  In this paper, we propose a novel adaptive decoding mechanism (ADM) for the
unmanned aerial vehicle (UAV)-enabled uplink (UL) non-orthogonal multiple
access (NOMA) communications. Specifically, considering a harsh UAV
environment, where ground-to-ground links are regularly unavailable, the
proposed ADM overcomes the challenging problem of conventional UL-NOMA systems
whose performance is sensitive to the transmitter's statistical channel state
information and the receiver's decoding order. To evaluate the performance of
the ADM, we derive closed-form expressions for the system outage probability
(OP) and system throughput. In the performance analysis section, we provide
novel expressions for practical air-to-ground and ground-to-air channels, while
taking into account the practical implementation of imperfect successive
interference cancellation (SIC) in UL-NOMA. Moreover, the obtained expression
can be adopted to characterize the OP of various systems under a Mixture of
Gamma (MG) distribution-based fading channels. Next, we propose a sub-optimal
Gradient Descent-based algorithm to obtain the power allocation coefficients
that result in maximum throughput with respect to each location on UAV's
trajectory. To determine the significance of the proposed ADM in nonstationary
environments, we consider the ground users and the UAV to move according to the
Random Waypoint Mobility (RWM) and Reference Point Group Mobility (RPGM)
models, respectively. Accurate formulas for the distance distributions are also
provided. Numerical solutions demonstrate that the ADM-enhanced NOMA not only
outperforms Orthogonal Multiple Access (OMA), but also improves the performance
of UAV-enabled UL-NOMA even in mobile environments.
