1. Introduction
In recent years, the limitations of the radio frequency (RF) spectrum for mobile communications have become very evident [
1]. This is a great challenge to overcome in light of the traffic load demands associated with fifth-generation (5G) mobile communication, with alarming projections beyond 5G and into the sixth generation (6G). Due to these limitations, the dense deployment of RF access points leads to high competition for available channels [
2], which entails degradation of the quality of service (QoS).
As an alternative solution, visible light communication (VLC) has been proposed, an optical wireless communication technology that uses the visible light spectrum with wavelengths between 375 and 780 nm [
3]. This technology uses light-emitting diodes (LEDs) that produce incoherent light to illuminate and transmit data simultaneously, achieving high data rates of hundreds of megabits per second [
4]. With the goal of secure and efficient communications, the VLC Consortium (VLCC) was established to promote and standardize VLC technology in 2007 [
5]. VLC is characterized by high security, as information cannot be filtered [
6]. In addition, it is immune to RF interference, meaning that the system can be used freely in environments sensitive to electromagnetic signals [
7]. VLC has experienced rapid development and has attracted much interest from researchers [
8]. In 2011, the Visible Light Communications Task Group created the IEEE 802.15.7 standard [
9] that establishes three physical (PHY) VLC operation modes, as shown in
Table 1. The three PHY modes can coexist with each other, thereby mitigating LED flicker and reducing dimming [
10]. In 2019, the ITU-T Telecommunication Standardization Sector finalized recommendation G.9991 that established the first commercially ready light fidelity (Li-Fi) standard [
11]. Both IEEE 802.15.7 and G.9991 have now converged into IEEE 802.11bb to refine the medium access control (MAC) and physical layer (PHY) of VLC [
12]. One of the essential features of the IEEE 802.11bb standard [
13] is that there is a single medium access control (MAC) sublayer common to all physical layers. This feature enables easier interoperability between the different physical layers, making cooperation between RF and visible light communications technologies possible [
2].
On the other hand, the implementation of the B5G cellular networks has brought with it an increase in mobile traffic with applications such as virtual reality, augmented reality, and online video games [
14]. Ultra-high capacity wireless connectivity is considered a key technology to support end-to-end delivery in the era of beyond 5G and 6G [
15,
16]. Therefore, there is a need for a network infrastructure capable of supporting multiple new services that demand ultra-high transmission speeds, device connectivity, and high quality of experience (QoE) [
17].
In the last few years, researchers have shown great interest in hybrid VLC/RF systems, and this type of network is expected to be the basis for the development of B5G and the next generations of cellular networks [
18,
19,
20]. Hybrid VLC/RF networks are presented as a complementary technology capable of realizing cooperation between RF and VLC networks without interfering with each other [
18]. They are able to significantly increase the system data rate and achieve better load balancing by distributing the data load between the two networks [
19]. Additionally, they have a very low implementation cost due to the extensive use of LEDs in lighting infrastructure around the globe that can be reused for mobile communication [
20].
In this sense, a complete search was carried out in the Scopus database [
21] using the keywords of Visible Light Communication/Ratio Frequency. A total of 727 English-language papers were collected from the years 2012–2022. A bibliometric analysis was performed using the VOSviewer software tool [
22] (see
Figure 1). The analyzed items were the published documents, keywords, country distribution, and affiliation.
Figure 1 shows a clear increase in the number of research papers published in the last decade in the field of hybrid VLC/RF networks, which demonstrates the growing scientific interest in this area of research. The countries appearing in the search according to the affiliation of the main authors were China (257), the United Kingdom (99), India (92), the USA (54), and Turkey (44). The universities with more than five publications related to the VLC/RF field were Ozyegin University, Istanbul; Southeast University, China; and Northumbria University, United Kingdom. This analysis shows that research related to this area is greater in the northern hemisphere than in the southern hemisphere, Chile being one of the countries with the lowest number of publications in this area (2).
The emergent Internet of Things (IoT) has boosted the need for innovative communication solutions that provide reliable and efficient data transmission methods. An intelligent, efficient, and reliable system can be obtained by combining VLC and RF technologies [
23]. This integrated system can support various IoT applications, such as smart lighting, home automation, e-healthcare, and industrial IoT. For example, in smart lighting the VLC network can be used to provide high-speed communication through luminaries while the RF network can be used to control luminaries from a remote location [
24]. Similarly, in e-healthcare applications the VLC network can provide reliable communications between medical devices, while the RF network can transmit data to healthcare providers or emergency services. Therefore, the implementation of hybrid VLC/RF networks offers a reliable and efficient solution for IoT applications, especially indoors [
25].
