A Review of Cognitive Hybrid Radio Frequency/Visible Light Communication Systems for Wireless Sensor Networks
Abstract
:1. Introduction
2. The Basics of CRSNs
- Spectrum resources: The range of electromagnetic frequencies employed for wireless communication is of paramount importance in CRSNs. As the spectrum hosts multiple critical applications, it is tightly regulated. CR is distinctive in its ability to dynamically access and utilize underutilized spectrum segments, known as “white spaces” or “spectrum holes”. The ability to intelligently detect unoccupied communication channels and adapt spectrum usage enables CR to coexist efficiently with other wireless systems.
- Time resources: Time synchronization is integral to CRSNs, as it facilitates coordination among sensor nodes and optimizes resource allocation. Through precise time alignment, CRSNs mitigate collisions and ensure efficient communication.
- Power resources: Power management is vital in CRSNs due to the limited energy resources of sensor nodes. By dynamically adjusting transmission power and routing paths, CR optimizes energy consumption, prolongs network lifespan, and enhances energy efficiency.
- Space resources: CRSNs leverage the spatial characteristics of wireless environments to optimize resource allocation and interference management. The strategic deployment of sensor nodes based on their physical locations aids in maximizing spectrum usage and enhancing network performance.
- Spectrum sensing: CR devices detect unused or underutilized frequency bands;
- Spectrum decision: based on sensed spectra, CR devices select the most suitable frequency bands for transmission;
- Spectrum sharing: spectrum sharing strategies, including underlay, overlay, and hybrid, manage the allocation and sharing of the spectrum between PUs and SUs;
- Interference mitigation: techniques such as power control mechanisms and adaptive modulation and coding schemes are employed to mitigate interference;
- Spectrum mobility: CR devices utilize spectrum mobility techniques to efficiently use the available spectrum resources in dynamic spectrum environments;
- Spectrum management database: a centralized repository of spectrum utilization data which informs about the availability of spectrum resources;
- Dynamic spectrum Access: allows CR devices to dynamically adapt frequency usage based on real-time conditions and spectrum availability;
- Spectrum monitoring and enforcement: involves continuous spectrum monitoring to maintain its integrity and ensure fair and efficient use.
2.1. Performance Metrics
2.1.1. Fairness
2.1.2. Outage
2.1.3. Sum Rate
2.1.4. Throughput
2.1.5. QoS
3. The Basics of VLC
- Spectrum availability: VLC, categorized under free-space optical communication, provides an expansive spectrum, offering substantially larger bandwidth than the RF spectrum utilized by conventional wireless technologies [40];
- Resistance to RF interference and overcrowding: VLC is immune to electromagnetic interference that can disturb sensitive electronic systems. This unique property enables VLC to function even in environments where RF usage is restricted, such as hospitals, airplanes, power plants, and sensitive industrial settings [41];
- Cost-effective implementation: VLC leverages readily available, cost-effective LEDs. Existing infrastructures can be conveniently adapted for VLC by incorporating LEDs into the existing lighting systems, thereby providing concurrent communication and illumination. This is particularly beneficial in scenarios such as vehicular communication where LED lights are already in operation, and introducing RF systems would entail additional equipment and associated costs [42,43];
- Energy saving and potential environmental impact: VLC utilizes existing LED lighting systems, which can be more energy-efficient compared to some radio wave technologies, depending on the specific implementation and energy source. LEDs might also offer certain environmental advantages, such as potentially reduced use of hazardous materials in their design and possibly lower heat generation, although these benefits may vary widely based on the particular technology, application, and lifecycle considerations [46,47].
- Distance range: The range of VLC is largely dictated by the power of the light source signal and the sensitivity of the receiver. The presence of obstacles and diverse environmental factors can further curtail range by blocking or degrading the light signals [50].
- Interference: Interference, a significant challenge in VLC, can disrupt data signal transmission. Interference can originate from ambient light sources (both natural and artificial), cross-talk among VLC systems, multi-path interference due to reflections and scattering, and other light sources, all of which can impact the reliability and quality of VLC communication [51].
- Uplink challenges: The uplink, crucial for transmitting collected data from sensor nodes to a central controller or sink node, poses challenges in VLC systems due to weaker signal strength, irradiance interference from ambient light sources, limited receiver sensitivity and bandwidth, LoS requirements, and mobility constraints. These factors hinder the achievement of efficient and reliable uplink transmission [52].
