Smart Radio System Lab.

Digital Pre-Distortion system for GaN power amplifier on base station

Sang-Hyun Chun, Kyu-Jin Choi, Jin-Ho Kim and Jong-Heon Kim

< Kwangwoon University >Seoul, Korea

2010. 6. 30

Smart Radio System Lab.

• Introduction

• Principle of Digital Pre-distortion Linearization

• Connected Solution

• Linearization Test of DPD

• Result (Linearization Performance)

• Conclusion

Contents

Smart Radio System Lab.

Introduction

• Requirements of PA for Wireless Communication

- High quality of transmission (High Linearity)

- Low power consumption (High Efficiency)

- Require digital control on hardware platform to reset and reuse

(Improving reliability, Advantageous maintenance)

Requirements Solution

High Linearity

(Improving quality of Transmission)Using Linearization Technique(Feed Back, Feed Forward, Pre-Distortion.. etc))

High Efficiency

(Reduce Power Consumption)Structure of High Efficiency PA(Doherty, EER, Envelope Tracking.. etc)

Smart Radio System Lab.

Analog PD Feed back Feed forward Digital PD

Bandwidth very wide Narrow wide moderate

Linearity good good very good very good

Complexity medium medium high high

Power Efficiency High high low high

• Strength of Digital Pre-distorter (Digital PD)

- Better power efficiency than feed forward technique using widely

- Feed forward technique is optimized only single mode PA, but DPD can be applied multimode by re-

setting the system (maintenance widely use)

- The most suitable linearization technique for next generation linearized PA requiring the digitally

control hardware platform

Introduction

• Comparison Linearization Techniques

Smart Radio System Lab.

Principle of DPD linearization

• Compensate non-linearity of PA

- Self-characteristic of PA(Non-linearity) : Amplitude and Phase Distortion

(Characteristic of output power is compressed due to increasing input power)

- Non-linearity of PA : Decline efficiency of output spectrum

- Pre-distorted Linearization : Input signal is distorted intentionally

to cancel the non-linear output characteristic

- Improve the linearity of PA by expending output according to increase input

- Digital Pre-distorted linearizer is made up of base-band signal processing

Smart Radio System Lab.

Principle of DPD linearization

• DPD linearization using adaptive algorithm

- Modeling the PA property using Polynomial method

- Compute the error comparison Desired and Output signal

- Optimized linearization performance when DPD coefficient W(n) is minimum value

- Actively, adapt environmental change and TR property by using the real-time updating coefficients

Principle of DPD system

Smart Radio System Lab.

Connected Solution

• Conventional measurement method of DPD system

- Need a part of signal generation

- Need a part of digital control

- Need a converting device of digital / analog

- Need Up/Down Converter

• Measurement method of DPD system using Connected Solution

- Using control PC

ADS : Signal generation and Control the signal, DPD system

MATLAB : Compute the DPD coefficients

I/O Libraries Suite : Interlocking Control PC and Measurement Device

- PSG 8267D : Signal generator, D/A Converter, Up Converter

- PSA E4440A, VSA: Down Converter, A/D Converter

Smart Radio System Lab.

Connected Solution

Schematic of DPD system using Connected Solution

Smart Radio System Lab.

Connected Solution(role of each part)1. Signal Generator

• Generating WiBro 1FA Signal and Linking PSG

- Generating Signal using WiBro Downlink Block of ADS

- Using ESG Sink Block of ADS to link PSG

- Insert I/Q signal of base-band -> D/A Conversion -> Up-Conversion -> Generating RF test signal

DAC DUTI(t)Q(t)

PC(ADS) PSG8267D

LO

Smart Radio System Lab.

Connected Solution (role of each part)2. Spectrum Analyzer

• Analysis output signal of PA

- Output signal of PA converts base-band signal (Down Conversion)

- Analog signal converts to Digital Signal for Data processing of control PC

- Consider 3rd IMD and Bandwidth of WiBro 1FA signal(8.75MHz) to linearization

: Require 30MHz bandwidth which is 3 times wider than bandwidth of signal source

- Output signal of PA -> Convert base-band signal -> A/D Conversion -> Insert control PC

Smart Radio System Lab.

Connected Solution (role of each part)3. Software

• ADS (Advanced Design System)- Generating Input signal to insert the PA- Link signal generator as I/Q signal of base-band- Link control PC(ADS) with measured PA output signal data from spectrum analyzer

Using VSA Sink Block- Construct DPD system- Link to MATLAB for computing DPD coefficients

• MATLAB- Computing and Saving DPD coefficient

• VSA (Vector Signal Analyzer)- Receive output signal from spectrum analyzer

and save as data file to connect ADS

• I/O Libraries Suite- Connect measurement device and control PC

IO Libraries Suite

Smart Radio System Lab.

