# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Vehicular Visible Light Communication (V\({}^{2}\)Lc)
In observations conducted by the authors, in high-density road traffic situations, V\({}^{2}\)LC requires multiple routes to overcome packet collisions and if there is visible light blockage in traffic, V\({}^{2}\)LC can still use complex inter-vehicle gaps to provide effective transmissions. With the omnipresence of LEDs in streetlights,advertising displays and traffic signals, automotive manufacturers use V\({}^{2}\)LC to implement cooperative ITSs. In [320], an overview of a cooperative V\({}^{2}\)LC system was presented and its performance was analyzed. The authors considered the pattern of the headlamp beam and road-reflected light. The results provided a data rate of 50 Mbps between two vehicles up to 70 m. The authors also presented open-channel modelling issues and physical/upper layer architectures. A V\({}^{2}\)LC channel is influenced by various light forms; hence, the channel depends on its own unique characteristics. The most powerful source of the noise is sunlight. The shot noise dominates in the presence of background light. Furthermore, bright sunlight will saturate the receiver, rendering it blind and lowering the communication link's efficiency. Numerous weather conditions (fog, snowfall, heavy dust, and raindrops) and the nature of vehicle environments make the V\({}^{2}\)LC channel unpredictable. These influences can obstruct the transmission channel as well as sunlight transmission, lowering the signal's intensity. The most important requirement in automotive V\({}^{2}\)LC is increasing the robustness to noise and interference. Another main challenge in V\({}^{2}\)LC is communication distance; as distance increases, power levels decrease at the receiver, resulting in low SNR levels. One solution for improving the SNR is to narrow down the FoV of the receiver for the benefit of background noise. While this solution works well for SNR enhancement, V\({}^{2}\)LC requires a direct LoS connection between an LED and receiver, and the signal's narrow reception angle reduces mobility. To improve stability, a low-cost camera-based vehicle tracking system can be combined with an active position control procedure. This approach reasonably offers improved flexibility in terms of connecting distance, agility, and robustness to noise and interference. Using spatial diversity with DSP at the receiver and laser-based range detection would be another solution for mobility enhancement. In terms of traffic safety applications, vehicle location information and its surrounding data are essential either for driving-assisting systems or autonomous vehicles. The existing tools for location determination in the latest autonomous cars are global positioning systems (GPS) [321], radar [322; 323], and lidar [324]. Even though these methods gained popularity, a GPS gives poor performance due to link and multipath blockage, particularly inside tunnels or underground; radar has high cost and severe RF interferences; and lidar has a low scanning rate and high cost. This motivates the use of V\({}^{2}\)LC for positioning applications. A visible light positioning (VLP) system is a solution for outdoor positioning as it provides 3D positioning with an accuracy within centimeters. In V\({}^{2}\)LC applications, along with robustness to noise/interference, low latencies, good packet delivery ratio, increased communication link distance, and data rate are also the crucial factors. The data rates of VLC in outdoor environments (when using LEDs) are significantly lower, several tens of Mbps in the link range of a few tens of meters, while the laser diode source can provide data rates in Gbps. Data speeds can be improved by distributing data across parallel channels using V\({}^{2}\)LC based on spatial diversity, such as MIMO [301]. One of the most challenging issues in V\({}^{2}\)LC systems is the handover due to the vehicle's rapid movement, the small size of each streetlight cell, and one cell to another shifting signal patterns.