Spread-Spectrum Techniques for Bio-Friendly Underwater Acoustic Communications

Abstract

This paper investigates techniques to mitigate the impact of acoustic communication signals on marine life, by minimizing source level and designing waveforms with characteristics proven to reduce animal discomfort in bioacoustics studies. High-ratio spread spectrum transmission is employed with bandwidth-time products exceeding 1000. Signaling is based on the families of near orthogonal pseudonoise waveforms, generated by bandpass filtering of binary M-sequences. This enables reception of data, at very low SNR, over a radius many times greater than the radius of discomfort experienced by marine mammals. Computationally efficient receivers with novel synchronization structures needed to be developed to operate at very low SNR and with severe Doppler effects. Simulations show the proposed scheme is able to achieve 45 bit/s at -18-dB SNR and 140 bit/s at -12-dB SNR. Experimental system performance was assessed during realistic experiments in the North Sea, verifying performance over ranges up to 10 km with transmitted SL of <;170 dB re 1 μPa at 1 m and with Doppler effects induced by relative motion exceeding 2 m/s. Conclusion: The system developed compares favorably, in terms of SNR performance and channel utilization, with previously reported work aimed at covert communication but offers reduced transmitter/receiver complexity and discomfort to animals. This paper offers a way forward to more biofriendly acoustic modem devices for operation in regions with sensitive fauna and/or increasingly strict environmental controls.

Keywords

• Spread-spectrum
• underwater acoustic modem
• low-power
• low-received-SNR
• M-OCK
• bio-friendly