# A Review on LiFi Network Research: Open Issues, Applications and Future Directions
## 9 Open Issues and Discussion  
In this section, the issues and challenges faced by the LiFi system are discussed according to our classification, specifically C1, C2, and C3. Issues and challenges in C4 are not considered in this section. HO, mobility, interference, and blockages are considered and analyzed. Some issues trigger another issue; for instance, the occurrence of blockage results in HO. For every problem, a specific method should be used. Figure 8 shows the causes and effects of the factors that are considered problems in the LiFi system. Given that the coverage areas of LiFi and WiFi completely overlap each other, a HO may occur. Hybrid LiFi/WiFi systems are susceptible to frequent HOs, HOs [108] between LiFi and WiFi VHO, and HOs between two LiFi AP Horizontal handover HHO. Study [108], proposed a HO scheme for a hybrid LiFi network. Study [40] suggested HO skipping to avoid frequent HOs. Notably, when users are moving, the HO rate would increase according to the speed of the movement. The effect of HO on the system and users could be impactful and might lead to latency, the consumption of system resources, and UE. Study [66] considered the HO circle in their LB scheme by calculating the average HO efficiency and HO locations of moving users. The HO radius circle decreases when the system's throughput increases, and when WiFi throughput increases, the distance between HO locations and LiFi APs decreases. Study [31] proposed an adaptive HO scheme, where the HO rate was analyzed. However, the blockage was considered to have a relationship with HO. Figure 8: Relationship of issues in LiFi (**a**): LiFi/LiFi/**(b)** LiFi/Wi-Fi. In Figure 8, two cases are presented. The first case (Figure 8a) showing the issues that could be occurred in stand-alone LiFi network with multiple APs. The second case (Figure 8b) shows the issues in hybrid LiFi/Wi-Fi networks caused by similar factors. Various factors such as mobility, blockage, and interference can cause a particular effect on users served by the network. The effects can be VHO, HHO, and DoS may result from one or more factors simultaneously on connected users in both cases. For instance, user mobility and blockage could result in Denial-of-Service DoS or HHO for the connected user in the stand-alone LiFi network. However, in the hybrid system, no DoS occurs because the Wi-Fi signals are available as alternative connections when the LiFi user is blocked or during LiFi AP overload, which leads to VHO. The cause of a specific effect (issue) in the system. The colored area in the diagram represents the effect of multiple HO in (Figure 8b) could be one of both VHO and HHO. Each circle represents a factor or event which is the factors that occur to the connected user. Figure 9 depicts a hybrid LiFi/WiFi network system reflected signal NLOS, blocked signal, and connected signal [57; 82]. In fact, there are certain key drawbacks to the radio wave spectrum WiFi that encourage the use of LiFi and hybrid networks, such as (i) power, (ii) performance, (iii) accessibility, and (iv) security. Some problems and concerns should be resolved. First of all, LiFi needs LOS, which is critical to mobility and blockages. Thus, NLOS supports the connection but provides a much lower data rate [39; 105]. LiFi DLT does not work in the dark because light does not pass through artefacts; hence, the signal is automatically lost if the receiver is unintentionally blocked by a person walking in front of a light source [183; 184]. The LiFi service area is small, and this issue has two possible solutions, precisely one for indoors and one for outdoors. The first one suggests the addition of APs to provide lighter and signal density. The second one indicates the deployment of more powerful light bulbs. The first solution is suitable for indoor environments, such as houses, companies, industries, and inside vehicles, cars, and airplanes.