Least-Squares Phase Predistortion of a +30 dBm Class-D Outphasing RF PA in 65 nm CMOS

• Published in: IEEE transactions on circuits and systems. I, Regular papers Vol. 60; no. 7; pp. 1915 - 1928
• Main Authors: Jung, Ylva, Fritzin, J., Enqvist, M., Alvandpour, A.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.07.2013; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Amplifier; CMOS integrated circuits; complementary metal-oxide-semiconductor (CMOS); Gain; linearization; Multiaccess communication; Optimization; outphasing; Phase distortion; Predistortion; Radio frequency; Studies
• ISSN: 1549-8328; 1558-0806
• DOI: 10.1109/TCSI.2012.2230507

This paper presents a model-based phase-only predistortion method suitable for outphasing radio frequency (RF) power amplifiers (PA). The predistortion method is based on a model of the amplifier with a constant gain factor and phase rotation for each outphasing signal, and a predistorter with phase rotation only. Exploring the structure of the outphasing PA, the model estimation problem can be reformulated from a nonconvex problem into a convex least-squares problem, and the predistorter can be calculated analytically. The method has been evaluated for 5 MHz Wideband Code-Division Multiple Access (WCDMA) and Long Term Evolution (LTE) uplink signals with Peak-to-Average Power Ratio (PAPR) of 3.5 dB and 6.2 dB, respectively, applied to one of the first fully integrated +30 dBm Class-D outphasing RF PAs in 65 nm CMOS. At 1.95 GHz for a 5.5 V (6.0 V) supply voltage, the measured output power of the PA was +29.7 dBm (+30.5 dBm) with a power-added efficiency (PAE) of 27%. For the WCDMA signal with +26.0 dBm of channel power, the measured Adjacent Channel Leakage Ratio (ACLR) at 5 MHz and 10 MHz offsets were - 46.3 dBc and - 55.6 dBc with predistortion, compared to -35.5 dBc and -48.1 dBc without predistortion. For the LTE signal with +23.3 dBm of channel power, the measured ACLR at 5 MHz offset was - 43.5 dBc with predistortion, compared to -34.1 dBc without predistortion.