Patent Publication Number: US-8121018-B2

Title: Signal transmitting method, signal receiving method and signal receiving device in OFDM system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 098119840 filed in Taiwan, R.O.C. on Jun. 12, 2009, the entire contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a signal transmitting method, a signal receiving method, and a signal receiving device applied in an orthogonal frequency-division multiplexing (OFDM) system. 
     BACKGROUND 
     An OFDM system is a frequency-division multiplexing (FDM) system using digital multi-carrier modulation. A plurality of orthogonal sub-carriers is used to transfer data. The data are divided into a plurality of parallel data streams or channels corresponding to the sub-carriers respectively. Each sub-carrier carries a single symbol, each symbol corresponds to several sets of bits, and each symbol of 16 quadrature amplitude modulation (QAM) corresponds to 4 bits. Corresponding manners include, for example, QAM or phase shift modulation, which is also referred to as phase shift keying (PSK). 
     The OFDM technology is usually applied in the field of wireless communication, which possibly generates a multi-path effect. The multi-path effect may cause time-spreading and inter-symbol interference (ISI). This is a so-called frequency-selective channel. The frequency-selectivity is generally overcome by adding a guard interval in each OFDM symbol. 
     When a transmitter and a receiver of the broadband mobile communication applying an OFDM technology are respectively located in a base station and a vehicle moving at a high speed (for example, an express train), relative movement there-between may generate Doppler Effect. The Doppler Effect may cause a channel response in a transferred OFDM symbol to become time-varying. That is a so-called time-selective channel. Orthogonality between the sub-carriers is further damaged, which is called an inter-carrier interference (ICI). The faster the vehicle moves, the more apparent the ICI will be. 
     A common approach for eliminating the ISI is to add a guard interval in each OFDM symbol, which may prolong a symbol period. However, the longer the symbol period is, the greater the influences of the Doppler Effect will be. Therefore, in order to eliminate the wireless channel time-varying and frequency-selective effects that the broadband mobile communication encounters during a high speed movement, a channel estimation technology is usually utilized. 
     In addition, the channel estimation technology may also be obtained with reference to the following documents: 
     (i) an article published by S. Chen and T. Yao (S. Chen and T. Yao, “Intercarrier interference suppression and channel estimation for OFDM systems in time-varying frequency selective fading channels,” IEEE Trans. Consum. Electron., vol. 50, no. 2, pp. 429-435, May 2004.); 
     (ii) an article published by Y. Mostofi and D. C. Cox (Y. Mostofi and D. C. Cox, “ICI mitigation for pilot-aided OFDM mobile systems,” IEEE Trans. Wireless Commun., vol. 4, no. 2, pp. 765-774, March 2005); 
     (iii) an article published by H. S. Cho (H. S. Cho, “Midamble aided OFDM performance analysis in high mobility vehicular channel,” 802.11 WLAN WG, Jan. 14, 2008); and 
     (iv) an article published by H. C. Lee, C. W. Chen, S. M. Young, and Shyue-Win Wei (H. C. Lee, C. W. Chen, S. M. Young, and Shyue-Win Wei, “Matrix Channel Estimation for OFDM Systems with Two Training Symbols and High-Order Polynomial Fitting,” In Proc. 18th Annu. IEEE Int&#39;l Symp. on Personal, Indoor and Mobile Radio Communications, Athens, Greece, September 2007, pp. 1-5). 
     The accuracy of time-varying channel estimation may influence performance of an OFDM frequency domain equalizer or signal detection performance, which further influences the accuracy for demodulating transferred signals. Therefore, the quality of the channel estimation technology directly influences performance of the whole OFDM system. 
     SUMMARY 
     In an embodiment, the present disclosure provides a signal transmitting method in an OFDM system. The signal transmitting method is performed by a transmitter of the OFDM system. The signal transmitting method comprises: receiving and converting a serial digital signal into a plurality of OFDM symbols (or referred to as parallel sub-carriers Xi(k)); transforming the OFDM symbols into a plurality of time domain signals x(k); and serializing the time domain signals into a transmitted signal and then transmitting the signal. The OFDM symbols comprise a plurality of data symbols, a plurality of first training symbols, and a plurality of second training symbols. The above symbols comprise a plurality of data sub-carriers, a plurality of first pilot sub-carriers, and a plurality of second pilot sub-carriers. The data symbols comprise the data sub-carriers. The first training symbols comprise the first pilot sub-carriers. The second training symbols comprise the second pilot sub-carriers and the data sub-carriers. That is to say, active sub-carriers of the data symbols are all data sub-carriers. Active sub-carriers of the first training symbols are all first pilot sub-carriers. Active sub-carriers of the second training symbols are all second pilot sub-carriers and data sub-carriers. 
     In an embodiment, the present disclosure further provides a signal receiving method, suitable for receiving transmitted signals transmitted through the signal transmitting method. The signal receiving method is performed by a receiver of the OFDM system. The transmitted signal is transferred to the receiver through a plurality of paths to become a receiving signal with a channel response. The signal receiving method comprises: receiving and converting the receiving signal with a channel response into a frequency-domain serial signal, in which the frequency-domain serial signal comprises a plurality of data receiving signals, a plurality of first pilot receiving signals, and a plurality of second pilot receiving signals; estimating channel responses on the data receiving signals by using the adjacent first pilot receiving signals and second pilot receiving signals; and restoring the data receiving signals into data sub-carriers by using the channel responses. 
     In an embodiment, the present disclosure further provides a receiver in an OFDM system, suitable for receiving transmitted signals transmitted through the signal transmitting method. The receiver comprises an OFDM demodulator and an inter-subcarrier interference cancellation unit. The OFDM demodulator receives and demodulates a receiving signal with a channel response to a frequency-domain serial signal. The frequency-domain serial signal comprises a plurality of data receiving signals, a plurality of first training receiving signals, and a plurality of second training receiving signals. The inter-subcarrier interference cancellation unit estimates a plurality of channel responses by using the adjacent first pilot receiving signals and second pilot receiving signals, and restores the data receiving signals into data sub-carriers by using the channel responses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below for illustration only, and thus is not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic structural view of a transmitter and a receiver of an OFDM system according to an embodiment; 
         FIG. 2  is a schematic flow chart of a signal transmitting method in an OFDM system according to an embodiment; 
         FIG. 3  is a schematic view of configuring pilot sub-carriers to data sub-carriers through the signal transmitting method according to an embodiment; 
         FIG. 4  is a schematic view of an OFDM transmitter according to another embodiment; 
         FIG. 5  is a schematic view of a plurality of frequency-domain receiving signals Y(k) received through a signal receiving method according to an embodiment; 
         FIG. 6  is a schematic circuit block diagram of an inter-subcarrier interference cancellation unit of a receiver in an OFDM system according to an embodiment; 
         FIG. 7  is a schematic view of a multi-path channel time-varying curve; 
         FIG. 8  is a schematic flow chart of a signal receiving method in an OFDM system according to an embodiment; 
         FIG. 9  is a schematic flow chart of Step S 80  in the signal receiving method in an OFDM system according to an embodiment; 
         FIG. 10  is a schematic flow chart of Step S 82  in the signal receiving method in an OFDM system according to an embodiment; and 
         FIG. 11  is a schematic view of performance comparison between the signal transmitting method and the signal receiving method in an OFDM system according to an embodiment and that of the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     The above brief description of the contents of the embodiments and the following detailed description of the embodiments are used to illustrate and explain the spirit and principle of the present invention, and provide further explanations of the claims of the present invention. 
