Sagnac Loop Based Broadband Self-Interference Cancellation and Down-Conversion for Distributed Antenna Systems

Focusing on distributed antenna systems, we propose a photonics-assisted self-interference cancellation and down-conversion method based on a Sagnac loop structure. Two inversely cascaded modulators form a Sagnac loop through polarization multiplexing. To support in-band full duplex (IBFD) communication, a double parallel Mach-Zehnder modulator (DPMZM) operated in a clockwise direction is used as a self-interference canceller (SIC) to achieve the phase conversion in the optical domain. Another MZM modulator working in the counterclockwise direction is used to modulate the local oscillation (LO) signal for frequency down-conversion. The adjustment of bias voltage is analyzed to optimize the conversion efficiency of the system. The power fading effect generated by fiber dispersion is compensated by a polarization-sensitive phase modulator (PS-PM). The experimental results show that the average cancellation depth is 56.4 dB at a single frequency in the operating frequency range of 6-22 GHz, and 29.1 dB and 23.7 dB for 20 MHz and 100 MHz bandwidth signals, respectively. In addition, the error vector map is improved from 16.50% to 6.26% after dispersion compensation. The self-interference cancellation, down-conversion and dispersion compensation are integrated in a microwave photonics link. Two cascaded modulators are equivalent to a parallel structure by the Sagnac loop, simultaneously ensuring high LO isolation and conversion efficiency. The proposed method provides an alternative for IBFD-based distributed antenna communication applications.