# Disguised Full-Duplex Covert Communications
## 1 Introduction
For an FD unmanned aerial vehicle (UAV) that collects data from a scheduled user and interferes with unscheduled users with AN, the maximum lowest average covert rate was obtained in [27]. In [28], the location of a covert transmitter UAV was optimized with the help of an FD ground receiver that confuses the warden, and in [29], covert communications with a hovering FD UAV relay assuming Rician air-ground channel was studied. The authors of [30] considered an FD DF UAV relay to aid in covert communications where multiple sensors deliver messages to a remote base station in orthogonal time slots. Moreover, in a cognitive radio system, covert communications between secondary users in the presence of FD eavesdroppers that interfere with the secondary receiver with AN were examined by [31]. The work in [32] studied covert rate maximization with a half-duplex (HD)/FD mode switching device-to-device (D2D) covert receiver in the presence of an uplink spectrum-sharing cellular network. Both secrecy and covert rates were optimized when an untrusted FD AF relay delivers the covert message to an FD base station that emits AN to the warden in [33]. In the IoT environment, the authors of [34] studied a covert transmitter with an optimized transmission probability, which is wirelessly charged by the AN from an FD receiver. Furthermore, covert uplink transmissions of devices towards FD IoT gateways were optimized based on the mean-field Stackelberg game approach in [35]. With an ambient backscatter system, a radio frequency tag modulates an ambient signal into a covert signal for an FD receiver that concurrently broadcasts AN [36]. Most of past works have assumed that the surveillance nodes have a perfect knowledge of the hardware specifications of covert nodes. However, the covert nodes could disguise themselves as other functional entities for even more covertness. For instance, an original FD node that secretly transmits confidential messages may disguise itself as a receive-only HD node. To the best of the author's knowledge, there are not enough studies on covert communications that consider such disguised tactics. In this paper, we study a covert communications system where a source node communicates with a disguised FD destination node. Supposedly receive-only, the destination node covertly delivers critical messages with an invisible extra antenna to another hidden receiver while evading the surveillance of a warden node as much as possible. We identify the optimal public data rate and the transmit power of the FD destination node that maximizes the covert rate at the hidden receiver. Closed-form solutions are provided, and we investigate the effects of various system parameters on the covert rate through numerical results. Our main contributions are summarized as follows:  
* Different from past works which assume that the surveillance party is confident of the hardware specifications of covert nodes, we take a step further and consider a practical scenario in which a covert communications node disguises itself as another functional entity for even more covertness. * The worst-case DEP is derived in the presence of noise uncertainty at the warden node. * Noting that covert communications typically suffer from a low data rate due to the stringent DEP requirements, we focus on maximizing the covert rate at the hidden receiver by optimizing the public data rate and the transmit power of the FD destination node subject to the worst-case DEP of the warden. * We investigate the effects of various system parameters on the covert rate through numerical results, one of which reveals that applying more (less) destination transmit power achieves a higher covert rate when the source transmit power is low (high). * Since our work provides a performance guideline from the information-theoretic point of view, we suggest analyses with practical modulations and imperfect CSI as valuable future research topics.