Efficient Angle-Domain Processing for FDD-Based Cell-Free Massive MIMO Systems

Asmaa Abdallah*, Mohammad M. Mansour

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

36 Scopus citations


Cell-free massive MIMO communications is an emerging network technology for 5G wireless communications wherein distributed multi-antenna access points (APs) serve many users simultaneously. Most prior work on cell-free massive MIMO systems assume time-division duplexing mode, although frequency-division duplexing (FDD) systems dominate current wireless standards. The key challenges in FDD massive MIMO systems are channel-state information (CSI) acquisition and feedback overhead. To address these challenges, we exploit the so-called angle reciprocity of multipath components in the uplink and downlink, so that the required CSI acquisition overhead scales only with the number of served users, and not the number of AP antennas nor APs. We propose a low complexity multipath component estimation technique and present linear angle-of-arrival (AoA)-based beamforming/combining schemes for FDD-based cell-free massive MIMO systems. We analyze the performance of these schemes by deriving closed-form expressions for the mean-square-error of the estimated multipath components, as well as expressions for the uplink and downlink spectral efficiency. Using semi-definite programming, we solve a max-min power allocation problem that maximizes the minimum user rate under per-user power constraints. Furthermore, we present a user-centric (UC) AP selection scheme in which each user chooses a subset of APs to improve the overall energy efficiency of the system. Simulation results demonstrate that the proposed multipath component estimation technique outperforms conventional subspace-based and gradient-descent based techniques. We also show that the proposed beamforming and combining techniques along with the proposed power control scheme substantially enhance the spectral and energy efficiencies with an adequate number of antennas at the APs.

Original languageEnglish (US)
Article number8968400
Pages (from-to)2188-2203
Number of pages16
JournalIEEE Transactions on Communications
Issue number4
StatePublished - Apr 2020


  • angle-based beamforming/combining
  • array signal processing
  • cell-free massive MIMO
  • FDD mode
  • multipath component estimation
  • power control

ASJC Scopus subject areas

  • Electrical and Electronic Engineering


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