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, an Associate Editor for the IEEE Journal of Oceanic Engineering. She is a past Associate Editor for IEEE Transactions on Signal Processing and IEEE Transactions on Vehicular Technology, and has served on the Advisory Board of IEEE Communication Letters and the Editorial Board of IEEE Signal Processing Magazine. She 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 2019 IEEE WICE Outstanding Achievement Award, and she is the 2018 IEEE/OES Distinguished Lecturer. In 2022, she was awarded an honorary doctorate from the Aarhus University in Denmark, and she was elected to the Academy of Engineering Sciences of Serbia.

Abstract: Space division multiple access (SDMA) allows multiple users to communicate at the same time and in the same frequency band without reduction in throughput that is common to other multiple access techniques such as time, frequency, and code division multiple access. As such, it offers significant potential benefits for acoustic communications where bandwidth is severely limited.

In an SDMA system, the base station uses an array to separate the signals of different users through beamforming. To construct an optimal beamformer, the channel between the base array and each of the users has to be known. The challenge in an underwater acoustic setting lies in the fact that the channel is time-varying and may change significantly over the time it takes to close the uplink/downlink feedback loop (to/from the base station). As a result, the channel estimated on the uplink will differ from the channel present on the downlink, raising multiple questions: Is the acoustic delay large enough to prevent the use of optimal, channel-based transmit beamforming? If so, is there some feature of the channel that is changing slowly enough to withstand the feedback delay and warrant transmit beamforming? What is the resulting beamforming strategy?

We address these questions from first principles, using a model of a shallow water channel in a multicarrier communication setting. We show that the feedback delay can indeed be a showstopper for optimal channel-based beamforming, and design an alternative technique based on estimating the principal angle of signal arrival. The alternative technique shows promising results in numerical simulation, as well as in laboratory tests involving multiple users transmitting to and from a base array using over-the-air acoustic signaling.