## 3 Factors That Could Affect the Performances of Dimming in Visible Light Communications
In this case, the VLC receiver should be designed with components that have lower switching times, with operational amplifiers that have higher slew rates, with DSP-based architecture that havesignificantly higher sampling frequencies, and with considerably more performant data processing units. All these requirements will increase the overall cost and/or could affect the reliability of the VLC system as a whole. Being designed for VLC applications with low data rate requirements in outdoor conditions, the physical layer type I (PHY I) operating mode described in the IEEE 802.15.7 standard is the appropriate choice for vehicular VLC applications (Table 1). For such applications, the IEEE 802.15.7 standard specifies the use of OOK modulation with Manchester coding at data rates between 11.67 and 100 kb/s. Additionally, the standard also mentions the use of VPPM and of 4B6B coding providing data rates between 35.56 and 266.6 kb/s. In this case, the light dimming is achieved by modifying the duty cycle, whereas in the OOK case, this function is achieved by the insertion of compensation symbols and/or idle patterns (see Figure 2b). The insertion of compensation symbols and idle patterns accomplishes the light dimming at the cost of the data rate but ensuring at the same time a constant range. In such a case, the total duration of the compensation symbols or idle times is established based on the dimming percent. Thus, as the dimming percent decreases, the entire duration with no data transmission is increasing. Now, if one is to apply this approach to the PHY I OOK use case, it is clear that this will lead to extremely low data rates that can hamper the safety concept, which should be the main function of a communication-based vehicle application. Consequently, this approach can be considered unsuitable for such applications, pointing out that the use of the VPPM technique for brightness control is more adequate for the automotive industry. In order to achieve a dimming resolution of 0.1%, an algorithm is described in the IEEE 802.15.7 standard, the definition for VPPM mode being illustrated in Table 2. Because the VPPM affects the illumination of the transmitter LED, the communication performances must be expressed as a function of the distance, dimming ratio, and environment settings in a given VLC scenario [42]. The avoidance of flickering is mandatory in an automotive VLC system, so extra care is needed in this regard. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Modulation**} & \multirow{2}{*}{**RLL Code**} & \multirow{2}{*}{**Optical Clock**} & \multicolumn{2}{c}{**FEC**} & \multirow{2}{*}{**Data Rate**} \\ \cline{3-3} \cline{5-5}  & & & **Rate (kHz)** & & **Outer Code** \\  & & & **(RS)** & **(CC)** & \\ \hline \multirow{4}{*}{OOK} & \multirow{4}{*}{Manchester} & \multirow{4}{*}{200} & (15, 7) & 1/4 & 11.67 \\  & & & (15, 11) & 1/3 & 24.44 \\  & & & (15, 11) & 2/3 & 48.89 \\  & & & (15, 11) & none & 73.3 \\ \hline \multirow{4}{*}{VPPM} & \multirow{4}{*}{4B6B} & \multirow{4}{*}{400} & (15, 2) & none & 35.56 \\  & & & (15, 4) & none & 71.11 \\ \cline{1-1}  & & & (15, 7) & none & 124.4 \\ \cline{1-1}  & & & none & 266.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: PHY operating modes [16]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Logical Value**} & \multicolumn{2}{c}{**Physical Value**} \\  & & \(d\) **is the VPPM Duty Cycle (0.1 \(\leq\) d \(\leq\) 0.9)** \\ \hline \multirow{2}{*}{0} & High & 0\(\leq\)\(t\)\(<\)\(dT\) \\  & Low & \(dT\)\(\leq\)\(t\)\(<\)\(T\) \\ \hline \multirow{2}{*}{1} & Low & 0\(\leq\)\(t\)\(<\) (1\(-\)\(d\))\(T\) \\  & High & (1\(-\)\(d\))\(T\)\(\leq\)\(t\)\(<\)\(T\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Definition of data mapping for VPPM mode [16].