  In this paper, uplink transmission of a cell-free massive multiple-input
multiple-output (CF-mMIMO) system coexisting with device-to-device (D2D)
communication links is investigated, under the assumption that access points
(APs) are equipped with low-resolution analog-to-digital converters (ADCs).
Lower bounds of achievable rates for both D2D users (DUEs) and CF-mMIMO users
(CFUEs) are derived in closed-form, with perfect and imperfect channel state
information. Next, in order to reduce pilot contamination, greedy and graph
coloring-based pilot allocation algorithms are proposed and analyzed for the
considered scenario. Furthermore, to control interference and improve the
performance, two power control strategies are designed and their complexity and
convergence are also discussed. The first power control strategy aims at
maximizing CFUEs' sum spectral efficiency (SE) subject to quality of service
constraints on DUEs, while the second one maximizes the weighted product of
CFUEs' and DUEs' signal-to-interference-plus-noise-ratios (SINRs). Numerical
results show that the proposed pilot and power allocations bring a considerable
improvement to the network SE. Also, it is revealed that the activation of D2D
links has a positive effect on the system throughput, i.e. the network
offloading ensured by the D2D links overcomes the increased interference
brought by D2D communications.
