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

Today’s traditional radio systems exploit two techniques to transmit and receive which are Time Domain Duplexing (TDD) and Frequency Domain Duplexing (FDD). These two techniques allow bidirectional communication exploiting different frequencies or time-slots for the transmission and receiving operations. The ongoing research on wireless communications is currently active under in-band Full-Duplex (FD) radio communications, where co-located transmitter and receiver are operating simultaneously at the same central frequency doubling the spectral efficiency. The main technical challenge consists in suppressing the strong self-interference (SI) which is caused by the transmitting (TX) leakage into the receiving (RX) chain. Thus, FD transceivers need to provide high TX-RX isolation requirements in order to suppress the SI. Different methods are employed to mitigate the effect of the SI pursuing then FD operation. Generally, passive isolation between the TX and RX in the radio frequency (RF) domain can be built on specific antenna technologies or hybrid junction based electrical balance duplexers (EBD). On top of these, active SI cancellation is also typically required, either at analog/RF stages or digital baseband or both. This thesis work presents a novel digitally-controlled electrical balance duplexer prototype capable of inband FD 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 performances are measured when operating with different antennas and under a low-cost highly nonlinear power amplifier