       FIG. 1  is a schematic structural view of a transmitter  10  and a receiver  30  in an OFDM system according to an embodiment. The OFDM system comprises a transmitter  10  and a receiver  30 . The transmitter  10  is used for performing an embodiment of a signal transmitting method. The receiver  30  is used for performing an embodiment of a signal receiving method. The transmitter is used for converting a serial digital signal  94  into a transmitted signal  20  (also referred to as a radio frequency signal) and then transmitting the transmitted signal  20 . After being transmitted, the transmitted signal  20  reaches the receiver  30  through different paths. At this time, a signal  26  received by the receiver  30  is the transmitted signal  20  comprising a channel impulse response h(k) and a noise w(k) (hereinafter briefly referred to as a receiving signal with a channel response). The receiver  30  converts the receiving signal  26  with a channel response into the restored serial digital signal  95 . 
     The serial digital signal  95  is a serial digital signal  94  formed by data to be transferred. As seen from  FIG. 1 , the serial digital signal  94  is generated by a random signal generator  90 . In practical implementation, the serial digital signal  94  is not generated by the random signal generator  90 , but directly formed by the data to be transferred. The random signal generator  90  shown in the figure aims at illustrating the generation of the serial digital signal  94  and tests the embodiment, and it is not intended to limit the implementation. 
     The serial digital signal  94  generated by the random signal generator  90  may be, but not limited to, PRBS 2 7 -1, 2 23 -1 (pseudo random binary sequence), and the like. The pseudo random binary sequence is adapted to simulate various possible serial digital signals  94 . 
     As seen from  FIG. 1 , the transmitter  10  comprises a symbol mapping unit  11  and an OFDM modulator  18 . 
     The symbol mapping unit  11  receives the serial digital signal  94 , divides the serial digital signal  94  into a plurality of sets of bits with a predetermined length, and maps the sets of bits one by one into a plurality of serial data sub-carriers S(k). The sets of bits may be respectively formed by two, four, six, eight, ten or more bits. Although the above sets of bits respectively comprise, for example, even-numbered bits, they may also comprise odd-numbered bits. The data sub-carriers S(k) generated by the symbol mapping unit  11  are still serial data. 
     The OFDM modulator  18  comprises a serial to parallel converting unit (S/P converting unit)  12 , a pilot sub-carrier insertion unit  13 , an inverse fast Fourier transforming (IFFT) unit  14 , a guard interval adding unit  15 , a parallel to serial converting unit (P/S converting unit)  16 , and a transmitting filter  17 . 
     The S/P converting unit  12  receives the serial data sub-carriers (k), and converts the serial data sub-carriers (k) into parallel data sub-carriers S(k). 
     The pilot sub-carrier insertion unit  13  receives the parallel data sub-carriers S(k), and then arranges/inserts “pilot sub-carriers” between “data sub-carriers” in a suitable manner. The “data sub-carriers” and the suitably-configured “pilot sub-carriers” are combined into a plurality of OFDM symbols Xi(k) (also referred to as a plurality of parallel active sub-carriers). That is to say, all the parallel active sub-carriers Xi(k) comprise a plurality of “data sub-carriers” and a plurality of “pilot sub-carriers”. The OFDM symbols Xi(k) are frequency domain signals. 
     Referring to  FIG. 3 , the manner of configuring the “pilot sub-carriers” between the “data sub-carriers” is shown. 
     In  FIG. 3 , the transverse axis is time, in which the unit thereof is symbol, and the longitudinal axis is frequency. As seen from  FIG. 3 , 16 dots exist on each vertical column. Each vertical column is referred to as one symbol. Each dot represents one sub-carrier. For the example shown in the figure, each symbol has 16 sub-carriers, or referred to as active sub-carriers. The active sub-carriers may be “pilot sub-carriers” or “data sub-carriers”. In the figure, the solid dots represent pilot sub-carriers. The soft dots represent data sub-carriers. The symbols comprise a plurality of data symbols  401  and  402 , a plurality of first training symbols  421  and  422 , and a plurality of second training symbols  441  and  442 . As seen from the sub-carriers in each symbol, the symbols comprise a plurality of data sub-carriers  40   a ,  40   b ,  40   c ,  40   d , and  40   e , a plurality of first pilot sub-carriers  42   a  and  42   b , and a plurality of second pilot sub-carriers  44   a  and  44   b.    
     The active sub-carriers (also referred to as each of the parallel sub-carriers X(k)) in the data symbols  401 ,  402  all use the data sub-carriers  40   a ,  40   b . The active sub-carriers in the first training symbols  421 ,  422  all use the first pilot sub-carriers  42   a ,  42   b . The active sub-carriers of the second training symbols  441 ,  442  are formed by the second pilot sub-carriers  44   a ,  44   b  and the data sub-carriers  40   c ,  40   d , and  40   e . The number of the second pilot sub-carriers  44   a ,  44   b  of the second training symbols  441 ,  442  is smaller than that of the active sub-carriers of the second training symbols  441 ,  442 . 
     As seen from the schematic view of the second training symbols  441  and  442  in  FIG. 3 , one second pilot sub-carrier  44   a ,  44   b  exists for each four adjacent active sub-carriers. That is to say, the number of the second pilot sub-carriers  44   a ,  44   b  of the second training symbols  441 ,  442  is a quarter of that of the active sub-carriers X(k) of the second training symbols  441 ,  442 . That is to say, the number of the second pilot sub-carriers  44   a ,  44   b  of the second training symbols  441 ,  442  is one third of the number of the data sub-carriers  40   c ,  40   d , and  40   e  of the second training symbols  441 ,  442 . Although in this embodiment, the configuration is performed in a ratio of 1:4 (pilot sub-carriers to active sub-carriers), in practical implementation, it may be configured in a ratio such as ½, or ⅛, which may be optionally adjusted by applied environment condition, e.g. coherent bandwidth. A frequency spacing between the adjacent second pilot sub-carriers  44   a ,  44   b  in the second training symbols  441 ,  442  is smaller than a coherent bandwidth. 
     Adjacent training symbols represents first training symbols  421 ,  422  and second training symbols  441 ,  442 . For example,  421  and  441  may be referred to as adjacent training symbols.  441  and  422  are also referred to as adjacent training symbols. 
     As discussed above, the second pilot sub-carriers  44   a ,  44   b  of the second training symbols  441 ,  442  and the first pilot sub-carriers  42   a ,  42   b  of the first training symbols  421 ,  422  are distributed in the training symbols  441 ,  442 ,  421 , and  422  in different manners. As seen from  FIG. 3 , the second pilot sub-carriers  44   a ,  44   b  of the first training symbol  421  and the first pilot sub-carriers  42   a ,  42   b  of the second training symbol  441  are distributed within the corresponding symbol in an asymmetric status. This is referred to as an asymmetric training symbol method or an asymmetric pilot symbol method. After being received by the receiver  30 , such training symbols  421 ,  422 ,  441 , and  442  are used to evaluate the time-varying channel response. As the second pilot sub-carriers  44   a ,  44   b  of the second training symbols  441 ,  442  do not occupy all the active sub-carriers X(k) corresponding to the same symbol interval, the other active sub-carriers (that is, data sub-carriers  40   c ,  40   d , and  40   e ) corresponding to the symbol interval may be used to transfer data. In such a manner, the bandwidth for transferring data may be increased, and meanwhile, the bandwidth occupied by the pilot sub-carriers is decreased. 
     As seen from  FIG. 3 , the time duration of the data symbols  401 ,  402  between the first training symbols  421 ,  422  and the second training symbols  441 ,  442  is only one symbol duration, which is not intended to limit the implementation manner. On the time axis, the data symbols  401 ,  402  between the adjacent first training symbols  421 ,  422  and second training symbols  441 ,  442  may be two, three, four or more symbol durations (that is, symbol intervals), as long as the accuracy for evaluating the time-varying channel response carried out after the symbols are received at the receiver  30  can achieve a predetermined effect. 
     Next, the IFFT unit  14  is used for transforming the OFDM symbols X(k) into parallel first time domain signals x(k) in an IFFT manner. The first time domain signals x(k) are orthogonal with respect to each other. Subsequently, the guard interval adding unit  15  is used for copying a specific time duration of each first time domain signal x(k) to a position before or after the first time domain signal in a unit of a symbol, so as to become second time domain signals  96 . The guard intervals added by the guard interval adding unit  15  may be cyclic prefix or cyclic post-fix. Taking the addition of the cyclic prefixes as an example, it is assumed that the duration of each symbol is 128 dots (the duration of 128 dots is a size of the IFFT unit  14 , that is, an IFFT size), and if the copied rear signals are 40 dots, the guard interval adding unit  15  copies signals of the symbol at the last 40 dots on the time axis to the foremost part of the symbol. That is to say, after the copying process, the length of each symbol becomes 168 dots. 
