Practical Implementation of Adaptation in a Digital Predistorter for RF Power Amplifiers

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Practical Implementation of Adaptation in a
Digital Predistorter for RF Power Amplifiers
IEEE Topical Workshop on Power Amplifiers for
Wireless Communications September 9 and 10, 2002
San Diego, CA

• A. Cesari, D.Dragomirescu, P. Lacroix, J.M. Dilhac

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INTRODUCTIONNon Linearity of the Power Amplifier
LPF
I
Q
16-QAM signal
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INTRODUCTIONNon Linearity of the Power Amplifier
LINEARISATION MECHANISM
Analog/Digital Adaptative or not Baseband/RFBand
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LINEARISATION STRATEGIESDigital Adaptative

• Digital
• use of DSP chips
• Low cost
• Low power consumption
• Small size
• NEWER APPROACH

• Adaptative
• a feedback path exists
• Generic approach
• Intended for low development time
• Ability for self training
• Ability to account for changes in the system and
  consequent adaptation

? DEALING WITH FEEDBACK ? BETTER PERFORMANCES
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LINEARISATION STRATEGIESGeneral Predistortion
Strategy
Output signal
Forward signal
Desired, linear characteristic
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LINEARISATION STRATEGIESGeneral Predistortion
Strategy
Well known techniques
or
or
or
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LINEARISATION STRATEGIES Adaptive Digital
predistortion

• Aging, temperature drifts
• Changes in work frequency
• Part to part variation, load mismatch

• Need for perfect synchronisation
• Tradeoffs Bandwidth, fsample, complexity

z
x
y
fPD(x)
gPA(y)

O

adaptation
comparison
Feedback signal
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COMPARISON ADAPTATIONSample-by-Sample basis
FORWARD signal
x
adaptation
Calculate fPD(x,z,e)
error signal
comparison

-
z
FEEDBACK signal
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COMPARISON ADAPTATIONOverview of Delays
delay
fsx
fsx
xi,xq
yi,yq
Mod
Nyquist filter
PD
DAC
zi,zq
DeMod
ADC
FIFO
fsz
delay kTsz ? k 0,1,2, ? lt Tsz/2
After digital delay estimation
Residual delay ?
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COMPARISON ADAPTATIONExample
Worst case delay, ? Tsz/2
FORWARD signal
error
FEEDBACK signal
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COMPARISON ADAPTATIONParameters of Interest
1 . Fsx sampling rate at FWD path, digital
stage As high as possible, BW of PreDistortion 2
. Fsz sampling rate at feedback path, digital
stage As high as possible, accuracy of delay
error 3 . SSx samples/symbol at Tx, digital
stage As high as possible accuracy of delay
error But if SSxs up, Fsx needs to be
incremented by the same amount in order to
maintain BW!
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COMPARISON ADAPTATIONOur proposal Multirate
DSP
Keep Fsx as high as possible Keep SSx as low
as possible (i.e. IS-95, SSx 4) Increment Fsz
oversampling
Feedback signal
FWD signal
fsz 2fsx
amplitude
amplitude
samples
samples
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COMPARISON ADAPTATIONMultirate DSP

• Objective Study the impact of
• oversampling multirate DSP
• over the delay error magnitude

Experience 1 measure of the (worst case) delay
error along the Dynamic Range of a QAM
signal Experience 2 improvements after
oversampling
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COMPARISON ADAPTATIONExperience 1

• Measure of the (worst case) delay error
• along the Dynamic Range of a 16-QAM signal

Px - Pz
Delay kTsz ?
x
?
Delay ? Tsz/2
Px
FORWARD signal
digital
_at_ fsx fsz
_at_ fsz
z
comparison

-
Digital Delay adjust
FEEDBACK signal
Worst Case sampling
rms2
rms2
analogic
Px
Pz
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COMPARISON ADAPTATIONExperience 1
fsx fsz 4 SSx
fsx fsz 16 SSx
Px - Pz
Px - Pz
Px
Px
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COMPARISON ADAPTATIONExperience 2

• Oversampling Multirate
• Keep the samples/symbol ratio at minimum
• fsz gt fsx at ADC DAC stages

Technique 2A drop samples from the higher rate
source (Z) to match the samples of X Technique
2B true multirate, smoothing through average
calculation in the domain of Z
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COMPARISON ADAPTATIONExperience 2
Technique 2A drop samples from the higher rate
source (Z-feedback signal) to match the samples
of X-forward signal
It results in exactly the same (poor)
improvement obtained by increasing both fsx and
fsz. Why? fsz sets the limit ? Tsz/2 in spite
of Tsx
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COMPARISON ADAPTATIONExperience 1

• Technique 2B average filtering (by convolution)
  in the domain of
• the oversampled signal, then matching with the
  signal X-FWD signal

Px - Pz
Delay kTsz ?
x
?
Delay ? Tsz/2
Px
FORWARD signal
digital
_at_ fsx lt fsz
_at_ fsz
comparison
z
Digital Delay Adjust DROP

-
FEEDBACK signal
Worst Case sampling
Avg. filter
rms2
rms2
analogic
Px
Pz
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COMPARISON ADAPTATIONExperience 2
Oversampling x 2, 4 SSx
Px - Pz
Px
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CONCLUSIONS FUTURE WORK

• Adaptive digital predistortion
• Feedback synchronisation problem ? Analog delay
• Traditional solution high rate samples/symbol
• Drawbacks
• ? decreased transmission bandwidth
• ? incompatibility with 3rd generation wireless
  telecommunication standards

Our proposal oversampling, multirate,
average filter Future work physical
implementation on a DSP system