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

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... erwater 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. INDEX TERMS Cooperative relaying strategy, ergodic rate, energy efficiency, outage probability, power-domain non-orthogonal multiple access, space-time block codes, underwater acoustic sensor networks. Recent advances in underwater wireless sensor networks (UWSNs) lead to a variety of oceanic applications, which include ocean exploration, underwater multimedia, military surveillance system, assisted navigation, pollution control, and much more. Nowadays, UWSNs also support the Internet of Things, which has been described as the Internet The associate editor coordinating the review of this manuscript and approving it for publication was Giovanni Pau . of underwater things (IoUT) [1] . IoUT consists of multiple underwater objects, which are interconnected through a wireless communication medium. In particular, IoUT allows applications to monitor the vast unexplored marine areas [1]- [3] . UWSNs may consist of both mobile and stationary nodes that exchange information, such as control, telemetry, speech, and video signals among themselves as well as to a central node located on or offshore. Such diverse and data-intensive underwater applications demand higher bandwidth as well as data rate [4] . Innovative physical (PHY) layer