Applying repeater-assisted cellular massive MIMO (RA-MIMO) to frequency-division duplex (FDD) systems

Investigate the opportunities and challenges of deploying repeater-assisted cellular massive MIMO (RA-MIMO) in frequency-division duplex (FDD) systems, and determine what design adaptations are required for effective operation under FDD.

Background

The paper proposes repeater-assisted cellular massive MIMO (RA-MIMO), where many small, low-cost repeaters act as active scatterers to improve coverage and macro-diversity while leveraging reciprocity-based multi-user beamforming. The core development and performance evaluation in the paper focus on time-division duplex (TDD) systems.

While RA-MIMO is envisioned primarily for TDD operation due to the benefits of reciprocity, the authors explicitly point out that using RA-MIMO in frequency-division duplex (FDD) systems is not resolved. Understanding how RA-MIMO can be adapted, configured, and optimized for FDD—where uplink/downlink reciprocity does not naturally hold—constitutes an open problem identified by the authors.