## 4 Framework and Principles for VHO Scheme
In cases where the VLC link remains blocked even after this dwell time has elapsed, the MT initiates a handover to the RF link. Figure 6: The overall handover process of the Ac-VHO scheme. When the MT enters the CCI overlap area without any active RTA, the estimated time _MCCI (t)_ required to traverse the overlap area can be calculated using the MT's current velocity information. Subsequently, the dwell time \(T_{dwell}\) is determined based on this estimated time:  
\[T_{dwell}=\left\{ \begin{array}{ll}M_{CCI}(t)&if\ M_{CCI}(t)<=t_{service}\\ 0&if\ M_{CCI}(t)>t_{service}\end{array} \right., \tag{12}\]  
_MCCI (t)_ is the expected time taken to pass through the CCI region at time t and is calculated as follows:  
\[M_{CCI}(t)=L/V(t), \tag{13}\]  
The average distance of the overlapping region \(L\) and the real-time velocity \(V\) of the MT collected at the current moment \(t\) are utilized to calculate the dwell time \(T_{dwell}\). Consequently, when the MT enters the CCI region and experiences a deterioration in link quality, it will prioritize handing over to the RF link. This preference is based on the MT's slower movement and the shorter interval between occurrences of RTAs, thus ensuring the maintenance of communication quality. Figure 7: The framework of the Ac-VHO scheme. In order to accommodate different user groups with varying psychological tolerance limits, we classify active RTAs into video, VoIP, and web categories. To determine the maximum user tolerance time interval \(\Phi\) for delays in each category, we consider both perceived and actual time delays. Based on the findings in Section 3.3, the video class RTA has a maximum user tolerance time of 1 s. Consequently, the tolerance time for video class RTA delay is set within the range of [0.2, 1] seconds. Additionally, research has shown that when the delay exceeds 10 s, more than half of users tend to abandon the task for all types of application [19]. Therefore, considering the maximum short interruption time \(t_{block}\), we aim to keep the time interval for VoIP and web classes below 2.6 s. Considering the priority order of sensitivity, the psychological tolerance time interval for the VoIP class RTA is set to be shorter, while the web class RTA is allowed a longer tolerance time. By incorporating the classification of different RTA types, the setting of VHO dwell time becomes more inclusive and adaptable. This enables the MT to select the optimal time for VHO with greater precision. The specific settings for the VHO dwell time are presented in Equation (14). \[\phi=\left\{ \begin{array}{ll}[0.2,1]&if\;servicetype=\{\textit{`Video^{ \prime}}\}\\ [1.1,1.9]&if\;servicetype=\{\textit{`VoIP^{\prime}}\}\\ [1.9,2.6]&if\;servicetype=\{\textit{`Web^{\prime}}\}\\ \end{array} \right., \tag{14}\]  
The handover dwell time and decisions are based on the specific active RTA type and the link status, considering factors such as VLC LoS link blockage and MT entering the CCI region. The dwell time is set differently for each RTA category (video, VoIP, and web), ensuring optimal switching decisions and maintaining communication quality. When the VLC LoS link is blocked, the significant drop in SINR necessitates a focus on link recovery. Considering the characteristics of different RTA types, we determine the waiting times for vertical handover as follows:  
\[T_{dwell}(servicetype)=\left\{ \begin{array}{ll}average(\phi,t_{block})&if\;servicetype =\{\textit{`Video^{\prime}}\}\\ \phi&if\;servicetype=\{\textit{`VoIP^{\prime}}\}\\ \max(\phi,t_{block})&if\;servicetype=\{\textit{`Web^{\prime}}\}\\ \end{array} \right., \tag{15}\]  
When the MT enters the CCI region, we consider both the estimated traversal time of the region and the MT's maximum tolerance time interval \(\varphi\) for different RTA delays. The VHO dwell time is primarily determined by considering the user's tolerance towards delays caused by different types of RTA. The calculation is as follows:  
\[T_{dwell}(servicetype)=\left\{