Impact of Sub-Array Configuration on Spectral Efficiency of Cell-Free Massive MIMO with Hybrid Beamforming in FR3

Cell-free massive multi-input multi-output (CF mMIMO) is one of the candidate elemental technologies for the sixth generation (6G) mobile communication system to provide excellent wireless quality every-where in a service area. In addition, utilization of frequency range (FR) 3, corresponding to around 7 to 24 GHz, has been expected as a candidate new spectrum in 6G. In the such FR, a hybrid beamforming (BF) architecture, which combines digital and analog BF, has been employed considering the power consumption and implementation cost of base stations. In this paper, we reveal the impact of the antenna configuration in sub-array architecture on the spectral efficiency of CF mMIMO deploying access points (APs) with hybrid BF functionality in the FR3 band through computer simulations. Concretely, we assume that the central processing unit performs digital BF, and each AP, which is equipped with multiple radio frequency chains and array antennas, utilizes analog BF using a connected sub-array, and clarify a suitable sub-array configuration in CF mMIMO deployed mainly towards terrestrial UEs through comparative study with simulations.