Bit error rate performances of M-QAM orthogonal multicarrier modulation in presence of time-selective multipath fading

Orthogonal frequency division multiplexing (OFDM) is considered as an effective technique for high speed digital transmission under multipath fading. We theoretically analyze the behaviour of a coherent pilot-symbol aided M-QAM-OFDM system in the presence of a time selective multipath channel. In the analyzed scheme, pilot symbols are periodically inserted to monitor the channel characteristics. The fading process experienced by the pilot symbols is used to estimate the fade characteristics at the symbol-rate through interpolation. The estimated channel is used to compensate for the random phase variation caused by the Doppler spread, so that the signal can be coherently demodulated. The bit error rate (BER) performance of this coherent OFDM modulation scheme is derived. The results obtained show that, with a reasonable amount of redundancy (frequency of the pilot symbols) introduced, the system performance approaches that of the ideal coherent demodulation, as long as a suitable channel estimate procedure is adopted.