     Subsequently, the P/S converting unit  16  is used for serializing the parallel second time domain signals  96  and transmitting the serialized second time domain signal  96  to the transmitting filter  17 . The transmitting filter  17  filters the serialized second time domain signal  96  and transmit it as a transmitted signal  20 . 
       FIG. 2  is a schematic flow chart of a signal transmitting method according to an embodiment of the present invention. The transmitting end  10  performs the embodiment of the signal transmitting method. The signal transmitting method comprises the following steps. 
     In Step S 60 , a serial digital signal  94  is received and converted into a plurality of OFDM symbols X(k). The OFDM symbols X(k) comprise a plurality of data symbols  401  and  402 , a plurality of first training symbols  421  and  422 , and a plurality of second training symbols  441  and  442 . The symbols  401 ,  402 ,  421 ,  422 ,  441 , and  442  comprise a plurality of data sub-carriers  40   a ,  40   b ,  40   c ,  40   d , and  40   e , a plurality of first pilot sub-carriers  42   a  and  42   b , and a plurality of second pilot sub-carriers  44   a  and  44   b . The data symbols  401  and  402  comprise the data sub-carriers  40   a  and  40   b . The first training symbols  421  and  422  comprise the first pilot sub-carriers  42   a  and  42   b . The second training symbols  441  and  442  comprise the second pilot sub-carriers  44   a  and  44   b  and the data sub-carriers  40   c ,  40   d , and  40   e.    
     In Step S 62 , the OFDM symbols are transformed into a plurality of time domain signals x(k). 
     In Step S 64 , the time domain signals are serialized into a transmitted signal for being transmitted. 
     Step S 60  is accomplished by the symbol mapping unit  11 , the S/P converting unit  12 , and the pilot sub-carrier insertion unit  13  in  FIG. 1 . In Step S 60 , the serial digital signal  94  is converted into OFDM symbols X(k), that is, OFDM symbols X(k) (also represented as Xi(k)) shown in  FIG. 3 . The OFDM symbols X(k) comprise a plurality of data symbols  401  and  402 , a plurality of first training symbols  421  and  422 , and a plurality of second training symbols  441  and  442 . 
     In Step S 62 , the OFDM symbols X(k) are transformed into a plurality of time domain signals x(k) by the IFFT unit  14 . The IFFT unit  14  transforms the OFDM symbols X(k) into a plurality of time domain signals x(k) in an IFFT manner. The plurality of time domain signals x(k) are orthogonal with respect to each other. 
     Then, in Step S 64 , the time domain signals x(k) are serialized into a transmitted signal  20  for being transmitted. Step S 64  is performed by the P/S converting unit  16  and the transmitting filter  17 . 
     Before Step S 64 , Step S 63  is further performed, in which a guard interval is added to each of the time domain signals x(k). Step S 63  is performed by the guard interval adding unit  15 . A guard interval is added before each symbol, so as to overcome the influences caused by the multi-path phenomenon. 
       FIG. 4  shows a transmitter  10  according to another embodiment of the present invention. An embodiment of the signal transmitting method is accomplished by a transmitter  10 ′ shown in  FIG. 4 . 
     As seen from  FIG. 4 , the transmitter  10 ′ comprises a symbol mapping unit  11 , a pilot sub-carrier insertion unit  13 ′, and an OFDM modulator  18 ′. A difference between the transmitter  10 ′ and the transmitter  10  in  FIG. 1  is that, the pilot sub-carrier insertion unit  13 ′ is moved from a position within the OFDM modulator  18  to a position behind the symbol mapping unit  11 . 
     The pilot sub-carrier insertion unit  13 ′ receives serial data sub-carriers S(k) and properly configures pilot sub-carriers  42   a ,  42   b ,  44   a , and  44   b  between the serial data sub-carriers S(k)  40   a ,  40   b ,  40   c ,  40   d , and  40   e . The configuration manner is similar to that in  FIG. 3 , and the difference there-between is that the pilot sub-carriers  42   a ,  42   b ,  44   a , and  44   b  are directly inserted in the serial data sub-carriers S(k) in this embodiment, whereas the above pilot sub-carrier insertion unit  13  inserts  42   a ,  42   b ,  44   a , and  44   b  in the parallel data sub-carriers S(k). 
     In both  FIGS. 4 and 1 , the transmitters  10 ,  10 ′ are used to implement the embodiment of the signal transmitting method, but the present invention is not limited here. 
     As discussed above, after being transmitted, the transmitted signal  20  reaches the receiver  30  through a plurality of paths. At this time, the transmitted signal  20  received by the receiver  30  comprises a channel impulse response h(k) and a noise w(k) in time domain, which is referred to as a receiving signal  26  with a channel response hereinafter. The noise w(k) comprises a thermal noise, a circuit noise, and the like. Therefore, a time-domain receiving signal  26  with a channel response received by the receiver  30  may be represented as x(k){circle around (x)}h(k)+w(k), where {circle around (x)} means convolution operations, and a frequency-domain receiving signal  26  may be represented as X(k)×H(k)+W(k), in which the X(k), H(k), and W(k) correspond to frequency domain signals obtained after performing the fast Fourier transform (FFT) on the time domain signals x(k), h(k), and w(k) respectively. 
     Referring to  FIG. 1  again, an embodiment of a signal receiving device in an OFDM system is further described. As seen from  FIG. 1 , the receiving signal  26  with a channel response is received by the receiver  30 . The receiver  30  comprises an OFDM demodulator  38 , an inter-subcarrier interference cancellation unit  39 , and a symbol inverse mapping unit  92 . 
     The OFDM demodulator  38  demodulates the receiving signal  26  with a channel response to obtain a frequency-domain serial signal  98 . Referring to  FIG. 5 , the frequency-domain serial signal comprises a plurality of data receiving signals  40   a′ ,  40   b′ ,  40   c′ ,  40   d′ , and  40   e′ , a plurality of first pilot receiving signals  42   a′  and  42   b′ , and a plurality of second pilot receiving signals  44   a′  and  44   b′ . A difference between the frequency-domain serial signal  98  of the receiver  30  and the sub-carrier serial signal  94  of the transmitter  10  is that, the frequency-domain serial signal  98  is obtained through the receiving and demodulating processes after being transferred via a multi-path channel. Therefore, the frequency-domain serial signal  98  has a channel impulse response and a noise. In contrast with the frequency-domain serial signal  98 , the sub-carrier serial signal  94  does not have a channel impulse response and a noise. The data receiving signals  40   a′ ,  40   b′ ,  40   c′ ,  40   d′ , and  40   e′ , the first pilot receiving signals  42   a′  and  42   b′ , and the second pilot receiving signals  44   a′  and  44   b′  respectively correspond to the data sub-carriers  40   a ,  40   b ,  40   c ,  40   d , and  40   e , the pilot sub-carriers  42   a  and  42   b  in the first training symbols, and the pilot sub-carriers  44   a  and  44   b  in the second training symbols, and the arrangement manner thereof is the same as that of the transmitting end shown in  FIG. 3 . 
     The inter-subcarrier interference cancellation unit  39  is used for estimating a plurality of channel responses (details of the estimation method are illustrated below in detail) by using adjacent first training receiving symbols and second training receiving symbols, and restoring the data receiving signals  40   a′ ,  40   b′ ,  40   c′ ,  40   d′ , and  40   e′  into the data sub-carriers  40   a ,  40   b ,  40   c ,  40   d , and  40   e  by using the channel responses. The restored data sub-carriers R(k) are serial data, and are the same as the serial digital signals S(k). 
     The symbol inverse mapping unit  92  maps the restored data sub-carriers R(k) back to the serial receiving data. The mapping rule of the symbol inverse mapping unit  92  is an inverse function of the mapping rule of the symbol mapping unit  90 . 
     The OFDM demodulator  38  comprises a receiving filter  31 , an S/P converting unit  32 , a guard interval removing unit  33 , a fast Fourier transforming (FFT) unit  34 , and a P/S converting unit  35 . 
     The receiving filter  31  receives and filters the receiving signal  26  with a channel response, so as to form a filtered receiving signal  99   a . The S/P converting unit  32  parallelizes the filtered receiving signal  99   a  to obtain a plurality of parallel first time-domain receiving signals  99   b . The guard interval removing unit  33  removes guard intervals of the first time-domain receiving signals  99   b  to obtain a plurality of second time-domain receiving signals y(k). If the manner of removing the guard intervals by the guard interval removing unit  33  follows the above example of removing the cyclic prefix, each symbol originally has 128 dots, and a length of the guard interval is 40 dots, the guard interval removing unit  33  removes the guard interval occupying 40 dots from each first time-domain receiving signal transferred from the S/P converting unit  32 . 
