Covert Communications in the RF Band of Primary Wireless Networks

Abstract

Covert systems are designed to operate at a low probability of detection (LPD) in order to provide system protection at the physical layer level. The classical approach to covert communications aims at hiding the covert signal in noise by lowering the power spectral density of the signal to a level that makes it indistinguishable from that of the noise. However, the increasing demand for modern covert systems that can provide better protection against intercept receivers (IRs) and provides higher data rates has shifted the focus to the design of Ad-Hoc covert networks (ACNs) that can hide their transmission in the RF spectrum of primary networks (PNs), like mobile networks. The early work on exploiting the RF band of other wireless systems has been promising; however, the difficulties in modeling such environments, analyzing the impact on/from the primary network, and deriving closed form expressions for the performance of the covert network have limited the work on this crucial subject. In this work, we provide the first comprehensive analyses of a covert network that exploits the RF band of an OFDM-based primary network to achieve covertness. A spectrum access algorithm is presented which would allow the ACN to transmit in the RF spectrum of the PN with minimum interference. Next, we use stochastic geometry to model both the OFDM-based PN as well as the ACN. Using stochastic geometry would also allow us to derive closed-form expressions and provide a comprehensive analysis for two metrics, namely an aggregate metric and a ratio metric. These two metrics quantify the covertness and performance of the covert network from the perspective of the IR and the ACN, respectively. The two metrics are used to determine the detectability limits of an ACN by an IR. The two metrics along with the proposed spectrum access algorithm will be used to provide a comprehensive discussion on how to design the ACN for a target covertness level, and analyze the effect of the PN parameters on the ACN expected performance. This work also addresses the question of trade-off between the ACN covertness and its achievable throughput. The overall discussion and results in this research work illustrate the strong potential for using man-made transmissions as a mask for covert communications. In addition, some of our results can be directly used for other applications such as device-to-device (D2D) and vehicle-to-everything (V2X) communications.