Patent ID: 7447277

Claim:
An OFDM demodulator apparatus for demodulating an orthogonal frequency division multiplex (OFDM) signal whose unit of transmission is a transmission symbol including effective symbols generated by making a time division of an information series and modulating the information into a plurality of sub-carriers and a guard interval generated by copying the signal waveform of a part of the effective symbols, the apparatus comprising: a clock generating circuit for generating a sampling clock that is asynchronous with respect to the OFDM signal; an analog-digital converting circuit for sampling the OFDM signal with the sampling clock of a predetermined frequency and converting the sampled OFDM signal into digital data; a symbol-boundary measuring circuit including a free-running counter that counts the sampling clock so that the count is cyclically repeated at each number of samples in one symbol period, calculating a correlation between the OFDM signal and a signal generated by delaying the OPDM signal by one symbol period, and outputting the count of the free-running counter at a peak timing as a measured value indicative of a boundary between transmission symbols of the OFDM signal with the sampling clock; a plurality of change-rate calculating circuits each calculating a time-change rate of the measured value; and a clock-frequency error calculating circuit for calculating a clock-frequency error that is a difference between the transmission clock for the OFDM signal and the sampling clock, each change-rate calculating circuit having respective time intervals set for calculation of the time change rate and being different in time interval from each other among the plurality of change-rate calculating circuits; and the clock-frequency error calculating circuit calculating the clock-frequency error based on a plurality of time-change rates from the plurality of change-rate calculating circuits, respectively.