This paper designs a novel opportunistic adaptive non-orthogonal multiple access (OA-NOMA) strategy, where a base station (BS) employs NOMA to serve a near user (NU)-far user (FU) pair opportunistically scheduled from $M$ NUs and $K$ FUs. In particular, the NOMA transmission to the scheduled NU-FU pair adaptively operates in one of two modes: Direct NOMA mode, in which the BS directly serves the scheduled NU-FU pair with using NOMA; Cooperative NOMA mode, in which the scheduled NU receives the messages intended by both scheduled users from the BS, and then forwards the message intended by the scheduled FU. For the OA-NOMA strategy, a scheduling candidate acquisition method and a probabilistic user pair scheduling scheme are proposed to guarantee the transmission reliability and improve the scheduling fairness, respectively. To evaluate the scheduling fairness, we develop a max-min fairness criterion and show that the OA-NOMA strategy approximately achieves max-min fairness. The reliability of the OA-NOMA strategy is also evaluated in terms of outage probability and diversity order. For the outage probability, we derive an approximate expression and numerically verify its tightness. For the diversity order, we show that the proposed OA-NOMA strategy achieves a diversity order of $M$ .

中文翻译：

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多用户无线系统中的机会自适应非正交多址：概率用户调度和性能分析

本文设计了一种新颖的机会自适应非正交多址 (OA-NOMA) 策略，其中基站 (BS) 使用 NOMA 来服务于从 百万美元 NU 和 $K$ FU。特别是，NOMA 传输到调度的 NU-FU 对以两种模式之一自适应运行：直接 NOMA 模式, 其中 BS 使用 NOMA 直接为调度的 NU-FU 对提供服务； 合作NOMA模式，其中被调度的NU从BS接收两个被调度用户想要的消息，然后转发被调度的FU想要的消息。针对OA-NOMA策略，分别提出了调度候选获取方法和概率用户对调度方案，以保证传输可靠性和提高调度公平性。为了评估调度公平性，我们开发了一个最大-最小公平性标准，并表明 OA-NOMA 策略近似地实现了最大-最小公平性。OA-NOMA 策略的可靠性也根据中断概率和多样性顺序进行评估。对于中断概率，我们推导出一个近似表达式并数值验证其紧密度。对于多样性顺序，我们表明所提出的 OA-NOMA 策略实现了多样性顺序 百万美元 .