Presentation on Adaptative Digital Pre Distortion of Power Amplifiers

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resentat on on apt ve g ta re stort on o

Power Amplifiers


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Seminar: Gain Wit hout PainNovember 2000

Adapt ive Digi t al Predist ort ion ofPower Ampli f ier s

Shawn StapletonAgilent Technologies1400 Fountaingrove Parkway

Santa Rosa, CA 95403


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Abstract

Linear modulation techniques possess good spectral eff iciency. However, their f luctuatingenvelopes in conjunction with nonlinear power amplifiers results in spectral spreading toadjacent channels. Linearization of the power amplifier by means of predistor tion is one meansof compensating for these nonlineari ties. One technique that is well suited to digital signalprocessing baseband implementations is adaptive digital predistortion. Adaptation is based onthe difference between the desired modulation and the actual power ampli fier output.

Biography

Dr. Shawn P. Stapleton has 17 years of experience in the design of RF and microwave circuitsand systems. He is presently professor of electr ical engineering at Simon Fraser University aswell as a consultant for Agilent EEsof. He has developed GaAs MMIC components, includingmixers, ampli fiers, frequency dividers and oscil lators. His most recent work includes digitalsignal processing, mobile communications and RF/microwave systems.


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Agenda & Topics

Introduction to Adaptive Digital Predistortion

Key Features: Digital Predistortion Techniques & Concepts

Digital Predistort ion Design Example

Conclusion

Digital Predistort ion of Power Ampli fiers

This section of the workshop provides an introduction to digital predistort ion. We wi llcover key features, technologies, and performance issues. Approaches to solving some of

the design challenges wi ll also be presented. An adaptive digital predistorter isdemonstrated using the Agilent Advanced Design System.


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Technology Overview

FeedForward Linearization

Based on inherently wideband technology

RF Predistortion

Limi ted accuracy of funct ion model

Implemented at RF wi th low complexi ty

Cartesian Feedback

Stabil it y considerations limit bandwidth and accuracy

LINC

Sensit ive to component drif t and has a high level of complexity

Dynamic Biasing

Limi ted ACI suppression Digit al Predistort ion

Limi ted Bandwidth (DSP implementation)

Good IMD suppression

Linearization approaches:

Of the various lineari zation techniques that have been developed, predistortion is themost commonly used. The concept behind predistor tion calls for the insert ion of a

nonlinear module between the input signal and the power amplif ier. The nonlinearmodule generates IM distortion that is anti-phase with the IM distortion produced by thepower amplifier, thereby reducing out-of-band emissions.

Feedforward linearization is the only strategy that simultaneously offers wide bandwidthand good IM distortion suppression. The price for this performance is higher complexity.

Automatic adaptation is essential to maintain performance.

RF-based predistortion offers reasonable IM distort ion reduction over moderatebandwidths.

Cartesian feedback is relatively less complex and offers reasonable IM distort ionsuppression, but stability considerations limit the bandwidth to a few hundred KHz.

The LINC technique converts the input signal into two constant envelope signals that areamplified by Class C amplifiers, and then combined, before transmission. Consequently,they are very sensitive to component drift.

Dynamic biasing is similar to predistortion, however the work function operates on thepower amplif iers operating bias.

Digital predistortion has two distinct advantages. First, the correction is applied beforethe power amplif ier where insert ion loss is less cri ti cal. Second, signif icant IMDreduction is achievable. Digital predistort ion techniques are more complex, but providebetter IM distortion suppression. However, bandwidths are low due to limi ted DSPcomputational rates.


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Digital Predistortion

ModemComplex Gain

Predistorter

D

A

C

QuadratureModulator

PowerAmplifier

QuadratureDemodulator

Adaptation

A

D

C

Local

Oscillator

Data

Digital Domain Analog Domain

The lineari zer creates a predistorted version of the desired modulation. The predistorterconsists of a complex gain adjuster that controls the amplitude and phase of the input

signal. The amount of predistor tion is control led by a look-up table (LUT) thatinterpolates the AM/AM and AM/PM nonlinearities of the power amplifier.

