  In this paper, we aim to secure the D2D communication of the D2D-underlaid
cellular network by leveraging covert communication to hide its presence from
the vigilant adversary. In particular, there are adversaries aiming to detect
D2D communications based on their received signal powers. To avoid being
detected, the legitimate entity, i.e., D2D-underlaid cellular network, performs
power control so as to hide the presence of the D2D communication. We model the
combat between the adversaries and the legitimate entity as a two-stage
Stackelberg game. Therein, the adversaries are the followers and aim to
minimize their detection errors at the lower stage while the legitimate entity
is the leader and aims to maximize its utility constrained by the D2D
communication covertness and the cellular quality of service (QoS) at the upper
stage. Different from the conventional works, the study of the combat is
conducted from the system-level perspective, where the scenario that a
large-scale D2D-underlaid cellular network threatened by massive spatially
distributed adversaries is considered and the network spatial configuration is
modeled by stochastic geometry. We obtain the adversary's optimal strategy as
the best response from the lower stage and also both analytically and
numerically verify its optimality. Taking into account the best response from
the lower stage, we design a bi-level algorithm based on the successive convex
approximation (SCA) method to search for the optimal strategy of the legitimate
entity, which together with the best response from the lower stage constitute
the Stackelberg equilibrium. Numerical results are presented to evaluate the
network performance and reveal practical insights that instead of improving the
legitimate utility by strengthening the D2D link reliability, increasing D2D
transmission power will degrade it due to the security concern.
