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Device-to-device (D2D) communications offers high spectral efficiency, lowenergy consumption and transmission latency. However, one of the mainlimitations of D2D communications is co-channel interference from underlayingwireless system. Reconfigurable intelligent surfaces (RIS) is a promisingtechnology because it can manipulate the electromagnetic waves in theirenvironment to overcome interference and enhance wireless communications. Thispaper considers RIS enhanced D2D communications underlaying unmanned aerialvehicle (UAV) networks with non-orthogonal multiple access (NOMA). Theobjective is to maximize the sum rate of NOMA D2D communications bysimultaneously optimizing the power budget of D2D transmitter, NOMA powerallocation coefficients of D2D receivers and passive beamforming of RIS whileguaranteeing the quality of services of UAV user. Due to non-convexity, theoptimization problem is intractable and challenging to handle. Therefore, it issolved in two parts using alternating optimization. Simulation results unvielthe performance of the proposed RIS enhanced D2D communications scheme. Resultsdemonstrate that the proposed scheme achieves 15\% and 27\% higher sum ratescompared to the fixed power D2D and orthogonal D2D schemes.

Reconfigurable Intelligent Surfaces Enhanced NOMA D2D Communications Underlaying UAV Networks