# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
###### Abstract
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). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Data Rate** & **Light Source** & **Modulation** \\ \hline PHY I & 11.67–266.6 [Kbps] & only one & OOK 1, VPPM 2 \\ PHY II & 1.25–96 [Mbps] & only one & OOK, VPPM \\ PHY III & 12–96 [Mbps] & multiple & CSK 3 \\ \hline \hline \end{tabular} 1
[FOOTNOTE:1]Footnote 1: OOK: On–off keying. 2
[FOOTNOTE:2]Footnote 2: VPPM: Variable pulse position modulation. 3  
\end{table} 
Table 1: IEEE 802.15.7 (PHY) parameters. 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 [26, 27, 28, 2, 17, 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 \(\star\) and \(\bigstar\) 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.