Article ID: 2021XBL0054
This letter proposes adaptive null-space expansion (NSE) for multiuser massive MIMO according to users’ mobility. NSE can improve the robustness of multi-user spatial multiplexing in a time-varying channel by expanding null-steering to multiple past channels. Its superiority was verified under the unified moving speed among users. In a realistic environment, users with different speeds coexist. Null-space dimension, which is the number of nullified channels per user terminal, should be optimized in accordance with users’ mobility so as to maximize the beamforming gain and nullification capability. In our proposal, we first show that user mobility can be classified by observing the singular value of time-varying channels. Computer simulation then clarifies that output SINR can be improved by assigning null space dimensions respective to user moving speeds.