Photonic-assisted wideband frequency downconverter with self-interference cancellation and image rejection

A photonic-assisted wideband frequency downconverter with self-interference cancellation and image rejection for in-band full-duplex radio-over-fiber systems based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator is proposed. The upper dual-parallel Mach–Zehnder modulator (DP-MZM) of the DP-QPSK modulator is used to cancel the self-interference directly in the optical domain and generate two first-order optical sidebands of the desired received radio frequency (RF) signal. The lower DP-MZM generates two optical sidebands of the local oscillator signal with the optical carrier suppressed via properly biasing the modulator. The optical signals generated from the two DP-MZMs are combined, and then the upper and lower optical sidebands of the combined signal are separated by a wavelength division multiplexer (WDM). The two outputs from the WDM are detected by two photodetectors and then combined at a 90° hybrid coupler to achieve frequency downconversion and image rejection. A simulation is performed. A quadrature phase-shift keying (QPSK)-modulated RF signal is successfully downconverted to a 1-GHz intermediate frequency signal with self-interference and image frequency cancelled. The performance of the frequency downconverter is also evaluated.