Cell-Free Massive MIMO with Underlay Spectrum-Sharing

In this paper, the coexistence of cell-free massive multiple-input multiple-output (MIMO) and underlay spectrum-sharing is investigated. Thereby, the fundamental performance limits are established to characterize the feasibility of this coexistence. A set of spatially-distributed secondary access points (S-APs), which are underlaid in a primary cell-free massive MIMO system, serves many secondary users (SUs) in the same licensed spectrum of the primary access points (P-APs). Stringent secondary transmit power constraints are defined for the S-APs to mitigate undesired secondary co-channel interference (CCI) at the primary users (PUs). The uplink channels are estimated locally at the P-APs and S-APs via a generalized pilot sharing scheme, and thereby, conjugate precoders are used to serve PUs/SUs simultaneously. The achievable rates for both primary and secondary systems are derived for imperfectly estimated channels at the P-APs/S-APs, and the impact of intra-system pilot contamination is investigated. User-fairness for SUs is guaranteed by designing an efficient transmit power control policy based on the max-min criterion and secondary transmit power constraints. Through a rigorous analysis, we reveal that massive distributed primary/secondary transmissions can be exploited to mitigate detrimental impact of secondary CCI on PUs, and thereby, the achievable rates of primary/secondary systems can be boosted.