A Two-Stage Wideband RF Cancellation of Coupled Transmit Signal for Bi-Static Simultaneous Transmit and Receive System

Wireless technology is growing at a fast rate to accommodate the expanding user demands. Currently, the radio frequency (RF) spectrum is overcrowded. Hence, it is more susceptible to signal fratricide and interference. To enhance spectrum access, in-band full duplex systems are implemented to achieve simultaneous transmission and reception (STAR) and double the spectral efficiency. However, STAR systems require suppression of the high power transmit signals that can leak into the receiver chain. This type of self-interference (SI) can significantly reduce the receiver.s dynamic range and often leads to its desensitization. Therefore, successful STAR implementation requires considerable isolation between the transmitter and receiver to reduce the SI signal. To do so, RF self-interference cancellation (RFSIC) stages are considered. In this paper, we present a two-stage SIC system that includes transmit and receive antennas isolation and RFSIC filter stages. Notably, the isolation between the transmit and receive antennas is based on a novel symmetric suppression technique. The RFSIC filter is based on a hybrid finite impulse response (FIR) filter and resonator architecture. Adding a resonator to the FIR filter provides improved matching to approximate and suppress the direct SI coupling. Our design achieves \sim52 dB isolation on average across a 500 MHz bandwidth ( viz. 1-1.5 GHz) in simulation. Simulation results show a minimum cancellation of 41 dB and a maximum cancellation of 65 dB. A prototype was fabricated and tested, showing an average of \sim 447 dB cancellation which is in good agreement with our simulation.