding symbol and the next guard interval  703 , that is, an interference occurs in the corresponding symbol  702 . 
         [0012]    In the OFDM communication system shown in  FIG. 6 , the subsidiary station  604  may miss any one of the base stations  601  to  603  to be next received in the hand-off mode. This requires the beginning of scanning all the frequencies f 1  to f 3 . The beginning of the scanning takes the time period for which the hand-off mode is completed, thus leading to a very inefficient operation. This leads to widening the range for radio communication. That is, an increasing number of base stations to be deployed results in an increased number of frequencies. As a result, the number of frequencies to be scanned is increased. Because the time period for which the operation retains a frequency is constant, the whole scanning time becomes longer considerably. Therefore, the problem is that the communication response becomes slow. 
         [0013]    Moreover, with an increase in the number of frequencies to be used, the mobile subsidiary station, which is scanning plural frequencies in a long period of time, cannot receive packets transmitted from one base station. In other words, the mobile subsidiary station cannot receive packets while receiving a different frequency. As a result, when considering probabilistically, the communication success probability decreases. In order to solve such problems, the communication traffic must be increased so that packets have to be re-transmitted for a relatively long period of time. 
         [0014]    In addition, an increased number of frequencies increases the number of frequency channels. The number of channels to be used for radio equipment is finite. As a result, a division of frequencies between plural systems has been demanded. Broadening the radio communicable range leads to tightening the number of channels. Moreover, the widened range is operated by dividing the channels without causing mutual interference between plural systems. Such a measure leads to further tightening of the number of channels. Moreover, the problem arises that when the communication range does not fall within a finite number of channels, an introduction of the system must be restricted. 
         [0015]    The roaming method, which uses the sophisticated modulation/demodulation diffusion technique, employed in the CDMA mobile telephones, may be considered. However, in the radio communication system, which includes base stations installed easily and at low costs, that method is not realistic because expenses for system configuration and for radio receiver development and manufacture become costly. 
         [0016]    In order to solve such a problem, the single frequency network (SFN) that includes base stations deployed in multiple different communication areas and establishes the communication between each base station and a subsidiary station at the same frequency has been developed. Example of such a method is found in Japanese Patent Publication No. 9-252278. According to that method, the simplified configuration allows a subsidiary station, which has traveled, to be handed off and the seamless communication to be established between a base station and a subsidiary station. However, with the SFN constructed in the conventional OFDM scheme, the problem on the multipath, as previously described, occurs so that it is difficult to realize accurate signal transmission. 
       SUMMARY OF THE INVENTION 
       [0017]    An object of the present invention is to provide more accurate signal transmission in an OFDM communication system employing the OFDM scheme. 
         [0018]    Another object of the present invention is to provide more accurate signal reception in an OFDM receiver employing the OFDM scheme. 
         [0019]    An OFDM communication system according to the present invention comprises a transmitter for transmitting a packet signal, the packet signal including a guard interval between plural symbols consisting of information and having a predetermined length, the guard interval using part of the symbols and a receiver for clipping the symbols from the packet signal received from the transmitter and decoding the information. The receiver clips a signal of the predetermined length from the packet signal received from the transmitter at a predetermined position with respect to the center position of the guard interval acting as a reference being set as a clipping start position, and decodes information transmitted from the transmitter based on the clipping signal. 
         [0020]    The receiver in the OFDM communication system comprises receiver means for receiving a packet signal from the transmitter, storage means for storing the clipping start position, clipping means for the signal of the predetermined length from the packet signal received by the receiver, the clipping start position stored by the storage means being set as a reference, and decoder means for decoding the information based on the signal clipped by the clipping means. 
         [0021]    The storage means in the OFDM communication system stores the center position of the guard interval as a clipping start position. The clipping means clips a signal of a predetermined length at a clipping start position being the center position of each guard interval. 
         [0022]    The receiver in the OFDM communication system includes a frequency domain equalizer for equalizing signals from the clipping means in a frequency region; and the decoder decodes the information based on the signal from the frequency domain equalizer. 
