# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
###### Abstract  
The expectation for communication systems beyond 5G/6G is to provide high reliability, high throughput, low latency, and high energy efficiency services. The integration between systems based on radio frequency (RF) and visible light communication (VLC) promises the design of hybrid systems capable of addressing and largely satisfying these requirements. Hybrid network design enables complementary cooperation without interference between the two technologies, thereby increasing the overall system data rate, improving load balancing, and reducing non-coverage areas. VLC/RF hybrid networks can offer reliable and efficient communication solutions for Internet of Things (IoT) applications such as smart lighting, location-based services, home automation, smart healthcare, and industrial IoT. Therefore, hybrid VLC/RF networks are key technologies for next-generation communication systems. In this paper, a comprehensive state-of-the-art study of hybrid VLC/RF networks is carried out, divided into four areas. First, indoor scenarios are studied considering lighting requirements, hybrid channel models, load balancing, resource allocation, and hybrid network topologies. Second, the characteristics and implementation of these hybrid networks in outdoor scenarios with adverse conditions are analyzed. Third, we address the main applications of hybrid VLC/RF networks in technological, economic, and socio-environmental domains. Finally, we outline the main challenges and future research lines of hybrid VLC/RF networks. hybrid networks; optical camera communication (OCC); radio frequency (RF); visible light communication (VLC); internet of things (IoT) 20232237545 202337545 202337545 202375  
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 802.11 bb to refine the medium access control (MAC) and physical layer (PHY) of VLC [12]. One of the essential features of the IEEE 802.11 bb 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].