  Standard cell-free (CF) multiple-input-multiple-output (mMIMO) systems is a
promising technology to cover the demands for higher data rates in
fifth-generation (5G) networks and beyond. These systems assume a large number
of distributed access points (APs) using joint coherent transmission to
communicate with the users. However, CF mMIMO systems present an increasing
computational complexity as the number of users increases. Scalable cell-free
CF (SCF) systems have been proposed to face this challenge. Given that the
cost-efficient deployment of such large networks requires low-cost
transceivers, which are prone to unavoidable hardware imperfections, realistic
evaluations of SCF mMIMO systems should take them into account before
implementation. Hence, in this work, we focus on the impact of hardware
impairments (HWIs) on the SCF mMIMO systems through a general model accounting
for both additive and multiplicative impairments. Notably, there is no other
work in the literature studying the impact of phase noise (PN) in the local
oscillators (LOs) of CF mMIMO systems or in general the impact of any HWIs in
SCF mMIMO systems. In particular, we derive upper and lower bounds on the
uplink capacity accounting for HWIs. Moreover, we obtain the optimal
hardware-aware (HA) partial minimum mean-squared error (PMMSE) combiner.
Especially, the lower bound is derived in closed-form using the theory of
deterministic equivalents (DEs). Among the interesting findings, we observe
that separate LOs (SLOs) outperform a common LO (CLO), and the additive
transmit distortion degrades more the performance than the additive receive
distortion.
