NOMA-Aided Cell-Free Massive MIMO with Underlay Spectrum-Sharing

We investigate the feasibility of employing non-orthogonal multiple-access (NOMA) in cell-free massive multiple-input multiple-output (MIMO) operating with underlay spectrum-sharing. In our proposed system model, multiple clusters of NOMA-enabled secondary users (SUs) are concurrently served by geographically distributed secondary access-points (S-APs) via conjugate beamforming. The uplink channels are estimated locally at each S-AP via pilots sent by SUs. A set of orthogonal pilots is shared among the secondary and primary clusters to strike a balance between the throughput and training overhead, while enabling massive connectivity in primary and secondary systems. We derive the achievable rates of the secondary system by capturing the adverse effects of inter/intra-cluster interference, primary/secondary pilot contamination, imperfect successive interference cancellation (SIC) and partial channel state information (CSI). We propose a transmit power allocation policy for the secondary system to mitigate the detrimental impact of near-far effects by virtue of max-min fairness criterion. Through an achievable rate analysis, we reveal that although the number of concurrently served users can be substantially boosted by employing the proposed system model, the achievable rates are adversely affected due to detection uncertainties with imperfect SIC and statistical CSI at NOMA-enabled SUs.