         [0023]    According to the OFDM communication system of the present invention, plural transmitters may be provided, each of the plural transmitters radio-transmitting the same packet signal using the same frequency and with the same timing. The receiver receives the packet signal from the transmitter located in a transmission area of the plural transmitters and then decodes the information. 
         [0024]    According to another aspect of the present invention, an OFDM receiver is provided. The OFDM receiver receives a packet signal which includes a guard interval between plural symbols consisting of information and having a predetermined length using part of the symbols, clips each of the symbols from the packet signal, and decodes the information. The signal of the predetermined length is clipped from the packet signal at a predetermined position with respect to the center position of each guard interval, the predetermined position being set as a clip starting position, and the received signal is decoded based on the clipping signal. 
         [0025]    The OFDM receiver further comprises receiver means for receiving the packet signal, storage means for storing said clipping start position, a clipper for clipping the signal of a predetermined length from the packet signal received by the receiver means with respect to a clipping start signal, as a reference, stored by the storage means, and a decoder for decoding the information based on the signal clipped by the clipper. 
         [0026]    The storage means in the OFDM receiver stores the center position of the guard interval as a clip starting position. The clipper clips a signal of a predetermined length, the center position of the guard interval being set as a clip staring position. 
         [0027]    The OFDM receiver further comprises a frequency domain equalizer for equalizing the signal from the clipper in a frequency domain and the decoder decodes the information based on the signal from the frequency domain equalizer. 
         [0028]    According to the present invention, the OFDM communication system can perform more accurate signal transmission, and the OFDM receiver is suitable for the OFDM communication system and can perform more accurate signal reception. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a diagram schematically illustrating an OFDM communication system according to an embodiment of the present invention; 
           [0030]      FIG. 2  is a block diagram schematically illustrating a base station used in the OFDM communication system according to an embodiment of the present invention; 
           [0031]      FIG. 3  is a block diagram schematically illustrating a subsidiary station used in the OFDM communication system according to an embodiment of the present invention; 
           [0032]      FIG. 4  shows a format for a packet signal used in the OFDM communication system according to an embodiment of the present invention; 
           [0033]      FIG. 5  is an explanatory diagram schematically illustrating operation of the OFDM communication system according to an embodiment of the present invention; 
           [0034]      FIG. 6  is a diagram schematically illustrating a conventional OFDM communication system; and 
           [0035]      FIG. 7  is an explanatory diagram showing operation of a conventional OFDM communication system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]      FIG. 1  is a diagram schematically illustrating an OFDM communication system according to the present invention. An example of a single frequency network (SFN) is shown, which establishes the communication between a base station and a subsidiary station at the same frequency.  FIG. 1  shows an example of the OFDM (SC-OFDM) employing the frequency domain equalization technique as an OFDM communication scheme. 
         [0037]    In  FIG. 1 , it is assumed that plural base stations, namely, three base stations  101  to  103  in the present embodiment are deployed. Each of the base stations  101  to  103  configures a transmitter. The subsidiary station  104  configures a receiver or an OFDM communication receiver. 
         [0038]    For synchronization, the base stations  101  to  103  are loop linked together with the communication cable  107 . The base stations  101  to  103  are synchronized and each base station transmits the same signal to the subsidiary station  104  at the same frequency and with the same timing. 
         [0039]    In the synchronizing method over the communication cable on which transmission information, for example, is multiplexed to reference frequency clock information, the oscillation frequency of the reference frequency generator in each of the base stations  101  to  103  is accurately synchronized with the reference frequency clock information. Since the transmission information is multiplexed to the clock information, each of the base stations  101  to  103  surely generates transmission triggers with the same timing. 
         [0040]    Using the transmission trigger, each of the base stations  101  to  103 , adjusts the transmission time finely with its own delay correction parameter. In such a configuration, it appears as if the mobile subsidiary station  104 , which receives signals, namely simultaneous delivery radio signals, transmitted from each of the base stations  101  to  103 , receives the signal transmitted one base station with a delay dispersion due to the multipath. 
