Abstract:
An OFDM communication apparatus may include a receiver that receives an OFDM signal in which a known signal and an information signal, following the known signal, are superimposed upon a plurality of subcarriers. A calculator of this apparatus calculates a propagation path estimated value using: (i) the known signal and the received version of the known signal or (ii) the received information signal and a recoded signal. A propagation path distortion compensator compensates the propagation path distortion of the received information signal, using the propagation path estimated value, to produce a distortion compensated signal. An error corrector error corrects the distortion compensated signal to produce an error corrected signal, and a recoder recodes the error corrected signal to produce the recoded signal. The calculator further calculates a correction value for updating the propagation path estimated value using the recoded signal and the received information signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an OFDM communication apparatus and propagation path estimation method in a digital radio communication system. 
     2. Description of the Related Art 
     A leading cause of deterioration of a transmission characteristic in a current ground wave transmission path is multi-path interference. An OFDM (Orthogonal Frequency Division Multiplexing) transmission system, which is resistant to this multi-path interference, is receiving attention in recent years. This OFDM is a system that multiplexes a multiple (several tens to several hundreds) of mutually orthogonal digital modulated signals in a certain signal segment. 
     A conventional OFDM communication apparatus calculates a frequency response estimated value of the propagation path by performing time-frequency conversion on a reception signal through an FFT circuit and performing a complex multiplication on a pilot symbol and known signal contained in the reception signal. Then, by carrying out a complex multiplication on the frequency response estimated value and information OFDM symbol, the conventional OFDM communication apparatus compensates propagation path distortion. This reception signal with propagation path distortion compensated is demodulated and subjected to error correction by an error detection circuit and in this way an information bit string, which is reception data, is obtained. 
     When long information is transmitted, the conventional OFDM communication apparatus above inserts, as shown in FIG. 1, propagation path response estimation pilot symbols (hatched areas) into information OFDM symbols at certain intervals to follow up variations of momentarily changing propagation path response. That is, as shown in FIG. 2, information OFDM symbols  1  to n are compensated using a propagation path estimated value obtained by pilot symbol A and information OFDM symbols n+1 to  2 n are compensated using a propagation path estimated value obtained by pilot symbol B. 
     However, when such long information is transmitted, following up time variations in the propagation path makes it necessary to frequently insert known signals such as pilot symbols, causing a problem of deteriorating the transmission efficiency. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an OFDM communication apparatus and propagation path estimation method capable of adaptively following up time variations of a transmission path and improving a reception characteristic without reducing the transmission efficiency even if there are considerable time variations in propagation path response. 
     A subject of the present invention is to adaptively follow up time variations of a transmission path and achieve an excellent reception characteristic by adaptively estimating propagation path response using an error-corrected signal, that is, using a judged value of the received information signal as a known signal without frequently inserting pilot symbols for propagation path estimation and without reducing the transmission efficiency even when long information is transmitted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
     FIG. 1 is a diagram showing a symbol configuration used by a conventional propagation path estimation method; 
     FIG. 2 is a diagram to explain the conventional propagation path estimation method; 
     FIG. 3 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 1 of the present invention; 
     FIG. 4 is a block diagram showing an internal configuration of a propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 1 of the present invention; 
     FIG. 5 is a block diagram showing an internal configuration of the propagation path estimated value update circuit shown in FIG. 4; 
     FIG. 6 is a diagram showing a symbol configuration used by a propagation path estimation method according to the present invention; 
     FIG. 7 is a diagram to explain the propagation path estimation method according to the present invention; 
     FIG. 8 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of an OFDM communication apparatus according to Embodiment 2 of the present invention; 
     FIG. 9 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of an OFDM communication apparatus according to Embodiment 3 of the present invention; 
     FIG. 10 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of an OFDM communication apparatus according to Embodiment 4 of the present invention; 
     FIG. 11 is another block diagram showing the internal configuration of the propagation path estimated value update circuit in the propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 4 of the present invention; 
     FIG. 12 is a block diagram showing an internal configuration of a propagation path estimation/compensation circuit of an OFDM communication apparatus according to Embodiment 5 of the present invention; 
     FIG. 13 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in the propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 5 of the present invention; 
     FIG. 14 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of an OFDM communication apparatus according to Embodiment 6 of the present invention; 
     FIG. 15 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of an OFDM communication apparatus according to Embodiment 7 of the present invention; and 
     FIG. 16 is another block diagram showing the internal configuration of the propagation path estimated value update circuit in the propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 7 of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the attached drawings, embodiments of the present invention will be explained in detail below. 
     Embodiment 1 
     FIG. 3 is a block diagram showing a configuration of an OFDM communication apparatus according to Embodiment 1 of the present invention. A signal used for an OFDM communication has a configuration shown in FIG.  6 . That is, the signal has a configuration with a preamble other than a pilot symbol followed by a propagation path response estimation pilot symbol, which is a known signal, and information OFDM symbols. Thus, a propagation path estimation pilot symbol is added only at the start of the information symbol to be transmitted. 
