Micro-Electromechanical Impedance Control for Electrical Balance Duplexing

The Electrical Balance Duplexer (EBD) implements a form of self-interference cancellation, and can provide high Tx-Rx isolation whilst allowing simultaneous transmit (Tx) and receive (Rx) trough a single antenna in In-Band Full-Duplex (IBFD) radio transceivers. This paper presents a novel Micro-ElectroMechanical Systems (MEMS) based implementation of the tunable balancing impedance component of the EBD. A prototype MEMS tunable impedance demonstrates excellent tuning range and accuracy, and can operate over wide frequency ranges, balancing the EBD at 800 MHz and 1900 MHz to provide >40 dB isolation over a 20 MHz bandwidth. However, the MEMS tunable impedance is observed to introduce non-linear distortion into the system. This non-linearity does not significantly degrade the Tx error vector magnitude, however it has a substantial detrimental impact on the performance of a second stage of linear active cancellation, which achieved just 16 dB of additional cancellation at a Tx power of 23 dBm.