Predistortion Techniques

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Predistortion Techniques

• Jonne Lindeberg
• jlindebe_at_ecdl.hut.fi

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Outline

• Introduction
• RF and IF predistortion
• Adaptive (BB) Predistortion
• Digital (BB) Predistortion
• Conclusions, References, and Homework

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Introduction

• Simplest method for RF PA linearization
• RF predistortion
• IF predistortion
• Baseband predistortion

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RF predistortion

• Traditional systems gt modest improvement (5 to
  15dB) compared to feedforward systems or
  cartesian feedback
• Predistortion depends on the behavior of the PA
• How to fabricate circuit with inverse
  characteristics
• Usually only odd order distortion precompensated
• Simplest and most widely used method is to
  predistort the 3rd order characteristics
• If higher order predistortion needed,
  predistoriting elements need to be designed
  individually for each PA or at least tuned

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Cubic predistorters

• Aim to eliminate third-order distortion to same
  level with higher order (usually 5th)
• Good in TWT amplifiers, because these usually
  generate third order distortion
• Beneficial in BP systems
• Modest degrees of of linearity improvement
• Poor matching between the predistorter and PA
• Increase the level of higher-order distortion

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Cubic predistorters
-Require high degree gain and phase matching
where ?A is amplitude matching and ?? phase
matching error
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Cubic predistorters
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RF predistorters
-Series diode is the simplest predistorting
nonlinear element -Schottky diode with separate
parallel capacitor (Cp) -Adjustment with bias
resistor Rbias -ACPR improvement of 4dB for
IS-95 CDMA have been reported at 1.96GHz
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RF predistorters
-FET source drain channel used for predistorting
element -Wide variety of configurations 1
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IF predistortion
-Same predistorter can be used for different
operating frequencies -Predistorter
significantly simpler than RF -Narrower band
predistortion than RF Some methods Curve-fittin
g predistorters Digital IF Cubic Complex Adap
tive Control Predistortion (plus several others)
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IF predistortion
DIGITAL IF -Relatively little used due to
complexity and speed requirements to achieve
reasonable BW -High power consumption -Improvement
s in device technologies will increase
attractiveness -Similar to adaptive
baseband -Reported 3.rd IMD improvement of 25dB
over 45MHz (limited by the DAC and ADC speed and
resolution)
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IF predistortion
CUBIC IF PREDISTORTION
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IF predistortion
COMPLEX IF PREDISTORTION -Complex predistortion
attempt to compensate the AM/AM and AM/PM
characteristics by operating in I/Q mode (upper
Fig.) or amplitude phase mode (lower Fig.)
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IF predistortion
ADAPTIVE IF PREDISTORTION Adjacent-channel method
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IF predistortion
ADAPTIVE IF PREDISTORTION Correlation-based
control
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RF and IF predistortion

• Efficiency of Predistortion Techniques
• The class of power amplifier to be linearized.
• The power consumption of the predistorter.
• The degree of output back-off necessary to meet
  required specification.
• The envelope characteristics of the signal to be
  amplified.
• In all cases, the efficiency of the predistorter
  system should be higher (usually significantly
  so) than the efficiency of a backed-off amplifier
  producing the same level of intermodulation
  distortion (or signal vector error, if this is
  more important in a given application).

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RF and IF predistortion
EXAMPLE Diode based RF predistorter
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RF and IF predistortion
EXAMPLE Diode based RF predistorter measurement
s with several modulation formats and BW.
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RF and IF predistortion

• Advantages and Disadvantages
• Simplicity of implementation
• few components gt low cost
• no stability problems (compare to FB)
• can be used in microwave region
• wide linearization BW
• - modest linearity improvement
• - difficulties with high order distortion
• gt increase higher order distortion
• - fabricate circuit with a desired transfer
  characteristic

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Baseband Predistortion

• Easier to fabricate (than the RF or IF)
• If complex versions of the signal constructed gt
  broadband audio phase shift required to create I
  and Q may prove difficult whit out Hilbert
  transform filter in DSP
• If Hilbert used gt the predistortion in DSP also
• Attractive since modern radio transmitters employ
  some DSP in BB

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Adaptive (BB) Predistortion

• Long been a promising technique for narrowband
  linearization
• Quadrature signals for both up- and
  downconversion gt gain and phase correction

