# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 1 Introduction
### Paper Outline and Contributions  
In this paper, we have proposed a novel adaptive routing protocol for emergency message dissemination in VANETs, which jointly utilizes RF and VLC technologies. In this protocol, first, we partition the coverage area into different zones based on road segments and traffic patterns. Then, the RSUs are assigned to these zones based on historic traffic data and zone dynamics. Unique RF channels and VLC channels/beams are then assigned to each zone, minimizing the interference between these zones, considering the bandwidth, throughput, and spill-over interference. The RSUs are responsible for building real-time topology maps to optimize the interference-aware assignments of vehicles to the zone resources. They also publish schedules that specify the channel hopping cycles and time slots. The vehicles are associated with the RSU that has the strongest broadcast signal, and the beacon periods alternate between the directional RF and LoS-based VLC communications. In an emergency situation, our protocol enables the source vehicle to simultaneously broadcast some warnings across all the channels to rapidly notify the surrounding vehicles. It should be emphasized that to extend the connectivity between the zones when the direct links are not available, multi-hop relays are considered. Our protocol aims to provide robust and low-latency dissemination of safety messages during incidents on congested roadways. Indeed, its design integrates a congestion detection and management scheme that works as follows: when an accident is detected, the RSU simulates the clearance times under different response strategies to score their impacts on the travel time, emissions, and driver stress levels. The highest-scoring strategy is then selected based on a consensus from the connected vehicles, and adaptive traffic signals prioritize the selected strategy while maintaining coordination across the zones. The latter action aims to proactively alleviate the traffic jams through optimized routing and signal control. It should also be noted that in our proposed approach, adding the VLC capability to vehicles and infrastructure involves additional hardware costs compared to solely relying on the RF. These include: using the LEDs, photo-detectors, and the associated drive/receive circuitry on all nodes; using transceivers that enable the switching between the RF and VLC modes; dealing with the potential high energy consumption when both radios are active simultaneously. However, there are also some benefits associated with our proposed hybrid VLC/RF approach. For instance,  
1. The VLC helps in offloading some traffic from the RF spectrum, thereby avoiding the need for additional RF access points in places where the LOS dominates. 2. Multi-hop relaying avoids the need for universal RF coverage, leading to infrastructure cost reduction. 3. The location privacy afforded by the VLC could reduce the costs of more complex identity/tracing systems on the RF side. To the best of our knowledge, this work is the first to:  
* Propose a zone-based resource optimization and topology mapping method enabling congestion-aware protocols. * Develop an emergency broadcasting mechanism leveraging dynamic resource assignments for reliable multicasting under disruption. * Introduce a vehicle consensus-based predictive response integrated with adaptive signaling for coordinating relief across network divisions. By developing these novel coordination techniques, our work provides a solution uniquely optimized for maximizing emergency alerting resilience and traffic management responsiveness under challenging impaired mobility conditions. Simulation experiments are conducted using realistic mobility traces to evaluate the protocol's performance in collision and congestion scenarios, showing the superiority of our hybrid VLC/RF-based protocol over the single VLC-based protocol and RF-based protocol, in terms of reduced total congestion exposure for vehicles, reduced packet loss, and reduced latency.