Optimized Design of Joint Mirror Array and Liquid Crystal-Based RIS-Aided VLC Systems

Abstract

Most studies of reflecting intelligent surfaces (RISs)assisted visible light communication (VLC) systems have focused on the integration of RISs in the channel to combat the line-ofsight (LoS) blockage and to enhance the corresponding achievable data rate. Some recent efforts have investigated the integration of liquid crystal (LC)-RIS in the VLC receiver to also improve the corresponding achievable data rate. To jointly benefit from the previously mentioned appealing capabilities of the RIS technology in both the channel and the receiver, in this work, we propose a novel indoor VLC system that is jointly assisted by a mirror array-based RIS in the channel and an LC-based RIS aided-VLC receiver. To illustrate the performance of the proposed system, a rate maximization problem is formulated, solved, and evaluated. This maximization problem jointly optimizes the roll and yaw angles of the mirror array-based RIS as well as the refractive index of the LC-based RIS VLC receiver. Moreover, this maximization problem considers practical assumptions, such as the presence of non-users blockers in the LoS path between the transmitter-receiver pair and the user's random device orientation (i.e., the user's self-blockage). Due to the non-convexity of the formulated optimization problem, a low-complexity algorithm is utilized to get the global optimal solution. A multi-user scenario of the proposed scheme is also presented. Furthermore, the energy efficiency of the proposed system is also investigated. Simulation results are provided, confirming that the proposed system yields a noteworthy improvement in data rate and energy efficiency performances compared to several baseline schemes.Index Terms-Reflecting intelligent surface (RIS), visible light communication (VLC), liquid crystals (LCs), random receiver orientation, achievable rate, energy efficiency (EE), line of sight (LoS) blockage. I. INTRODUCTIONT HE exponential increase in connected devices and the ongoing development of wireless applications are driving the need to explore alternative wireless communications options to radio frequency (RF) communications. Visible light communications (VLC) has emerged as a bandwidth-abundant, cost-effective, and secure communications technology. Subsequently, VLC is seen as a promising candidate for complementing RF communications in future wireless networks [1].In ordinary VLC systems, the line-of-sight (LoS) availability between the transmitter and the receiver is essential for

Sum Rate and Fairness Optimization in RIS-Assisted VLC Systems

“…The utilization of RISs in VLC networks enables strengthening the link quality and robustness by introducing enhancements in the NLOS channel gain, which depends on the characteristics of the elements used for building the RIS array [4]. In general, the RIS elements in the optical domain can be classified according to their impact on directing the incident signals into reflecting and refracting elements [4]- [6]. In this work we consider the use of reflecting elements, which can rely on specular or diffuse reflections for directing the incident signals, and evaluate the advantages provided by each type.…”

“…In particular, the use of RIS arrays in VLC networks enables increasing the achievable network throughput and minimizing the impacts of LOS link blockage and shadowing by enhancing the non-LOS (NLOS) channel gain. This could help in improving the VLC link quality and reliability [3], according to the characteristics of the used RIS array [4]- [6]. Based on the response induced by the RIS elements to the impinging light signals, optical RISs can be classified into specular reflecting, diffuse reflecting, and refracting RISs [4].…”

Relay Selection for Improving Physical Layer Security in D2D Underlay Communications

“…In RIS-aided VLC systems, phase control is not directly applicable, as these systems operate with intensity modulation and direct detection (IM/DD), focusing solely on the non-negative and real part of the signal [107]. There are three primary approaches to incorporating RIS into VLC systems, namely, mirror array-based RIS [107]- [111], metasurface-based RIS [112]- [116], and liquid crystal (LC)-based RIS [117]- [120]. Both the mirror array-based and metasurface-based optical RISs consist of nearly-passive elements in the form of mirror arrays or intelligent metasurfaces used to control incident lights based on the light reflection or refraction principle and the manipulation of the EM waves, while the latter uses LCs as front-row materials embedded in a layered structure to control light reflection and/or refraction.…”

“…3) Liquid Crystal-based RIS: This type of RIS primarily consists of LC materials due to their duplex transparency which can be used as reflectors and/or refractors [117], [118], since an RIS element can smoothly steer and amplify the light signal owing to its electrically controllable birefingence property. Furthermore, RIS-based LC materials can be placed (a) in front of the light emitting diode (LED) arrays of the VLC transmitter for high data rates and adequate illumination performance [119]; (b) in the VLC receiver for light amplification and beam steering [120]. The integration of LC in RIS-aided systems can greatly contribute to satisfying the joint illumination and communication needs of indoor VLC technology.…”