     The FFT unit  34  transforms the second time-domain receiving signals y(k) into a plurality of frequency-domain receiving signals Y(k) in an FFT manner. The P/S converting unit  35  serializes the frequency-domain receiving signals Y(k) into the frequency-domain serial signal  98 . 
     The configuration manner of the frequency-domain receiving signals Y(k) is the same as the configuration manner of the sub-carriers in the OFDM symbols X(k) sent from the transmitter  10 .  FIG. 5  is a schematic view of a plurality of frequency-domain receiving signals Y(k) received through a signal receiving method according to an embodiment of the present invention. In the figure, the frequency-domain receiving signals Y(k) comprise the data receiving signals  40   a′ ,  40   b′ ,  40   c′ ,  40   d′ , and  40   e′ , the first pilot receiving signals  42   a′  and  42   b′ , and the second pilot receiving signals  44   a′  and  44   b′ . As for the respective symbols, the frequency-domain receiving signals Y(k) comprise a plurality of first training receiving symbols  421 ′ and  422 ′, a plurality of second training receiving symbols  441 ′ and  442 ′, and a plurality of data receiving symbols  401 ′ and  402 ′. 
     The adjacent first and second training receiving symbols  421 ′,  422 ′,  441 ′, and  442 ′ refer to  421 ′ and  441 ′, or  441 ′ and  422 ′, and the like. 
     As seen from  FIG. 5 , in the frequency-domain receiving signals Y(k), one data receiving symbol  401 ′ exists between the adjacent first and second training receiving symbols  421 ′ and  441 ′. The second training receiving symbol  441 ′ further comprises the data receiving signals  40   c′ ,  40   d′ , and  40   e′ . The data receiving signals located within the adjacent first and second training receiving symbols  421 ′ and  441 ′ comprise the data receiving signal  40   a′  between the first and second training receiving symbols  421 ′ and  441 ′, and the data receiving signals  40   c′ ,  40   d′ , and  40   e′  within the second training receiving symbol  441 ′. 
     During implementation, the frequency-domain receiving signals Y(k) may correspond to the OFDM symbols X(k). That is to say, the OFDM symbols X(k) in  FIG. 5  may be consistent with the frequency-domain receiving signals Y(k) in  FIG. 3 , but the present invention is not limited here. That is to say, besides only one data receiving symbol  401 ′ as shown in  FIG. 3  or  FIG. 5 , a plurality of data receiving symbols may also exist between the adjacent first and second training receiving symbols  421 ′ and  441 ′. 
     The inter-subcarrier interference cancellation unit  39  estimates a channel response corresponding to each parallel frequency-domain receiving signal Y(k) by using the known adjacent first pilot receiving signal  42   a′  and second pilot receiving signal  44   a′ , and then restores the data receiving signal  40   a′  between the adjacent first training receiving symbol  421 ′ and second training receiving symbol  441 ′, and the data receiving signals  40   c′ ,  40   d′ , and  40   e′  within the second training receiving symbol  441 ′ by using the channel responses. 
     As seen from the figure, the inter-subcarrier interference cancellation unit  39  firstly performs a first estimation on the adjacent first pilot receiving signal  42   a′  of the first training receiving symbol  421 ′ and second pilot receiving signal  44   a′  of the second training receiving symbol  441 ′ on the left side of the figure (the first two), and restores the data receiving signal  40   a′  in the data receiving symbol  401 ′ and the data receiving signals  40   c′ ,  40   d′ , and  40   e′  in the second training receiving symbol  441 ′. Next, the inter-subcarrier interference cancellation unit  39  performs a second estimation by using the adjacent second pilot receiving signal  44   a′  of the second training receiving symbol  441 ′ and first pilot receiving signal  42   b′  of the first training receiving symbol  422 ′, and then restores the data receiving signal  40   b′  in the data receiving symbol  402 ′. Similar operations are performed subsequently. 
     Next,  FIG. 6  is a schematic circuit block diagram of an inter-subcarrier interference cancellation unit  39  of a receiver in an OFDM system according to an embodiment. 
     The inter-subcarrier interference cancellation unit  39  comprises a sub-carrier storing unit  390 , a pilot sub-carrier extracting unit  392 , a data sub-carrier extracting unit  393 , a polynomial coefficient estimating unit  394 , a data sub-carrier channel estimating unit  396 , and an equalizer unit  398 . 
     The sub-carrier storing unit  390  stores all the symbols to be estimated and restored in the frequency-domain serial signal  98 , that is, all the symbols between the first training receiving symbol  421 ′ and the second training receiving symbol  441 ′ (including the training receiving symbols  421 ′,  441 ′ and the data receiving symbol  401 ′). For example, during the first estimation and restoration, the sub-carrier storing unit  390  stores (or extracts) the first training receiving symbol  421 ′, the data receiving symbol  401 ′, and the second training receiving symbol  441 ′. During the second estimation and restoration, the sub-carrier storing unit  390  stores the second training receiving symbol  441 ′, the data receiving symbol  402 ′, and the first training receiving symbol  422 ′. Similar operations are performed subsequently. 
     The pilot sub-carrier extracting unit  392  extracts the pilot receiving signals  42   a′  and  44   a′ , that is, the first pilot receiving signal  42   a′  in the first training receiving symbol  421 ′ and the second pilot receiving signal  44   a′  in the second training receiving symbol  441 ′ from the sub-carrier storing unit  390 . For example, during the first estimation and restoration, the pilot sub-carrier extracting unit  392  extracts the first pilot receiving signal  42   a′  of the first training receiving symbol  421 ′ and the second pilot receiving signal  44   a′  of the second training receiving symbol  441 ′. During the second estimation and restoration, the pilot sub-carrier extracting unit  392  extracts the second pilot receiving signal  44   a′  of the second training receiving symbol  441 ′, and the first pilot receiving signal  42   b′  of the first training receiving symbol  422 ′. Similar operations are performed subsequently. 
     During the first estimation and restoration, the data sub-carrier extracting unit  393  extracts the data receiving signal  40   a′  in the data receiving symbol  401 ′ and the data receiving signals  40   c′ ,  40   d′ , and  40   e′  in the second training receiving symbol  441 ′ from the sub-carrier storing unit  390 . 
     The polynomial coefficient estimating unit  394  estimates a plurality of polynomial coefficients by using the adjacent first pilot receiving signal  42   a′  and second pilot receiving signal  44   a′ , the pilot sub-carriers  42   a  and  42   b  in the first training symbol, and the pilot sub-carriers  44   a  and  44   b  in the second training symbol. Subsequently, the data sub-carrier channel estimating unit  396  estimates the channel responses corresponding to the extracted data receiving signals  40   a′ ,  40   c′ ,  40   d′ , and  40   e′  by using the polynomial coefficients. The equalizer unit  398  restores the extracted data receiving signals  40   a′ ,  40   c′ ,  40   d′ , and  40   e′  into the data sub-carriers  40   a ,  40   c ,  40   d , and  40   e  by using the channel responses. 
     The estimation manners of the polynomial coefficient estimating unit  394  and the data sub-carrier channel estimating unit  396  are illustrated as follows. 
     First, the first pilot sub-carriers  42   a  and  42   b  of the first training symbols  421  and the second pilot sub-carriers  44   a  and  44   b  of the second training symbols  441  transferred from the transmitter  10  are represented by X t , X p  respectively. The first pilot receiving signal  42   a′  of the first training receiving symbol  421 ′ and the second pilot receiving signal  44   a′  of the second training receiving symbol  441  after being received and demodulated by the receiver  30  are represented by variables Y t , Y p  respectively. 
     An OFDM system with N active (parallel) sub-carriers is taken as an example (for example, as shown in  FIG. 3 , N is 16, and N is the number of the active sub-carriers, which corresponds to the dot number of the IFFT (IFFT size)). The first pilot receiving signal  42   a′  and the second pilot receiving signal  44   a′  of each of the N active sub-carriers are estimated by using a Qth-order polynomial coefficient estimator  394 . That is to say, the channel responses of a plurality of paths are firstly simulated with a plurality of (in the example, l) Qth-order polynomials with a plurality of coefficients (that is, Step S 820  below, which is illustrated in detail below). Each polynomial describes a channel response of one path.  FIG. 7  is a schematic view of a multi-path channel time-varying curve. k represents a sampling point on the time axis. One symbol interval comprises a plurality of sampling points. l represents a number of paths (i.e., path number). g i (k, l) represents a channel change of an l th  path at the k th  sampling point in the i th  pilot sub-carrier, that is, a gain of an l th  path of a channel response at the k th  sampling time.
 
