UAV-Enabled Hardware-Impaired Cell-free Massive MIMO With Spatially-Correlated Rician Fading

We consider an unmanned aerial vehicle (UAV) enabled cell-free (CF) massive multi-input multi-output (mMIMO) system, where multi-antenna access points (APs) assist various single-antenna UAVs and ground users (GUEs). Unlike, existing CF mMIMO works, we assume non-ideal transceiver hardware at the UAVs, GUEs and APs, and derive a closed-form downlink spectral efficiency (SE) expression by considering spatially-correlated Rician channels and channel estimation errors. We also propose a novel block quadratic transformation technique to optimize the system global energy-efficiency (GEE) by incorporating practical backhaul power, circuit and UAV propulsion power. We numerically show that severely hardware-impaired UAVs will have a higher SE when they operate at a larger height.