NOMA User Pairing and UAV Placement in UAV-Based Wireless Networks

In this paper, we investigate the integration of the Non-Orthogonal Multiple Access (NOMA) technology into the Unmanned Aerial Vehicle (UAV)-based wireless system. In particular, we study the joint NOMA power allocation, user pairing, and UAV deployment (placement) for this wireless system. To gain insight into the optimal structure of this problem, we derive the optimal power allocation and UAV placement to maximize the sum-rate of the two-user (one NOMA pair) network. We then address the general setting with multiple NOMA pairs where users must he paired into two-user groups using NOMA. For this setting, we optimize the user pairing, power allocation, and UAV placement to maximize the minimum sum rate for individual user pairs. Solving this optimization problem optimally requires exhaustive search over all possible pairing scenarios, which has very high complexity. To overcome this challenge, we propose a heuristic pairing algorithm based on the minimum sum-of-squared-distance criteria whose pairing result is then applied to perform optimal power allocation and UAV placement. Through numerical studies, we show the significance of UAV placement optimization and the fact that the proposed heuristic user pairing scheme achieves close-to-optimal performance.