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

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, decentralized (D-HBF) and semi-centralized (SC-HBF). the former, both digital analog beamformers are generated independently at each AP based on local channel state information (CSI). contrast, in latter, only beamformer is obtained locally access point (AP), whereas matrix central processing unit (CPU) global CSI received from all APs. show that these schemes provide approximately same achievable rates despite lower complexity of D-HBF its lack requirement. Furthermore, to reduce power consumption, a novel adaptive radio frequency (RF) chain-activation (ARFA) scheme, which dynamically activates/deactivates RF chains their connected analog-to-digital converters (ADCs) phase shifters (PSs) APs CSI. For activation chains, low-complexity algorithms proposed, can achieve significant improvement energy efficiency (EE) with marginal loss total rate.