## 1 Introduction  
Recent advances in connected car technology have pushed vehicle-to-everything (V2X) research toward faster, more reliable, and spectrum-efficient data links [1, 2]. Traditionally, dedicated short-range communications and cellular V2X have dominated this space, yet both rely on congested radio frequency (RF) bands and are vulnerable to multipath fading and electromagnetic interference [3, 4]. Against this backdrop, vehicle-to-vehicle (V2V) communication based on visible light communication (VLC) has emerged as a compelling alternative that exploits the unlicensed, gigahertz wide optical spectrum already available through automotive light-emitting diode (LED) head and tail lights [5, 6]. Since LEDs can be modulated at megabit per second rates without altering their primary illumination function, VLC offers inherently high data throughput, sub-millisecond latency, and a narrow optical beam that limits eavesdropping, thereby enhancing link security [7, 8]. These attributes are key enablers for the next generation of cooperative driving functiosn-platooning,cooperative adaptive cruise control, and collision avoidance maneuvers-that require the hard real-time exchange of kinematic data among neighboring vehicles. Tunnel environments magnify both the need and the opportunity for VLC-based V2V systems [9; 10]. First, global positioning system (GPS) signals are typically unavailable or severely attenuated in tunnels, depriving on-board units of absolute positioning and time references [11]. Second, the curved walls and low ambient light impair a driver's line of sight (LoS), so a single incident can quickly propagate into a multi-car pile up if the following vehicles receive no early warning [12]. VLC leverages the existing vehicle lighting infrastructure, which is always active in tunnels due to traffic regulations, to form an optical safety net that carries real-time speed, acceleration, and hazard flags beyond the driver's visual horizon. Despite these clear advantages, deploying VLC-for-V2V communication in practice poses non-trivial engineering challenges. The narrow emission patterns of automotive LEDs demand continuous beam alignment between the transmitter and receiver as vehicles weave through curved lanes or change their relative altitude. Optical channels inside tunnels also experience fluctuating background illumination from emergency lights, reflective signage, and oncoming traffic, which introduces shot noise and saturation effects in photodetectors. In addition, water droplets, exhaust haze, and surface contaminants on lamp covers scatter and attenuate the optical signal. Finally, the stringent cost and retrofit constraints of mass-market vehicles restrict the complexity and power budget of any add-on VLC module. To address these issues, several studies have explored the development and implementation of vehicular VLC systems, as demonstrated in [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. The study in [13] presented a V2V communication system utilizing VLC technology, where communication is established via LED transmitters and a camera-based receiver. The LED transmitter transmits various forms of data through optical signals. The camera receiver is equipped with a specialized optical communication image sensor that features a communication pixel array for high-speed optical signal reception, as well as a flag image output function that enhances LED detection, particularly in outdoor environments. The work of [14] experimentally analyzed the effect of the fog condition in the vehicular VLC system. The experimental results demonstrated that the vehicular VLC system introduced in [14] offered a reliable V2V data transmission over a fog-impaired optical channel with a relatively high signal-to-noise ratio (SNR), even under heavy-fog conditions. The authors in [15] demonstrated an IEEE 802.15.7-compliant VLC physical layer that repurposes commercial LED fog lamps as transmitters of low-latency safety beacons in LoS V2V links.