Receiver Maximum Eigenmode Beamforming-Based Null-Space Expansion for Multi-User Massive MIMO in Time-Varying Channel

This paper proposes a novel joint weight design scheme for pre/post-coding in multiuser massive multiple-input multiple-output (MU-mMIMO) downlink in time-varying channel environments. It combines Null-Space Expansion (NSE) and Maximum Eigenmode Beamforming (MEB), mainly focusing on user terminals (UTs) with multiple antenna elements. NSE steers additional nulls for interfering UTs using multiple past channel state information (CSI). Since NSE does not predict future channels, it performs excellent interference suppression in high-mobility environments for UTs. However, the limited antenna degrees of freedom (DoF) on the base station (BS) side make it challenging to form nulls effectively for interference suppression in NSE. Therefore, this paper utilizes maximum eigenmode beamforming (MEB). MEB utilizes singular value decomposition (SVD) to form a single stream corresponding to the maximum singular value on the UT side. MEB can supplement the beamforming gain reduction on BS side caused by NSE. Moreover, when a dominant line-of-sight (LoS) component exists in the channel, MEB also performs beamforming for this LoS component on each UT side. Since the LoS component is not highly sensitive to channel fluctuations, the nullification by NSE works more effectively. Computer simulations demonstrate that NSE incorporated with MEB can significantly enhance the Signal-to-Interference-plus-Noise Ratio (SINR) performance compared to conventional channel prediction-based precoding schemes. Its improvement is particularly notable in high-speed UT movement scenarios. This study presents MEB-NSE as an effective solution for stable multiuser spatial multiplexing in mobility environments.