## 1 Introduction
### Meteorological Conditions Associated Issues and Solutions  
In addition to the optical interference, automotive VLC applications are also affected by the mobility and unpredictability of the outdoor VLC channel [2]. Due to its intrinsic nature, vehicular applications involve frequent and unexpected direction changes, highly variable inter-vehicle distances, highly variable velocities, and so on. Thus, as the VLC technology requires a mandatory Line of Sight (LoS), connectivity problems might arise. Additionally, the variable emitter receiver distance significantly influences the power of the received optical signals. To compensate the input power variance, automotive VLC receivers usually integrate an Automatic Gain Control (AGC) stage [2]. In addition, the unpredictability of the vehicular VLC channel is further amplified by the unpredictability of the multitude of meteorological conditions and phenomena that can influence the passage of the optical data carrier. Thus, the water particles from rain or fog can influence the light passage through a combination of reflection, refraction, absorption, and scattering [2; 11; 15; 41; 42], whereas the snow or heavy dust block part of the optical channel [2; 43; 42]. All these phenomena eventually lead to a lower optical power detected by the VLC receiver [2; 11; 15; 41; 42; 43], which in turn affect the SNR, the communication distance, and the BER. Based on the simulation results performed in [15], the authors consider that the heavy fog reduces the maximum V2V communication distance by more than 60%. The high impact of fog is also determined experimentally in [41] for a 5.5 m V2V VLC link. In this case, the results also show that the attenuation of the light is higher as the modulation frequency is increasing. This work also determines the fact that the attenuation introduced by the heavy fog is stronger than the one introduced by the rain or the thermal turbulences. To address the problems associated with heavy fog, the authors of [11] considered the usage of a Fresnel lens and multiple photodiodes. The experimental results performed at 1 m in laboratory conditions demonstrated that the proposed approach enables a VLC system to have a reliable connection in a fog-impaired optical channel and a relatively high SNR. The authors of [42] performed a simulation analysis concerning the effect of rain and snow on I2V VLC, showing that the attenuation introduced by snow is higher than the one due to rain for the same precipitation rate. The results also showed that the communication distance of an I2V VLC link can be decreased by 20%-80% due to snow and by 8%-30% due to rain. In the upper-mentioned context, one can see that the adverse meteorological conditions affect VLC mainly by introducing a signal attenuation which depends on the meteorological phenomena and its intensity. Nevertheless, from a VLC developer point of view, one cannot contribute to the reduction of the attenuation factor introduced by any of these phenomena. A possible solution to increase the optical power level is to use optical lens and multiple photosensitive elements as suggested in [11]. It is expected that future work concerning the effect of meteorological conditions on automotive VLC would be focused on developing highly accurate SNR prediction models in order to have the most exact data concerning the power of the received optical signal. Based on such information, future automotive VLC systems could use artificial intelligence and neural networks [44] to develop high performance environment-adaptive VLC systems [22]. Table 1 provides the summary of the issues associated to the usage of VLC in vehicular applications and of some of the existing solutions to address them. One can see that from a hardware approach, there is much more to be done in terms of increasing the VLC systems robustness to light interferences rather than on mitigating the signal attenuation due to meteorological phenomena.