  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.
