# A Hybrid Handover Scheme for Vehicular VLC/RF Communication Networks
## 4 Simulation Results  
In this section, the performance of the proposed hybrid handover scheme, labeled as O-HO, is compared with several benchmarks, listed as follows:  
* Immediate Handover (I-HO): Handover is immediately performed when the current VLC link is interrupted. HHO is executed with high priority if the next VLC AP is accessible. * Dwell Handover (D-HO): When blockage of the current VLC link is detected, the VTM waits for link recovery for a period of dwell time. If the VLC link is not recovered during waiting, handover is performed in the order of HHO and VHO. When the blocked VLC link is recovered, transmission with the host VLC AP continues. * Immediate Vertical Handover (I-VHO): VHO is immediately performed once the current VLC link is interrupted. * Dwell Vertical Handover (D-VHO): When blockage of the current VLC link is detected, the VTM waits for a period of dwell time. When the dwell time expires, VHO is performed if the VLC link has not been recovered. Otherwise, the transmission continues by the recovered VLC link to the host VLC AP. In the benchmark handover schemes, the VTM is supposed to switch back to the VLC AP once the VLC link is recovered, because the VLC link is always preferable due to its high transmission rate. Thus, the dwell time helps avoid the potential ping-pong effect. Four performance metrics were adopted to evaluate the handover schemes: _the packet loss rate_\(\rho\), defined as the ratio of the number of the dropped packets to the total number of the arrived packets; _the average delay_\(d\), defined as the average time a packet waits to be processed; _the average queue length_\(l\), defined as the average number of packets waiting in the queue; and _the number of VHOs_\(c\). The number of HHOs was not taken as a performance metric, because HHO would have been performed when the VTM moves and the signaling cost of HHO is much less than that of VHO. Monte Carlo simulation was carried out over a period of continuous time. The simulation parameters are summarized in Table 1. The packet arrival and departures of RF and VLC links were modeled as Poisson processes with rates \(\lambda\), \(\mu_{0}\), and \(\mu_{1}\), as stated before. The VLC-LoS link blockages of the host VLC AP and the next VLC AP were modeled as independently negatively exponentially distributed blocking events with duration parameters \(\gamma_{1}\) and \(\gamma_{-1}\). Similarly, the non-blocking duration for the LoS link of the host VLC AP and the next VLC AP were modeled with parameters \(\beta_{1}\) and \(\beta_{-1}\)[29]. The processes of VTM driving into the VLC APs' overlapping areas and non-overlapping areas were also modeled as Poisson distributions with parameters \(\alpha_{12}\) and \(\alpha_{21}\), respectively. Since we focused on the switching cost of the handover process, the energy consumption of the VLC and RF transmission was assumed to be the same. The handover cost of HHO was assumed to be zero, because HHO would have been performed when driving. Finally, the performance metrics of the proposed hybrid handover scheme and the benchmarks were counted and calculated statistically. As shown in Figure 3, the four considered performance metrics were evaluated with respect to the link recovery rate of the VLC APs denoted by \(\beta_{1}\) and \(\beta_{-1}\), ranging from 0.3 to 1.1 (s\({}^{-1}\)). The handover delays for HHO and VHO were set to \(\tau_{H\Omega}=0.1\) s\({}^{-1}\) and \(\tau_{H\Omega}=0.3\) s\({}^{-1}\) in the simulations. Larger values of \(\beta_{1}\) and \(\beta_{-1}\) indicate a shorter VLC link blockage duration and a greater chance of link recovery. Therefore, the average delay time, the average queue length, and the packet loss rate of all handover schemes decreased remarkably. It can be observed that the I-HO, I-VHO, and the O-HO schemes significantly outperformed the others in terms of the average delay, the average queue length, and the packet loss rate.