# On IRS-Assisted Covert Communication with a Friendly UAV
## 1 Introduction
To improve the security of wireless CCS, friendly nodes can be utilized to jam adversaries, as in [19; 20]. Due to their fast deployment with flexible configurations and the presence of short-range line-of-sight links, unmanned aerial vehicles (UAVs) are frequently utilized as friendly nodes. A UAV equipped with a base station operates as a moving base station to provide comprehensive communication services. However, the exposure of the transmitted wireless signals to open space threatens the information security of communication systems that rely on UAVs [21]. To reduce this security threat, a communication protocol was proposed in [22] to ensure secure communication between UAVs and other UAVs as well as between UAV and ground users. For CCS, when the locations of legal users and monitors cannot be determined, the UAV can operate as a relaying node to deliver messages between the legal users covertly in order to avoid detection by monitors. In a pioneering work, a UAV-aided CCS was designed in [23], where the UAV trajectory and transmit power for the UAV were alternately optimized to maximize the average covert rate. Based on this, [PERSON] proposed a model for UAV-assisted CCS with multiple users [24]. Similarly, the authors of [25] developed a multi-user mmWave covert communication system wherein proper covert beam training and data transmission between legitimate parties was designed. In order to cope with the complex communication environment, in [26] a UAV with multiple antennas was designed to jam the monitor in the CCS; this approach can account for multiple monitors colluding to listen to adjacent users. Similarly, [PERSON] proposed a multi-antenna UAV to assist a covert communication system, along with a power allocation algorithm that can be used to avoid detection from a monitor with multiple antennas [27]. Owing to the substantial enhancements in performance obtained by combining UAVs and IRS, numerous frameworks incorporating these technologies have been proposed [28; 29; 30]. However, the effective use of UAVs with IRS to design an efficient wireless covert communication system remains a challenging problem. In this paper, we propose an IRS-assisted covert communication system with a friendly UAV, where the UAV can be utilized to reinforce message security and the IRS can be employed to improve the covert rate. Moreover, thanks to the high mobility of UAVs, the proposed CCS can be deployed in complex communication environments with various communication barriers, resulting in improved reliability. The contributions of this paper are listed as follows:  
* We propose an IRS-assisted covert communication system in which a UAV operates as a jammer to interfere with the monitor and reduce the detection rate. * We derive the minimum detection error probability (DEP) for the monitor by modeling the artificial noise (AN) power of the UAV as an uncertainty model, under the assumption that the coordinates of both the UAV and the transmitter are available. * We formulate the optimization problem to maximize the covert rate for a covert communication system by optimizing the trajectory and transmitting power of the UAV. Furthermore, we develop an iterative algorithm that uses the SCA and Dinkelbach methods to solve the optimization problem. The rest of this paper is organized as follows: Section 2 describes the system model; Section 3 formulates the optimization method, considering the goal of maximizing the covert rate for [PERSON]; Section 4 presents the optimal algorithm designed to solve the problem described in Section 3; and Section 5 presents the simulation results of the algorithm. Finally, Section 6 concludes the paper.