Determining the relationship between the performance of an underwater acoustic data communications system and the operating environmental conditions is a problem that continues to plague researchers. The complexity of the time-varying channel is difficult to measure and model. Therefore an approach that uses metrics measured from data collected at sea to characterize the channel is attractive. As expected, preliminary assessments on limited data have shown that performance depends not only on environmental conditions, but also on system implementation. By extracting a variety of metrics, a better understanding of the subset that discriminate between good and bad performance can be developed. Also by analyzing the relationship between certain metrics and performance, system limitations can be identified for re-evaluation. For example, a surprising result of the initial assessment of performance using a multichannel decision feedback equalizer on real data showed that sparseness of multipath arrivals may be an arbiter of performance [Richman et al., J. Acoust. Soc. Am. 110, 2619 (2001)]. Therefore changes to the algorithm that allows for sparse arrivals may improve performance. In this paper, a larger number of metrics from greater quantities of real-data and system configurations are measured and evaluated against equalizer results.
One method for introducing time diversity into a decision feedback equalizer (DFE) spatial diversity communications receiver operating in an underwater acoustic environment is presented and analyzed. Time diversity refers to the repetition of transmitted packets and the subsequent batch processing of these packets by an appropriately modified multichannel receiver. To overcome the effects of the time-varying nature of the channel, the modifications made to the spatial diversity-based multichannel DFE (SD-DFE) include appropriate Doppler shift compensation for each received packet and determination of the proper alignment among the time-displaced packets to allow for successful equalization. The proposed time diversity DFE (TD-DFE) receiver is shown to achieve a high rate of packet success under a variety of channel conditions. More significantly, the TD-DFE algorithm is able to equalize packets when a comparable SD-DFE might fail. The packet-to-packet coherence of a series of transmissions is analyzed to provide insight into how the characteristics of the channel affect the performance of the TD-DFE. This analysis illustrates why a time diversity-based equalization approach can be more robust for the underwater acoustic channel as compared to a standard TD-DFE algorithm at the possible expense of reducing throughput.
The relationship between the performance of an underwater acoustic data communications system and operating conditions is investigated. There has been much work devoted toward this end focused on developing mathematical models of the underwater acoustic channel. However, the underwater acoustic environment is a complicated one, and current models do not fully account for the challenging, time-varying conditions. The analysis here is based on actual data transmissions collected during several at-sea tests involving a variety of operating conditions. Metrics have been developed that attempt to characterize properties relating to signal-to-noise ratio, Doppler, multipath spread, channel coherence, and channel complexity. By applying these metrics to data transmissions in conditions with different water depths, carrier frequencies, and source/receiver geometries, a better understanding can be developed of the factors that determine the performance of an underwater communications system. The ultimate goal of this work is to provide a systematic prediction method by computing those metrics that are the true arbiters of performance. This work will also aid in the development and validation of mathematical models of the underwater acoustic environment by providing practical measures with which to compare the simulated channels.
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