Texas instruments linearization fundamentals driving digital pre distortion and the gc5322
Download
1 / 37

Texas Instruments Linearization Fundamentals Driving Digital Pre-Distortion and the GC5322! - PowerPoint PPT Presentation


  • 102 Views
  • Uploaded on

Texas Instruments Linearization Fundamentals Driving Digital Pre-Distortion and the GC5322!. April 2006. Agenda. Introduction and Impact Origin and History of the Problem Linearization Fundamentals Polynomial Power Amplifier Modeling Crest Factor Reduction Digital Pre-Distortion

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Texas Instruments Linearization Fundamentals Driving Digital Pre-Distortion and the GC5322!' - varana


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Texas instruments linearization fundamentals driving digital pre distortion and the gc5322

Texas InstrumentsLinearization Fundamentals Driving Digital Pre-Distortionand the GC5322!

April 2006


Agenda
Agenda

  • Introduction and Impact

  • Origin and History of the Problem

    • Linearization Fundamentals

    • Polynomial Power Amplifier Modeling

    • Crest Factor Reduction

    • Digital Pre-Distortion

  • System Implementation

    • Crest Factor Reduction and Digital Pre-Distortion

    • Adaptive Memory Pre-distortion of Power Amplifiers

  • Conclusions


Introduction and impact
Introduction and Impact

4G Cellular

&

WiMAX

3G – Digital

Wideband

Cellular

2G - Digital

Cellular

1G - Analog

Cellular

1980

1985

1990

1995

2000

2005

2010

2015

  • The demands for spectrally efficient modulation schemes have increased; however these schemes are subject to severe intermodulation distortion (IMD) when the power amplifiers (PA) are operated near saturation

  • Unfortunately, PAs are most efficient when operated near saturation

20MHz BW@ 2.1GHz

Super 3G

& 4G

10-40MHz BW

@ 2.5, 3.5 & 5GHz

WiMAX

.16a/d/e

5MHz BW@ 2.1GHz

WCDMA

Cellular Channel

BW @ Band

Increased signal bandwidth and complexity

1.25MHz BW

@ 1.9GHz

CDMA

2000

A big challenge forMCPA designers!

200kHz BW

@ 800MHz

EDGE/

CDMA

<=200kHz BW

@ 8-900MHz

TDMA/

GSM

30kHz BW

@ 800MHz

AMPS/

D-AMPS


Introduction and impact1
Introduction and Impact

  • High Power RF PA’s (>10W) use multiple driver stages to amplify an input signal.

  • Different PA architecture’s (Class A, AB, C, etc …) offer various degrees of linearity, cost and efficiency.

    • RF PA’s are notoriously inefficient – Air is a convenient but poor transmission medium.

  • RF PA’s are designed (tuned) for specific frequency range and bandwidth

    • MCPA ~= wideband RF PA, does not have to process multiple carriers

  • PA Gain is usually fixed – so pre-amps may be required to drive the PA input.

TX Board

Antenna

RFout =50dBm(100W)

RFin ==20dBm@ >800MHz

FromBaseband

PA

DUC

DAC

IF->RF

A

50 OhmTypicalInput

3 to 4 gain stages typical

If Gain = 30dB,

Pre-Amp


Introduction and impact2
Introduction and Impact

  • Linearization techniques allow a PA to be operated at higher power with minimal IMD increases, thus greater efficiency

  • Recent technological advances have made digital pre-distortion the focus of research efforts

  • Crest factor reduction (CFR) further increases the efficiency of the PA by reducing the peak-to-average ratio (PAR) of the transmitted signal

Theoretical Performance of Class AB PA


Origin and history of the problem
Origin and History of the Problem

1. Linearization Fundamentals

  • The trade-off between efficiency and linearity is the primary concern for PA designers

  • A PA operating at a high percentage of its power rating requires external linearization to maintain linearity

  • The linearization of the PA reduces back-off, thus increasing efficiency


Origin and history of the problem1
Origin and History of the Problem

2. Polynomial Power Amplifier Modeling

  • Accurate representation of the nonlinear effects in PAs is achieved using a polynomial expression, as follows

