A joint blind timing and frequency offset estimator for OFDM systems over doubly selective fading channels

A novel joint timing and frequency estimator based on maximum likelihood (ML) criterion, is derived for OFDM systems over a doubly selective fading (time and frequency selective) channel. In this estimator, the unwanted channel fading gains and transmitted signals are modeled as unknown deterministic variables and extracted from the received signals employing linear minimum mean square error (LMMSE). An algorithm in closed form expressions is derived, with its implementation complexity comparable to the estimators proposed in J.J. van de Beek et al. (1997). Simulation results show our timing estimator performs significantly better than estimators reported in J.J. van de Beek et al. (1997) in time invariant frequency selective fading channel, and our frequency estimator achieves a lower MSE than the estimators in J.J. van de Beek et al. (1997) at higher SNR (15 dB). For doubly selective fading channels, at high SNR (20 dB) and with only three OFDM symbols, our frequency estimator performs close to the estimator studied in H. Bolcskei (2001) but the latter has employed 25 symbols. We also demonstrate how the performance of our algorithm changes with the increase in Doppler frequency spread.