     An OFDM signal received through antenna  101  is subjected to normal radio reception processing by radio reception circuit  102  and converted to a baseband signal. This baseband signal is subjected to coherent detection processing by a coherent detector, stripped of the unnecessary frequency component by a low-pass filter and converted from analog to digital. Here, the reception signal is divided into an in-phase component and quadrature component by the coherent detection processing, but these two components are expressed as one signal path in the figure. 
     This baseband signal is subjected to an FFT (Fast Fourier Transform) operation by FFT circuit  103  and in this way signals assigned to different subcarriers are obtained. The signal subjected to the FFT operation by FFT section  103  is sent to propagation path estimation/compensation circuit  104  where a first propagation path estimated value (initial value) is obtained by performing propagation path estimation by carrying out a complex multiplication on a pilot symbol and known signal contained in the reception OFDM signal. 
     Propagation path estimation/compensation circuit  104  performs propagation path distortion compensation of information OFDM symbols one by one using the first propagation path estimated value. The information symbols subjected to propagation path distortion compensation are sent to correction circuit  105  one by one where these symbols are subjected to error correction. Error correction circuit  105  outputs an information bit string subjected to error correction for every unit of transmission path coding. This information bit string is sent to error detection circuit  106  where the information bit string is subjected to error detection and output as reception data. 
     The information bit string after error correction is periodically sent to re-coding circuit  107 . Re-coding circuit  107  performs transmission path coding processing, modulation processing and rearrangement processing on the error-corrected information bit again. Thus, this re-coded and error-corrected information bit string is sent to propagation path estimation/compensation circuit  104 . Propagation path estimation/compensation circuit  104  uses this re-coded information bit as a known signal, performs propagation path estimation by carrying out a complex multiplication with the FFT-operated signal and obtains a propagation path estimated value. This propagation path estimated value is updated to a first propagation path estimated value. 
     Propagation path distortion compensation is carried out by performing a complex multiplication on this new first propagation path estimated value and the information OFDM symbol. The reception signal with propagation path distortion compensated is sent to error correction circuit  105  where the reception signal is subjected to error correction. The information bit string output from error correction circuit  105  is sent to error detection circuit  106  where the information bit string is subjected to error detection and output as reception data. 
     The propagation path estimated value can also be updated for every information bit or for every two or more information bits. When the propagation path estimated value is updated for every two or more information bits, a switch, etc. can be provided after error correction circuit  105  so that the output to re-coding circuit  107  and output to error detection circuit  106  are switched through a control signal. 
     On the other hand, an information signal, which is transmission data for every subcarrier, is subjected to digital modulation processing by, for example, QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), etc. and subjected to an IFFT operation by IFFT (Inverse Fast Fourier Transform) circuit  108  and converted to an OFDM signal. This OFDM signal is converted from digital to analog, sent to radio transmission circuit  109  where it is subjected to normal radio transmission processing and transmitted via antenna  101  as a transmission signal. 
     Next, the operation of the OFDM communication apparatus with the configuration above will be explained. An OFDM signal received by antenna  101  is subjected to normal radio reception processing by radio reception circuit  102 , converted to a baseband signal, subjected to an FFT operation by FFT circuit  103  and converted to signals assigned to subcarriers. 
     This signal is sent to propagation path estimation/compensation circuit  104 . As shown in FIG. 4, propagation path estimation/compensation circuit  104  includes register  201  that stores the output from FFT circuit  103 , multiplier  203  that carries out a complex multiplication on this FFT output and a known signal or the output from re-coding circuit  107 , propagation path estimated value update circuit  204  that stores a propagation path estimated value, which is the output of multiplier  203 , and updates it to a new propagation path estimated value and multiplier  202  that carries out a complex multiplication on the propagation path estimated value and FFT output. 
     Furthermore, propagation path estimation/compensation circuit  104  also includes switch  205  to switch between multiplier  203  and multiplier  202  to output the FFT output, switch  206  to switch between the output from FFT circuit  103  and FFT output stored in register  201  to output to multiplier  203  and switch  207  to switch between the known signal and the output from re-coding circuit  107  to output to multiplier  203 . 
     Furthermore, propagation path estimated value update circuit  204  includes register  301  as shown in FIG.  5 . 
     First, propagation path estimation/compensation is performed using a pilot symbol. A signal sent to propagation path estimation/compensation circuit  104 , that is, FFT output is sent to multiplier  203  and multiplier  203  performs a complex multiplication on a pilot symbol and known symbol of the FFT output. In this way, a first propagation path estimated value (initial value) is obtained. At this time, switches  205  to  207  are set so that the FFT output and known signal are input to multiplier  203 . This propagation path estimated value is stored in register  301  of propagation path estimated value update circuit  204 . 
     Furthermore, this propagation path estimated value is sent to multiplier  202  and multiplier  202  multiplies the propagation path estimated value by the information symbol of the FFT output. In this way, propagation path distortion compensation is performed on the information symbol. The information symbol subjected to such propagation path distortion compensation is sent to error correction circuit  105 . 
