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An OFDM Design for Underwater Acoustic Channels with Doppler Spread

In this paper, we study the performance of orthogonal frequency division multiplexing (OFDM) over underwater acoustic multipath channels with different Doppler scales on different paths. We first derive an exact inter-carrier-interference (ICI) expression after incorporating the compensation of nonuniform Doppler shifts across OFDM sub-carriers. Based on the assumption that the residual ICI is dominantly from immediate neighbors, we suggest a practical design that divides subcarriers into groups, where each group of eight subcarriers consists of three contiguous data subcarriers, one pilot subcarrier, and five carefully spaced null subcarriers. We use the orthogonal matching pursuit (OMP) algorithm for sparse channel estimation that identifies distinct physical paths with different Doppler scales. System performance is evaluated using data recorded from the GLINT08 and SPACE08 experiments. Relative to the receiver that ignores the residual ICI, we observe that explicitly suppressing the residual ICI induced by Doppler spread leads to improved performance for the SPACE08 data, while not for the GLINT08 data.