 g   i ( k,l )= a   0,l   +a   1,l   k+a   2,l   k   2   + . . . , k= 0,1,2, . . . ,  N− 1
 
     The channel response matrix G is as follows. 
     
       
         
           
             G 
             = 
             
               [ 
               
                 
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           0 
                           , 
                           0 
                         
                         ) 
                       
                     
                   
                   
                     0 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           0 
                           , 
                           
                             L 
                             - 
                             1 
                           
                         
                         ) 
                       
                     
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           0 
                           , 
                           
                             L 
                             - 
                             2 
                           
                         
                         ) 
                       
                     
                   
                   
                     … 
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           0 
                           , 
                           1 
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           1 
                           , 
                           1 
                         
                         ) 
                       
                     
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           1 
                           , 
                           0 
                         
                         ) 
                       
                     
                   
                   
                     0 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           1 
                           , 
                           
                             L 
                             - 
                             1 
                           
                         
                         ) 
                       
                     
                   
                   
                     … 
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           1 
                           , 
                           2 
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     ⋮ 
                   
                   
                     ⋱ 
                   
                   
                     
                         
                     
                   
                   
                     ⋱ 
                   
                   
                     ⋱ 
                   
                   
                     
                         
                     
                   
                   
                     
                         
                     
                   
                   
                     ⋮ 
                   
                 
                 
                   
                     0 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           
                             N 
                             - 
                             1 
                           
                           , 
                           
                             L 
                             - 
                             1 
                           
                         
                         ) 
                       
                     
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           
                             N 
                             - 
                             1 
                           
                           , 
                           
                             L 
                             - 
                             2 
                           
                         
                         ) 
                       
                     
                   
                   
                     … 
                   
                   
                     … 
                   
                   
                     
                       g 
                       ⁡ 
                       
                         ( 
                         
                           
                             N 
                             - 
                             1 
                           
                           , 
                           0 
                         
                         ) 
                       
                     
                   
                 
               
               ] 
             
           
         
       
       
         
           
             G 
             = 
             
               [ 
               
                 
                   
                     
                       
                         a 
                         0 
                       
                       + 
                       
                         
                           b 
                           0 
                         
                         · 
                         0 
                       
                     
                   
                   
                     0 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     
                       
                         a 
                         
                           L 
                           - 
                           1 
                         
                       
                       + 
                       
                         
                           b 
                           
                             L 
                             - 
                             1 
                           
                         
                         · 
                         0 
                       
                     
                   
                   
                     
                       
                         a 
                         
                           L 
                           - 
                           2 
                         
                       
                       + 
                       
                         
                           b 
                           
                             L 
                             - 
                             2 
                           
                         
                         · 
                         0 
                       
                     
                   
                   
                     … 
                   
                   
                     
                       
                         a 
                         1 
                       
                       + 
                       
                         
                           b 
                           1 
                         
                         · 
                         0 
                       
                     
                   
                 
                 
                   
                     
                       
                         a 
                         1 
                       
                       + 
                       
                         
                           b 
                           1 
                         
                         · 
                         1 
                       
                     
                   
                   
                     
                       
                         a 
                         0 
                       
                       + 
                       
                         
                           b 
                           0 
                         
                         · 
                         1 
                       
                     
                   
                   
                     0 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     
                       
                         a 
                         
                           L 
                           - 
                           1 
                         
                       
                       + 
                       
                         
                           b 
                           
                             L 
                             - 
                             1 
                           
                         
                         · 
                         1 
                       
                     
                   
                   
                     … 
                   
                   
                     
                       
                         a 
                         2 
                       
                       + 
                       
                         
                           b 
                           2 
                         
                         · 
                         1 
                       
                     
                   
                 
                 
                   
                     ⋮ 
                   
                   
                     ⋱ 
                   
                   
                     
                         
                     
                   
                   
                     
                         
                     
                   
                   
                     ⋱ 
                   
                   
                     ⋱ 
                   
                   
                     