     The information symbol subjected to propagation path distortion compensation is sent to error correction circuit  105  where the information symbol is subjected to error correction and then sent to error detection circuit  106  where the information symbol is subjected to error detection and output as reception data. 
     Then, propagation path estimation/compensation is carried out using the error-corrected information bits. The error-corrected information bits are periodically sent to re-coding circuit  107 . Re-coding circuit  107  performs transmission path coding processing, modulation processing and rearrangement processing on the error-corrected information bits again. Thus, the re-coded and error-corrected information bit string is sent to multiplier  203  of propagation path estimation/compensation circuit  104 . Propagation path estimation/compensation circuit  104  uses this re-coded information bit instead of a known signal and performs a complex multiplication on this re-coded information bit and FFT output. At this time, the FFT output is stored in register  201 . In this case, switches  205  to  207  are set so that the FFT output stored in register  201  and re-coded output are output to multiplier  203 . 
     In this way, a propagation path estimated value is obtained by performing a complex multiplication on the re-coded information bit and FFT output. This propagation path estimated value is sent to propagation path estimated value update circuit  204 . Then, using this propagation path estimated value, the propagation path estimated value (initial value) stored in register  301  of propagation path estimated value update circuit  204  is updated. 
     Furthermore, the updated propagation path estimated value is sent to multiplier  202  and multiplier  202  multiplies the updated propagation path estimated value by the information symbol of the FFT output. In this way, propagation path distortion compensation is carried out on the information symbol. The information symbol subjected to such propagation path distortion compensation is sent to error correction circuit  105 . 
     The information symbol with propagation path distortion compensated is sent to error correction circuit  105 , subjected to error correction, then sent to error detection circuit  106  where the information symbol is subjected to error detection and output as reception data. 
     According to such a propagation path estimation method, as shown in FIG. 7, propagation path distortion compensation is performed on information bits  1  to n with propagation path estimated value (X) calculated using a pilot symbol (hatched area), propagation path distortion compensation i s performed on information bits n+1 to  2 n with propagation path estimated value (Y) calculated using the error correction outputs of information bits  1  to n as a known signal, and propagation path distortion compensation is performed on information bits  2 n+1 to  3 n with propagation path estimated value (Y) calculated using the error-corrected outputs of information bits n+1 to  2 n as a known signal. Therefore, even if long information is transmitted, it is possible to estimate propagation path response without inserting pilot symbols between information OFDM symbols, which are continuously transmitted, which allows an excellent reception characteristic to be obtained without reducing the transmission efficiency. 
     Moreover, when the error-corrected output of information bits is used as a known signal, it is also possible to obtain quality information on a plurality of information bits, input the quality information to propagation path estimated value update circuit  204  and determine which information bit error-corrected output is appropriate for a known signal to calculate the transmission path estimated value. This allows an optimal propagation path estimated value to be obtained and appropriate propagation path distortion compensation to be performed for the information bits. Thus, it is possible to maintain a low error rate by adaptively following up time variations of the transmission path without reducing the transmission efficiency even when long information is transmitted or when there are considerable time variations of propagation path response. 
     Embodiment 2 
     In the OFDM communication apparatus according to this embodiment, propagation path estimated value update circuit  204  updates a propagation path estimated value using both the propagation path estimated value obtained using information bits after error correction and a past propagation path estimated value. 
     The configuration of the OFDM communication apparatus according to this embodiment is the same as that of Embodiment 1 except the propagation path estimated value update circuit, and therefore the propagation path estimated value update circuit will be explained. 
     FIG. 8 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 2 of the present invention. This propagation path estimated value update circuit includes register  601  that stores a propagation path estimated value and outputs it to multiplier  202 , multipliers  603  and  604  that multiply the propagation path estimated value stored in register  601  by a weighting factor, adder  605  that adds up the multiplication results of multipliers  603  and  604  and factor per subcarrier selection section  602  that selects the output of multiplier  203  or a weighting factor of a past propagation path estimated value stored in register  601  according to a control signal. 
     The propagation path estimated value update circuit shown in FIG. 8 updates a propagation path estimated value using both the propagation path estimated value obtained using the error-corrected information bits and a past propagation path estimated value, and the propagation path estimated value to be updated follows, for example, expression (1) below: 
     
       
         (Updated estimated value)= W ×(output of multiplier  203 )+(1 −W )×(immediately preceding estimated value)  expression (1) 
       
     
     where, W is a weighting factor and given by factor per subcarrier selection section  602 . Factor per subcarrier selection section  602  gives a weighting factor for every subcarrier based on the past propagation path estimated values. Factor per subcarrier selection section  602  selects a preset weighting factor according to a control signal based on information such as channel quality. Here, an identical weighting factor can be used in all cases. 
     More specifically, propagation path estimated value update circuit  204  outputs a past (here immediately preceding) propagation path estimated value from register  601  to multiplier  604 . On the other hand, a propagation path estimated value (output of multiplier  203 ) obtained using the error-corrected current information bit as a known signal is output to multiplier  603 . 