  • The coefficients represent the linear gain, and the gain constants for the quadratic and cubic nonlinearities

  • A system with memory (phase) versus memory effects (non-linearities)

  • Envelope and frequency memory effects


Origin and history of the problem2
Origin and History of the Problem

2. Power Amplifier Characterization

  • Two tone test is useful for measuring spectral regrowth in a nonlinear and memoryless system


Origin and history of the problem3
Origin and History of the Problem

2. Power Amplifier Characterization

  • Theoretically, only odd-degree nonlinearities generate in-band distortion products

  • The simplified polynomial PA model is expressed as follows


Origin and history of the problem4
Origin and History of the Problem

2. Power Amplifier Characterization

  • A PA is often characterized by its amplitude-amplitude and amplitude-phase transfer characteristics

  • The simple polynomial is unable to model AM-PM effects

  • Both AM-AM and AM-PM effects are represented by the complex baseband model

where


Origin and history of the problem5
Origin and History of the Problem

2. Power Amplifier Characterization

  • A simple case considering only 3rd degree nonlinearities in the AM-AM and AM-PM transfer characteristics is represented by the following

  • In the linear range, the PA can be characterized by the following

and


Origin and history of the problem6
Origin and History of the Problem

2. Power Amplifier Characterization

AM-AM Characteristic

AM-PM Characteristic


Origin and history of the problem7
Origin and History of the Problem

3. Crest-Factor Reduction

  • The DPD optimal performance depends greatly on signal characteristics

  • Multi-carrier signals can have a PAR as high as 13dB increasing the level of back-off to maintain acceptable IMD levels

  • The application of CFR allows the PA to operate at higher input/output power levels while maintaining linearity at the output of the PA

  • Achieved through pulse generation and digital clipping


Origin and history of the problem8
Origin and History of the Problem

3. Crest-Factor Reduction

  • Preferred PA bias point for a typical modulated signal


Origin and history of the problem9
Origin and History of the Problem

3. Crest-Factor Reduction

  • Preferred PA bias point for a CFR signal


Origin and history of the problem10
Origin and History of the Problem

4. Digital Pre-Distortion

  • Pre-distortion effectively performs a mathematical inversion of the Volterra PA model

  • The output of the pre-distortion processor is described by the following

  • The PA is linearized when


Origin and history of the problem11
Origin and History of the Problem

4. Digital Pre-Distortion

  • Digital pre-distortion (DPD) has become an effective linearization technique due to the renewed possibilities offered by DSP

  • Adaptive PD designs use feedback to compensate for PA variations

  • Look-up tables are updated to achieve optimal pre-distortion by comparing PD input to PA output

  • The PD function is expressed as a complex polynomial

where


Origin and history of the problem12
Origin and History of the Problem

4. Digital Pre-Distortion

  • Digital pre-distortion (DPD) requires feedback for sample-by-sample adaptation 5 times that of the signal bandwidth

  • Multi-carrier systems use signal bandwidths of up to 20MHz today, thus the feedback bandwidth must be 100MHz to compensate 3rd and 5th order IMD

  • Least-mean-square (LMS) is a popular gradient based optimization algorithm that requires wideband feedback


System implementation
System Implementation

1. Crest-Factor Reduction and Digital Pre-Distortion

  • The combination of CFR and digital pre-distortion were investigated

  • In this case, linearization was achieved with a traditional wideband feedback LMS algorithm

  • The CFR technique used was proposed by Texas Instruments using the GC1115 signal pre-processor

  • Four stages ensure that the output PAR is reduced to values from 5 to 8dB, as specified by the user

  • Performance results were compared using a Cree Microdevices 30W PA operating at 1.96GHz and a signal bandwidth of 1.25MHz

