Time synchronization for OFDM systems in very dispersive channels

Orthogonal frequency division multiplexing (OFDM) systems guarantee transmissions free from intersymbol interference (ISI) and interchannel interference (ICI) over frequency selective channels, provided that the channel delay spread is less than the duration of the cyclic-prefix (or the zero-padding guard interval). However, it is realistic to consider practical implementation scenarios in which the channel delay spread exceeds this limit so that both ISI and ICI do arise. We show that in such cases, the objective of symbol synchronization should be to maximize the ratio of the total useful received power over all subcarriers to the total power of ISI and ICI for the given channel realization, and that this aim can be achieved through a proper choice of the symbol timing. We also present a practical low-complexity synchronization scheme for this case and show that its performance tops the results obtained by the best existing correlation-based timing estimators. Furthermore, we determine upper bounds on the achievable signal-to-interference ratio based on the ideal assumptions of either perfect channel state information or at least knowledge of the channel statistical description, and show that they are very closely approached by the proposed method.