Distributed Cell-Free Massive MIMO Versus Cellular Massive MIMO Under UE Hardware Impairments

This paper first investigates and compares the uplink spectral efficiency (SE) of distributed cell-free massive multiple-input multiple-output (mMIMO) and cellular mMIMO networks, both with user equipment (UE) hardware impairments. We derive a lower bound on the uplink ergodic channel capacity of the cellular mMIMO with UE hardware impairments, based on which we determine the optimal receive combining that maximizes the instantaneous effective signal-to-interference-and-noise ratio. Then, a lower bound on the uplink capacity of a distributed cell-free mMIMO with UE hardware impairments is derived using the use-and-then-forget technique. On this basis, the optimum large-scale fading decoding vector is found using generalized Rayleigh entropy. By using three combining schemes of minimum mean-square error (MMSE), regularized zero-forcing (RZF), and maximum ratio, the uplink SEs of distributed cell-free mMIMO and cellular mMIMO networks are analyzed and compared. The results show that the two-layer decoding distributed cell-free mMIMO network with MMSE combining outperforms the cellular mMIMO network, and the advantage is more evident as the hardware impairment factor increases. Finally, the uplink energy efficiency (EE) of the distributed cell-free mMIMO networks is analyzed and evaluated through the established realistic power consumption model with hardware impairments. Simulation results show that two-layer decoding provides higher SE and EE than single-layer decoding. In addition, RZF achieves almost the same SE and EE as MMSE in a two-layer decoding architecture.