Performance Analysis of Multihop MIMO-EGC Vertical UWOC Systems With Layered Cascaded Turbulence Model

As a promising underwater communication technique for the Internet of Underwater Things (IoUT), underwater wireless optical communication (UWOC) has attracted increasing interest due to its low-link delay, high-transmission rate, and wide applications. In underwater environments, path loss, ocean turbulence fading, and pointing error fading seriously damage UWOC performance. Thus, relaying schemes have been proposed to mitigate fading in wireless optical links. In this article, we present a relay-assisted multihop vertical UWOC system employing multiple-input-multiple-output (MIMO) technology in conjunction with the equal gain combing (EGC) method over layered cascaded Gamma-Gamma turbulence channels with pointing errors. For the proposed system, we consider both fixed gain amplify-and-forward (AF) relaying and decode-and-forward (DF) relaying. The cumulative distribution function (CDF) and the probability density function (PDF) of the end-to-end instantaneous signal-to-noise ratio (SNR) are derived with the help of the single-variate Fox-H function and the bivariate Fox-H function. Further, we give the closed-form expressions of the outage probability and the ergodic channel capacity. Moreover, we provide the asymptotic analysis of the outage probability to obtain more insights into the diversity order. The diversity order is shown to depend on the turbulence parameters of each layer, the number of laser sources and photodetectors, the pointing error parameters, and the number of hops. Finally, Monte-Carlo simulation results are utilized to validate our derived results.