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Hybrid RF/VLC Systems: A Comprehensive Survey on Network Topologies, Performance Analyses, Applications, and Future Directions

Hisham Abuella, Mohammed Elamassie, Murat Uysal, Zhengyuan Xu, Erchin Serpedin, Khalid A. Qaraqe, Sabit Ekin

arXiv:2007.02466v3eess.SPcs.SYeess.SY

TL;DR

The paper addresses the need to synthesize research on hybrid RF/VLC systems across their topologies, analyses, and applications. It surveys these systems and concludes that application needs should guide topology and resource choices, while further analyses and simulations remain needed.

  • Problem

    The survey addresses the need to compare hybrid RF/VLC studies across network topologies, employed technologies, features, and research challenges.

    Source: §I · p. 3

  • Method

    The paper surveys hybrid RF/VLC topologies, performance analyses, applications, implementations, challenges, and future directions.

    Source: §I · p. 3

  • Results

    Hybrid RF/VLC networks use varied topologies, handover and load-balancing schemes, modulations, and access methods; further access-method and load-balancing analyses and simulations are still needed.

    Source: §V · p. 25

  • Takeaways & Limitations

    Network specifications should match each use case's application needs and available resources, with future networks envisioned as smarter cognitive systems.

    Source: §V · p. 25

Abstract

from arXiv · show

Wireless communications refer to data transmissions in unguided propagation media through the use of wireless carriers such as radio frequency (RF) and visible light (VL) waves. The rising demand for high data rates, especially, in indoor scenarios, overloads conventional RF technologies. Therefore, technologies such as millimeter waves (mmWave) and cognitive radios have been adopted as possible solutions to overcome the spectrum scarcity and capacity limitations of the conventional RF systems. In parallel, visible light communication (VLC) has been proposed as an alternative solution, where a light source is used for both illumination and data transmission. In comparison to RF links, VLC links present a very high bandwidth that allows much higher data rates. VLC exhibits also immunity to interference from electromagnetic sources, has unlicensed channels, is a very low power consumption system, and has no health hazard. VLC is appealing for a wide range of applications including reliable communications with low latency such as vehicle safety communication. Despite the major advantages of VLC technology and a variety of its applications, its use has been hampered by its cons such as its dependence on a line of sight connectivity. Recently, hybrid RF/VLC systems were proposed to take advantage of the high capacity of VLC links and better connectivity of RF links. Thus, hybrid RF/VLC systems are envisioned as a key enabler to improve the user rates and mobility on one hand and to optimize the capacity, interference and power consumption of the overall network on the other hand. This paper seeks to provide a detailed survey of hybrid RF/VLC systems. This paper represents an overview of the current developments in the hybrid RF/VLC systems, their benefits and limitations for both newcomers and expert researchers.

I. INTRODUCTION

The introduction motivates hybrid RF/VLC networks as complementary systems that combine VLC’s strengths with RF connectivity. It positions the survey as a synthesis of hybrid-system designs, analyses, applications, and open directions.

I. INTRODUCTION

  • The introduction describes OCC as a camera-based optical communication approach covered by IEEE 802.15.7-2018, with applications including V2X and positioning.Commercial cameras operating at 30 FPS achieve rates from several bps to several kbps, while high-speed cameras can address this limitation.Source: §I · p. 2
  • Hybrid RF/VLC systems combine RF with VLC to address VLC limitations while using the technologies as complements.The introduction points to standalone RF and VLC strengths and weaknesses as motivation for combining them.Source: §I · p. 2
  • Prior surveys leave gaps in coverage of hybrid-system design, performance, applications, or general RF/VLC networks.The introduction distinguishes earlier work focused on VLC, Li-Fi/Wi-Fi optimization, or optical hybrid systems from a general RF/VLC survey.Source: §I · pp. 2–3
  • The survey covers VLC and RF technologies, hybrid topologies and performance analyses, simulations and implementations, applications, and future research directions.It also compares studies by employed technologies, network topologies, and main features.Source: §I · p. 3

II. VLC AND RF TECHNOLOGIES

This section introduces VLC components, standards, and channel constraints. It highlights commercial LED bandwidth limits and the effects of line-of-sight conditions on VLC links.