         [0041]    For example, when the subsidiary station  104  represented by  104   b  is located in the communication area  106  of the base station  103 , the subsidiary station  104   b  establishes SC-OFDM communication to the base station  101  at the frequency f 1 . When the subsidiary station  104  travels within the communication area  105  of the base station  101 , SC-OFDM communication is carried out at the frequency f 1 . This allows a seamless hand-off to be realized using a single frequency. 
         [0042]    By realizing the above operation and applying the frequency domain equalization technique to the receiving circuit of the subsidiary station  104 , the simultaneous delivery radio signal can be decoded without any trouble. The originally raised problem on a large number of base stations deployed and disposed over a wide area can be solved as described above. 
         [0043]    Each of  FIGS. 2 and 3  is a block diagram illustrating an OFDM communication system using the frequency domain equalization according to the present invention.  FIG. 2  is a block diagram illustrating a base station, namely OFDM transmitter used in the OFDM communication system.  FIG. 3  is a block diagram illustrating a subsidiary station, namely OFDM receiver used in the OFDM communication system. Since each of the base stations  101  to  103 , shown in  FIG. 1 , has the same configuration, the base station  101  is shown as a typical example in  FIG. 2 . 
         [0044]    Referring to  FIG. 2 , the base station  101  includes an access controller  201  connected to the communication cable  107 , for establishing the synchronization with other base stations  102  and  103 , an encoder  202  for CRC (Cyclic Redundancy Check) encoding data bits forming information, a mapping section  203  for mapping signals encoded by the encoder  202 , for example, performing symbol-to-symbol mapping, a pilot insertion section  204  for inserting a phase correction pilot signal to the signal from the mapping section  203 , a guard interval adder  205  for adding guard intervals respectively to plural symbols included in the signals from the pilot insertion section  204 , and an over sampling section  206  for over sampling and outputting the signal from the guard interval adder  205 . 
         [0045]    The base station  101  also includes a filtering section  207  on the transmitter side for filtering signals from the over sampling section  206  to pass a necessary signal of the signals from the over sampling section  206  and creating and outputting a packet signal or an OFDM signal, and a transmitter  208  for outputting the packet signal from the filtering section  207  as a packet signal of the frequency f 1 . 
         [0046]    Referring to  FIG. 3 , the subsidiary station  104  includes a receiver section  209  for receiving and outputting the packet signals of the frequency f 1  from the base stations  101  to  103 , a filtering section  210  passing and outputting only the necessary signal of signals from the receiver section  209 , an AGC (Automatic Gain control)  211  for controlling the signal level at a constant value, a symbol synchronizer  212  for symbol synchronizing the signal from the AGC  211 , and down-sampling section  213  for down-sampling the signal from the symbol synchronizer. 
         [0047]    The subsidiary station  104  also includes a frame synchronizer  215 , a carrier synchronizer for carrier synchronizing and outputting the signal from the down sampling section  213  based on the signal from the frame synchronizer  215 , a memory  227  for storing the clip starting position of a signal, a guard interval deletion section  226  for referring to the clip starting position stored in the memory  227  and clipping and outputting a signal of a predetermined length from the output signals of the carrier synchronizer  214 , a serial/parallel (S/P) converter  216  for converting a serial signal from the guard interval remover  226  into a parallel signal, a frequency domain equalizer  225  for performing frequency domain equalization to the signal from the serial/parallel converter  216 , a serial/parallel (P/S) converter  221  for converting the parallel signal from the frequency domain equalizer  225  into a serial signal, a phase compensator or corrector  222  for correcting and aligning the phase of a serial signal from the serial/parallel converter  221 , de-mapping section  223  for de-mapping the signal from the phase compensator  222 , for example, symbol-to-symbol de-mapping, and a detector  224  for detecting the signal from the de-mapping section  223 , namely, the signal matching the information from a base station. 