Due to the great interest shown in hybrid VLC/RF networks, several authors have conducted reviews of the history of these networks [
2,
17,
26,
27,
28,
29]. However, the dynamic and changing characteristics depending on the deployment scenario and the extensive evolution of VLC/RF networks in recent times have produced the need to delve deeper into issues that have not been fully analyzed by the existing reviews.
Table 2 compares this work with the most prominent reviews in the last ten years relating to hybrid VLC/RF networks. We used the markers ⋆ and ★ to represent partially and deeply analyzed topics, respectively. In our criterion, we mark as partially analyzed those topics that were only commented on in a general way without going deeper into characteristics, problems, and possible solutions. On the other hand, the papers marked as deeply analyzed are those that we consider to address each of the topics mentioned in detail, providing characteristics of the technologies, pending challenges, an analysis of what has been achieved by other authors, and possible solutions that can be carried out to optimize VLC/RF networks.
Therefore, in this work an extensive study of the state of the art of hybrid VLC/RF networks in ideal indoor scenarios in the scientific literature is carried out and the characteristics and implementation of these networks in outdoor scenarios with harsh conditions are analyzed. The main applications of hybrid VLC/RF networks in the technological, economic, and social-environmental fields are considered. In addition, the main challenges faced by hybrid VLC/RF networks and future research directions are discussed.
The rest of this paper is organized as follows. In
Section 2, the state of the art of hybrid VLC/RF networks in indoor scenarios with ideal conditions is reviewed, including analysis of aspects such as lighting requirements, hybrid channel model, load balancing, network selection, and resource allocation, and hybrid network topologies. In
Section 3, a review of the state of the art of hybrid VLC/RF networks in outdoor scenarios with harsh conditions is presented.
Section 4 analyzes the possible applications of hybrid VLC/RF networks in the technological, economic, and social-environmental fields.
Section 5 establishes the main challenges and future directions of these hybrid networks. Finally, the conclusions of this review are presented in
Section 6.
3. Hybrid VLC/RF Networks in Outdoor Scenarios with Harsh Conditions
In an outdoor scenario with harsh conditions (e.g., sunlight, rain, dust, wind) the communication channel of hybrid VLC/RF networks becomes very complex and dynamic. These conditions distort the VLC optical beam, causing amplitude and phase fluctuations along with optical losses. The level of loss of the VLC signal is higher than the losses suffered by the RF network, as the VLC signal is very sensitive to atmospheric conditions such as sun, rain, and fog [
136]. There are several factors (scattering, absorption, non-alignment, and turbulence) that can cause signal loss in both the VLC and RF networks, which complicate the design of the hybrid communication channel model [
137]. Among the most important phenomena responsible for signal loss in VLC/RF networks are:
Scattering corresponds to an angular spread of the optical field, which is independent of frequency and wavelength in certain cases. It can be caused by solar radiation, rain, or particles and objects in the environment, which create a divergence in the transmission beam and cause the size of the received transmission beam to be larger than the receiver aperture, resulting in part of the signals being lost.
Absorption takes place when a collision occurs between the photons of the propagating beam of the VLC network and the particles or molecules existing in the environment. The absorption coefficient depends on the type of molecules in the air and their concentration. There are areas of minimum absorption in the atmosphere called transmission windows. One way to avoid or decrease this phenomenon is to select the wavelengths of the grid to be within the wavelengths of the transmission window.
The distance between the transmitter and receiver in an outdoor scenario can cause non-alignment of the transmitter and receiver, resulting in a loss of signal power. Non-alignment between the transmitter and receiver can be due to the presence of obstacles, buildings, and trees, as well as changes in the weather. As the distance between the transmitter and receiver increases, these factors must be taken into account.
Ambient turbulence corresponds to a divergence in the optical signal, which can be caused by random temperature variations, solar radiation, increased wind speed, and atmospheric pressure. Divergence caused by turbulence during transmission results in random variations of signal amplitude and phase, causing fading of the received power and decreased performance of hybrid systems.
In an outdoor scenario, multiple sources of noise and interference can affect hybrid VLC/RF networks, including shot noise, thermal noise, and environmental interference. Among these, sunlight is considered the most serious co-channel interference source for the VLC network, and thus for the hybrid network, as it can significantly reduce the SINR of the system. One way to reduce the level of sunlight interference in the VLC network is to use lenses and filters on the receivers; however, this is not an optimal solution.
