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LiFi Through Reconfigurable Intelligent Surfaces: A New Frontier for 6G?

Hanaa Abumarshoud, Lina Mohjazi, Octavia A. Dobre, Marco Di Renzo, Muhammad Ali Imran, Harald Haas

arXiv:2104.02390v3cs.ITcs.NImath.IT

TL;DR

LiFi capacity and reception reliability are constrained by LED bandwidth, line-of-sight blockage, and receiver orientation. This article surveys RIS architecture and potential roles in LiFi transceivers and environments. It presents RIS-assisted LiFi as a route to adaptive operation and resilient connectivity, while identifying practical research needs.

  • Problem

    LiFi capacity is limited by LED modulation bandwidth and line-of-sight dependence, with blockage and receiver orientation affecting signal quality.

    Source: §I · p. 1

  • Method

    The article surveys RIS architecture and potential transceiver and environment functions for RIS-assisted LiFi.

    Source: §I · p. 1

  • Results

    The article identifies RIS-assisted LiFi as a potential means of supporting resilient connectivity and adaptive transceiver and environment functions.

    Source: A. Non-LoS Links for Resilient… · p. 5, §VII · p. 8

  • Takeaways & Limitations

    RIS could make the propagation medium an additional degree of freedom for LiFi applications in transceiver operation and network environments.

    Source: §VII · p. 8

    Realistic channel models and quantified RIS efficiency and response time are needed to assess functionalities and practical limits.

    Source: A. Modelling and Characterisation · p. 7

Abstract

from arXiv · show

Light fidelity (LiFi), which is based on visible light communications (VLC), is celebrated as a cutting-edge technological paradigm that is envisioned to be an indispensable part of 6G systems. Nonetheless, LiFi performance is subject to efficiently overcoming the line-of-sight blockage, whose adverse effect on wireless reception reliability becomes even more pronounced in highly dynamic environments, such as vehicular application scenarios. Meanwhile, reconfigurable intelligent surfaces (RIS) emerged recently as a revolutionary concept that transfers the physical propagation environment into a fully controllable and customisable space in a low-cost low-power fashion. We anticipate that the integration of RIS in LiFi-enabled networks will not only support blockage mitigation but will also provision complex interactions among network entities, and is hence manifested as a promising platform that enables a plethora of technological trends and new applications. In this article, for the first time in the open literature, we set the scene for a holistic overview of RIS-assisted LiFi systems. Specifically, we explore the underlying RIS architecture from the perspective of physics and present a forward-looking vision that outlines potential operational elements supported by RIS-enabled transceivers and RIS-enabled environments. Finally, we highlight major associated challenges and offer a look ahead toward promising future directions.

I. TOWARDS INTELLIGENT LIFI

LiFi’s capacity and reliability face limits from LED bandwidth, line-of-sight dependence, and receiver orientation, motivating a move beyond fixed transceiver designs. The article introduces RIS as a way to make LiFi operation and propagation more adaptable, and surveys its opportunities and challenges.

I. TOWARDS INTELLIGENT LIFI

  • LiFi capacity is limited by LED modulation bandwidth and dependence on line of sight, while MIMO can be hindered by correlated spatial subchannels.Blockage and random receiver orientation also affect signal quality.Source: §I · p. 1
  • The article presents RIS integration as a way to adapt LiFi operation to blockage and receiver orientation and surveys its opportunities, applications, and challenges.The authors describe this as the first overview of RIS integration in LiFi.Source: §I · p. 1

A. RIS: A Disruptive Concept

  • A RIS metasurface alters wireless propagation by tuning each element’s electromagnetic response through electrical voltage stimulation.Prior work cited in the article considers RIS for maintaining line of sight in free-space optical systems and focusing light in indoor VLC.Source: A. RIS: A Disruptive Concept · pp. 1–2

B. Metasurfaces From the View of Physics

Metasurfaces can engineer visible-light propagation through several forms of light manipulation. RIS tunability could also make transceiver optics adaptable, though the optics described for current LiFi transceivers are fixed.

B. Metasurfaces From the View of Physics

  • Metasurfaces can tune refractive index, steer anomalous reflections, amplify or attenuate light, and control different wavelengths independently.The article presents these capabilities as a basis for its vision of metasurfaces in LiFi transceivers.Source: B. Metasurfaces From the View of Physics · p. 2

II. RIS FOR ADAPTIVE TRANSCEIVERS

  • Large photodetectors can improve detection by collecting more received photons, but their increased capacitance lowers 3 dB bandwidth and limits high-data-rate use.LiFi receivers therefore pair small-area photodetectors with lenses that focus light onto the detector.Source: §II · p. 2
  • Current LiFi transceiver optics have predetermined fields of view, amplification factors, and operating wavelengths, whereas RIS tunability could make these characteristics dynamically adjustable.A wider transmitter field of view improves coverage and illumination uniformity but reduces received optical intensity; a narrow field of view preserves optical power but requires precise alignment.Source: §II · p. 2

A. Beam Steering and Amplification

RIS concepts for LiFi transceivers include receiver-side steering and amplification and wavelength-tunable WDM. The WDM concept uses RIS tuning to adjust filtering and wavelength weights for the transmission scheme.

