[MWJ0405] Predicting the EVM Performance of WLAN Power Amplifiers With OFDM Signals

7/28/2019 [MWJ0405] Predicting the EVM Performance of WLAN Power Amplifiers With OFDM Signals

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WLAN IEEE 802.11a/gEVM DEFINITION

EVM is a measure of the deviation of thedemodulated received symbol (I,Q) from the

original transmitted data symbol (Io, Qo). Theratio of the error vector magnitude (see Fig-ure 1) to the original symbol magnitude de-fines the EVM as

In practical specifications, a statistical aver-age is taken for the EVM. The IEEE802.11a/g standards1 define the root-mean-square EVM for an OFDM signal as

EVME

Po= ( )1

PATRICK NARAINESkyworks Solutions Inc.

Irvine, CA

The 802.11a/g WLAN standards stipulatethe use of orthogonal frequency divisionmultiplexing (OFDM) signals withM-QAM symbols modulation, where M canbe as high as 64. These wideband digital mod-ulations demand PAs with high linearity, dueto the inherent high peak-to-average ratio(PAR) of these signals. This article describesthe PAR and cumulative complementary dis-tribution function (CCDF) of WLAN signalsand their implications for PAs.

A method is proposed to determine the PAnonlinear response, and, in turn, the EVMperformance. A computer algorithm is de-scribed, which uses the proposed model topredict EVM for WLAN PAs driven withOFDM signals.

Finally, predicted versus measured EVMresults are presented for two high power andhigh linearity WLAN power amplifiers(SKY65130 and SKY65131).

PREDICTING THE EVM

PERFORMANCE OF WLAN

POWER AMPLIFIERSWITH OFDM SIGNALSThis article details a method of predicting the error vector magnitude (EVM) of

wireless local area network (WLAN) power amplifiers (PA). It starts with a

definition and explanation of EVM, and explains the EVM specification as

defined by the IEEE 802.11a/g standards.1 The article then describes the

importance of PA linearity and the method of determining linearity based on the

gain compression response (AM/AM) and the phase distortion (AM/PM)

characteristics of the PA.

Reprinted with permission ofMICROWAVE JOURNAL from the May 2004 issue.2004 Horizon House Publications, Inc.


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whereLp = length of the package

(must be > symbols)Nf = number of frames used

in the EVM measurement(must be > 20)

Io, Qo= I and Q values of the originalOFDM symbol

I, Q = received I and Q valuesk = kth subcarrier of the OFDM

symbolPo = average power of the

constellation

PA LINEARITY(AM/AM AND AM/PM)The linearity of a PA amplifier can

be assessed from its AM/AM andAM/PM response. The AM/AM re-sponse is usually plotted as the inputpower versus the resulting PA gain;the AM/PM response is the inputpower versus the resulting phase shiftintroduced by the PA.

A good indication of the PA linear-ity is the input (or output) P1dBpoint. The input P1dB point is de-

fined as the input power at which thePA gain drops by 1 dB from its small-signal gain value (see Figure 2).When the PA is operated close to itsP1dB point, the output amplitudewill start to be distorted (that is, itwill not linearly follow changes in itsinput power). The same is true forthe insertion phase response. There-fore, when selecting a PA for linearoutput power operation, it is very im-portant to select a PA with the appro-priate P1dB point.

PAR FOR 802.11a/gOFDM SIGNALS

The IEEE 802.11a and IEEE802.11g standards specify the use ofOFDM 64-QAM signals for in-creased data rates. The signal enve-lope of these signals is inherently verypeaky (that is, it shows large devia-tion from the average power level).The variation of the input signal fromits average level is described by the

PAR term. The statistical distributionof the PAR is usual-ly represented bythe signal CCDF.Figure 3 shows theCCDF for an802.11a/g input sig-nal with the maxi-mum specified datarate of 54 Mbps,along with a refer-ence signal, whichhas a Gaussian dis-

tribution (such asAWGN).A review of the

CCDF summary

data (left side of the figure) indicatesthat the signal PAR will be equal to orgreater than 3.66 dB 10 percent ofthe time; the PAR will be equal to orgreater than 6.97 dB 1 percent of thetime.

Depending on the specified EVM

(or bit error rate), a system may onlybe able to tolerate 1 percent of its to-tal envelope being distorted, in whichcase the average power input to thePA should be set at least 6.97 dB be-low the input P1dB compressionpoint.

For the maximum data rate of 54Mbps, IEEE 802.11a/g specificationscall for an average EVM of 5.6 per-cent. If the PA is backed-off by atleast 5 dB from its P1dB point, theamplitude will be compressed by lessthan 5.6 percent.

