Simultaneous wideband radio-frequency self-interference cancellation and frequency downconversion for in-band full-duplex radio-over-fiber systems.

A photonic approach for simultaneous wideband radio-frequency (RF) self-interference cancellation and frequency downconversion in an in-band full-duplex radio-over-fiber system is proposed based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. The upper dual-parallel Mach-Zehnder modulator (DP-MZM) in the DP-QPSK modulator is used to cancel the self-interference directly in the optical domain and generate two sidebands of the desired RF signal, whereas the lower DP-MZM generates two sidebands of the local oscillator. After detecting the combined optical signal at a photodetector, the desired RF signal is downconverted to an intermediate frequency (IF) signal with the self-interference cancelled. The proposed approach can overcome the reappearance of the interference signal and the power fading effect of the received signal caused by fiber dispersion. An experiment is performed. The cancellation depth for single frequency interference cancellation is around 58 dB, whereas that for the wideband interference cancellation is larger than 25 dB. The fading effect of the proposed IF transmission system is also evaluated compared with the traditional RF transmission system.