OTFS and OCDM Based Underwater Acoustic Communication: System Design and Evaluation

Modulation schemes tolerant to large delay spreads and high Doppler shifts are needed for reliable underwater acoustic communication (UAC). Due to the sensitivity of orthogonal frequency division multiplexing (OFDM) to Doppler shifts, two alternatives, namely, orthogonal time frequency space (OTFS) and orthogonal chirp division multiplexing (OCDM) are studied to evaluate their performance in UAC chanenls. The OTFS and OCDM schemes are seen to achieve significantly lower bit error rates (BERs) compared to OFDM in the ideal scenario, i.e., when the channel information at the receiver is perfect. Even with estimated channel knowledge at receiver, time-variations of the channel between the estimation phases, and reduced guard intervals, OTFS and OCDM outperformed OFDM. A linear channel interpolation of the estimated channel gains is proposed to account for the time-variations of the channel and is seen to improve the BERs of OTFS and OCDM schemes.