Dept. of Elec. and Comput. Eng., Northeastern Univ., Boston, MA 02115
Josko A. Catipovic
Woods Hole Oceanographic Inst., Woods Hole, MA 02543
Recent advances in coherent underwater communications technology are the backbone of an Acoustic Local Area Network (ALAN), which is designed for a real-time underwater telemetry with multiple sensors or unmanned underwater vehicles. In shallow-water areas ALAN is planned to cover 100--1000 km[sup 2] with nodes designed to communicate at 10 kbit/s at ranges of 3--4 nm. The communication protocol is optimized for a shallow-water acoustic environment, minimizing retransmission from finite-energy transceivers and taking into account time-variant link reliability and long propagation time between the nodes. One of the key elements of this protocol is a multiuser receiver capable of simultaneously demodulating signals coming from several asynchronous cochannel modems. Unlike spread-spectrum systems the narrow-band transmission is used. The propagation conditions in shallow water are severe due to the highly dynamic multipath structures and the adaptive multiuser receiver performs joint multiple-access cancellation, equalization, phase tracking, and bit timing recovery. It consists of fractionally spaced feedforward section, coupled digital phase-locked loops, and feedback sections, which utilize previous decisions both from the desired and the interfering user. The adaptive algorithm is the combination of the recursive least-squares algorithm for filter tap weights and a second-order phase update for the carrier recovery loop. Extensive testing has been carried out to confirm successful operation of the proposed receiver in a variety of propagation conditions using the experimental data provided by the Woods Hole Oceanographic Institution. The results indicate significant performance gain over the conventional receiver at the expense of a modest increase in complexity.