# A Preemptive-Resume Priority MAC Protocol for Efficient BSM Transmission in UAV-Assisted VANETs
## 1 Introduction
### Preliminaries
Vehicles periodically transmit BSM messages that encompass fundamental vehicle information, including position, speed, and acceleration. Given the time-critical nature and substantial volume of BSM messages, effectively managing network traffic load and mitigating congestion have emerged as pivotal research areas [10]. Furthermore, in situations where UAV resources are constrained or cost considerations come into play, multiple vehicles may require shared access to these resources. Consequently, the allocation of resources needs to be prioritized based on the urgency and importance of the demands to ensure their efficient utilization [11]. In particular, in emergency scenarios like traffic accidents or urgent rescue operations, specific vehicles may require expedited communication and assistance. Through the establishment of priority levels, it becomes feasible to guarantee prompt responsiveness and preferential allocation of resources to address these critical circumstances [12]. In the case of limited resources, constructing an appropriate Media Access Control (MAC) layer protocol is an important method for effectively managing network traffic load and alleviating congestion. The most popular media access control protocol used for wireless local area networks in the context of vehicular networks is IEEE 802.11P Distributed Coordination Function (DCF) [13]. It employs the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol with binary exponential backoff. DCF defines two channel access methods: basic access and the Request-to-Send/Clear-to-Send (RTS/CTS) mechanism. DCF not only plays a crucial role in new standards like IEEE 802.11e for enhanced Quality of Service (QoS), but it has also been widely implemented in various wireless test beds and simulation packages for wireless multi-hop self-organizing networks. Its low complexity has been widely recognized. Therefore, making reasonable improvements to this MAC layer protocol can provide a significant solution and innovation point for optimizing network architecture and addressing network congestion issues. Moreover, when considering safety aspects, it has been shown that higher vehicle speeds on highways correlate with increased accident rates [14]. This insight offers a viable approach for establishing a priority criterion based on vehicle speed. In comparison to other metrics such as vehicle type or communication needs, vehicle speed is readily obtainable and can be promptly updated, enabling real-time reflection of the vehicle's present motion state [15]. By prioritizing vehicles according to their speed, efficient resource allocation can be achieved. In general, vehicles traveling at lower speeds tend to have relatively lower requirements for real-time communication [16] as they can maintain a considerable distance from neighboring vehicles for a longer duration. Conversely, vehicles traveling at higher speeds necessitate timely communication support to promptly adjust their driving states and effectively respond to sudden situations. The prioritization of vehicles based on speed can further be leveraged for traffic flow control and coordination purposes. During instances of road congestion, the prioritization of faster-moving vehicles as a high priority can facilitate UAVs in providing real-time traffic feedback and route suggestions to enable these vehicles to avoid congested regions or opt for quicker routes [17]. This approach can potentially alleviate traffic congestion and enhance overall traffic efficiency.