  Cell-free massive multiple-input multiple-output (MIMO), with a large number
of distributed access points (APs) that jointly serve the user equipments
(UEs), is a promising network architecture for future wireless communications.
To reduce the cost and power consumption of such systems, it is important to
utilize low-quality transceiver hardware at the APs. However, the impact of
hardware impairments on cell-free massive MIMO has thus far not been studied.
In this paper, we take a first look at this important topic by utilizing
well-established models of hardware distortion and deriving new closed-form
expressions for the spectral and energy efficiency. These expressions provide
important insights into the practical impact of hardware impairments and also
how to efficiently deploy cell-free systems. Furthermore, a novel
hardware-quality scaling law is presented. It proves that the impact of
hardware impairments at the APs vanish as the number of APs grows. Numerical
results validate that cell-free massive MIMO systems are inherently resilient
to hardware impairments.
