Linearizing Radio Frequency Power Amplifiers Using an Analog Predistortion Technique
As critical elements of the physical infrastructure that enables ubiquitous wireless connectivity, radio frequency power amplifiers (RFPAs) are constantly pushed to the limits of linear but efficient operation. Digital predistortion, as a means of achieving unprecedented breakthroughs in this linearity – efficiency trade-off, has been a subject of prolific research for well over a decade. However, to sustain the uncontrolled growth of broadband mobile traffic, wireless networks are expected to rely increasingly on heterogeneously-sized small cells which necessitate new predistortion solutions operating at a fraction of the power consumed by digital predistortion.
This thesis pertains to an emerging area of research involving analog predistortion (APD) – a promising, low-power alternative to digital predistortion (DPD) for future wireless networks. Specifically, it proposes a mathematical function that can be used by the predistorter to linearize the RFPA. As a preliminary step, the challenges of transitioning from DPD to APD are identified and used to formulate the constraints that APD imposes on the predistorter function. Following an assessment of mathematical functions commonly used by DPD, and an analysis of the physical mechanisms of RFPA distortion, a new candidate function is proposed that is both compatible with and feasible for an APD implementation while offering competitive performance against more complex predistorter functions (that can only be implemented in DPD).
The proposed predistorter function and its associated coefficient identification procedure are experimentally validated for the linearization of an RFPA stimulated with single-band carrier aggregated signals of progressively wider bandwidths. The solution is then extended to the case of dual-band transmission, and subsequently validated on an RFPA as well. The proposed function is a cascade of a finite impulse response filter and an envelope memory polynomial (abbreviated as FIR-EMP for single-band, 2D-FIR-EMP for dual-band) and has the potential to deliver far better linearization results than what has been demonstrated in the APD literature.