A. VLC Systems

  • IEEE 802.15.7 defines PHY and MAC layers, with PHY I for outdoor low-rate applications and PHY II and III supporting up to 96 Mbps.PHY II supports OOK and VPPM, while PHY III supports CSK; the MAC layer assigns guaranteed time slots during contention-free periods.Source: A. VLC Systems · pp. 3–4
  • Commercial LED modulation bandwidth is limited to MHz-to-hundreds-of-MHz levels, while micro LEDs have reported 1 Gbps.A post-equalization circuit extended phosphor-converted LED bandwidth to 150 MHz.Source: A. VLC Systems · p. 4
  • VLC links can detect reflected rays in NLOS scenarios, but measured channel gain drops severely compared with LOS.The section also notes challenges from light-source interference, uplink provision, dimming, and LED access-point internet connectivity.Source: A. VLC Systems · p. 4
  • A VLC transmitter uses an LED with analog and DC inputs, while a receiver uses a photodetector, amplification, and ADC before demodulation.The section presents this system structure as analogous to RF baseband processing, with LEDs and photodetectors replacing antennas.Source: A. VLC Systems · p. 4, Figure 2 · p. 4

1) VLC Front-Ends (Transmitter/Receiver):

The section describes VLC transmitter and receiver choices, then summarizes indoor and outdoor VLC path-loss modeling approaches. Its channel descriptions distinguish geometry-sensitive models from simpler distance-based models.

1) VLC Front-Ends (Transmitter/Receiver):

  • Phosphor white LEDs are low-cost but bandwidth-limited by phosphor response, whereas RGB LEDs offer bandwidth an order of magnitude higher and support CSK.The survey assumes blue phosphor LEDs and photodetector receivers unless otherwise stated.Source: 1) VLC Front-Ends (Transmitter/Receiver) · pp. 4–5
  • VLC receivers include photodetectors, solar cells, and imaging sensors, compared in the survey by cost, data rate, sensitivity, size, power, and device availability.The cited comparison table covers these three receiver types, and the survey describes the photodetector as supporting very high data rates.Source: 1) VLC Front-Ends (Transmitter/Receiver) · p. 5, Table I · p. 5

2) VLC Path Loss Models (Channel Models):

  • Indoor VLC commonly models LED emission as Lambertian, with channel gain depending on beam and receiver geometry.The described parameters include detector size, distance, irradiance and incidence angles, LED half-angle, optical-filter gain, and concentrator gain.Source: 2) VLC Path Loss Models (Channel Models) · p. 5, Figure 3 · p. 5
  • A simple VLC path-loss model uses channel gain, aggregate system gains and transmitted power, and a path-loss exponent for static links with changing LOS distance.Outdoor models are also developed through simulation, analytical, and experimental methods, including high- and low-beam vehicle headlights.Source: 2) VLC Path Loss Models (Channel Models) · pp. 5–6

3) VLC Modulation Methods (PHY):

The section surveys VLC modulation and multiple-access choices, including constraints arising from optical intensity modulation, illumination, and channel access. It describes how these schemes encode data or share the medium, and notes practical requirements for NOMA.

3) VLC Modulation Methods (PHY):

  • VLC uses intensity modulation and direct detection, requiring a nonnegative real-valued transmitted signal; OFDM can address frequency selectivity but must be modified for this constraint.The passage also notes OFDM’s adaptive bit and power loading and frequency-domain single-tap equalization.Source: 3) VLC Modulation Methods (PHY) · p. 6
  • Dimming support and flicker mitigation are two main challenges because VLC must provide both illumination and data communication.The passage relates flicker to continuous on-off switching during transmission.Source: 3) VLC Modulation Methods (PHY) · p. 6
  • OOK represents bits through light presence or absence, while pulse modulation conveys data as pulses using width, amplitude, or position.The section also identifies OFDM and color-shift keying (CSK), which transmits data using different colors at the same intensity.Source: 3) VLC Modulation Methods (PHY) · p. 6

4) VLC Multiple Access Schemes:

