Two-Step User Selection Algorithm for Multi-User Massive MIMO with Hybrid Beamforming

The fifth-generation (5G) mobile communication system will introduce the use of high frequency bands with wide bandwidth and Massive MIMO technology for realizing higher bit rate than 20 Gbps. One of practical ways to realize Massive MIMO in high frequency bands is hybrid beamforming (BF) that combines analog BF with digital precoding. In multi-user Massive MIMO system employing hybrid BF, user selection is an important function because its performance is highly affected by inter-user interference. However, the computational complexity of the user selection in multi-user Massive MIMO is high because a size of NUM() channel matrix is also large. 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 user-fair and low complexity user selection algorithm for high SHF wideband multi-user Massive MIMO employing hybrid BF, this paper proposes a two-step user selection algorithm which is a combination of PF based user selection and chordal distance user selection. Results of computer simulations show that the proposed two-step user selection algorithm can achieve higher system performance than the conventional user selection algorithms do with higher user fairness and lower computational complexity.