Photon-assisted RF self-interference cancellation with OFC-based time/amplitude difference measurement and real-time feedback

The simultaneous in-band full-duplex (IBFD) technology holds the promise of significantly improving spectrum utilization efficiency, thereby easing the mounting strain on spectrum resources. Nevertheless, large-scale IBFD platforms encounter a formidable obstacle in the form of substantial and problematic time differences of arrival (TDOA), which impede the effectiveness of existing photonics-assisted radio frequency self-interference cancellation (RF SIC) techniques. To address these challenges, this study proposes an optical frequency comb(OFC)-assisted RF SIC method. This methodology utilizes an OFC to assist in the measurement and real-time feedback of TDOA and amplitude differences. Initially, the original RF signal undergoes optical undersampling by an OFC to generate an Intermediate Frequency (IF) replica, facilitating precise calculation of the arrival time difference between the interfering and reference branches. The calculated result is then used to guide the adjustment of delay and attenuation, enabling RF SIC across the continuous-wave optical link. We have formulated and rigorously derived the optical path matching condition, establishing a correspondence between RF and IF SIC. The developed spectrum-correlated residual SI (SCRSI) energy algorithm leverages this equivalence to provide real-time feedback and adjustment, ensuring the system achieves and maintains optimal performance. The system achieved high-precision TDOA estimation with an error margin of 150 ps over a 100 ns range within the 50 MHz-10 GHz frequency band, enabling accurate delay alignment beyond the capability of manual tuning reported in previous works. When deviations occur in TDOA and amplitude, the system adaptively adjusts to the optimal delay and attenuation settings, achieving a performance improvement of approximately 16 dB. Within the 0.3-10.2 GHz frequency range and a 200 MHz instantaneous bandwidth, the system attains an RF SIC depth of around 30 dB, and over 20 dB at 400 MHz bandwidth. Moreover, the error vector magnitude (EVM) of the recovered QPSK signal meets the 3GPP international standard requirements. This study provides a significant step toward advancing photonic-assisted RF-SIC technologies from laboratory research to real-world implementation.