# Disguised Full-Duplex Covert Communications
## 1 Introduction  
Wireless technology has revolutionized the way people live in various ways [1]. However, behind the proliferation of wireless communications are cyberattacks that leave users open to information leakage [2]. To cope with this, cryptography has widely been adopted, which encrypts and decrypts data using secret keys [3]. Nevertheless, this approach has certain limitations, e.g., high complexity for generating secret keys and vulnerability to eavesdroppers with stronger computational power, which are particularly unfavorable for the Internet of Things (IoT) devices. These downsides have led researchers to examine the possibility of utilizing physical layer security [4]. Its main characteristic is that a wireless link from legitimate entities to eavesdroppers can be effectively obstructed, either by nullifying beamforming with multiple antennas, or by disruption with artificial noise (AN) [5]. Hence, the dependency on secret key agreements and the need of avoiding high-powered adversaries can be greatly alleviated. Still, some applications require an even more strict level of confidentiality. A reconnaissance troop would require reporting its surroundings to the operation center without being detected by enemies in the middle of a military mission [6], or closed networks in security facilities need to make sure that any classified information over the air is concealed from any external party. An adequate technology for such situations is _covert communications_ or _low-probability-of-detection_ communications that hide the existence of a critical communications link [7]. Covert communications have also been extensively studied for full-duplex (FD) systems. A basic three-node system with a covert transmitter and an FD receiver that simulaneously emits AN was studied in [8]. The authors of [9; 10] further treated specific channel properties where only channel distribution information of the transmitter-warden link is available, and where every link is non-coherent with channel distribution information for slow or fast fading, respectively. An antenna selection at the receiver was studied in [11], and a transmission time selection and power control strategy were presented by [12] based on channel state information (CSI). The authors in [13] also verified the effectiveness of a truncated channel inversion power control that ceases covert transmission when the channel of the transmitter-receiver is low. A delay-constrained covert communications with a fixed AN power was investigated in [14], and joint AN power and receiver position optimization problems were discussed in [15; 16]. Uncertain locations of a warden node were also taken into consideration in [17]. On top of the AN, a random covert channel selection by the transmitter was studied in [18] to aggravate the confusion of the warden. The maximum detection error probability (DEP) subject to the age of information constraint was identified by [19]. As for more complex FD systems, an FD amplify-and-forward (AF) relay was considered by [20], and the authors in [21] designed an energy harvesting FD decode-and-forward (DF) relay-based covert communications protocol in which the relay forwards and harvests energy simultaneously. Moreover, for integrated satellite-terrestrial communications, an FD relay-aided covert communications from a satellite to a ground node was explored in [22]. For intelligent reflecting surface (IRS)-aided covert communications, a transmit beamforming vector and the reflecting coefficients are jointly optimized when an FD receiver broadcasts random AN to confuse the warden in [23]. An uplink covert communications with the aid of an IRS was also investigated by [24]. Utilizing an active IRS, which is naturally FD, for covert communications between a pair of users was discussed in [25]. The age of information was minimized in [26] when a receiver covertly transmits confidential messages to the transmitter, shielded under public transmissions from the transmitter to the receiver with the aid of an IRS.