Digital self-interference cancellation in inter-band carrier aggregation transceivers: Algorithm and digital implementation perspectives

In this paper we study and analyze the problem of self-interference in transceivers performing inter-band carrier aggregation, where separate power amplifiers (PAs) are used for each component carrier (CC). The self-interference stems from the nonlinear behaviour of the passive RF components at the transmitter, which results in passive intermodulation terms that in some cases fall onto the RX band. Moreover, also the individual PAs distort the CCs in a nonlinear fashion, which means that the self-interference is in fact produced by a cascade of two nonlinearities. This is something that has largely been ignored in earlier literature. Hence, in this work, a signal model is derived that considers both the nonlinearity of the PAs and the passive components, resulting in a highly efficient digital cancellation solution. Using realistic waveform simulations, it is shown to outperform the existing digital cancellers that neglect the PA-induced nonlinear distortion. Also the computational complexity of the proposed digital canceller is analyzed in detail. All in all, the findings indicate that the developed digital cancellation solution is a feasible option for improving the receiver sensitivity of mobile devices utilizing inter-band carrier aggregation.