Impact of UE Hardware Impairments on Uplink Spectral Efficiency of Cell-Free Massive MIMO Network

In this paper, the Cell-Free (CF) massive MIMO (mMIMO) network with user equipment (UE) hardware impairments in spatially correlated channels is investigated. By using the established generic UE hardware impairment model, a minimum mean-squared error (MMSE) channel estimator is derived. Then, a lower bound on the uplink ergodic capacity is derived for CF mMIMO network with UE hardware impairments, and an optimal receive combining vector is also derived for obtaining maximum instantaneous signal-to-interference-and-noise ratio (SINR). Considering the computational complexity of the MMSE combining, the regularized zero-forcing (RZF) and maximum ratio (MR) combining schemes are presented as alternatives. Our results show that the RZF combining scheme has little loss in sum spectral efficiency (SE) compared to MMSE combining under different hardware impairments. Based on the use-and-then-forget (UatF) bound, a new closed-form uplink SE expression with MMSE estimator is derived for MR combining. Finally, we evaluate the tightness of the capacity bounds under different hardware impairments.