Exploiting Underlay Spectrum Sharing in Cell-Free Massive MIMO Systems

We investigate the coexistence of underlay spectrum sharing in cell-free massive multiple-input multiple-output (MIMO) systems. A primary system with geographically distributed access points (P-APs) serves a multitude users (PUs), while secondary large number (SUs) same primary/licensed by exploiting sharing. To mitigate co-channel interference inflected at PUs, stringent transmit power constraints are defined for (S-APs). generalized pilots scheme is used to locally estimate uplink channels P-APs/S-APs, and thereby, conjugate precoders adopted serve PUs/SUs time-frequency resource element. Moreover, effect user-centric AP clustering investigated assigning suitable set APs particular user. The impact estimated downlink (DL) via DL beamformed P-APs/S-APs investigated. achievable primary/secondary rates derived statistical CSI cases. User-fairness achieved designing efficient control policies based on multi-objective optimization problem formulation joint max-min criteria. proposed orthogonal multiple-access analytical framework also extended facilitate non-orthogonal multiple-access. Our analysis numerical results manifest that performance can be boosted virtue average reduction powers/path-losses, uniform coverage/service, macro-diversity gains, which inherent transmissions/receptions MIMO.