TEMPERATURE COMPENSATED DIGITAL PREDISTORTER FOR3G POWER AMPLIFIERS

Oualid Hammi and Fadel Ghannouchi*

Poly-GRAMES Research Center, Department of Electrical Engineering, Ecole Polytechnique,Montreal, Quebec, H3V lA2, Canada

* Intelligent RF Radio Laboratory, Department of Electrical and Computer Engineering,University of Calgary, Calgary, Alberta, T2N IN4, Canada

ABSTRACT

This paper proposes a new digital baseband predistorterarchitecture for 3G RF power amplifiers. The proposedarchitecture compensates for temperature drifts using asimple temperature sensoring feedback path coupled witha temperature dependant predistortion function. Theproposed predistorter is composed of a static look-uptable in parallel with an adjustment module thatimplements the temperature compensation function.Measured AMIAM and AM/PM characteristics of 100-Wpeak envelop power LDMOS power amplifier were usedto evaluate the performances of the proposed predistorter.The results show 15dB improvement in theintermodulation distortion level at the power amplifieroutput under a CDMA2000 SR3 excitation. Thisimprovement was maintained for amplifier's casetemperature variations between 25°C and 45°C.

1. INTRODUCTION

The RF power amplifier (PA) is one of the most criticaland expensive components in modem radio systems. Itsperformances are evaluated in terms of linearity andpower efficiency that have to meet stringentspecifications especially within the 3G wireless standard.Indeed, 3G signals have envelop varying waveforms withhigh peak to average power ratio that call for the use ofhighly linear PAs. To achieve the required linearityspecification, class AB amplifiers have to operate at largeback-off levels that result in very low power efficiency.Such power amplification stages will be oversized, bulkyand costly to manufacture. In order to improve the powerefficiency while keeping satisfactory linearityperformances, the PA has to be linearized. Three majorlinearization techniques have been reported in theliterature: feedback, feedforward and predistortion [1].Among these techniques and their numerous derivatives,the digital predistortion is perceived as the most valuablesolution since it offers the best trade-off betweenimplementation complexity and linearity and efficiencyimprovement.

Predistortion consists in using a linearizer upstream theamplifier. The linearizer's non-linear characteristics arecomplementary to those of the PAso that the cascade ofthe predistorter and the amplifier behaves as a linearamplification system. Consequently, the predistortion hasto be adaptive in order to compensate for the changes inthe PA characteristics due to aging, temperature drifts, ...Several adaptive predistortion techniques have beenproposed [2]-[5]. These techniques are mainly basedeither on the minimization of an error signal [2]-[3] or thereal-time modeling of the power amplifier [4]-[5]. Bothapproaches are powerful but require relatively complexfeedback loops.In this paper, we propose an innovative adaptive digitalpredistorter architecture suitable for temperature driftscompensation. This technique is based on offline PAcharacterization versus temperature. These measurementsare then used to synthesize a temperature dependantpredistortion function. In the first section, we present theexperimental characterization of the PA behavior fordifferent case temperatures. In the second one, weintroduce the proposed predistorter architecture. In thethird section, we present the simulation results anddiscuss the performances of the new predistorter.

2. POWER AMPLIFIER CHARACTERIZATION

A power amplifier for 3G base stations applications wasused in this work. The Class AB amplifier is composedof three amplification stages using LDMOS transistorsand operating around 2140 MHz. This PA line-up has asmall signal gain of 51 dB and its output power at PI dBis 100 Watts peak envelop power. The PA's non linearcharacteristics were measured for different casetemperatures varying from 25°C to 45°C and controlledusing a cold and hot plate of a thermal platform. ACDMA2000 SR3 signal having a chip rate of 3.6864Mcps and a peak to average ratio of 9.7 dB was used.For each case temperature, the AM/AM and AM/PMcharacteristics were extracted from the measured inputand output waveforms using the measurement setuppresented in figure 1. First, the baseband waveform of theinput signal was synthesized using Aglient's AdvancedDesign System (ADS) software and downloaded into thesignal generator. The corresponding RF signal was then

generated and fed to the PA. The resulting signal at theoutput of the amplifier was down-converted and its I andQ components extracted using a vector signal analyzer.After delay and gain adjustment, the I and Q componentsboth at the input and output of the power amplifier wereused to calculate the complex gain of the power amplifierversus the input power. These AM/AM and AMIPMcurves are presented in figure 2. For clarity purposes,only three of the five measured characteristics are shown.

Drivers Power Stage

3. PROPOSED PREDISTORTER

The proposed adaptive digital predistorter architectureuses the PA's case temperature to control and update thepredistortion function. As a result, the feedback path isgreatly simplified. The resulting transmitter architectureis presented in figure 3. The I and Q components of theinput signal are predistorted and digitally modulated.After the digital to analog conversion, the signal is up­converted before feeding the power amplifier. Atemperature sensor is used to track the PA's casetemperature variations and control the temperaturedependant adjustment function of the predistorter.

