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In this paper, uplink transmission of a cell-free massive multiple-inputmultiple-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 stateinformation. Next, in order to reduce pilot contamination, greedy and graphcoloring-based pilot allocation algorithms are proposed and analyzed for theconsidered scenario. Furthermore, to control interference and improve theperformance, two power control strategies are designed and their complexity andconvergence are also discussed. The first power control strategy aims atmaximizing CFUEs' sum spectral efficiency (SE) subject to quality of serviceconstraints on DUEs, while the second one maximizes the weighted product ofCFUEs' and DUEs' signal-to-interference-plus-noise-ratios (SINRs). Numericalresults show that the proposed pilot and power allocations bring a considerableimprovement to the network SE. Also, it is revealed that the activation of D2Dlinks has a positive effect on the system throughput, i.e. the networkoffloading ensured by the D2D links overcomes the increased interferencebrought by D2D communications.

Coexistence of D2D Communications and Cell-Free Massive MIMO Systems With Low Resolution ADC for Improved Throughput in Beyond-5G Networks