Two-Step User Selection Algorithm in Multi-User Massive MIMO with Hybrid Beamforming for 5G Evolution

In order to realize the higher bit rates compared for the fifth-generation (5G) mobile communication system, massive MIMO technologies in higher frequency bands with wider bandwidth are being investigated for 5G evolution and 6G. One of practical method to realize massive MIMO in the high frequency bands is hybrid beamforming (BF). With this approach, user selection is an important function because its performance is highly affected by inter-user interference. However, the computational complexity of user selection in multi-user massive MIMO is high because MIMO channel matrix size excessive. Furthermore, satisfying user fairness by proportional fairness (PF) criteria leads to further increase of the complexity because re-calculation of precoding and postcoding matrices is required for each combination of selected users. To realize a fair and low-complexity user selection algorithm for multi-user massive MIMO employing hybrid BF, this paper proposes a two-step user selection algorithm that combines PF based user selection and chordal distance user selection. Computer simulations show that the proposed two-step user selection algorithm with higher user fairness and lower computational complexity can achieve higher system performance than the conventional user selection algorithms.