Photonics-based radio frequency self-interference cancellation for radio-over-fiber systems

A photonics-based radio frequency (RF) self-interference cancellation approach for radio-over-fiber (RoF) systems is proposed and demonstrated. An intermediate frequency (IF) signal and a local oscillator (LO) signal are separately modulated to a pair of optical carriers with orthogonal polarization states in the central station. The generated optically carried RF signal is then transmitted to remote base stations over fibers. After performing photodetection at the two outputs of the polarization beam splitter (PBS), a pair of upconverted RF signals with inverse phases are generated. One is used as the transmitted signal, while the other is used as the reference signal for the self-interference cancellation. In this way, the phase inversion between them is realized in the optical domain. In addition, the adjustments with the time delay and the amplitude of the reference signal are both achieved in the optical domain, guaranteeing the self-interference cancellation in a wide bandwidth. Meanwhile, the independent optical system for self-interference cancellation is avoided, simplifying the complex hardware requirements with the base stations. The cancellation depths of 33, 30, and 22 dB over 0.5-, 1-, and 3-GHz bandwidths are experimentally achieved, respectively. The ranging and imaging functions of a frequency modulated continuous wave (FMCW) radar system using the established self-interference cancellation structure are experimentally verified.