AM/AM AND AM/PMMODEL FOR WLAN PA

The EVM performance of a PAcan be calculated using its AM/AMand AM/PM responses. As discussedin the previous sections the AM/AMand AM/PM responses indicate theextent to which the amplitude andphase of the input signal is distorted.If the input signal level is well below

the P1dB compression point of thePA, very little AM/AM or AM/PMdistortion occurs and the EVMshould be very low. As the input sig-nal approaches the 1 dB compressionpoint of the PA, significant amplitudeand phase distortion occurs and theEVM becomes larger. Therefore, topredict the EVM performance, theAM/AM and AM/PM responses ofthe PA must be modeled.

A PA model was developed, basedon the TWT nonlinear model pro-

posed by A. Saleh.2 The originalSaleh model used the following equa-tions to model AM/AM and AM/PM:

where

r = input signal voltage level

a, a= AM/AM response constants,typically 2 and 1, respectivelyp, p= AM/PM response constants,

typically 2 and 3, respectively

A rr

r

P rr

r

a

a

p

p

( ) =+

( ) =+

13

14

2

2

2

( )

( )

TECHNICAL FEATURE

EVM

I i j k I i j k Q i j k Q i j k

LpP

NRMS

o o

kj

Lp

oi

N

f

f

=

( ) ( )( ) + ( ) ( )( )

==

, , , , , , , ,

( )

2 2

1

52

1

1 52 2

NEW SYMBOLVECTOR, Pe

PHASEERROR

ORIGINALSYMBOL

VECTOR, Po

ERROR VECTOR, E

I Io

Q

QQo

I

v Fig. 1 I-Q constellation diagramshowing the error vector magnitude.

OUTPUT P1dB

=29 dBm32

30

28

26

24

22

209753

Pin(dBm)

1-1-3

Pout

dBm

1 dB

GAIN

DROP

INPUT

P1dB

LINEARRESPONSE

LINE

v Fig. 2 Poutvs. Pin of a WLAN power

amplifier showing the P1dB point.

v Fig. 3 CCDF of an IEEE 802.11 a/g 64-QAM OFDM signal witha data rate of 54 Mbps.


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It was found that a slight modifica-

tion to Equations 3 and 4 would bet-ter model the AM/AM and AM/PMresponse of HBT PAs optimized forlinearity and efficiency (class A/Bbias). The modified equations are

The models for AM/AM and

AM/PM were tested using two Sky-works WLAN power amp lifier s(SKY65130 and SKY65131). Thesepower amplifiers were built using In-

A rr

r

P rr

r

a

am

p

pn

( ) =+

( ) =+

15

16

2

2

2

( )

( )

GaP HBT technolo-gy, and were biasedfor maximum lin-earity (high P1dB)and maximum DCPAE. Table 1shows the main RF

characteristics ofthese power ampli-fiers using 802.11gsignal conditions.For these class A/BHBT power ampli-fiers, the model er-ror was found to beminimum when m =1.8 and n = 1.4.

Using Equations5 and 6, the predicted AM/AM andAM/PM responses were plotted ver-sus the measured values, as shown inFigures 4 and 5, respectively.

EVM PREDICTION PROGRAMA Matlab program was developed

to predict the EVM performance ofWLAN PAs. The program generatesa random bit sequence, and thenmaps the bits into 64-QAM symbols.The I and Q values of this sequenceare stored as the Io and Qo array. TheIo and Qo array is next used to form

the OFDM frequency domain signal,So(f). An inverse FFT function isthen used to compute the time do-main signal, So(t). The input time do-main signal is then scaled to the re-quired average input power level.The resulting PA output time domainsignal is calculated using Equations 5and 6. Using the FFT function, thePA output frequency domain signal,Se(f), is calculated.From Se(f), the re-sulting OFDM sym-

bols, Ie and Qe, aremapped. Equation2 is then used tocalculate the EVMof the PA. The com-plete program algo-rithm is shown inFigure 6.

PREDICTED EVMvs. MEASUREDEVM

The WLAN PA

test samples weremeasured using thedefined IEEE802.11g signal un-

der maximum data rate conditions of54 Mbps (that is, 64-QAM, OFDM).Figure 7(bottom right table) showsthe RMS EVM which was measuredat 2.5 percent for +22 dBm averageoutput power. On the top left is thede-coded IQ constellation for 64-QAM; the bottom left shows the out-put spectrum mask (also shown inFigure 8, with the IEEE 802.11gspecification limits).

To determine the accuracy of theprediction program, the EVM of the

TECHNICAL FEATURE

24232221201918

108642

Pin (dBm)

CALCULATED

MEASURED

0-2-4

GAIN(

dB)

v Fig. 4 Calculated vs. measured AM/AMresponse.

