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RIS-Assisted Visible Light Communication Systems: A Tutorial

Sylvester Aboagye, Alain R. Ndjiongue, Telex M. N. Ngatched, Octavia Dobre, H. Vincent Poor

arXiv:2204.07198v2eess.SP

TL;DR

B5G and optical wireless systems face skip zones from signal obstructions, while RIS research for OWC remains limited. This paper provides a tutorial on RIS-assisted indoor VLC, covering optical RIS technologies, deployment, blockage and orientation mitigation, integrations, and open design challenges. It identifies RIS element orientation, AP/user assignment, array positioning, and accurate channel or position information as important research issues.

  • Problem

    Signal obstructions create skip zones in RF, VLC, and FSO systems, while relatively few studies have integrated RISs into VLC and optical wireless communication.

  • Method

    The paper presents a comprehensive tutorial reviewing VLC fundamentals, optical RIS technologies, deployment configurations, comparisons with RF-RISs and optical relays, applications, and integrations with emerging technologies.

  • Results

    The tutorial details how RISs can address line-of-sight blockages and device-orientation issues in VLC while identifying element orientation, AP/user assignment, and RIS positioning as open design problems.

  • Takeaways & Limitations

    RIS-assisted VLC is presented as a research direction for reconfigurable optical wireless systems, including applications involving MIMO, physical-layer security, NOMA, diversity, and simultaneous lightwave and power transfer.

Abstract

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Recent development of the fifth-generation (5G) of cellular networks has led to their deployment worldwide. As part of the implementation, one of the challenges that must be addressed is the skip-zone problem, which occurs when objects obstruct the transmission of signals. A signal obstruction can significantly reduce the signal-to-noise ratio in radio frequency (RF) and indoor visible light communications (VLC) systems, whereas the obstruction can completely disrupt data transmission in free-space optical (FSO) systems. Therefore, the skip-zone dilemma must be resolved to ensure the efficient operation of 5G and beyond networks. In recent years, reconfigurable intelligent surfaces (RISs) that are more efficient than relays have become widely accepted as a method of mitigating skip-zones and providing reconfigurable radio environments. However, there have been limited studies on RISs for optical wireless communication (OWC) systems. This paper aims to provide a comprehensive tutorial on indoor VLC systems utilizing RISs technology. The article discusses the basics of VLC and RISs and reintroduces RISs for OWC systems, focusing on RIS-assisted indoor VLC systems. We also provide a comprehensive overview of optical RISs and examine the differences between optical RISs, RF-RISs, and optical relays. Furthermore, we discuss in detail how RISs can be used to overcome line-of-sight blockages and device orientation issue in VLC systems while revealing key challenges such as RIS element orientation design, RIS elements to access point/user assignment design, and RIS array positioning design problems that need to be studied. Moreover, we discuss and propose several research problems on integrating optical RISs with other emerging technologies and highlight other important research directions for RIS-assisted VLC systems.

I. INTRODUCTION

B5G and OWC systems face severe signal blockage and skip-zone challenges, motivating RIS-assisted VLC. This tutorial reviews VLC fundamentals, optical RIS technologies, deployment configurations, applications, and research directions.

  • Motivation: B5G networks using high-frequency mmWave, THz, and optical bands are vulnerable to blockage, shadowing, and non-line-of-sight penetration loss.These effects can create skip zones that reduce coverage and system performance.
  • Motivation: RISs address skip-zone situations by creating reconfigurable signal paths when obstacles prevent the transmitter-to-receiver line of sight.They are presented alongside relays and cooperative communications as solutions for obstructed links.
  • VLC background: VLC transmits data through visible light and offers higher bandwidth than RF, but short range and high blockage susceptibility limit its standalone competitiveness.VLC is therefore used together with RF to alleviate RF-spectrum saturation.
  • RIS-assisted VLC: RIS-assisted VLC differs from RF-RIS because optical signals, RIS materials, and functionalities create distinct design challenges.VLC RISs must account for intensity modulation, direct detection, and simultaneous illumination and communication.
  • VLC background: VLC receivers accept data only when the incident optical signal remains within the receiver field of view.The FoV is specified by the semi-angle at half power.
  • Tutorial scope: The tutorial reviews VLC principles, channels, noise, modulation, applications, standardization, and RIS fundamentals, including IMR, IMA, and liquid-crystal RISs.It also examines optical RIS configurations inside transmitters, receivers, and along the channel, and compares them with RF-RISs and optical relays.