                         
                     
                   
                   
                     ⋮ 
                   
                 
                 
                   
                     0 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     
                       
                         a 
                         
                           L 
                           - 
                           1 
                         
                       
                       + 
                       
                         
                           b 
                           
                             L 
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             N 
                             - 
                             1 
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       
                         a 
                         
                           L 
                           - 
                           2 
                         
                       
                       + 
                       
                         
                           b 
                           
                             L 
                             - 
                             2 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             N 
                             - 
                             1 
                           
                           ) 
                         
                       
                     
                   
                   
                     … 
                   
                   
                     … 
                   
                   
                     
                       
                         a 
                         0 
                       
                       + 
                       
                         
                           b 
                           0 
                         
                         · 
                         
                           ( 
                           
                             N 
                             - 
                             1 
                           
                           ) 
                         
                       
                     
                   
                 
               
               ] 
             
           
         
       
     
     The frequency-domain and time-domain transmitted signals at the i th  symbol are represent by X i (p) and x i (k). The time-domain receiving signal obtained after a guard interval is removed is y i (k). Next, the demodulation is performed in an FFT manner. After the demodulation process, Y i (p) represents the changes from the l th  path of the p th  active sub-carrier in the signal received by the i th  training symbol, and through analysis, the influence of the m th  active sub-carrier may be represented as the following Equation (1): 
     
       
         
           
             
               
                 
                   
                     
                       
                         Y 
                         i 
                       
                       ⁡ 
                       
                         ( 
                         p 
                         ) 
                       
                     
                     = 
                     
                       
                         
                           
                             X 
                             i 
                           
                           ⁡ 
                           
                             ( 
                             p 
                             ) 
                           
                         
                         · 
                         
                           
                             ∑ 
                             
                               l 
                               = 
                               0 
                             
                             
                               L 
                               - 
                               1 
                             
                           
                           ⁢ 
                           
                             
                               a 
                               
                                 0 
                                 , 
                                 l 
                               
                             
                             ⁢ 
                             
                               exp 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     - 
                                     j 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   π 
                                   ⁢ 
                                   
                                     pl 
                                     N 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           ∑ 
                           
                             q 
                             = 
                             1 
                           
                           Q 
                         
                         ⁢ 
                         
                           
                             ∑ 
                             
                               l 
                               = 
                               0 
                             
                             
                               L 
                               - 
                               1 
                             
                           
                           ⁢ 
                           
                             
                               a 
                               
                                 q 
                                 , 
                                 l 
                               
                             
                             · 
                             
                               
                                 ∑ 
                                 
                                   m 
                                   = 
                                   0 
                                 
                                 
                                   N 
                                   - 
                                   1 
                                 
                               
                               ⁢ 
                               
                                 
                                   
                                     X 
                                     i 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     m 
                                     ) 
                                   
                                 
                                 ⁢ 
                                 S 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     C 
                                     i 
                                     q 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       p 
                                       - 
                                       m 
                                     
                                     ) 
                                   
                                 
                                 ⁢ 
                                 
                                   exp 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         - 
                                         j 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       π 
                                       ⁢ 
                                       
                                         ml 
                                         N 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           W 
                           i 
                         
                         ⁡ 
                         
                           ( 
                           p 
                           ) 
                         
                       
                     
                   
                   , 
                   
                     i 
                     = 
                     1 
                   
                   , 
                   2 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the above equation, W i (p) represents noise component of the p th  active sub-carrier in the i th  training symbol. 
     A sub-carrier correlation (SCC) vector is defined as follows. D is a number of data symbols between adjacent training symbols (that is, the number of symbols occupied by the data sub-carrier  401  between the adjacent training symbols  421  and  441 ). Accordingly, the active sub-carriers of the i th  training symbol X i  (that is, the pilot sub-carriers  421 ,  441 ) from (i−1)(D+1)(N+L) to (i−1)(D+1)(N+L)+(N−1) are represented as follows. 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           C 
                           i 
                           q 
                         
                         ⁡ 
                         
                           ( 
                           p 
                           ) 
                         
                       
                     
                     = 
                     
                       
                         1 
                         N 
                       
                       ⁢ 
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             0 
                           
                           
                             N 
                             - 
                             1 
                           
                         
                         ⁢ 
                         
                           
                             
                               ( 
                               
                                 k 
                                 + 
                                 
                                   
                                     ( 
                                     
                                       i 
                                       - 
                                       1 
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     ( 
                                     
                                       D 
                                       + 
                                       1 
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     ( 
                                     
                                       N 
                                       + 
                                       L 
                                     
                                     ) 
                                   
                                 
                               
                               ) 
                             
                             q 
                           
                           ⁢ 
                           
                             exp 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   - 
                                   j 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 π 
                                 ⁢ 
                                 
                                   pk 
                                   N 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   , 
                   
                     
 
                   
                   ⁢ 
                   
                     k 
                     = 
                     0 
                   
                   , 
                   1 
                   , 
                   … 
                   ⁢ 
                   
                       
                   
                   , 
                   
                     N 
                     - 
                     1 
                   
                   , 
                   
                     
 
                   
                   ⁢ 
                   
                     q 
                     = 
                     1 
                   
                   , 
                   2 
                   , 
                   … 
                   ⁢ 
                   
                       
                   
                   , 
                   Q 
                   , 
                   
                     
 
                   
                   ⁢ 
                   
                     i 
                     = 
                     1 
                   
                   , 
                   2 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The frequency-domain receiving signals Y t (p)=Y 1 (p), Y p (p(v))=Y 2 (p), p=0, 1, . . . , N−1, v=0, 1, . . . , N p −1 in Equation (1) are represent as follows in a matrix form. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             Y 
                             t 
                           
                         
                       
                       
                         
                           
                             Y 
                             p 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         [ 
                         
                           
                             
                               
                                 A 
                                 t 
                               
                             
                             
                               
                                 B 
                                 t 
                                 1 
                               
                             
                             
                               
                                 B 
                                 t 
                                 2 
                               
                             
                             
                               … 
                             
                             
                               
                                 B 
                                 t 
                                 Q 
                               
                             
                           
                           
                             
                               
                                 A 
                                 p 
                               
                             
                             
                               
                                 B 
                                 p 
                                 1 
                               
                             
                             
                               
                                 B 
                                 p 
                                 2 
                               
                             
                             
                               … 
                             
                             
                               
                                 B 
                                 p 
                                 Q 
                               
                             
                           
                         
                         ] 
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               
                                 a 
                                 
                                   0 
                                   , 
                                   0 
                                 
                               
                             
                           
                           
                             
                               
                                 a 
                                 
                                   0 
                                   , 
                                   1 
                                 
                               
                             
                           
                           
                             
                               ⋮ 
                             
                           
                           
                             
                               
                                 a 
                                 
                                   0 
                                   , 
                                   
                                     L 
                                     - 
                                     1 
                                   
                                 
                               
                             
                           
                           
                             
                               
                                 a 
                                 
                                   1 
                                   , 
                                   0 
                                 
                               
                             
                           
                           
                             
                               
                                 a 
                                 
                                   1 
                                   , 
                                   1 
                                 
                               
                             
                           
                           
                             
                               ⋮ 
                             
                           
                           
                             
                               
                                 a 
                                 
                                   1 
                                   , 
                                   
                                     L 
                                     - 
                                     1 
                                   
                                 
                               
                             
                           
                           
                             
                               ⋮ 
                             
                           
                           
                             
                               
                                 a 
                                 
                                   Q 
                                   , 
                                   
                                     L 
                                     - 
                                     1 
                                   
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                     + 
                     
                       [ 
                       
                         
                           
                             
                               W 
                               t 
                             
                           
                         
                         
                           
                             
                               W 
                               p 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     H asy  dimension is defined as (N+N p )×(Q+1)L, which is represented as follows in a matrix form. 
     
