Inter-user interference (IUI) mitigation has been an essential issue for multi-user multiple-input multiple-output (MU-MIMO) communications. The commonly used linear processing schemes include the maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean squared error (MMSE) beamforming, which may result in the unfavorable performance or complexity as the antenna number grows. In this paper, we introduce a low-complexity linear beamforming solution for the IUI mitigation by using the convolutional beamspace (CBS) technique. Specifically, the dimension of channel matrix can be significantly reduced via the CBS preprocessing, thanks to its beamspace and spatial filtering effects. However, existing methods of the spatial filter design mainly benefit from the Vandermonde structure of channel matrix, which only holds for the far-field scenario with the uniform plane wave (UPW) model. As the antenna size increases, this characteristic may vanish in the near-field region of the array, where the uniform spherical wave (USW) propagation becomes dominant. To gain useful insights, we first investigate the beamforming design and performance analysis of the CBS-based beamforming based on the UPW model. Our results unveil that the proposed CBS-based MMSE beamforming is able to achieve a near-optimal performance but demands remarkably lower complexity than classical ZF and MMSE schemes. Furthermore, our analysis is also extended to the near-field case. To this end, a novel optimization-based CBS approach is proposed for preserving spatial filtering effects, thus rendering the compatibility of the CBS-based beamforming. Finally, numerical results are provided to demonstrate the effectiveness of our proposed CBS-based beamforming method.

翻译：用户间干扰抑制一直是多用户多输入多输出通信中的关键问题。常用的线性处理方案包括最大比合并、迫零和最小均方误差波束成形，这些方案在天线数量增加时可能导致性能不佳或复杂度升高。本文通过采用卷积波束空间技术，提出了一种用于用户间干扰抑制的低复杂度线性波束成形方案。具体而言，凭借其波束空间特性和空间滤波效应，通过CBS预处理可显著降低信道矩阵的维度。然而，现有空间滤波器设计方法主要依赖于信道矩阵的范德蒙结构，该结构仅适用于采用均匀平面波模型的远场场景。随着天线尺寸增大，这一特性在阵列的近场区域可能消失，此时均匀球面波传播将占据主导地位。为获得有效见解，我们首先基于UPW模型研究了CBS波束成形的设计与性能分析。结果表明，所提出的基于CBS的MMSE波束成形能够实现接近最优的性能，且复杂度显著低于传统的ZF和MMSE方案。此外，我们的分析还扩展到近场场景。为此，提出了一种基于优化的新型CBS方法以保持空间滤波效应，从而使基于CBS的波束成形方案具备兼容性。最后，通过数值结果验证了所提CBS波束成形方法的有效性。