## 5 Experimental Results  
### Experimental Procedure and Methods  
The objective of the experimental procedure is to evaluate the effect of light dimming on the performances of a vehicular VLC system. As already discussed in Section 4, the same software subroutine is used for duty cycle values greater than 50%, as well as for duty cycle values less than 50%. As such, the experiments carried out were done only for light dimming less than 50%, the results being valid for the other approach as well. For this purpose, two different setups have been chosen: an indoor setup and an outdoor setup. For each situation, the V2V VLC prototype has been tested for variable distances starting from 1 m and a duty cycle of 40%. From this point, the distance has been gradually increased until a maximum communications distance was achieved while maintaining a BER lower than \(10^{-4}\)-\(10^{-3}\). At each point, the BER has been determined in real-time, without using any error-correcting protocol. After that, the duty cycle has been gradually decreased from 40% to 1% and the measurements were repeated for each of these steps. For the indoor setup, the communication distance was restricted by the limited length of the laboratory (i.e., 40 m). In this case, the purpose was to evaluate the system in controlled conditions, in order to have a clear view concerning the light dimming effects. Thus, the indoor tests have been performed in high SNR conditions, with limited effect from natural daylight coming through the windows, and in low SNR conditions with an incandescent noise source consisting of six bulbs of 70 W each. At maximum power, this noise source can introduce an optical noise of up to 28,000 \(\upmu\)W/cm\({}^{2}\). Figure 9 presents the spectral analysis of the incandescent light source used to test the system's noise resilience. One can see from this analysis that the incandescent noise source has its peak in the red region which is the spectral region for which PIN photodiodes have the highest sensitivity. The outdoor tests have been performed in uncontrolled daylight conditions on a mostly sunny day. In this case, the range could be increased up to 50 m, while the sun generated a variable noise component which reached values between 3200 and 50,000 \(\upmu\)W/cm\({}^{2}\) at the VLC receiver level. The summary of the experimental setup, the parameters of the parasitic light source, and the materials used during the experimental evaluation are presented in Tables 4-6, whereas Figure 10 illustrates the experimental methodology. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Feature/Values** \\ \hline \hline \multirow{4}{*}{Testing conditions} & High SNR indoor conditions \\  & Indoor conditions \\  & In the presence of parasitic lights (low SNR) \\  & Outdoor, uncontrolled conditions \\ \cline{1-1} Dimming factor & 1–40\% \\ \cline{1-1} VLC emitter & LED-based vehicle rear lights \\ \cline{1-1} Emitter-Receiver (V2V) distance & 1–50 m \\ \cline{1-1} VLC receiver & PIN Photodiode-based \\ \cline{1-1} VLC receiver height & 74 cm \\ Modulation technique & VPPM \\ \cline{1-1} Data rate & 10 kb/s \\ \cline{1-1} Measured parameter & Real-time BER determination without the use of forward error correcting protocols \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the experimental parameters. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Feature/Value** \\ \hline \hline Light source & Incandescent light source \\ Light source power & 6 \(\times\) 70 W \\ Light color temperature & 3254 K \\ Light source irradiance & Up to 28,000 \(\upmu\)W/cm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of the light source parameters. Figure 9: Spectral analysis of the incandescent light noise source used during the tests.