- Outdoor environment: VLC is vulnerable to atmospheric turbulence, leading to signal fading and degradation. It confronts multiple challenges such as elevated noise levels, interference, and restricted mobility owing to the requirement for a LoS configuration. Moreover, the SNR is adversely affected by environmental conditions like fog, rain, snow, dust, haze, and sunlight [53]. The technology also grapples with limited transmission range and the intricacies of integration with other communication systems, such as RF. Furthermore, there is a necessity for specialized modulation and coding schemes. These constraints render outdoor VLC less versatile than its indoor counterpart [54,55].
4. Heterogeneous Network (HetNet)
- Dual-hop hybrid RF/VLC system: this system operates in two hops; the first hop is RF for long-distance communication, and the second hop is VLC for short-distance communication;
- Opportunistic separate networks (RF/VLC): these networks operate RF and VLC independently, selecting the network opportunistically based on network conditions and user requirements;
- Heterogeneous Networks (HetNets) with a centralized unit: These networks comprise RF and VLC networks with a centralized control unit, which assigns resources to the users based on the network conditions. This network is needed when a high number of users are present in a small area network.
- Spectrum utilization: the unique frequency ranges of RF and VLC allow hybrid RF/VLC systems to promote efficient spectrum utilization;
- Coverage and mobility: RF and VLC provide complementary characteristics in coverage and mobility;
- Interference mitigation: by integrating RF and VLC, hybrid systems can offload communication tasks to VLC in high RF interference areas, thereby ensuring reliable communication;
- Data offloading and resource management: hybrid RF/VLC systems facilitate efficient data offloading and resource management;
- Reliability and adaptability: the complementarity of RF and VLC in hybrid systems enhances their reliability and adaptability.
- Noise and multipath propagation: hybrid systems must address noise and multipath propagation phenomena in specific home environments to ensure reliable communication;
- Non-determinism in wireless communication: hybrid systems should strive to eliminate as many sources of non-determinism as possible;
- Co-existence with multiple wireless units: hybrid systems should coexist with other wireless units without causing or suffering interference;
- Backward and forward compatibility: hybrid systems should ensure compatibility with both older and newer devices and technologies;
- Fault tolerance: hybrid systems should be able to tolerate faults and prevent performance degradation.
5. Hybrid Systems Review
- Spectrum use and management: In contrast to the regulated RF spectrum, VLC operates on the visible light segment of the electromagnetic spectrum, which is unlicensed and devoid of regulatory constraints. The visible light spectrum surpasses the RF spectrum in terms of bandwidth, thereby offering a stronger data transfer capacity. VLC systems typically utilize LED light sources, modulating the light intensity to encode and convey data.
- Propagation characteristics: Light waves, unlike RF waves, cannot permeate walls or other opaque structures, necessitating VLC to predominantly rely on LoS communication. This mandates that the transmitter and receiver maintain a direct visual pathway. Nonetheless, the propensity of light to reflect off surfaces enables NLoS communication. For instance, a CR-inspired approach could dynamically optimize the VLC system parameters in response to prevailing propagation conditions. Additionally, CR tactics for mitigating multi-path fading, such as employing diversity techniques, can be adapted to handle reflections in VLC systems [67].
- Interference management: VLC systems are susceptible to interference from many light sources, including sunlight, incandescent lamps, and other LED units. This interference can be curtailed by implementing filters or similar techniques to attenuate its effects [68]. Cooperative communication in VLC might encompass sharing interference source data and coordinating transmissions to circumvent interference. Furthermore, adaptive power control can be harnessed to adjust the VLC light source intensity according to the ambient light conditions.
- Security concerns: The security measures in VLC systems are distinctive due to their unique transmission medium. The communication is restricted within a confined space, limiting the possibility of eavesdropping, and hence enhancing the security.
- Spatial reuse: The inability of light to infiltrate through walls empowers the same frequency (or color) to be deployed for communication in adjacent spaces without inciting interference. This principle, known as spatial reuse, can be further enhanced in a cooperative VLC environment through judicious frequency coordination.
- Beamforming and MIMO: Beamforming, a technique refined in RF communication to concentrate signals towards specific antennas, can be implemented in VLC through the focus or shape modulation of the light beam. Likewise, Multiple-Input Multiple-Output (MIMO) techniques, employing multiple transmitters and receivers for performance enhancement, are transferable to VLC systems. Nonetheless, the actualization of beamforming and MIMO in VLC might require modification due to the distinct propagation characteristics of light.Cooperative communication in VLC could involve the strategic positioning and orientation of devices to maximize the benefits of both LoS and NLoS communication paradigms.
