On the Design of Noncoherent Acoustic Underwater Communication

QC 231003

Abstract

The underwater domain is an environment hostile to humans due to the hydrostatic pressure that rapidly increases with water depth, which has led to underwater robotics becoming an emerging technological field with many commercial-, environmental-, and security-related applications. A major challenge to untethered autonomous underwater vehicles (AUVs) is communicating robot-to-robot and robot-to-topside operator since it must, in most cases, be done acoustically. Meanwhile, the underwater acoustic (UWA) channel is widely considered one of nature's most difficult communication mediums due to the limited frequency range, complicated sound propagation physics, prolonged- and time-varying multipath, and, in some situations, non-Gaussian background noise. The wide variety of UWA channels observed in different locations, and in the same location at different times, also poses a challenge to the research methodology since sea experiments become inherently difficult to repeat. However, replay simulation of recorded channels using the public benchmark Watermark allows direct comparison between publications and is employed extensively in this thesis, complemented by sea experiments for verifying the internal validity of simulation results.

No link solution is perfect for all channels, and a toolbox consisting of methods with varying information rates and robustness is necessary for an adaptive network to exploit the full capability of the channel encountered in situ. A link is called channel-agnostic if its robustness is limited by the ratio of signal power to noise (SNR), rather than the channel characteristics, thereby being robust to many channels one might encounter. The proven information rates for such link methods are relatively low; this, together with recent advancements in other areas, such as error correction codes and joint synchronisation and decoding, motivates the focus on noncoherent methods in this thesis. The second focus is link adaptation and the necessary mechanisms for its implementation in adaptive UWA networks.

The thesis provides a background on various approaches to acoustic signalling, both coherent and noncoherent, and other key components of a noncoherent UWA link, such as symbol alphabets, receiver data models, error correction codes, and time-Doppler synchronisation. The topic of link adaptation in a UWA network is discussed, as is the methodology for research in UWA communication. The included papers provide a set of channel-agnostic link methods with spectral efficiencies in the range 0.02-0.22 (bit/s/Hz), with varying requirements on the SNR and the length of the communication frame, which are enabled by the presented improvements to link methods. Using a Rice-fading model for soft decoding, the robustness to parameter time variation is found to increase substantially by limiting the SNR of the likelihood parametrisation; this result applies to all methods that employ frequency shift keying (FSK). Furthermore, a novel noncoherent symbol alphabet with 1 (bit/s/Hz) maximum spectral efficiency is presented, whose dimensionality M increases the soft decoder performance, specialising to on-off keying (OOK) for M=1. A joint synchronisation and decoding framework is proposed, allowing robust time-Doppler detection with low overhead; its viability is demonstrated in an adverse shallow-water channel with relative platform velocities in the range +/- 4 (m/s). Moreover, a framework for efficient evaluation of link adaptation algorithms is presented, and a link-adaptive ad-hoc UWA network using low-latency implicit feedback is demonstrated through sea experiments.

The research presented herein has been conducted as part of the Swedish Maritime Robotics Centre (SMaRC), a national cross-disciplinary research centre funded by the Swedish Foundation for Strategic Research (SSF). 

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-337374