## 4 Description of the Visible Light Communications System
### Discussion of the VLC Receiver Structure  
The proposed VLC receiver consists of three main stages, each with its own component blocks. The front stage is responsible for the optical signal collection and primary conditioning of the optical signal, the signal conditioning stage is responsible for the analog signal treatment and the digital signal regeneration, whereas the data processing stage is responsible for the analysis of the reconstructed signal, message decoding, and data extracting.  
The optical front end is mainly based on a commercial PDA100A silicon optical detector. The optical detector consists of a PIN photodiode connected in a transimpedance circuit which transforms the incident optical signal into a proportional voltage. Since outdoor applications involve a multitude of optical noise sources, the design of the front stage becomes very important. Therefore, to improve the SNR, the front end uses a 2-inch  
Figure 8: The structure of the proposed data frame: (**a**) Average brightness less than or equal to 50%; (**b**) Average brightness greater than 50%.  
optical lens, which focuses the incident light onto the optical receiver's photosensitive surface. The optical lens reduces the VLC receiver's FOV to \(\pm 20^{\circ}\), diminishing the effect of the parasitic light sources. Additionally, to further enhance the SNR, the VLC receiver englobes a band-pass optical filter, which only allows the passage of the wavelengths of interest. In this case, as the VLC emitter is based on a set of red rear lights, the optical filter only allows the passage of 600-680 nm signals. The optical front end based on this approach significantly improves the SNR, enhancing the results.  
The front end has the purpose of conditioning the optical signal and transforming it into an electrical signal. From this point on, the electrical signal is being processed by the signal conditioning stage. At this level, the signal is filtered, gradually amplified from a few millivolts level to a 3.3 V level, and regenerated to its original square shape. The high-pass filter eliminates the DC component introduced by unmodulated light sources, such as the sun or unmodulated LED sources, as well as the low-frequency spectral components introduced by fluorescent and incandescent light sources, which introduce strong 100 Hz components. The low-pass filter eliminates the high-frequency noise components mainly represented by shot noise and thermal noise. To achieve this, a second order high-pass Bessel filter with a cut-off frequency of 500 Hz and a low-pass fourth order Bessel filter with a cut-off frequency of 500 kHz were used. This stage also englobes an automatic gain control stage which enables the VLC receiver to tolerate the variation of the incident optical power, caused by variable emitter-receiver distances or by different signal propagation conditions.  
After that, the regenerated signal is processed in real-time by a 1008 MHz microcontroller board. Based on the length of the compensation time and knowing the structure of the synchronization header, the VLC receiver is able to determine the duty cycle, in order to correctly decode the received signal. At this stage, the data are decoded based on rising and falling edge identification and on pulse-width measurement. The high frequency of the microprocessor allows a high-resolution pulse-width measurement and adequate data decoding performances.