In this letter, we propose a two-step user-paring scheme for an orthogonal frequency-division multiplexing (OFDM)-based cooperative non-orthogonal multiple-access (NOMA) system where far users cannot communicate with the base station directly. The first step is for allocating near users, and the second step is for allocating far users. To evaluate the performance of the proposed scheme, we derive the closed-form expression for the upper bound of the outage probability, and then obtain the achievable diversity gain. Numerical results show that the proposed scheme can achieve the optimal diversity gain confirmed by exhaustive search.

中文翻译：

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基于 OFDM 的协作 NOMA 系统的两步用户配对

在这封信中，我们为基于正交频分复用 (OFDM) 的协作非正交多址 (NOMA) 系统提出了一种两步用户配对方案，其中远方用户无法直接与基站通信。第一步是分配近端用户，第二步是分配远端用户。为了评估所提出方案的性能，我们推导出中断概率上限的闭式表达式，然后获得可实现的分集增益。数值结果表明，所提出的方案可以实现穷举搜索确定的最优分集增益。