The harsh conditions in outdoor scenarios are important factors to take into account in the design of hybrid VLC/RF networks. A study of the degrading effects of physical phenomena on the quality of VLC communications in an industrial environment, where dust, other particles, and artificial light sources increase signal attenuation and cause optical interference, was carried out in [
138]. In such cases, it is necessary to design and implement a robust model capable of considering the existing environmental conditions in order to improve the overall performance of the hybrid system [
139]. The use of transmitters and receivers with complementary metal oxide semiconductor-based cameras and image sensors is a robust way to realize VLC network connection in outdoor scenarios, as complementary metal oxide semiconductors are capable of converting the modulated optical signal into an electrical waveform. As in indoor scenarios, a hybrid VLC/RF network can be implemented through different topologies in outdoor scenarios, mainly depending on the type of VLC network link, i.e., line-of-sight (LOS) or non-line-of-sight (NLOS). A very suitable scheme for an outdoor scenario with a line-of-sight link is the Point-to-Point topology. In this topology, the transmitters and receivers are in a fixed position and oriented towards each other. Otherwise, if the link is NLOS, a fuzzy topology can be used in which the transmitter and receiver are not require to be aligned. In this case, a receiver with a panoramic Lambertian pattern that can rotate within the radiation field of the transmitter is used to receive the signal [
140].
One of the main challenges in hybrid VLC/RF networks stems from difficult outdoor conditions, mainly due to the high mobility of background noise caused by sunlight or other artificial light sources. A study of the impact of sunlight noise sources on the packet delivery rate in a VLC network was carried out in [
141]. The results showed that when the sun is not in the receiver’s field of view (FOV) a packet delivery rate of
can be achieved for distances of less than 100 meters. On the other hand, Turan performed a time-domain characterization of the VLC channel by conducting real experiments in a static outdoor scenario during different times of the day [
142]. The results showed that sunlight both affects the effective useful bandwidth of the VLC channel and reduces the channel gain when direct sunlight is present in the receiver FOV. To counteract the adverse effects of sunlight on the VLC channel, the receiver FOV can be optimized by adjusting the receiving angle. It should be noted that FOV adjustment is usually a compromise between noise reduction and the possible reception angle. To avoid reducing the receiver FOV too much, in [
143] the use of a high-pass filter was proposed to filter the sunlight intensity in a one-channel VLC experiment. This filter was composed of a biconvex lens adaptive optical receiver, allowing high reduction of background noise without affecting the FOV angle. In addition, the authors coated the entire area from the optical receiver to the photodiode in order to block unwanted light angles, as shown in
Figure 8. Finally, they added a DC blocker after the photodiode to block strong DC components. Experiments showed that data transmission is possible up to a distance of 40 m under perfect lighting conditions; however, the connection distance decreases in strong sunlight.
In visible light communication, due to the propagation and directionality characteristics of light it is important to establish a clear line-of-sight (LOS) link between the transmitter and receiver. However, maintaining this LOS link becomes a challenge when users are in motion. This is especially true in outdoor environments, where light beams can travel long distances and reflected components lose energy, making reliable communication difficult. In [
144], the authors suggested that non-line-of-sight (NLOS) communication via ground reflections can be advantageous in VLC. The effectiveness of NLOS communication depends on factors such as weather conditions and ground surface materials, which influence ground reflectivity. Unlike traditional wireless communication, VLC communication does not suffer from multipath fading caused by NLOS reflections. This is due to the inherent spatial diversity resulting from the carrier wavelength of visible light waves, which is significantly smaller compared to the size of typical receivers [
43].
Based on the conditions of the outdoor VLC channel and the solutions proposed by [
141,
142,
143], we propose designing a hybrid outdoor VLC/RF network as shown in
Figure 9. This hybrid VLC/RF network can provide service to users through both networks at times when the solar intensity direction does not coincide with the direct FOV angle, thereby taking advantage of the multiple benefits of hybrid networks. At times when it is not possible to connect through the VLC network due to saturation of the photodiode by high background noise caused by sunlight, the service can be provided only by the RF network to ensure that users are not left without connection at any time. At night, the VLC network can be affected by artificial light sources such as lights used by cars and street lights. These light sources have much lower luminance values than sunlight, and over long distances they should not affect the VLC network connection [
145]. In case the artificial lights are at a relatively short distance from the user connected to the VLC network, they can be blocked by the filter of the optical receiver as long as they have a different direction from the FOV angle.
