# VLC Network Design for High Mobility Users in Urban Tunnels
###### Abstract
Previous research works on VLC for vehicular communications have mainly focused on the physical layer aspects of point-to-point communication. Few studies have been done to explore the higher layers issues when deploying a vehicular VLC network. For instance, in V2I cases, the network restructures rapidly, preventing users from staying connected to an AP for more than a few seconds. As consequence, the vehicle reduces its available connection time. This barrier has produced a growing interest in studying handover mechanisms for continuous communication in the vehicular network. The user could tackle this problem by holding the session while moving through the different Road-Side Units (RSUs). The handover process requires the coordination of the APs in the area with upper-layer entities. The handover is a set of steps to detect in advance when the user's node is on the coverage area bounds. Then, it informs the next AP to prepare resources before the communication link breaks. Depending on the movement of the vehicular node, it can move to a neighbor VLC AP (horizontal scheme) or migrate to another communication technology (vertical handover). The implementation of handover schemes for VLC is still at a very early stage, with only a few works within this open area of research [20; 21]. Section 2 provides a more detailed analysis of this problem. In this paper, we propose a VLC design solution that provides reliable connections for high mobility environments. The main contributions are twofold: firstly, we develop a promising VLC network topology that restructures the uplink connection process to ensure the reliability of the handover decision. These modifications allow increasing the redundancy on the uplink to avoid abrupt interruptions. Then, we introduce a novel handover scheme with a fast resolution protocol that copes with high mobility in VLC based networks. The scheme takes advantage of the predictable channel response variation to trigger the handover process opportunely. Moreover, we create a system-level simulator to validate the proposed solution in a simplified but still accurate representation of a vehicular VLC network. As an example, we evaluate the operation of the VLC network over a realistic vehicular tunnel scenario. Simulation results show that the proposed method allows a vehicle to connect longer times in high mobility scenarios, as much as 99%, regardless of the number of vehicular nodes on the network. Moreover, the FER is used to measure the system reliability after the re-transmission and detection mechanism of the MAC layer. In general, the system can attain FER lower than \(10^{-3}\). The remainder of this paper is structured as follows. Section 2 presents the comprehensive details of the handover process and an overview of the related works applied to V2I links. A summary of the IEEE 802.15.7 MAC and PHY layers is discussed in Section 3, which provides a framework for our work. Section 4 introduces the proposed system design, it first describes the network topology changes to fasten the handover resolution, and then it explains the proposed handover strategy for this problem. Section 5 describes the evaluation methodology, and Section 6 discusses the obtained results. Finally, we draw the conclusions and future work in Section 7.