Scalable Photonic-Assisted Multipath Self-Interference Cancellation Based on Optical Frequency Comb

In order to increase the scalability and reduce the complexity of photonic multipath SIC systems, a photonic multipath self-interference cancellation (SIC) method based on optical frequency comb (OFC) is proposed. The OFC is generated by continuously frequency-shifting the seed laser in the re-circulating frequency shifter (RFS) loop. The reference signal is modulated to the multiple combs of the OFC via a Mach-Zehnder modulator (MZM), and the received signal is loaded into another MZM. These two MZMs work at opposite quadrature bias points to reverse the phase of the received signal and the reference signal. After splitting and filtering with a dense wavelength division multiplexer, the reference modulation signals are injected into a multipath amplitude-delay matched link, where the amplitude and delay time are accurately controlled by several sets of optical variable attenuators and tunable delay lines. Both the inversion and amplitude-delay matching of the reference signals are realised in the optical domain, contributing to a broad bandwidth and high operational frequency. Experimental results show that the cancellation depth is greater than 45.9 dB for single-frequency signals, and 23.9 dB for signals with a bandwidth of 100 MHz in a setup with three self-interference paths. The proposed method is characterized by easy scalability, good flexibility, and low cost, making it suitable for complex multipath SIC in in-band full-duplex wireless systems.