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Presentation on Simulation of WLAN System

1

Simulation of WLAN System

• Motivation• Basic System• WLAN Design Library (DL) in

ADS• Simulation of WLAN System• Test and Verification of WLAN

Power Amplifier• Conclusions

2

Simulation of WLAN SystemNovember 2001

Page 2

Motivation

• 5-GHz WLAN systems based on IEEE 802.11a deliver higher datarates, better spectral efficiency, improved multipath performance,and less interference in low mobility wireless conditions.

• The demand for WLAN systems is rapidly growing. WLAN systemsare being marketed all over the world.

• To design WLAN system with non-linear components under multipathchannel environment, simulation tools must be used. ADS WLAN DLserves this purpose.

• High Peak-to-average ratio is a known design problem that requiresaccurate models-- behavioral and circuit

• ADS T&V templates using key measurements, such as EVM, CCDF,and ORFS are very useful to test and verify key components such asPower Amplifiers (PA).

.

.

.The purpose of this presentation is to show the environment and the appropriatesetup and results that could assist in selecting the right components such as PA.

FIRST part of the presentation talks in general about the OFDM signaland then we will show some of the models that constitute the WLAN Design Library,in the signal source and the receiver- The library has all the baseband models neededfor an ideal transmitter and receiver.we will see some key measurements like EVM- The same measurements are alsodone with the VSA SW for validation

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Basic System

• OFDM Signal• Simulation Framework• Test and Verification Environment• Summary

For supporting 5-GHz WLAN systems with high-rate data transmission, multi-carriermodulation, orthogonal frequency division multiplex (OFDM) is proposed. The basicprinciple of OFDM is to split a high data rate data stream into a number of lower ratestreams that are transmitted simultaneously over a number of subcarriers. Becausethe symbol duration increases for the lower rate parallel subcarriers, the relativeamount of dispersion in time caused by multipath delay spread is decreased.Intersymbol interference (ISI ) is eliminated almost completely because the OFDMallows us to insert adequate guard interval between successive OFDM symbols.The concepts behind OFDM have been around for a long time. It’s only been withinthe last few years that the baseband processing has been cheap enough to allowpractical implementations. Besides WLAN, OFDM is used for digital audiobroadcasting, digital video broadcasting, and xDSL. The primary advantage of OFDMis improved performance under multipath conditions.One of the problems with single carrier modulations (SCM) is that, in a givenenvironment, the symbol interval becomes much shorter than the delay spread as thesymbol rate is increased. To solve this problem with multi-carrier modulation formats,the symbol rate is instead decreased, and the number of carriers is increased. Thesymbol interval for each of the lower-rate carriers is made very long compared to thedelay spread. To increase robustness, should a subset of the carriers be unusablebecause of nulls or interference, the information is interleaved between carriers. Theinterfering tone shown here would do little damage to the OFDM signal

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OFDM Transmission and Receiving

IQModulator

IQModulator

QAMMappingQAM

MappingPilot

InsertionPilot

Insertion

IFFT(TX)

FFT(RX)

IFFT(TX)

FFT(RX)

G.I.Addition

&Windowing

G.I.Addition

&Windowing

DACDAC

HPA

RemoveG.I.

RemoveG.I.

ChannelCorrectionChannel

CorrectionDe-interleaving/FEC Decoding/De-Scrambling

De-interleaving/FEC Decoding/De-Scrambling

LNA

AGC Amp

Rx Lev. Det.

