## 2 Vehicular Visible Light Communications and Related Work  
In the early 2000s, VLC technology emerged as a technology initially envisioned for automotive applications [34]. After a promising beginning, there was almost a decade when the development of vehicular VLC prototypes was rather neglected, with very few research groups investigating the topic. In the 2010s, the development of vehicular VLC systems gained momentum [35; 36; 37] and so the performances in this area have gradually improved. In terms of light-sensing, the design of automotive VLC receivers followed two paths, one based on photodiodes, especially PIN photodiodes, and the other one based on camera systems, each one with its advantages and drawbacks, as underlined in [34] and [38]. In recent times, the performances of vehicular VLC prototypes significantly improved in terms of communication distance, and data rates [34; 39]. Furthermore, in terms of reliability, it has been demonstrated that using proper optical systems [30; 40; 41] and signal processing techniques, automotive VLC prototypes can maintain the connectivity link in fog conditions [42; 43], in snowfall conditions [23; 24] or in direct exposure to natural and artificial light sources [25; 26; 27], while providing extremely low latencies that go down to a few milliseconds [23; 28]. In terms of coverage, the communication distance of existing prototypes varies from an average of 50 m [23; 26; 27; 28; 29; 44] to distances of around 100 m [31; 45; 46], where the reliability of the link is determined by the lighting conditions and by the data rate. Automotive VLC systems are able to provide relatively high data rates over short distances, such as 427.5 Mb/s over 1.2 m, as seen in [32], but rarely provide more than a few tens of megabits per second rates over useful distances. Megabits per second data rates over a few tens of meter distances have been demonstrated in [45; 46]. On average, such systems provide reliable middle-range links while supporting tens to hundreds of kilobits per second data rates [23; 26; 27; 28; 29; 44]. Since the performances of automotive VLC systems are influenced by a wide range of factors, an adequate solution would be to use environment or context-adaptive VLC architectures [47; 48], which evaluate the existing circumstances and determine an optimal trade-off for each communication parameter. Although remarkable progress has been made in addressing the main issues affecting the automotive VLC system performances, most of these challenges remain active topics of interest, as their further improvement is still being researched [34]. Therefore, in order to be suitable for communication-based vehicle applications or for autonomous vehicles, the reliability, the communication distance, and the data rates are still on the list, while an intrinsic challenge begins to be re-exposed. On this topic, in order to have an active link between the VLC emitter and the VLC receiver, a mandatory Line-of-Sight (LoS) condition is required. Nevertheless, in mobile scenarios associated with the vehicular environment, there could be certain situations when the mandatory LoS condition is not always satisfied. Previous research based on the analysis of the VLC channel has indicated that the VLC technology could be used in platooning applications [49; 50]. Moreover, other studies showed that the VLC technology, in a hybrid architecture, can significantly enhance the resilience to malicious attacks and the reliability of the link for 802.11p 5.9 GHz inter-vehicle communications [51; 52; 53]. This observation further highlights the important role of VLC in automotive applications, as it has a high complementarity with the RF-based Wireless Access in Vehicular Environments (WAVE) solutions. In such hybrid scenarios, the VLC technology is suitable for short to middle-range direct Vehicle-to-Vehicle (V2V) connections, whereas RF-based solutions are more adequate for long-range non-LoS situations and for broadcasting messages that are relevant for the entire chain of vehicles.