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（ELAA）技术，包括超大规模多输入多输出（UM-MIMO）或可重构智能表面（RISs），正逐渐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每秒太比特级极端数据速率的需求。对于工作在太赫兹（THz）频段的ELAA，瑞利距离扩展，用户可能同时位于远场（FF）和近场（NF）区域。一方面，需要表征近场传播和空间非平稳性等新特性；另一方面，远场与近场边界附近的特性转变值得探索。本文基于采用最多4096个阵元的虚拟均匀平面阵列（UPA）结构的多输入单输出信道测量，利用太赫兹虚拟天线阵列对远近场信道特性进行了完整的实验分析。具体而言，在近场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。随后，提出了一种新型跨场路径损耗模型，该模型可表征UPA中天线阵元的功率变化，且同时适用于远场和近场场景。