Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 13, NO. 2, FEBRUARY 2001

The emphasis on higher data rates and spectral efficiency has driven the wireless industry towards linear modulation techniques, such as quadrature phase shift keying (QPSK), quadrature amplitude modulation (64 QAM) and multicarrier configurations. Although these linear modulation techniques provide good spectral efficiency, they produce a signal with a fluctuating envelope that generates intermodulation distortion (IMD) at the system s power amplifiers. Most of the IM power appears as interference between adjacent channels, which requires the use of highly linear power amplifiers.

One way to compensate for these nonlinear distortions is linearization of the power amplifier via predistortion. In most applications, linearization is a more desirable approach than backing off a Class A amplifier, which lowers power efficiency and increases heat dissipation. Recently, active linearization has emerged as a critical technology in modern wireless communications systems that addresses this problem.

Adaptive digital predistortion, in which the adaptation mechanism is based on the difference between the desired modulation and the power amplifier s output, is one linearization technique that is well-suited to baseband applications that employ digital signal processors (DSPs). A digital predistorter provides significant IMD reduction over low to moderate bandwidths, while continuously adjusting for component drift and power variations. This predistortion technique operates independently of the chosen modulation scheme.

This article introduces the concepts of power amplifier linearization and digital predistortion, looking in...