How Stability of Hybrid Coupler Characteristic Affects Front-End Isolation of In-Band Full Duplex System

In In-Band Full Duplex (IBFD) transceivers, Electrical Balance Duplexers (EBDs) provide Transmit (TX)-Receive (RX) isolation to implement a form of self-interference (SI) cancellation to facilitates simultaneous transmission and reception from a single antenna. EBD works by coupling transmitter, receiver, antenna, and balancing impedance using a hybrid coupler. The balancing impedance in the EBD needs to be equal as much as possible to the antenna impedance to achieve a high isolation while the antenna impedance variations limit the isolation bandwidth. Hybrid couplers are also not ideal elements, and their S parameters are variable in frequency domain. An antenna with a stable impedance, designed by authors, has been connected in this work to two different hybrid couplers in the EBD stages. One coupler is a commercial coupler and the other one, designed by the authors, has more S parameter stability in the frequency domain than the commercial one. It is shown that by using the designed coupler in the front-end EBD stage more stable isolation bandwidth and 45% better isolation value at ultra-wideband range of 1.5-3.5GHz are obtained, in comparison when using the commercial one.