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4G For EveryoneExtended RF Performance with Digital Pre-Distortion

Live webinar presented June 17, 2010http://www.agilent.com/find/eesof-systemvue

Copyright Agilent Technologies 2010

1

Dr. Jinbiao XuAgilent TechnologiesMaster System Engineer

John BaprawskiRF/MW System Consultant

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1. Introduction and Problem Statement

2. Digital Pre-Distortion (DPD) Concepts

3. Example using DPD

4. Crest Factor Reduction (CFR) Concepts

5. Example using CFR

6. DPD Hardware Implementation

7. Using X-Parameters with PA and DPD

8. Practical Considerations

9. Summary

Agenda

Copyright Agilent Technologies 2010SystemVue DPD webcast

Jinbiao XU

2 6/17/2010

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3

Improved productivity throughmodel-based design

Provides top-down ESL cockpit forRadar design

Unites Algorithm & Baseband withAgilent leadership in RF, Test, and

Radar IP for superior cross-domain effectiveness earlier design maturity higher performance, lower margins

About Agilent SystemVue

For system architects and baseband algorithm developers

3

SystemVue DPD webcastJinbiao XUCopyright Agilent Technologies 2010

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SystemVue The shortest path from imaginationto hardware for communications & defense

Easily assemble Virtual PHYs

Quickly move Ideas to proven,real-world HardwareWiMAX

DVB-S2ZigBeeOFDM

DPDRADARMIMO ChannelmmWave WPAN

HARDWARE &

MEASUREMENTS

REFERENCE

IP & APPS

BASEBAND

ALGORITHMS & IP

ACCURATE RF &

CHANNEL EFFECTS

SystemVue DPD webcastJinbiao XUCopyright Agilent Technologies 2010

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Modern communication systems:

Signals have high peak-to-average power ratios (PAPR). Must operate with high power-added efficiency (PAE).

High PAPR is a consequence of high spectral efficiency. Multiple-Carrier Signals (MC GSM, MC WCDMA) CDMA (WCDMA, CDMA2000) OFDM (LTE, WiMAX)

High PAE is achieved when the RF power amplifier (PA) is driventowards saturation.

Operation near saturation inherently results in higher signal distortion.

Problem Statement


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Conflicting requirements

Problem Statement


Jinbiao XU6

How to handle signals with high PAPR, while driving thePA to operate with high PAE, while also having lowsignal distortion?

HighPAE

Signalswith high

PAPR

HighSpectral

Efficiency

HigherDrivelevels

HighDistortion

levels

Back offthe drive

levels

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High-efficiency PA design typically relies on :

Constraining the PA energy into a bandpass characteristic Peaking the PA energy by dynamically biasing the PA versus input

power

These techniques inherently result in PA nonlinearities with memory

Techniques used to reduce distortion when operating at high efficiency:

PA linearization enables driving the PA closer to saturation for agiven level of distortion

Signal preconditioning Reduces signal peaks without significant

signal distortion

Problem Statement



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Solution Approach


Jinbiao XU8

Solution: Preconditioning the signal (CFR) andcorrectingfor the hardware (DPD) will both be discussed in thispresentation

HighPAE

Signalswith high

PAPR

HighSpectral

Efficiency

HigherDrivelevels

HighDistortion

levels

Higherthroughputlevels for

subscribers

CFR

DPD

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Two key power amplifier characteristics

AM-to-AM Change in output power gain (in dB) versus input power,

relative to small signal gain AM-to-PM

Change in output phase (in degrees) versus input power,

relative to small signal conditions

Without memory effects There is a one-to-one relationship between the AM-to-AM and

AM-to-PM values vs. input power

With memory effects There are multiple AM-to-AM, AM-to-PM values associated with

input power due to the PA memory and the previous signal values

Definitions



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Todays emerging 4G signals use spectrally efficient formats withhigh PAPR and require PAs with high PAE The highest performance modes of these standards are sensitive to

PA memory effects

AM-AM characteristics for a PAWith and without memory effects

SystemVue DPD webcastJinbiao XU

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AM-to-AM for PA with memoryAM-to-AM for PA without memory


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9. Summary

Agenda



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DPD enables operating the PA in its high PAE region near saturation

without significant signal distortion.

