# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Vehicular Visible Light Communication (V\({}^{2}\)Lc)
In this context, light sources such as streetlights, traffic lights, vehicle taillights, and headlights offer a communication environment. Hence, V\({}^{2}\)LC has become a part of the solutions required for intelligent transport systems (ITS) [303]. Yuan Zhuang et al. investigated a vehicle positioning system using V\({}^{2}\)LC for VIC and V\({}^{2}\)C in the underwater and tunnel positioning [306]. To improve road safety, Narmanlioglu and their team used brake lights to enable point-to-point V\({}^{2}\)LC and relay-based V\({}^{2}\)LC decoding and forwarding (DF) cooperative techniques. To test the performance in LoS and multi-hop V\({}^{2}\)LC systems (and beyond), the authors considered a MIMO transmission scheme with direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM). The findings show that the efficiency of the communication system varies from one mode to another. Authors also showed the advantages of multi-hop transmission with a different number of relay vehicles over direct transmission [307]. In Ref. [308], an ITS-V\({}^{2}\)LC system demonstrated using headlamp LEDs. The authors built a V\({}^{2}\)LC system using an inverse 4-PPM scheme, with 75% dimming, and prototype headlight LEDs at a data rate of 10 Kbps for a link distance of 20 m. Peng Ji proposed a prototype V2 LC system that works with both a taillight LED and a rolling shutter camera. The authors mentioned that smartphone cameras and photodetectors could be used as V\({}^{2}\)LC receivers. The proposed V2 LC prototype system utilizes under-sampled frequency shift OOK (UFSOOK) modulation. The authors found that the prototype's performance is stable; it can withstand common optical interferences/noises in the image, and it may thus be used in automobiles [309]. Xiao Tong Shen and Harald Haas have developed a V\({}^{2}\)LC channel model for VIC; the authors considered V\({}^{2}\)LC as a fascinating similar technology to conventional RF technology and used the passive car suspension method to evaluate the instantaneous position and orientation of a vehicle, determined by the road surface. The results indicate that a 4 dB drop in the SNR would occur on rough roads [310]. Understanding the propagation characteristics of VLC and RF technologies towards V\({}^{2}\)LC is essential for investigating their benefits and shortcomings. In Ref. [311], the salient features of RF and V\({}^{2}\)LC channels, such as channel time variation, path loss modelling, radiation pattern, and noise influence and interference were analyzed. The higher coherence time suggests that the V\({}^{2}\)LC is the best technology to ensure tolerance to faults, since V\({}^{2}\)LC extends its linking duration; V\({}^{2}\)C can be used in emergency braking, platooning, and overtaking. The directional (i.e., LOS contact) beam reduces the number of V\({}^{2}\)LC interferences. However, in terms of efficiency and coverage, VLC and RF communication channels will complement each other in their capabilities, thereby developing fully cooperative ITSs. A new 2D/3D regular-shaped geometry-based stochastic model (RS-GBSM) was proposed for V\({}^{2}\)LC channels. The authors evaluated the performance of the system using a confocal  
Figure 35: (**a**) Vehicle–VLC communication scenarios; and (**b**) continuous data connectivity via street lights. ellipse model and a combined two-ring model, in which optical power was obtained by adding the optical power from single-bounced (SB) and double-bounced (DB) components. The 3D model uses von-Mises-Fisher (VMF) distribution to find issues close to the 2D model, including the azimuth and elevation angles [312; 313]. If two LEDs overlap in the captured image, there is data loss and a BER increase. In Ref. [314], the authors suggested a three-step detection algorithm to isolate the overlapping LEDs to overcome these problems. The authors evaluated the efficiency of the proposed algorithm in a real-time outdoor environment under different parametric conditions.