2) VLC signal detection:

VLC detection commonly uses photodiodes and transimpedance amplifiers with intensity modulation and direct detection. Its channel modeling covers line-of-sight and first-reflection paths, while limited receiver field of view, blockages, noise, and orientation affect reception.

  • Receiver components: Photodiodes detect wavelength-matched waveforms, while transimpedance amplifiers convert detector current into voltage for VLC processing.The IM/DD receiver structure offers advantages in cost and complexity.
  • Channel modeling: Line-of-sight VLC channel gain depends on transmitter divergence, receiver area, distance, irradiance and incidence angles, optical-filter gain, concentrator gain, and receiver field of view.The concentrator gain applies within the receiver’s field of view and is zero otherwise.
  • Channel modeling: Indoor non-line-of-sight modeling typically includes the line-of-sight path and first-reflected links because multiply reflected optical power is negligible.First-reflection gain incorporates wall reflectivity, path distances, and irradiance and incidence angles.
  • Channel impairments: Opaque obstacles can create shadows and communication outages, while photodiode field-of-view limits make reception sensitive to device orientation.These effects are characteristic vulnerabilities of VLC channels.
  • Channel impairments: Shot and thermal noise are prominent VLC noise sources, and natural or artificial light sources can interfere with the message signal.Shot noise is associated with photon-generated electron variation, while strong interfering light can drive the photodiode toward saturation.

D. Modulation and Coding Schemes

VLC modulation and coding must accommodate optical signals that are positive because LEDs and laser diodes do not permit negative current. The section distinguishes standardized schemes from optical OFDM variants and related complex or spatial modulation methods.

  • Signal constraints: VLC signals are optical and positive because LEDs and laser diodes do not allow negative current to flow.This positivity constrains signal modulation choices.
  • Modulation schemes: Standardized VLC modulation schemes include OOK, VPPM, and CSK, alongside optical variants assigned across IEEE 802.15.7 physical layers.Examples include UFSOOK, twinkle VPPM, offset VPPM, C-OOK, and HA-QL.
  • Modulation schemes: OFDM cannot be directly applied to VLC under IM/DD restrictions, motivating DC-biased, clipped, fast, and polar optical OFDM variants.These variants target real and positive transmitted signals while supporting optical adaptations of schemes such as quadrature amplitude modulation.
  • Modulation schemes: MIMO VLC systems use dual-LED and quad-LED complex modulation, while other methods include quadrature spatial modulation and dual-mode index modulation.

3) Coding schemes:

VLC coding schemes provide error correction and line coding, while VLC supports applications including LiFi, indoor positioning, and underwater optical communication research. RIS deployment is emerging as a way to reconfigure VLC environments.

  • Coding schemes: Reed Solomon, Manchester, and convolutional codes are used to correct VLC transmission errors across IEEE-associated physical layers.For PHY I, Reed Solomon serves as an outer code, convolutional coding as an inner code, and Manchester as the line code.
  • Applications: VLC applications include Internet broadcasting, LiFi, and indoor positioning in both indoor and outdoor contexts.Illumination is necessary outdoors only at night because sunlight provides daytime lighting.
  • Applications: LiFi uses VLC for the downlink and another technology, commonly RF, for the uplink in a duplex hybrid network.RF supports a fully mobile receiving node, while VLC combines lighting with downlink data transfer.
  • Applications: Indoor positioning uses the same access-point light bulbs for illumination and for determining a user’s location within a building.
  • Applications: Acoustic underwater links have low bandwidth and large delays, while RF offers higher data rates but has higher energy consumption, deployment cost, and limited distance.
  • RIS-assisted VLC: RIS use in VLC is recent and is attracting research interest because it may support reconfigurable future VLC-based wireless networks.

G. Standardization Efforts for VLC Technology

VLC standardization defines physical-layer operating modes, modulation, coding, and data-rate parameters, while RIS-assisted VLC combines reconfigurable metasurface structures with tunable optical functions. The section also identifies implementation and comparison considerations for optical RISs.

