## 4 Framework and Principles for VHO Scheme  
Designing effective VHO schemes for VLC heterogeneous networks involves addressing the challenges posed by MT mobility and diverse types of RTA. In light of these considerations, our proposed VHO scheme takes into account various RTA types and factors contributing to VLC link quality degradation, enabling adaptive handover decisions tailored to specific scenarios. Figure 5: The probability distribution model of RTAs. The overall handover process facilitated by our scheme is depicted in Figure 6. Initially, the MT enters the designated area and establishes a connection with the VLC link, subsequently engaging in random movement within the predefined time window. The multimode terminal then captures information regarding the active RTAs and link changes, followed by the configuration of dwell time. Finally, VHO is executed based on the determined dwell time, enabling a seamless transition between VLC APs. Disruptions in the VLC link can introduce delays in the execution of RTAs on the MT. As different types of RTA exhibit varying degrees of sensitivity to delay, users have distinct psychological tolerance thresholds for delays associated with different applications. To optimize the QoE, it is imperative to make informed handover decisions that consider the specific type of active RTA and aim for enhanced stability and performance. When confronted with changes in the link state, the MT must make reasonable VHO decisions that align with the active RTA type. These decisions serve the dual purpose of avoiding unnecessary and detrimental back-and-forth switching while ensuring optimal data transfer rates. Hence, it is crucial to determine an appropriate dwell time before initiating a switch. Moreover, this dwell time should be determined based on the current link state information and the specific RTA type, with the aim of satisfying user requirements and facilitating satisfactory handover decisions. In our proposed Ac-VHO scheme, we not only consider the classification of VLC LoS link influencing factors in setting the pre-switching residence time, but also account for the specific RTA type to guide the decision-making process. The schematic framework of the Ac-VHO scheme is illustrated in Figure 7.  
Initially, the MT is positioned in an indoor environment where a VLC-RF heterogeneous network has been deployed, establishing a connection with the VLC link. Subsequently, as the MT undergoes random movement within the indoor environment, the link state experiences variations. To begin, we ascertain the current state of the link, which can be influenced by factors such as blockage in the VLC LoS link and the MT's entry into CCI areas. When these factors impact the link state, it becomes necessary to determine whether an RTA is actively running at that moment. In instances where no RTA is active, the handover dwell time is determined solely based on the two factors associated with the degradation of link quality. If \(t_{service}\) represents the average duration for which an RTA remains active, we set this value as the reciprocal of the probability \(\rho\) that the RTA is active, denoted as \(1/\rho\). During the period of \(t_{service}\), no RTA is actively running. When the VLC link is subject to masking and no RTA is active, the dwell time \(T_{dwell}\) is calculated as follows:  
\[T_{dwell}=\left\{ \begin{array}{ll}average(\gamma,t_{block})&if\ \gamma<=t_{service}\\ 0&if\ \gamma>t_{service}\end{array} \right., \tag{11}\]  
The parameter \(t_{block}\) represents the maximum duration of a short interruption and is set to 2 s [3]. If the average interruption time \(\gamma\) exceeds the average interval \(t_{service}\) during which the RTA is active, the dwell time \(T_{dwell}\) is set to 0, prompting an immediate switch to the RF link. Conversely, \(T_{dwell}\) is determined as the average value between the average interrupt time and the maximum short interrupt time.