A broadband UWA communication system

A new broadband underwater acoustic communication (UWA) system which uses coherent pseudo-random phase carrier COFDM modulation is proposed. The configuration of the new system shares the advantages of (i) an autocorrelation characteristic similar to the direct sequence spread spectrum (DS-SS) system for synchronisation and, (ii) the benefits of inherent time and frequency diversities offered by the COFDM modulation. A key feature of the new system is the use of a pseudo-random Gaussian noise (PRGN) sounding signal to measure the complex time varying channel transfer function for adaptive frequency domain signal equalisation. In order to improve the BER at low transmission powers, a new form of equaliser which is based upon the least squares auto-regressive (AR) algorithm has been developed and the method has been modified to accommodate the multi-carrier OFDM signal structure for frequency domain equalisation. In the context of adaptive channel equalisation, the receiver configuration is arranged to operate as a coded decision feedback loop which takes advantage of maximum likelihood Viterbi decoding to continuously update the measured time varying channel transfer function. In this paper, a brief outline of the structure of the COFDM system and the method of parallel frequency domain equalisation and the adaptive receiver structure are described. The performance of the system is examined by comparing the bit-error-rate (BER) number different types of multipath/Doppler channels