Optical STAR-RIS-Aided VLC Systems: RSMA Versus NOMA

TL;DR

This paper tackles blockage and device-orientation challenges in indoor VLC by deploying an optical STAR-RIS (OSTAR-RIS) that combines mirror-array reflectors and LC-based refractors. It analyzes two multiple-access schemes, RSMA and NOMA, and solves a joint optimization of STAR-RIS angles and refractive index via a sine-cosine meta-heuristic to maximize the sum rate and energy efficiency. Results show RSMA-aided OSTAR-RIS VLC consistently outperforms the NOMA counterpart across power, wavelength, and user configurations, with significant gains in $R_{sum}$ and energy efficiency. The work demonstrates the practical potential of optical STAR-RIS for robust, high-rate indoor VLC in the presence of blockers and user orientation effects.

Abstract

A critical concern within the realm of visible light communications (VLC) pertains to enhancing system data rate, particularly in scenarios where the direct line-of-sight (LoS) connection is obstructed by obstacles. The deployment of meta-surface-based simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) has emerged to combat challenging LoS blockage scenarios and to provide 360 coverage in radio-frequency wireless systems. Recently, the concept of optical simultaneous transmission and reflection reconfigurable intelligent surface (OSTAR-RIS) has been promoted for VLC systems. This work is dedicated to studying the performance of OSTAR-RIS in detail and unveiling the VLC system performance gain under such technology. Specifically, we propose a novel multi-user indoor VLC system that is assisted by OSTAR-RIS. To improve the sum rate performance of the proposed system, both power-domain non-orthogonal multiple access (NOMA) and rate splitting multiple access (RSMA) are investigated in this work. To realize this, a sum rate maximization problem that jointly optimizes the roll and yaw angles of the reflector elements as well as the refractive index of the refractor elements in OSTAR-RIS is formulated, solved, and evaluated. The maximization problem takes into account practical considerations, such as the presence of non-users (i.e., blockers) and the orientation of the recipient's device. The sine-cosine meta-heuristic algorithm is employed to get the optimal solution of the formulated non-convex optimization problem. Moreover, the study delves into the sum energy efficiency optimization of the proposed system. Simulation results indicate that the proposed OSTAR-RIS RSMA-aided VLC system outperforms the OSTAR-RIS NOMA-based VLC system in terms of both the sum rate and the sum energy efficiency.

Figures (6)

• Figure 1: An illustration of the proposed -aided system.
• Figure 2: Sum rate versus optical transmit power. At $\lambda = 510$ nm.
• Figure 3: Sum rate versus wavelength of the transmitted light signal. At $p = 3$ Watts.
• Figure 4: Sum rate versus number of intended users. At $\lambda = 510$ nm, and $p = 3$ Watts.
• Figure 5: Sum rate versus optical transmit power for different power allocation strategies for the scheme. At $\lambda = 510$ nm and $p = 3$ Watts.