On Joint Covert and Secure Communications in D2D-Enabled Cellular Systems

This paper explores the joint covert and secure communications in a device-to-device (D2D)-enabled cellular system (DCS) consisting of a base station BS, an eavesdropper Eve, and two user equipments UE and UR. To conduct secure communications with UE against Eve, BS works either under the cellular mode using direct transmission or under the D2D mode replying through UR, while UR is greedy since it opportunistically transmits its own covert message to UE against the detection from BS. To understand the fundamental performance of secrecy rate and covert rate in DCS, we first develop theoretical models to depict the detection probability/secrecy rate of BS and covert rate of UR under different modes (i.e., underlay, overlay, or cellular). Based on these models, we further explore the secrecy rate maximization (SRM) for BS subject to the constraints of detection probability at BS and transmit power at both BS and UR, as well as the covert rate maximization (CRM) for UR subject to the constraints of covertness requirement and covert transmit power. Finally, we employ the Newton-based searching method to solve the SRM/CRM problems and illustrate via numerical results the achievable secrecy rate and covert rate of BS and UR under various DCS scenarios.