Hybrid Beamforming and Adaptive RF Chain Activation for Uplink Cell-Free Millimeter-Wave Massive MIMO Systems
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In this work, we investigate hybrid analog–digital beamforming (HBF) architectures for uplink cell-free (CF) millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) systems. We first propose two HBF schemes, namely, centralized HBF (C-HBF) and semi-centralized HBF (SC-HBF) with the assumptions of a capacity-achieving advanced receiver for digital processing and perfect channel state information (CSI) at the access points (APs). For analog processing, in C-HBF, the analog beamforming matrix is generated at the central processing unit (CPU) based on the CSI received from all APs, whereas that in SC-HBF is generated independently at each AP based on the local CSI. We show that the SC-HBF scheme can achieve approximately the same total achievable rate as that given by the C-HBF scheme with lower computational complexity and no global CSI. 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 connected analog-to-digital converters (ADCs) and phase shifters at the APs based on the CSI. For the activation of RF chains, four low-complexity algorithms are proposed, which can achieve up to 236% improvement in energy efficiency (EE) with only a marginal loss in the total achievable rate.
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