# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 3 System Model  
### Description  
We propose a hybrid VLC-RF system for emergency messaging in vehicular networks, as shown in Figure 1.  
Figure 1: Layout of our Model. The system utilizes both VLC and RF to disseminate safety messages across vehicles and RSUs. An IEEE 802.11p RF system provides wide-area coverage of hundreds of meters through omnidirectional signals. Due to fast fading in dynamic vehicular environments, a frequency-selective RF channel model is considered. On the other hand, VLC uses forward-facing transmitters and receivers to enable LoS communication within tens of meters. We consider Avalanche Photo Diodes (APD) for reception. All vehicles and RSUs are equipped with both VLC and RF transceivers. A few RSUs are co-located with traffic lights at intersections. This provides additional RF coverage directly within intersections where vehicles merge and cross paths. Intersections experience more obstructions of LoS VLC links. Co-locating RSUs helps boost RF strength in these dense areas to improve connectivity when direct VLC may be blocked. Each RSU has a fixed broadcast zone/region based on its RF transmission power and propagation conditions. Vehicles can only be associated with one RSU at a time, whose broadcast zone they are located within. The model assumes sufficient but limited RSU deployment along roads/intersections such that vehicle movement between zones is possible but zones do not overlap. RSU schedules are transmitted via broadcast coordinate channel/frequency hopping across zones to neighboring vehicles as they hand off between cells. RF broadcast links from RSUs are used to partition the coverage area into zones, then transmit schedules specifying RF/VLC channel/frequency hopping cycles and duration of the time slots. Vehicles associated with the RSU with the strongest signal, ensuring synchronization and receiving the correct schedule. RF signals are also used for emergency message broadcasts from any vehicle detecting an incident, allowing rapid dissemination to nearby vehicles. Visible light links establish direct communication from vehicles to their associated RSU when LoS connectivity is available. This enables the real-time exchange of beacon messages containing location, speed, direction, and other data. This powers the RSU's ability to build dynamic topology maps reflecting current traffic conditions. To preserve privacy, vehicles would not broadcast identifying information like license plates over VLC/RF. Safety messages would contain only cryptographically protected non-traceable IDs. Location data in broadcasts would also be coarse-grained to prevent the identification of individual vehicles. VLC provides some inherent location privacy due to its directional/short-range nature. Beacon transmissions alternate between directional RF and reliable LoS-based VLC in each time slot to maximize dissemination under differing radio environments. In dense intersections, not all vehicles will necessarily have LoS VLC links to each other due to obstructions. Multi-hop relaying via both RF and VLC further extends the reach of critical messages if direct connections are obstructed. The hybrid VLC-RF system allows for both direct communication between vehicles and RSUs when LoS is available, as well as multi-hop relaying and reflective NLoS connections when direct links are obstructed. While the system model focuses on the direct V2I links, the multi-hop functionality is implicitly included through the consideration of both VLC and RF technologies working in tandem. Relaying behaviors can leverage the same VLC/RF channel models. Multi-hop relaying can utilize either VLC or RF links. For example, if a vehicle's view of the RSU is blocked by another vehicle, it may act as a VLC relay node to pass messages to the vehicles that follow. Alternatively, parked vehicles alongside the road can act as reflective surfaces to enable NLoS VLC links between obstructed vehicles. Or if vehicles are out of RF range, multi-hop forwarding via RF extends the coverage.