     According to a control signal based on information such as channel quality, factor per subcarrier selection section  602  selects a weighting factor (W) to be multiplied on a current propagation path estimated value and past propagation path estimated value, outputs the current propagation path estimated value to multiplier  603  and outputs the past propagation path estimated value to multiplier  604 . 
     Multipliers  603  and  604  perform weighting on the current propagation path estimated value and past propagation path estimated value, respectively and their results are output to adder  605 . Adder  605  adds up the weighted propagation path estimated values and calculates a propagation path estimated value to be updated. The calculated propagation path estimated value is sent to register  601  and the propagation path estimated value stored in the register is updated. 
     According to this embodiment, a new propagation path estimated value is obtained also using a past propagation path estimated value, and therefore it is possible to achieve high estimation accuracy using this propagation path estimated value and perform propagation path distortion compensation on information bits more accurately. 
     Embodiment 3 
     The OFDM communication apparatus according to this embodiment adds a process of averaging propagation path estimated values corresponding to n symbols using error-corrected information bits. 
     The configuration of the OFDM communication apparatus according to this embodiment is the same as that of Embodiment 1 except the propagation path estimated value update circuit, and therefore the propagation path estimated value update circuit will be explained. 
     FIG. 9 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 3 of the present invention. This propagation path estimated value update circuit includes register  701  that stores a propagation path estimated value and outputs it to multiplier  202  and averaging section  702  that averages the propagation path estimated values corresponding to n symbols obtained using error-corrected information bits. Furthermore, the propagation path estimated value update circuit includes switch  703  that switches whether to output a propagation path estimated value (output of multiplier  203 ) directly to register  701  or to averaging section  702 . 
     With this configuration, when a propagation path estimated value is calculated using a pilot symbol, switch  703  is set so that the output of multiplier  203  is sent to register  701  and the propagation path estimated value is sent to register  701  and stored in register  701 . Moreover when a propagation path estimated value is calculated using the error-corrected information bit, switch  703  is set so that the output of multiplier  203  is sent to averaging section  702  and a propagation path estimated value is sent to averaging section  702  and propagation path estimated values corresponding to n symbols are averaged. The averaged propagation path estimated value is sent to register  701  and the propagation path estimated value stored in register  701  is updated. When the amplitude of the transmission signal contains information as in the case of multi-value QAM, averaging section  702  can also be configured in such a way as not to include values of signal points with a small amplitude in averaging and to reduce deterioration by additive noise. 
     According to this embodiment, newly acquired propagation path estimated values are averaged for a plurality of symbols, and therefore it is possible to reduce estimated errors due to additive noise, achieve high estimation accuracy using this propagation path estimated value and perform propagation path distortion compensation on information bits more accurately. 
     Embodiment 4 
     In the OFDM communication apparatus according to this embodiment, propagation path estimated value update circuit  204  adds a process of averaging propagation path estimated values corresponding to n symbols obtained by using error-corrected information bits and further updates propagation path estimated value using both the averaged propagation path estimated value and past propagation path estimated value. 
     Since the configuration of the OFDM communication apparatus according to this embodiment is the same as that of Embodiment 1 except the propagation path estimated value update circuit, the propagation path estimated value update circuit will be explained. 
     FIG. 10 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 4 of the present invention. This propagation path estimated value update circuit includes register  801  that stores a propagation path estimated value and outputs it to multiplier  202 , multipliers  803  and  804  that multiply the propagation path estimated value stored in register  801  by a weighting factor, adder  805  that adds up the multiplication results of multipliers  803  and  804 , factor per subcarrier selection section  802  that selects the output of multiplier  203  and weighting factors of the past propagation path estimated values stored in register  801  according to a control signal and averaging section  806  that averages propagation path estimated values corresponding to n symbols obtained by using error-corrected information bits. Furthermore, the propagation path estimated value update circuit includes switch  807  that selects whether to directly output the propagation path estimated value (output of multiplier  203 ) to multiplier  803  or output it to averaging section  806  and then output to multiplier  803 . 
     The propagation path estimated value update circuit shown in FIG. 10 averages propagation path estimated values corresponding to n symbols obtained by using error-corrected information bits and further updates propagation path estimated values using both the averaged propagation path estimated value and the past propagation path estimated value, and the propagation path estimated value to be updated follows, for example, expression (2) below. 
     
       
         (Updated estimated value)= W ×(output of averaging section)+(1 −W )×(immediately preceding estimated value)  expression (2) 
       
     
     where, W is a weighting factor and is given by factor per subcarrier selection section  802 . Factor per subcarrier selection section  802  gives a weighting factor for every subcarrier based on the past propagation path response estimated value. Factor per subcarrier selection section  802  selects a preset weighting factor according to the control signal based on information such as channel quality. Here, an identical weighting factor can also be used in all cases. 
     More specifically, propagation path estimated value update circuit  204  outputs a past (here, immediately preceding) propagation path estimated value from register  801  to multiplier  804 . 
