The explosion in demand for wireless capacity has been a strong driver for cellular network modification. Previous solutions to this problem have involved an increase in available channel spectrum to each user and an increase in bands used for cellular communication. This practice has led to an extremely crowded low-frequency spectrum, where the significant problems now are not processing high bandwidths in a mobile device, but instead managing the interference caused by high-traffic scenarios.
One of the most challenging instantiations of this interference mitigation problem is Frequency Division Duplex (FDD) communication in the Long Term Evolution (LTE) standard, where a cellular device both transmits and receives information simultaneously, but in separate frequency bands. Globally, there are 40 LTE FDD bands, and in the worst cases, the center-to-center spacing of transmit (TX) and receive (RX) bands is only twice the bandwidth. With 120dB of dynamic range between the TX and RX signals, without significant isolation, the RX is heavily desensitized or even damaged by the high power TX signal. This problem is currently solved with fixed external duplexers, which provide high TX/RX isolation, but at the cost of frequency tunability. Due to the large number of TX/RX frequency band pairings present in the LTE standard globally, using fixed filters for isolation is infeasible if a phone is to operate well internationally.
In this thesis, a fully-integrated, highly frequency-flexible method for TX self-interference cancellation is proposed, where a mixed-signal canceller is employed at the input of the RX. This technique is shown to allow the receiver to tolerate high TX power levels over a large variety of channel and transceiver nonidealities. The deterministic TX-band interference signal is shown to be mitigated within this system, along with the non-deterministic sources of interference from TX phase noise and canceller thermal noise. Finally, further reduction in TX interference in the digital backend using digital modelling of the PA, canceller, and duplex network is shown.