  • The PAR of the IS-95 signal was reduced from 9.6dB to 5dB


System implementation1
System Implementation

1. Crest-Factor Reduction and Digital Pre-Distortion

Complex Canceling Pulse


System implementation2
System Implementation

1. Crest-Factor Reduction and Digital Pre-Distortion

Corrected and uncorrected signal with canceling peaks and detection threshold


System implementation3
System Implementation

1. Crest-Factor Reduction and Digital Pre-Distortion

Typical Peak Detection and Cancellation through Pulse Injection

Cancellation Signal

Input Signal

Output Signal

-

+


System implementation4
System Implementation

X

PA

PC

1. Crest-Factor Reduction and Digital Pre-Distortion

  • Hardware Implementation of Wideband Pre-Distortion

Waveform Generator

Attenuator

Down-Converter

Analog

RF

Agilent 4432B

~20dB

DUT

Pre-Distorted Input Signal

LO

Tektronics TDS224 Oscilloscope

Analog

IF


System implementation5
System Implementation

1. Crest-Factor Reduction and Digital Pre-Distortion

ACPR improvement with respect to output power


System implementation6
System Implementation

1. Crest-Factor Reduction and Digital Pre-Distortion

  • The ACPR measurements were recorded according to specifications with a 30kHz marker at and offset of 885kHz

  • Results were limited by the performance limitations of the test bed

Power and efficiency improvement


System implementation7
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • The term memory effects refer to the bandwidth-dependant nonlinear effects often present in PAs.

  • These encompass envelope memory effects and frequency response memory effects.

  • Envelope memory effects are primarily a result of thermal hysteresis and electrical properties inherent to PAs.

  • Frequency memory effects are due to the variations in the frequency spacing of the transmitted signal and are characterized by shorter time constants.


System implementation8
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • Memory Polynomial Pre-Distortion Implementation

Where (K=7)

And (D=2)


System implementation9
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • Simulated Performance of Wideband Pre-Distortion

  • This traditional approach uses and LMS algorithm to adapt the PD coefficients on a sample-by-sample basis.

  • The memory PA model has D=1 (delay) and K=5 (order).


System implementation10
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • Simulated Performance of Wideband Pre-Distortion

  • The memory PA model is characterized by the following AM-AM and AM-PM curves


System implementation11
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • Simulated Performance of Wideband Pre-Distortion

    • DPD = 0: the LMS algorithm indicates an ACPL improvement of -3dB and an ACPH improvement of 3dB.

    • DPD = 1: the LMS algorithm indicates an ACPL improvement of -15dB and an ACPH improvement of -11dB.


System implementation12
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • Simulated Performance of Wideband Pre-Distortion

    • DPD = 2: the LMS algorithm indicates an ACPL improvement of -24dB and an ACPH improvement of -23dB.

    • DPD = 3: the LMS algorithm indicates an ACPL improvement of -24dB and an ACPH improvement of -20dB.


System implementation13
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • Hardware Implementation of Wideband Pre-Distortion

  • TI offers the complete high-performance signal chain including: DAC5687, CDCM7005, TRF3761, ADS5444, and TRF3703.


System implementation14
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

Typical Doherty Amplifier configuration and Performance Results


System implementation15
System Implementation

2. Adaptive Memory Pre-distortion of Power Amplifiers

  • Hardware Implementation of Wideband Pre-Distortion


Conclusions
Conclusions

  • CFR improves DPD performance

  • CFR uses EVM and ACLR to tradeoff for added efficiency

  • Depending on modulation schemes the relative percentages may vary

  • OFDM modulations are sensitive to EVM

  • 3GPP modulations are sensitive to ACLR

3GPP Relative Tradeoffs

OFDM Relative Tradeoffs

EVM

ACLR

EVM

ACLR

Efficiency

Efficiency


Conclusions1
Conclusions

  • Relative to a PA that operates normally under backoff, DPD adds additional hardware (cost) and system complexity to tradeoff for added efficiency

  • DPD can effectively remove the negative effects of CFR enabling even greater levels of efficiency

DPD Relative Tradeoffs

Cost

Complexity

DPD

EVM

ACLR

CFR+DPD

CFR+DPD

Efficiency



ad