  • CSMA makes a node sense for an ongoing transmission before sending, whereas OFDMA assigns different subcarriers to different users for simultaneous transmission.Source: 4) VLC Multiple Access Schemes · p. 6
  • CDMA lets multiple users transmit simultaneously over one VLC channel using assigned codes to share frequencies with limited interference.Source: 4) VLC Multiple Access Schemes · p. 6
  • NOMA shares time and frequency resources simultaneously, but practical power splitting and successive interference cancellation require channel-state information at transmitters and receivers.The section notes that exact VLC capacity is unknown and that optical-capacity bounds are used; the lower-bound gap can be neglected at high SNR.Source: 4) VLC Multiple Access Schemes · pp. 6–7

B. RF Systems

The section surveys the RF technologies used alongside VLC, covering Wi-Fi, mmWave, and DSRC. It describes their frequency ranges and capabilities, as well as constraints relevant to their use.

B. RF Systems

1) Wi-Fi:

  • Wi-Fi refers to IEEE 802.11 wireless local-area-network standards; the passage reports a maximum data rate of 1300 Mbps for 802.11ac.It also lists earlier variants, including 802.11n at 150 Mbps per channel.Source: 1) Wi-Fi · p. 7

2) mmWave:

  • The 57–64 GHz range is used for short-range Gbps links, but 60 GHz communication is limited to a few meters and cannot pass through walls like conventional Wi-Fi.Atmospheric oxygen absorption around 60 GHz can cause attenuation on the order of 15 dB/km.Source: 2) mmWave · p. 7

3) Dedicated Short Range Communications (DSRC):

  • DSRC is designed for short- and medium-range vehicular communications, including V2V, V2I, and I2V, and uses microwave ranges of 5.805–5.815 GHz and 5.795–5.805 GHz.The passage describes DSRC as a complete communication protocol, not only PHY and MAC layers.Source: 3) Dedicated Short Range… · p. 7

C. Comparison of RF and VLC Technologies

VLC offers wide bandwidth, unlicensed spectrum, low power use, and other advantages that complement RF. Its limited coverage, line-of-sight dependence, and integration challenges constrain its use.

Comparison and limitations

  • VLC offers immunity to electromagnetic interference, unlicensed bands, and low power consumption, but its operating range is smaller than RF’s.The passage identifies real-time channel-model estimation and performance across lighting and environmental scenarios as future-work considerations.Source: C. Comparison of RF and VLC Technologies · p. 8

Motivating factors

  • VLC provides a 10,000-times-larger spectrum range than the entire RF wireless spectrum, with no license required.The passage gives VL as 400–790 THz and traditional RF as 300 KHz–300 GHz.Source: C. Comparison of RF and VLC Technologies · p. 8
  • LED infrastructure can support VLC with communication modules added to existing lighting, and existing power lines can interconnect VL networks.The passage also describes VLC as compatible with the smart power grid.Source: C. Comparison of RF and VLC Technologies · p. 8
  • VLC channels have coherence times at least an order of magnitude longer than RF channels, reducing channel-estimation frequency.The passage attributes this to significantly less multipath and notes its relevance to continuous, stable links.Source: C. Comparison of RF and VLC Technologies · p. 8

Limitations

  • Strong ambient light can increase shot noise or saturate the receiver, preventing VLC systems from functioning properly.The passage identifies unmodulated artificial and natural light sources in the same spectral band as the interference sources.Source: C. Comparison of RF and VLC Technologies · p. 9
  • Indoor VLC must balance communication and lighting requirements; intensity changes can cause flicker, while mobility also requires channel updates and handover.Coverage is limited by small cell sizes and signal attenuation, and obstacles can block the line-of-sight link.Source: C. Comparison of RF and VLC Technologies · p. 9

III. HYBRID SYSTEMS

The survey organizes hybrid RF/VLC research by network topology and describes dual-hop systems in which VLC and RF serve different links. It reviews work on relay-assisted performance, security, and application-specific designs.