5.1. Hybrid RF/VLC
- Hybrid RF/VLC system architectures: the paper discusses three types of hybrid RF/VLC network topologies, namely the dual-hop hybrid RF/VLC system, opportunistic separate networks (RF/VLC), and heterogeneous networks (HetNets) with a centralized unit;
- Cognitive strategies: the paper provides an in-depth analysis of cognitive strategies applied in hybrid RF/VLC systems, including spectrum sensing, spectrum management, and spectrum mobility;
- Machine learning applications: the paper discusses the application of machine learning techniques in hybrid RF/VLC systems, including reinforcement learning, deep learning, and federated learning.
- Practical considerations: the paper discusses the practical considerations in implementing hybrid wireless networks, including noise and multipath propagation, non-determinism in wireless communication, co-existence with multiple wireless units, backward and forward compatibility, and fault tolerance;
- Challenges: the paper provides an in-depth analysis of the challenges faced in implementing hybrid wireless networks, including interference mitigation, data offloading and resource management, and reliability and adaptability;
- Applications: the paper discusses the potential applications of hybrid wireless networks, including vehicular communication, indoor positioning, and IoT.
5.2. Free Space Optics (FSO)
5.3. Energy Efficient Resource Allocation
5.4. Industrial Scenarios
- Channel characteristics: Investigation of the VLC channel in IIoT scenarios was executed using a ray-tracing simulation method. The study analyzed and modeled large-scale fading and multipath-related characteristics through distance-dependent and statistical distribution models;
- Density of surrounding objects: The impact of surrounding object density on VLC channel characteristics was evaluated under a single transmitter. The findings suggest that users in denser environments receive more multipath components, resulting in reduced optical path loss and increased RMS DS;
- User heights: A comparison of large-scale fading and multipath-related characteristics was performed at different user heights under multiple transmitters. Results indicate increased optical path loss and reduced RMS DS at lower receiver heights;
- Link adaptation method: The study proposes a two-step link adaptation method combining luminary adaptive selection and delay adaptation to mitigate multipath interference. Simulation-based verification of this method showed optimization in several parameters such as SNR, RMS DS, Channel Impulse Responses, and BER of a DCO-OFDM system.
- Luminary adaptive selection: a strategy utilizing a greedy algorithm to select the optimal LED index for maximizing SNR;
- Delay adaptation technique: a strategy that controls effective signal transmission time, ensuring LoS components from variable transmitters arrive at the receiver simultaneously.
5.5. Energy Harvesting
- denotes the downlink transmission time from LED i to device j;
- is the thermal voltage of the photodetector;
- signifies the DC component of the output current from the photodetector;
- L represents the side length of a room model;
- indicates the energy harvesting efficiency at the jth device;
- stands for the proportion of the room’s side length not covered by the distance from the LED to the device;
- is the dark saturation current of the photodetector.
5.6. Simultaneous Wireless Information and Power Transfer (SWIPT) and Simultaneous Lightwave Information and Power Transfer (SLIPT) Incorporation in CRSNs
5.7. Resource Allocation in Multiple Access
5.8. Machine Learning
6. Discussion
7. Conclusions
Author Contributions
Funding
Conflicts of Interest
Abbreviations
AP | Access Point |
BER | Bit Error Rate |
CR | Cognitive Radio |
CRLB | Cramer–Rao Lower Bound |
CRSNs | Cognitive Radio Sensor Networks |
CSI | Channel State Information |
DCO-OFDM | Direct-Current-Biased Optical Orthogonal Frequency Division Multiplexing |
DSS | Dynamic Spectrum Sharing |
DSA | Dynamic Spectrum Access |
D2D | Device-to-Device |
FoV | Field of View |
HetNets | Heterogeneous Networks |
KKT | Karush–Kuhn–Tucker |
kNN | k-Nearest Neighbors |
LU | Licensed User |
ILP | Integer Linear Programming |
IoT | Internet of Things |
LoS | Line-of-Sight |
MDP | Markov Decision Process |
MDPI | Multidisciplinary Digital Publishing Institute |
m-MIMO | Massive Multiple-Input, Multiple-Output |
MIMO | Multiple-Input, Multiple-Output |
NLoS | Non-Line-of-Sight |
NOMA | Non-Orthogonal Multiple Access |
