CORDIC-Based Multi-Gb/s Digital Outphasing Modulator for Highly Efficient Millimeter-Wave Transmitters

Dixian Zhao,Pingyang He

doi:10.1155/2018/7216870

Abstract

This paper describes a high-speed CORDIC-based digital outphasing modulator. Fixed-point Matlab model of the outphasing modulator is developed to evaluate the system performance and define the circuit design parameters. Design issues such as signal quantization error, delay mismatch, and phase overflowing are addressed to enable hardware implementation. The complete outphasing modulator is fully custom designed in 40 nm CMOS, which can be integrated in a millimeter-wave outphasing transmitter to enhance the system average efficiency. Tested with 10.56 Gb/s 64-QAM, this work achieves an EVM of 3.2% and fulfils the IEEE 802.11ad spectral mask requirements.

Highlights

Linearization and efficiency enhancement techniques are always the focus in the design of power amplifiers (PAs) [1]
At millimeter-wave frequencies, such issue becomes even more severe as the transistor operates at a large fraction of the fT/fMAX [2]
The work in [10] presents a digital outphasing modulator based on piece-wise linear functional approximation, but its speed will be limited if high-order modulation (e.g., 64-QAM) is applied due to the increased memory size of the look-up tables (LUTs) in the system

Summary

Introduction

Linearization and efficiency enhancement techniques are always the focus in the design of power amplifiers (PAs) [1]. The work in [6] first time presents the fully integrated outphasing transmitter front-end at mm-Wave It shows that even at mm-Wave the outphasing TX is able to perform linear amplification using switching or saturated power amplifiers (PAs), achieving nearly two times better average efficiency [6] than that of a conventional I/Q TX. The work in [10] presents a digital outphasing modulator based on piece-wise linear functional approximation, but its speed will be limited if high-order modulation (e.g., 64-QAM) is applied due to the increased memory size of the look-up tables (LUTs) in the system. Coordinate rotation digital computer (CORDIC) [11] is employed to realize the conversion between Cartesian and Polar coordinates, and different trigonometric functions Issues such as signal quantization error, delay mismatch, and phase overflowing are Wireless Communications and Mobile Computing.

Outphasing Transmitter System Architecture

CORDIC-Based Outphasing Modulator

A DOUBLE CORDIC

Conclusion