This paper proposes a dimensionality reduction of interference subspace to improve the interference suppression capability of null-space expansion (NSE) for multiuser massive multiple-input-multiple-output (MIMO) systems. The fundamental concept of the NSE is to nullify the interference subspace generated by the combined multiple past channel state information (CSI) vectors. It can suppress inter-user interference (IUI) leaked due to outdated CSI in a mobility environment. It is enabled by plentiful degrees of freedom (DoF) brought by massive antenna elements the base station has, and hence NSE can outperform existing channel prediction based precoding approaches. However, additive noise-induced CSI estimation error limits its effectiveness; expanded dimension of null-space cannot be appropriately spanned to interference subspace. The key idea of our new proposal is to extract principal interference subspace channel. Null-space can be appropriately expanded with reduced its dimension, and remaining DoF can be assigned to beamforming. It results in improving overall signal-to-interference-plus-noise power ratio (SINR) performance which is verified via numerical simulation.

为了提高多用户大规模多输入多输出（MIMO）系统的零空间扩展（NSE）的干扰抑制能力，本文提出了干扰子空间的降维方法。NSE的基本概念是使由组合的多个过去信道状态信息（CSI）向量生成的干扰子空间无效。它可以抑制由于移动环境中的CSI过期而导致的用户间干扰（IUI）泄漏。基站拥有大量天线元件带来的大量自由度（DoF）可以实现此功能，因此NSE可以胜过现有的基于信道预测的预编码方法。但是，加性噪声引起的CSI估计误差限制了其有效性。空空间的扩展维不能适当地扩展到干扰子空间。我们新提案的关键思想是提取主要干扰子空间信道。零空间可以通过减小其尺寸适当地扩展，并且剩余的DoF可以分配给波束成形。这可以改善整体信噪比和噪声功率比（SINR）性能，这已通过数值仿真得到了验证。