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

Abstract

Abstract-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 PA in 65nm 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.

A Low-Power CMOS Class-E Chireix RF Outphasing Power Amplifier for WLAN Applications

“…Recently, the outphasing technique has received much attention for the superiority of high data rates and low-power and because it can achieve high output power [25,26] and wide power control range while maintaining high back-off efficiency [27][28][29][30][31][32]. The outphasing architectures can employ highly efficient nonlinear PAs to amplify the two constant envelope signals, increasing the overall efficiency without degrading linearity [31].…”

“…The isolated combiner, like the Wilkinson, can result in great linearity since the two branches of outphasing topology have less interactions, but it leads to low efficiency because the isolation is at the expense of wasted RF power when signals have large phase angles [29]. On the other hand, the non-isolating combiners, such as transmission line-based combiner, broadband floating load-based combiner and transformer-based combiner [25,27,28], do not create perfect isolation between the two branches of outphasing architecture, thereby eliminating the associated losses and achieving high efficiency. However, the conventional non-isolating combiner, especially the transformer-based combiner, requires bulky quarter-wave transmission lines, which are hard to integrate.…”

Linearization of an Outphasing Amplifier for Wide-band Multi-carrier Signals

“…In [3]a phase polynomial for each outphasing path is derived, correcting amplitude and phase distortion of the output signal. This method is then further improved in [4]with a more complex model for the outphasing amplifier including the phase polynomial as well as amplitude and path delay effects. These methods have greatly improved the linear PA performance however wide band operation still remains a challenge for base station specifications.…”

Estimating models of inverse systems

“…One application example of this is power amplifiers in communication devices, which are often nonlinear and/or dynamic, causing interference in adjacent transmitting channels [1]. To be useful, linearization is needed.…”