## 3 Description of the Proposed VLC Prototype
### Hardware Design and Implementation
#### 3.1.2 Discussions on the Design of the VLC Receiver  
A VLC receiver architecture is proposed to achieve the objective of providing a superior VLC system. The VLC sensor is developed while considering its future integration into commercial vehicles and, thus, the cost constraints were considered to be a fundamental aspect. Furthermore, the development of each functional block has been made while considering the challenging outdoor scenario and the requirements of vehicular communication applications. Hence, each functional block of the sensor is specially designed to be able to cope with a multitude of scenarios and conditions. The VLC sensor uses a PIN photodiode working in the photoconductive mode and coupled in a transimpedance circuit. It is well known that avalanche photodiodes (APD) provide improved sensitivity when compared to PIN photodiodes. Nevertheless, the SNR is usually affected, as they also amplify the noise. Based on the fact that this system is intended to work in variable uncontrolled outdoor conditions, the use of a PIN photodiode is preferred. This decision can reduce the communication range in night circumstances, but it significantly improves the SNR in daytime conditions. Because the system is envisioned to provide a holistic approach, focusing on the overall system performances, a PIN photodiode seems to be a better choice. The photodiode generates an electrical current proportional to the incident light. The transimpedance outputs a proportional voltage that is further processed until the data are recovered. Selecting the sensor's FOV represents an important decision, as VLC are a direct line of sight technology. A narrow FOV reduces the influence of the parasitic light on the system performances [33], but it limits the mobility [17]. Because the VLC sensor is envisioned  
Figure 3: Schematic of the frame building at the VLC receiver level. for the automotive domain, whereas vehicular communications involve continuous mobility, a wide angle FOV of \(\pm 45^{\circ}\) was considered. Consequently, in daylight conditions, the parasitic light will influence the generated photocurrent, while the data extracting process becomes more challenging. In order to reduce this influence, the receiver's bandwidth is limited to 300 kHz, while a logarithmic transimpedance configuration [31] is used instead of the classical one. Next, two amplification stages amplify the output of the transimpedance circuit. However, as vehicular communications involve high dynamicity, the power of the received signal can vary significantly, as the emitter-receiver distance is changing. Moreover, in high background brightness conditions, the sensitivity of the logarithmic transimpedance circuit is automatically adjusted in order to prevent photoelectron saturation, further limiting the amplitude of the useful signal, as mentioned above. An additional automatic gain control (AGC) stage is used to provide a constant amplitude signal to address these issues. This solution is able to improve the BER results within the entire communication service area. As the sensor is envisioned to receive data modulated at variable frequencies between few kHz and few hundreds of kHz (i.e., although the bandwidth is currently limited to 300 kHz, higher frequencies are envisioned for low background light conditions), a specific condition that was imposed for the AGC stage was to provide a constant signal gain, whatever the frequency of the data (i.e., for frequencies within the interest band). Subsequently, the signal passes through a filtering stage that limits the passage of low and high frequency noise. As the sensor is envisioned to work with changeable frequencies, this block is designed to be able to commute between several frequency bands. The high pass filter is set at 1 kHz cutoff frequency and it has the purpose of removing low frequency interferences mainly introduced by incandescent and fluorescent lighting sources.