OCC | Optical Camera Communication |
PCA-kNN | Principal Component Analysis with k-Nearest Neighbors |
PCA-NN | Principal Component Analysis with Neural Network |
PSO | Particle Swarm Optimization |
PUs | Primary Users |
QoS | Quality of Service |
RSMA | Rate-Splitting Multiple Access |
RF | Radio Frequency |
RMS DS | Root Mean Square Delay Spread |
SLIPT | Simultaneous Lightwave Information and Power Transfer |
SNR | Signal-to-Noise Ratio |
SPIE | The International Society of Optics and Photonics |
SUs | Secondary Users |
SWIPT | Simultaneous Wireless Information and Power Transfer |
URLLC | Ultra-Reliable Low-Latency Communications |
VLC | Visible Light Communication |
WSNs | Wireless Sensor Networks |
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Paper | Discussion and Focus | Strengths | Limitations |
---|---|---|---|
[62] | Comprehensive survey on hybrid wireless networks, covering both RF and VLC technologies. | Extensive coverage of hybrid wireless networks. Detailed discussion on network topologies and performance metrics. | Does not delve into the specific challenges and opportunities in the context of wireless sensor networks. |
[63] | Focuses on hybrid LiFi and Wi-Fi networks. Discusses design considerations, performance metrics, and applications like IoT. | Detailed framework of system design. Discusses key performance metrics and applications. | Lacks a discussion on the adaptability and reconfigurability of the systems. |
[31] | Detailed survey of hybrid RF/VLC systems, discussing network topologies, performance analyses, and applications. | Comprehensive overview of hybrid RF/VLC systems. Discusses both benefits and limitations of the technology. | Does not address the energy efficiency challenges that are critical for wireless sensor networks. |
[64] | Discusses hybrid VLC and RF systems, focusing on access frameworks and application scenarios. | Detailed discussion on access frameworks. Addresses challenges in resource allocation and handover. | Does not discuss the scalability issues that could arise when deploying these systems in more complex environments. |
[65] | Proposes a hybrid RF/VLC network architecture for IoT, particularly in remote areas. | Addresses the challenge of providing IoT connectivity in remote areas. Explores the interoperability of different communication technologies. | Does not discuss the robustness and resilience of the proposed architecture. |
[66] | Comprehensive state-of-the-art study of hybrid VLC/RF networks. Discusses indoor and outdoor scenarios, and applications in IoT. | Extensive coverage of both indoor and outdoor scenarios. Discusses applications in IoT and future challenges. | Lacks a focus on the trade-offs between performance and complexity in hybrid systems. |
Topic | References |
---|---|
Energy-efficient resource allocation | [65,69,70,71,72,73,74,75,76,77,78,79] |
Communication in industrial scenarios | [65,67,72,80,81] |
Using M-MIMO | [73,82] |
SWIPT (simultaneous wireless information and power transfer) | [74,83,84] |
Artificial intelligence (AI)-based | [61,69,72,73,81,85,86,87,88] |
Energy harvesting and management | [65,69,72,74,83,84] |
Resource allocation in multiple access | [69,73,78,86,87,89,90] |
Paper | System Proposed | Performance Analysis | Metrics | Type of Problem Formulation |
---|---|---|---|---|
[76] | Hybrid RF/VLC system for energy-efficient wireless access. | Competitive ratio of . | Total power consumption of the system. | Optimization problem using ILP. |
[75] | Hybrid RF/VLC with shared backhaul and imperfect CSI. | Two-layer approach using KKT conditions and Lagrange multipliers. | Weighted proportional fairness. | Convex optimization for joint resource allocation. |
[70] | Hybrid RF/VLC network for energy-efficient resource allocation. | Evaluated via simulations. | Energy efficiency, total data rate, and sum rate. | Concave-convex fractional program, using KKT conditions. |
[72] | Heterogeneous RF/VLC system for URLLC in Industrial IoT networks. | Evaluated via simulations using PDS-ERT. | Energy efficiency, reliability, latency, and data transmission rates. | Decision-making problem, modeled as an MDP. |
[71] | Multi-cell cognitive VLC model with a hybrid underlay/overlay power assigning strategy. | Simulations, focusing on sum-rate and area spectral efficiency. | Sum-rate, area spectral efficiency, and Field of View (FoV) angle. | Optimization problem to maximize spectral efficiency of VLC systems. |
