RF chain-wise Clustering for Centralized mm-wave Cell-Free massive MIMO with Hybrid Beamforming.

This paper proposes a clustering scheme suitable for a centralized millimeter wave Cell-Free massive MIMO with hybrid beamforming (BF). Conventionally AP-wise clustering schemes have been proposed for a decentralized architecture, that suppresses multi-user interference at each access point (AP) with local digital precoding and analog BF of each radio frequency (RF) chain, in order to obtain better performance efficiently. However in the centralized architecture, designing the precoder and cluster at central processing unit (CPU) with considering all RF chains of all APs together, the AP-wise clustering could not fit well especially when a large number of UEs are multiplexed. Because the RF chains with high coupling loss (the sum of the path loss and BF gain of the analog beam) could be included in the APs of the cluster to each UE. The proposed method forms a cluster on a per-RF-chain basis, and can select RF chains with lower coupling loss for each UE at the CPU regardless of the AP by incorporating the analog BF gain of each RF chain to form a cluster for each UE. Through simulation evaluations, we show that the hybrid BF with the proposed clustering scheme can obtain the superior spectral efficiency while effectively reducing the complexity of the centralized digital precoding.