  In this work, a delay-tolerant unmanned aerial vehicle (UAV) relayed covert
and secure communication framework is investigated. In this framework, a
legitimate UAV serves as an aerial relay to realize communication when the
direct link between the terrestrial transmitter and receiver is blocked and
also acts as a friendly jammer to suppress the malicious nodes presented on the
ground. Subsequently, considering the uncertainty of malicious nodes'
positions, a robust fractional programming optimization problem is built to
maximize energy efficiency by jointly optimizing the trajectory of the UAV, the
transmit power of the transmitter, and the time-switching factor. For the
extremely complicated covert constraint, Pinsker's inequality, Jensen's
inequality, and the bisection search method are employed to construct a
tractable shrunken one. After this, an alternate optimization-based algorithm
is proposed to solve the fractional programming optimization problem. To
achieve low complexity, we design the primal-dual search-based algorithm and
the successive convex approximation-based algorithm, respectively, for each
sub-problem. Numerical results show the effectiveness of our proposed
algorithm.
