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][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].

2. VLC/RF Hybrid Network Applications

Thanks to the varied characteristics of VLC and RF networks and their complementarity, there are a wide range of applications in the technological, economic, and social-environmental fields for these types of networks. VLC/RF hybrid networks have demonstrated great improvements in overall system performance in terms of data rate, load balancing, and lower time delay in both indoor and outdoor scenarios ^[18][158]. The implementation cost of these networks is relatively low, and they allow for the reuse of the entire existing lighting infrastructure ^[19][159]. The energy efficiency of VLC/RF networks and the possibility of distributing multiple access points without interference or environmental waste can facilitate the decrease of energy consumption and environmental pollution, helping to meet the Sustainable Development Goals for the year 2030 established by the UN ^[20][160]. Figure 1 0 shows an example of the applications of VLC/RF networks and the relationship between them.

2.1. Technological Applications

VLC/RF hybrid technology could be a key tool to achieve smart homes. The data speed and security of VLC networks in conjunction with the ubiquity of RF coverage could be used to interconnect all electronic equipment within smart homes. In this way, any command could be executed on any electronic equipment in the house from a mobile terminal or simply by voice, even if far away from the house. The use of a solar panel on the roof of an IoT smart house represents a way to store energy and transmit information at the same time through the VLC network, which could then be transferred to the interior of the house through the RF network. The authors of ^[21][161] investigated the applications of hybrid VLC/RF networks in a three-dimensional indoor IoT system, taking into account temperature sensors, indoor air quality sensors, indoor sensors, and humidity sensors. IoT applications associated with factories and production have attracted the attention of researchers in recent years ^[21][22][23][103,161,162]. The goal is to use the performance of VLC/RF networks to interconnect production equipment and automation technologies to enable the generation and sharing of information between all equipment. This can enable big data generation to identify certain production patterns and even predict inefficiencies and future events. Vehicular and traffic control applications on the road are another promising options for the development of automotive technology. Due to the random nature of car traffic on the road, it is necessary to implement a hybrid technology that allows communication between cars and organizes their passage. Several authors have investigated this topic and proposed different vehicle control applications using hybrid VLC/RF networks ^[24][25][26][163,164,165]. In ^[24][163], the authors proposed an outdoor cognitive network with electric vehicles using mixed VLC and RF channels to establish interference-free communication between vehicles. On the other hand, in ^[25][164] the authors presented a hybrid VLC/RF cooperative system in vehicle-to-vehicle communication networks and analyzed the performance of the hybrid system in terms of outage probability and bit error rate. The authors of ^[26][165] investigated resource management in hybrid VLC/RF systems for wireless communication between vehicles and infrastructures.

2.2. Economic Applications

VLC networks offer a great guarantee of security, as the transmitter and receiver must be in the line of sight, the coverage area is very small, and the transmission power is focused only on the light beam. In places where it is necessary to send and receive data that must be protected (e.g., companies, banks, hospitals) the implementation of hybrid VLC/RF networks can provide great security in data transmission ^[27][28][119,127]. In scenarios where there is a high density of users connected to the internet (e.g., stadiums, airports, and parks) the use of multiple RF access points can cause interference and reduce system performance. The use of hybrid VLC/RF networks can solve this problem by reusing the entire existing lighting infrastructure for VLC data transmission. These hybrid networks provide higher data rates, reduce RF interference, and have a very low implementation cost, making them are ideal for high user density scenarios ^[29][30][31][18,63,89]. Hybrid VLC/RF networks have demonstrated a great improvement in the characteristics and capabilities of wireless communications systems. Research has focused on increasing the data rate ^[32][33][34][78,84,166] to improve the overall capacity of hybrid system, while other authors have focused on decreasing the system time delay ^[35][36][37][66,81,167]. Mining communication systems are an application that safeguards the safety of workers in mines and increases the efficiency of their work. In recent years, VLC links have been widely investigated as a complementary technology to RF links for subway mining applications due to their high data rate and freedom from RF interference ^[38][39][168,169]. Subway mines represent a challenging scenario due to their irregular walls and many shadows and dust particles, which produce the scattering phenomenon. For all of these reasons, this industry requires an optimal transverse communication system that can be supplemented by hybrid VLC/RF networks. The use of indoor, motion, and ambient dust particle sensors would be of great help in the implementation of VLC/RF networks in these and similar hostile scenarios. Technology is a great ally of agriculture, helping to achieve greater efficiency in farming with the objective of supplying the population without depleting the planet’s resources. The implementation of hybrid VLC/RF networks in conjunction with sensors dedicated to agricultural activity can be of great help in monitoring and sending information about events concerning crops and their surroundings ^[40][41][170,171].

2.3. Social-Environmental Applications

VLC/RF networks have been presented as a reliable alternative to obtain the location of mobile terminals in deep indoor scenarios (tunnels, mines, and subways) where GPS has no connection. The RF network can be used to obtain the external location of the site, while the VLC network can provide the internal location of the user. Several proposals for positioning applications using VLC/RF networks have been put forward ^[42][43][44][172,173,174], and a minimum estimation error of 5.8 cm has been demonstrated. Several authors have investigated the process of power transfer from a VLC network to the nearest users ^[27][45][118,119]. This power transfer allows the energy transmitted by the LED lights to be stored for later use in other transmissions. This application has demonstrated high energy efficiency for hybrid VLC/RF networks, and could have a large impact on overall energy consumption ^[46][47][175,176]. The growth and development of cities is an aspect that has been taken into account in technological research throughout history. The implementation of hybrid networks, especially VLC/RF networks, represents a key parameter to achieve efficient management of each area of the city, thereby improving the life of its inhabitants. Hybrid VLC/RF technology can allow energy optimization, reduce household consumption, organize the movement of vehicles, and increase the flow of mobile traffic, among many other aspects necessary to achieve the development of sustainable cities ^[48][49][50][83,131,132]. The main advantages of hybrid VLC/RF networks over conventional RF networks are their ability to increase transmission bandwidth and alleviate congestion in RF networks. In addition, these hybrid networks can be deployed and operated satisfactorily in locations where a pure RF network is insufficient. Table 25 comparatively summarizes the performance of VLC/RF and conventional RF networks for the applications proposed in this section. The ⋆ and ★ marks respectively indicate good and excellent performance considering the criteria of previous works and the corresponding performance metrics.