Receiver

Transmitter

Scrambling/FEC Coding/Interleaving

Scrambling/FEC Coding/Interleaving

DataIn

ADCADC

Timing &Frequency

Synchronisaton

Timing &Frequency

Synchronisaton

QAMDemapping

QAMDemapping

DataOut

Frequencycorrectedsignal

Symbol Timing

In the Figure, a simplified block diagram of OFDM transmitter and receiver is shown.The information data are serial-parallel converted then modulated by the allocatedsubcarrier using linear modulation such as BPSK, QPSK,16-QAM, and 64- QAM. TheOFDM signal is generated as the IFFT of modulated subsymbols. To combat ISIadequate guard interval between successive OFDM symbols is inserted by using the‘guard Interval’ block.The time discrete channel model provides multipath channel environmentsas well as channel noise to simulate practical WLAN systems.In the OFDM receiver block, the ‘Sync’ block is for timing, frequencysynchronization as well as phase compensation. Through DFT anddemodulation and parallel to serial conversion the information data will bedetected.

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WLAN Burst Structure

In the WLAN system, packetized burst signals are transmitted without scheduling.Therefore, synchronization must be established burst by burst. The proposed burststructure based on IEEE 802.11A is shown in Figure 2. The OFDM burst actuallyhas four distinct regions shown in Figure 2. The first is the Short preamble (trainingsequence). This is followed by a Long preamble (training sequence) and finally bythe Signal and Data symbols. Between each burst section, there are some guardintervals

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WLAN Library in ADSSystem Models

• Channel coding• convolutional coder/decoder, Puncture coder/decoder,

interleaver/deinterleaver, Scrambler.• Modulation

• BPSK, QPSK, 16-QAM, 64-QAM modulation/demodulation.• Framing

• Burst framing, OFDM symbol multiplex/demultiplex, Guardinterval inserter.

• Receiver• Symbol synchronization, Frequency synchronization, Phase

Estimate, Phase tracking, channel estimator, OFDMEqualizer.

To simulate WLAN system the WLAN library provides simulation models for dataand signal generation, channel coding, modulation, Burst framing, Receiving, aswell as measurements.

Main functions of WLAN models can meet the system requirements by IEEE802.11a standard. 6-54 Mb/s rates

The detailed information can be found in WLAN library manual.

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WLAN DL in ADSMeasurements

• Error Vector Magnitude (EVM)• Output RF Spectral With Mask (ORFS- Spectral Mask)• Complementary Cumulative Distribution Function (CCDF)• Power Vs Time

First three tests(EVM, ORFS, CCDF) are most useful in the 802.11a measurementswith presence of PA.These tests are part of the test and verifications of the WLAN library.Preconfigured test and verification setups help the designer quickly evaluate aDUT(Design Under Test)Built in pass no pass criterions facilitate the evaluations

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Simulation of WLAN SystemConfiguration

Source

ComponentMeasurement

Hierarchical structures and simulation templates are used for easy usage.

1. Test and verify the designed power amplifier (or other RF device) to see if thiscomponent can meet the WLAN standard.

A Unified look of the top hierarchy enable easy usage. Specs/Parameters can beentered at the level where it is most useful and flexible.

Next we will look into the source.

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Simulation of WLAN System: Source Configuration:Source Example: A 36 Mbps Signal Source

For the hierarchical structured source, users can push into the block and see thelower level structure. The second level of the hierarchy includes a baseband sourceand a RF modulator. Push into the baseband source, the third level of the hierarchy isshown how to generate WLAN signals based on IEEE 802 11a standard.Generating the short training sequence section of the preamble by using W1, W2 andF1 in the lowest branch.Generating the long preamble sequence section of the preamble by using W3, W5 andF2 in the third branch.Generating the SIGNAL field bits, Coding, interleaving, modulating, multiplexing withData section by using B2, ConvCoder, Interleaver, BSKmod in the lower branch.Forming the Data, Scrambling, Convolutional coding, interleaving, 16 QAM modulationand multiplexing with signal section data by using B1, Data, Scrambler, L1, Tail,PuncCoder, Interleaver, 16 QAM, MuxSigandData in the upper branch.Mapping the SIGNAL and Data into frequency domain, then transformation fromfrequency to time by using MuxSym and IFFTBuffer, F3 in the upper branch.Forming the PPDU frame by multiplexing short preamble, long preamble, signal andData to the OFDM burst by using MuxBurst model.

