Sixth generation (6G) wireless networks are envisioned to include aspects of energy footprint reduction (sustainability), besides those of network capacity and connectivity, at the design stage. This paradigm change requires radically new physical layer technologies. Notably, the integration of large-aperture arrays and the transmission over high frequency bands, such as the sub-terahertz spectrum, are two promising options. In many communication scenarios of practical interest, the use of large antenna arrays in the sub-terahertz frequency range often results in short-range transmission distances that are characterized by line-of-sight channels, in which pairs of transmitters and receivers are located in the (radiating) near field of one another. These features make the traditional designs, based on the far-field approximation, for multiple-input multiple-output (MIMO) systems sub-optimal in terms of spatial multiplexing gains. To overcome these limitations, new designs for MIMO systems are required, which account for the spherical wavefront that characterizes the electromagnetic waves in the near field, in order to ensure the highest spatial multiplexing gain without increasing the power expenditure. In this paper, we introduce an analytical framework for optimizing the deployment of antenna arrays in line-of-sight channels, which can be applied to paraxial and non-paraxial network deployments. In the paraxial setting, we devise a simpler analytical framework, which, compared to those available in the literature, provides explicit information about the impact of key design parameters. In the non-paraxial setting, we introduce a novel analytical framework that allows us to identify a set of sufficient conditions to be fulfilled for achieving the highest spatial multiplexing gain. The proposed designs are validated with numerical simulations.

翻译：第六代（6G）无线网络的设计愿景，除网络容量与连接性外，更需在规划阶段纳入降低能耗足迹（可持续性）的考量。这一范式转变要求根本性的新型物理层技术。其中，大孔径阵列的集成与高频段（如亚太赫兹频谱）传输是两种极具前景的技术路径。在许多实际通信场景中，亚太赫兹频段大天线阵列的应用常导致短距离传输，其特征表现为视距信道，且收发对彼此处于（辐射）近场区域内。这些特性使得基于远场近似的传统多输入多输出（MIMO）系统设计在空间复用增益方面呈现次优性。为突破此局限，需构建新的MIMO系统设计方案，该方案须考虑近场电磁波特有的球面波前特性，从而在不增加功耗的前提下确保最高的空间复用增益。本文提出了一种用于优化视距信道中天线阵列部署的解析框架，可适用于傍轴与非傍轴网络部署场景。在傍轴配置下，我们构建了更简明的解析框架，与现有文献相比，该框架能明确揭示关键设计参数的影响机制。在非傍轴配置下，我们提出了一种新颖的解析框架，通过该框架可识别实现最高空间复用增益所需满足的一组充分条件。数值仿真验证了所提设计方案的有效性。