Beamforming Strategies for In-Band Full-Duplex Wireless Systems

Licentiate thesis, 2025

In-band full-duplex (IBFD) wireless communication promises to revolutionize future wireless systems by enabling simultaneous transmission and reception over the same frequency band, effectively doubling spectral efficiency. However, its practical implementation is challenged by strong self-interference (SI) from the transmitter to the receiver, which can saturate the receiver front-end and degrade overall system performance. While various SI cancellation techniques have been developed across the propagation, analog, and digital domains, they are often categorized based on the domain in which cancellation occurs. This domain-specific design approach can limit scalability and robustness, especially in antenna array-based systems.

This thesis addresses these limitations through a novel joint transmit-receive (Tx-Rx) beamforming framework specifically designed for IBFD antenna arrays. Unlike conventional methods that treat Tx and Rx beamforming separately, the proposed strategy jointly optimizes both to maximize antenna gain while maintaining SI below a critical threshold level. The constraints on the Tx-side ensure the receiver operates in its linear regime, while the Rx-side optimization further suppresses residual SI to enhance the signal-to-self-interference ratio and support reliable reception.

The framework is built upon electromagnetic modeling based on scattering parameters and embedded element patterns, and incorporates newly defined figures of merit to evaluate system-level isolation and performance trade-offs. A systematic optimization strategy is developed, and its effectiveness is demonstrated through full-wave simulations of a dual-aperture IBFD array using Method of Moments analysis. Key insights into beamforming flexibility, frequency selectivity, and isolation vs. gain trade-offs are revealed through eigenvalue analysis and pattern shaping studies.

To validate the proposed concept, an integrated IBFD testbed was developed using 3D-printed, silver-coated Vivaldi arrays operating in the 3-6 GHz band, together with the Zynq UltraScale+ RFSoC ZCU216 platform. Measurements closely match simulation results despite practical hardware impairments such as DAC/ADC nonlinearities. With a conventional Tx maximum-gain beamformer, the measured isolation between the closely spaced 3x5 Tx and Rx Vivaldi arrays is approximately 22 dB. In contrast, the proposed Tx beamforming approach achieves over 45 dB of isolation while maintaining high Tx array gain. Combined with the optimized Rx beamformer, the joint Tx-Rx approach provides more than 80 dB of overall IBFD system isolation, with minimal Rx gain loss---demonstrating both the effectiveness and practicality of the method.

Altogether, this work demonstrates that joint Tx-Rx beamforming is a powerful approach to overcoming the core SI challenge in IBFD systems. By directly addressing the scalability and performance trade-offs inherent in full-duplex operation, it brings us a significant step closer to realizing the full potential of next-generation, spectrally efficient wireless communication.