Covert Communication in D2D Underlaying Cellular Networks With Multiple Colluding Wardens

This article investigates the covert communication in a D2D underlaying cellular network consisting of a cellular transmitter, a base station, a cellular receiver, multiple colluding wardens, and an underlaid D2D pair with a transmitter and a receiver. For multiple colluding wardens, they fuse their observations to a fusion center (FC) with the maximum ratio combining (MRC) scheme and the equal gain combining (EGC) scheme. To protect the covert communication between the cellular transmitter and the base station from being detected by multiple colluding wardens, the underlaid D2D transmitter reuses the spectrum of the cellular user to transmit its signal to the D2D receiver with a random power to deliberately confuse multiple colluding wardens. We first provide the basic theoretical results for the detection performance of FC under MRC and EGC, respectively, i.e., the optimal detection threshold, the minimum detection error probability and its average value. We then explore the modeling of the average covert rate as well as the optimal power control for average covert rate maximization to enhance the covert performance under MRC and EGC. Finally, extensive numerical and simulation results are presented to validate theoretical analysis and also to illustrate the effects of some important system parameters on the average minimum detection error probability, the average covert rate, and the maximum average covert rate under MRC and EGC, respectively. We can find that the detection performance of multiple colluding wardens under the MRC scheme is better than that under the EGC scheme.