A. Beam Steering and Amplification

  • A liquid-crystal-array RIS receiver concept replaces the conventional convex lens with a surface that steers and locally amplifies the incident beam, extending coverage by a few meters.The RIS is globally passive, and amplification must avoid driving the photodetector into saturation; RIS beam steering is also proposed for transmitters.Source: A. Beam Steering and Amplification · p. 2

B. Configurable Wavelength Division Multiplexing (WDM) Transceivers

  • RIS elements could be tuned for different wavelengths to provide dynamic interference filtering and wavelength weights that optimise effective signal-to-interference ratio in WDM transceivers.Realising tunable WDM transceivers requires metasurfaces whose responses can be finely tuned across wavelengths.Source: B. Configurable Wavelength Division… · p. 4

C. Dynamic Angular Diversity

Fixed angular-diversity transmitters may not provide enough subchannel separation or beam adaptation for changing user distributions. The section proposes RIS-based adjustment of beam width and divergence to serve more users with less interference.

C. Dynamic Angular Diversity

  • Angular-diversity transmitters use multiple LEDs aimed in different directions to create multi-user sub-cells, but overlapping beams can cause interference and weak diversity gains.The section identifies adapting beam separation and width to user number and location as a need.Source: C. Dynamic Angular Diversity · p. 4
  • RIS could dynamically change optical-beam width and divergence, allowing more simultaneous users with less interference and higher area spectral efficiency..Source: C. Dynamic Angular Diversity · p. 4

III. RIS-ENABLED ENVIRONMENTS

  • The article frames RIS-enabled LiFi environments as a setting for potential applications.Source: §III · p. 4

A. Non-LoS Links for Resilient Connectivity

LiFi links rely heavily on line of sight, while typical indoor non-line-of-sight paths are weak and uncontrolled. RIS reflections are presented as a way to compensate for blockage and improve coverage for cell-edge users.

A. Non-LoS Links for Resilient Connectivity

  • LiFi link quality depends on line-of-sight paths, and typical indoor non-line-of-sight signals are weak and nearly isotropic.Blockage can significantly deteriorate or interrupt reception, particularly for mobile users.Source: A. Non-LoS Links for Resilient… · p. 4
  • RIS reflections could compensate for line-of-sight blockage and support connectivity despite mobility, link blockage, and random device orientations.Source: A. Non-LoS Links for Resilient… · p. 5

B. Uniform Coverage for Seamless User Experience

  • Wall-mounted RIS reflections can provide meaningful link quality to room-edge users whose direct received power from a central access point may be low or absent.LiFi's typical beam spans 2–3 m in diameter; weak cell-edge links can prompt frequent horizontal or vertical handovers.Source: B. Uniform Coverage for Seamless User… · p. 5

C. Spatial Decorrelation for Enhanced MIMO

LiFi MIMO channels can be rank-deficient or ill-conditioned because optical propagation is strongly determined by transceiver geometry. RIS-controlled reflections are proposed both to improve spatial decorrelation and to provide an additional modulation dimension, with practical performance dependent on distinguishable channel combinations.

C. Spatial Decorrelation for Enhanced MIMO

  • Geometry-driven, deterministic optical channels can be rank-deficient or ill-conditioned, creating a bottleneck for LiFi MIMO configurations.The issue is especially relevant to massive MIMO, where front-ends are closely spaced.Source: C. Spatial Decorrelation for Enhanced… · p. 5
  • RIS-controlled reflected paths could make otherwise equivalent MIMO subchannels more distinguishable, improving channel rank and enabling higher spectral efficiency and data rates.Source: C. Spatial Decorrelation for Enhanced… · p. 5

D. RIS-Based Modulation

  • RIS-based modulation generalizes spatial modulation by encoding data through reflecting-element states, adding Nr bits per symbol to an M-ary LED signal's spectral efficiency.The resulting total spectral efficiency is Nr + log2(M); Fig. 6 depicts a LiFi transceiver with an RIS containing Nr reflecting elements.Source: D. RIS-Based Modulation · p. 5, Figure 6 · p. 6
  • Theoretical spectral-efficiency growth with reflecting-element count does not guarantee better error performance unless the resulting channel combinations are distinguishable.The receiver must estimate each reflecting element's channel state and be trained with pilots for all RIS combinations.Source: D. RIS-Based Modulation · p. 5
  • Fig. 5 compares simulated channel-gain distributions for LED line-of-sight-only operation and LED operation assisted by RIS on all four walls.The caption specifies a 5 × 5 × 3 m³ room and 1 m² of RIS coverage on each wall.Source: Figure 5 · p. 6
  • A fixed RIS can receive its data through the LiFi backhaul, such as a wall-mounted indoor unit connected to the LEDs' power-line link.The paper also describes a traffic-light-mounted RIS for reflecting modulated signals to vehicular receivers.Source: D. RIS-Based Modulation · p. 6

E. Non-Orthogonal Multiple Access (NOMA)

In LiFi, users can have similar channel conditions because optical channel gain depends largely on their locations, which can hinder power-domain NOMA. RIS-assisted control of multipath propagation is proposed to adjust users' perceived gains for reliability and fairness benefits.

