Performance analysis of intelligent reflecting surface assisted underwater wireless optical communication systems

Abstract

The rapid growth of underwater applications such as ocean exploration, environmental mon itoring, offshore industries, and maritime defense has created a strong demand for reliable and high-data-rate underwater communication systems. Among the available technologies, underwater wireless optical communication (UWOC) has emerged as a promising solution due to its large bandwidth and low latency compared to conventional acoustic and radio-frequency (RF) systems. However, the performance of UWOC is significantly affected by absorption, scattering, oceanic turbulence, beam misalignment, and frequent link blockages. In addition, maintaining reliable communication in non-line-of-sight (NLoS) conditions remains a critical challenge in practical underwater environments. To overcome these limitations, this thesis investigates intelligent reflecting surface (IRS) assisted architectures for enhancing the performance of UWOC systems. The work begins with the analysis of a hybrid RF–UWOC framework, where a programmable RF-based IRS is employed to improve the terrestrial RF link prior to underwater optical transmission. The results demonstrate that IRS-assisted systems provide improved spectral efficiency (SE) and energy efficiency (EE) compared to conventional relaying approaches. Building on this foundation, the thesis develops optical intelligent reflecting surface (OIRS) assisted UWOC systems using mechanically adjustable mirror arrays (MAs) to enable passive beam steering and efficient NLoS communication. A comprehensive analytical framework is established to evaluate key performance metrics, including SE, EE, outage probability (OP), and bit error rate (BER) under realistic underwater channel conditions. The results show that OIRS-assisted systems significantly outperform conventional techniques such as plane mirror surfaces (PMS) and total internal reflection (TIR)-based approaches. The work is further extended to multi-user UWOC scenarios, where efficient resource allocation becomes essential. To address this, intelligent mirror allocation strategies, namely distance-based mirror assignment (DMA) and equal mirror assignment (EMA) , are proposed to improve system throughput and user fairness. The performance of these strategies is analyzed in conjunction with multiple access schemes such as orthogonal multiple access (OMA), non orthogonal multiple access (NOMA), and rate-splitting multiple access (RSMA), demonstrating improved multi-user performance under varying channel conditions. Furthermore, advanced optical architectures, including optical simultaneously transmitting and reflecting IRS (OSTAR-IRS) and liquid-crystal-based OSTAR-IRS (LC-OSTAR-IRS), are introduced to enable enhanced flexibility and adaptive beam control. Optimization frameworks are developed to efficiently design transmission and reflection parameters, improving overall system performance in dynamic environments. Extensive simulation results validate the proposed analytical models and demonstrate significant improvements in reliability, SE, and EE. Overall, this thesis establishes OIRS assisted UWOC systems as effective and practical solutions for next-generation underwater communication networks, providing both theoretical insights and design guidelines for high capacity and energy-efficient underwater connectivity.