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Simulation of WLAN SystemMeasurement Configuration: EVM

To estimate the EVM the following steps are performedStart of frame is detected by using W1 (WLAN_BurstReceiver)..Using the short preambles to synchronize the in WLAN_BurstSync.Frequency offsets are estimated by using the WLAN_FreqSync model.The packet is derotated according to estimated frequency offset by using theWLAN_DemuxBurst.The complex channel response coefficients are estimated for each subcarrier by usingthe WLAN_PhaseEst and the WLAN_ChannelEst.Each data OFDM symbol is transformed into subcarrier received values; the phasefrom the pilot subcarriers is estimated; subcarrier values are rotated according to theestimated phase; and, each subcarrier value is divided with a complex estimatedchannel response coefficient by using the WLAN_MuxDataChEst, WLAN_PhaseTrackand WLAN_Equalizer.For each data-carrying subcarrier the closest constellation point is determined and theEuclidean distance from it is calculated. For the EVM, the RMS average of all errorsin a packet is calculated.

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EVM

Ideal(t)

Actual(

t)

Error(t)

Magnitude Error(t)

Error Vector Magnitude(t)

Phase Error(t)

I

Q

Carrier Leakage

EVM represents the distance between the measured and expected carriermagnitude and phase at some point in time after it has been compensated in timing,amplitude, frequency, phase and DC offset.

For IEEE 802.11a, the error vector between the vector representing the transmittedsignal and the vector representing the error-free modulated signal defines modulationaccuracy. The magnitude of the error vector is called error vector magnitude (EVM).The purpose of this test is to verify that the RMS EVM measured on the specific partof the burst meets the conformance requirement. (does not exceed the conformancerequirement.)

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Basic Test: WLAN Waveform

The system output waveforms can be captured from the simulation data shown in theslide. As can be seen, the 10 short preambles (SP) can be viewed during the first 8us, then two long preambles can be found in the second 8 us. The WLAN SIGNAL(Sig) can be observed in the next 4 us. The rest of waveforms are the WLAN DATA(Data).

Useful part of the measurement is the DATA portion.

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Test and Verification of WLAN Power Amplifier

• Purpose: Test a PowerAmplifier (PA) for WLANterminals from Product listof Vendors.

• Candidate PA: MGA-82563(0.1-6 GHz), low noiseeconomic GaAs PA fromAgilent.

• Test and Verify to see if thecandidate can meet therequirement by the standard

MGA-82563

Tested PA from Agilent Wireless Semiconductor Group.This part is a couple years old and at the time of the print was the only available PAthat we could access to with the full circuit.We are working with a new part from Agilent WDS Group and will show the resultssoon.

14


Page 14

MGA-82563 Output Power / Gain Vs Input Power

This slide shows the MGA-82563 output power performance expressed by the redcurve, Output power Vs Input power. The PA Gain performance also is shown.

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Page 15

Test and Verification of WLAN Power Amplifier

• Purpose: T&V of a candidate WLAN Amplifier, Agilent’s MGA-82563

• Simulation configuration• Signal Source: 36 Mbps 16-QAM Data, 50 kHz frequency

offset• Tested PA: Agilent’s MGA-82563• Measurements: ORFS with Mask, EVM, CCDF

Device_To_Be_Tested

dBc1out=dbmtow(18.256)GCType=dBc1Gain=dbpolar(DUT_Gain,0)

SignalSource

WLANSignalSource

SignalMeasurement

EVM WLAN EVM

MGA-82563

Power Amplifier AM/AM and AM/PM compression is implemented in a complexvalued equation.Gain Compression type is selected as 1 dB compression point.We found the 1dB compression point of the MGA-82563 PA to be 18.256 dBm

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Page 16

Modeling MGA-82563 for WLAN Simulation

• As a System Behavior Model

• As a circuit Model

A M P 1

P 2 D F i le = " L i n e U p . p 2 d "F r e q = F _ X m i t H z

O p t i o n s 1

M a x W a r n i n g s = 1 0G i v e A l lW a r n i n g s =I _ R e lT o l= 1 e - 6V _ R e lT o l = 1 e - 6T o p o lo g y C h e c k =T e m p = 2 5

