# UAV-Assisted Hybrid Scheme for Urban Road Safety Based on VANETs
## 3 UAV Assisted (UAVa) Reactive Protocol Strategy
### UAVa's System Model  
UAVa is proposed to reduce the dependence on the traffic density for reducing the congestion and intermediate intersections selection between the source node and the destination node, and increase data packet transmission rate, thereby enhancing intersection-based routing mobility by sending data at any time and anywhere irrespective of traffic density. The novelty of this selection mechanism is that the roadside units (RSUs) and unmanned aerial vehicles (UAVs) at road intersections are among the selection metrics, where RSUs are very important in VANETs. Yet, RSUs were not previously used as metric-assisted UAVs for intersection. Installing RSUs every 200 m is a lot of capital cost. Reducing the capital cost of deploying RSU is justified by adding a limited number of RSUs to create the intersections. As an independent network, it is assumed that RSUs communicate with each other through the existing cellular network, i.e., there is no direct wireless or wired contact from RSU to RSU. UAVa assumes a vast and continuous traffic density at a certain period of time cycle, and that each node can download a digital map through the vehicle to roadside (V2R) communications. Let us illustrate the different assumptions considered by the system model of UAVa. Consider VANETs stationed in a geographically small urban area where a group of RSUs is implemented on the ground, and UAVs are hovering in the congested intersection. Both RSUs and UAVs communicate with vehicles on the ground and exchange data. As shown in Figure 1, the road intersection is RSU based. UAVs will be assisted up to its demands. Assume that it is divided into fixed intersections with an ID. The communication range of these intersections is defined based on the RSUs capacity. For understanding our proposed network, different static connections are characterized by a unique ID, since our routing process is not primarily based on static nodes, but rather depends on the intersection. This research is essential for understanding relevant information, such as the present traffic conditions, the uniform allocation of the RSUs, and the destination location. The geographical positions and the identity of every intersection should be known by all nodes and UAVs utilizing their downloaded digital city map through V2R communications. Based on the embedded on-board unit (OBU) with GPS, every node can determine its current geographical location. In addition, we assume that there is a limitation of vitality for all nodes since they can be equipped with rechargeable batteries at first. In our proposed scheme, each node energy is 300 joules. It should be emphasized that UAVs float very well at low altitudes to operate on the ground vehicles. We assume that RSUs are allocated uniformly across the network and moving vehicles which do not exceed 300 m during the flight of UAVs. Through following cellular standards, RSUs can communicate with each other on a wireless interface. As depicted in Figure 1, we use six forms of wireless configuration to build the entire network communications we propose:  
1. Vehicle-to-Vehicle (V2V): Vehicles on the ground can communicate according to the IEEE 802.11p wireless standard within the line-of-sight (LoS) communication range with each other. In the event of an obstacle, communication is not possible. 2. Vehicle-to-Unmanned aerial vehicle (V2U): Vehicles and UAVs can communicate with each other according to the IEEE 802.11p wireless standard. In general, UAVs in urban areas do not fly at high altitudes, because majority of their applications in our chosen simulation scenario must be carried out at low altitudes. 3. Vehicle-to-Roadside unit (V2R): For most instances, this form of contact exists for applications requiring direct access to the information provided by the gateways (i.e., RSUs). Vehicle and RSU at a road intersection can communicate with each other based on the inclusion of a wireless protocol that adopts the IEEE 802.11p standard, which can have a high likelihood of line-of-sight (LoS). 4.