  Motivated by diverse secure requirements of multi-user in UAV systems, we
propose a collaborative secret and covert transmission method for multi-antenna
ground users to unmanned aerial vehicle (UAV) communications. Specifically,
based on the power domain non-orthogonal multiple access (NOMA), two ground
users with distinct security requirements, named Bob and Carlo, superimpose
their signals and transmit the combined signal to the UAV named Alice. An
adversary Willie attempts to simultaneously eavesdrop Bob's confidential
message and detect whether Carlo is transmitting or not. We derive close-form
expressions of the secrecy connection probability (SCP) and the covert
connection probability (CCP) to evaluate the link reliability for wiretap and
covert transmissions, respectively. Furthermore, we bound the secrecy outage
probability (SOP) from Bob to Alice and the detection error probability (DEP)
of Willie to evaluate the link security for wiretap and covert transmissions,
respectively. To characterize the theoretical benchmark of the above model, we
formulate a weighted multi-objective optimization problem to maximize the
average of secret and covert transmission rates subject to constraints SOP,
DEP, the beamformers of Bob and Carlo, and UAV trajectory parameters. To solve
the optimization problem, we propose an iterative optimization algorithm using
successive convex approximation and block coordinate descent (SCA-BCD) methods.
Our results reveal the influence of design parameters of the system on the
wiretap and covert rates, analytically and numerically. In summary, our study
fills the gaps in joint secret and covert transmission for multi-user
multi-antenna uplink UAV communications and provides insights to construct such
systems.
