## 5 Description of the Vehicle-to-Vehicle Visible Light Communications Prototype  
As illustrated in Figure 6, the V2V VLC prototype consists of a VLC emitter and a VLC receiver separated by the VLC channel. The VLC emitter mainly consists of a 180 MHz microcontroller board, an LED driver and an LED-based vehicle lighting system. The 180 MHz microcontroller is the main part of the VLC emitter. The microcontroller processes the data to send, transforms it into a binary string, encodes the data, creates the data frame and modulates it. In this case, the VLC prototype is able to work with Direct Sequence Spread Spectrum (DSSS) and On-Off-Keying (OOK) modulations, Sequence Inverse Keying (SIK) [21], Manchester and Miller [65] coding techniques and variable data rates between 3 and 200 kb/s. The data frame begins with a synchronization header that informs the VLC receiver that a new data frame will begin, continues with a physical header that provides  
Figure 5: (**a**) Taillight irradiance distribution pattern for a vehicle with an LED-based lighting system; (**b**) stolight irradiance distribution pattern for vehicle with an LED-based lighting system; (**c**) taillight irradiance distribution pattern for vehicle with incandescent based lighting system; and (**d**) stolight irradiance distribution pattern for vehicle with incandescent based lighting system. the VLC receiver with information concerning the selected modulation, coding technique, data rate, and message length, and ends-up with a variable length data field. After the data frame is constructed, the information is modulated and the signal is applied to the LED driver, which in turn commands the LED vehicle rear lights, the data to send being transformed into a modulated light beam. The VLC emitter system is developed based on the characteristics of the LED vehicle backlights, in order to have an average transmitted optical power similar to the one of existing commercial vehicles. The VLC receiver is the main component of a VLC system, as its design is the one that has the strongest impact on the VLC system performances. The VLC receiver consists of three main blocks. The front-end contains an optical collecting system that determines the VLC receiver FOV and its optical collecting area. As already mentioned in Section 2, the FOV strongly influences the VLC receiver's SNR and its ability to work in mobile conditions. In order to evaluate the effect of the VLC receiver FOV on the system's performances, two different FOVs have been used during the tests. Therefore, in the first case, the VLC receiver has been evaluated without an optical collecting system that limits its reception area, having this way a wide \(\pm 53^{\circ}\) FOV. Next, in order to evaluate the effect of a narrower FOV, a 2-inch optical lens with a \(\pm 20^{\circ}\) FOV has been used. The wide angle FOV should enable a wider communication area at the cost of a lower SNR, whereas the \(\pm 20^{\circ}\) FOV should facilitate a SNR enhancement, contributing to a longer communication distance. The front-end also englobes an optical filter that eliminates part of the unwanted signals. In this case, an 80 nm band-pass optical filter with a center wavelength of 645 nm has been used. This optical filter is adequate, as the LED vehicle rear lights have only the red color lamps wired for communications. The selected optical filter eliminates up to 80% of the ambient optical noise. After that, the optical signal reaches the PDA100A PIN photodiode-based optical detector. The optical detector generates an electrical current, which is directly dependent on the power of the incident light. The internal transimpedance circuit transforms the electrical current into a voltage that will be further processed in order to extract the information. The next stage of the VLC receiver is responsible for signal conditioning. At this stage, the low-amplitude signal is amplified, and then is passed through a band-pass filter.