## 5 Final Discussions Concerning the Proposed VLC Prototype  
The proposed VLC system has been evaluated under variable scenarios, distances, and noise conditions in order to demonstrate the compatibility of VLC systems with vehicular communication applications. As this work involves the development of a context-adaptive VLC system, different modulation techniques, coding techniques, and data rates have also been investigated. Thus, the proposed VLC architecture aims to simultaneously address all of the main issues affecting vehicular VLC, while providing an optimal trade-off between different parameters of the system. Accordingly, in order to facilitate the deployment of automotive VLC systems, their future development should be focused on improving specific abilities (i.e., resilience to noise, communication range, data rate, mobility), but also on a holistic approach that addresses the overall performances. In this context, the proposed VLC design brings together in one system performances of state of the art VLC systems. Thus, it provides noise resilience that is comparable with [21; 23; 33; 54; 55; 56; 57], but longer communication distance than [21; 23], better mobility performances [33], and simpler design than [54; 55; 56; 57]; it ensures alike communication distances as [16; 21; 30; 58; 59], but improved noise resilience when compared with [16; 21; 30; 58; 59] and improved mobility as compared with [16; 58; 59]. On the downside, in the current configuration the proposed VLC prototype is able to provide 3-50 kb/s data rates. Although these data rates are comparable with the ones achieved by other middle-long range automotive VLC systems [16; 18; 19; 20; 30], they are lower when compared to the 5.9 GHz DSRC systems, which can achieve few Mb/s data rates. Thus, this aspect should be further improved in order to provide higher data rates as long as these advances do not affect noise resilience and link reliability. The experimental verification has confirmed the high potential of VLC systems in vehicular applications. It should be remembered here that these tests have been made while using a standard VLC traffic light and a wide FOV VLC receiver (i.e., suitable in mobile applications). Thus, in such conditions, the SNR becomes significantly lower, complicating the signal reconstruction process. According to the existing standards, 200 mm traffic lights are fixed at 250 cm above the road, whereas 300 mm traffic lights are fixed at 500 cm above the road. In most cases, the traffic lights are not parallel with respect to the road, but they are slightly orientated toward it. In our case, the height of the traffic light (i.e., 190 cm) is not similar to the height specified by the standards, as it is quite difficult  
Figure 10: Communication latency measurement. For a clear presentation this example illustrates a 56 bit data message transmitted at 24 kb/s using on-off keying (OOK) modulation and Manchester coding. time and data processing time at the VLC receiver level. Channel 1 (orange) represents the message at the VLC emitter, Channel 2 (blue) shows the beginning of a message where the high pulse represents header transmission and the low pulse represents data message transmission, Channel 3 (purple) illustrates the received message, whereas Channel 4 (green) shows header processing. to handle such equipment in laboratory conditions. Therefore, the height and the orientation of the traffic light influence the power of the received signal and, in consequence, the SNR and the BER. Nevertheless, as the vehicular outdoor environment is highly dynamic and very unpredictable, an on vehicle VLC receiver will experience, in certain conditions, similar SNR levels, whereas, in other cases, the SNR could differ slightly. Even so, these results are still representative for traffic light to vehicle communication scenario, especially when the emitter-receiver distance is increasing. In such a case, the solid angle between the traffic light and the vehicle is decreasing and the influence of this height is reduced.