# A Survey of Vehicular VLC Methodologies
## 7 Vehicular VLC Research Trends and Future Directions  
In this section, we discuss various relevant research topics for vehicular VLC systems, highlight some lessons learned, and illustrate some future research directions.  
* Research Trends
* Multi-hop Relaying  
The performance of vehicular VLC systems is heavily dependent on LoS links, which may not always be feasible in practical scenarios. Consequently, several crucial technologies have emerged to address this challenge. One key enabler for connectivity in vehicular VLC is multi-hop transmission, where the signal transmitted from a source car can reach a destination car through a series of intermediate cars known as relays. The authors of [36] examined the deployment of relay-assisted V2V systems using a linear path loss model to extend transmission ranges. Moreover, the authors in [136] analyzed the error rate and coverage range of multi-hop V2V VLC systems under different weather conditions, employing a more realistic channel model. Their findings confirmed that multi-hop relaying can significantly enhance performance. In [118], focused on a VLC-based platoon, incorporating longitudinal and lateral control to maintain a constant inter-distance between cars. [PERSON] et al. presented experimental results for a dual-hop vehicular VLC system in [135]. Their work aimed to connect a distant car to a roadside access point through a closer car. [PERSON] et al. explored the medium access control (MAC) layer performance of multi-hop vehicular VLC networks using carrier sense multiple access (CSMA) protocols in [55]. To enhance connectivity in an I2V link, the authors in [137] proposed a relay-assisted approach, where streetlamps can act as access points and other vehicles can function as nodes. This setup helps maintain communication links in NLoS conditions.  
* Reconfigurable Intelligent Surfaces  
Another promising technology is reconfigurable intelligent surfaces (RISs), which show great potential in mitigating LoS constraints in vehicular VLC systems. While there is an increasing body of research on RISs in indoor VLC, investigations into the performance limits and advantages of RISs in outdoor VLC are still in their early stages. For instance, in [138], researchers explored the signal-to-noise ratio (SNR) with varying numbers of mirrors and transmission distances. They also employed an exhaustive search method to determine the maximum distance between adjacent RISs. In [139], the authors examined the outage probability, throughput, and delay outage rate of an RIS-aided hybrid VLC/RF V2V system, although the RIS was only considered for the RF link. In [140], an optimization algorithm was utilized to determine the optimal number of mirrors that maximize energy efficiency while satisfying data rate and bit error rate requirements. Recently, in [141], researchers derived closed-form expressions for energy efficiency and spectral efficiency in RIS-assisted V2V VLC systems. The effect of random misalignments was then considered in [142]. These findings demonstrated the significant performance enhancements made possible by RIS technology in vehicular VLC systems.