The DPD approach discussed here addresses PAs with memory Realistic training signals are used Actual PA output is measured The DPD network pre-distorts the PA input signal so that

combined [DPD+PA] behaves more linearly

Based on Agilent Technologies Measurement Research Labs OFDM MIMO research drove the need for DPD development Agilent Multi-Carrier Power Amplifier (MCPA) test system

Custom set-ups for high power basestation PAs

Digital Pre-Distortion (DPD) Concepts



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1. Characterize DUT PA using DUT input and

output waveforms

2. Calculate complex memory coefficients usingQR and SVD algorithms

3. Apply memory polynomial coefficients toconstruct memory polynomial pre-distorter

4. Pre-distorted signal cancels nonlinear distortionin PA (including major memory effects)

Steps to Achieving Digital Pre-distortion (DPD)



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Theory of Operation Desired Pout

Uncorrected PA:

Pout vs. Pin curve saturates at Psat (in dBm)

DPD+PA:Pout-pd vs Pin curve is linear, up to a higher limit,

where Max(Pin

) is the maximum correctible input power Linear Output

Input PowerPin Pin- d

Output Power

PoutPout- d

DesiredOutput Saturation

Psat

Max Correctable Pin



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Theory of Operation Operating Region

o- d = kout

Input Power

Output Phase

Pi Pi-pd

Desired OutputLinearOutput



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Memory Polynomial Algorithm

As the signal (such as 3GPP LTE) bandwidth gets wider, power amplifiersbegin to exhibit memory effects. Memoryless (LUT) pre-distortion can achieveonly very limited linearization performance.

Volterra series is a general nonlinear model with memory. It is unattractive forpractical applications because of its large number of coefficients.

Memory polynomial reduces Volterras model complexity. It is interpreted as a

special case of a generalized Hammerstein model. Its equation is as follows:

Kis Nonlinearity order and Qis Memory order

Copyright Agilent Technologies 2010 SystemVue DPD webcastJinbiao XU16

= =

=

K

k

Q

q

k

kq qnyqnyanz1 0

1)()()(

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Signal Training to derive the Memory Polynomial

1. Pre-distorter training: Nonlinear coefficients are extracted from the

PA input and PA output waveforms (ie on real physical behavior)

2. Copy of PA : The DPD model accurately captures the nonlinearitywith memory effects


zUUUaHH 1

)(

=

[ ]TNzzzz )1(,),1(),0( = KQQK uuuuU ,,,,,, 1010 =[ ]

T

kqkqkqkq Nuuuu )1(,),1(),0(=

1)()(

)(

=

k

kqG

qny

G

qnynu

Memory Polynomial Coefficients

[ ]TKQQK aaaaa ,,,,,, 1010 =

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9. Summary

Agenda

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5-Step Measurement-Based DPD Modeling Flow

1. Create DPD Stimulus

2. Capture PA Response

3. Extract DPD Model

4. Capture DPD+PA Response

5. Verify DPD+PA Response


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DPD Hardware Verification Platform

10MHz Reference

10MHz Reference

External Trigger

External Trigger

Attenuator

(20dB)

1. Instrument setup to capture PA input signal

2. Instrument setup to capture PA output signal

Agilent E4438C Agilent E4440A

Agilent E4440AAgilent E4438C


Thru measurement

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SystemVue DPD Hardware Flow for LTE

The download power and length ofthe waveform can also be set.


Step 1. Create DPD stimulus waveform LTE parameters such as bandwidth, Resource Block allocation and others

can be set

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Connect the ESG directly to thePXA and click the Capture

Waveform button. The capturedsignal is the input of the PA.

Connect the ESG to the PA,connect the PA to the PXA, andclick the Capture Waveformbutton. The captured signal is theoutput of the PA DUT.

The measured I/Q files arestored and used in followingsteps.