     On the other hand, when a propagation path estimated value is calculated using a pilot symbol, switch  807  is set so that the output of multiplier  203  is sent to multiplier  803  and the propagation path estimated value is sent to multiplier  803  and multiplier  803  multiplies the propagation path estimated value by a weighting factor. Moreover when a propagation path estimated value is calculated using the error-corrected information bit, switch  807  is set so that the output of multiplier  203  is sent to averaging section  806  and a propagation path estimated value is sent to averaging section  806  and propagation path estimated values corresponding to n symbols are averaged. The averaged propagation path estimated value is sent to multiplier  803  and multiplier  803  multiplies the averaged propagation path estimated value by a weighting factor. 
     At this time, factor per subcarrier selection section  802  selects a weighting factor (W) to be multiplied on the averaged output of the current propagation path estimated value and past propagation path estimated value according to a control signal based on information such as channel quality, and outputs the weighting factor of the averaged output of the current propagation path estimated value to multiplier  803  and outputs the weighting factor of the past propagation path estimated value to multiplier  804 . 
     Multipliers  803  and  804  perform weighting on the current propagation path estimated value and past propagation path estimated value, respectively and output their results to adder  805 . Adder  805  adds up the weighted propagation path estimated values and calculates a new propagation path estimated value for updating. The calculated propagation path estimated value is sent to register  801  and the propagation path estimated value stored in the register is updated. When the amplitude of the transmission signal contains information as in the case of multi-value QAM, averaging section  806  can also be configured in such a way as not to include values of signal points with a small amplitude in averaging and to reduce deterioration by additive noise. 
     According to this embodiment, newly acquired propagation path estimated values are averaged for a plurality of symbols, and therefore it is possible to reduce estimated errors due to additive noise and achieve higher estimation accuracy because it obtains a new propagation path estimated value using a past propagation path response estimated value. As a result, it can perform propagation path distortion compensation on information bits more accurately. 
     In this embodiment, it is also possible to input a CRC (Cyclic Redundancy Check) result to factor per subcarrier selection section  802  as external quality information as shown in FIG.  11 . This setting is intended to prevent an averaging block including information bits for which the CRC result shows some errors from not being used as an averaging output. At this time, weighting factor W in expression (2) above becomes 0. 
     Thus, applying external quality information to weighting factor selection makes it possible to reduce estimation errors due to bit errors and drastically improve estimation accuracy. 
     Embodiment 5 
     The OFDM communication apparatus according to this embodiment uses a signal after propagation path distortion compensation as the information OFDM symbol stored to be used for successive propagation path estimation. More specifically, the OFDM communication apparatus according to this embodiment takes a difference between the information OFDM symbol after propagation path distortion compensation stored in the register and re-coded output and only updates the past propagation path estimated value by the portion corresponding to the difference. 
     Since the configuration of the OFDM communication apparatus according to this embodiment is the same as that of Embodiment 1 except the propagation path estimated value update circuit, the propagation path estimated value update circuit will be explained. 
     FIG. 12 is a block diagram showing an internal configuration of the propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 5 of the present invention. 
     Propagation path estimation/compensation circuit  104  includes multiplier  1001  that carries out a complex multiplication on the output from FFT circuit  103  (FFT output) and a known signal, propagation path estimated value update circuit  1002  that stores the output of multiplier  1001 , that is, a propagation path estimated value and updates it to a new propagation path estimated value, multiplier  1003  that carries out a complex multiplication on the output from propagation path estimated value update section  1002  and the FFT output, register  1004  that stores the information bit after propagation path distortion compensation, which is the output of multiplier  1003 , and subtractors  1005  and  1006  that calculate a difference between the information bit after propagation path distortion compensation and the output of re-coding circuit  107 . Furthermore, propagation path estimation/compensation circuit  104  includes switches  1007  and  1008  to switch between multipliers  1003  and  1001  to output the FFT output. Here, the FFT output, known signal and re-coded output are expressed by an I component and Q component. 
     As shown in FIG. 13, this propagation path estimated value update circuit  1002  includes registers  1101  and  1102  that store a propagation path estimated value (output of multiplier  1001 ) and output it to adders  1103  and  1104 , multipliers  1105  and  1106  that multiply the outputs of subtractors  1005  and  1006  by weighting factors and adders  1103  and  1104  that adds up the multiplication results of multipliers  1105  and  1106  and the propagation path estimated values stored in registers  1101  and  1102 . Furthermore, propagation path estimated value update circuit  1002  includes switches  1107  and  1108  that switch between the output of register  1101  to adder  1103  and the output of multiplier  1001  to adder  104 . 
