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In this work, we investigate hybrid analog-digital beamforming (HBF)architectures for uplink cell-free (CF) millimeter-wave (mmWave) massivemultiple-input multiple-output (MIMO) systems. {We first propose two HBFschemes, namely, decentralized HBF (D-HBF) and semi-centralized HBF (SC-HBF).In the former, both the digital and analog beamformers are generatedindependently at each AP based on the local channel state information (CSI). Incontrast, in the latter, only the digital beamformer is obtained locally at theAP, whereas the analog beamforming matrix is generated at the centralprocessing unit (CPU) based on the global CSI received from all APs. We showthat the analog beamformers generated in these two HBF schemes provideapproximately the same achievable rates despite the lower complexity of D-HBFand its lack of CSI requirement.} Furthermore, to reduce the power consumption,we propose a novel adaptive radio frequency (RF) chain-activation (ARFA)scheme, which dynamically activates/deactivates RF chains and their connectedanalog-to-digital converters (ADCs) and phase shifters (PSs) at the APs basedon the CSI. For the activation of RF chains, low-complexity algorithms areproposed, which can achieve significant improvement in energy efficiency (EE)with only a marginal loss in the total achievable rate.

Hybrid Beamforming and Adaptive RF Chain Activation for Uplink Cell-Free Millimeter-Wave Massive MIMO Systems