TABLE IMAIN RF PERFORMANCE

OF EXAMPLE WLAN PAS USED TO VERIFY THE EVM MODEL

RF Parameters SKY65130 SKY65131

Frequency range (MHz) 24002500 24002500

Small-signal gain (dB) 23 27

P1dB (dBm) 29 28

PAE for output power = P1dB (%) 34 38

Noise figure (dB) 4 4

Power output for EVM = 3% (dBm) 23 22

Power output for IEEE802.11a/gstandard transmit spectrum mask 27 26(see Figure 8) (dBm)

10

9

8

7

6

5108642

Pin (dBm)

CALCULATED

MEASURED

0-2-4

PHASE

v Fig. 5 Calculated vs. measured AM/PMresponse.

MAP TO 64-QAM SYMBOLS

STORE ORIGINAL Io & Qo VALUES

GENERATE OFDM SIGNAL INFREQUENCY DOMAIN, So(f)

COMPUTE INVERSE-FFT OF So(f)-> So(t)

SCALESo(t) TO AVERAGE INPUT POWER

COMPUTE OUTPUT SIGNAL, Se(t), USINGAM/AM AND AM/PM RESPONSE

CALCULATEDFROM EQUATIONS 5 AND 6

COMPUTE FFT OF Se(t) -> Se(f)

USE AMPLITUDE AND PHASE OF Se(f)TO CALCULATE NEW Ie & Qe

USE EQUATION 2 TO COMPUTE EVM

GENERATE PSEUDORANDOM BIT SEQUENCE

v Fig. 6 Matlab algorithm for computingEVM from the modeled PAs AM/AM andAM/PM responses.

v Fig. 7 WLAN PA measurements top left: 64-QAMconstellation, top right: EVM vs. carrier number, bottom left:Tx spectral mask, bottom right: RMS EVM table.


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PA was measured at different levelsof average input power. Figure 9shows the predicted and measuredEVM versus output power. The EVM

plot shows that the predicted EVMclosely matches the actual measuredvalues. The prediction model actuallyhas a slight over-estimation of theEVM around the inflection point ofthe curve. This is due to the modelgoing into compression before the ac-tual device. Note that the PA AM/AMresponse shows a slight gain expan-sion (~0.5 dB) about the inflectionpoint. At this location the model ispredicting a higher level of gain com-pression.

TECHNICAL FEATURE

CONCLUSIONThis article describes the EVM re-

quirements for WLAN OFDM appli-cations and the importance of PA lin-earity in order to achieve adequateEVM, along with a review of the 1 dBcompression point and the CCDF

and PAR characteristics of WLANOFDM signals. It further proposes amethod for predicting the amplitudeand phase distortion introduced by aWLAN PA when driven into its non-linear region of operation. A comput-er algorithm is described that usesthe proposed PA nonlinear model topredict the EVM performance.

The EVM model was compared tothe measurements taken from sampleWLAN power amplifiers using a 64-QAM OFDM signal with a maximum

data rate of 54 Mbps. The resultsshow the model very closely predictsthe measured performance of thepower amplifiers. s

References1. IEEE Std. 802.11a1999, Part 11: Wireless

LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications.2. A. Saleh, Frequency-independent and

Frequency-dependent Nonlinear Modelsof TWT Amplifiers, IEEE Transactions onCommunications , Vol. 29, No. 11, Novem-ber 1981.

Patrick Naraineearned his BSEE andMSEE degrees fromMcMaster University,Hamilton, Ontario,Canada. He wastechnical payloadmanager of Canadas

first remote sensingsatellite built for theCanadian SpaceAgency in 1996. He

then joined Nortel Networks, where he was agroup leader in charge of RF system designsfor AMPS, TDMA and EDGE base stationtransceivers. From 1999 to 2002, he worked atAT&T Wireless on the system design, testingand deployment of the first US commercialfixed wireless voice and high speed Internetsystem using OFDM modulated signals. He iscurrently working on system designs forRFICs, MMICs and PAMs at SkyworksSolutions Inc. He can be reached via e-mail [email protected].

1412108642

0 2624222018

OUTPUT POWER (dBm)

MEASURED PREDICTED

16141210

EVM(

%)

v Fig. 9 Predicted vs. measured EVMfor a WLAN PA.

10

0

-10

-20

-30

-40

-502.4732.4612.4482.436

FREQUENCY (GHz)

MEASURED TOTALOUTPUT POWER= +26 dBm

SPECIFICATION MASK

2.4232.411OUTPUTPOWER

dBc

v Fig. 8 Output spectrum response ofSKY65131 using an 802.11a/g 64-QAM, 54Mbps input signal.

[MWJ0405] Predicting the EVM Performance of WLAN Power Amplifiers With OFDM Signals

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