Non-orthogonal multiple access (NOMA) has been integrated with beamspace multi-input multi-output (MIMO) to enhance system spectrum efficiency (SE) by serving more than one user per dedicated radio frequency (RF) chain. However, balancing the system performance with significant RF reduction is still challenging. In this study, the authors propose beamspace MIMO-NOMA system based on signal alignment (SA) concept, where they will be able to tackle the issue of hardware complexity with a significant performance. Spectrum and energy efficiencies are analysed through two main stages. First, the excess degrees-of-freedom of beamspace MIMO-NOMA are exploited by SA to suppress the inter-beam/pair interference through designing user detection vectors and precoding matrix. Accordingly, a significant RF-chains reduction is provided. Second, power allocation coefficients are evaluated upon the optimal SE in order to mitigate intra-beam/pair interference. Moreover, the authors derive a tight closed formula for optimal SE based on Karush-Kuhn-Tucker analysis, which is validated by a heuristic-based optimal solution. Simulation results show a superior performance gain over orthogonal multiple access technique. Furthermore, the Monte-Carlo numerical solution depicts the marginal performance gap between the analytical and heuristic-based optimisation approach. This confirms the accuracy and rigidity of the proposed system.

中文翻译：

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基于信号对准的波束空间毫米波MIMO-NOMA系统的频谱效率分析

非正交多路访问（NOMA）已与波束空间多输入多输出（MIMO）集成在一起，通过为每个专用射频（RF）链服务多个用户来增强系统频谱效率（SE）。但是，在系统性能与大幅降低RF之间取得平衡仍然是一项挑战。在这项研究中，作者提出了基于信号对准（SA）概念的波束空间MIMO-NOMA系统，他们将能够以显着的性能解决硬件复杂性问题。通过两个主要阶段来分析频谱和能效。首先，SA利用波束空间MIMO-NOMA的多余自由度，通过设计用户检测向量和预编码矩阵来抑制波束间/对干扰。因此，提供了显着的RF链减少。第二，根据最佳SE评估功率分配系数，以减轻光束内/对干扰。此外，作者基于Karush-Kuhn-Tucker分析得出了最佳SE的严格封闭公式，并通过基于启发式的最佳解决方案进行了验证。仿真结果表明，该性能优于正交多址技术。此外，蒙特卡洛数值解描述了分析性和基于启发式的优化方法之间的边际性能差距。这证实了所提出系统的准确性和刚性。仿真结果表明，该性能优于正交多址技术。此外，蒙特卡洛数值解描述了分析性和基于启发式的优化方法之间的边际性能差距。这证实了所提出系统的准确性和刚性。仿真结果表明，该性能优于正交多址技术。此外，蒙特卡洛数值解描述了分析性和基于启发式的优化方法之间的边际性能差距。这证实了所提出系统的准确性和刚性。