Underwater acoustic communications: Fundamentals and new results
Underwater wireless communications rely on transmission of acoustic waves, since electro-magnetic waves propagate only over very short distances. Acoustic waves, however, are confined to low frequencies (usually up to a few tens of kHz), and the communication bandwidth is limited. Sound travels underwater at a very low speed (1500 m/s) and propagation occurs over multiple paths. Delay spreading over tens of milliseconds results in a frequency-selective signal distortion, while motion creates an extreme Doppler effect. The worst properties of radio channels—poor link quality of a mobile terrestrial channel, and long delay of a satellite channel—are thus combined in an underwater acoustic channel, which is often said to be the most difficult communication medium in use today.
The quest for bandwidth-efficient acoustic communications has progressed over the past few decades from an initial feasibility proof of phase-coherent detection, to the development of the first high-speed acoustic modem, and finally to a plethora of innovative solutions on both the signal processing and the networking fronts.
In this presentation, we begin with an overview of channel characteristics, focusing on the major differences between acoustic and radio channels. We follow with a discussion of signal processing methods, over viewing single-carrier broadband equalization used in an existing acoustic modem, and focusing on recent research results in multi-carrier signal detection on Doppler-distorted channels. The performance of various techniques is illustrated through experimental results, which include transmissions over few kilometers in shallow water to hundreds of kilometers in deep water, at highest bit-rates demonstrated to date. We conclude by outlining the open research problems.