# A Survey of Vehicular VLC Methodologies
## 5 Vehicular VLC Applications
### Lane Changing  
A fundamental assumption in the majority of research on vehicular VLC systems is that two cars traveling in the same direction will perfectly align with each other. However, in practical scenarios, achieving perfect alignment is typically not feasible. Moreover, it is commonly believed that the leading car in a vehicular VLC system is equipped with one or two photodetectors mounted on its trunk. Even when two cars are traveling in the same lane, there will inevitably be some misalignment between them, regardless of whether they are positioned side by side. Similarly, maintaining a stable vehicular VLC connection during lane changes is crucial. One of the key design challenges in vehicular VLC systemsis determining the minimum number of photodetectors (PDs) required and their optimal positions to ensure reliable communication in VLC-based V2V links, enabling continuous connectivity. Recent research has focused on addressing this challenge, as summarized in Table 4. These studies propose vehicular VLC methods that utilize PDs at the receiver side to enhance the VLC system while considering lane changes. For instance, in V2V VLC systems, it is common to assume the use of one or two PDs placed on the rear of the vehicles [36; 39]. This configuration is typically sufficient for communication between two cars traveling in a straight line without horizontal displacement. However, for curved roads or wider roads (e.g., two-lane roads), additional PDs may be required to maintain effective communication.  
For instance, in the study of [108], three PDs are employed, and the PD device with the highest received power is selected. In [109], a proposal is made to utilize four PDs in an angle diversity receiver, where the PDs are positioned in different directions. The work described in [37] focuses on maintaining V2V connectivity during lane changes on a straight two-lane road, employing a configuration with four PDs. On the other hand, ref. [38] takes it a step further by employing nine PDs for V2V and I2V VLC systems in order to achieve omnidirectional coverage at intersections and various road types. The proposed method was validated in different scenarios, including cruising in different lanes or the same lane, lane changes, and more. Regarding I2V systems, some studies, such as [35; 80; 110; 111], concentrate on using a single PD, while others, like [80; 110], investigate the location of the PD device on the front hood of the car. Certain works, such as [35; 111], propose placing the PD at the top of the vehicle, citing its convenience for connecting with streetlights. The use of a single PD can be attributed to the presence of a clear line of sight on a single-lane road.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Refs.** & **Transmitter** & **Receiver** & **Scenario** & \multicolumn{2}{c}{**Observations**} \\ \hline
[36] & Headlights & One PD & V2V & - & Single lane \\  & Headlights & One PD & V2V & - & Straight road \\  & Headlights & One PD & V2V & - & Perfect alignment \\ \hline
[73; 112] & Taillight & One PD & V2V & - & Single lane \\  & Headlights & One PD & V2V