Abstract

Although acoustic energy has been used effectively for point-to-point communications in deep-water channels, it has had limited success for horizontal transmissions in shallow water. Time-varying multipath propagation and non-Gaussian snapping shrimp noise are two of the major factors that limit acoustic communication performance in shallow water. Rapid time variation in the channel can limit the use of equalizers to compensate for frequency selective fading introduced due to multipath propagation. OFDM (orthogonal frequency division multiplexing), a communication technique widely used in wired and wireless systems, divides the available bandwidth across a number of smaller carriers, each of which experiences flat fading. This simplifies the equalizer structure and provides robustness against time-varying frequency-selective fading. Another source of signal degradation is impulsive noise from snapping shrimp, which affects several OFDM carriers at the same time. OFDM, when coupled with coding, can provide robustness against impulsive noise by distributing the energy for each bit over a longer period of time. We tested coded OFDM in a very shallow water channel in Singapore waters. The results show that it is a promising technique for use in very shallow, warm water channels.