## 5 Final Discussions Concerning the Proposed VLC Prototype
This article also intended to investigate and propose a VLC system that is able to work with different modulation techniques, coding techniques, and data rates adjustable in accordance with the existing context. Yet, the experimental results showed that, under the investigated conditions, rather similar BER results have been obtained for the three coding techniques, with the exception of the 50 kb/s data rate. Although different from what we expected, we consider that this is mainly due to the performances of the VLC receiver: (i) thus, the logarithmic transimpedance circuit is able to significantly reduce the influence of the ambient light, providing expanded dynamic range and preventing the photodiode saturation; (ii) the automatic gain control circuit is able to maintain a constant signal amplitude output, although its input signal varies significantly; (iii) the adaptive filters are optimally adjusted on the signal frequency; (iv) the decoding algorithm that is based on pulse width measurement allows for variations of the data signal period. Nevertheless, we still consider that the use of DSSS modulation is suitable for high priority data in low SNR conditions [35; 36], the usage of OOK along with Manchester coding is suitable in medium SNR [5; 24], whereas the Miller code could be an efficient solution in MIMO applications [33; 34], based on previous experience and on the existing literature. Another important contribution of this work is provided by one of the first experimental verifications of an automotive VLC system in snowfall conditions. These tests have showed and reconfirmed the highly disruptive effect of snowfall on outdoor VLC. For these particular tests, snowfall and heavy gust have increased the number of bit errors by up to 10,000 times. Nevertheless, even so, a raw BER of \(10^{-4}\)-\(10^{-3}\) has been achieved. Thus, these tests show that communication is still possible, whereas, with the help of error correcting codes, the BER can be significantly improved. Based on an intense experimental evaluation of the proposed VLC system, one can conclude that this work provided important evidence concerning the future use of the VLC technology in vehicular applications. Thus, such systems can provide BER results that can go down to \(10^{-7}\) values. Nevertheless, in unfriendly working conditions, the BER can increase up to \(10^{-3}\). However, the purpose of this work was to evaluate the physical layer of VLC applications. Accordingly, even if strong sunlight, snowfall, and blizzard depreciate BER performances, the use of error correcting codes can significantly improve these performances in order to provide reliable inter-vehicle connections. Last but not least, the experimental tests have also confirmed that the proposed system is able to comply with the latency requirements. These tests have showed that the VLC prototype is able to provide latencies below 20 ms for messages of 400 bits. This aspect becomes very important, as it has been shown that, in high traffic densities, IEEE 802.11p 5.9 GHz DSRC systems can have difficulties in complying with this requirement. Furthermore, numerous other studies indicate the complementarity between the two wireless communication solutions [27], and they suggest that, for improved reliability, 5.9 GHz DSRC and VLC should be used together [17; 27]. In such circumstances, the VLC technology can provide ultra-low latency communications between neighboring vehicle, while offloading the DSRC channel and contributing this way to improved performances in communication-based vehicle safety applications.