# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 6 Conclusions  
In this paper, we have proposed an adaptive routing protocol for emergency message dissemination in vehicular networks using both RF and VLC communications. When an accident or congestion happens, the source vehicle and the RSU disseminate the information to the incoming vehicles as fast as possible using a combination of VLC and RF channels. The receiving vehicles immediately switch to emergency modes. The multi-hop relays extend the connectivity if the direct links are blocked. In order to minimize interference, the coverage area is partitioned into zones based on the road segments, intersections, and traffic flows. The RSUs are assigned to zones and analyze the historical traffic data to characterize each zone. Unique RF channels and VLC channels/beams are assigned by the RSUs, considering the bandwidth, throughput, and interference. The RSU schedules the channel hopping cycles and time slots, and the beacon periods alternate between directional RF and LoS VLC. The RSUs also build the topology maps and run the optimization algorithms for the resource assignments in order to minimize interference while maximizing the throughput. We also proposed a novel congestion detection scheme utilizing traffic simulations to forecast the clearance times under different response strategies. The strategy that yields the lowest impact on travel time, emissions, and driver stress is selected. The selected \begin{table}
\begin{tabular}{l l} \hline \hline
**RF Parameter** & **Value** \\ \hline Vehicle speed limit & 60 km/h \\ \hline Vehicle distribution type & Clustered \\ \hline Congestion Simulation time & \(\approx\)100 s \\ \hline Bit rate & 18 Mbps \\ \hline RF MAC protocol & IEEE 802.11P \\ \hline Physical layer model & Shadowing propagation model \\ \hline Path loss scenario & Urban Macrocell \\ \hline RF Carrier Frequency & 5.89 GHz \\ \hline Packet size & 256 bytes \\ \hline VLC Parameter & Value \\ \hline VLC Modulation type & OOK \\ \hline VLC Model & Empirical light model \\ \hline Head light Max Tx Angle & 45\({}^{\circ}\) \\ \hline Tail light Max Tx Angle & 60\({}^{\circ}\) \\ \hline Carrier Frequency & 666,000 GHz \\ \hline Bandwidth & 1.0 MHz \\ \hline Packet Byte Length & 1024 bytes \\ \hline Beaconing Frequency & 1 Hz \\ \hline Optical transmitted power & 72 W \\ \hline VLC Transmitted and received angles & 5–45\({}^{\circ}\) \\ \hline APD model & S8664-1010 (Si) Hamamatsu \\ \hline Responsivity of PD \(\gamma\) & 0.54 A/W \\ \hline Gain of the optical concentrator g & 5 \\ \hline Visible light wavelength \(\lambda\) & 350 to 750 nm \\ \hline APD gain (M) & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation Parameters. strategy is then proactively used to alleviate the predicted congestion through optimized routing and control. Both prioritized dissemination and dynamic congestion mitigation are evaluated to optimize the safety and traffic flow. Simulation findings show that the hybrid RF-VLC system achieves an improvement in the system metrics compared to other techniques. It demonstrates the lowest packet loss and transmission delay, with the highest coverage range and the probability of emergency messages being rebroadcast. Also, the results show an improvement in quick congestion alleviation using our congestion management algorithm compared to other approaches. However, more complex environments with additional interference sources were not modeled. Also, a larger simulation area and duration would be needed to fully evaluate the scalability and robustness of the proposed algorithm. The obtained results are applicable specifically to the used configurations, and may differ for other settings. Applying this algorithm for diverse vehicle applications also needs more investigation. In summary, this work presents an adaptive routing protocol for emergency dissemination across partitioned zones, where the RSUs dynamically assign the RF/VLC resources to optimize the coverage while minimizing interference.