  Intelligent reflecting surface (IRS) enhanced multi-unmanned aerial vehicle
(UAV) non-orthogonal multiple access (NOMA) networks are investigated. A new
transmission framework is proposed, where multiple UAV-mounted base stations
employ NOMA to serve multiple groups of ground users with the aid of an IRS.
The three-dimensional (3D) placement and transmit power of UAVs, the reflection
matrix of the IRS, and the NOMA decoding orders among users are jointly
optimized for maximization of the sum rate of considered networks. To tackle
the formulated mixed-integer non-convex optimization problem with coupled
variables, a block coordinate descent (BCD)-based iterative algorithm is
developed. Specifically, the original problem is decomposed into three
subproblems, which are alternatingly solved by exploiting the penalty method
and the successive convex approximation technique. The proposed BCD-based
algorithm is demonstrated to be able to obtain a stationary point of the
original problem with polynomial time complexity. Numerical results show that:
1) the proposed NOMA-IRS scheme for multi-UAV networks achieves a higher sum
rate compared to the benchmark schemes, i.e., orthogonal multiple access
(OMA)-IRS and NOMA without IRS; 2) the use of IRS is capable of providing
performance gain for multi-UAV networks by both enhancing channel qualities of
UAVs to their served users and mitigating the inter-UAV interference; and 3)
optimizing the UAV placement can make the sum rate gain brought by NOMA more
distinct due to the flexible decoding order design.
