Pairing and Power Allocation for Downlink Nonorthogonal Multiple Access Systems

In this paper, a greedy-search-based user pairing and selection method as well as an optimal power allocation approach is proposed to maximize the weighted sum rate of a downlink nonorthogonal multiple access (NOMA) system. The NOMA system consists of one base station and multiple users. By exploiting superposition coding and successive interference cancellation techniques at the receiver, multiple users can be multiplexed into transmit power domain, and then nonorthogonally scheduled for transmission on the same spectrum resources. The proposed greedy-search-based user pairing method has low computational complexity and the power allocation method is shown to be equivalent to a weighted minimum mean square error based problem, which is solved using an iterative procedure. The subproblem in each step of the iteration has a closed-form solution. Also, the convergence of the iteration is proved. Simulation results show that the proposed method can significantly improve the sum rate of the system over the other methods.