Optimization design of liquid crystal-based reconfigurable intelligent surface-assisted indoor visible light communication systems

Visible light communication (VLC) is a promising technology for sixth-generation (6G) networks, offering a broad, license-free spectrum, enhanced security, and protection from radio frequency (RF) interference. This makes VLC an excellent complement to traditional RF communication. VLC is mainly used indoors, typically covering distances ranging between 2 and 5 meters. A major challenge in VLC is increasing data rates, especially when obstacles block the direct line of sight (LoS). To overcome such obstacles, reconfigurable intelligent surfaces (RISs) have been effectively used in wireless networks. Meta-surface-based simultaneous transmission and reflection reconfigurable intelligent surfaces (STAR RISs) have emerged to address LoS blockage and provide 360° coverage in radio-frequency wireless systems. The latest advancement is the proposal of optical simultaneous transmission and reflection reconfigurable intelligent surfaces for VLC systems. Liquid crystal (LC) RISs can electronically tune their physicochemical properties, such as the refractive index, by altering the orientation of the LC molecules using an external electric field. This fine-tuning capability enables LC RISs to precisely direct incident light beams, ensuring that the refracted beams perfectly align with the photodetector's field of view. This paper introduces a VLC downlink communication system based on indoor power domain nonorthogonal multiple access technology. The system is designed and optimized to include a STAR RIS in the channel and LC RIS-based receivers with co-assisted composition. By utilizing the energy splitting (ES) protocol, STAR RIS is deployed in the transmission channel, while LC RIS-based VLC receivers are employed at the receiver side. Both components significantly enhance the optical signal, improving overall system performance. The system performance is evaluated by solving a sum-rate maximization problem that accounts for practical scenarios such as the presence of nonuser-obscured LoS paths between the transmitter and the receiver. A low-complexity algorithm using fractional programming and step-by-step optimization is recommended for managing perfect channel state information. Owing to the nonconvexity of the objective function, it is first transformed into a multinomial fractional planning problem using the Lagrangian dual transform, making it more tractable. Following this, a step-by-step optimization scheme combined with the CVX tool is used to obtain a feasible solution. This paper also evaluates how the number of light-emitting diodes (LEDs), the optical signal wavelength, and the LC refractive index affect the performance of indoor VLC systems. The final numerical simulation results show that the sum rate of the indoor VLC system incorporating both STAR RIS and LC RIS is significantly higher compared to systems using only reflective RISs and no LC RISs, especially under the ES operation protocol. Additionally, system performance is influenced by the number of LEDs, incident light wavelength, and LC refractive index. These findings provide an important reference for the design and optimization of indoor VLC systems.