Underwater acoustic telemetry: Looking at the last decade and the next

Progress in underwater acoustic telemetry since 1982 is reviewed within a framework of five major areas: coherent signaling adaptive equalization and carrier recover, spatial and frequency diversity exploitation, error control coding and networked systems. Advances in each of these areas as well as perspectives on the future challenges facing them are presented. The ascension of coherent systems dominates the published research in recent years with a commensurate emphasis on equalization. Unfortunately, many efforts focus exclusively on the signal processing aspects of coherent communication rather than the underlying channel physics which, ultimately, limit its use. A primary thesis of this paper is that future performance gains (measured either as complexity reduction or an expansion of the environments supporting coherent communications) will draw heavily on understanding the channel structure and dynamics. Perhaps as a consequence of the incomplete channel models, incoherent modulation methods have received incoherent renewed modulation attention. Both spatial and frequency diversity has proven to be quite effective in increasing system performance margins. Most investigations have taken an empirical approach to evaluating diversity by designing particular receivers and reporting on their effectiveness. The stability of adaptive multichannel combining algorithms in a real, dynamic underwater channel however, is uncertain, primarily due to a poor characterization of those dynamics. The promise held out by information theory is that channel coding provides fundamental tools in increasing transmission rates and reliability. Channel coding for incoherent systems has been closely examined in the gears since 1982 and is a relatively mature area of research that has received renewed interest as covert applications are considered Channel coding for coherent systems, however, may well pose a substantial challenge primarily due to the prevalent use of decision feedback based approaches requiring an integrated approach to decoding and equalization. Finally, research in underwater acoustic networks is increasing in recent years as a result of growing interest in distributed sensor architectures. The work in this area has been largely analytic to date with few experimental efforts undertaken.