In this paper, we investigate the near-field modelling and analyze the beam focusing pattern for modular extremely large-scale array (XL-array) communications. As modular XL-array is physically and electrically large in general, the accurate characterization of amplitude and phase variations across its array elements requires the non-uniform spherical wave (NUSW) model, which, however, is difficult for performance analysis and optimization. To address this issue, we first present two ways to simplify the NUSW model by exploiting the unique regular structure of modular XL-array, termed sub-array based uniform spherical wave (USW) models with different or common angles, respectively. Based on the developed models, the near-field beam focusing patterns of XL-array communications are derived. It is revealed that compared to the existing collocated XL-array with the same number of array elements, modular XL-array can significantly enhance the spatial resolution, but at the cost of generating undesired grating lobes. Fortunately, different from the conventional far-field uniform plane wave (UPW) model, the near-field USW model for modular XL-array exhibits a higher grating lobe suppression capability, thanks to the non-linear phase variations across the array elements. Finally, simulation results are provided to verify the near-field beam focusing pattern and grating lobe characteristics of modular XL-array.

翻译：本文研究了模块化超大规模阵列（XL-Array）通信中的近场建模与波束聚焦模式分析。由于模块化XL-Array在物理和电气尺度上通常较大，精确表征其阵元间的幅度和相位变化需要采用非均匀球面波（NUSW）模型，但该模型难以用于性能分析与优化。针对这一问题，首先利用模块化XL-Array独特的规则结构提出了两种简化NUSW模型的方法，分别称为基于子阵的具有不同或相同角度的均匀球面波（USW）模型。基于所建模型，推导了XL-Array通信系统的近场波束聚焦模式。研究表明，与具有相同阵元数量的现有共址XL-Array相比，模块化XL-Array能显著提升空间分辨率，但代价是会产生非期望的栅瓣。值得注意的是，与传统远场均匀平面波（UPW）模型不同，模块化XL-Array的近场USW模型因阵元间相位非线性变化而具有更强的栅瓣抑制能力。最后，通过仿真验证了模块化XL-Array的近场波束聚焦模式与栅瓣特性。