Antenna design and channel modelling for in-band full-duplex radios

Abstract

In-band full-duplex (IBFD) radios have the potential to double the throughput by improving the spectral efficiency. The main bottleneck in its implementation is the self-interference (SI) at the receiving side of the transceiver due to its own transmission. The main focus of this thesis is 1) to develop novel solutions to improve isolation between separate transmit and receive antennas, to mitigate the SI due to direct coupling, and 2) to model the multipath SI in different deployment environments of IBFD relays, so that robust analog and digital cancellation solutions can be designed to suppress the SI sufficiently. The main contributions of the thesis are as follows. First, novel antenna decoupling methods are proposed for improving the port-to-port isolation between closely-spaced antenna elements for bi-directional IBFD transmission. A new decoupling method inserting lumped resistive and reactive elements between the antenna feeds is proposed to improve the wideband isolation, also considering the impact on total efficiency. This is extended to a T-shaped decoupling circuit configuration to improve the wideband isolation further, at the cost of increased circuit complexity. The proposed decoupling circuit configurations are designed between two closely spaced printed monopole antennas and a prototype is fabricated to demonstrate the improvement of port-to-port isolation. Secondly, two techniques are proposed to improve the isolation between compact back-to-back antennas for IBFD relaying. First, the so-called neutralization technique is applied to compact back-to-back antennas at 2.6 GHz to improve the port-to-port isolation. In this method, a portion of the signal is transferred through a transmission line from one antenna to the other antenna causing destructive interference with the electromagnetically coupled signal between the antennas. The second technique uses a T-shaped decoupling circuit connected between the antenna feeds to improve the port-to-port isolation. The decoupling circuit uses only lumped reactive elements to maintain the total efficiency. The proposed decoupling technique is demonstrated experimentally for compact back-to-back antennas in the 900 MHz band. Third, the multipath self-interference channel has been measured for outdoor-to-indoor relaying in different domains. An office, coffee room and street-canyon scenario was covered. This is followed by the characterization of the SI for a street-canyon scenario. A site-specific geometry-based stochastic channel model is developed for modelling the SI in the delay, Doppler, spatial and polarization domains jointly. Finally, the benefit of using a compact back-to-back IBFD relay is demonstrated, compared to using a similar half-duplex relay in enhancing coverage for IEEE802.11ah Wireless Local Area Networks. The COST 2100 channel model is used to generate the coverage map for the analysis. The developed decoupling circuits are strong enablers of IBFD transceivers and the SI channel model allows us to design and evaluate the IBFD transceivers, links and systems.