Mitigation of intercarrier interference for OFDM over time-varying underwater acoustic channels

Orthogonal frequency division multiplexing (OFDM) has emerged as a promising modulation scheme for underwater acoustic (UWA) communications, thanks to its robustness to channels with severe time dispersion. Compared to conventional single-carrier systems, for which complicated equalization schemes are usually required, OFDM systems are in general much simpler to implement as detection can be carried out symbol-by-symbol over time-dispersive channels. In this paper, we focus on cyclic-prefixed OFDM over time-varying UWA channels. To cope with the intercarrier interference (ICI) that arises at the receiver side because of the time variations in the channel, we consider two ICI-mitigation techniques. In the first scheme, the ICI coefficients are explicitly estimated, and minimum mean square error linear equalization based on such estimates is performed. In the second approach, no explicit ICI estimation is performed, and detection is based on an adaptive decision-feedback equalizer applied in the frequency domain across adjacent subcarriers. To cope with the phase variations of the ICI coefficients, phase-tracking loops are introduced in both ICI-mitigation schemes. The effectiveness of the presented schemes is demonstrated through simulation results, as well as real data collected in a recent experiment conducted in shallow water off the western coast of Kauai, HI, in June 2008.