1) Dual-Hop Hybrid RF/VLC System:

  • Dual-hop systems use VLC for the fixed base-station-to-relay link and low-power directional RF for the relay-to-user access link.The RF access area can be divided into cells to serve multiple users and save power; handover, multi-relaying, and beamforming may also be used.Source: 1) Dual-Hop Hybrid RF/VLC System · p. 9
  • Studies analyze dual-hop systems’ secrecy, outage, and bit-error performance under relay energy harvesting, power allocation, node-location randomness, and timing errors.The survey reports derived exact, asymptotic, approximate, or closed-form expressions for several of these performance measures.Source: 1) Dual-Hop Hybrid RF/VLC System · p. 9
  • Further dual-hop work includes paired-user VLC-to-RF forwarding, context-aware learning, relay-jammer selection, IoT downlink/uplink designs, and medical indoor/outdoor links.The IoT example uses VLC downlink, RF-NOMA uplink, and harvested optical energy for device transmissions.Source: 1) Dual-Hop Hybrid RF/VLC System · p. 11

2) Opportunistic Separate Networks (RF/VLC):

Opportunistic separate networks let users choose between RF and VLC, whereas centralized heterogeneous networks allocate resources through a control unit. These topologies trade control overhead and resource management against user flexibility and network scale.

2) Opportunistic Separate Networks (RF/VLC):

  • In opportunistic separate networks, users select RF or VLC according to factors such as SNR, application requirements, and mobility, but the model is hard to control and optimize.Its low network overhead and user flexibility motivate its use.Source: 2) Opportunistic Separate Networks… · p. 11
  • Maximum-received-power selection suits scenarios without dominant interference, but can worsen performance when interference leads users to choose RF over VLC.A separate study addressed user grouping for NOMA with a merge-and-split coalitional-game algorithm.Source: 2) Opportunistic Separate Networks… · p. 11

3) Heterogeneous Networks (HetNet) with Centralized Unit:

  • A centralized unit assigns resources based on network conditions, potentially using user location; its overhead supports fair treatment in dense, small-area networks.Without a control unit dividing resources, opportunistic networks become difficult to adopt as users compete in the same place.Source: 3) Heterogeneous Networks (HetNet)… · pp. 11–12, 2) Opportunistic Separate Networks… · p. 11
  • Hybrid users may aggregate RF and VLC simultaneously for higher data rate and reliability, or use only one technology to optimize network conditions and manage interference.The survey notes that topology choice depends on link-level design, user count, and system complexity.Source: 3) Heterogeneous Networks (HetNet)… · p. 12

B. Resource Allocation

Hybrid RF/VLC resource-allocation work addresses single and multiple objectives, including energy efficiency, throughput, load balancing, and fairness. Its optimization must account for lighting and link-coupling constraints, while RF/VLC channel coupling remains an open issue.

1) Single-Objective Optimization (SOO):

  • Single-objective studies optimize goals including energy efficiency, power consumption, throughput, and load balancing subject to requirements such as QoS, illumination, or outage constraints.One study leaves handover energy consumption for future work.Source: 1) Single-Objective Optimization (SOO) · pp. 12–13

2) Multi-Objective Optimization (MOO):

  • Multi-objective studies address load balancing, capacity, fairness, outage, energy, and queue lengths, using approaches including game theory and two-phase RF/VLC selection.The two-phase method reduces outage probability first and then improves system capacity; its performance is reported close to exhaustive search with lower computational complexity.Source: 2) Multi-Objective Optimization (MOO) · pp. 13–14
  • Lighting constraints include nonnegative VLC signals, peak optical power or PAPR, dimming, flicker avoidance, and white-light requirements for multi-color VLC.These conditions address illumination and communication requirements and LED linear operation.Source: 2) Multi-Objective Optimization (MOO) · p. 14
  • Serial dual-hop data rates are limited by the slower system, while parallel heterogeneous rates combine RF and VLC; both links share a total-power budget.The survey states that RF/VLC channel coupling constraints have not yet been investigated and identifies them as an important future problem.Source: 2) Multi-Objective Optimization (MOO) · p. 14

C. Performance Analyses

The section groups hybrid RF/VLC performance analyses by shared metrics to support comparison across studies. It covers user throughput, delay, errors and outages, coverage, and fairness.