Temperature measurement

Figure 1. Measurement setup for PA characterization. Figure 3. Transmitter's architecture using the proposedpredistorter.

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The detailed block diagram of the proposed basebanddigital predistorter is shown in figure 4. This predistorteris made up with two parallel paths. The first one consistsof a static look-up table (LUT), while the second one is atemperature dependant adjustment function.First, the I and Q components of the input signal are usedto calculate the index of the static look-up table. ThisLUT is indexed using the square magnitude of the inputsignal for a higher resolution in the compression region.The predistortion function implemented in this staticLUT has been derived using the measured AM/AM andAM/PM characteristics of the power amplifier at thereference case temperature (25°C). The LUT index isalso used as an input for the temperature dependantadjustment function. Indeed, when the PA's casetemperature changes, this predistortion function adjuststhe static predistorter characteristics to fit those of theideal predistorter at the new case temperature. Thus, thisfunction is dependant not only on the PA's casetemperature but also on the input signal's power as anypredistortion function.The main objective of the temperature dependantpredistortion function is to compensate for the variationsof the PA characteristics due to temperature drifts. Sincethe PA non linearities are primarily expressed in polarformat (AM/AM and AMIPM characteristics), thetemperature dependant function is used to adjust thepredistortion coefficients in polar format. As aconsequence, the static LUT was designed to use thissame format. After adding the static LUT's predistortioncoefficients and those of the adjustment function, a polarto rectangular conversion is applied to get the correctioncoefficients (Ic and Qc). These coefficients are applied tothe input I and Q components to get the predistortedoutput signal.

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Figure 4. Block diagram of the proposed baseband digitalpredistorter.

4. RESULTS AND PERFORMANCEEVALUATION

First, the ability of the proposed temperature dependantpredistortion function to track the changes in the PA'scharacteristics was investigated. As a first attempt, weused an adjustment function that is linear with respect tothe case's temperature variation for both the gain and thephase predistortion functions. Figure 5 presents theAM/AM and AMIPM curves of the proposed predistorterand those of the ideal predistorter for case temperaturesof 35°C and 45°C. This figure also shows the AM/AMand AMIPM curves of the static LUT (without anyadjustment function) derived for a case temperature of25°C. For a given case temperature, the ideal predistortercurves are those that are directly derived from themeasured PA characteristics at that temperature.According to this figure we can conclude that as thecase's temperature drifts away from its nominal value(25°C), the predistortion function implemented using thestatic LUT is not optimal any more. However, theproposed temperature dependant function adjusts theAM/AM and AMIPM curves of the static LUT lito fitthose of the ideal predistorter.

The proposed predistorter has been simulated alongwith the behavioural model of the PA using ADSsoftware in order to evaluate its performances in terms ofintermodulation distortions reduction. For a casetemperature of 35°C, the non linear and the linearizedspectrum at the power amplifier's output are presented infigure 6. One can observe a linearity improvement ofmore than 15dB. Moreover, unlike the spectrum obtainedbefore linearization, the linearized output spectrum iscompliant with the spectrum emission mask. Thesecurves were obtained using a CDMA2000 SR3 inputsignal with a peak to average ratio of 9.7dB and theamplifier was operated with 14dB back-off from itssaturation.

5. CONCLUSION

This paper proposes a new architecture for adaptivebaseband digital predistortion suitable for temperaturedrifts compensation in 30 power amplifiers. Theproposed predistorter uses a static LUT coupled with a

temperature dependant adjustment function that tracksthe PA's behaviour changes due to temperature drifts.The validation of the predistortion architecture wascarried on measured characteristics of an LDMOS 100Watts power amplifier using a CDMA2000 SR3 signal.The results demonstrate the ability of the predistorter tocorrect for the temperature drifts and a linearityimprovement of 15dB was also obtained.

6. REFERENCES

[1] F. H. Raab, and aI, "Power amplifiers and transmitters forRF and microwave," IEEE Trans. Microwave Theory & Tech.,vol. 50, no. 3,pp. 814-826, Mar. 2002.

[2] Y. Nagata, "Linear amplification technique for digitalmobile communications," in Proc. IEEE Veh. Technol. Conf.,San Francisco, CA/USA, May 1989, pp. 159-164.

[3] 1. K. Cavers, "Amplifier linearization using a digitalpredistorter with fast adaptation and low memoryrequirements," IEEE Trans. Veh. Technol., vol. 39, no. 4, pp.374-382, Nov. 1990.

[4] E. G. Jeckeln, F. M. Ghannouchi, and M. A. Sawan, "A newadaptive predistortion technique using software-defined radioand DSP technologies suitable for base station 3G poweramplifiers," IEEE Trans. Microwave Theory and Techn., vol.52, no. 9,pp.2139-2147,Sep. 2004.

[5] S. Boumaiza, and F. M. Ghannouchi, "Realistic power­amplifiers characterization with application to baseband digitalpredistortion for 3G base stations," IEEE Trans. MicrowaveTheory and Techn., vol. 50, no. 12, pp. 3016-3021, Dec. 2004.

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