Self-interference suppression improvement by employing circular polarized antennas

This paper presents a novel method for suppressing the self-interference due to multipath in WiFi nodes of full-duplex systems based on employing two circular polarized antennas. Three different techniques have been used in order to suppress the interference generated by the direct wave: antenna separation, antenna pattern and analogue cancellation circuit comprised of a balun, a phase shifter and a variable attenuator. As a result, an initial isolation of 90 dB was achieved inside an anechoic chamber. In order to beat the multipath interference circular polarization was employed. With the aim of analyzing the robustness of the circular polarization in comparison with the linear one, the self-interference between transmitter and receiver was measured in nine different environments. The measured level of self-interference was also compared with the results achieved inside the anechoic chamber. In this way, the degradation of the self-interference suppression due to multipath was evaluated. Results show better isolation when employing circularly polarized antennas in seven of nine environments. In addition, with the circular polarization self-interference suppression higher than 70 dB was achieved independently of the environment.