# Outdoor Visible Light Communication in Intelligent Transportation Systems: Impact of Snow and Rain
## 2 System Model
### LED-Based Traffic Emitter Model  
A VLC emitter (also known as a transmitter) is an optoelectronic transducer that transmits data using visible light waves via a wireless transmission medium. According to Figure 2, the traffic details, which are either gathered in real time or prerecorded, will be modulated with the goal of changing LEDs at the required data transmission rate. The selective modulation scheme must provide high strength to background noise, and at the same time, it should be as bright as possible.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline Length of traffic arm & L & 2.0 [m] \\ Height of traffic light & H\({}_{\mathrm{I}}\) & 5.3 [m] \\ Height of receiver & H\({}_{\mathrm{r}}\) & 1.0 [m] \\ Distance in lane direction & X & [m] \\ Distance in width direction & Y & [m] \\ Difference between H\({}_{\mathrm{I}}\) and H\({}_{\mathrm{r}}\) & Z & 4.3 [m] \\ Width of lane & & 3.5 [m] \\ Width of vehicle & & 1.8 [m] \\ Distance between transmitter and receiver & d & [m] \\ Half power semi-angle & \(\varphi_{1/2}\) & 15 [°] \\ Vertical inclination & \(\theta\) & \(0^{\circ}\leq\theta\leq 90^{\circ}\) \\ FOV of receiver & \(\Psi_{\mathrm{c}}\) & \(0^{\circ}\leq\Psi_{\mathrm{c}}\leq 90^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: System parameters [12]. FOV: field of view.  
Figure 1: Two-lane traffic light system model [12].  
In practice, to ensure that the transmitter is synchronized, the modulator often receives information from the traffic information unit so that it can retain information while the light color varies. Then, the switching of the LED is controlled by the output signal through the output driver. In order to obtain the foreseeable range of communication, the output driver coupled with the control signal should provide enough optical power. The VLC data rate is limited by the switching speed of LEDs. On the other side, long-distance communication is restricted by transmitting power and ambient light sources [14].  
The traffic light consists of an array of LEDs tightly wired to match into the standard traffic light with either a 200-mm or 300-mm diameter. Each LED is designed on the basis of a generic Lambertian pattern of radiation. The emission power from LED transmitters, \(P_{tr}\), can be calculated as [13]  
\[P_{tr}=\left\lfloor\frac{m+1}{2\pi}\right\rfloor P_{t}\cos^{m}(\varphi) \tag{4}\]  
where \(P_{t}\) is the average transmission optical power, and the order of Lambertian emission m is related to a half power semi-angle \(\varphi_{1/2}\) by [13]  
\[m=-\frac{\ln(2)}{\ln(\cos(\varphi_{1/2}))}. \tag{5}\]  
The chosen \(\varphi_{1/2}\) angle has a noticeable impact on the coverage range and pattern shape of the Lambertian light source, according to Figure 3[15].  
Thus, by choosing an LED with a narrower \(\varphi_{1/2}\) or by adding an optical focusing stage just after the LED illumination surfaces to absorb and guide the emitted beams, the illumination intensity can be explicitly improved, resulting in a long range of communication.