Coherent and differential acoustic communication in shallow water using transmitter and receiver arrays

Underwater acoustic communication link performance is investigated under strong time-frequency variation. A state of the art split-step parabolic equation program was used to generate channel responses under a moving rough sea surface. A shallow water environment with 47.5 m depth at 1 km range was considered, with the sea surface characterized as a mature sea under a 10 m/s (at 10 m height) wind. OFDM signal structure with a reasonable trade-off between symbol duration and subcarrier bin width is found to address only partially the severely overspread channel. It is found that beamforming at the transmitter and at the receiver significantly improves both coherence time and frequency, allowing doubling of the effective throughput. Further improvement of the rate is achieved through differential modulation which avoids training overhead. A link with 4 transmitters and 10 receivers was found to support 1.8 bps/Hz effective rate at 1 km using differential modulation on each OFDM subcarrier, more than 4 times the corresponding rate of a 1×1 link using pilot-based estimation and non-differential modulation on each OFDM subcarrier.