Dept. of Elec. and Comput. Eng., Northeastern Univ., Boston, MA 02115
Woods Hole Oceanographic Inst., Woods Hole, MA 02543
John G. Proakis
Northeastern Univ., Boston, MA 02116
In this presentation the issues concerning the impact of channel structure and dynamics on the design and performance of a decision feedback equalizer (DFE) type of the receiver for coherent underwater acoustic (UWA) telemetry is discussed. The receiver, developed as a part of the feasibility analysis of coherent communications over UWA telemetry channels, consists of a fractionally spaced DFE and a passband digital phase-locked loop. The adaptive algorithm for updating the receiver parameters is a combination of the recursive least-squares algorithm for the equalizer tap weights and a second-order phase update for carrier recovery. The receiver jointly performs channel equalization, symbol timing, and carrier phase synchronization, using the minimum mean-squared error criterion. Besides processing the single-channel signal, the receiver structure and the algorithm are extended to the multichannel case in which coherent diversity combining of different spatially distributed signals is performed. This receiver algorithm was applied to the experimental data originating from the three fundamentally different ocean environments, namely long range--deep water, long range--shallow water, and short range--shallow water channels. The experiments, conducted by the Woods Hole Oceanographic Institution, were performed in the deep waters off the coast of northern California (long range), at New England Continental Shelf (long range--shallow water), and in the Buzzards Bay (short range). Propagation in deep water, which occurs in convergence zones, results in long, but fairly stable multipath structures. Channel impulse responses, which are typically nonminimum phase, span several tens of milliseconds, requiring at least 40 taps in the feedforward section of the equalizer at data rates of 300 symbols per second. In the cases of strictly maximum phase channel responses, some reduction in complexity can be achieved by time-reversed equalization. The multipath structure of the shallow water channel is much less stable than that of the deep water channel, due to the presence of large amount of bottom and surface reverberation. The impulse responses are again nonminimum phase, typically characterized by extended multipath propagation following the main arrival. Equalizer lengths of as many as 100 feedforward and 80 feedback taps were needed in some cases of transmission at 1000 symbols per second. In all of the mentioned UWA channels, successful operation of the receiver algorithm for joint synchronization and equalization has been achieved. Experimental results assert the feasibility of coherently combining multiple arrivals at the expense of high computational complexity of the receiver. With the transmitted power constraint, performance limitations encountered at very high data rates lie both in the increased noise levels, as well as in the increased computational complexity of extremely long equalizer structures needed to overcome the intersymbol interference problem caused by multipath propagation. Future work should be directed towards finding the ways to reduce the equalizer complexity, which will result in the improved performance by virtue of reducing the noise enhancement in the equalizer.