Linearization Test of DPDTest Method

• Test Method1. Run PSG Simulation (Initialization)

: Signal generated by control PC(ADS) is inserted as input signal through signal generator2. Run VSA Simulation

: Link output signal of PA to control PC, Synchronize input and output signal3. Generate Memory Polynomial Coefficients

: Generating DPD coefficient using in/output signal of PA4. Run PSG Simulation (with Pre-distortion)

: Insert pre-distorted signal to the PA

Test Process

Smart Radio System Lab.

Linearization Test of DPD1. Run PSG Simulation (Initialization)

• Run PSG Simulation (Initialization)

- Process of insert signal source to signal generator after generating test input signal of PA

- To Link WiBro signal source block of ADS and PSG by using ESG sink block

- Insert scale factor to Control between amplitude of signal source and PSG source

- Test input signal converts I/Q signal to link PSG

Run PSG Simulation (Initialization)

Spectrum and CCDF of input signal

Smart Radio System Lab.

Linearization Test of DPD2. Run VSA Simulation

• Run VSA Simulation

- Process of converting the available data for

PC. Captured output signal of PA from PSA

is converted through VSA.

- Synchronization after Evaluating delay time

of generated In/Output data rows

- Increase samples of primary input data

rows to evaluate accuracy delay time

- Convert primarily number of data samples

after compensation of evaluating delay time

using Cross-correlation

Run VSA Simulation

Synchronized In/Output Data

Smart Radio System Lab.

Linearization Test of DPD3. Generate Memory Polynomial Coefficients

• Generate Memory Polynomial Coefficients- Synchronized In/Output data rows on ADS save and transfer to MATLAB data rows through the ADS to

MATLAB sink- Modeling the PA using synchronized In/Output data rows from MATLAB- Compute and save the DPD coefficients to apply DPD algorithm which Consider the Memory Effect to

improve linearity of PA

Run VSA Simulation

Smart Radio System Lab.

Linearization Test of DPD4. Run PSG Simulation (with Pre-distortion)

• Run PSG Simulation - Generating Input signal using DPD coefficient from Generate Memory Polynomial- Insert pre-distorted signal to signal generator using ADS- Conform linearized output signal to insert pre-distorted to non-linear PA

Run PSG Simulation

Smart Radio System Lab.

ResultsGaN Power Amplifier

• Doherty Power Amplifier using GaN power transistor

- Fc : 2.345GHz

- Max. output power 51.5dBm

- Ave. output power 42 dBm (9.5dB Back-off)

Structure of Doherty Power Amplifier

Smart Radio System Lab.

Output Spectrum of GaN PA Output Spectrum of PA with DPD(P:7, M:3)

Results Before Linearization (dBr) After Linearization (dBr) Quantity of Improvement(dBr)

Lower (@ -4.77 MHz) -35.47 -46.57 11.1

Upper (@ +4.77 MHz) -36.53 -49.9 13.37

ResultsGaN Power Amplifier

Smart Radio System Lab.

Conclusion

• Construct Test Bed using Connected Solution

- Complex test environment can be simple using the connected solution

- Effective and time-saving test environment

- Problem resolve easily and quickly through connecting MATLAB and ADS

- Available for DPD test for various signal and different method

• Digital Pre-distorted Linearization System

- Develop DPD algorithm with Memory Polynomial for WiBro base station GaN PA using MATLAB

- Construct DPD linearization system using ADS(Ptolemy) and Connected Solution Test Bed

- Conform the linearization performance about 12dB result from test for WiBro base station GaN PA

Smart Radio System Lab.

Reference

[1] Agilent Application Note 5989-8309EN

[2] Agilent Application Note 5988-6044EN

[3] Agilent Datasheet 5988-0697EN

[4] Agilent Configuration Guide 5989-1326EN

[5] Agilent Datasheet 5980-1284E

[6] Agilent Configuration Guide 5989-2773EN

[7] Agilent Technical Overview 5989-1679EN

[8] TTAS.KO-06.0098, “2.3GHz Radio Conformance Test Specification for 2.3GHz band

Portable Internet Service” Dec.2005

[9] J. Vuolevi, T. Rahkonen, J. Manninen, "Measurement technique for characterizing memory

effects in RF power amplifiers," IEEE Trans. Microwave Theory and Tech, vol. MTT-49, no. 8,

pp. 1383-1389, Aug. 2001.