Due to the attractions and advantages of VLC networks, such as offering greater data security, being less susceptible to interference from external signals, offering greater bandwidth for data transmission, and making efficient use of the frequency spectrum, thereby helping to alleviate congestion in the radio frequency spectrum, multiple studies [
140,
141,
142,
143,
146,
147] have been conducted on the study of VLC networks in outdoor scenarios regardless of the presence or absence of sunlight. The deployment of outdoor VLC networks aims to extend the multiple advantages of this technology to all users, regardless of their locations and scenario. A study of the impact of sunlight noise sources on the packet delivery rate in a VLC network was carried out in [
141]. In [
142], a time-domain characterization of the VLC channel was performed by conducting experiments in a static outdoor scenario during different times of the day. In [
143], a high pass filter was proposed to filter the sunlight intensity in a one-channel VLC experiment. Outdoor VLC network access points could be powered by solar panels or other renewable energy sources, meaning that there would be no additional energy costs during the day. In addition, VLC technology could use directional LEDs with a small light cone that transmit only the desired information and would not be occupied by unnecessary lighting. On the other hand, the VLC network saves energy at night, as the lighting LEDs act as data transmitters, eliminating the need for separate devices for lighting and communication. However, outstanding issues related to interference caused by solar illumination on VLC devices must be resolved. Consequently, several investigations on outdoor VLC networks have focused on this issue [
140,
141,
142,
143].
Other external factors that greatly affect the VLC channel include difficult weather conditions such as fog, rain, and snow. Normally, molecules and particles in the atmosphere interact with the light, deflecting the beams and attenuating the transmitted signal. In adverse weather conditions these effects are much more severe, as fog, rain, and snow are made up of larger particles that have a large impact on the range and reliability of VLC. The study conducted in [
148] showed that fog has a greater negative impact on VLC; however, the study conducted in [
149] showed that dry snow can be even more detrimental to VLC system performance than fog. For this type of scenario in which weather conditions are adverse, it is possible to establish reliable communication up to 15 m using the highest possible modulation scheme [
150].
The hybrid VLC/RF networks and their deployment unlock multiple possibilities for IoT applications in various outdoor scenarios [
151]. The high-precision communication capabilities of VLC technology make it an effective solution for interfacing with smart sensors that require accurate real-time data transmission. At the same time, the RF network ensures ubiquitous connectivity and uninterrupted communication. Among the applications with potential for innovative systems are smart outdoor streetlights that autonomously adjust to changing traffic patterns, security cameras that detect movements with high precision, and traffic control systems that can effectively manage traffic congestion in real time. With the integration of VLC/RF networks, exciting new possibilities that expand the realms of IoT become a reality [
152].
Several authors have started to investigate this topic, and the first steps in the design of hybrid VLC/RF systems in outdoor scenarios can already be found in the scientific literature [
153,
154,
155,
156]. In [
153], an optimization problem was established to obtain the maximum possible information rate for users through the DC offset and AC peak amplitude of the VLC network access points along with the electrical power supplied to LEDS and relay relays. On the other hand, in [
154] the energy efficiency of a Non-Orthogonal-Multiple-Access (NOMA) scheme of hybrid VLC/RF networks with imperfect hybrid channel state information was investigated. It was shown that a non-orthogonal multiple access scheme has higher performance compared to the VLC/RF network in an outdoor scenario where LOS availability is limited. The authors of [
155] evaluated a Co-NOMA-based cooperative transmission scheme for hybrid VLC/RF systems in outdoor scenarios where users with weak VLC connections can connect to the network through users with strong VLC connections using the RF network. An increase in the sum rate of the system was demonstrated, as was increased user fairness. Recently, a hybrid FSO-VLC/RF system capable of providing communication services to emergency responders for an outdoor fronthaul network based on a machine learning algorithm was experimentally demonstrated in [
156]. A study of outdoor VLC for vehicle-to-vehicle security applications was conducted in [
157]. The authors introduce the privacy and security fields in the physical layer of VLC to improve the vehicle-to-vehicle power distribution.
