Performance Analysis of Reconfigurable Intelligent Surface-Assisted Underwater Wireless Optical Communication Systems

With the in-depth exploration of the ocean, the Internet of Underwater Things (IoUT) technology has attracted increasing attention. Underwater wireless optical communication (UWOC) provides the possibility of massive data transmission for IoUT. The underwater optical link of UWOC can be interrupted by the obstruction of marine animals and plants, seamounts, and some underwater equipment. In this paper, we present a reconfigurable intelligent surface (RIS) assisted UWOC system to solve the problem of link occlusion. The cascaded turbulence channel fading coefficients from source to destination through RIS are modeled as Gamma-Gamma distribution, and the pointing errors caused by beam jitter and RIS jitter are considered. In the RIS-assisted UWOC systems with intensity modulation and direct detection (IM-DD), we apply the approximate distribution of the sum of Gamma-Gamma random variables to derive the probability density function (PDF) of the instantaneous received signal-to-noise ratio (SNR) for the first time. Based on the PDF, the novel closed-form expression of outage probability is given, and the outage probability analysis results show that when the number of reflecting elements is very large (i.e., tends to infinity), the outage probability asymptotically approaches zero. Subsequently, the closed-form expressions of the finite-SNR diversity order, the asymptotical diversity order, and the convergence speed of finite-SNR diversity order to asymptotical diversity order are proposed. Additionally, the ergodic channel capacity expression is investigated. Finally, we give simulation results to validate our derived results and analyze the impacts of the number of reflecting elements of RIS and the pointing errors on the above system performance indicators.