     The operation of the OFDM communication apparatus with the above configuration will be explained. A signal sent to propagation path estimation/compensation circuit  104 , that is, the FFT output is first sent to multiplier  1001  and multiplier  1001  carries out a complex multiplication on the I component and Q component of the FFT output and the I component and Q component of a known signal. In this way, a propagation path estimated value is obtained. At this time, switches  1007  and  1008  are set so that the FFT output and known signal are input to multiplier  1001 . This propagation path estimated value is stored in registers  1101  and  1102  of propagation path estimated value update circuit  1002 . At this time, switches  1107  and  1108  of propagation path estimated value update circuit  1002  are set so that the output of multiplier  1001  is sent to registers  1101  and  1102 . 
     Furthermore, this propagation path estimated value is sent to multiplier  1003  and multiplier  1003  multiplies the I component and Q component of the FFT output and the I component and Q component of the information symbol. In this way, propagation path distortion compensation is carried out on the information symbol. The information symbol subjected to such propagation path distortion compensation is sent to error correction circuit  105 . Moreover, the information symbol subjected to propagation path distortion compensation is stored in register  1004 . 
     The information symbol subjected to propagation path distortion compensation is sent to error correction circuit  105  where the information symbol is subjected to error correction, and then sent to error detection circuit  106  where the information symbol is subjected to error detection and output as reception data. 
     Next, propagation path estimation/compensation is performed using the error-corrected information bit. The error-corrected information bit is periodically sent to re-coding section  107 . Re-coding circuit  107  performs transmission path coding processing, modulation processing and rearrangement processing on the error-corrected information bit again. The I component of this re-coded and error-corrected information bit string is sent to subtractor  1005  of propagation path estimation/compensation circuit  104  and the Q component is sent to subtractor  1006  of propagation path estimation/compensation circuit  104 . 
     Subtractor  1005  finds a difference between the I component of the re-coded and error-corrected information bit string and I component of the information bit stored in the register and subjected to propagation path distortion compensation and the difference value is input to multiplier  1105  of propagation path estimated value update circuit  1002 . Subtractor  1006  finds a difference between the Q component of the re-coded and error-corrected information bit string and Q component of the information bit stored in the register and subjected to propagation path distortion compensation and the difference value is input to multiplier  1106  of propagation path estimated value update circuit  1002 . 
     In multipliers  1105  and  1106 , the difference value is multiplied by a weighting factor (0&lt;W≦1). Thus, multiplying weighting factor W reduces the difference value, making it possible to prevent influences by large estimation errors. This weighting factor W can be either fixed or made variable according to the channel condition as appropriate. 
     The difference values multiplied by weighting factor W are sent to adders  1103  and  1104 . Adder  1103  adds up the I component of the difference value and I component of the propagation path estimated value (output of multiplier  1001 ) and adder  1104  adds up the Q component of the difference value and Q component of the propagation path estimated value (output of multiplier  1001 ) and in this way a new propagation path estimated value is obtained. This new propagation path estimated value is sent to registers  1101  and  1102 , updated and at the same time sent to multiplier  1003  of propagation path estimation/compensation circuit  104 . 
     Multiplier  1003  performs a complex multiplication on the I component and Q component of the information symbol of the FFT output and the I component and Q component of the propagation path estimated value. In this way, the information symbol is subjected to propagation path distortion compensation. The information symbol subjected to propagation path distortion compensation is sent to error correction circuit  105 . 
     The information symbol subjected to propagation path distortion compensation is sent to error correction circuit  105  where the information symbol is subjected to error correction, then sent to error detection circuit  106  where the information symbol is subjected to error detection and output as reception data. 
     Thus, this embodiment allows propagation path response to be estimated without inserting a pilot symbol between continuously transmitted information OFDM symbols, making it possible to obtain an excellent reception characteristic without reducing the transmission efficiency. Moreover, even if there is a residual phase error, this embodiment only corrects the difference while compensating the residual phase error, making it possible to reduce deterioration of the estimation accuracy due to the residual phase error. 
     Embodiment 6 
     In the OFDM communication apparatus according to this embodiment, propagation path estimated value update circuit  1002  adopts a variable weighting factor using a past propagation path estimated value as quality information. 
     Since the configuration of the OFDM communication apparatus according to this embodiment is the same as that of Embodiment 5 except the propagation path estimated value update circuit, the propagation path estimated value update circuit will be explained. 
     FIG. 14 is a block diagram showing an internal configuration of a propagation path estimated value update circuit of a propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 6 of the present invention. 
     This propagation path estimated value update circuit  1002  includes registers  1201  and  1202  that store a propagation path estimated value (output of multiplier  1001 ) and output it to adders  1204  and  1205 , multipliers  1206  and  1207  that multiply the outputs of subtractors  1005  and  1006  by a weighting factor, adders  1204  and  1205  that add up the multiplication results of multipliers  1206  and  1207  and the propagation path estimated values stored in registers  1201  and  1202  and factor per subcarrier selection section  1203  that selects weighting factor Wk using the propagation path estimated value stored in registers  1201  and  1202  as the quality information. Propagation path estimated value update circuit  1002  includes switches  1208  and  1209  that switch between the output of register  1201  to adder  1204  and the output of multiplier  1001  to adder  1204 . 
