A review of progress in acoustic telemetry and the challenges of the underwater channel

Abstract

High data rate acoustic telemetry in the ocean is limited by both bandwidth and reverberation. Attenuation limits the available bandwidth and most acoustic environments are dominated by reverberation which is spread in both delay and Doppler. The availability of low power VLSI electronics has had an enormous impact upon acoustic telemetry because the algorithms of digital communication systems could be implemented in oceanographic sensors which have severe constraints upon available energy and space. The first demonstration was the DATS (Digital Acoustic Telemetry System) for reverberant acoustic channels. It used MFSK with an (8,4) Hamming code, frequency hopping, and explicit synchronization of time and Doppler. Since then, (i) coherent signaling, (ii) source and channel coding, (iii) adaptive equalizers, and (iv) frequency and spatial diversity and networked configurations all have been used. Bit rates of 5–20 kbs at short ranges and 0.5–1 kbs at long ranges have been demonstrated. Higher rates can probably be obtained with forward error control methods; however, there the relatively slow speed of sound leads to some unique latency issues. Nevertheless, the availability of two-way links leads to acoustic telemetry networks which can be addressed just as any other node on a net. Some other interesting issues in acoustic telemetry concern increasing the data rate within the same bandwidth, low probability of intercept covert communications, methods for high Doppler situations, and network topologies which are robust to node outages. This presentation will provide a brief overview of acoustic telemetry and the evolving research areas, and present some of the challenges of the underwater channel.