A stochastic underwater acoustic channel model

The overall aim of the research outlined in this paper is to realise a flexible microprocessor channel simulator utilising measured characteristics as the basis for the model. Experimentation carried out in the coastal area of Cap Ferrat in the south of France, described in a companion paper elsewhere [1], indicated the necessity for a channel simulator which could reproduce the multifaceted phenomena experienced by the received signal but under controlled laboratory conditions. A communication system can thereby be tested before undertaking yet more costly field experiments. The Differentially Phase Shift Keyed (DPSK) system under investigation employs parametric transduction with primary frequencies equal to 50 & 55 kHz to develop a modulated carrier at the difference frequency. The transmitter was deployed in deep water, and propagation was in free field conditions. The received signal suffered from severe fading (rapid fluctuations in both amplitude and phase) and high levels of noise. It is the characteristics of these fluctuations which are of most interest and are used as the basis for the simulation. These fluctuations are modelled by the generation of specific stochastic processes using a combination of linear and nonlinear transformations on a Gaussian variable. The ensuing performance, characterised by the bit error rate for various symbol rates, can then be calculated. The validity of this approach is confirmed by comparing the predicted performance under such conditions with established theory for phase modulated signals operating in a Rayleigh fading channel.