Design and Digital Predistortion Linearization of a Wideband Outphasing Amplifier Supporting 200 MHz Bandwidth

This paper presents a holistic approach to design and linearize a wideband outphasing power amplifier (PA) to efficiently amplify a 200 MHz bandwidth (BW) signal compliant with the Data Over Cable Service Interface Specification (DOCSIS) standard. The proposed outphasing PA, based on a load insensitive GaN HEMT class-E topology, was designed to face the trade-off between the output power control range to be covered with high efficiency and the desired BW. Operated following an isogain approach, the linearity performance is ensured by applying digital predistortion linearization, which combined with crest factor reduction (CFR) techniques can enhance the overall power efficiency. An 18.7^% average efficiency has been measured when fitting the standard in-band and out-of-band linearity requirements for a 26.3 dBm 64 QAM DOCSIS signal with 12.2 dB of PAPR.

I. Introduction

The power amplifier (PA) is a nonlinear critical building block in a data transmission link. In cable distribution systems (following the Data Over Cable Service Interface Specification (DOCSIS) standard), the signal goes from a few tens or hundreds MHz to about 1.2 GHz. In contrast to narrow band systems where the critical intermodulation distortion products are the 3^rd and the 5^th (the rest may be filtered out), in cable applications the fractional bandwidth (FBW) is much higher and the even order products may fall in-band. To deal with both out-of-band and in-band distortion one straightforward strategy consists in operating with certain back-off. Nevertheless, when considering orthogonal frequency division multiplexing (OFDM)-like waveforms with high peak-to-average power ratios (PAPR), back-off operation leads to very low system efficiencies, typically below 2%.