Optimal User Grouping for Downlink NOMA

In this letter, we study the user grouping for the downlink nonorthogonal multiple access system. The optimal user grouping problem is formulated to maximize the sum rate. Due to the combinatorial nature, the computational complexity to find the optimal solution is generally very high. To address the complexity issue, we propose faster optimal and suboptimal schemes for the user grouping problem. We then obtain the useful and important insight of the optimal user grouping. Numerical results demonstrate that the proposed schemes significantly outperform the existing schemes and the proposed suboptimal algorithm works very close to the optimal algorithm.