Underwater acoustic channel characterisation for medium-range shallow water communications

The ability to effectively communicate underwater has numerous applications for researchers, marine commercial operators and defence organizations. As electromagnetic waves cannot propagate over long distances in seawater, acoustics provides the most obvious choice of channel. Although acoustics has been used effectively for point-to-point communications in deep-water channels, acoustics has had limited success for horizontal transmissions in shallow water. Time-varying multipath propagation and nonGaussian noise are two of the major factors that limit acoustic communication performance in shallow water. Although it is known that medium-range shallow water propagation is dominated by time-varying multipath arrivals, very few measurements of the variability of the multipath structure are available. In this paper, we present channel measurements made in a shallow water channel (depth 15-20 m) up to a range of 1 km. An analysis of the temporal variability of the arrival structure is presented. Most communication systems make the assumption that the noise is additive and Gaussian. Snapping shrimp dominate the ambient noise spectrum above a few kHz in warm shallow waters. It is known that snapping shrimp noise is impulsive and highly nonGaussian. These noise characteristics need to be taken into account when designing communication systems if robust and near-optimal performance is desired. An analysis of the ambient noise characteristics from some warm shallow water channels is also presented.