Tracking of time misalignments for OFDM systems in multipath fading channels

In this paper, new time offset tracking techniques for OFDM are proposed without devoting specific pilot symbols for synchronization. This can be attained since, in coherent OFDM, channel estimation is usually performed employing pilot symbol assisted modulation (PSAM), based on inserting known symbols in the time-frequency grid. We show how this two dimensional embedded signalling can also be used for tracking time-frequency offsets. A two-stage procedure is proposed, focusing this work on the time correction step. The ML estimator has been derived, but though optimal, may not be practical due to its complexity. Hence, a frequency-domain low-complex 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 has been proven to be efficient against time dispersive channels.

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