  We exploit both covert communication and friendly jamming to propose a
friendly jamming-assisted covert communication and use it to doubly secure a
large-scale device-to-device (D2D) network against eavesdroppers (i.e.,
wardens). The D2D transmitters defend against the wardens by: 1) hiding their
transmissions with enhanced covert communication, and 2) leveraging friendly
jamming to ensure information secrecy even if the D2D transmissions are
detected. We model the combat between the wardens and the D2D network (the
transmitters and the friendly jammers) as a two-stage Stackelberg game.
Therein, the wardens are the followers at the lower stage aiming to minimize
their detection errors, and the D2D network is the leader at the upper stage
aiming to maximize its utility (in terms of link reliability and communication
security) subject to the constraint on communication covertness. We apply
stochastic geometry to model the network spatial configuration so as to conduct
a system-level study. We develop a bi-level optimization algorithm to search
for the equilibrium of the proposed Stackelberg game based on the successive
convex approximation (SCA) method and Rosenbrock method. Numerical results
reveal interesting insights. We observe that without the assistance from the
jammers, it is difficult to achieve covert communication on D2D transmission.
Moreover, we illustrate the advantages of the proposed friendly
jamming-assisted covert communication by comparing it with the
information-theoretical secrecy approach in terms of the secure communication
probability and network utility.