[65] | Hybrid RF/VLC network architecture for IoT, leveraging solar-powered lighting systems. | Validated through fragmentation, in-depth VLC tests, and real-scenario experiments. | Success rate of fragmentation, Bit Error Rate (BER), error probabilities, propagation times, and transfer times. | Need for high connectivity and interoperability in IoT systems. |
[79] | 5G network with passive reflectors for enhanced connectivity. | Evaluated via simulations demonstrating improved connectivity and energy efficiency. | SNR and total power consumption. | Optimization problem focusing on maximizing SNR and minimizing power consumption. |
Paper | Proposed System | Key Features | Problem Formulation | Performance Evaluation |
---|---|---|---|---|
[83] | Hybrid RF/VLC system with SLIPT and SWIPT protocols. | Two-way cooperative communications. | Outage minimization for LU and IoT networks. | Based on outage probabilities and throughput. |
[84] | Dual-hop hybrid RF/VLC IoT system with SLIPT. | Evaluated via mathematical analyses and simulations | PDF of Harvested Energy at the relay node, and the relationship between its lower and upper thresholds | Dynamic balance between information and energy flow. |
[74] | Hybrid RF/VLC system with SLIPT. | Cognitive-based resource allocation to serve RF user without burdening VLC user. | Optimization problem to maximize harvested energy under constraints. | Performance evaluated in terms of outage probability and harvested energy. |
Paper | Proposed System | Key Features | Problem Formulation | Results |
---|---|---|---|---|
[89] | RSMA in VLC to improve sum rate. | Joint power allocation and beamforming design. | Non-convex optimization problem, transformed using SCA. | Outperforms traditional NOMA and OMA in sum rate. |
[78] | RSMA in VLC for energy efficiency. | Joint user pairing and power allocation. | Non-convex optimization problem, solved using bisection search. | Achieves better energy efficiency compared to traditional NOMA and OMA. |
[90] | RSMA in VLC with optimal beamformer design. | Derives the lower bounds of the achievable rate of each user | Sum rate maximization under power constraints. | Superior performance compared to several baseline schemes. |
Paper | Proposed System | Key Features | Problem Formulation | Performance Evaluation |
---|---|---|---|---|
[86] | ML Algorithms for Applications in CRSNs | Discusses ML for spectrum sensing, auction, prediction in DSA applications. | Focuses on CRN challenges, robustness, scalability. | Overview of advancements in ML for CR. |
[85] | Reinforcement Learning (RL) for Hybrid WiFi-VLC Networks | Centralized Q-learning algorithm, new reward function considering user location. | Maximize system throughput by reassigning users to APs. | Numerical results show improvement in throughput, fairness. |
[81] | VLC and D2D Heterogeneous Network Optimization using RL | Single-agent Q-learning for optimal routing, maximizing rewards. | Single-agent RL scenario for optimal routing in VLC-D2D network. | Details Q-learning algorithm, no specific performance results. |
[87] | CNN-Based Signal Demodulator in NOMA-VLC | CNN-based demodulator to mitigate linear and nonlinear distortions. | Focuses on error propagation, multipath distortions, nonlinearity in NOMA-VLC. | Simulation and experiment results show mitigation of distortions, improved performance. |
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Miranda, R.F.; Barriquello, C.H.; Reguera, V.A.; Denardin, G.W.; Thomas, D.H.; Loose, F.; Amaral, L.S. A Review of Cognitive Hybrid Radio Frequency/Visible Light Communication Systems for Wireless Sensor Networks. Sensors 2023, 23, 7815. https://doi.org/10.3390/s23187815
Miranda RF, Barriquello CH, Reguera VA, Denardin GW, Thomas DH, Loose F, Amaral LS. A Review of Cognitive Hybrid Radio Frequency/Visible Light Communication Systems for Wireless Sensor Networks. Sensors. 2023; 23(18):7815. https://doi.org/10.3390/s23187815
Chicago/Turabian StyleMiranda, Rodrigo Fuchs, Carlos Henrique Barriquello, Vitalio Alfonso Reguera, Gustavo Weber Denardin, Djeisson Hoffmann Thomas, Felipe Loose, and Leonardo Saldanha Amaral. 2023. "A Review of Cognitive Hybrid Radio Frequency/Visible Light Communication Systems for Wireless Sensor Networks" Sensors 23, no. 18: 7815. https://doi.org/10.3390/s23187815
APA StyleMiranda, R. F., Barriquello, C. H., Reguera, V. A., Denardin, G. W., Thomas, D. H., Loose, F., & Amaral, L. S. (2023). A Review of Cognitive Hybrid Radio Frequency/Visible Light Communication Systems for Wireless Sensor Networks. Sensors, 23(18), 7815. https://doi.org/10.3390/s23187815