On the effect of S-Parameter Stability of Antenna and Coupler on Electrical Balance Duplexing

Electrical Balance Duplexers (EBDs) provide transmit-receive isolation to implement a form of self-interference cancellation to facilitate simultaneous transmission and reception from single antenna in systems like In-Band Full duplex (IBFD) transceivers. EBD works by coupling transmitter, receiver, antenna, and balancing impedance using hybrid coupler. In recent literatures, antenna impedance variations are considered as main factor limiting the EBD isolation bandwidth while the EBD balancing impedance needs to be equal as much as possible to the antenna impedance to achieve high isolation. But hybrid couplers are also not ideal elements, and their S-parameters are not stable in frequency domain. In this work, five broadband RF-devices (two antennas, two couplers and a 50-Ohm RF-load) are used to form four EBD setups. One of the antennas, designed by authors, has more impedance stability in frequency domain than the other one which is a commercial antenna. Also, one of the couplers, designed by authors, has more S-parameter stability in Ultra-Wideband (UWB) frequency domain than the other one which is a commercial UWB coupler. Results of implemented EBDs show that when both antenna and coupler have strong S-parameter stabilities in frequency domain wider isolation bandwidth (UWB 1.5–3.5GHz range) and higher EBD isolation is obtained.