3-D Fourier Series Based Digital Predistortion Technique for Concurrent Dual-Band Envelope Tracking With Reduced Envelope Bandwidth

In this paper, the design, implementation, and measurement results of a new digital predistortion (DPD) method for a concurrent dual-band envelope tracking (ET) power amplifier (PA) system is presented. The PA gain is represented using a set of 3-D orthogonal Fourier series basis functions, which accounts for the distortions introduced by the two signal bands, as well as the supply modulation voltage. Here, the new Fourier series basis is shown to substantially outperform the traditional memory polynomial approach in terms of its linearization capability for subsequent data for which it was not trained for (prediction) due to its well-defined numerical rank and stability. The new DPD system was implemented using a 10-W peak gallium-nitride (GaN) ET PA operating with a dual-band input based on long-term evolution and WCDMA signals center frequency (1.89 and 2.2 GHz) spaced by 310 MHz. The linearization results are compared to the 3-D memory polynomial in two steps: extraction and prediction. In the DPD coefficient extraction phase, tested under the same signal, the traditional memory polynomial and the Fouries series reach close results in both normalized mean square error (NMSE) and adjacent channel power ratio (ACPR). In the prediction phase, however, the proposed method provides a significant performance improvement. A performance improvement as high as 17 dB in terms of NMSE and 4.6 dB in terms of ACPR in comparison to the conventional dual-band 3-D memory polynomial method implementing ET. Furthermore, the well-defined numerical rank of the Fourier series approach allows for a substantial reduction in coefficients using principal component analysis without detrimenting performance.