Theoretical Bounds of the Self-Interference Cancellation-Bandwidth Integral in Full Duplex RF Front Ends

This paper studies the behavior of lumped self-interference cancellation (SIC) filters in simultaneous transmit-receive (STR) RF front ends. The crux that enables a different approach in this work for finding an optimal network solution is converting the SIC circuit design flow into an equivalent impedance-matching problem. As a result, we analyze a reflective SIC filter that complies with electric-balance duplexers utilizing known techniques. Bode, Fano, Matthaei, and others studied lumped network theory considerations and devised methodical steps for maximizing the impedance-matching bandwidth for reactive loads. Accordingly, a systematic approach that considers the analysis of the SIC-Bandwidth mutual relationship, and the optimal lumped networks, is detailed in this work. We indicate that the Chebyshev band-pass filter can provide optimal and lossless SIC filter performance, which is crucial for the overall STR performance. The realistic RF front-end in this work, which includes a simulated quadrature hybrid, and a measured Wi-Fi antenna with VSWR of 3:1, achieves > 40 dB of TX-RX isolation over 200 MHz around 5.5 GHz, utilizing a second order filter that employs only two realizable inductors.