## 1 Introduction
### Long-Range Vehicular Communications State-of-the-Art  
Concerning the communication distances, one can see that the above mentioned works present results mainly determined based on simulations. For the cases when experimental results are reported, the testing setups are generally laboratory setups with demonstrational prototypes tested at relatively short distances [11; 16; 41]. In other cases, these works present outdoor experimental results in favorable conditions [20; 38], or they present rather short communication distances [37]. In recent years, interest in the usage of VLC technology in vehicular applications has increased significantly. This has led to the development of numerous vehicular VLC systems. Nevertheless, a problem with some of the existing VLC prototypes comes from the fact that in most cases researchers aim to achieve top performances focusing on one specific issue while almost completely ignoring other important factors. For example, in order to increase the data rate of vehicular VLC systems to unprecedented values, the communication distance and the robustness to noise are left aside. Thus, the V2V VLC systems presented in [45; 46] are able to achieve data rates up to 427 Mb/s, but are only able to achieve a 1.2-2.5 communication distance while using a narrow beam optical signal. On the other hand, the vehicular VLC systems presented in [47; 48; 49; 50; 51; 52] are developed while considering the communication range as the main priority. Again, one can see that even if the communication distance is significantly increased, in most cases the experimental testing procedure is totally different compared to the real life use case. So, many of these systems are evaluated in rather friendly conditions, compared to a worst case scenario imposed for vehicular applications. So, referring to communication distances, the VLC system presented in [47] is able to achieve a range of 70-80 m at a bit rate of 30 b/s in the absence of direct incident light by using a low cost camera based VLC receiver. Also based on a camera VLC receiver, the authors of [48] report a 100 m communication range using off-line data processing. The authors of [49] aim to determine the maximum communication distance that could be achieved using a camera-based VLC receiver approach, in non-LoS conditions. Based on an analytical process and on simulations, the authors consider that such systems could achieve a 2000 m range. Nevertheless, their experimental evaluation showed a range of 300 m, significantly lower than the range indicated by the simulations. Although this work is not referring to vehicular VLC, it points out again that in general, experimental evaluations are lower in performance than the results provided by simulations. The system presented in [50] is able to provide an effective I2V transmission for a 66 m communication distance, while providing a 10 Mb/s data rate. Although the 10 Mb/s data rate is quite high considering the 66 m range, the indoor testing involved only low level artificial light interferences that can be approximated to only 200-800 lux. A high performance automotive VLC system, compatible to the IEEE 802.15.7 standard [5] is presented in [51]. This prototype is able to provide up to 200 kb/s data rates and reliable communication distances of 50 m at 99.9% confidence level, while having latencies lower than 10 ms. Although the system performances are quite impressive in terms of confidence level for 50 m range, again, the system is tested only in indoor conditions, with limited noise directly incident on the VLC receiver. The authors of [52] present the first existing V2V VLC system capable of simultaneous inter-vehicle range determination and communication. Regarding the communication part, the system is able to provide a 30 m range, at a 500 kb/s data rate and at a BER lower than \(10^{-6}\).