## 4 Experimental Evaluation of the VLC System and Experimental Results
### Communication Latency Evaluation  
In addition to noise resilience, communication ranges, and data rates, wireless vehicular communications also impose strict requirements for communication latencies. Accordingly, a message transmitted by a vehicle must reach the destination vehicle within a certain time limit. As defined in [73] and summarized in [27], this limit is established as a function of the application. For example, a message containing a curve speed warning must reach its destination within a 1000 ms limit, whereas a message referring to a traffic signal violation must have its latency lower than 100 ms (latencies that are below 100 ms are imposed for the majority of high priority safety application messages). The strictest latency requirement is imposed for the scenario when two vehicles are on a collision trajectory. In such circumstances, in order to prevent accident occurrence, latencies below 20 ms must be provided. Nevertheless, existing research has shown that, in high traffic density scenarios, 5.9 GHz DSRC systems are not able to guarantee such requirements [27] and, so, the usage of the VLC technology as a complement technology can further improve the communication reliability.  
In these circumstances, the next test is performed in order to evaluate the proposed VLC prototype in terms of latency performances. Similar as in [29], the latency of the system is considered to be equal to the time difference between the moment that the VLC emitter begins to transmit the data and the moment the VLC receiver decodes the entire message. Based on this definition, one can see that there are several factors influencing the latency. The main ones are given by the transmission data rate and by the length of the data message. According to [73], messages transmitted in vehicular communications high priority safety applications have a length between 208 and 904 bits, whereas a 400-bit message is conserved as an average message length. Accordingly, these tests were performed for a 400 bits message length. Figure 10 shows the latency measurement procedure. One can see here that the 180 MHz microcontroller is able to process the data in real time, whereas the electronic circuits introduce a negligible latency (i.e., below 200 us). Table 8 summarizes the results of these tests. One can see that, when the VLC system is working at data rates of 24 kb/s or above, it is able to satisfy even the strictest latency requirements (i.e., below 20 ms) for a message of up to 400 data bits. For lower OOK data rates, the system is suitable for most applications with the exception of pre-crash sensing cooperative collision mitigation. On the other hand, these tests have showed that the usage of the 3 kb/s DSSS modulation should be used only for shorter messages to ensure latency below 100 ms. Overall, these experimental results confirm that the proposed VLC system is able to satisfy the latency requirements imposed in communication-based vehicle safety applications. Moreover, the system latencies will be further reduced, as these data rates will be further improved. Accordingly, from this point of view, VLC systems are definitely suitable for automotive applications.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Modulation**} & \multirow{2}{*}{**Coding**} & **Data Rate** & **VLC Distance** & **Message** & **Latency (ms)** \\  & & **(kb/s)** & **(m)** & **Length (bits)** & **Latency (ms)** \\ \hline DSSS & SIK & 3 & 18 & 400 & 118 \\ OOK & Manchester & 11 & 18 & 400 & 32.8 \\ OOK & Miller & 11 & 18 & 400 & 32.8 \\ OOK & Manchester & 24 & 18 & 400 & 15.6 \\ OOK & Miller & 24 & 18 & 400 & 15.6 \\ OOK & Manchester & 50 & 18 & 400 & 8.12 \\ OOK & Miller & 50 & 18 & 400 & 8.12 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Summary of the experimental results showing the VLC prototype latency performances for different modulation techniques, coding techniques and data rates.