     The operation of the OFDM communication apparatus with the above configuration will be explained. The propagation path estimated value (output of multiplier  1001 ) is stored in registers  1201  and  1202  of propagation path estimated value update circuit  1002 . At this time, switches  1208  and  1209  of propagation path estimated value update circuit  1002  are set so that the output of multiplier  1001  is sent to registers  1201  and  1202 . 
     The difference values from subtractors  1005  and  1006  are input to multipliers  1206  and  1207 , respectively. Multipliers  1206  and  1207  multiply the difference values by weighting factor Wk. This weighting factor Wk is selected by factor per subcarrier selection section  1203 . Factor per subcarrier selection section  1203  selects weighting factor Wk using the propagation path estimated value stored in registers  1201  and  1202  as quality information. 
     Thus, multiplying the difference values by weighting factor Wk reduces the difference values, making it possible to prevent influences due to large estimation errors. 
     The difference values multiplied by weighting factor Wk are sent to adders  1204  and  1205 . Then, adder  1204  adds up the I component of the difference value and the I component of the propagation path estimated value (output of multiplier  1001 ) and adder  1205  adds up the Q component of the difference value and the Q component of the propagation path estimated value (output of multiplier  1001 ) and a new propagation path estimated value is obtained. This new propagation path estimated value is sent to registers  1201  and  1202 , updated and at the same time sent to multiplier  1003  of propagation path estimation/compensation circuit  104 . 
     Thus, this embodiment allows propagation path response to be estimated without inserting a pilot symbol between continuously transmitted information OFDM symbols and furthermore allows the percentage of updated low-reliability difference value to be reduced by changing the weighting factor for every subcarrier, thus making it possible to obtain an excellent reception characteristic without reducing the transmission efficiency. Moreover, even if there is a residual phase error, this embodiment only corrects the difference while compensating the residual phase error, making it possible to reduce deterioration of the estimation accuracy due to the residual phase error. 
     Embodiment 7 
     In the OFDM communication apparatus according to this embodiment, propagation path estimated value update circuit  1002  averages the outputs of subtractors. 
     Since the configuration of the OFDM communication apparatus according to this embodiment is the same as that of Embodiment 6 except the propagation path estimated value update circuit, the propagation path estimated value update circuit will be explained. 
     FIG. 15 is a block diagram showing an internal configuration of a propagation path estimated value update circuit in a propagation path estimation/compensation circuit of the OFDM communication apparatus according to Embodiment 7 of the present invention. 
     In propagation path estimated value update circuit  1002 , the I component of a difference value from subtractor  1005  is input to averaging section  1301  and the Q component of a difference value from subtractor  1006  is input to averaging section  1302 . Averaging sections  1301  and  1302  perform averaging processing on the difference values corresponding to n symbols. This I component of the averaged difference value is sent to multiplier  1206  and the Q component of the averaged difference value is sent to multiplier  1207 . The processing hereafter is the same as that in Embodiment 6. When the amplitude of the transmission signal contains information as in the case of multi-value QAM, averaging sections  1301  and  1302  can also be configured in such a way as not to include values of signal points with a small amplitude in averaging and reduce deterioration by additive noise. 
     According to this embodiment, averaging the subtractor outputs allows the estimated value of propagation path variations to be obtained more accurately, making it possible to obtain an excellent reception characteristic without reducing the transmission efficiency. Moreover, even if there is a residual phase error, this embodiment only corrects the difference while compensating the residual phase error, making it possible to reduce deterioration of the estimation accuracy due to the residual phase error. 
     In this embodiment, it is also possible to input a CRC (Cyclic Redundancy Check) result to factor per subcarrier selection section  1203  as external quality information as shown in FIG.  16 . This setting is intended to prevent an averaging block for which the CRC result shows some errors from not being used as a difference value of variations in the estimated propagation path. 
     Thus, applying external quality information to weighting factor selection makes it possible to obtain more accurate estimated values of variations in the propagation path, further eliminate difference value estimation errors due to bit errors and thereby obtain an excellent reception characteristic without reducing the transmission efficiency. Moreover, even if there is a residual phase error, this embodiment only corrects the difference while compensating the residual phase error, making it possible to reduce deterioration of the estimation accuracy due to the residual phase error. 
     The present invention is not limited to Embodiments 1 to 7 above, but can be implemented with various modifications. For example, Embodiments 1 to 7 above can be implemented in combination as appropriate. 
     The OFDM communication apparatus of the present invention comprises an estimated value calculation section that calculates a propagation path estimated value using a known signal contained in an OFDM signal, a propagation path distortion compensation section that compensates propagation path distortion for the information signal obtained from the OFDM signal using the propagation path estimated value above, an error correction section that performs error correction processing on the information signal with propagation path distortion compensated, a re-coding section that performs re-coding processing on the error-corrected signal, and the estimated value calculation section calculates a propagation path estimated value using the re-coded information signal instead of the known signal. 
     This configuration allows the propagation path estimated value to be calculated using the re-coded information signal instead of the known signal, making it possible to estimate propagation path response without inserting a pilot symbol between continuously transmitted information OFDM symbols even if long information is sent, thus obtaining an excellent reception characteristic without reducing the transmission efficiency. 
