## 3 Factors That Could Affect the Performances of Dimming in Visible Light Communications  
As described in Section 2, several solutions to enable VLC in light dimming conditions have been proposed. Nevertheless, some initial conclusions can be drawn from these studies, as starting points for some guidelines. First of all, there is no intensive study (to the best of our knowledge) that analyzes the dimming and/or \"lights-off\" visible light communication in the automotive field. The overwhelming majority of these papers focused on indoor applications. Moreover, most of the existing concepts have only been evaluated analytically or by simulation means. However, these studies can be very useful in researching the applicability in the vehicular field, even if the practical implementation of such VLC solutions is rather challenging. For example, some of these techniques lead to other situations that are very difficult to handle, whereas in other cases, these lead to a very complex signal processing approach. Additionally, the use of VPPM to enable light variable dimming between 1% and 50% generates a significant variation of the pulse width, which significantly complicates the signal filtering process. Thus, the brightness dimming of a pulse train of a certain frequency leads to a proportional decrease in the width of each of its pulses (see Figure 2), and in the end, it is similar to a message transmitted using a higher frequency. Consequently, such a pulse will be affected by a low-pass filter having a cut-off frequency calculated based on the initial frequency. Thus, if we have a 400 kHz message, and apply a 1% dimming, the positive pulse width decreases from 1.25 us to 25 ns. In such a case, the limit of the low-pass filter should be adapted as well. A possible solution to this problem would be to use an adaptive filter, of which the cut-off frequency is modified as a function of the dimming percentage. Nevertheless, as the standard for light dimming imposes a 0.1% resolution, the hardware complexity of an optimally matched analog filter significantly increases due to the high number of possible pulse widths. In such a case, the size of the circuit and the cost of the device significantly increase. A possible solution to this new problem would be the use of variable value components (i.e., digitally-programmable potentiometers, such as MCP41100) that would enable software control of the band-pass frequency. Nevertheless, in many cases, such components are not compatible with high-frequency applications. A different solution would be to use a wider band-pass filter. This approach has the advantage of a simpler hardware configuration that will allow the passage of a wider frequency range. On the downside, as the bandwidth increases, the additional noise components will be allowed to pass, affecting, in turn, the SNR of the received data signal. This light dimming effect on the pulse width generates additional hardware problems. From the VLC emitter perspective, the narrow pulses involve LED drivers with a faster response, whereas in some cases, the light dimming performances could be limited by the LED's characteristics. It is well-known that LEDs have switching times that can go down to a few nanoseconds. Nevertheless, the LED's switching times are influenced by their capacitance. Thus, high-power LEDs, such as the ones used in vehicle lighting systems or in street lighting systems, have higher surfaces and in turn higher capacitances, affecting significantly the switching times, which can be hundreds of times higher. In such conditions, it is highly possible that most of the high-power LEDs will not be able to sustain a reliable link communication for light dimming levels below certain limits. The problems caused by light dimming become more stringent at the VLC receiver level.