Downlink Training in Cell-Free Massive MIMO: A Blessing in Disguise

Giovanni Interdonato, Hien Quoc Ngo, Pål Frenger, Erik G. Larsson

Research output: Contribution to journalArticle

4 Citations (Scopus)
131 Downloads (Pure)

Abstract

Cell-free Massive MIMO (multiple-input multiple-output) refers to a distributed Massive MIMO system where all the access points (APs) cooperate to coherently serve all the user equipments (UEs), suppress inter-cell interference and mitigate the multiuser interference. Recent works [1], [2] demonstrated that, unlike co-located Massive MIMO, the channel hardening is, in general, less pronounced in
cell-free Massive MIMO, thus there is much to benefit from estimating the downlink channel. In this study, we investigate the gain introduced by the downlink beamforming training, extending the analysis in [1] to non-orthogonal uplink and downlink pilots. Assuming single-antenna APs, conjugate beamforming and independent Rayleigh fading channel, we derive a closed-form expression for the per-user achievable downlink rate that addresses channel estimation errors and pilot contamination both at the AP and UE side. The performance evaluation includes max-min fairness power control, greedy
pilot assignment methods, and a comparison between achievable rates obtained from different capacity bounding techniques. Numerical results show that downlink beamforming training, although increases pilot overhead and introduces additional pilot contamination, improves significantly the achievable
downlink rate. Even for large number of APs, it is not fully efficient for the UE relying on the statistical channel state information for data decoding.
Original languageEnglish
JournalIEEE Transactions on Wireless Communications
Early online date14 Aug 2019
DOIs
Publication statusEarly online date - 14 Aug 2019

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