Step 2. Capture PA response SystemVue interfaces directly to the ESG (source) and PXA (analyzer). Instrument parameters such as number of signal, trace assignment and file

name can be set


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DPD AM-to-AM Characteristic



Step 3. DPD Model Extraction DPD model parameters such as number

of training samples, memory order andnonlinear order can be set

PA AM-to-AM Characteristic

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Set the RF power



Step 4. Capture DPD+PA Response The signal is predistorted by the DPD model and downloaded into the ESG.

DPD+PA output (blue) andoriginal raw signal (green) isdisplayed

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Step 5. Verify DPD+PA response LTE performance for the DPD model used with the PA hardware is verified

Spectrum, EVM andACLR are calculatedand plotted automatically


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Spectrum:

ACLR (dB): DPD-PA restores performance ~to the original ACLR.

EVM: DPD-PA output signal has same (-26.8 dB) as input signal

LTE Results for PA with Memory


ACLR -2BW

Lower

-1BW

Lower

+1BW

Upper

+2BW

Upper EVM (dB)Raw signal 127.35 103.75 103.66 127.30 -26.80

PA output 51.08 38.90 38.96 51.15 -24.97

DPD-PA output 84.65 80.07 79.96 84.62 -26.78

RED - PA output

BLUE - DPD+PA outputGREEN Original raw signal

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GSM Results with 4-Carriers for PA with Memory4-Carrier GSM,30 MHz Bandwidth

No DPD

DPD+PANonlinearOrder=9.

DPD+PANonlinearOrder=11.


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9. Summary

Agenda


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Spectrally efficient wideband RF signals may have PAPR >13dB. CFR preconditions the signal to reduce signal peaks without significant

signal distortion CFR allows the PA to operate more efficiently it is not a linearization

technique CFR supplements DPD and improves DPD effectiveness Without CFR and DPD, a basestation PA must operate at significant

back-off from saturated power to maintain linearity. The back-offreduces efficiency

Benefits of CFR1. PAs can operate closer to saturation, for improved efficiency (PAE).

2. Output signal still complies with spectral mask and EVMspecifications

Crest Factor Reduction (CFR) Concepts


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Crest Factor Reduction (CFR) Concepts

If you can reduce the Peak-to-Average Ratio of the signal, then for a givenamplitude Peak, you can raise the Average power (up & to the right, above)with no loss in signal quality.

Thus, CFR enables higher PA efficiency by reducing the back-off, often by 6dB


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Crest Factor Reduction for Multiple-Carrier Signals

Multiple-Carrier Signals (such as GSM, WCDMA, WiMAX) already have highPAPR.

In the future, they will also include multiple waveforms (ie - LTE with 3G WCDMA). Therefore CFR will increase in importance for Multi-Carrier PA (MCPA)

linearization.


CFR algorithm for multiple carrier signals PW (Peak Windowing)-CFR NS (Noise-Shaping) -CFR PI (Pulse Injection)-CFR PC (Peak Cancellation)-CFR

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CFR for 3GPP LTE DL OFDM Signal

Controls EVM and band limits in the frequency domain. Constrains constellation errors, to avoid bit errors.

Constrains the degradation on individual sub-carriers. Allows QPSK sub-carriers to be degraded more than 64 QAM sub-

carriers. Does not degrade reference signals, P-SS and S-SS. All control channels (PDCCH, PBCH, PCFICH and PHICH) adopts

QPSK threshold.


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9. Summary

Agenda


WCDMA E l ith 4 C i

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CFR Clipping CFR Filtering

WCDMA Example with 4-Carriers


WCDMA E l ith 4 C i

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WCDMA Example with 4-CarriersSimulation results


