Wideband Self-Interference Canceller for Monostatic Co-Circularly Polarized In-Band Full-Duplex Systems

This study presents a wideband self-interference (SI) canceller based on a cross-shaped symmetric hybrid coupler network for co-circularly polarized (co-CP) in-band full-duplex (IBFD) systems using a single antenna. The proposed SI canceller is implemented with four hybrid couplers arranged concentrically in a cross-shaped symmetric configuration, enabling the transmitting (Tx) and receiving (Rx) signals to share the same feed network and radiate with co-CP through a single antenna. In this configuration, SI signals leaking from the Tx to Rx paths—including components due to antenna reflection and mutual coupling between antenna ports—arrive with appropriate amplitude and phase such that they cancel out at the hybrid coupler preceding the Rx port without requiring multiple antennas, orthogonal polarizations, or active circuits. This enables high Tx/Rx isolation over a wide frequency band. The implemented SI canceller has compact dimensions of 68.2 mm × 81.3 mm and is realized on a single-layer substrate. Experimental results show that the proposed canceller achieves better than –35 dB isolation across the 2.86–3.52 GHz band (approximately 20.3% fractional bandwidth) while maintaining robust isolation performance when integrated with a single CP patch antenna. These results represent a significant improvement in bandwidth and isolation compared to previously reported single-antenna IBFD designs.