A Full Duplex RF Front End Employing an Electrical Balanced Duplexer and a Chebyshev Load-Balancing Filter

This paper presents a new Chebyshev load-balance (LB) circuit for electrical-balanced RF front-ends targeted for simultaneous transmit-receive (STR) operation. TX reflection of a commercial Wi-Fi antenna with near 3:1 VSWR and direct TX leakage of a commercial quadrature-hybrid are considered and cancelled concurrently in simulations and measurements. We analyze the reflective LB as the RF front-end optimal load and propose a theoretically lossless impedance-matching network that achieves self-interference cancellation (SIC) over more than ± 5 % bandwidth relative to the carrier. A PCB design with two transmission lines and lumped RLC elements is realized to demonstrate the potential of the architecture, achieving > 25 dB of total TX-RX isolation over 300 MHz around 2.65 GHz where the antenna reflection is 7 dBr. This is the first published work that reports cancellation of such a reflective antenna employing a low-cost LB circuit. The proposed Chebyshev LB is theoretically superior to the networks proposed in previous papers.