Fano-Bode Cancellation-Bandwidth Limit in Full-Duplex Wireless Transceivers

This paper explores the attainable cancellation bandwidth for practical TX-RX self-interference (SI) in simultaneous transmit and receive (STAR) RF front-ends. We present a passive, lumped feed-forward self-interference cancellation (SIC) network to neutralize a measured SI of a commercial single antenna interface. The proposed SIC transfer function is a reflection coefficient linearly related to the SI, enabling SIC filter design and analysis following impedance matching theory and techniques. Accordingly, the SIC circuit adheres to the Fano-Bode limit, where the TX-RX SIC is inversely proportional to the achieved bandwidth (BW). Lumped Chebyshev SIC equalizers enable efficient extraction of the Fano-Bode limit, ensuring low insertion loss, linear response, and on-chip integration. Furthermore, we establish that the rational function approximation of the SI sets a numerical SIC-BW bound corresponding to the SIC Chebyshev equalizer transfer function of the same order. We present a practical application of the proposed SIC design approach in a commercial single antenna circulator RF front-end around 5.3 GHz, which comprises a Wi-Fi antenna. The cancellation path includes commercial Wilkinson and quadrature hybrid SMA components, and two simulated
$4^{th}$
order LC Chebyshev equalizer networks. The realizable SIC filter of this work achieves an insertion loss of 6 dB and enables 40 dB of TX-RX isolation over 300 MHz. The 40 dB numerical TX-RX isolation BW bound for the same order of the Chebyshev equalizer is 450 MHz, further validating the SIC-BW conclusions of this work.