With CFR

With CFR

PAPR With CFR PAPR No CFR5.89dB 9.49dB

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LTE Downlink

10MHz BW Sampling Rate=61.44MHz QPSK EVM threshold < 10%

LTE Example with Crest Factor Reduction


CFR Design with

Clip and Filter

PAPR With CFR PAPR No CFR6.867dB 9.05dB

CCDFwith CFR

CFR causes

little ACLRdegradation

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9. Summary

Agenda


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Real-Time SystemVue DPD

SystemVue for DPD Investigation / Design

Integrated algorithm design, validation and real-time implementation solution

HDL, C++ code generation available

Shortens time to market for DPD designs

FPGA implementation challenges

High FPGA computational resource requirement

High numerical range for model coefficients

10-20 ~ 1020

High requirement on accuracy, # bits

High computation load

10s~100s GFLOPS Additional implementation complexity if

accommodating different PAs

Real-timeDPD IP

FPGA(Real-time DPD)

Basebandsignal

RF card

DPD hardware board

DPD algorithmdesign

DPD real-timeimplementation

DPD verification


R l Ti S V DPD IP10

DPD gain improvement

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Real-Time SystemVue DPD IP

Generic FPGA core with customizedparameters to shorten time to market

and R&D cost

Variable memory order & nonlinearorder to cover different PAs

Variable FPGA footprint depending on

model orders Accurate real-time implementation

needed to achieve the sameperformance as simulated DPD

About 20dB ACLR improvement

depending on the PA

-60 -50 -40 -30 -20 -10 0 10-15

-10

-5

0

5

Pout [dBm]

Pin/Pout[dB]

SystemVue

FPGA

No DPD (red),DPD in SystemVue (green)Real-Time DPD FPGA (blue)


DPD AM-to-AMSystemVue (blue)FPGA (red)

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A d

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9. Summary

Agenda


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X-Parameters for System Design


Accurate: X-parameters are the mathematically correct superset of S-

parameters

Versatile: Applicable to both small and large signal conditions for linear andnonlinear components vs. Power, Freq, Bias, Zload, etc

Design flow closure: Brings finished RF designs back up to the architecturelevel, and makes the IP safely transportable

Measure and view X-parameterswith the Agilent NVNA and ADS

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Using X-Parameters with SystemVue DPD


X-parameters (Version 2 and earlier) accurately models frequency-domain nonlinearities, but not nonlinearity memory effects

Research to add nonlinearity memory effects to X-parameters iscurrently in development at Agilent (keynote IMS 2009 paper)

SystemVue DPD can work directly on the current generation of X-parameters and P2D type models

Set the DPD model memory order = 0

Gives quick estimate of achievable system performance

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EVM (dB)

Frame 1

EVM (dB)

Frame 2PA input -26.016 -26.016

Raw PA output -25.810 -25.807

DPD-PA output -26.013 -26.013

LTE DPD Simulation with X-parameter PA (1W)Spectrum, ACLR and EVM results


SystemVue DPD webcastJinbiao XU

43

EVM (dB)

-2BW

Lower

-1BW

Lower

+1BW

Upper

+2BW

UpperPA input 71.01 64.34 63.43 70.84

Raw PA output 65.39 49.06 49.28 65.33

DPD-PA output 67.47 62.84 62.20 67.34

ACLR (dB)

Low power PA (1 W) X-parameters were measured with

Agilent NVNA Same SystemVue DPD flow used for

these LTE spectrums, EVM and

ACLR results.

SystemVue DPD webcastJinbiao XU6/17/2010

A d

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9. Summary

Agenda


P ti l C id ti

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DPD and CFR are more likely to be needed when

The PAE must be maximized

The PA must be operated at high power

The nonlinearity has memory

The analog part is used with wideband or multiband signals

DPD and CFR are needed for modern systems that operate with highRF spectral efficiency

Multiple-Carrier Signals (Multiple-Carrier GSM, Multiple-CarrierWCDMA)

CDMA (WCDMA, CDMA2000)

OFDM (LTE, WiMAX)

Practical Considerations


A d

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7.. Using X-Parameters with PA and DPD


9. Summary

Agenda


Summary

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Modern communication systems with high spectral efficiency andwide bandwidths typically

Use signals with high peak-to-average power ratios (PAPR).

Operate RF PAs with high power added efficiency (PAE).

However, high PAPR results in driving the PA into higher distortion

levels that requires PA back-off in drive level, which reduces PAE. The key problem is: How to handle signals with high PAPR, with the

PA operating at high PAE, while maintaining low signal distortion?

