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The mass adoption of smartphones has led to a huge demand for wireless services, and 5G mobile is expected to deliver a 1000 fold capacity increase over the coming decade. However, this increase in demand will not been accompanied by a corresponding increase in the spectrum allocated for mobile networks, and therefore significant technological advances. will be required in order to reach this goal. Furthermore, due to the fragmented availability of spectrum globally, the number of frequency bands in use around the globe continues to increase, and thus a tunable radio frequency front end is required to enable roaming and cost effective manufacture. Duplexing techniques based on self-interference cancellation can increase spectral efficiency by enabling simultaneous transmission and reception on the same frequency, and have the potential to replace fixed frequency duplexing filters to create a tunable radio frequency front end. This thesis investigates self-interference cancellation technologies in the context of frequency division and division free duplexing in future mobile devices. The focus of this thesis is the Electrical Balance Duplexer (EBD), which implements a form of self-interference cancellation to provide high transmit-to-receive (Tx-Rx) isolation whilst allowing simultaneous transmission and reception through a single antenna. This contribution offers several advancements concerning the design and operation of the EBD, placing emphasis on investigating practical issues that may affect the suitability and performance of this circuit in mobile device applications. The results and analysis presented in this thesis have demonstrated that time-domain and frequency domain antenna impedance variation can have a substantial impact on the Tx-Rx isolation achieved by the EBD. A realistic evaluation of EBD isolation bandwidth is presented, and a novel method for maximising isolation is proposed. It is shown that circuit adaptation is required to mitigate time-domain antenna impedance variation which arises due to environmental interaction, and requirements for antenna impedance tracking in user interaction and indoor mobile scenarios are determined. A novel EBD circuit architecture combining passive and active cancellation methods is proposed and implemented in hardware, demonstrating a substantial improvement by providing >80 dB of isolation over an 80 MHz bandwidth. This thesis also presents an in-depth theoretical analysis of self-interference coupling in non-ideal EBDs, and from this derives a novel circuit adaptation algorithm which can achieve the required accuracy but is orders of magnitude faster and computationally less expensive than existing methods. It can be concluded that the EBD is a promising candidate technology for implementing tunable duplexing functionality in mobile devices, and that the environmental effects and non-ideal circuit characteristics which are typical of this application can be successfully mitigated. |