         [0048]    The clip starting position is set by an operation means (not shown) and then is stored in the memory  227 . For example, since the ingress time of multipath waves may be often delayed or quickened due to communication environments, the operation means sets the clip starting position in consideration of such environments. However, the content of clipping position designation information for designating the clipping position is set to a predetermined position with respect to the center position of a guard, such as “the position led by a predetermined number bits from the center position of a guard interval” or “the position delayed by a predetermined number of bits from the center position of a guard interval”, using parameters. 
         [0049]    As described above, the memory  227  stores as a clip starting position a predetermined position with respect to the center position of each guard interval acting as a reference point, for example, the center position of each guard interval. Thus, the memory  227  stores the clip starting position in accordance with the communication environment. 
         [0050]    The frequency domain equalizer  225  includes a FET (high-speed Fourier transformation) section  217  for FET (high-speed Fourier transformation) processing the signal from the serial/parallel converter  216 , a channel estimation section  219  for channel estimating the signal from the FET processing section  217 , a channel equalizer  218  for channel equalizing the signal from the FET processing section  217  based on the channel estimation by the channel estimation section  219 , and a IFET (inverse high-speed Fourier transformation) section for IFET (inverse high-speed Fourier transformation) processing the signal from the channel equalizer  220 . 
         [0051]    The receiving means is configured of a receiving section  209 , a filtering section  210 , an AGC section  211 , a symbol synchronizing section  212 , a down sampling section  213 , a carrier synchronizing section  214 , and a frame synchronizing section  215 . The clipping means is configured of a guard interval deletion section  226 . The memory  227  configures storage means for storing the clip starting position. The frequency area equalizer  225  configures frequency domain equalizing means. Decoding means is configured of a parallel/serial converter  221 , a phase compensator  222 , a de-mapping section  223 , and a detector  224 . 
         [0052]      FIG. 4  is a diagram illustrating a format of a packet signal, which is created by each of the base stations  101  to  103  and transmitted to the subsidiary station  104 . 
         [0053]    Referring to  FIG. 4 , the packet signal includes a preamble section  401 , a unique word section (UW)  402 , a channel signal section (CHAN)  403 , plural data section (DATA)  405 , a signal section (SIGNAL)  404 , plural data sections (DATA)  405 , and a pilot signal section inserted between data sections  405 . 
         [0054]    The preamble section  401  is the signal section representing the starting portion of a packet signal for AGC, symbol synchronization, and carrier synchronization. The unique word section  402  is the signal section including frame synchronization information or information about a symbol length. The channel signal section  403  is the signal section including information for estimation of radio transmission path or channel, phase and amplitude. The signal section  404  is the signal section including information representing transmission rate, data amount, or the like. 
         [0055]    Each data section  405  is the signal section including information transmitted from the base station  101  to  104  to the subsidiary station  104  or the signal section formed of a predetermined length of symbols and guard intervals. In the present embodiment, the length of the data section corresponds to 20 symbols. The last four symbols are copied every 16 symbols and are added to the forefront portion. The four symbols added to the forefront portion become a guard interval. These 20 symbols are transmitted as one block. 
         [0056]    The above mentioned value has been shown as one example. A different number of symbols or a different guard interval length may be realized. The pilot signal section  406  inserted between data sections  405  corresponds to a pilot symbol inserted every predetermined length, namely, every 20 symbols in the present embodiment, that is, every data section  405  for carrier phase compensation. 
         [0057]      FIG. 5  is an explanatory diagram illustrating operation of an OFDM communication system according to the present embodiment. Referring to  FIG. 5 , the symbol section  502  represents a symbol for the multicarrier OFDM scheme or an effective symbol for SC-OFDM scheme. Numeral  501  represents a guard interval in the symbol section  502 ,  503  represents a guard interval in the next symbol section, and  504  represents a symbol clip starting position. 