  • G. Standardization Efforts for VLC Technology: VLC regulation includes IEEE 802.15.7-2011 and later drafts, including IEEE P802.15.7/D2a for short-range optical wireless communications.
  • G. Standardization Efforts for VLC Technology: IEEE 802.15.7 organizes VLC physical layers into PHY I through PHY VI, each with specified modulation, coding, data-rate, and clock-rate characteristics.
  • G. Standardization Efforts for VLC Technology: VLC is presented as a candidate for addressing RF spectrum shortage, particularly in indoor environments.
  • RIS structure and tuning: An RIS is a reconfigurable metasurface or mirror composed of individually configurable reflecting elements that manipulate incident signals through reflection, refraction, or focusing.Element control can be real-time and coordinated through a central controller using switching components such as varactors, PIN diodes, or MEMS switches.
  • RIS structure and tuning: Optical RIS implementations include intelligent metasurface reflectors and intelligent mirror arrays, with IMRs combining a metasurface, conducting back plane, and control circuit.
  • RIS structure and tuning: Metasurfaces use subwavelength meta-atoms on flat substrates to reconfigure incident-signal phase, amplitude, and polarization, while tunable designs can change functionality without refabrication.
  • RIS structure and tuning: Liquid-crystal tuning varies permittivity or refractive index using external stimuli, whereas graphene tuning adjusts surface conductivity or chemical potential through voltage-controlled Fermi-level changes.

2) Typical tunable functionalities and applications in communication systems:

Metasurfaces and controllable mirrors provide tunable optical and electromagnetic functions, including filtering, absorption, refraction, beam steering, amplification, and polarization transformation.

  • Typical tunable functionalities and applications in communication systems:: Metasurfaces support spectral filtering, perfect absorption, refractive-index tuning, beam steering, amplification, and polarization transformation.Filtering can support multi-color VLC detection and outdoor-light-noise suppression, while perfect absorbers absorb all incident wave power under specified conditions.
  • Typical tunable functionalities and applications in communication systems:: Beam-steering metasurfaces can direct transmitted or reflected signals, supporting coverage extension and anomalous reflection.Transmitarrays steer beams from transmitters, while reflectarrays control the main direction of reflected signals.
  • Typical tunable functionalities and applications in communication systems:: Table III compares optical-RIS tuning mechanisms by their typical functionalities, operating frequency ranges, and maturity.The comparison supports selecting tuning materials for communication-system requirements.
  • Typical tunable functionalities and applications in communication systems:: Controllable mirror arrays realize RIS functionality through specular or anomalous reflection and electronically tuned yaw and roll orientations.The mirror-array orientation determines the reflected-beam direction.

D. Summary and Lessons Learned

Optical RISs can be deployed at transmitters or receivers to reconfigure light, while liquid-crystal RISs provide filtering, amplification, focusing, and field-of-view expansion in VLC.

  • Summary and Lessons Learned: Optical RIS technologies for OWC include liquid crystals, graphene, mirror arrays, and photoconductive semiconductor tuning materials.Deployment is organized by transmitter-side, receiver-side, and channel locations.
  • Summary and Lessons Learned: Transmitter-side RISs can steer beams, amplify light, extend coverage, and support configurable wavelength-division multiplexing.These functions exploit tunable optical materials and reconfigurable wavefront control.
  • Summary and Lessons Learned: Receiver-side RISs can filter incoming wavelengths, focus light onto photodetectors, improve SNR, and adjust receiver field of view.RIS configurations include parallel beams, frequency-selective filtering, and focusing onto the detector surface.
  • Summary and Lessons Learned: LC-enabled receiver RISs remove unwanted colors, improve photodetector SNR, extend transmission range, and increase receiver field of view.The reported field of view can reach approximately 90°, with LC amplification compensating for intensity reduction near that angle.
  • Summary and Lessons Learned: LC-RIS transmittance and emerged light power depend on externally applied voltage, with dye addition increasing the transmission coefficient.Experimental transmittance, emerged-power, and extended-range profiles are presented for selected LC samples.

C. RIS between the Transmitter and the Receiver

RISs placed between transmitters and receivers can create virtual optical paths, steer and shape beams, and improve coverage or SNR under blockage conditions.

