Underwater acoustic communication
Date
1989
Authors
Wu, Lixue
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Abstract
Underwater acoustic communication has been developed since it was recognized that the ocean could support sound wave propagation. Requirements for commercial applications have been rapidly growing. In this thesis the theoretical work serves to develop the realistic acoustic communication systems which operate in shallow water environment.
Methods of reduction of multipath interference in a shallow water channel are discussed. It is suggested that a vertically suspended linear array is a useful configuration for middle range acoustic communications and its performance is superior to that of a planar array with the same number of elements.
The system aspects of a narrowbeam acoustic communication system are presented. The practical upper limit of system performance has been obtained. It has been shown that a communication channel with a relatively broad bandwidth is possible using a narrowbeam receiver. A higher transmission rate can be achieved using frequency and beam diversity.
Based on these results, the high data rate acoustic communication system using VLSI technique has been proposed. It has been shown that an acoustic communication link suitable for transmitting data from bottom instrumentation to the surface receiver is feasible using a fixed, broad transmitting beam and a steerable, narrow receiving beam. A vertically suspended linear array is suggested to generate an umbrella-type narrow beam which offers superior performance in rejecting multipath interference. Because of a high directivity index associated with such an array, a longer transmission range is possible in comparison with broad-beam systems with the same transmitted powers. Such an array can be conveniently utilized to estimate the angular direction (elevation) of the arriving signals needed for proper positioning of the beam. A high data transmission rate acoustic communication system can be relatively easily implemented using a multi-transmitted-beam and a broad band receiving array. A non-coherent BFSK data transmission scheme is postulated and a FFT algorithm is suggested to implement both direction finding and demodulation algorithms. The system can be used to derive information from sea bottom in shallow water environment.
In order to achieve high data transmission rate, dealing with broadband signal in shallow water environment has been addressed. A method of broadband direction finding applicable to broadband acoustic communication, navigation and remote sensing is proposed. Problems associated with shading function approximation, multipath interference and ambient noise are addressed and their effects on system performance are analysed. The possible solutions to improve the performance are suggested.