# VLC Network Design for High Mobility Users in Urban Tunnels
## 4 Proposed VLC Network for High Mobility
### Handover Strategy  
The new handover approach is based on SNR information from downlink and uplink channels. The downlink SNR is measured when the AP transmits a beacon frame, and the uplink SNR by sending pilot frames periodically every 5 ms. In our previous work [57], the system design (topology configuration) was evaluated by triggering the handover process using only uplink information. However, this information is highly affected by the lateral displacements of the vehicle that contaminates the estimation. As a consequence, the system does not perform the handover on time, and the vehicular node loses the link. Therefore, in this paper, we formulate and solve a new handover decision scheme based on the SNR variations in both uplink and downlink channels. To better cope with the handover sensitivity, we use two different criteria to trigger the handover, one over the downlink channel and another for the uplink channel. The handover will initiate if at least one has fulfilled its conditions. The handover scheme is a three-stage process. The first stage is the signal gathering process which is depicted in Figure 7. During this stage, the current site \(AP_{n}\) transmits a beacon frame to synchronize the users. When the vehicular node receives this signal, it estimates the instantaneous SNR of the downlink defined by \(P_{DL}(t)\). Then, this node broadcasts a periodic pilot frame that includes the measured \(P_{DL}(t)\). This pilot frame arrives at multiple AP receivers, which individually estimate the instantaneous SNR of the uplink channel, i.e., \(P_{UL}^{m}(t)\) for \(AP_{n}\), \(P_{UL}^{n+1}(t)\) for \(AP_{n+1}\), etc. The links information is then shared with the network switch in a single frame. After that, the information is sent to the HME to analyze if it is necessary to initiate a handover process. In our scheme, this entity only considers the instantaneous SNR values from the current site, \(P_{DL}(t)\), and next site, \(P_{UL}^{n+1}(t)\), for the decision stage. Additional benefits could be gained by incorporating the extra SNR inputs from other receivers, however, it needs a further study that is out of the scope of this paper. Figure 6: Cells distribution over the evaluation scenario (**top**), and cells distribution on the superframe structure (**bottom**). This is followed by the handover decision stage, where the vehicular node updates the SNRs information and the system decides if such node is initializing a handover process. Figure 8 shows the flowchart of the handover decision algorithm that consists of two independent loops. The first loop updates the downlink information at the vehicular node when a beacon frame arrives from its associated coordinator. Then, it calculates \(P_{DL}(t)\), and after that, it registers the reference time, \(t_{b}\), of the transmitted beacon frame in the last synchronization process that helps to detect an outage. In the second loop, the vehicular node creates a pilot frame every \(t_{pilot}=5\) ms, which includes the instantaneous SNR of the downlink \(P_{DL}(t)\). Next, the vehicular node checks if the current time does not exceed the value of \(t_{b}+t_{dis}\) with \(t_{dis}\) as the threshold time of disconnection. Since the coverage area of two adjacent APs is not always overlapped, it is not possible to guarantee a soft handover. To consider this case, the system extends the disconnection time to allow the vehicle to cross this gap area by using the recovery time \(t_{recovery}\). If the current time is greater than \(t_{b}+t_{recovery}\) then the vehicular node forces the pilot transmission without using a CSMA/CA mechanism. The instantaneous SNR of the uplink channels are then calculated by those APs that receive the pilot frame, and only the value of \(P_{UL}^{n+1}(t)\) is passed out to the HME entity. To complete the handover decision stage, the HME entity checks the criteria of \"condition 1\".