  • RIS between the Transmitter and the Receiver: OWC performance is limited by obstruction of the direct line-of-sight path, motivating RIS deployment between transmitters and receivers.Blockages may be moving human or mobile obstacles, stationary objects, or outdoor structures.
  • RIS between the Transmitter and the Receiver: Between-link RIS deployments vary in the numbers of panels, elements, light sources, and photodetectors.The surveyed scenarios range from single-element, single-panel systems to multiple-panel, multi-element configurations.
  • RIS between the Transmitter and the Receiver: Optical RISs and relays can forward obstructed light, solve outdoor VLC skip zones, and improve indoor receiver SNR by focusing reflected components.Outdoor obstructed laser signals may otherwise have no reflected path, whereas indoor reflected signals remain available but weak.
  • RIS between the Transmitter and the Receiver: Optical relays use amplify-and-forward or decode-and-forward processing, whereas optical RISs manipulate light through reflection or refraction in two-dimensional mirror or metasurface structures.Mirror-based RISs can behave like unity-gain relays, but decode-and-forward RIS operation has not been demonstrated.
  • RIS between the Transmitter and the Receiver: RIS elements can be independently manipulated to create beams, giving RISs a beamforming capability beyond relays that typically create one beam.Liquid crystals provide tunable reflection and refraction through controllable refractive index.
  • RIS between the Transmitter and the Receiver: Receiver-side RISs can amplify, steer, and filter OWC signals while improving VLC receiver field of view.The cited receiver designs include convex lenses with approximately 36.2° FoV, gradient-index lenses and compound parabolic concentrators with 40° FoV, and other spherical-lens configurations.

B. Summary and Lessons Learned

RIS-assisted VLC addresses line-of-sight blockage and receiver-orientation problems while offering lower-complexity optical alternatives to relays and motivating several design challenges.

  • Summary and Lessons Learned: Optical RISs and relays can provide coverage extension, SNR improvement, and beamforming, while RISs use less energy, complexity, hardware cost, and signal processing.The paper presents this comparison as guidance for VLC performance-enhancement system design.
  • RIS for LoS Blockages and Skip Zone Problem: VLC deployment is constrained by direct-LoS requirements, limited photodetector field of view, blockages, and random receiver orientation.RISs are motivated both to address skip zones and orientation problems and to enhance transmitter and receiver performance.
  • RIS for LoS Blockages and Skip Zone Problem: RIS-enhanced VLC scenarios address self-blockage, non-user blockage, dead-zone coverage, and receiver-orientation effects.The scenarios are illustrated through transmitter-to-receiver optical redirection.
  • RIS for LoS Blockages and Skip Zone Problem: About half of users with a direct LoS path still cannot receive data when receiver incidence angles exceed the photodetector’s 85° field of view.Thus, an unobstructed LoS path alone does not guarantee successful transmission.
  • RIS for LoS Blockages and Skip Zone Problem: Open RIS design problems include element orientation, element-to-AP/user assignment, and RIS-array positioning for improving data rate, energy efficiency, spectral efficiency, and secrecy rate.These topics target practical use of RIS elements in blockage- and orientation-affected VLC systems.

1) RIS elements orientation design:

RIS element orientation is a central VLC design problem because mirror angles determine network utility, yet the resulting optimization is generally non-convex. Existing studies use metaheuristics to optimize rate, secrecy, or energy efficiency under blockage and orientation conditions.

  • RIS elements orientation design:: RIS mirror-array orientation angles are optimized because they determine VLC network utility, including data rate, secrecy rate, or energy efficiency.The orientation variables are yaw and roll angle matrices, and the objective is generally non-convex.
  • RIS elements orientation design:: The orientation optimization is generally non-convex, making globally optimal solutions difficult to obtain.Metaheuristic methods are identified as appropriate for overcoming this difficulty.
  • RIS elements orientation design:: A sine-cosine metaheuristic optimized RIS mirror angles to maximize NLoS VLC data rate under self-blockage, non-user blockage, and random device orientation.The approach addresses the high-dimensional nature of the data-rate optimization problem.
  • RIS elements orientation design:: A secrecy-rate study transformed mirror-orientation optimization into reflected-spot positioning and used modified particle swarm optimization.The objective was to enlarge the channel-gain difference between the legitimate user and eavesdropper.
  • RIS elements orientation design:: RIS mirror orientation and LED beamforming weights jointly achieved the highest secrecy rate in an RIS-assisted VLC system.The result was reported for a deep deterministic policy gradient solution.

2) RIS elements to AP/user assignment design:

RIS-assisted multi-user VLC requires assigning RIS elements to access points or users while accounting for geometry, placement, and optical propagation. Existing assignment methods improve rate or secrecy, but assumptions and joint orientation-placement design remain open issues.