C. Performance Analyses

  • Table VI groups studies by performance analysis to make comparisons easier.The section then reviews the main studies under these metrics.Source: C. Performance Analyses · p. 14, Table VI · p. 15
  • Average user throughput depends on assigned bandwidth and time-varying SINR, which varies with user location and whether the link uses RF or VLC.Reported work includes two-stage Wi-Fi/VLC AP selection with near-optimal simulated throughput, and experimental parallel-link systems with improved coverage and capacity.Source: C. Performance Analyses · p. 14
  • Delay analyses account for retransmissions, errors, and handover, and compare aggregated with non-aggregated network scenarios.The reported studies find lower minimum average delay for aggregation in one scenario, but better non-aggregated performance in a contention-based scenario.Source: C. Performance Analyses · pp. 14–15
  • The surveyed analyses also examine packet loss and BER, coverage and outage probability, and user satisfaction and fairness.Outage is related to whether user data rate falls below a threshold; fairness is assessed among users, including with Jain’s fairness index [143].Source: C. Performance Analyses · pp. 15–16

(NAP ) PNAP

The section describes metrics used to assess resource use, handover, energy, area spectral efficiency, and secrecy in hybrid RF/VLC networks. It reviews studies applying these metrics to allocation and network design.

(NAP ) PNAP

  • Resource-allocation studies combine throughput and fairness goals, including iterative joint power allocation and load balancing across access points.Other work considers decentralized allocation under diverse QoS requirements and NOMA user grouping with coalitional game theory.Source: (NAP ) PNAP · p. 16
  • Handover overhead is control data for coordinating handovers, and it reduces network efficiency and users’ data rates.A cited on-off vertical handover scheme reports an approximately 87.6% reduction in average handover delay cost versus I-VHO and D-VHO.Source: (NAP ) PNAP · p. 16
  • Energy-efficiency studies optimize power use subject to user-rate, AP-power, request, or illumination constraints.One study reports hybrid RF/VLC networks to be more power-efficient than standalone RF networks.Source: (NAP ) PNAP · p. 16
  • Area spectral efficiency accounts for cell area when comparing hybrid networks with standalone RF or VLC systems.A three-tier network study reports average hybrid RF/VLC ASE at least two orders of magnitude higher than standalone RF.Source: (NAP ) PNAP · p. 17
  • Secrecy analyses examine outage and secrecy rate, including beamforming and power allocation under secrecy-capacity requirements.A comparison of DF-based hybrid RF/VLC with standalone RF and VLC reports better secrecy capacity for the hybrid system.Source: (NAP ) PNAP · p. 17

D. Hybrid Network Experimental Implementation

The paper reviews experimental implementations of hybrid RF/VLC systems and their reported performance, then surveys application areas and research trends. Hybrid systems are studied across indoor and outdoor settings, with applications spanning mobility, energy, security, and connectivity.

D. Hybrid Network Experimental Implementation

  • Real-time VLC implementation faces LED-driver cost and bandwidth constraints, while lens combinations have been investigated to improve received SNR.The passage notes early hardware bandwidth of 45 kHz, a later 100 Mbps implementation, and the narrow bandwidth of common commercial white LEDs.Source: D. Hybrid Network Experimental… · pp. 17–18
  • Experimental hybrid systems report improved throughput, coverage, and capacity compared with standalone networks.Wi-Fi/VLC coexistence experiments include parallel link bonding, while another platform integrates multiple links, mobility management, and handover.Source: D. Hybrid Network Experimental… · p. 18, Figure 8 · p. 18, Table VII · p. 18
  • A practical two-link system combining duplex RF and asymmetric duplex RF/VLC outperforms standalone RF and an asymmetric system.Other implementations switch rapidly between VLC and RF and validate the system with low-latency video streams,.Source: D. Hybrid Network Experimental… · p. 19