       
         
           
             
               
                 
                   
                     H 
                     asy 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               A 
                               t 
                             
                           
                           
                             
                               B 
                               t 
                               1 
                             
                           
                           
                             
                               B 
                               t 
                               2 
                             
                           
                           
                             … 
                           
                           
                             
                               B 
                               t 
                               Q 
                             
                           
                         
                         
                           
                             
                               A 
                               p 
                             
                           
                           
                             
                               B 
                               p 
                               1 
                             
                           
                           
                             
                               B 
                               p 
                               2 
                             
                           
                           
                             … 
                           
                           
                             
                               B 
                               p 
                               Q 
                             
                           
                         
                       
                       ] 
                     
                     
                       
                         ( 
                         
                           N 
                           + 
                           
                             N 
                             p 
                           
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           Q 
                           + 
                           1 
                         
                         ) 
                       
                       ⁢ 
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The parameters A t , A p , B t   q , and B p   q  of known asymmetric training symbols (X t , X p ) are listed in the following Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Parameter Settings for Estimator H asy   
               
            
           
           
               
               
            
               
                   
                 Variable Definition 
               
               
                   
                   
               
            
           
           
               
               
            
               
                 A t   
                 
                   
                     
                       
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     
                                       X 
                                       1 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       0 
                                       ) 
                                     
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       ( 
                                       
                                         
                                           - 
                                           j 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                           
                                             0 
                                             · 
                                             0 
                                           
                                           N 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       X 
                                       1 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       0 
                                       ) 
                                     
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       ( 
                                       
                                         
                                           - 
                                           j 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                           
                                             0 
                                             · 
                                             1 
                                           
                                           N 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                               
                                 … 
                               
                               
                                 
                                   
                                     
                                       X 
                                       1 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       0 
                                       ) 
                                     
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       ( 
                                       
                                         
                                           - 
                                           j 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                           
                                             0 
                                             · 
                                             
                                               ( 
                                               
                                                 L 
                                                 - 
                                                 1 
                                               
                                               ) 
                                             
                                           
                                           N 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     
                                       X 
                                       1 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       1 
                                       ) 
                                     
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       ( 
                                       
                                         
                                           - 
                                           j 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                           
                                             1 
                                             · 
                                             0 
                                           
                                           N 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       X 
                                       1 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       0 
                                       ) 
                                     
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       ( 
                                       
                                         
                                           - 
                                           j 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                           
                                             1 
                                             · 
                                             1 
                                           
                                           N 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                               
                                 … 
                               
                               
                                 
                                   
                                     
                                       X 
                                       1 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       1 
                                       ) 
                                     
                                   
                                   ⁢ 
                                   
                                     ⅇ 
                                     
                                       ( 
                                       
                                         
                                           - 
                                           j 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                           
                                             1 
                                             · 
                                             
                                               ( 
                                               
                                                 L 
                                                 - 
                                                 1 
                                               
                                               ) 
                                             
                                           
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     q is the order number of the Qth-order polynomial. Equation (3) is then solved through a least-square method to obtain the coefficients in the time-varying channel curve estimation. 
     
       
         
           
             
               
                 
                   
                     
