Cell-Free Massive MIMO: Uniformly great service for everyone

TLDR

We consider the downlink of Cell‑Free Massive MIMO systems, where a very large number of distributed access points simultaneously serve a much smaller number of users. The goal is to design an optimal max‑min power‑control scheme that ensures equal quality of service for all users. Each access point uses locally estimated channels from uplink pilots and applies conjugate beamforming, and the authors derive a closed‑form expression for the achievable rate. Compared with a conventional small‑cell network, the Cell‑Free Massive MIMO system delivers a much more concentrated throughput distribution, achieves an almost 20‑fold increase in 95‑percentile per‑user throughput, and is more robust to shadow‑fading correlation.

Abstract

We consider the downlink of Cell-Free Massive MIMO systems, where a very large number of distributed access points (APs) simultaneously serve a much smaller number of users. Each AP uses local channel estimates obtained from received uplink pilots and applies conjugate beamforming to transmit data to the users. We derive a closed-form expression for the achievable rate. This expression enables us to design an optimal max-min power control scheme that gives equal quality of service to all users. We further compare the performance of the Cell-Free Massive MIMO system to that of a conventional small-cell network and show that the throughput of the Cell-Free system is much more concentrated around its median compared to that of the smallcell system. The Cell-Free Massive MIMO system can provide an almost 20-fold increase in 95%-likely per-user throughput, compared with the small-cell system. Furthermore, Cell-Free systems are more robust to shadow fading correlation than smallcell systems.