  • RIS elements to AP/user assignment design:: RIS elements can be assigned to LEDs, APs, or users to maximize utility in multi-user, multi-AP VLC systems.Because VLC channels are highly sensitive to AP and user geometry, assignment depends strongly on physical positions.
  • RIS elements to AP/user assignment design:: A projected-gradient method followed by greedy recovery assigned RIS elements to LEDs for sum-rate maximization in cell-free TDMA VLC.The formulation relaxed binary association indicators before recovering a binary solution.
  • RIS elements to AP/user assignment design:: Joint user association, power allocation, and RIS-AP assignment produced significant spectral-efficiency gains over VLC without RIS and other benchmarks.The proposed solutions used frozen-variable and minorization-maximization algorithms.
  • RIS elements to AP/user assignment design:: An iterative Kuhn-Munkres method transformed LED-RIS secrecy-rate maximization into successive assignment subproblems and significantly improved secrecy performance.The transformation used objective-function approximation.
  • RIS elements to AP/user assignment design:: Only three studies address RIS array and AP/user association, while known-location and ignored-reflection assumptions may not hold in practice.Future designs should relax these assumptions and jointly consider association with orientation, placement, and multiple-access schemes.
  • RIS elements to AP/user assignment design:: RIS placement should keep the array within the AP illumination region and determine whether deployment is better near the receiver or transmitter.Array positioning greatly impacts indoor VLC performance.

VII. INTEGRATING RIS WITH OTHER TECHNOLOGIES IN VLC

RIS integration extends VLC capabilities across adaptive beam steering, physical-layer security, and other emerging technologies, but optical implementations differ fundamentally from RF RISs. The section identifies reconfigurable transmitter designs and unresolved CSI and security challenges.

  • A. Angle Diversity Transmitters and RISs: Fixed LED-array elevation angles optimize performance only for particular configurations, whereas RIS steering adapts spatial separation and beam width to changing environments.Relevant changes include user positions, user numbers, and line-of-sight blockages.
  • A. Angle Diversity Transmitters and RISs: RISs at transmitter sides can create reconfigurable angle-diversity transmitters by dynamically changing beam direction according to user number and position.This can create varying numbers of smaller, interference-free cells for multi-user access.
  • B. Physical Layer Security and RISs: RIS-assisted VLC security can use transmitter-side or channel-side deployment to focus light toward legitimate users and improve robustness to eavesdropping.Channel-side RIS placement is described as focusing reflected signals at the intended receiver.
  • B. Physical Layer Security and RISs: Transmitter-side RIS security lacks performance analysis, and suitable RIS materials for LED-front-end deployment remain unclear.The open questions concern channel-gain improvement for legitimate users and restriction of eavesdropper gain.
  • B. Physical Layer Security and RISs: For channel-side security, an eavesdropper may have a comparable or better direct LoS channel than the legitimate user, especially when closer to the transmitter.This motivates further security design for RIS-assisted VLC.
  • B. Physical Layer Security and RISs: CSI acquisition for RIS-user links remains a research issue because RIS controllers require this information to configure elements reliably.RISs are low-powered structures and cannot initiate transmission to facilitate accurate channel estimation.

C. MIMO and RISs

RIS-assisted MIMO OWC models distribute signals across multiple light sources, reflect them through multiple RIS elements, and detect them at multiple photodetectors. Capacity depends on optical constraints, physical arrangement, beam overlap, and RIS placement.

  • C. MIMO and RISs: A generalized RIS-assisted MIMO OWC system distributes data across L light sources, reflects beams through QN RIS elements, and directs them toward P photodetectors.The model captures multiple transmitter, RIS, and receiver configurations.
  • C. MIMO and RISs: The MIMO OWC input and output vectors are real-valued and non-negative, with additive independent and identically distributed receiver noise.The input represents source light intensities, while the output represents RIS-to-destination subchannel observations.
  • C. MIMO and RISs: Channel matrices depend strongly on the physical arrangements of light sources, photodetectors, and RIS elements.The transmitted signal also obeys non-negative and peak-intensity constraints with a total average optical-power bound.
  • C. MIMO and RISs: Multiple RIS elements mitigate geometric losses by converging reflected beams toward a receiver in RIS-assisted MIMO OWC configurations.The four illustrated scenarios include SISO and MIMO RIS arrangements.
  • C. MIMO and RISs: Capacity bounds are evaluated across overlapping and non-overlapping scenarios under the constraint p_o/X relative to L/2.The analysis reports upper and lower bounds while varying the common number of light sources and photodetectors.
  • C. MIMO and RISs: QR decomposition is used for overlapping VLC beams because, unlike singular-value decomposition, it does not generate negative values unsuitable for the VLC channel.For non-overlapping beams, total capacity is modeled as the sum of individually optimized channel capacities.
  • C. MIMO and RISs: Optimal RIS positioning and capacity optimization remain open research challenges for RIS-assisted MIMO OWC systems.The capacity analysis explicitly identifies both design problems.

