Abstract

As electromagnetic waves propagate poorly in sea water, acoustics provides the most obvious choice of medium to enable underwater communications. Underwater acoustic (UWA) communications has been a difficult problem due to unique channel characteristics. Attempts at adapting communication techniques developed for other channels have yielded successful implementations in vertical deep water channels, but have had limited success in shallow water channels. One of the most challenging UWA communication channels is the medium-range very shallow warm water channel, common in tropical coastal regions such as Singapore. This warm shallow water acoustic (WSWA) channel exhibits two key features - extensive time-varying multipath and high levels of non-Gaussian ambient noise due to snapping shrimp - both of which limit the performance of traditional communication techniques.

We have developed a physics-based time-varying statistical channel model for the WSWA channel. The model is calibrated and its accuracy validated using channel measurements made at sea. In contrast with most UWA communications research which assumes Gaussian noise, we model the ambient noise as a symmetric alpha-stable (S-alpha-S) noise. This has profound implications on the design of communication systems for the WSWA channel. Many commonly used techniques such as the linear correlator and the soft-decision Viterbi algorithm with Euclidean branch metric perform poorly in presence of S-alpha-S noise. We have developed computationally efficient near-optimal solutions such as the sign correlator for detection and the 1-norm Viterbi decoder for decoding convolutional codes in the presence of S-alpha-S noise.

In this talk, we present an orthogonal frequency division multiplexing (OFDM) based communication scheme for use in WSWA channels. The scheme uses differential QPSK and a cyclic prefix to eliminate the need for an equalizer. It uses error correction coding and a channel interleaver to benefit from the time-frequency diversity available in the channel. The 1-norm Viterbi decoder ensures good decoding performance in impulsive noise. We have tested this scheme via experiments in Singapore waters. We obtained reliable data transmission at a rate of 7.6 kbps at 1 km range and 15 kbps at 800 m range. This is a large improvement when compared to the performance of the best commercial modems in similar environments.