In future B5G/6G broadband communication systems, non-linear signal distortion caused by the impairment of transmit power amplifier (PA) can severely degrade the communication performance, especially when uplink users share the wireless medium using non-orthogonal multiple access (NOMA) schemes. This is because the successive interference cancellation (SIC) decoding technique, used in NOMA, is incapable of eliminating the interference caused by PA distortion. Consequently, each user's decoding process suffers from the cumulative distortion noise of all uplink users. In this paper, we establish a new and tractable PA distortion signal model based on real-world measurements, where the distortion noise power is a polynomial function of PA transmit power diverging from the oversimplified linear function commonly employed in existing studies. Applying the proposed signal model, we characterize the capacity rate region of multi-user uplink NOMA by optimizing the user transmit power. Our findings reveal a significant contraction in the capacity region of NOMA, attributable to polynomial distortion noise power. For practical engineering applications, we formulate a general weighted sum rate maximization (WSRMax) problem under individual user rate constraints. We further propose an efficient power control algorithm to attain the optimal performance. Numerical results show that the optimal power control policy under the proposed non-linear PA model achieves on average 13\% higher throughput compared to the policies assuming an ideal linear PA model. Overall, our findings demonstrate the importance of accurate PA distortion modeling to the performance of NOMA and provide efficient optimal power control method accordingly.

翻译：在未来的B5G/6G宽带通信系统中，发射功率放大器（PA）损伤引起的非线性信号失真会严重恶化通信性能，尤其是当上行用户采用非正交多址（NOMA）方案共享无线媒介时。这是由于NOMA中使用的串行干扰消除（SIC）解码技术无法消除PA失真引起的干扰。因此，每个用户的解码过程都会受到所有上行用户累积失真噪声的影响。本文基于实际测量建立了新的易处理的PA失真信号模型，其中失真噪声功率是PA发射功率的多项式函数，这与现有研究中普遍采用的过度简化的线性函数不同。应用所提出的信号模型，我们通过优化用户发射功率刻画了多用户上行NOMA的容量速率区域。研究结果揭示了由于多项式失真噪声功率，NOMA的容量区域会出现显著收缩。面向实际工程应用，我们在个体用户速率约束下构建了通用加权和速率最大化（WSRMax）问题，并进一步提出了一种高效功率控制算法以达到最优性能。数值结果表明，与假设理想线性PA模型的策略相比，所提出的非线性PA模型下的最优功率控制策略平均可提升13%的吞吐量。总体而言，我们的研究结果证明了精确的PA失真建模对NOMA性能的重要性，并据此提供了高效的最优功率控制方法。