Underwater wireless communications rely on transmission of acoustic waves as 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, overviewing 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. Finally, we discuss several issues important for the design of underwater acoustic networks. We conclude by outlining the open research problems.
Milica Stojanovic graduated from the University of Belgrade, Serbia, in 1988, and received the M.S. and Ph.D. degrees in electrical engineering from Northeastern University in Boston, in 1991 and 1993. She was a Principal Scientist at the Massachusetts Institute of Technology, and in 2008 joined Northeastern University, where she is currently a Professor of electrical and computer engineering. She is also a Guest Investigator at the Woods Hole Oceanographic Institution. Her research interests include digital communications theory, statistical signal processing and wireless networks, and their applications to underwater acoustic systems. Milica is a Fellow of the IEEE and serves as an Associate Editor for its Journal of Oceanic Engineering (and in the past for Transactions on Signal Processing and Transactions on Vehicular Technology). She also serves on the Editorial Board of IEEE Signal Processing Magazine and chairs the IEEE Ocean Engineering Society’s Technical Committee for Underwater Communication, Navigation and Positioning. Milica is the recipient of the 2015 IEEE/OES Distinguished Technical Achievement Award, the 2018 IEEE/OES Distinguished Lectureship, and the 2019 IEEE WICE Outstanding Achievement Award. In 2022, she was awarded an honorary doctorate from the Aarhus University in Denmark, and she was elected into the Academy of Engineering Sciences of Serbia.