E. Current Applications for Hybrid Systems

  • Applications also include energy harvesting, RF-sensitive indoor settings, and improving indoor data rate, capacity, mobility, and handover.Some work uses VLC downlink connectivity in RF-sensitive environments; other studies optimize resources or handover protocols for throughput, delay, and connectivity.Source: E. Current Applications for Hybrid… · p. 19
  • Hybrid RF/VLC studies also explore physical-layer security and asymmetric links with VLC downlink and RF uplink.The paper notes that most newer studies focus on implementation, while older studies emphasize performance analysis and simulation.Source: E. Current Applications for Hybrid… · pp. 19–20
  • Hybrid RF/VLC applications include IoT and device-to-device communication, vehicular communications, and power-line backhauling.Vehicular work considers hybrid systems as complements to RF standards, while PLC studies optimize power and subcarrier allocation for user data rates.Source: E. Current Applications for Hybrid… · p. 19

IV. RESEARCH DIRECTIONS AND FUTURE WORK

The paper identifies future research needs for hybrid RF/VLC systems, emphasizing practical evaluation, adaptable network design, and unresolved optimization challenges. It highlights backhaul, resource allocation, mobility, security, and energy-harvesting systems as directions for further study.

A. Current Research Directions

  • Energy-harvesting dual-hop systems need analysis across multiple constraints, reliable handover schemes, and real-hardware testing.The surveyed simulations often optimize a single constraint, such as delay, data rate, or secrecy.Source: A. Current Research Directions • · p. 20
  • Resource-allocation research should target energy efficiency, fairness, and throughput while limiting control overhead, delay, and transmit power.The paper points to machine learning for non-convex optimization and calls for efficient, low-cost algorithms and AP and bandwidth allocation.Source: A. Current Research Directions • · p. 20
  • Mobility work should develop dynamic load balancing and fast handover, including use of shared user information to reduce control overhead and transmit power.Candidate schemes can be compared using maximum user data rate, control overhead, fairness, and outage probability.Source: A. Current Research Directions • · pp. 20, 23
  • Security research needs to account for eavesdropper location while minimizing power under secrecy requirements across network scenarios and topologies.The paper identifies eavesdropper location as an important parameter in secrecy analysis.Source: A. Current Research Directions • · p. 23
  • Lower-cost, flexible testbeds are needed to validate algorithms and support more users, network designs, and applications.The paper calls for optimization of existing testbeds and easier-to-set-up hardware.Source: A. Current Research Directions • · p. 23
  • Future modulation and access work should improve resource optimization while addressing hardware updates and competition among NOMA users.The paper discusses HRO-OFDM and NOMA, including coalitional-game approaches to user grouping, –.Source: A. Current Research Directions • · pp. 23–24
  • Networks need environment- and application-specific parameters, algorithms, and topologies for indoor and outdoor settings.The paper anticipates new applications as testbeds become more accessible.Source: A. Current Research Directions • · p. 24
  • Backhaul links may become a bottleneck as user numbers and data-rate demands rise, warranting comparison of VLC backhaul with fiber.The paper also proposes examining dual-hop architectures in which VLC backhauls RF access points.Source: A. Current Research Directions • · p. 24

B. Expected Future Research Directions

The survey identifies future work on improving hybrid RF/VLC networks and expanding their applications. It calls for greater RF/VLC coexistence to improve user experience and reduce network delay and power consumption.

B. Expected Future Research Directions

V. CONCLUSIONS

The survey presents hybrid RF/VLC networks as dynamic, reconfigurable systems with potential benefits across network performance and applications. It concludes that their design should match each use case and its available resources, while further analysis and simulation remain needed.

V. CONCLUSIONS

  • The listed benefits also include stable channels under different circumstances, improved mobility and security, and less RF interference.Source: §V · p. 24
  • The survey associates hybrid RF/VLC networks with improved power consumption, delay, and capacity, as well as reliability with many users and high data rates.Source: §V · pp. 24–25
  • The survey covers network topologies, optimization techniques, performance-analysis tools, and applications where hybrid networks outperform standalone systems.It notes that topologies are being tested in recent hardware implementations.Source: §V · p. 25
  • Network specifications should fit the intended use case, user needs, and available resources; future analyses and simulations are still needed for access methods and load balancing.The survey anticipates smarter cognitive networks that maximize user performance while maintaining efficient resource use.Source: §V · p. 25
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