Lossless Decoupling Networks for RF Self-Interference Cancellation in MIMO Full-Duplex Transceivers

The article discusses a radio-frequency-based self-interference cancellation (SIC) technique to handle the self-interference signal segment associated with the self-interference caused by antennas’ mutual coupling, particularly for dedicated-transmit-and-receive antenna configurations. The technique is a novel SIC technique based on antenna decoupling utilizing a lossless network in the radio frequency domain. At first, the authors present a fundamental physical and mathematical description of the self-interference radio channel, employing antenna scattering matrix representation and spherical vector wave expansion. Then, the article presents the experimental results of two antenna mutual coupling measurements – including a single-input-single-output setup and a $2\times 2$ dual-polarized multiple-input-multiple-output (MIMO) setup. Afterward, the authors discuss the lossless network decoupling technique for MIMO full-duplex wireless transceivers. Finally, the authors present a generalized- $\Pi $ synthesizing topology of the lossless decoupling network and empirically evaluate its SIC performance by leveraging the conducted antenna mutual coupling measurement results. The empirical evaluation validates the effectiveness of the lossless decoupling network in canceling the self-interference entirely for a specific design frequency – narrowband systems. Moreover, the evaluation reveals the degradation of the network SIC performance with respect to system bandwidth and the deviation of antennas’ separation distance from its nominal design value.