Note that the inputs in this adaptation process include a delayed version of the output andthe input signal. The input is delayed and then subtracted from the power amplifiersoutput signal. The dif ference should contain only the distor tion components.


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ModemComplex Gain

Predistorter

D

A

C

QuadratureModulator

PowerAmplifier


Adaptation

A

D

C

Local

Oscillator

Data


Spectrum at the Nodes

Given a two-tone input signal, we can observe the spectral response at various nodes inthe digital predistorter. The complex gain adjuster, once optimized, provides the inverse

of the nonlinear characteristics from the power ampli fier. Thus, the spectral growth fromthe predistorter can be observed at the input node to the power ampli fier. Ideally, the IMproducts wil l be equal in amplitude, but anti-phase to the IM products created as the twotones pass through the power amplif ier. The funct ion of the adaptation process is toquickly adjust the LUT entries, so that distortion is minimized.


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Design Techniques

Generic digital predistortion techniques

Complex vector mapping LUT

Complex gain LUT

Cartesian feedback

Generic adaptation techniques

Secant method

Linear convergence

Digital Predistortion

There are three distinct digital predistortion techniques.

The complex vector mapping LUT technique translates the input vector by adding an error

vector to compensate for the AM/AM and AM/PM distort ion.

The complex gain LUT approach mult ipl ies the input signal by a complex gain vectoroptimized and stored in the LUT. The LUTs index is the envelope of the input signal.

Cartesian feedback is another approach that does not require a LUT, but tends to be lessstable.

Adaptation based on the use of gradient signals requires a continuous computation toestimate the gradient of a three-dimensional power surface. The surface for the digitalpredistorter ci rcui t is the difference between the input signal and the scaled output signal.The adjacent channel interference power is minimized when this error signal i scompletely suppressed. The gradient is continually updated, so deliberate misadjustment

is not required. Two common gradient techniques are linear convergencea fi rst orderfeedback loopand the secant method, which is based on estimating the gradient usingNewtons classical method.


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Complex Gain LUT

D

A

C

Quadrature

Modulator

Power

Amplifier

Local

Oscillator


Modem

Data

phase

shift

F(x)

RAM


A

D

C

Delay Adjust

Delay

Ve(t)

|Vm(t)|

up-date

Vd(t)

Vm

(t)

Va(t)

The complex gain LUT technique is depicted in this slide. The input signal is mult ipl ied bythe gain signal derived from the RAM. This gain term is dependent on the input-signal

envelope, which is quanti zed to a finite number of entries (64 in this example). These entr iesare optimized by finding the difference between the input signal and the power ampli fiersoutput. The results should contain only the distortion, provided that weve established thefeedback delay. There are a number of techniques that operate in the time or frequencydomain that are available to adaptively compensate for this delay. Updating the RAM entriescan be accompl ished using approaches such as linear convergence or the secant method.


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Complex Gain LUT

Q

I

Quadrant of I-Q Plane

Ve(ti)

Vm(ti)

Va(ti)

rotation error

scaling error

VVdd( t ) =( t ) = VVmm( t ) F { |( t ) F { |VVmm( t ) | }( t ) | }

VVee( t ) =( t ) = VVaa( t ) -( t ) - VVmm( t )( t )

||VVee( t ) | =( t ) | = scal ing err orscal ing err or

VVee( t ) =( t ) = rot at ion err orrot at ion err or

The input signal vector generated from the modem is mult ipl ied by a gain function. The gainfunct ion is a complex quanti ty that i s dependent on the envelope of the input signal. This

dependence is because we only need to compensate for the AM/AM and AM/PM distortion ina power amplif ier. The gain function can be stored in the LUT in either polar coordinateform or rectangular form.

The LUT entries are derived from the resulting error vectors, which come from subtractingthe input signal from the power amplif iers output signal. The result is that the poweramplifiers distortion produces a scaling and rotation of the input vector.


