# Trajectory Optimization for UAV-Aided IoT Secure Communication Against Multiple Eavesdroppers +
## 1 Introduction  
In recent years, the rapid advance of the Internet of Things (IoT) has led to ubiquitous connectivity among a vast number of smart devices, posing significant challenges to both the reliability and real-time performance of IoT communications. Unmanned aerial vehicles (UAVs), due to their flexible maneuverability and fast deployment capabilities, are progressively emerging as a pivotal enabler of IoT communication networks [1, 2]. In IoT applications, UAVs may swiftly access target locations and establish solid line-of-sight (LoS) connections with ground terminals, significantly enhancing signal quality and data transmission efficacy. Furthermore, optimizing UAV trajectories enables IoT networks to attain superior energy efficiency and broader coverage. In [3, 4], UAVs serve as airborne base stations, facilitating swift 3D deployment to ensure ubiquitous air-to-ground connection. In [5], UAV functions as a mobile relay, maintaining robust wireless connectivity for ground users in environments where line-of-sight (LoS) is hindered by physical obstructions such as buildings or trees. To address the resource allocation issue, Ref. [6]proposes enhancing the energy efficiency of beamforming in IoT systems by optimizing UAV trajectories. To address suboptimal channel state information (CSI), leveraging the inherent advantages of LoS connections established by UAVs is beneficial. In a reconfigurable intelligent surface (RIS)-assisted UAV communication environment, a three-dimensional geometric dynamic channel model and a deep-learning-based channel tracking method are developed, leading to a significant improvement in channel estimation precision [7]. The swift progression of IoT has led to UAVs becoming vital nodes for collecting, transferring, and relaying data, hence broadening their applications and enhancing their significance in interconnected systems. Nonetheless, UAVs encounter substantial communication security issues despite their considerable potential. Specifically, UAV communication networks face several critical security challenges. Firstly, when a UAV functions as a lawful broadcaster, its air-to-ground LoS communication links, although advantageous for authorized receivers, simultaneously increase the risk of potential eavesdropping. Secondly, the elevated mobility of UAVs creates dynamic communication environments, allowing eavesdroppers to capitalize on channel fluctuations for malicious activities. Furthermore, the extensive utilization of UAVs in IoT communication services forces UAV networks to address not only traditional threats but also new risks stemming from device heterogeneity and network scalability, driven by the extensive interconnection of IoT devices and the continuous transmission of real-time data. The increased threat of eavesdropping exacerbates security weaknesses, highlighting the need for secure UAV network communications [8; 9]. Existing research has provided numerous strategies to solve these challenges. For example, to address security risks in UAV networks, researchers have created a security protocol tailored for UAV-to-ground communication, exhibiting markedly improved security compared to conventional protocols [10]. Furthermore, to address the need for resource-efficient and lightweight solutions, a mutual authentication system utilizing physical unclonable functions (PUFs) has been implemented. This technique facilitates mutual authentication and user anonymity while maintaining comparatively low computing costs [11]. Additionally, to address the robustness deficiencies of existing lightweight encryption techniques, a lightweight symmetric encryption algorithm and key agreement mechanism based on SM4 have been presented, improving data transmission security [12]. Notwithstanding these developments, current lightweight encryption systems continue to demonstrate constraints regarding flexibility and authentication. Due to the inherent resource limitations of UAVs, proper resource usage is essential.