Digitally-Controlled Electrical Balance Duplexer for Transmitter-Receiver Isolation in Full-Duplex Radio

This paper presents a novel digitally-controlled electrical balance duplexer (EBD) prototype capable of inband full-duplex radio communications. The developed EBD prototype works in the ISM-Band, at 2.4-2.48 GHz, and can achieve TX to RX isolation of 53 dB across an 80 MHz instantaneous bandwidth. The prototype is composed of three parts, namely coupled line hybrid junction, triple-Pi balancing impedance and antenna tuning unit (ATU) which are all realized with commercial off-the-shelf components and implemented over a two layer FR4 board. The EBD contains also a self-adaptive or self-healing digital control system enabling automatic tracking of time-varying antenna impedance characteristics, providing robustness against fast changes in the surrounding environment and against user interactions. In addition to the architecture and operating principle descriptions, we also provide actual RF measurements at 2.4 GHz ISM band with real antenna connected, demonstrating the achievable isolation levels with different bandwidths and when operating in different environmental conditions. Furthermore, isolation performance is measured when operating with different antennas and under a low-cost highly nonlinear power amplifier.