Cell-Free Massive MIMO Versus Small Cells

TLDR

Cell‑Free Massive MIMO comprises many distributed access points that simultaneously serve a smaller number of users over the same time/frequency resources based on directly measured channel characteristics. The system uses single‑antenna APs and users that acquire channel state via uplink pilots in TDD, perform conjugate beamforming and matched filtering, and employ closed‑form throughput expressions to implement max‑min power control for uniform service, with pilot assignment further reducing contamination. Cell‑Free Massive MIMO yields markedly higher 95‑%‑likely per‑user throughput and resilience to shadow‑fading correlation compared to conventional small‑cell schemes, achieving nearly fivefold improvement under uncorrelated fading and tenfold under correlated fading.

Abstract

A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a very large number of distributed access points (APs), which simultaneously serve a much smaller number of users over the same time/frequency resources based on directly measured channel characteristics. The APs and users have only one antenna each. The APs acquire channel state information through time-division duplex operation and the reception of uplink pilot signals transmitted by the users. The APs perform multiplexing/de-multiplexing through conjugate beamforming on the downlink and matched filtering on the uplink. Closed-form expressions for individual user uplink and downlink throughput lead to max-min power control algorithms. Max-min power control ensures uniformly good service throughout the area of coverage. A pilot assignment algorithm helps to mitigate the effects of pilot contamination, but power control is far more important in that regard. Cell-Free Massive MIMO has considerably improved performance with respect to a conventional small-cell scheme, whereby each user is served by a dedicated AP, in terms of both 95%-likely per-user throughput and immunity to shadow fading spatial correlation. Under uncorrelated shadow fading conditions, the cell-free scheme provides nearly fivefold improvement in 95%-likely per-user throughput over the small-cell scheme, and tenfold improvement when shadow fading is correlated.