                       
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     The data sub-carrier channel estimating unit  396  utilizes the Qth-order polynomial and the polynomial coefficients (or referred to as channel time-varying characteristic coefficients), a q, l , q=0, 1, 2, . . . , estimated by the polynomial coefficient estimating unit  394  to calculate channel changes of the l th  path at the k th  sampling point in the i th  training symbol. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     With reference to the above estimation method,  FIG. 5  is a schematic view of a plurality of frequency-domain receiving signals Y(k) received through the signal receiving method. As seen from the figure, a plurality of data receiving symbols  401 ′ exists between the adjacent first training receiving symbol  421 ′ and second training receiving symbol  441 ′ on the time axis (the transverse axis in the figure). Consecutive data receiving symbols  401 ′ at each parallel frequency-domain receiving signal Y(k) occupy a plurality of symbol intervals. That is to say, the transmitter  10  configures a plurality of data symbols  401  between the first training symbol  421  and the second training symbol  441 , so as to obtain higher bandwidth for transferring data. 
     As the first pilot receiving signal  42   a′  and the second pilot receiving signal  44   a′  are known data contents, as for each parallel frequency-domain receiving signal Y(k), four parameters (A t , A p , B q   t , B q   p ) corresponding to the frequency-domain receiving signal Y(k) may be calculated through two known asymmetric pilot sub-carriers (X t , X p ). X t  is the transferred first pilot sub-carrier  42   a . X p  is the transferred second pilot sub-carrier  44   a . Next, the channel time-varying characteristic coefficient a q, l  is calculated by using Equation (5) with the received two known asymmetric training receiving symbols (Y t , Y p ). Y t  is the received first pilot receiving signal  42   a′ . Y p  is the received second pilot receiving signal  44   a′ . In the end, a channel response of the data sub-carrier is interpolated through interpolation, so as to accomplish the channel estimation. 
     Referring to the curve below  FIG. 5 , the curve marked as  45  is a schematic curve of a real channel response corresponding to one frequency-domain receiving signal Y(k). The curve marked as  46  is a schematic curve of an estimated channel response obtained through a first-order polynomial estimation manner (Q=1). The curve marked as  47  is a schematic curve of an estimated channel response obtained through a second-order polynomial estimation manner (Q=2). The real-line parts at two ends of the schematic curves  46 ,  47  of the estimated channel response represent estimation values practically estimated by using the first and second pilot receiving signals  42   a′  and  44   a′ . The dotted-line parts in the middle of the schematic curves  46 ,  47  of the estimated channel response represent values calculated through interpolation based on the estimated coefficients. In such a manner, the data receiving signals  40   a′ ,  40   c′ ,  40   d′ , and  40   e′  corresponding to the frequency-domain receiving signal Y(k) are restored into the transferred data sub-carriers  40   a ,  40   c ,  40   d , and  40   e  by using the estimated channel response H(k). 
     Comparing the schematic curves  46 ,  47  of the estimated channel response with the schematic curve  45  of the practical channel response, it may be seen that, the schematic curve  47  of the estimated channel response obtained through the second-order polynomial is much closer to the schematic curve  45  of the practical channel response. Although  FIG. 5  shows that error values between the schematic curve  45  of the practical channel response and the schematic curve  47  of the estimated channel response obtained through the second-order polynomial in the middle section of the consecutive data receiving symbols  401 ′ are relatively large, such a problem can be solved by decreasing the number of the data symbols  401  and  402  between the first training symbols  421  and  422  and the second training symbols  441  and  442 . In addition, the larger the order number of the polynomial estimation is (that is, Q=3, 4 or even larger), the smaller the error from the schematic curve  45  of the practical channel response will be. This may be adjusted according to practical application situation. 
     An embodiment of a signal receiving method in an OFDM system is further provided. The signal receiving method is suitable for receiving the transmitted signal  20  transmitted through the signal transmitting method. The OFDM system comprises a receiver  30  for performing the signal receiving method. The transmitted signal  20  is transferred to the receiver  30  through a plurality of paths to become a receiving signal with a channel response. Referring to  FIG. 8 , the signal receiving method comprises the following steps. 
     In Step S 80 , the receiving signal  26  with a channel response is received and converted into a frequency-domain serial signal  98 . The frequency-domain serial signal  98  comprises a plurality of data receiving symbols  401 ′ and  402 ′, a plurality of first training receiving symbols  421 ′ and  422 ′, and a plurality of second training receiving symbols  441 ′ and  442 ′. A plurality of data receiving signals  40   a′  and  40   b′  of the receiving symbols  401 ′ and  402 ′, a plurality of first pilot receiving signals  42   a′  and  42   b′ , and a plurality of second pilot receiving signals  44   a′  and  44   b′  correspond to the data sub-carriers  40   a  and  40   b , the first pilot sub-carriers  42   a  and  42   b , and the second pilot sub-carriers  44   a  and  44   b  respectively. 
     In Step S 82 , a plurality of channel responses is estimated by using the adjacent first pilot receiving signal  42   a′  of the first training receiving symbol  421 ′ and second pilot receiving signal  44   a′  of the second training receiving symbol  441 ′, as well as the first pilot sub-carrier  42   a  in the first training symbol  421  and the second pilot sub-carrier  44   a  in the second training symbol  441 . 
     In Step S 84 , the data receiving signals  40   a′ ,  40   c′ ,  40   d′ , and  40   e′  in the adjacent first and second training symbols  421 ′ and  441 ′ are restored into the data sub-carriers  40   a ,  40   c ,  40   d , and  40   e  by using the channel responses. 
     The process of receiving and converting the receiving signal  26  with a channel response into a frequency-domain serial signal  98  in Step S 80  is accomplished by the OFDM demodulator  38 .  FIG. 9  is a schematic flow chart of Step S 80 . Step S 80  may further comprise the following steps. 
     In Step S 800 , the receiving signal  26  with a channel response is received and parallelized into a plurality of parallel first time-domain receiving signals  99   b.    
     In Step S 802 , the guard intervals are removed from the first time-domain receiving signals  99   b  to obtain a plurality of second time-domain receiving signals y(k). 
     In Step S 804 , the second time-domain receiving signals y(k) are transformed into a plurality of frequency-domain receiving signals Y(k) in an FFT manner. 
     In Step S 806 , the frequency-domain receiving signals Y(k) are serialized into the frequency-domain serial signal  98 . 
     Step S 800  is accomplished by the receiving filter  31  and the S/P converting unit  32 . Steps S 802 , S 804 , and S 806  are performed by the guard interval removing unit  33 , the FFT unit  34 , and the P/S converting unit  35  respectively. 
     The process of estimating a plurality of channel responses by using the adjacent first pilot receiving signal  42   a′  of the first training receiving symbol  421 ′ and second pilot receiving signal  44   a′  of the second training receiving symbol  441 ′, as well as the first pilot sub-carrier  42   a  in the first training symbol  421  and the second pilot sub-carrier  44   a  in the second training symbol  441  in Step S 82  is performed by the polynomial coefficient estimating unit  394  and the data sub-carrier channel estimating unit  396 . In Step S 82 , when a channel response of each of the paths is calculated, the channel response corresponding to each of the paths is approximated by using the same polynomial. The polynomial is a first-order polynomial, a second-order polynomial, or a high-order polynomial. 
     The example of Step S 82  may be obtained with reference to  FIG. 10 .  FIG. 10  is a schematic flow chart of Step S 82  in the signal receiving method in an OFDM system according to an embodiment of the present invention. As seen from  FIG. 10 , Step S 82  comprises the following steps. 
     In Step S 820 , a plurality of polynomials is defined. The polynomials are used to describes the channel responses of the paths corresponding to the polynomials. Each of the polynomials comprises a plurality of polynomial coefficients. 
     In Step S 822 , the polynomial coefficients are calculated by using the adjacent first pilot receiving signal  42   a′  of the first training receiving symbol  421 ′ and second pilot receiving signal  44   a′  of the second training receiving symbol  441 ′, as well as the first pilot sub-carrier  42   a  in the first training symbol  421  and the second pilot sub-carrier  44   a  in the second training symbol  441 . 
     In Step S 824 , the channel responses are estimated by using the polynomials. 
     The polynomial coefficients (or referred to as channel time-varying characteristic coefficients) in Step S 820  are a q, l,  q=0, 1, 2, . . . , Q, l=0, 1, 2, . . . , L−1. Each polynomial is used to simulate a channel response of each path. That is to say, the number of polynomials is equal to the number of the paths. That is, from the above example, totally L paths and L polynomials exist. If each polynomial is a Qth-order polynomial (a channel response of each path is simulated with a Qth-order polynomial), each Qth-order polynomial has (Q+1) coefficients. Therefore, the number of the polynomial coefficients in Step S 820  is totally L*(Q+1). 
     The estimation manner in Step S 822  is obtained based on Equations (1) to (4). That is, the calculation is performed by using the four parameters (A t , A p , B q   t , B q   p ) of the frequency-domain receiving signals Y(k). 
     In Step S 824 , the channel responses (that is, channel changes) may be estimated according to Equation (6). 
     In Step S 84 , the process of restoring the data receiving signals  40   a′ ,  40   c′ ,  40   d′ , and  40   e′  into the data sub-carriers  40   a ,  40   c ,  40   d , and  40   e  by using the channel responses may be performed by the equalizer unit  398 . 
       FIG. 11  is a schematic view of performance comparison between the signal transmitting method and the signal receiving method in the OFDM system according to an embodiment of the present invention and that of the prior art. The estimation method (i), method 1 in the estimation method (ii), method 2 in the estimation method (ii), the estimation method (iv) in the prior art and the estimation method in the embodiment of the present invention are mainly compared. Environment parameters and settings in the comparison are listed in Table 3 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 Carrier Frequency 
                 2G 
               
               
                   
                 Bandwidth (BW) 
                 5 MHz 
               
               
                   
                 Power delay profile (COST207 
                 Bad urban (BU) 
               
               
                   
                 Tapped delay line model) 
               
               
                   
                 Max delay spread 
                 10 us 
               
               
                   
                 Path Number 
                 12 
               
               
                   
                 Active sub-carriers (N) 
                 512 
               
               
                   
                 Active sub-carrier spacing 
                 9.76 kHz 
               
               
                   
                 Guard interval overhead 
                 1/8 Symbol 
               
               
                   
                 OFDM symbol duration (Ts) 
                 (w/o GI) 102.4 us 
               
               
                   
                 Normalized Doppler frequency (fdT) 
                 0.1 
               
               
                   
                 Velocity 
                 400 km/h 
               
               
                   
                 Doppler spread (fd) 
                 1000 Hz 
               
               
                   
                 Environment 
                 High speed rail 
               
               
                   
                 Channel Estimation Method 
                 Second-order polynomial 
               
               
                   
                   
                 2 nd  order (Q = 2) 
               
               
                   
                   
               
            
           
         
       
     
     In  FIG. 11 , the transverse axis is signal-to-noise ratio (SNR) per bit (the energy per bit to noise power spectral density ratio, E b /N o  (dB)), and the longitudinal axis is a normalized sub-carrier power (normalized mean square error, NMSE), that is, the difference between the estimated channel response and the practical channel response. A smaller value on the longitudinal axis represents a smaller estimation error, and the estimation accuracy is higher. As seen from the figure, the higher the SNR in an environment is, the smaller the error obtained through the method according to the embodiments will be. In the situation that the SNR per bit is about higher than 16 dB, the smallest error is generated through the method according to the embodiments.