         [0058]    In the present embodiment, the guard intervals  501  and  503  is each formed of four symbols and the symbol section  502  is formed of 16 symbols as described above. The guard interval  501  added to the forefront section of the symbol section  502  corresponds to the symbol to which a portion of the symbol section  502 , namely, the rearmost four symbols in the present embodiment, are copied and added. 
         [0059]    In the frequency domain equalization to be described later in detail as to the operation, the channel equalization is carried out along the frequency axis using FET. In each data section  405  shown in  FIG. 4 , the last four symbols are copied every 16 symbols and added to the forefront portion so that the resultant structure is handled as four symbol guard interval. For that reason, when the transmission rate is 1M symbols/second, the guard interval is 4μ seconds. Even when radio waves and multipaths propagated from plural base stations have the above-mentioned delay dispersion amount, the frequency domain equalization can be realized. 
         [0060]    The operations of the OFDM communication system and OFDM receiver described above will be explained below. 
         [0061]    As described with reference to  FIG. 1 , the base stations  101  to  104  which are synchronously operated transmit the same signal to the subsidiary station  104  at the same frequency and with the same timing. 
         [0062]    The subsidiary station  104  communicates with one of the base stations  101  to  103  in the communication area, in which the subsidiary station  104  is located, of the base stations  101  to  103 . The example will be explained below where the subsidiary station  104  is located in the communication area  105  of the base station  101  and communicates with the base station. However, when the subsidiary station  104  is located in another communication area, the same communication is established with the base station, which controls the corresponding communication area. 
         [0063]    In the base station  101  shown in  FIG. 2 , the encoder  202  CRC encodes data bits, which constructs information to be transmitted to the subsidiary station  104 . The mapping section  203  maps the signal encoded by the encoder  104 . The pilot insertion section  204  inserts the pilot signal section (refer to  FIG. 4 ) for phase compensation between data sections  405  of the signal from the mapping section  203 . 
         [0064]    The guard interval addition section or adder  205  adds and outputs the guard interval in the previously described format to the respective symbols included in the signal from the pilot insertion section  204 . 
         [0065]    The over sampling section  206  over-samples and outputs the signal from the guard interval adder  205 . The filtering section  207  filters the signal from the over sampling section  206  and thus passes necessary signals of the signals from the over sampling section  206  and creates and outputs the packet signal or SC-OFDM signal. The transmitter section  208  outputs the packet signal from the filtering section  207 , as a packet signal of the frequency f 1 , to the subsidiary station  104 . 
         [0066]    In the subsidiary station  104  shown in  FIG. 3 , the receiver section  209  wirelessly receives the packet signal of the frequency f 1  from the base station  101  and outputs an encoded packet signal. The filtering section  210  passes and outputs only the necessary signal of the signals from the receiver section  209 . The AGC section  211  controls such that the signal level from the filtering section becomes constant. The symbol synchronizing section or synchronizer  212  symbol-synchronizes the signal from the AGC section  211 . The down sampling section  213  down-samples the signal from the symbol synchronizer  212 . 
         [0067]    The frame synchronizer  215  refers to the frame synchronous information described in the unique word section  402  of the packet signal from the down sampling section  21  to perform frame synchronization. The carrier synchronizer  214  performs carry synchronization based on the frame synchronization, thus outputting the signal to the guard interval delete section  226 . 
         [0068]    The guard interval deletion section  226  refers to the clip starting position stored in the memory  227 , clips the signal of a predetermined length from the signals sent by the carrier signal synchronizer  214 , with the clip starting position acting as a reference, and discards the remaining signals. For example, the guard interval deletion section  226  discards the number of symbols, namely four symbols, corresponding to the guard interval from the signal of a predetermined length of 16 symbols. 
         [0069]    The serial/parallel converter  216  converts a serial signal into a parallel signal as one FET block corresponding to 16 symbols of the signal from the guard interval deletion section  226 . 
