Asymmetry Index for Memory Effects Quantification in Wireless Transmitters and Power Amplifiers Driven by Broadband-Modulated Signals

Abstract

This paper presents a novel metric for the quantification of memory effects in wireless transmitters and radio frequency power amplifiers driven by broadband-modulated signals. The proposed metric, labeled “spectrum asymmetry index”, is based on the spectrum asymmetry at the output of the transmitter/power amplifier (PA). The effectiveness and reliability of the proposed metric are experimentally validated using a Doherty PA and a class AB PA driven by WCDMA signals having various bandwidths and power levels. The experimental results show that the proposed metric is consistent with the previously reported memory effects intensity metric for bandwidths up to 20 MHz since both metrics predict increasingly strong memory effects as the signal bandwidth or average power increases. However, experimental validation with signals having wider bandwidths (up to 40 MHz) illustrates the robustness and advantage of the proposed metric compared to its state of the art counterpart. Indeed, for these test conditions, the spectrum asymmetry index is able to predict the memory effects strengthening, while the memory effects intensity metric is unable to track this trend. It is also shown that the proposed metric can be used to compare the memory effects strength in PA prototypes. In fact, comparing the spectrum asymmetry index of two different power amplifiers provides a reliable information on the relative strength of their memory effects. Furthermore, the proposed metric is derived directly from the measured output signal without requiring cancelation of the static nonlinearity. This presents an additional major advantage compared to previously reported methods for memory effects quantification in transmitters and power amplifiers driven by wideband-modulated signals.