# A Survey of Vehicular VLC Methodologies
## 6 Vehicular VLC Channel Modeling and Propagation Characteristics
### Receiver aperture size  
Vehicular VLC systems employ two primary methods for receiving light signals transmitted by vehicle headlights and taillights: camera image sensors and PDs. These devices capture the received light signal and convert it into an electric current signal, facilitating subsequent decoding and demodulation processes. The utilization of both camera-based and PD-based receivers has been extensively documented in vehicular VLC  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Weather Condition**} & \multirow{2}{*}{**Visibility (km)**} & \multicolumn{2}{c}{\(\lambda\) = **785 nm**} & \multicolumn{2}{c}{\(\lambda\) = **850 nm**} & \multicolumn{2}{c}{\(\lambda\) = **1550 nm**} \\  & & **Attenuation (dB/km)** & **Attenuation (dB/km)** & **Attenuation (dB/km)** \\ \hline Clear air & 23 & 0.5 & 0.4 & 0.1 \\ \hline Haze & 2 & 6.7 & 6.4 & 4.2 \\ \hline Light fog & 0.8 & 19.1 & 18.6 & 15.5 \\ \hline Moderate fog & 0.6 & 27.3 & 27 & 25.5 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Attenuation of incoming electromagnetic waves by the atmosphere under a variety of weather conditions and wavelengths [129].  
research [17; 29; 130; 131; 132]. Camera-based and PD-based receivers are considered due to their distinct design and engineering characteristics, as well as the differences in how they capture and process optical signals. These differences significantly influence the overall architecture and performance of vehicular VLC systems. Various aspects, such as media access control, modulation techniques, coding schemes, noise reduction, and interference mitigation, are tailored based on whether the receiver utilizes an image sensor or a PD.  
Typically, two types of PDs are used in vehicular VLC: Avalanche Photodiode (APD) and PIN (p-n diode). The APD devices have better gain and higher sensitivity than PIN PDs. Moreover, a type of APD with higher sensitivity, such as a Single-Photon Avalanche Diode (SPAD), works better in simulation than traditional APD devices, especially in bad weather conditions, at a lower cost, in addition to having temperature tolerance. The main point of the vehicular VLC systems when choosing a PD receiver is that the characteristics of the receivers should be taken into account to achieve optimal performance, such as suitable bandwidth, low noise, high sensitivity, and wide linear range; the main factor is the size of the active area of the PD device. All these parameters significantly affect the performance of the vehicular VLC systems. For example, when the size of the PD receiving area is large, or the field of view (FOV) is large, it will allow the capture of a large number of optical signals in addition to allowing the reception of other unwanted signals, which causes system degradation and thus reduces the signal-to-noise ratio (SNR) [17; 18; 133]. Researchers and academics have considered the effect of the active area size parameter for vehicular VLC [37; 38; 56; 69; 84; 134; 135]. It is possible to improve VLC receiver performance by utilizing an optical filter. It is also possible to reduce the impact of interference by limiting the FOV on the side of the recipient device. However, it will significantly influence the service area [11]. The path loss, considering the active aperture area of PD, can be calculated as follows:  
\[PL=10\ log(A_{r}(m+1))-10\ log(2\pi)-20\ log\ D+10(m+1)log\ cos(\varphi) \tag{6}\]  
where \(\varphi\) denotes the irradiance azimuth (horizontal) angle, \(\psi\) denotes the angle of an incident at the recipient side, \(D\) denotes the distance between vehicles, \(Ar\) represents the active aperture area, \(\Psi_{1/2}\) indicates the [half-power angle] of radiation, and denotes the index of Lambertian order.