## 2 Vehicular Visible Light Communication Issues, Related Work, and Specificity of this Work  
### Automotive VLC Specific Issues  
Visible light communications in general and automotive VLC applications, in particular, are a highly complex research areas, as numerous issues influence the communication performance, especially in outdoor conditions. In the most popular approach, the VLC sensor mainly consists of a photosensitive element, usually, a reversed bias positive intrinsic negative (PIN) photodiode, connected in a transimpedace circuit [31]. The photodiode generates an electrical current that is directly proportional to the incident light. Thus, significant challenges already begin to arise:  
1. as the generated photocurrent is directly proportional to the incident (i.e., data signal carrying light), whereas, as the _optical power is decreasing proportionally to the distance' square_, middle to long-range automotive VLC applications must cope with _very low power optical signals_ that can go down to tens of nW/cm\({}^{2}\) levels;
2. unlike other wireless communication technologies, in VLC the _optical carrier is perceivable to the human eye_, and so, the _optical transmission power is strictly determined by the lighting device primary purpose_ and by light regulations, with no possibility to increase it;
3. automotive VLC applications must cope with a _multitude of weather phenomena that influence the light passage_; thus, heavy dust can block part of the light [35], fog and rain influence the light passage by means of reflection, refraction, absorption, and scattering [36; 37; 38], whereas sleet and snow also affect the light passage affecting the received optical power [39; 40]. Hence, weather phenomena block part of the light affecting, in turn, the SNR and the communication range. The effect of the snow is probably the most disruptive on automotive VLC, as dense snowfall can reduce the VLC communication distance by up to 60-80%, as demonstrated by simulation means [40];
4. in order to have connectivity between the VLC transmitter and the VLC receiver, the emitted light must reach the photosensitive surface and, thus, the _line-of-sight condition is mandatory_ and affects the mobility and the connectivity of the automotive VLC link. In order to address this issue the complexity of the system design must be increased [17; 41]. A relay assisted approach to address this issue is under research consideration [42; 43]; and,
5. as the photodiode generates an electrical current proportional to all the incident light, one of the main problems that emerge is generated by the _multitude of light sources_ that are _present in the outdoor environment_. Hence, the photodiode will be exposed to very strong sunlight and artificial light sources (i.e., incandescent, fluorescent, LED). Most likely, the power of the parasitic light is considerably higher than the power of the data carrier light (i.e., thousands of times higher) significantly affecting the SNR. Thus, the effect of parasitic light sources is the most important problem affecting the performances of automotive VLC. Nevertheless, these aspects are not detailed in this work again, as this topic has been widely debated [16; 17; 18; 31; 44; 45], whereas additional information concerning optical noise modeling and its influence on the VLC system can be found in [46; 47; 48].  
One can see that numerous issues in the outdoor environment affect automotive VLC applications. Most of these perturbing elements affect the light passage and introduce additional noise to the system, significantly affecting the SNR. Thus, in the real use case scenario, automotive VLC applications must cope with low SNR signals while providing a highly reliable link.