A Novel Reconfigurable Duplexer Supporting FDD, TDD, and FD Operation With Controllable Center Frequency and Enhanced Self-Interference Isolation

In this article, a novel reconfigurable duplexer topology supporting time-division duplex (TDD), frequency-division duplex (FDD), and full-duplex (FD) operations under spatiotemporal modulation (STM) is proposed. The proposed reconfigurable duplexer structure is realized by seven shunt LC resonant tanks with controllable center frequencies. Controllable electromagnetic mixed coupling (EMC) is employed as the interstage coupling to achieve flexible switching operation between different duplex modes such as TDD and FDD modes. The first three resonators of the proposed structure are connected by three EMC LC tanks in a circular loop configuration, forming a delta circuit architecture. By modulating the nonlinear capacitors in the EMC LC tanks of delta circuit loop with low-frequency STM signals, nonreciprocal transmission is obtained within the delta circuit loop, resulting in an integrated circulator and enabling the FD mode operation. Meanwhile, self-interference cancellation (SIC) technique is employed in the proposed structure to realize the tunable isolation between transmitting (Tx) and receiving (Rx) paths in the FD mode. A higher Tx–Rx isolation with adjustable center frequency and bandwidth (BW) is obtained by utilizing the nonresonating-node-based (NRN-based) amplitude–phase control circuit in proposed SIC circuits. To validate the proposed reconfigurable diplexer topology, a prototype using microstrip technology is fabricated and measured. Good agreement between the simulated and measured results can be observed.