The performance of acoustic modems in the ocean is strongly affected by the ocean environment. A storm can drive up the ambient noise levels, eliminate a thermocline by wind mixing, and whip up violent waves and thereby break up the acoustic mirror formed by the ocean surface. The combined effects of these and other processes on modem performance are not well understood. The authors have been conducting experiments to study these environmental effects on various modulation schemes. Here the focus is on the role of the thermocline on a widely used modulation scheme (frequency-shift keying). Using data from a recent experiment conducted in 100-m-deep water off the coast of Kauai, HI, frequency-shift-key modulation performance is shown to be strongly affected by diurnal cycles in the thermocline. There is dramatic variation in performance (measured by bit error rates) between receivers in the surface duct and receivers in the thermocline. To interpret the performance variations in a quantitative way, a precise metric is introduced based on a signal-to-interference-noise ratio that encompasses both the ambient noise and intersymbol interference. Further, it will be shown that differences in the fading statistics for receivers in and out of the thermocline explain the differences in modem performance.
Matched-field or model-based processing has now been widely demonstrated for improving source localization and detection in ocean waveguides. Most of the processing approaches become increasingly sensitive to fluctuations or uncertainties as the frequency increases. As a result, there has been very limited work above 1 kHz and there is a perception that above several kilohertz the technique cannot be applied. We have conducted acoustic communications experiments in a variety of shallow water sites around coastal areas of the United States. These experiments show that a clear multipath structure is readily observed even in the 8-16 kHz band. Furthermore, it is shown that model-based processing can then be exploited to localize sources at these high frequencies out to ranges of several kilometers.
In order to achieve high data rate digital communications, multiple-input/multiple-output (MIMO) techniques have attracted growing interests in the underwater acoustic communication studies. In this paper, multichannel combining and decision feedback equalization (MCC/DFE) has been proposed for underwater acoustic MIMO channels. In order to overcome the difficulties introduced by the fast fluctuating channel, Doppler tracking and frequent channel estimation are performed. Then time reversal combining followed by a single channel DFE is used to demodulate individual symbol sequences transmitted by the multiple element source. To improve the performance, successive interference cancellation is also incorporated into the receiver structure.Using data from the Makai experiment conducted around Kauai Island, HI, 2005, we have shown that the achievable data rate can be increased up to 4 times using the same bandwidth as single source systems. For example, 32 kilobits/s could have been achieved by simultaneous transmission of four 4 kilosymbols/s 4-phase shift keying (QPSK) symbol sequences when both the source and the receiver were drifting at a 2 km range in the ocean.
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