Covert Rate Study for Full-Duplex D2D Communications Underlaid Cellular Networks

Device-to-device (D2D) communications underlaid cellular networks have emerged as a promising network architecture to provide extended coverage and high data rate for various Internet of Things (IoT) applications. However, because of the inherent openness and broadcasting nature of wireless communications, such networks face severe risks of data privacy disclosure. This article investigates the covert communications in such networks for providing enhanced privacy protection. Specifically, this article explores the critical covert rate performance in a full-duplex (FD) D2D communication underlaid cellular network consisting of a base station, a cellular user, a D2D pair with a transmitter and an FD receiver, and a warden, where the D2D receiver (DR) can operate over either the FD mode or the half-duplex (HD) mode. We first derive transmission outage probabilities of cellular and D2D links under the FD and HD modes, respectively. Based on these probabilities, we further provide theoretical modeling for the covert rate under each mode and explore the corresponding covert rate maximization by jointly optimizing the transmit powers of the D2D pair and the cellular user. To improve the covert rate performance, we propose a general mode in which the DR can flexibly switch between these two modes. Under the general mode, we also investigate the theoretical modeling and maximization problems of covert rate. Finally, we present extensive numerical results to illustrate the covert rate performances under the FD, HD, and general modes.