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ADS RF Predistortion Simulation

Simulation Parameters:Simulation Parameters:

1) Multi -tone modulation (1) Multi -tone modulation (FcFc=800=800 MHz, BW=25 MHzMHz, BW=25 MHz))

2) 64-entry LUT2) 64-entry LUT

3) RAM write enable period is 0.43) RAM write enable period is 0.4 nsns

4) Linear convergence parameter is -0.14) Linear convergence parameter is -0.1

5) Behavioral model for power ampli fier5) Behavioral model for power amplifier

6) Ideal passive components assumed6) Ideal passive components assumed

Now well look at an example of a digital predistor ter simulation based on the complexgain LUT technique, carried out using the Advanced Design System. In this case, we

uti li ze the linear convergence technique to adjust the LUT entr ies to minimize the ACP.The adaptation coeff icient is set to -0.1 for fast optimization. We use a 64-entry LUT toquantize the input envelope. Passive components, such as the power spli tters andcombiners are assumed to be ideal. For demonstration purposes, we use a ten-tone inputcentered on 800 MHz, spanning a bandwidth of 25 MHz.


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ADS Digital Predistortion Circuit

LUT Clock TimingLUT Clock TimingLUT Clock Timing

InputInputInput

OutputOutputOutput

Here is the circuit schematic for the digital predistor ter as displayed in the Advanced DesignSystem. The adaptation technique is based on the linear convergence method, and the

rectangular implementation is used for the complex gain adjuster. The input consists of aten-tone modulation. Timing clocks are used to read and wri t t o the RAM.


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LUT for Digital Predistorter

In-Phase LUTIn-Phase LUTIn-Phase LUT

Quadrature LUTQuadratureQuadrature LUTLUT

I Error SignalI Error SignalI Error Signal

Q Error SignalQ Error SignalQ Error Signal

LUT I updateLUT I updateLUT I update

LUTQ updateLUTQ updateLUTQ update

The error signal derived from the dif ference between the input and output signals isscaled by the adaptation constant, and the result is latched in the data registers. The

index for the RAM is established by passing the input envelope through an A/D converter.The in-phase (I) and quadrature (Q) signals are stored in their respective LUTs.

The fixed-point summation provides the update for the new table entry based on theprevious value at the corresponding index.


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Optimized LUT Phase for Predistorter

Time ( 20 ns/div)Time ( 20Time ( 20 nsns/div)/div)

Ind

ex

In

dex

Signal EnvelopeSignal Envelope

PhasePhase

Radians

(0.

005/div)

Radians

Radians

(0.

005/div)

(0.

005/div)

These plots demonstrate the envelope for the ten-tone input signal and the correspondingLUT entries for the phase.


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Optimized LUT Gain for Predistorter

Signal EnvelopeSignal Envelope

Time (20 ns/div)Time (20Time (20 nsns/div)/div)

GainGain

Index

Ind

ex

Magnitude

Magnitude

These plots demonstrate the envelope for the ten-tone input signal, and the correspondingLUT entries for the magnitude. We observe that a nominal amount of gain is required to

compensate for the AM/AM compression that occurs because of the power amplif ier.


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Multi-Tone Simulation of Predistorter

-34 dBc-34 dBc

BeforeBefore

AfterAfter

Frequency (MHz)Frequency (MHz) Frequency (MHz)Frequency (MHz)

-54 dBc-54 dBc

dBmdBm

dBmdBm

The plot on the left shows the power amplif ier dr iven at 5dB back-off , which generateshigh levels of intermodulation power and high levels of harmonics. The plot at right

shows the output from the digital predistort er once the LUT entr ies have been optimized.We can observe the spectral growth that occurs using a digital predistorter. The adjacentchannel power is spread over a wider bandwidth, but the mask requirements can now bemeet.


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Summary

G The ADS digital predistorter design exampledemonstrates the performance that can be achieved withlinearization.

G

System level simulation provides a solid starting point forbuilding an implementation quickly.

G Designed components can be integrated into a system towi tness the impact on overall performance.

Design Solutions

Digital PredistortionG Adaptive digital predistorters have moved from the

research to the development phase.


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Presentation on Adaptative Digital Pre Distortion of Power Amplifiers

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