## 2 Vehicular Visible Light Communications and Related Work
On the other hand, for messages that are only relevant in a relatively small area, the VLC solution is more adequate, as it contributes to spectrum offloading due to the inherent properties of light, which in this case provide a spatial isolation between neighboring communication channels. Thus, this feature enables an efficient location-based data distribution. One can see that the use of VLC in vehicular applications can bring some benefits, making this technology a suitable candidate in providing inter-vehicle connectivity in autonomous vehicle applications [54; 55; 56]. However, due to the mandatory LoS conditions imposed by the intrinsic features of this technology, the reliability of the link connection is hard to be maintained, as there are many variables that influence it in V2V applications. To overcome this challenge, different approaches have been made for various driving scenarios. For instance, the authors of [57] propose the usage of a lane-centering (LC) technique that maintains the vehicles aligned in the middle of the lane, helping in this way to maintain the connectivity of the V2V VLC link. The problem of vehicle misalignment was also addressed in [58]. In this case, the authors focused on the case when the misalignment is caused by the vehicle trajectory at curved sections of the road. The analytical evaluation based on simulations showed that in such conditions, the connectivity of the link is affected. Therefore, in the case of a VLC emitter with a half value angle of \(\pm 15^{\circ}\) and a receiver with a FOV of \(\pm 30^{\circ}\), the connectivity is lost at angles above \(17^{\circ}\). To overcome this issue, the authors propose the usage of relay-assisted communications supported by an intermediate vehicle. Although the analysis provided by [58] is interesting, it should be mentioned that the analyzed scenario is based on a Lambertian radiation pattern and not on a real vehicle light distribution pattern, which can have a great impact on the final result, as demonstrated in [59] and in [60]. The VLC technology should provide reliable communications while maintaining the lighting device's primary purpose. Thus, the light radiation pattern and the output optical power must be in compliance with the regulations, even if their adjustment would significantly optimize the SNR and the communication coverage. On the other hand, the performances of automotive VLC receivers can be significantly improved by narrowing the FOV. A narrow FOV limits the amount of optical noise (i.e., from sun and/or from artificial light sources with no data transmission capabilities) that reaches the optical detector, being one of the simplest solutions to enhance the SNR and to prevent the photosensitive element saturation. Nevertheless, narrowing the FOV also limits the mobility of the system, as the useful light can be also obstructed from reaching the VLC receiver. Therefore, the problem of maintaining the direct LoS between vehicles is very complex and challenging and requires further investigation. Analysis Concerning the Influence of the Incidence Angle on the Quality of the Vehicle-to-Vehicle Visible Light Communications Link  
In order to highlight the communication differences between two vehicles on the same lane and two vehicles on adjacent lanes, the simulation scenario considers a straight road with two lanes traveling in the same direction. This test road is considered to have a 3.50 m lane width \(w\) (motorways have at least two through-traffic lanes in each direction, with a typical width between 3.50 m and 3.75 m each [61]) and the assumption made is that the vehicles will occupy the center of its corresponded lanes, at 1.75 m and, respectively, at 5.25 m projected on the \(y\)-axis. Another assumption is that the photodetector is centered along the longitudinal axis of the receiving car.