# Trajectory Optimization for UAV-Aided IoT Secure Communication Against Multiple Eavesdroppers +
## 1 Introduction
Consequently, researchers have developed a blockchain-based distributed security solution specifically built for UAV computing networks. This system allows UAVs to independently manage authentication, attribute administration, and access control, while adaptively modifying access policies according to real-time network conditions, thereby enhancing both security and resource efficiency [13]. Although data encryption remains a pivotal focus in communication security research, its high cost, computational complexity, and reliance on stable channels make it challenging to ensure adequate security in resource-constrained and dynamic IoT environments. In contrast, as a supplementary approach to traditional encryption methods, physical layer security (PLS) techniques, founded upon information-theoretic security principles, provide significant advantages. With low computational overhead and no need for complex encryption or decryption operations, PLS is well-suited for resource-limited scenarios. By preventing eavesdroppers from extracting meaningful information while ensuring reliable communication, PLS has emerged as a critical approach for countering ground-based eavesdroppers and safeguarding UAV communication systems [14; 15; 16]. Investigating how to secure information transmission for UAVs in IoT environments is of critical importance. The introduction of PLS technology not only provides essential safe guards for data sharing in UAV networks, but also enhances communication reliability and security in the context of large-scale IoT device interconnectivity. In UAV secure communications, secrecy rate is recognized as the principal metric for assessing the effectiveness of physical layer security (PLS) systems. Secure UAV communication is deemed feasible when the received signal quality at the legitimate link exceeds that at all eavesdropping nodes [17]. Generally, studies on UAV-assisted secure communication can be categorized into two main directions: UAV-enabled secure communication [18; 19] and UAV-aided secure cooperation [20; 21]. Due to their ability to dynamically reposition in response to changing radio propagation environments, UAVs can establish highly efficient LoS communication links for legitimate users, making their deployment and trajectory optimization fundamental to PLS design. For instance, one study proposed a buffer-aided UAV secure communication system, where optimized UAV trajectories significantly improved the overall secrecy rate [22]. In UAV-assisted relay communication systems, the joint optimization of the base station's transmit power and the UAV's flight trajectory can significantly improve the secrecy rate of communications [23]. Short-packet communication and UAVs play pivotal roles in 5G and future wireless networks, particularly in scenarios with stringent latency requirements and security challenges. Accordingly, researchers have developed a secure SPC-IoT system leveraging UAVs as mobile decode-and-forward relays. By exploiting their agile 3D mobility, UAVs periodically receive and forward short packets from remote IoT devices in a two-hop manner, ensuring end-to-end secure and reliable transmission. This approach not only mitigates risks posed by ground-based eavesdroppers in uncertain locations, but also enhances the secrecy and reliability of SPC networks [24]. Multi-UAV cooperative PLS has also attracted significant attention. For example, a secure communication system assisted by two UAVs has been proposed to support the mobility of ground devices. By leveraging reinforcement learning to jointly optimize the UAVs' trajectories and transmission rates, the system enables secure bidirectional communication [25]. Unlike conventional methods limited to unidirectional security with static devices, this work innovatively supports device mobility while ensuring mutual security between UAVs and ground nodes. PLS can be further strengthened by integrating interference injection and beamforming techniques.