  Device-to-device (D2D) communications offers high spectral efficiency, low
energy consumption and transmission latency. However, one of the main
limitations of D2D communications is co-channel interference from underlaying
wireless system. Reconfigurable intelligent surfaces (RIS) is a promising
technology because it can manipulate the electromagnetic waves in their
environment to overcome interference and enhance wireless communications. This
paper considers RIS enhanced D2D communications underlaying unmanned aerial
vehicle (UAV) networks with non-orthogonal multiple access (NOMA). The
objective is to maximize the sum rate of NOMA D2D communications by
simultaneously optimizing the power budget of D2D transmitter, NOMA power
allocation coefficients of D2D receivers and passive beamforming of RIS while
guaranteeing the quality of services of UAV user. Due to non-convexity, the
optimization problem is intractable and challenging to handle. Therefore, it is
solved in two parts using alternating optimization. Simulation results unviel
the performance of the proposed RIS enhanced D2D communications scheme. Results
demonstrate that the proposed scheme achieves 15\% and 27\% higher sum rates
compared to the fixed power D2D and orthogonal D2D schemes.
