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 pseudo-noise 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 $1~\mu $ Pa 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 bio-friendly acoustic modem devices for operation in regions with sensitive fauna and/or increasingly strict environmental controls.
Highlights
U NDERWATER acoustic modems play an increasingly important role in underwater sensor networks and the control/tracking of underwater vehicles
These were conceptualised in a model for categorising the effect of noise on marine mammals devised by Richardson et al in 1995 [1] and included in the OSPAR report on impacts of anthropogenic underwater sound [2]
These results show that the receiver structure and signal resamplers are a viable solution to compensating for Doppler effects on the transmitted signal
Summary
U NDERWATER acoustic modems play an increasingly important role in underwater sensor networks and the control/tracking of underwater vehicles. Behavioural studies by Kastelein et al have shown the effect of acoustic data transmissions around 12 kHz on Harbour Porpoises (Phocoena phocoena) [4]. Chirp sounds have been shown to affect harbour porpoises [6] From this it can be determined that continuous broadband noise-like signals have less impact than chirps and impulsive or on-off switching sounds. The work by Kastelein et al shows that the signal should be long-duration and noise-like rather than tonal, chirp or burst transmissions in order to minimise behavioural response [4]–[6]. The channel capacity at −12 dB is 324.84 bit/s indicating a channel utilisation of 23.09% These schemes make use of complex receiver structures and computationally-intensive turbo decoders in order to achieve low-received-SNR communications at these data rates and SNR levels.
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