Phase Optimization and Performance Evaluation of Multi-STAR-RIS-Aided Underwater Optical Wireless Communication System

This article proposes a multiple simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided underwater optical wireless communication (UOWC) system, where the STAR-RISs can transmit and reflect data simultaneously in two modes of operation: 1) energy splitting (ES) and 2) mode switching (MS). Such a system enables us to broadcast data and enhance the coverage at reduced error rates even for moderate turbulence in UOWC systems. The phase shifts of the multiple STAR-RISs are intricately coupled, for which the optimization problem is formulated for both ES and MS modes, and solved by particle swarm optimization (PSO) and differential evolution (DE) techniques. It is demonstrated that PSO can obtain more sum-rate as compared to DE. Hence, the phase shift values obtained from PSO are utilized to validate the ensuing performance evaluation in terms of average bit error rate (ABER) and channel capacity, for which tractable expressions are derived in terms of bivariate Fox-H function. Further, outage probability expressions are derived in terms of Meijer-G function. Finally, the ABER results reveal that our proposed system can generate a minimum signal-to-noise ratio (SNR) gain of 5 dB over a conventional single RIS system.