# Optimal Position and Target Rate for Covert Communication in UAV-Assisted Uplink RSMA Systems
## 1 Introduction  
The rapid development of intelligent wireless terminals promotes a large amount of wireless private information, bringing broader attention to information security. The study of communication security at the physical layer has been segregated into two directions, namely physical layer security (PLS) [1, 2] and covert communications [3, 4]. The purpose of PLS is to ensure that the transmitted information is not intercepted by eavesdroppers, i.e., protecting the transmitted content. However, it's not always sufficient to focus solely on protecting information security, as the exposure of communication behavior can also pose potential risks and threats. For example, the exposure of the signal transmission would disclose the existence and position of a device to an adversary, ultimately resulting in an attack on the device. Different from PLS, covert communications focus on shielding the transmission behavior from potential watchful adversaries. [PERSON] et al. initiated covert communication research and proposed a square root law as the fundamental limit in the additive white Gaussian noise (AWGN) channels [3]. In [4], the authors proved that it is possible to transmit \(\mathcal{O}(n)\) bits covertly and reliably in \(n\) uses of AWGN channel when the monitor has uncertainty about the received power. With the growing number of connected devices and increasing data traffic, there is a need for more efficient and effective ways to manage the available resources. Non-orthogonal multiple access (NOMA) is a promising technique that offers improvements over conventional orthogonal multiple access techniques in terms of spectral efficiency [5, 6]. In [7], the PLS in NOMA systems assisted with a HAP and UAVs was studied. Rate-splitting multiple access (RSMA), which can further increase the sum rate, has recentlyemerged as a more general and robust transmission framework compared to NOMA [8; 9; 10; 11; 12]. In particular, the performances of uplink RSMA systems were studied in [10; 11]. In [10], the authors investigated a rate-splitting (RS) strategy in uplink cognitive radio systems, where a secondary user splits its rate to guarantee the primary user's transmission. In [11], the optimal decoding order and maximum sum rate in uplink RSMA systems were studied. To protect the privacy information, the authors in [13] studied the security and energy efficiency of the cognitive RSMA-based satellite-terrestrial networks, where a beamforming scheme was proposed to prevent eavesdropping and increase energy efficiency. Covert communications in NOMA systems have also been widely studied [14; 15]. The author in [14] achieved covert communication in an uplink NOMA system via random power jamming generated by channel inversion power control. The study in [15] explored an intelligent reflecting surface (IRS)-assisted covert communication in both downlink and uplink NOMA systems. The randomness was brought about by the phase-shift uncertainty of IRS and the overlapping signal transmission. In addition, unmanned aerial vehicles (UAVs) have been used by virtue of their high mobility, which provides new degrees of freedom to enhance the covertness of communications [16; 17]. In [16], the optimal transmit power and location for the UAV were studied to achieve covert communications. In [17], the authors used the geometric method to solve the trajectory problem. Most recently, covert communication in UAV-aided NOMA systems was investigated in [18]. To further increase the sum rate, we investigate covert communication in UAV-assisted uplink RSMA systems in this paper. In this system, a ground user and a UAV simultaneously communicate with a base station (BS), suffering the surveillance of an evil monitor. The UAV acts as both the transmitter and the friendly jammer, covering the ground user's transmission with random power. This work aims to maximize the expected sum rate (ESR) by designing the UAV's power allocation, position, and target rate while guaranteeing the ground user's covertness and throughput.