# VLC Network Design for High Mobility Users in Urban Tunnels
###### Abstract  
Current vehicular systems require real-time information to keep drivers safer and more secure on the road. In addition to the radio frequency (RF) based communication technologies, Visible Light Communication (VLC) has emerged as a complementary way to enable wireless access in intelligent transportation systems (ITS) with a simple design and low-cost deployment. However, integrating VLC in vehicular networks poses some fundamental challenges. In particular, the limited coverage range of the VLC access points and the high speed of vehicles create time-limited links that the existing handover procedures of VLC networks can not be accomplished timely. Therefore, this paper addresses the problem of designing a vehicular VLC network that supports high mobility users. We first modify the traditional VLC network topology to increase uplink reliability. Then, a low-latency handover scheme is proposed to enable mobility in a VLC network. Furthermore, we validate the functionality of the proposed VLC network design method by using system-level simulations of a vehicular tunnel scenario. The analysis and the results show that the proposed method provides a steady connection, where the vehicular node is available more than 99% of the time regardless of the number of vehicular nodes on this network. Additionally, the system is able to achieve a Frame-Error-Rate (FER) performance lower than \(10^{-3}\). visible light communications; vehicular communication; handover; MAC/PHY simulation 202222880  
spectrum (from 380 nm to 780 nm) using solid-state lighting. It modulates optical signals by adjusting the power at high-frequency intervals to be undetected by the human's eyes. The reception can be possible using photodiode-based receivers or even image sensors. This technology is also recognized as a green technology that does not interfere with radio-frequency-based communications. The incorporation of VLC in vehicular scenarios has been studied for several years. Vehicular communication systems require wireless data transmission between vehicles (V2V), vehicle-to-infrastructure (V2I/I2V), or both simultaneously. In all these cases, the communication channel is the most critical problem, which has been characterized and modeled to understand its limitations. These channels have been explored following the classical Monte Carlo simulation approach [6; 7; 8; 9], which provides accurate estimations of the channel characteristics such as the channel gain or its bandwidth. Other works study practical measurements to characterize the link and help to get the impact in the VLC communication from different external agents in outdoor environments. Furthermore, the different weather phenomena have been studied before such as snow, light interference, fog, raining, dust storms, among others have been studied in [10; 11; 12; 13; 14]. During the design of the Vehicular VLC network is important to consider that the uplink and downlink will be asymmetrical in most cases. Further to this, the signal propagation depends on the radiation pattern of the lamp, which is different even in the V2V case where the vehicle from behind uses a headlamp while the car in front of it transmits with a taillamp. These factors create an unequal link range between the nodes, which leads to some open issues [15]. On the other hand, the VLC channel is more robust to variations than the DSRC channel. It has a longer coherence time and it is not highly affected by Doppler effects [16]. Moreover, the average link duration depends on the type of street, highway, urban street, or rural road. In urban areas, the V2V link lifetime is around 6 s [17]. Although VLC does not substitute DSRC, it will operate when traditional RF technologies can not provide this service. For example, the vehicular tunnel is a scenario where RF technologies degrade their performance due to multipath propagation and the continuous variations of the channel [18; 19].