# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 4 Algorithm Description
### Proposed Routing Protocol
\[C_{ROI_{i}}=f(T_{i},I_{i}) \tag{15}\]3  
\[C_{ROI_{i}}=f(T_{i},I_{i}) \tag{15}\]4  
**Topology Map Building**: The RSU uses the beacons to build topology maps M. The maps contain node locations L and connectivity links \(\epsilon\) as \(M=L,\epsilon\). The RSUs will then run optimization algorithms to maximize throughput \(T\) while meeting interference, \(I\) less than a threshold \(\zeta\). \[C_{ROI_{i}}=f(T_{i},I_{i}) \tag{15}\]5  
The model interference \(I\) refers to predicted/estimated interference between nodes based on their locations, connectivity, and assigned resources. \(I\) is modeled/calculated based on the topology map \(M\), which contains nodes' positions \(L\) and links \(\epsilon\). The goal is to maximize throughput \(T\) across the network. However, resource assignments cannot exceed a certain interference threshold, captured by \(I\leq\zeta\). So it is indeed a maximization of \(T\), subject to the constraint that interference \(I\) stays below a set level. **Incident Handling**: In emergencies, the source s simultaneously broadcasts accident message \(A\) over all assigned RF/VLC channels \(C\) and frequencies \(F\) using maximum transmit power \(P_{max}\). This is to ensure the message reaches the greatest possible number of vehicles immediately. Here, \(P_{max}\) is the maximum allowable transmit power that devices are permitted to use for emergency broadcasts according to regulations. This is usually higher than the typical/normal transmit power level used under regular conditions. The \(P_{max}\) is predefined based on things like device antenna class/power class, allowed equivalent Isotropic radiated power, proximity to safety-critical systems, and interference considerations. **Mode Switching**: When a receiver \(r\) detects an emergency message \(A\) from source \(s\), it immediately switches its radio to an emergency mode \(M_{r}\). The goal of \(M_{r}\) is to maximize the range \(R\) over which \(r\) can receive/relay the emergency message. The \(R\) depends on factors like transmit power, antenna gain, and radio hardware limits. The \(M_{r}\) is defined as a function of \(R\) that configures the radio parameters to achieve the increased range \(R\). **Message Rebroadcasting**: Receivers \(r\) rebroadcast messages \(A\) containing hop count \(H\) simultaneously over assigned resources Cand \(F\):  
**Multi-hop Relaying**: Vehicles participate in multi-hop relaying to extend connectivity. The optimal next hop \(n_{h}\) for message forwarding is chosen to maximize network coding gains N:  
\[C_{ROI_{i}}=f(T_{i},I_{i}) \tag{15}\]6  
If the VLC link is disrupted, a low-bandwidth RF link will be used. Then the RF beam will reorient (realign) to the optical transmitter/receiver with angle \(\theta\) to the RSU:  
\[C_{ROI_{i}}=f(T_{i},I_{i}) \tag{15}\]7  
Multi-hop relays extend connectivity between zones if direct links are obstructed, allowing store-and-forward rerouting of critical messages. The RSU uses its maps M to determine the optimal alternate path \(p_{opt}\) that maximizes the probability of delivery P.  
\[C_{ROI_{i}}=f(T_{i},I_{i}) \tag{15}\]8