Optimal and Sub-Optimal Uplink NOMA: Joint User Grouping, Decoding Order, and Power Control

In this letter we explore the user grouping, decoding order and power control in an uplink non-orthogonal multiple access (NOMA) system. We formulate a joint optimization problem to maximize the achievable sum rate (ASR) of the multiple users, where each user has a minimum rate requirement. The problem is a combinatorial integer programming problem, due to the discreteness and coupling of the decoding order and user grouping. We then first derive the closed-form optimal solution of the decoding order and power control. By exhaustively searching the user grouping variables, we can obtain the globally optimal solution of the formulated problem. To achieve the tradeoff between the computational complexity and performance, we propose a sub-optimal user grouping algorithm with a linear complexity. Simulation results show that the proposed sub-optimal solution can achieve a near-optimal performance in terms of ASR.