Transmit and Receive Beamforming for In-Band Full-Duplex MU-MIMO under Self-Interference Constraints with Measurement-Based Validation

Artikel i vetenskaplig tidskrift, 2026

In-band full-duplex (IBFD) multi-user multiple-input multiple-output (MU-MIMO) systems are fundamentally limited by self-interference (SI) caused by strong transmitter-to-receiver coupling, which can degrade receiver performance and restrict simultaneous uplink and downlink operation. This work presents a measured-coupling-based transmit and receive beamforming framework for IBFD MU-MIMO arrays with separate transmit and receive apertures. The transmit design restricts excitation to a low-coupling spatial subspace derived from measured coupling characteristics, ensuring controlled SI levels while maintaining sufficient spatial degrees of freedom for multi-user downlink beamforming. Channel state information is assumed available at the transmitter and represented in the angular domain, enabling beamforming based on dominant spatial directions without requiring uplink–downlink reciprocity. At the receiver, residual SI is mitigated through subspace projection before uplink combining. Full-wave simulations and experimental validation on an RFSoC-based testbed with strongly coupled Vivaldi transmit/receive arrays at 3 GHz demonstrate that imposing a 45 dB isolation threshold at the transmit beamforming stage increases perelement isolation from approximately 20–40 dB to 60–80 dB while maintaining multi-beam operation. Subsequent receive beamforming further suppresses residual SI, yielding over 80 dB of system-level isolation. Frequency-dependent evaluation shows that the center-frequency beamformers maintain effective isolation over the operating bandwidth. These results confirm a scalable and hardware-efficient approach for SI mitigation in IBFD MU-MIMO systems under strong mutual coupling conditions.