Outage Probability Analysis of RSMA-Based Multihop UWOC Systems With Relay Selection Over Thermocline Turbulence

Underwater wireless optical communication (UWOC) is a promising technology for the massive data transmission of the Internet of Underwater Things (IoUT) due to its high speed, low latency, and wide range of applications. However, UWOC systems face significant challenges, such as oceanic turbulence and pointing errors. Given the increasing number of underwater optical devices, interference is another challenge for the UWOC. Therefore, this paper proposes a multihop UWOC system based on rate-splitting multiple access (RSMA) with relay selection over the Weibull-generalized Gamma (WGG)-distributed oceanic thermocline turbulence channel with pointing error. The RSMA scheme is used to realize communication between a source and multiple users. Multihop relays are deployed to extend the communication distance, and relay selection is applied to improve the channel quality of underwater wireless optical links. The statistical characterization of the system is analyzed, including the probability density function (PDF) and the cumulative distribution function (CDF) of the instantaneous signal-to-noise ratio (SNR) under relay selection. Based on the statistical results, an exact closed-form expression for the outage probability is given. Moreover, the asymptotic expression of the outage probability under high SNR is provided, and the diversity order is obtained based on the asymptotic analysis of the outage probability. Simulation results are given to verify our derived expressions. The outage performance of the proposed system under various system configurations, including air bubble levels, thermohaline gradients, pointing error levels, target rates, power allocation, the number of parallel relays, and the number of hops, is analyzed. Additionally, the RSMA-based system is compared with the non-orthogonal multiple access (NOMA)-based system. It has been demonstrated that the RSMA-based system outperforms the NOMA-based system.