Joint Position, Decoding Order, and Power Allocation Optimization in UAV-Based NOMA Downlink Communications

Unmanned aerial vehicles (UAVs) make mobile base stations possible in future wireless communication systems. In this article, we propose to employ a UAV-enabled base station to serve multiple users in a nonorthogonal multiple access (NOMA) network. We consider two practical goals. The first one is to minimize the transmit power of the UAV subject to the minimum achievable rate requirements, and the second one is to maximize the achievable rate of a specific user subject to the minimum achievable rate requirements of other users. Because of the NOMA scheme, our optimizing variables consist of the decoding order, the transmit power allocation, and the position of the UAV. For both problems, given a decoding order, we propose globally optimal solutions to the joint optimization problems of the power allocation and the position of the UAV. By searching all possible different decoding orders, we obtain globally optimal solutions to both problems. Numerical results show that our proposed joint optimization algorithms perform better than the orthogonal multiple access scheme and the NOMA scheme with the fixed UAV position.