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Adaptive (BB) Predistortion
Quadrature BB predistorter
-LUTs accessed by algorithms which receives its
input from downconverter -Coefficients may be
updated -Drawback gt power consumption of the
ADC, DAC and LUT memories -Power efficiency poor
until tech. is sufficiently advanced
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Adaptive (BB) Predistortion
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Adaptive (BB) Predistortion
Sampling rate considerations -Since most signals
I/Q components gt FsBimd -Bimd is the full BW
of the RF channel including all significant
orders of distortion
-Reduction of Fs gt reduce the level of the of
higher order IMD -Loop delay must be considered.
Loop delay is dominated by the D/A converter
reconstruction filters which change with
temperature
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Adaptive (BB) Predistortion

• Quantization and Resolution
• Source (input) signal
• Calculation of the table index
• The LUT coefficient
• The modulator error correction process

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Digital (BB) Predistortion 2

• Adaptive Linearization of PA in Digital Radio
  Systems, Saleh and Salz, 1982.
• Linearizer utilizes a real-time data-directed
  recursive algorithm for predistorting the signal
  constellation
• Predistortion accomplished within a digital
  memory, which is used to generate the desired
  baseband signal

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Digital (BB) Predistortion 2
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Digital (BB) Predistortion 2

• LUT and DAC have to operate full signaling rate
• ADC and linearizing processor can operate much
  reduced rate since updating is needed for
  compensating drifts that occur much lower rate
• System assumes that the amplifier is memoryless
  gt No filtering inside the loop gt not good

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Digital (BB) Predistortion 3

• Linear Amplification Technique for Digital
  Mobile Communications, Nagata, 1989.
• Suitable for Narrow band system
• Linearization by adding a Predistorting signal to
  BB modulating signal
• Adapts to the fast and frequent changes of
  nonlinearity
• Power consumption can be reduced small enough for
  use in mobile station

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Digital (BB) Predistortion 3
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Digital (BB) Predistortion 3
Simulation results
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Digital (BB) Predistortion 4

• A GSM-EDGE High Power Amplifier utilising
  Digital Linearisation, Kenington, Cope, Bennett,
  and Bishop, 2001.
• Method of improving both efficiency and signal
  vector error
• Simple third-order linearizer will not achieve a
  sufficient level of mask improvement for EDGE

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Digital (BB) Predistortion 4

• Input output comparison made in frequency domain

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Digital (BB) Predistortion 4
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Digital (BB) Predistortion 5

• Digitally Controlled RF input/output
  Predistortion for A Class AB Power Amplifier,
  Teikari, Vankka, and Halonen, 2003.
• Digitally controlled analog predistortion system
• Linearizes an class AB PA
• Signal is 16-QAM, with 18kHz BW around 420MHz

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Digital (BB) Predistortion 5
-Phase and amplitude detection -Comparison made
in time domain
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Conclusions

• In RF and IF predistortion problem is low
  improvement
• In digital predistortion problem becomes the
  power consumption
• Predistortion methods interesting in the future
  with the development of different technologies

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References

• 1 Peter B. Kenington, "High-Linearity RF
  Amplifier Design, Chapter XXX.
• 2 A.A.M. Saleh and J. Salzm, Adaptive
  Linearization of PA in Digital Radio Systems,
  BSTJ, Vol. 62, No. 4, April 1983.
• 3 Yoshimori Nagata, Linear Amplification
  Technique for Digital Mobile Communications,
• 4 P. B. Kenington, M. Cope, R. M. Bennett, and
  J Bishop, A GSM-EDGE High Power Amplifier
  utilising Digital Linearisation.
• 5 I. Teikari, J. Vankka, and K. Halonen,
  Digitally Controlled RF input/output
  Predistortion for A Class AB Power Amplifier,
  URSI 2003.

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Homework

• Your PA has 3rd. IMD at level 30dBc. Using RF
  cubic predistortion with amplitude error of 0.5
  dB and phase error of 1 degree respect to ideal
  case. What is the theoretical maximum of 3rd. IMD
  result you can achieve with these parameters?
• Your job is to choose a linearization method for
  two different transmitter. What method would you
  prefer and WHY?
• You need to transmit 4 WCDMA carriers (20MHz BW),
  and your transmitter chain is 3dB over
  specifications (ACPR)?
• You need to transmit GSM carrier and the signal
  does not fit to the mask at the antenna (3rd. and
  5th. order distortion causes signal to rise 20dB
  over mask limit)
• (Hint. There are no correct solutions. Just
  explain your desicions.)