1.  Field of the Invention
The present invention relates to an OFDM communication apparatus used in a radio communication system. 
2.  Description of the Related Art
A conventional OFDM communication apparatus is explained using FIG.  1.  FIG.  1 is a block diagram showing a configuration of a conventional OFDM communication apparatus. 
In the OFDM communication apparatus shown in FIG.  1, an information signal for every subcarrier is subjected to digital modulation processing by modulation section 1 through, for example, QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) and a synchronization symbol is then inserted into this signal by synchronization symbol insertion section 2. 
The signal with the synchronization symbol inserted is subjected to an IFFT (Inverse Fast Fourier Transform) calculation by IFFT section 3 and converted to an OFDM signal. 
A guard interval is inserted into this OFDM signal by guard interval insertion section 4.  As shown in FIG.  2, this signal frame is configured by synchronization symbol 21, phase reference symbol or pilot symbol 22, guard interval 23 and valid symbol 24. 
The signal with the guard interval inserted is D/A-converted to a baseband signal by D/A converter 5.  This baseband signal is subjected to normal radio transmission processing by a radio transmission section, which is not shown in the figure, and transmitted from an antenna as a transmission signal. 
A signal received via an antenna is subjected to normal radio reception processing by a radio reception section, which is not shown in the figure, and converted to a baseband signal.  This baseband signal is subjected to quasi-coherent detection by a quasi-coherent detector and stripped of an unnecessary frequency component by a low-pass filter (the quasi-coherent detector and low-pass filter are not shown in the figure).  This baseband signal is A/D-converted by A/D converter 6.  In addition, the reception signal is divided into an in-phase component and quadrature component through quasi-coherent detection processing but the figure only shows a single signal path. 
This baseband signal is subjected to an FFT (Fast Fourier Transform) calculation by FFT section 12 and signals assigned to different subcarriers are obtained.  At this time, the baseband signal is delayed by delay section 7, sent to multiplier 8 and the multiplication result is accumulated by accumulator 9.  The accumulation result is sent to subtractor 10 to be subjected to a subtraction with a threshold and subjected to threshold judgment by determination section 11.  This determination result is sent to FFT section 12. 
The signal FFT-calculated by FFT section 12 is sent to demodulation section 13, subjected to delay detection, judged by determination section 14 as to whether the signal is different from a signal one bit ahead and output as a demodulated signal. 
When symbol synchronization is established in the OFDM communication apparatus with the configuration above, the baseband signal prior to FFT calculation and the signal prior to FFT calculation delayed by one symbol by delay section 7 are sent to multiplier section 8 to be subjected to complex multiplication. 
Then, the output of multiplier 8 is sent to accumulator 9 where the complex multiplication results are accumulated.  Here, since the phase reference symbol is identical to the synchronization symbol, the result of accumulation of both signals has a peak on the phase reference symbol of the signal delayed by one symbol as shown in part A of FIG.  3.  Therefore, it is possible to establish symbol synchronization by detecting timing at which the accumulation result exceeds a threshold. 
However, signals with high power may be included under circumstances under which tens of delay signals are received.  In this case, since threshold judgment is carried out using power of correlation results, it is assumed that a signal with high power may exceed the threshold.  In such a case, symbol synchronization may be established using a signal with high power, failing to correctly detect FFT processing start timing and resulting in desynchronization.