Direct-sequence spread-spectrum underwater acoustic communications are analyzed in this paper between communication nodes, at least one of which is moving. At-sea data are analyzed which show that the phase change due to source motion is significant. The differential phase between two adjacent symbols is often larger than the phase difference between symbols. One finds that the simple cross-correlation method that detects the inter-symbol phase change without requiring channel equalization does not work when the source or receiver is moving. A pair of energy detectors that are insensitive to the phase fluctuations, are proposed, whose outputs are used to determine whether adjacent symbols are of the same kind or opposite kind. The method shows good results for input signal-to-noise ratio (SNR) as low as 8 dB. The purpose of low SNR communications is to minimize the probability of detection (PD) by an interceptor. PD is analyzed as a function of range to the interceptor assuming a source level high enough to communicate to an intended (friendly) receiver. The analysis is conducted in a typical shallow water environment. A broadband energy detector is employed assuming signal bandwidth is known.
Citation: Journal of the Acoustical Society of America
Pub Type: Journals
Low-Probability of Detection Underwater Acoustic Communications