D. NOMA and RISs

RISs can enhance NOMA-based VLC by reshaping users’ channel conditions, improving pairing opportunities, and controlling channel-gain ordering and achievable rates. However, increasing user numbers raises SIC complexity and residual-interference sensitivity.

  • NOMA fundamentals: NOMA allocates signals non-orthogonally and relies on channel-gain ordering, power allocation, and successive interference cancellation (SIC).Users with better channel conditions receive less power, while the power-allocation coefficients satisfy a total-power constraint.
  • Challenges and prior work: As the number of users increases, NOMA decoding becomes more complex because K−1 users perform SIC, while inaccurate CSI increases residual interference.The paper also summarizes prior VLC-NOMA work reporting sum-rate and BER gains over OFDMA-based VLC systems.
  • NOMA fundamentals: In VLC, user channel ordering typically follows location because channel gain depends on geometry, with nearer users receiving stronger gains.Relevant factors include incidence and irradiance angles, LED semi-angle, and AP-user distance; most factors except distance are typically fixed.
  • RIS-assisted NOMA-VLC: RIS elements can enhance distant users’ channels, thereby changing channel-gain ordering and which users achieve higher data rates according to QoS requirements.This gives the VLC network control over channel conditions that otherwise largely follow user location.
  • RIS-assisted NOMA-VLC: RISs can create more distinct channel conditions for closely located users, supporting NOMA pairing when similar VLC channels would otherwise reduce performance.NOMA performance depends strongly on pairing users with sufficiently different channel conditions.

E. Simultaneous Lightwave and Power Transfer and RISs

RIS-assisted VLC can combine lighting, data transmission, and energy harvesting to support simultaneous lightwave and power transfer. The receiver harvests energy from illumination while decoding the transmitted information.

  • Motivation: RIS-assisted simultaneous lightwave and power transfer addresses a gap in optical wireless communication, where RIS-based simultaneous data and energy transfer remains underexplored.The incident optical energy is reflected with the same orientation as the light.
  • System principles: An RIS-supported SLPT VLC system integrates lighting, data transmission, and energy harvesting through a photodetector, with or without a transimpedance amplifier.The transmitter illuminates the environment while broadcasting information, and the receiver harvests energy while decoding.
  • Energy harvesting: LEDs provide photon-to-current conversion in RIS-assisted VLC, potentially reaching 5 V with substantial current for charging a smart-device battery.The passage presents this as an example capability of the integrated system.

F. Summary and Lessons Learned

The paper identifies practical and architectural research directions for RIS-enabled VLC, including better channel prediction, hardware validation, new deployments, optical materials, and hybrid-network integration.

  • Learning approach for blockage prediction in RIS-based VLC systems: Accurate CSI assumptions make blockage prediction, user positioning, and RIS configuration computationally demanding and sensitive to estimation errors.Large RIS element counts can require significant pilot overhead, motivating sensing, deep learning, and ray-casting approaches.
  • Practical implementation and analysis: Prototype platforms are needed because existing RIS-enabled VLC studies largely report theory and simulations under idealized hardware conditions.Such platforms could compare optimization algorithms and system metrics under temperature, response, and hardware-error effects.
  • Transmitter-channel-receiver (TCaR)-RIS-assisted VLC systems: TCaR-RIS-assisted VLC systems could place RIS modules at transmitters, receivers, and channel locations for beam generation, dynamic FoV, coverage, illumination, security, and power enhancement.The paper identifies channel models, performance metrics, and low-complexity optimization as research needs for this unstudied architecture.
  • Advances in optical RISs materials: Optical RIS materials and tuning mechanisms remain insufficiently understood across wavelength, refractive-index, birefringence, and temperature conditions.Open issues include material selection, channel and noise modeling, and optimum operating conditions for metasurfaces, mirrors, and liquid crystals.
  • RIS-enabled hybrid RF/VLC and standalone VLC systems: RISs could support hybrid RF/VLC and standalone VLC networks, but simultaneous uplink and downlink enhancement with RISs remains unstudied.Potential designs include shared RIS elements using time sharing or dynamic clustering.
  • Conclusion: The review connects optical RIS physics, deployment scenarios, blockage mitigation, transmitter and receiver enhancement, optimization, and emerging-technology integration.Simulations indicate that selecting and optimizing RIS technology can reduce operational complexity and improve data-rate performance.
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