  In Massive MIMO base stations (BSs), the hardware design needs to balance
high spectral efficiency (SE) with low complexity. The level of hardware
impairments (HWIs) indicates how strong the signal distortion introduced by
hardware imperfections is. In particular, the analog-to-digital converters
(ADCs) have an important impact on signal distortion and power consumption.
This article addresses the fundamental problem of selecting the optimal
hardware quality in the Massive MIMO space. In particular, we examine the
optimal HWI and ADC bit allocation per BS antenna to maximize the SE. The
results show that in co-located arrays with low channel gain variations across
antennas, equal ADC bit allocation is optimal. In contrast, cell-free Massive
MIMO systems benefit the most from optimizing the ADC bit allocation achieving
improvements in the order of 2 [bit-per-channel-use] per user equipment when
using regularized zero-forcing (RZF). In addition, when including the impact of
power consumption in cell-free Massive MIMO with RZF, allocating low values of
mixed ADC bit resolutions across the BS antennas can increase the energy
efficiency up to 30% compared to equal ADC bit allocation.
