In this paper, we investigate multi-user modular extremely large-scale multiple-input multiple-output (XL-MIMO) communication systems, where modular extremely large-scale uniform linear array (XL-ULA) is deployed at the base station (BS) to serve multiple single-antenna users. By exploiting the unique modular array architecture and considering the potential near-field propagation, we develop sub-array based uniform spherical wave (USW) models for distinct versus common angles of arrival/departure (AoAs/AoDs) with respect to different sub-arrays/modules, respectively. Under such USW models, we analyze the beam focusing patterns at the near-field observation location by using near-field beamforming. The analysis reveals that compared to the conventional XL-MIMO with collocated antenna elements, modular XL-MIMO can provide better spatial resolution by benefiting from its larger array aperture. However, it also incurs undesired grating lobes due to the large inter-module separation. Moreover, it is found that for multi-user modular XL-MIMO communications, the achievable signal-to-interference-plus-noise ratio (SINR) for users may be degraded by the grating lobes of the beam focusing pattern. To address this issue, an efficient user grouping method is proposed for multi-user transmission scheduling, so that users located within the grating lobes of each other are not allocated to the same time-frequency resource block (RB) for their communications. Numerical results are presented to verify the effectiveness of the proposed user grouping method, as well as the superior performance of modular XL-MIMO over its collocated counterpart with densely distributed users.

翻译：本文研究了多用户模块化超大规模多输入多输出（XL-MIMO）通信系统，其中基站（BS）部署了模块化超大规模均匀线性阵列（XL-ULA）以服务多个单天线用户。通过利用独特的模块化阵列架构并考虑潜在的近场传播，我们针对不同子阵列/模块间到达/离开角（AoAs/AoDs）的异同，分别建立了基于子阵列的均匀球面波（USW）模型。在此USW模型下，我们利用近场波束赋形技术分析了近场观测位置处的波束聚焦特性。分析表明：相较于传统共址天线单元的XL-MIMO，模块化XL-MIMO凭借更大的阵列孔径能提供更优的空间分辨率，但过大的模块间间距也会导致不期望的栅瓣出现。进一步研究发现，在多用户模块化XL-MIMO通信中，波束聚焦模式的栅瓣可能导致用户可达信干噪比（SINR）性能下降。为解决该问题，本文提出一种高效的用户分组方法用于多用户传输调度，使得彼此位于对方栅瓣覆盖范围内的用户不会被分配至同一时频资源块（RB）进行通信。数值结果验证了所提用户分组方法的有效性，以及在用户密集分布场景下模块化XL-MIMO相较于共址架构的优越性能。