Smart Element Allocation Strategies for Dynamic Optical IRS in Underwater Wireless Communication

Optical intelligent reflecting surfaces (OIRSs) represent a paradigm shift towards improving the optical wireless communications capabilities. The application of this developing technology in the field of underwater wireless communication (UWC) systems is a promising way to overcome the limitations associated with such challenging environments. In this research, we analyze the performance improvement provided by OIRS in the context of non-line-of-sight (NLOS) underwater wireless optical communication (UWOC). The OIRS support multiple users by grouping and allocating different OIRS elements to different users based on their distance from the transmitter. To increase the aggregate spectral efficiency (ASE) and preserve user fairness, techniques based on equal mirror assignment (EMA) and distance-based mirror assignment (DMA) are presented. The outcomes are evaluated against conventional orthogonal multiple access (OMA), non-orthogonal multiple access (NOMA) and rate split multiple access (RSMA) schemes for UWOC. The performance variation of the overall system is also analyzed with respect to the ocean depth. We have also derived the analytical expressions for the probability density function (PDF) of the signal-to-noise ratio (SNR) considering exponential and generalized gamma (EGG) turbulent model and zero bore-sight model for pointing errors. We use Monte-Carlo simulation results to validate our analytical expressions. Furthermore, an asymptotic analysis of average spectral efficiency is performed to provide a deeper understanding of the proposed system's performance at high SNR. From the results obtained, it is analyzed that proposed DMA based scheme outperforms the EMA, OMA, NOMA and RSMA even with a large number of users. Furthermore, it is shown that the system performance improves with the oceanic depth. The results of this work provide valuable insights for the design of UWOC systems in challenging underwater environments.

I. Introduction

Underwater communication, essential for various industrial, military, and scientific endeavors, has witnessed significant advancements over time. Initially, communication relied heavily on wired techniques, employing cables or fiber-based systems. While these methods offer high speed and reliability, they face challenges in difficult-access locations and deep-sea environments, limiting the range and maneuverability of underwater vehicles [1]. To overcome these limitations, researchers have turned to wireless communication methods. These techniques utilize radio frequency (RF) or acoustic waves to transmit data within underwater environments. However, each approach comes with its own set of challenges. RF waves suffer from significant attenuation in ocean water, while acoustic waves encounter issues such as multipath interference, bandwidth limitations, and propagation delays [2].