         [0070]    This operation will be explained below by referring to  FIG. 5 . As shown in  FIG. 5(   c ), the number of symbols of a predetermined length corresponding to the symbol length is clipped, with the center position of the guard interval  501  acting as the symbol clip starting position. That is, the front edge corresponding to half of the guard interval length is removed and the rear edge corresponding to half of the guard interval length is removed. In this manner, the signal of the corresponding symbol length is clipped and extracted. Thus, even under a plural multipath environment and in the SFN scheme, the signal can be suitably applied to FET decoding of OFDM or frequency domain equalization. Moreover, as described above, equalization can be suitably implemented under various conditions by varying the clip starting position with parameters. 
         [0071]    The corresponding clipped symbol portion includes the guard interval of the corresponding symbol. However, because the guard interval is the signal of which the rear portion of the symbol is copied and the signal corresponding to the clipped symbol is the signal of which the symbol phase is shifted, accurate symbols can be obtained by performing the phase correction. The guard interval deletion section  226  outputs the symbol to the parallel/serial converter  216  after the phase correction. 
         [0072]    The FET section  217  performs the FET process to the signal from the serial/parallel converter  216 , in which the guard interval is removed. In this case, 16-point FET, for example, is used. Thus, the signal having delay information on the time axis is converted into the signal on the frequency axis. The channel equalizer  218  performs channel equalization to the information converted on the frequency axis, based on the channel information estimated by the channel estimation section  219  based on the information in the channel signal section  403 . The IFFT section  220  performs the inverse Fourier transformation to the signal from the channel equalizer  218  and then outputs the result. In this manner, the frequency domain equalizer  225  performs the frequency domain equalization to the signal from the serial/parallel converter  216  and then outputs the result. 
         [0073]    The parallel/serial converter  221  converts the parallel signal from the IFFT section  220 , in other words, from the frequency domain equalizer  225 , into a serial signal. The phase compensator  222  removes the pilot signal section  406  to phase correct the signal from the parallel/serial converter  221 . The de-mapping section  223  de-maps the signal from the phase compensator  222  and then outputs the signal, which is obtained by decoding the signal transmitted from the base station  101 . The detector  224  detects the signal matching the information from the base station  101 . 
         [0074]    As described above, in the OFDM communication system according to the embodiment of the present invention, the clip starting position of a symbol can be varied to the center position of the guard interval or to a predetermined amount in the vicinity of the center position acting as a reference using parameters. Thus, multi paths can be removed appropriately. 
         [0075]    That is, by removing (or varying) the front edge and the rear edge corresponding to half of a guard interval are removed, a FET block of OFDM can be extracted without being influenced due to the multipath even if the timing extracted at the preamble varies. Accordingly, when an interference due to multipath as shown in  FIG. 5(   b ) occurs in the original signal shown in  FIG. 5(   a ), the interference to the symbol portion  502  due to the multipath in the symbol portion is removed. As a result, only the information in the symbol portion  502  can be extracted. When the extraction timing is shifted largely as shown in  FIG. 5(   c ), the signal corresponding to the symbol portion  502  can be clipped. 
         [0076]    Moreover, when SNF to be transmitted simultaneously from plural base stations is formed, the possibility that the strongest wave of the multipath signal particularly is not an initial wave is strong considerably. The possibility that an extraction shift of the symbol timing occurs increases more. The present embodiment can favorably suppress an adverse effect due to multipath. 
         [0077]    In the present embodiment, an example of the communication between a base station and a subsidiary station has been explained. However, the present embodiment can be utilized in various communication field where information communication performs unit-directionally or bi-directionally, such as radio LAN (Local Area Network), digital broadcast, data communications, and the like. 
         [0078]    Moreover, the present embodiment has been explained with the example of SC-OFDM. However, the present embodiment can be applied to OFDM of a multicarrier. Also, the present invention can be applied to the communication between a base station and a subsidiary station or network communication, in which different frequencies are used. It is to be understood that the present invention can be utilized in various communication fields, where information communications are performed uni-directionally or bi-directionally, commencing with networks, broadcasts, data communications, which use OFDM or SC-OFDM. 
         [0079]    While there has been shown and described what are at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention defined by the appended claims.