Harnessing Mutual Coupling in Cell-Free mMIMO
This letter investigates a cell-free massive MIMO system with $L$ access points (APs), each employing a small fixed-form-factor, uniform linear array of $N$ potentially closely spaced antennas. Utilizing multiport communication theory, we analyze the impact of mutual coupling on uplink and downlink spectral efficiency (SE) under various impedance-matching strategies. We compare two deployment regimes: ($i$) increasing $N$ while $L$ is kept fixed, and ($ii$) increasing $N$ while fixing the total antenna count $M = NL$. Numerical results show that, with a full matching network, mutual coupling itself becomes beneficial when an AP serves several user equipments, which is the typical operating regime of user-centric cell-free networks. Furthermore, with full matching, antenna densification can enhance SE or allow for a reduction in the number of APs without performance loss. Since full matching networks remain of manageable size in the small arrays of distributed APs, unlike in large centralized arrays, cell-free networks offer a natural setting for exploiting mutual coupling.