James B. Bowlin
Josko A. Catipovic
Woods Hole Oceanogr. Inst., Woods Hole, MA 02543
John G. Proakis
Northeastern Univ., Boston, MA 02115
Latest achievements in underwater acoustic communications are leading the way to modern instrumentation for acoustical oceanography by eliminating the need for tethering or diver's contact with submerged sensors. To meet the various telemetry needs, an acoustic link must provide both reliability and a high rate of data transfer, requirements that are often conflicting. The first approximation of an acoustic channel is that of a time-invariant multipath communication channel. It is well known that by increasing the transmission rate on such a channel, multipath propagation causes longer intersymbol interference, ultimately limiting system performance. Recent experimental results obtained with a newly developed receiver algorithm, however, showed a seemingly surprising result: an increase in transmission rate resulted in improved system performance. An explanation for this phenomenon is found in the time variation of the ocean multipath. Notably emphasized in the fully saturated shallow water channels, higher transmission rates allow finer sampling of the rapidly varying channel, and therefore better tracking. Experimental results obtained in the Woods Hole harbor are presented showing a dramatic improvement in performance of QPSK coherent detection over a one-mile range as the data rate is increased from 5 to 30 kilobits-per-second. A theoretical analysis based on stochastic channel modeling supports these observations.