# Cell-Free Massive MIMO Versus Small Cells

“Cell-Free Massive MIMO Versus Small Cells” (2017), by Hien Quoc Ngo et al. Engineering research paper. Original abstract below, unchanged. A free PDF is available.

- Authors: Hien Quoc Ngo, Alexei Ashikhmin, Hong Bing Yang, Erik G. Larsson, Thomas L. Marzetta
- Published: January 19, 2017
- Field: Engineering
- DOI: https://doi.org/10.1109/twc.2017.2655515
- Free PDF: https://arxiv.org/pdf/1602.08232

## 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 throughputs 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.

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