Self-Interference Cancellation in Interband Carrier Aggregation Transceivers

The increasing amount of data usage by mobile phone users requires higher data rates which, in turn, calls for employing larger bandwidths and flexible transmission schemes. To meet these demands, advanced solutions such as higher-order modulation techniques, multiantenna and multicarrier transmission shemes, among others, have been developed with time. Carrier aggregation (CA) is also one of the techniques that has been proposed, where multiple component carriers (CCs) are aggregated together to form larger transmission bandwidth in order to support high data rate requirements, while also enabling efficient spectrum utilization. In this thesis, the problem of self-interference in radio transceivers supporting interband CA, where separate transmitter (TX) and receiver (RX) chains are used for each CC, is studied and analyzed. At the transmitter the nonlinear behavior of the radio front-end components generates spurious intermodulation distortion components of the transmit CCs which, in some cases, may appear in one of the configured RX operating bands, causing self-interference. Moreover, the CCs are also distorted in a nonlinear fashion by each of the individual power amplifiers (PAs) in the transmitter chain, which means that the self-interference is actually produced by a cascade of two nonlinearities. The resulting self-interference may lead to the blockage of desired received signal, thus causing own receiver desensitization. In order to investigate and formulate a solution to this problem, in this thesis, a signal model for self-interference is first derived incorporating the nonlinearities of the TX PAs and passive front-end components. Then, a digital cancellation solution, utilizing the original transmit data and estimated parameters, is proposed to efficiently suppress the self-interference at the receiver band. The performance of the suggested method is first tested through waveform simulations, and then through practical radio frequency (RF) measurements adopting LTE-Advanced mobile transceiver components. It is observed that the self-interference is efficiently suppressed close to the receiver thermal noise floor by the proposed digital cancellation solution and sufficient cancellation of about 18.2 dB is achieved. All the results based on the waveform simulations and practical RF measurements are presented in this thesis work. The obtained results suggest that the digital cancellation is one potential approach for improving the receiver sensitivity in interband CA transceivers while also relaxing the RF components' linearity requirements