| Summary: | In this paper, we propose non-orthogonal multiple access based cooperative relaying strategy (NOMA-CRS) for underwater acoustic sensor networks (UASNs). We analyse the performance of NOMA-CRS for both shallow and deep water scenarios, under imperfect channel state information (I-CSI) as well as imperfect successive interference cancellation (I-SIC). We derive mathematical expressions for ergodic rate, outage probability as well as the energy efficiency of NOMA-CRS in UASNs by considering the underwater specific characteristics, such as distance-dependent usable bandwidth, acoustic spreading, propagation loss, and fading effects. We compare the performance of NOMA-CRS with the widely used decode-and-forward based CRS in the UASNs. From the results, it is evident that NOMA-CRS can achieve significant improvement in ergodic sum rate and energy efficiency. But the outage performance is slightly degraded for the proposed scheme. Our results show that I-CSI and I-SIC have a significant impact on the performance of the NOMA-CRS. We also investigate the impact of relay position, wind speed as well as shipping activities on the performance of NOMA-CRS under the realistic underwater scenario. Results show that high-speed winds and high shipping activities severely degrade the performance of ergodic sum rate of the NOMA-CRS. Implementation of NOMA-CRS requires CSI at the transmitter. However, acquiring perfect CSI at the transmitter is a challenging task in time-varying multi-path underwater acoustic channels. As a solution, we also propose space-time block coded NOMA-CRS (STBC-NOMA-CRS) for UASNs, which can be implemented without CSI at the transmitter. Extensive simulation studies are conducted to corroborate the analytical findings.
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