Digital Pre-Distortion (DPD) and Crest Factor Reduction (CFR)techniques together help overcome these conflicting requirements.

SystemVue offers a practical DPD Design Flow usable with real PAhardware that includes integration with Agilent ESG/MXG andPXA/MXA instruments.

Summary


Questions & Answers

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Questions & Answers

48

Copyright Agilent Technologies 2010 SystemVue DPD webcastJinbiao XU6/17/2010

4G For Everyone

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4G For EveryoneNow you can use DPD, too!

49

THANK YOUFor more information

www.agilent.com/find/eesof-systemvue

www.agilent.com/find/eesof-systemvue-videoswww.agilent.com/find/eesof-systemvue-evaluationwww.agilent.com/find/ims2010-microapps

Or, contact your regional Agilent resource www.agilent.com/find/eesof-contact

Copyright Agilent Technologies 2010 SystemVue DPD webcastJinbiao XU6/17/2010

Appendix

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Appendix


LTE R lt ith D h t PA (30W)

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LTE Results with Doherty PA (30W)AM-AM characteristics


AM-AM30W Doherty PA

AM-AMDPD correction network

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LTE R lt ith D h t PA (30W)

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LTE Results with Doherty PA (30W)Spectrum, ACLR and EVM results


Jinbiao XU52

EVM (dB)

-2BW

Lower

-1BW

Lower

+1BW

Upper

+2BW

UpperPA input 51.61 50.99 50.01 51.82

PA output 43.59 27.64 27.42 42.26

DPD-PA output 50.15 40.43 40.08 47.78

BLUE Raw PA outputRED DPD+PA output

ACLR (dB)

6/17/2010

References

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1. Lei Ding, Zhou G.T., Morgan D.R., Zhengxiang Ma, Kenney J.S., Jaehyeong Kim, Giardina C.R., A robust digital

baseband predistorter constructed using memory polynomials, Communications, IEEE Transactions on, Jan. 2004,

Volume: 52, Issue:1, page 159-165.

2. Lei Ding, Digital Predistortion of Power Amplifiers for Wireless Applications, PhD Thesis, March 2004.3. Roland Sperlich, Adaptive Power Amplifier Linearization by Digital Pre-Distortion with Narrowband Feedback using

Genetic Algorithms, PhD Thesis, 2005.

4. Helaoui, M. Boumaiza, S. Ghazel, A. Ghannouchi, F.M., Power and efficiency enhancement of 3G multicarrier

amplifiers using digital signal processing with experimental validation, Microwave Theory and Techniques, IEEE

Transactions on, June 2006, Volume: 54, Issue: 4, Part 1, page 1396-1404.

5. H. A.Suraweera, K. R. Panta, M. Feramez and J. Armstrong, OFDM peak-to-average power reduction scheme with

spectral masking, Proc. Symp. on Communication Systems, Networks and Digital Signal Processing, pp.164-167, July

2004.

6. Zhao, Chunming; Baxley, Robert J.; Zhou, G. Tong; Boppana, Deepak; Kenney, J. Stevenson, Constrained Clipping for

Crest Factor Reduction in Multiple-user OFDM, Radio and Wireless Symposium, 2007 IEEE Volume , Issue , 9-11 Jan.

2007 Page(s):341- 344.

7. Olli Vaananen, Digital Modulators with Crest Factor Reduction Techniques, PhD Thesis, 20068. Boumaiza, et a, On the RF/DSP Design for Efficiency of OFDM Transmitters , IEEE Transactions on

Microwave Theory and Techniques, Vol. 53, No. 7, July 2005, pp 2355-2361.

9. Boumaiza, Slim, Advanced Memory Polynomial Linearization Techniques, IMS2009 Workshop WMC(Boston, MA), June 2009.

10. Amplifier Pre-Distortion Linearization and Modeling Using X-Parameters, Agilent EEsof EDA


http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=26http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=22http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=22http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=22http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=22http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=26

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