STAR Front-End Using Two Circulators in a Differential Connection

In-band full duplex (IBFD) communication systems have attracted much interest due to their ability to double the spectrum efficiency by simultaneously transmitting and receiving (STAR) over the same bandwidth. Modern communication technologies have to adapt to be able to meet the ongoing high demand capacity over existing radio channels. This paper proposes a system-level approach based on physical symmetry to improve the electromagnetic performance of a 3-port circulator. A system-level design consisting of two matched circulators connected in a differential configuration is proposed to cancel out residual RF scattering and leakage that causes self-interference in a STAR front-end. The method is validated using a custom designed microwave circulator based on microstrip technology, which forms a building block that operates in the 3–8 GHz band with 20 dB isolation. The proposed RF front-end operates within an extended band of 3–8 GHz while simultaneously exhibiting improvement in isolation by about 10 dB (isolation $30\pm 4$ dB) between the transmitter and the receiver ports of the STAR system.