Doppler experiment for shallow underwater acoustic communication using QPSK and turbo corder

Abstract

A continuous data link under shallow and towing conditions is essential for UUV operation. In shallow water, inter-symbol interference (ISI) due to multipath fading strongly distorts the carrier signal. In addition, the phase of the signals shifts considerably due to the Doppler shift at a high relative speed (~5-kt) between the mother ship and the UUV. The communication performance degrades significantly compared to static communication. Actually, a 1-kt relative speed between the transmitter and receiver causes a phase shift of more than 2 Mπ within 1000 symbols of transported data. In this study, we investigated QPSK acoustic telecommunication systems that incorporate a Turbo corder and digital phase lock loop (DPLL) for the Doppler shift compensation. A towing experiment was conducted in shallow water, and the performance of the system was evaluated. When the receiver was 3 m deep, at speeds of 1.49 and 2.02 kt, we see a considerable number of errors. When the receiver depth was 15 m from the surface, all of the transmitted data were completely reproduced after applying our acoustic communication algorithm.