Underlaid Spectrum Sharing for Cell-Free Massive MIMO-NOMA

We investigate the achievable rate of a hybrid cell-free (CF) massive multiple-input multiple-output (MIMO) non-orthogonal multiple-access (NOMA) underlaid below a primary massive MIMO. We propose a low complexity sub-optimal user-clustering method and derive the closed-form sum rate expression for Rayleigh fading channels by considering the effects of intra-cluster pilot contamination, inter-cluster interference, imperfect successive interference cancellation (SIC) and statistical downlink channel state information (CSI) at secondary users (SUs). Our results reveal that NOMA based underlay CF massive MIMO-NOMA can exploit the scarce spectrum bands more efficiently than its counterpart orthogonal multiple-access (OMA).