This paper investigates a novel unmanned aerial vehicles (UAVs) secure communication system with the assistance of reconfigurable intelligent surfaces (RISs), where a UAV and a ground user communicate with each other, while an eavesdropper tends to wiretap their information. Due to the limited capacity of UAVs, an RIS is applied to further improve the quality of the secure communication. The time division multiple access (TDMA) protocol is applied for the communications between the UAV and the ground user, namely, the downlink (DL) and the uplink (UL) communications. In particular, the channel state information (CSI) of the eavesdropping channels is assumed to be imperfect. We aim to maximize the average worst-case secrecy rate by the robust joint design of the UAV’s trajectory, RIS’s passive beamforming, and transmit power of the legitimate transmitters. However, it is challenging to solve the joint UL/DL optimization problem due to its non-convexity. Therefore, we develop an efficient algorithm based on the alternating optimization (AO) technique. Specifically, the formulated problem is divided into three sub-problems, and the successive convex approximation (SCA), S\mathcal {S} -Procedure, and semidefinite relaxation (SDR) are applied to tackle these non-convex sub-problems. Numerical results demonstrate that the proposed algorithm can considerably improve the average secrecy rate compared with the benchmark algorithms, and also confirm the robustness of the proposed algorithm.

中文翻译：

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借助可重构智能表面实现稳健安全的无人机通信


本文研究了一种借助可重构智能表面（RIS）的新型无人机（UAV）安全通信系统，其中无人机和地面用户相互通信，而窃听者则倾向于窃听他们的信息。由于无人机的能力有限，采用RIS来进一步提高安全通信的质量。无人机与地面用户之间的通信采用时分多址(TDMA)协议，即下行链路(DL)和上行链路(UL)通信。特别地，假设窃听信道的信道状态信息（CSI）是不完善的。我们的目标是通过无人机轨迹、RIS 的无源波束成形和合法发射机的发射功率的稳健联合设计，最大限度地提高平均最坏情况保密率。然而，由于其非凸性，解决联合UL/DL优化问题具有挑战性。因此，我们开发了一种基于交替优化（AO）技术的高效算法。具体来说，将公式化的问题分为三个子问题，并应用逐次凸近似（SCA）、S\mathcal {S} -过程和半定松弛（SDR）来解决这些非凸子问题。数值结果表明，与基准算法相比，该算法能够显着提高平均保密率，同时也证实了该算法的鲁棒性。