# A Preemptive-Resume Priority MAC Protocol for Efficient BSM Transmission in UAV-Assisted VANETs
## 1 Introduction  
### Preliminaries  
In recent years, with the in-depth research and gradual popularization of autonomous driving technology, modern intelligent transportation systems (ITS) have become a focus of attention for governments, industry, and academia. Vehicle-to-vehicle (V2V) communication, as a core component of ITS, causes technical difficulties and challenges in the entire system. Each vehicle is equipped with a wireless interface called an On-Board Unit (OBU). The traditional method is that vehicles exchange and collect vehicle information and road condition information through the OBU and hand it over to the Road Side Unit (RSU) to complete communication between vehicles and infrastructure (V2I) [1]. The RSU then sends the information to the information processing center for data processing and integration. This leads to various challenges in Vehicular Ad hoc Networks (VANET). The communication coverage of VANETs is limited by the direct communication capability between vehicles and RSUs, which is further affected by factors such as terrain, buildings, and other obstacles. Consequently, certain regions or specific conditions may experience suboptimal communication quality or even a complete lack of continuous communication. Vehicles within VANETs are required to exchange a significant volume of data, encompassing crucial information such as location details, traffic status, event reports, and more [2]. Nevertheless, the data transfer speed in VANETs tends to be relatively slow due to the mobility of vehicles and the constrained transmission bandwidth. Consequently, this may result in increased communication latency and delayed data updates [3]. Ensuring the utmost reliability of vehicle data in VANETs is crucial for facilitating safety applicationsand enabling well-informed decision-making processes. However, ensuring data reliability presents certain challenges, such as signal attenuation, interference, and packet loss. These factors can undermine the reliability of data transmission. Unprocessed unreliable data can have detrimental effects, leading to erroneous decision-making or the dissemination of inaccurate traffic information [4]. However, the development of Unmanned Aerial Vehicle (UAV) technology has led to the replacement of RSUs by UAVs, which have become a topic of great interest for operators. UAVs, with their advantages of flexible deployment and wide coverage, offer faster transmission rates and can meet the demands of increasing user numbers and application messages. Compared to static relay stations, UAVs have a higher cost-performance ratio and economic efficiency due to their mobility, wide coverage, and low operation and maintenance costs [5]. Furthermore, UAVs can easily return to the air for inspection and maintenance, further reducing operational and maintenance costs. Low-altitude UAVs can fly in the air, free from the constraints of ground obstacles, providing a wider communication coverage range, filling the \"blind spots\" in communication between vehicles, and potentially offering better communication channels due to the existence of short-range line-of-sight (LOS) links [6]. Moreover, UAVs can expand network coverage and enhance network connectivity in various environments [7]. In VANETs, messages are characterized by their immediacy and time sensitivity, necessitating the timely transmission of messages from the source vehicle to the destination within a constrained time frame, while placing high demands on low latency. This requirement becomes particularly stringent in light of the advancements in autonomous driving technologies. Data packets that fail to meet the time constraints are discarded, leading to data loss [8]. In general, the vehicular networking domain follows the standardized framework established by the Society of Automotive Engineers (SAE) known as the J2735 standard [9]. At the heart of this standard lies the Basic Safety Message (BSM), thus making BSM messages the primary focus of discussion in this study.