Low-Power Environmentally Friendly Underwater Acoustic Communication Using Pseudo-Noise Spreading Sequences

This paper explores possible approaches to very low power, environmentally friendly underwater acoustic communications with signals that are well disguised in the background noise. While the underwater acoustic spectrum is currently not regulated, it is likely that it will be in the future, both to ensure human activities have minimal impact on marine wildlife and to minimise interference with other acoustic systems. Two variations on a low bit rate spread spectrum modulation scheme using multiple pseudo-noise codes are compared, the first using conventional BPSK modulation and the second using "carrierless" modulation (bandlimited noise). Simulation shows that for a given time-bandwidth product and data rate, the carrierless modulation outperforms the BPSK system. Additional benefits from the carrierless approach are a simpler transmitter and receiver structure, reduced cost of implementation and better noise-like statistics for blending into background noise. The carrierless system is tested in recent trials in the North Sea and was effective for communication at ranges up to 4km with only 0.1W of acoustic power transmitted (160 dB re 1 mu Pa at 1m). The maximum bit rate tested for systems using length 4095 PN codes was 70bps, while for a system with length 8191 codes 35bps, with the latter system being capable of 48bps if it's full codebase capacity is used. The authors conclude that this is a potentially powerful technique that, combined with advanced error correction codes and Doppler compensation methods, could form the basis of very robust, long range, environmentally friendly acoustic modems for a variety of command/control and low volume sensor data gathering applications.