Fast predistorter adaptation to varying antenna load

The effect of varying power amplifier (PA) output VSWR is studied in a workfunction predistorter system. The resistive load of a commercially available W-CDMA PA is tuned and the large-signal S-parameters measured. From the measurements, the optimum gain and phase predistortion functions are derived and studied. The predistorter coefficients are parameterized with a single variable that roughly fits optimum predistortion functions for other than nominal PA loading conditions. The adaptation speed is evaluated in a complete predistorter system simulator and it is found that the number of iterations needed to adapt is reduced from 4 to 1.