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Intelligent User Clustering and Robust Beamforming Design for UAV-NOMA Downlink

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Document pages: 30 pages

Abstract: In this work, we consider a downlink NOMA network with multiplesingle-antenna users and multi-antenna UAVs. In particular, the users arespatially located in several clusters by following the Poisson Cluster Processand each cluster is served by a hovering UAV with NOMA. For practicalconsiderations, we assume that only imperfect CSI of each user is available atthe UAVs. Based on this model, the problem of joint user clustering and robustbeamforming design is formulated to minimize the sum transmission power, andmeanwhile, guarantee the QoS requirements of users. Due to the integervariables of user clustering, coupling effects of beamformers, and infinitelymany constraints caused by the imperfect CSI, the formulated problem ischallenging to solve. For computational complexity reduction, the originalproblem is divided into user clustering subproblem and robust beamformingdesign subproblem. By utilizing the users position information, we propose ak-means++ based unsupervised clustering algorithm to first deal with the userclustering problem. Then, we focus on the robust beamforming design problem. Toattain insights on solving the robust beamforming design problem, we firstlyinvestigate the problem with perfect CSI, and the associated problem is showncan be solved optimally. Secondly, for the problem in the general case withimperfect CSI, an SDR based method is proposed to produce a suboptimal solutionefficiently. Moreover, we provide a sufficient condition under which the SDRbased approach can guarantee to obtain an optimal rank-one solution, which istheoretically analyzed. Finally, an alternating direction method of multipliersbased algorithm is proposed to allow the UAVs to perform robust beamformingdesign in a decentralized fashion efficiently. Simulation results demonstratethe efficacy of the proposed algorithms and transmission scheme.

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