5. Challenges and Future Directions for Hybrid VLC/RF Networks
This section discusses important issues facing hybrid VLC/RF networks as well as their future directions, including the use of Software Defined Networking (SDN) platforms, multiple access techniques, the development of new modulation and coding schemes, the design of efficient resource allocation and handover algorithms, the development of security mechanisms, the hybrid channel model, load balancing, uplink communication in VLC, outdoor scenarios, and Optical Camera Communication (OCC).
The use of SDN platforms has been booming in recent years due to their ability to efficiently manage all access points in the network. Several papers have been presented on SDN [
177] in which the advantages of its use in resource allocation and network scalability have been demonstrated. The use of SDN platforms in hybrid VLC/Femtocell networks remains in its infancy, and it remains necessary to study this topic in greater depth. The use of SDN platforms in hybrid VLC/RF networks must overcome challenges such as the coexistence of technologies and hybrid topology management [
178]. Hybrid VLC/RF networks must be able to interoperate efficiently with existing technologies, which can pose compatibility and interference management challenges. In addition, the management of hybrid topologies may require a more dynamic and flexible approach to ensure optimal connectivity between the two technologies. Therefore, new standards and algorithms for VLC/RF networks on SDN platforms that facilitate interoperability and coexistence of the technologies are required. This can help minimize compatibility issues and manage interference effectively [
179].
Supporting user mobility is vital in hybrid networks. The user may move in the scenario with multiple obstacles, which makes the location of the mobile receiver and the coordination mechanism between the LED transmitter and the RF base station a key challenge for multiple access techniques [
51]. The CSMA/CA access technique has been presented separately for single network scenarios instead of hybrid networks [
180] making a CSMA/CA access technique especially designed for hybrid VLC/RF networks required in order to support user mobility. On the other hand, non-orthogonal multiple access (NOMA) techniques have been presented in recent works as a promising option for multiple access between users via power multiplexing [
97,
118].
The communication channel is the medium responsible for transmitting information between the transmitter and receiver. Efficient channel modeling is vital because it reduces multi-project and interference effects [
40]. It is necessary to optimally design a hybrid channel model that integrates VLC and RF technologies and takes advantage of the benefits provided by each channel independently. The design of hybrid communication channels for VLC/RF technologies is considered a major challenge due to the marked differences in the communication channel of each network. The channel model, modulation and propagation techniques, data rate, transmission capacity, and coverage are among of the characteristics that differentiate VLC and RF networks [
45]. Despite this, because VLC and RF networks do not interfere with each other it is possible to design a common channel for both technologies. The correct selection of the modulation techniques of the networks and the synchronization between the channels available for the transmission of both technologies is fundamental to optimizing the design of a hybrid channel [
46]. Although several channel designs have been proposed for hybrid VLC/RF networks, it remains necessary to optimally design a hybrid channel model that integrates VLC and RF technologies and takes advantage of the benefits provided by each channel independently.
A technical concern of hybrid VLC/RF networks is to achieve efficient load balancing. The main question is how to allocate users between VLC access networks and RF femtocells. The first step to finding an answer to this question involves solving the problem of joint association and resource allocation [
50]. Although several authors have investigated these problems [
53,
55,
56,
57,
59], several parameters remain to be solved before efficient load balancing can be achieved. It is necessary to delve deeper into issues such as the fair distribution of user load; dynamic allocation algorithms could be developed to ensure this balance [
51]. In addition, it is necessary to develop load-balancing algorithms that analyze the state of each base station and dynamically assign users to base stations with less load. This can help to avoid congesting the stations and make the best use of the network resources [
53]. During a call session a load balancing mechanism must be performed periodically, and users may need to switch to an AP with better service if they are on the move [
70].
Due to the particular characteristics of each technology, it is essential to develop new modulation and coding schemes to improve the performance of hybrid VLC/RF networks [
181]. While traditional RF networks have limitations such as electromagnetic interference and signal attenuation, VLC networks are not subject to these limitations and can offer higher transmission rates. However, their sensitivity to physical obstacles and variations in light intensity are relevant challenges. It is necessary to develop modulation and coding schemes to overcome these limitations of hybrid RF/VLC networks. These schemes have to mitigate the adverse effects of interference and attenuation on data transmission in order to improve the quality and reliability of optical communication [
182]. These new schemes should be able to adapt to environmental conditions and optimize spectral efficiency to take full advantage of the available bandwidth [
183].
