# A Hybrid Handover Scheme for Vehicular VLC/RF Communication Networks
## 4 Simulation Results
The average delay, the average queue length, and the packet loss rate for the I-HO, I-VHO, and the O-HO schemes were about a half of the corresponding performances for the Dwell handover scheme with a dwell time of 0.3 s and about a third for the Dwell time of 0.6 s. However, the VHO times of the I-HO and I-VHO schemes were much higher than the others, consuming too high a signaling cost of the heterogeneous networks, which might cause the ping-pong effect. The proposed optimal handover scheme, denoted by O-HO, achieved similar performance to the I-HO scheme, but performed a much lower number of VHOs (from two-thirds to lower than a half). Consequently, the O-HO scheme achieved a balance between the switching cost and the quality of service. In addition, the hybrid handover schemes (including O-HO, I-HO, and D-HO) obtained superior performance in respect of average delay time and average queue length compared to the counterpart VHO schemes (including I-VHO and D-VHO). This is because the proposed hybrid handover scheme is more effective than the other schemes. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Value** \\ \hline Packet arrival rate \(\lambda\) & 2 Packets/s \\ Packet departure rate of VLC \(\mu_{1}\) & 3.8 Packets/s \\ Packet departure rate of RF \(\mu_{0}\) & 2.2 Packets/s \\ Changing rate of the host VLC AP from On \(\rightarrow\) OFF \(\gamma_{1}\) & 0.4 s\({}^{-1}\) \\ Changing rate of the next VLC AP from On \(\rightarrow\) OFF \(\gamma_{-1}\) & 0.4 s\({}^{-1}\) \\ Changing rate of the host VLC AP from OFF \(\rightarrow\) On \(\beta_{1}\) & 0.3–1.1 s\({}^{-1}\) \\ Changing rate of the host VLC AP from OFF \(\rightarrow\) On \(\beta_{-1}\) & 0.3–1.1 s\({}^{-1}\) \\ Changing rate from the overlapping area of VLC APs to the & \(\alpha_{21}=0.4\) s\({}^{-1}\) \\ non-overlapping area & \\ Changing rate from the non-overlapping area of VLC APs to the & \(\alpha_{12}=0.2\) s\({}^{-1}\) \\ overlapping area & \\ Energy consumption for transmissions by VLC links \(E_{VLC}\) & \(5\times 10^{-4}\) kwh \\ Energy consumption for transmissions by RF links \(E_{RF}\) & \(5\times 10^{-4}\) kwh \\ Handover delay for HHO \(\tau_{H\Omega}\) & 0.1 s \\ Handover delay for VHO \(\tau_{H\Omega}\) & 0.1–0.5 s \\ Signaling cost for VHO \(E_{V\Omega}\) & 418 J \\ Signaling cost for HHO \(E_{H\Omega}\) & 0 J \\ Buffer size \(B\) & 10 Packets \\ Scaling parameter \(\theta\) & 1000 J/Packet \\ Trade-off Parameter \(\zeta\) & 500 J \\ Dwell time \(t_{0}\) & 0.3 s, 0.6 s \\ Simulation Period & 40,000 s \\ Learning rate \(\alpha\) & 0.99 \\ Convergence criterion parameter \(\epsilon\) & \(10^{-5}\) \\ Balancing coefficient \(\xi_{1}\) & 1 \\ Balancing coefficient \(\xi_{2}\) & 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. scheme introduced an early HHO decision that could avoid VHO signaling and provide continuous transmission links considering the driving state of the VTM. Figure 4 illustrates the performances of all handover schemes versus the VHO delay \(\tau_{\text{VHO}}\) with fixed VLC-link recovery rate \(\beta_{1}=\beta_{-1}=1\). The VLC HHO delay between VLC APs was set to \(\tau_{\text{HHO}}=0.1\) s and the VHO delay \(\tau_{\text{VHO}}\) was increased from 0.1 s to 0.5 s. The increment in the gap between \(\tau_{\text{VHO}}\) and \(\tau_{\text{HHO}}\) degraded the performance of the average delay time, average queue length, and the packet loss rate. Because the cost function in (11) contained a positive weighted item of \(\tau_{\text{VHO}}\), the value iteration algorithm tended to perform HHO or keep the transmission links unchanged so as to lower the total cost. As a result, the evaluated performance deteriorated but the VHO number was reduced.