# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 2 Hybrid VLC/RF Networks in Indoor Scenarios under Ideal Conditions
### Load Balancing  
One of the most important elements in hybrid networks is load balancing, as it manages traffic and resources in the network. Load balancing helps to evenly distribute a load of operations and traffic among each device connected to the hybrid network. This reduces network congestion, prevents overheating of devices, and improves performance in terms of speed, latency, and reliability [50]. Part of the literature on VLC/RF systems focuses on the development of algorithms to optimize network load balancing. The goal is for each user to find the best VLC or RF access point which provides the highest data rate depending on the location and mobility of each user. In [51], a protocol combined with horizontal and vertical handover mechanisms was proposed for a mobile terminal to solve the mobility of users between access points and the OFDMA system. In addition, a VLC network scheme was presented to solve multi-user access problems at each access point in the network. The authors of [52] proposed a load-balancing algorithm to manage hybrid VLC/RF network resources and determine the association of users to each system, and additionally proposed a decentralized optimization method based on Lagrangian multipliers. In [53], a bidirectional selection handover algorithm that takes load balancing into account was developed to improve the overall performance of the hybrid system. Such an algorithm selects the suitable target AP based on the hybrid technique of fuzzy analytic hierarchy process and order preference technique based on the similarity to the ideal solution. Figure 5 shows a load balancing scheme of a hybrid VLC/RF network in an indoor scenario. Here, users are associated with an access point, either VLC (red triangles) or RF (blue squares), depending on the location and possible movement of each one. The association of users with the AP only takes place within the association zone (yellow shadow). In order to save network resources, access points outside of this zone (far from the users) do not transmit. Another part of the research focuses on the objective of minimizing the handover latency and increasing the overall network throughput using load-balancing techniques such as Handover. Handover in hybrid VLC/RF networks is the process through which a user switches from one network to another without losing connectivity. Thus, if a user connected to the VLC network moves to an area without access to the VLC network, the device automatically connects to the available RF network [54]. Similarly, if the user returns to a VLC access zone, the device automatically reconnects to the VLC network without losing connectivity. During this process, the quality of the received signal and the availability of the networks must be monitored to ensure that the connection transfer from one network to another is optimal. In [55], the authors proposed a dynamic handover scheme based on fuzzy logic with the objective of reducing the handover overhead. This scheme takes into account channel state information, user rate, and desired data rate to determine whether or not to request a handover. If a mobile terminal moves from one access point to another, a handover of the mobile load occurs between the access points. This handover can be classified as horizontal if the change occurs intra-system or vertical if  
Figure 5: Load balancing scheme of a hybrid VLC/RF network in an indoor scenario. the change occurs inter-system. Vertical handover is a fundamental type of handover to improve the performance of hybrid VLC/RF networks. The authors of [56] formulated a vertical handover scheme as a Markov decision process problem. Moreover, they adopted a dynamic approach to obtain a compromise between the switching cost and the delay requirement. On the other hand, the authors of [57] proposed an algorithm based on decision selection, taking into account dynamic network parameters and actual traffic preferences.