Resource allocation and handover are essential to transmit large volumes of data quickly and reliably in hybrid VLC/RF networks [
53]. These algorithms are crucial to the effective and balanced allocation of transmission resources such as bandwidth, transmitting power, and processing capacity. Moreover, they ensure continuous and uninterrupted transmission by managing the handover of data between VLC and RF technologies. Resource allocation and handover have a direct impact on the quality of service offered to users by maximizing network capacity and avoiding congestion. In addition, proper resource allocation helps to reduce energy consumption and costs associated with data transmission. Although several algorithms have been proposed for hybrid VLC/RF networks [
26,
60,
66], there is a need to design new algorithms that optimize resource usage and decrease handover latency between networks. The use of tools such as artificial intelligence can be of great help in the evolution towards more efficient algorithms.
The growing dependence on communication technologies and the need to protect the integrity and confidentiality of the information being transmitted makes security mechanisms in hybrid VLC/RF networks of vital importance today [
184]. Multiple vulnerabilities and security risks can arise from the use of different communication technologies in hybrid VLC/RF networks. Therefore, it is necessary to develop security mechanisms that protect the privacy of the information transmitted, prevent unauthorized interception of data, and prevent attacks and malicious manipulation. These security mechanisms should include data encryption techniques, user and device authentication, and intrusion detection and prevention. In addition, the integrity of transmitted data must be guaranteed, while access and control policies must be established to ensure that only authorized users can access the network [
185,
186].
To date, VLC systems lack acceptable uplink performance; for this reason, hybrid networks are forced to use different direct and return communication paths [
93,
111]. For uplinks, these systems require the beam to be in a fixed direction; thus, shifting the MTs can greatly impair system performance and make the link inadequate. One of the main reasons for this is that smart mobile devices have limited power; it has not yet been possible to integrate a light source for communications [
115,
116]. Therefore, supporting uplink transmission using VLC is a challenge in the implementation of hybrid systems.
Outdoor scenarios are characterized by their very complex and dynamic communication channel. There are several factors that cause fading of the transmission signal, such as scattering, absorption, non-alignment, turbulence, and sunlight. Sunlight is considered to be the most severe co-channel interference source for VLC networks, and coupled with other sources of signal attenuation results in a very low SNR. Another major challenge for the VLC/RF network is the support of mobility in outdoor scenarios where obstacles and time changes can affect the direct link between the transmitter and receiver. To counteract the adverse effects of sunlight on the VLC channel, the receiver FOV can be optimized by adjusting the receiving angle. However, FOV adjustment is usually a compromise between noise reduction and the possible reception angle [
155]. To avoid reducing the receiver FOV too much, in [
143] the authors proposed using a high-pass filter to filter the sunlight intensity. In addition, it was proposed to coat the entire area from the optical receiver to the photodiode in order to block unwanted light angles [
143]. An optimal design of the communication channel of VLC/RF networks in harsh environments capable of adapting to the existing environmental conditions and enhancing overall system performance is required [
153,
154].
Hybrid VLC/RF networks could be a key point in long-distance (satellite) communication. This could be achieved through the use of solar panels designed in such a way that they can receive satellite information via sunlight or detecting light from lasers [
140]. This is a very interesting open research topic, as it would make sunlight a source of illumination, energy, and data transmission.
Optical camera-to-camera communication technology (OCC) is a branch of visible light communication that has emerged in recent years. OCC communication, like VLC, takes place between an optical source and a camera composed of image sensors. OCC can be implemented in any type of camera, smartphone, or any device that has integrated image sensors [
187]. No changes are needed to the current infrastructure of imaging sensors that act as signal receivers; therefore, the complexity and installation cost are very low. Several papers have presented OCC technology as a fundamental key for the development of 5G communication systems in both indoor and outdoor scenarios [
188,
189,
190]. Therefore, it would be very interesting to combine OCC technology with hybrid VLC/RF networks as a way to improve system performance.
Hybrid VLC/RF networks have a great deal of potential for IoT applications. In the future, these networks are expected to be used to improve connectivity in areas with weak RF signals [
191]. Additionally, hybrid networks can overcome indoor positioning challenges by combining the high resolution of VLC with the ubiquity of RF networks. Another area in which these networks can be applied is smart agriculture, where VLC can be used to monitor crop growth and RF can be used for data communication. The integration of VLC and RF technologies can lead to the development of more efficient energy-saving solutions. In summary, we anticipate a bright future for hybrid VLC/RF networks in IoT applications in light of their ability to solve many of today’s limitations and offer innovative solutions for a wide range of industries [
192].