# A Survey of Vehicular VLC Methodologies
## 6 Vehicular VLC Channel Modeling and Propagation Characteristics  
### Key Parameters of Vehicular VLC  
A. Asymmetrical pattern  
In general, the main feature of the vehicular VLC is the variable intensity of the headlights and taillights (i.e., asymmetry) [114; 122; 123]. In addition, the headlights and taillights in the vehicular VLC serve two purposes simultaneously: illuminating and transmitting information. Hence, the intensity of the light emitted from the headlights is several times stronger than that of the taillights. In contrast, it creates an asymmetrical association between vehicles with this light unit. Moreover, the intensity of illumination of the headlights of cars is irregular. On the other hand, the ideal Lambert model is widely used for modeling VLC channels and obtaining CIRs. In this method, any light source is assumed to have a uniform radiation pattern, with the intensity of the light defined as a function of the radiation angle (\(\alpha\)). \(n_{L}=-\ln(2)/\ln\Bigl{(}cos\theta_{\frac{1}{2}}\Bigr{)}\) must be in the Lambertian sequence at the half-power angle \(\theta_{1/2}\). The channel impulse response (CIR) can be calculated using (\(\beta\)) angle of incidence and propagation distance (\(d\)) [124]:  
\[h(t)=\frac{\left(n_{L}+1\right)A}{2\pi d^{2}}\cos^{n_{L}}(\alpha)\ cos(\beta) \delta\left(t-\frac{d}{c}\right) \tag{2}\]  
where (\(A\)) denotes the area of the photodetector and (\(c\)) represents the speed of light. It should be noted, however, that this ideal model should be only used for indoor VLC scenarios due to several reasons. For instance, this ideal model does not consider that the  
Figure 8: Platoon V2V VLC architecture. vehicular light sources have a symmetrical radiation pattern [29] that is designed to achieve specific illumination functions following the road regulations. Additionally, outdoor lights, such as HLs [76] and TLs [112], have asymmetrical radiation patterns because they are intended for different illumination purposes. The Lambertian model fits well with most indoor light sources. Still, it does not work well with outdoor light sources. Moreover, experimental measurements or non-sequential ray tracing methods can be used where the measured radiation pattern considers the properties of outdoor light sources. The channel impulse response (CIR) is calculated using the following equation [125]:  
\[h(t)=\sum_{K=1}^{M}P_{K}\delta(t-\tau_{K}), \tag{3}\]  
where \(M\) denotes the total ray number captured within the photodiode, \(\delta\) denotes the Dirac delta function, \(P_{K}\), and \(\tau_{K}\) respectively represent power and delay in the transmission of the signal of the \(k^{th}\) ray, and \(k=1,2,M\). The PL is given by  
\[PL=10 log_{10}\left[\int_{0}^{\infty}h(t)dt\right] \tag{4}\]  
B. Mobility  
Another critical parameter for VLC technology is mobility, due to the required Line of Sight (LoS). A precise spatial alignment is required between the optical devices transmitting and receiving data in a VLC link. This is because classic optical receivers that use photodiodes offer a field of view (FOV) or narrow cones on the receiver side. Performing an alignment of this nature becomes a challenging task. It is possible to enlarge the field of view on the side of the receiver by placing a lens in front of the receiver components, but to do so will compromise the work of the VLC system due to the increase in the amount of received noise, as the system must collect this noise, and preserve the field of view, and reduce the proportion of this noise spatially by using reception components inspired by the camera's components [34; 38]. On the other hand, selective filtering in the receiver improves the signal-to-noise ratio of the background noise in the environment. The spatial solution allows the dotted structure to filter this ambient noise. Thus, the large field of view gives both the receiver and the transmitter great flexibility in their mobility. In this manner, camera-inspired receivers present a variety of distinct benefits to address the problem of mobility.