Non-orthogonal multiple access (NOMA) is a potential candidate to further enhance the spectrum utilization efficiency in beyond fifth-generation (B5G) standards. However, there has been little attention on the quantification of the delay-limited performance of downlink NOMA systems. In this paper, we analyze the performance of a two-user downlink NOMA system over generalized {\alpha}-{\mu} fading in terms of delay violation probability (DVP) and effective rate (ER). In particular, we derive an analytical expression for an upper bound on the DVP and we derive the exact sum ER of the downlink NOMA system. We also derive analytical expressions for high and low signal-to-noise ratio (SNR) approximations to the sum ER, as well as a fundamental upper bound on the sum ER which represents the ergodic sum-rate for the downlink NOMA system. We also analyze the sum ER of a corresponding time-division-multiplexed orthogonal multiple access (OMA) system. Our results show that while NOMA consistently outperforms OMA over the practical SNR range, the relative gain becomes smaller in more severe fading conditions, and is also smaller in the presence a more strict delay quality-of-service (QoS) constraint.

中文翻译：

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$\alpha$-$\mu$衰落信道上下行NOMA的延迟违反概率和有效率

非正交多址 (NOMA) 是进一步提高超五代 (B5G) 标准中频谱利用效率的潜在候选者。然而，很少有人关注下行链路 NOMA 系统的延迟受限性能的量化。在本文中，我们从延迟违反概率 (DVP) 和有效率 (ER) 方面分析了两用户下行链路 NOMA 系统在广义 {\alpha}-{\mu} 衰落上的性能。特别是，我们推导出 DVP 上限的解析表达式，并推导出下行链路 NOMA 系统的精确总和 ER。我们还推导出高和低信噪比 (SNR) 近似值的解析表达式到总和 ER，以及表示下行链路 NOMA 系统的遍历总速率的总和 ER 的基本上限。我们还分析了相应的时分复用正交多址 (OMA) 系统的总和 ER。我们的结果表明，虽然 NOMA 在实际 SNR 范围内始终优于 OMA，但相对增益在更严重的衰落条件下变得更小，并且在存在更严格的延迟服务质量 (QoS) 约束时也更小。