E. Non-Orthogonal Multiple Access (NOMA)

  • LiFi users can have similar channel conditions because optical channel gain depends almost entirely on user location, hindering power-domain NOMA.NOMA's spectral-efficiency gain depends on distinct channel conditions among users.Source: E. Non-Orthogonal Multiple Access (NOMA) · p. 6
  • RIS control of multipath propagation could dynamically alter the channel gains perceived by different users.Source: E. Non-Orthogonal Multiple Access (NOMA) · p. 6
  • Such gain control is proposed to improve reliability through effective power allocation and successful successive interference cancellation, and fairness by changing users' decoding order.Users with lower decoding order otherwise decode with interference, which may leave their rates short of QoS requirements.Source: E. Non-Orthogonal Multiple Access (NOMA) · p. 6

F. Physical Layer Security (PLS)

The section outlines RIS-assisted physical-layer security and light-based monitoring, including how reconfigurable reflections may support secrecy and localization.

F. Physical Layer Security (PLS)

  • RIS-assisted physical-layer security could use dynamic path tuning, jamming, and secure beamforming to protect LiFi transmissions.LiFi links remain vulnerable to eavesdroppers beneath the same access point, including in public and vehicular settings.Source: F. Physical Layer Security (PLS) · pp. 6–7
  • Further work must assess secrecy capacity against RIS locations and capabilities, outdoor ambient noise, and RIS-based user authentication.Source: F. Physical Layer Security (PLS) · p. 7

A. RIS-Aided Light-based Monitoring

  • Existing light-sensing methods use paths between multiple optical transmitters and receivers to reconstruct point clouds for localization.Source: A. RIS-Aided Light-based Monitoring · p. 7
  • RIS-controlled reflections create object-dependent spatial signatures that can improve position-to-measurement mapping for indoor and outdoor localization.Examples include rehabilitation, gesture recognition, manufacturing, and distance measurement for safety and surveillance.Source: A. RIS-Aided Light-based Monitoring · p. 7

B. Simultaneous Light-Wave Information and Power Transfer (SLIPT)

The section discusses RIS-assisted SLIPT, in which LiFi supports information decoding and energy harvesting, and identifies transceiver tuning and resource allocation as ways to balance them.

B. Simultaneous Light-Wave Information and Power Transfer (SLIPT)

  • SLIPT uses visible light from LEDs for simultaneous information decoding and energy harvesting, extending the lifetime of energy-constrained terminals.Source: (SLIPT) · p. 7
  • SLIPT transceivers must select a suitable splitting factor for decoding and harvesting, while energy-harvesting efficiency also depends on receiver light-collection area.Source: (SLIPT) · p. 7
  • RIS-enabled beam steering and amplification, combined with splitting-factor optimization, could balance decoding and harvesting demands.Joint active and passive beamforming could configure multiple streams and control transmission rates, light power, and inter-user interference dynamically.Source: (SLIPT) · p. 7

A. Modelling and Characterisation

The section identifies practical modelling and control requirements for RIS-assisted LiFi, including accurate channel characterization and real-time estimation and optimization.

A. Modelling and Characterisation

  • Realistic channel models, response-time and efficiency measurements, and practical implementations are needed to characterize performance across outdoor and underwater settings.The discussion highlights vehicle links, underwater attenuation, and outdoor ambient noise as relevant conditions.Source: A. Modelling and Characterisation · p. 7

B. Real-Time Estimation and Optimisation

  • Dynamic RIS configuration requires accurate channel-state information for direct and reflected paths, while distributed and centralized control trade local autonomy against energy use, hardware simplicity, and signaling overhead.The trade-off needs quantification for practical strategies across applications.Source: B. Real-Time Estimation and Optimisation · pp. 7–8

A. Interplay of machine learning (ML) and RIS-Assisted LiFi

The section discusses machine-learning needs and hybrid LiFi-RF integration for RIS-assisted networks, concluding that further research is needed to establish capabilities, limitations, and practical deployment requirements.

A. Interplay of machine learning (ML) and RIS-Assisted LiFi

  • RIS coordination and time-critical processing involve many parameters and sensed data, while conventional machine learning may not meet the real-time needs of highly dynamic, ultra-low-latency applications.The paper calls for mechanisms addressing training and communication overhead and processing delays.Source: A. Interplay of machine learning (ML)… · p. 8

B. RIS-Assisted Hybrid LiFi-RF Networks

  • RIS-assisted LiFi and RF networks could combine their respective capabilities, but require load balancing, resource and energy management, cross-layer tools, and cross-band selection methods.Source: B. RIS-Assisted Hybrid LiFi-RF Networks · p. 8

VII. CONCLUSION

  • The paper frames propagation as an additional degree of freedom for wireless applications and calls for more research on RIS-LiFi capabilities, limitations, and practical deployment.Source: §VII · p. 8
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