Underwater Acoustic Communication

Underwater wireless communications over distances in excess of about 100 m are established using acoustic signals. Acoustic signals propagate as pressure waves, whose energy absorption limits the available bandwidth. As a result, existing technology provides bit rates on the order of several kilobits per second for transmission over distances on the order of several kilometers. Additional challenges are presented by multipath propagation that causes frequency selectivity, random time variation, and Doppler effects that occur due to low speed of sound (1500 m/s). This article overviews the development of acoustic modem technology, which evolved over the past several decades from noncoherent modulation/detection techniques to bandwidth-efficient phase-coherent modulation/detection. A survey of techniques for channel equalization in single-carrier systems as well as recent advances in multicarrier acoustic communications is also presented. Keywords: Acoustic; communications; coherent; equalization; channel estimation; phase synchronizations; multipath; Doppler; sparse channels; diversity combining; beamforming; multiuser detection; interference suppression; time reversal; multi-input–multi-output (MIMO) processing; multicarrier modulation; OFDM; adaptive modulation; underwater networks