Digital predistortion using quadrature ΔΣ modulation with fast adaptation for WLAN power amplifiers

This paper presents a low-power, low-complexity adaptive digital predistortion system based on a quadrature ΔΣ modulator for linearizing WLAN PAs. To decrease the modulator oversampling rate, second-order ΔΣ modulation is employed with phase alignment and gain normalization, which ensure the stability of the modulator. The feedback path of the predistortion system is comprised of an RF subsampler and a 32-MSPS ADC. An experimental system linearizes a commercial 2.4-GHz SiGe PA transmitting a 20.2-dBm WLAN signal with 10-MHz channel bandwidth. The background adaptation of a predistortion look-up table in the ΔΣ modulator is achieved in 524 milliseconds. The experimental predistortion system improves the EVM of the unlinearized PA from 5.90% to 0.96%, and enhances the ACPR from -33.3 dBc to -38.0 dBc.