DAP2 - Multi-point Broadcast Linearisation

Digital Adaptive Predistortion (DAP) has become the industry standard for linearity improvement in digital amplifier designs. By measuring transmitter distortion and applying an inverse correction at the digital modulator output, adaptive pre-distortion provides predictable transmitter linearity, low out of band emissions, improved efficiency and lower overall amplifier cost.

Overview

The Affarii DAP3 predistortion processor is a third generation adaptive pre-distortion engine optimised specifically for the linearization of Class-AB and Doherty amplifiers in Broadcast applications.

The DAP3 core provides complete signal control including modulation preconditioning with peak reduction, linear output filter correction, non-linear amplifier distortion correction, and output automatic gain control (AGC) functions.

The DAP3 non-linear correction capability includes extended length reactive memory compensation designed specifically to enhance performance in large scale combined amplifier arrays. The inclusion of reactive memory compensation allows DAP to achieve up to 30dB of shoulder correction, improved MER and higher output power.

The DAP3 core includes optional monitor capability for modulation output shoulder, flatness, CCDF, and MER parameters allowing detailed real time reporting of amplifier operational status.

Block Diagram

Figure 1: Multi-point Digital Adaptive Predistortion.

Optimised for Broadcast Systems

The implementation of adaptive predistortion in digital broadcast systems varies significantly from that deployed in mobile telephony and cellular radio systems.

Mobile systems use short data packets with rapid power changes that demand high performance memory compensation and rapid parameter adaption to achieve high adjacent channel linearity. In contrast broadcast systems have constant transmission power and channel specific filtering with a focus on maximum output power and low modulation error (MER).

The DAP3 predistortion solution brings together the performance of cellular memory compensation with key features of broadcast operation:

Multi-point Linear and Non-Linear Correction

The DAP3 predistortion solution integrates both a high performance non-linear memory effect predistortion engine with powerful LMS algorithm for filter compensation that provide transmit power and band flatness to 0.1dB accuracy.

Compression Detection and Saturation Operation

To maximise output power it is normal for broadcast amplifiers to be operated with peak power saturation of modulation signal. DAP3 includes saturation detection and peak compression limiting that allows amplifiers to be safely operated at or in saturation for maximum output power while maintaining non-linear and linear correction convergence.

Integrated Modulation Analysis

DAP3 includes the ability to perform modulation detection and synchronised sampling of the transmitter output for monitoring and performance analysis. Analysis personalities are available for DAB, DVB-T and DVB-T2 COFDM formats.