Unmanned aerial vehicles (UAVs) can provide wireless access to terrestrial users, regardless of geographical constraints, and will be an important part of future communication systems. In this paper, a multi-user downlink dual-UAVs enabled covert communication system was investigated, in which a UAV transmits secure information to ground users in the presence of multiple wardens as well as a friendly jammer UAV transmits artificial jamming signals to fight with the wardens. The scenario of wardens being outfitted with a single antenna is considered, and the detection error probability (DEP) of wardens with finite observations is researched. Then, considering the uncertainty of wardens' location, a robust optimization problem with worst-case covertness constraint is formulated to maximize the average covert rate by jointly optimizing power allocation and trajectory. To cope with the optimization problem, an algorithm based on successive convex approximation methods is proposed. Thereafter, the results are extended to the case where all the wardens are equipped with multiple antennas. After analyzing the DEP in this scenario, a tractable lower bound of the DEP is obtained by utilizing Pinsker's inequality. Subsequently, the non-convex optimization problem was established and efficiently coped by utilizing a similar algorithm as in the single-antenna scenario. Numerical results indicate the effectiveness of our proposed algorithm.

翻译：无人机能够突破地理限制为地面用户提供无线接入，将成为未来通信系统的重要组成部分。本文研究了多用户下行链路双无人机协同隐蔽通信系统，其中一架无人机向地面用户传输安全信息，同时存在多个监控者，而另一架友好干扰无人机发送人工干扰信号以对抗监控者。考虑监控者配备单天线场景，分析了有限观测条件下监控者的检测错误概率。随后，针对监控者位置不确定的情况，构建了以最坏情况隐蔽性约束为条件的鲁棒优化问题，通过联合优化功率分配与轨迹以最大化平均隐蔽速率。为解决该优化问题，提出了一种基于逐次凸逼近的算法。进一步将研究成果推广至所有监控者配备多天线的场景。在分析该场景下的检测错误概率后，利用平斯克尔不等式推导出检测错误概率的易处理下界，继而建立非凸优化问题，并采用与单天线场景类似的算法高效求解。数值结果表明了所提算法的有效性。