Extremely large-scale antenna array (ELAA) technologies consisting of ultra-massive multiple-input-multiple-output (UM-MIMO) or reconfigurable intelligent surfaces (RISs), are emerging to meet the demand of wireless systems in sixth-generation and beyond communications for enhanced coverage and extreme data rates up to Terabits per second. For ELAA operating at Terahertz (THz) frequencies, the Rayleigh distance expands, and users are likely to be located in both far-field (FF) and near-field (NF) regions. On one hand, new features like NF propagation and spatial non-stationarity need to be characterized. On the other hand, the transition of properties near the FF and NF boundary is worth exploring. In this paper, a complete experimental analysis of far- and near-field channel characteristics using a THz virtual antenna array is provided based on measurement of the multi-input-single-output channel with the virtual uniform planar array (UPA) structure of at most 4096 elements. In particular, non-linear phase change is observed in the NF, and the Rayleigh criterion regarding the maximum phase error is verified. Then, a new cross-field path loss model is proposed, which characterizes the power change at antenna elements in the UPA and is compatible with both FF and NF cases.

翻译：超大规模天线阵列技术（如超大规模多输入多输出系统或可重构智能表面）正逐步兴起，以满足第六代及未来通信无线系统对增强覆盖和高达太比特/秒的极速数据速率的需求。对于工作在太赫兹频段的超大规模天线阵列，瑞利距离随之扩展，用户可能同时位于远场与近场区域。一方面，需对近场传播和空间非平稳性等新特性进行表征；另一方面，远场与近场边界附近的特性过渡也值得探索。本文基于最多4096个阵元的虚拟均匀平面阵列结构的多输入单输出信道测量，提供了利用太赫兹虚拟天线阵列对远场与近场信道特性的完整实验分析。具体而言，在近场中观测到非线性相位变化，并验证了关于最大相位误差的瑞利准则。进而提出一种新的跨场路径损耗模型，该模型表征了均匀平面阵列中天线阵元的功率变化，并兼容远场与近场场景。