     The OFDM communication apparatus of the present invention comprises an estimated value calculation section that obtains a propagation path estimated value using a known signal contained in an OFDM signal, a propagation path distortion compensation section that compensates propagation path distortion for the information signal obtained from the OFDM signal using the propagation path estimated value above, an error correction section that performs error correction processing on the information signal with propagation path distortion compensated, a re-coding section that performs re-coding processing on the error-corrected signal, and the estimated value calculation section calculates a propagation path estimated value using a difference between the re-coded information signal and the information signal subjected to propagation path distortion compensation. 
     This configuration allows propagation path response to be estimated without inserting a pilot symbol between continuously transmitted information OFDM symbols, making it possible to obtain an excellent reception characteristic without reducing the transmission efficiency. Moreover, even if there is a residual phase error, this embodiment only corrects the difference while compensating the residual phase error, making it possible to reduce deterioration of the estimation accuracy due to the residual phase error. 
     In the OFDM communication apparatus of the present invention with the above configuration, the estimated value calculation section calculates a new propagation path estimated value using a propagation path estimated value obtained from the current re-coded information signal and a past information signal. 
     The OFDM communication apparatus of the present invention with the above configuration comprises a weighting section that performs weighting on the current re-coded information signal and the past information signal. 
     These configurations obtain a new propagation path estimated value also using a past propagation path response estimated value, and therefore using this propagation path estimated value can achieve high estimation accuracy and perform propagation path distortion compensation for information bits more accurately. 
     In the OFDM communication apparatus of the present invention with the above configuration, the weighting section performs weighting based on external quality information. 
     This configuration applies the external quality information to selection of weighting factors, and therefore can reduce estimation errors due to bit errors, improving estimation accuracy drastically. 
     In the OFDM communication apparatus of the present invention with the above configuration, the estimated value calculation section comprises an averaging section that averages re-coded information signals of a plurality of symbols. 
     According to this configuration, newly obtained propagation path estimated values corresponding to a plurality of symbols are averaged, making it possible to reduce estimation errors due to additive noise and using this propagation path estimated value makes it possible to obtain high estimation accuracy and perform propagation path distortion compensation for information bits more accurately. 
     The communication terminal apparatus of the present invention is characterized in that it comprises the OFDM communication apparatus with the above configuration. Furthermore, the base station apparatus of the present invention is characterized in that it comprises the OFDM communication apparatus with the above configuration. 
     According to these configurations, it is possible to estimate propagation path response even if long information is transmitted without inserting a pilot symbol between continuously transmitted information OFDM symbols and implement a radio communication system capable of obtaining an excellent reception characteristic without reducing the transmission efficiency. 
     The propagation path estimation method of the present invention comprises an estimated value calculation step of calculating a propagation path estimated value using a known signal included in an OFDM signal, a propagation path distortion compensation step of compensating propagation path distortion for the information signal obtained from the OFDM signal using the propagation path estimated value, an error correction step of performing error correction processing on the information signal with propagation path distortion compensated, a re-coding step of performing re-coding processing on the error-corrected signal, and in the estimated value calculation step, a propagation path estimated value is calculated using the re-coded information signal instead of the known signal. 
     According to this method, a propagation path estimated value is calculated using the re-coded information signal instead of the known signal, and therefore it is possible to estimate propagation path response even if long information is transmitted without inserting a pilot symbol between continuously transmitted information OFDM symbols and obtain an excellent reception characteristic without reducing the transmission efficiency. 
     The propagation path estimation method of the present invention comprises an estimated value calculation step of calculating a propagation path estimated value using a known signal included in an OFDM signal, a propagation path distortion compensation step of compensating propagation path distortion for the information signal obtained from the OFDM signal using the propagation path estimated value, an error correction step of performing error correction processing on the information signal with propagation path distortion compensated, a re-coding step of performing re-coding processing on the error-corrected signal, and in the estimated value calculation step a propagation path estimated value is calculated using a difference between the re-coded information signal and information signal with propagation path distortion compensated. 
     This method allows propagation path response to be estimated without inserting a pilot symbol between continuously transmitted information OFDM symbols, making it possible to obtain an excellent reception characteristic without reducing the transmission efficiency. Moreover, even if there is a residual phase error, this embodiment only corrects the difference while compensating the residual phase error, making it possible to reduce deterioration of the estimation accuracy due to the residual phase error. 
     As described above, the OFDM communication apparatus of the present invention adaptively estimates propagation path response using an error-corrected signal, that is, using a judged value of the received information signal, and therefore can adaptively follow up time variations of the transmission path and maintain a low error rate without reducing the transmission efficiency even when long information is transmitted or there are considerable time variations in propagation path response. 
     The present invention is not limited to the above described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
     This application is based on the Japanese Patent Application No. HEI 11-245098 filed on Aug. 31, 1999, entire content of which is expressly incorporated by reference herein.