Tracking of time-frequency misalignments in 2D-pilot-symbol-aided coherent OFDM systems

Pilot-symbol assisted-modulation (PSAM) is employed in coherent OFDM for channel estimation and it is based on inserting known symbols in the time-frequency grid. We show how this two-dimensional embedded signalling can be used for time and frequency offset tracking. A two-stage procedure is proposed, focusing in this work on the time correction step. The ML estimator has been derived, but though optimal, it may not be practical due to its complexity. Hence, a frequency-domain low-complexity estimator has been proposed, based on only the samples at pilot positions. Also, the stability of the generated estimate can be improved by averaging over a few number of OFDM symbols. This estimator is efficient against time dispersive channels