# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 2 Hybrid VLC/RF Networks in Indoor Scenarios under Ideal Conditions
### Network Selection and Resource Allocation
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref** & **Year** & **Hybrid Network** & **Contributions** \\ \hline
[51] & 2014 & VLC/OFDM & A combined protocol with horizontal and vertical handover mechanisms is proposed for the mobile terminal to solve the mobility of users between different access points and the OFDMA system. A VLC network scheme and its frame format are presented to solve multi-user access problems at each access point. A new metric is defined to evaluate the capacity of the hybrid network based on the assumption of homogeneous Poisson point process distribution of users. \\ \hline
[67] & 2015 & LIFI/WIFI & Load balancing is studied taking into account user mobility and handover signaling overhead. A dynamic load balancing scheme is proposed considering system throughput and fairness. The service areas of different APs are studied and the performance of each AP is analyzed using the proposed load-balancing scheme. \\ \hline
[56] & 2015 & VLC/WIFI & An efficient vertical handover scheme is investigated by formulating it as a Markov decision process problem. A dynamic approach is adapted to obtain a compromise between the switching cost and the delay requirement. The proposed scheme determines whether to perform vertical handover given the queue length and the wireless optical channel condition. \\ \hline
[68] & 2015 & RF/VLC & A cooperative load balancing that achieves proportional fairness using centralized and distributed resource allocation algorithms is studied. Performance is analyzed in terms of both throughput and fairness under various cell formation scenarios. \\ \hline
[55] & 2016 & LIFI/RF & A dynamic handover scheme based on fuzzy logic is proposed, which uses channel state information, user rate and desired data rate to determine if the handover is necessary. The objective is to reduce the handover overhead by properly assigning users to the RF or LiFi access point. \\ \hline
[57] & 2017 & VLC/Femto & An approach that takes into account multiple attributes, including dynamic network parameters and actual traffic preferences, is performed using the analytical hierarchy process. The degree of rationality of each decision is calculated by combining the results and the decision with the highest degree of rationality is selected. \\ \hline
[69] & 2017 & LIFI/RF & A dynamic load balancing scheme that takes into account the handover overhead is proposed to improve the overall system performance. A joint optimization algorithm and a separate optimization algorithm, which jointly and separately optimize the access point allocation and resource allocation, are proposed. \\ \hline
[50] & 2018 & VLC/RF & An iterative algorithm is proposed to distribute users into APs and to distribute the powers of the APs to their users. An optimization problem is formulated to allocate the power of each AP to the connected users in order to maximize the total achievable data rate. Two approaches are proposed to perform joint AP and load balancing: a main approach that considers the exact interference information for all users, and a suboptimal approach that aims to decrease the complexity of the first approach by considering only the approximate interference information of the users. \\ \hline
[70] & 2018 & LIFI/RF & A handover algorithm based on the received signal strength indicator is considered. A theoretical analysis of the handover probability based on a Markov chain model is presented. \\ \hline
[71] & 2019 & LIFI/WIFI & An overview of the concept of hybrid handover in LiFi/WIFI networks is given and related work done in this field is discussed. \\ \hline
[52] & 2020 & VLC/RF & A load balancing algorithm is proposed to manage the hybrid network resources and determine the association of users to each system. A decentralized optimization method based on Lagrangian multipliers is proposed. \\ \hline
[53] & 2021 & VLC/RF & A bidirectional selection handover algorithm is developed that takes load balancing into account to improve system performance. The appropriate destination AP is selected based on the hybrid technique of fuzzy analytic hierarchy process and order preference technique by similarity to the ideal solution for each user that needs to handover. \\ \hline
[72] & 2021 & VLC/WLAN & A vertical handover technique that uses the user’s location information to make a handover decision is proposed. The proposed algorithm is contrasted with the immediate vertical handover algorithm and the permanent vertical handover algorithm with two different dwell times. \\ \hline
[26] & 2022 & VLC/MMW & An event-triggered adaptive handover mechanism for centralized hybrid systems is proposed. The handover variables are self-adjusted using rule-based algorithms with the number of connected users and the data transmission rate as input variables. \\ \hline
[60] & 2022 & LIFI/WIFI & A higher layer handover solution known as “Li-Wi” is proposed to address handover issues due to mobility, shadows and obstacles. Robust connectivity with shorter handover outage duration and high network throughput is provided. \\ \hline \hline \end{tabular}
\end{table}