O P T I O N S

E n v 1

S te p = T i m e S t e p s e cS to p = 1 0 0 n s e cE n v S k i p D C _ F i t=E n v U s e P o o r F i t = n oE n v W a r n P o o r F i t = n oE n v B a n d w i d t h = 1F u n d O v e r s a m p le = 2O r d e r [ 1 ] = 1F r e q [ 1 ] = F _ X m i t H zM a x O r d e r = 2

E N V E L O P E

P 2N u m = 2

P 1N u m = 1

D e v i c e _ T o _ B e _ T e s t e d

d B c 1 o u t = d b m t o w ( 1 8 . 2 5 6 )G C T y p e = d B c 1G a i n = d b p o l a r ( D U T _ G a i n , 0 )

There are two approaches for modeling MGA-82563 to test and verify theperformance.Top Model as a system model in hptolemy. A RF_Gain component is used.Bottom Model as a circuit in envelope test bench, then using co-simulation to findout the performance.

An alternative to the detailed circuit model is the power-dependent S-parameter orP2D model. It is derived from a Harmonic Balance circuit simulation which generatespower-dependent S-parameters, assuming the use of 50-ohm terminations. A powersweep template enables the user to enter the power levels with start and stop points[5]. In this scheme, the circuit simulation engine runs using the P2D file-based model,co-simulating with the DSP behavioral model simulator. This co-simulation is fasterthan the detailed circuit model co-simulation.

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Page 17

Time

40% (0 dB above Avg)

21%

5%

EnvelopePower2dB/div

Power Statistics

As mentioned the power envelope of the OFDM burst is not constant. Due to the largepeaks that are characteristic of the OFDM signal, a single PAP ratio is not very useful.It would be more meaningful to associate a percentage probability with a power level.For example in the plot shown, the signal exceeds the average power(red line) 40% ofthe time. It exceeds a level that is 4 dB above average, 5% of the time.CCDF(Complementary Cumulative Distribution Function) which is the common CDFsubtracted from 1 is shown.It shows dB above average power on horizontal axis and percent probability onvertical axis. The point that the signal is clipped is shown at 7.2 dB , 0.09 percent.Signal exceeds 7.2 dB above average 0.09 percent of the time. Furthermore, If anamplifier with 7.2 dB headroom is used in the system it will go to saturation 0.09percent of the time.The waterfall curve represents the statistics for Gaussian noise. Most OFDM signalswill follow this statistic closely.Next we will see the ADS measurement of the IEEE802.11a signal with a linear PAwhich shows the waterfall curve.

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Page 18

CCDF in ADS

OFDM CCDF- Gaussian OFDM - clippednon-linear PA

Shows normal OFDM and saturated OFDM signal

19


Page 19

Spectral Mask

Taken from IEEE802.11a standard- Modulated RF spectrumThe output RF spectrum due to modulation is the relationship between the frequencyoffset from the carrier and the power measured in a specified bandwidth. Themeasurement provides information about distribution of the transmitter's channelspectral energy due to modulation.

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Page 20

Output RF Spectrum

OFDM Signal Spectrum

5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 MHz

5725 5745 5765 5785 5805 5825 MHz

20MHz 20MHz

30MHz 30MHz

Lower and Middle U-NII Band – 8 carriers in 200MHz / 20MHz spacing

Upper U-NII Band – 4 carriers in 100MHz / 20MHz spacing Transmit Spectrum Mask

-30 –20 –11 –9 fc 9 11 20 30 MHz

-20dBr

-28dBr

-40dBr

Power Spectral Density (dB)

12 total bands of the lower, middle and upper U-NII are shown with the spectrummask.IEEE802.11a dictates three output power measured at three different frequencychannels:5.18 GHz 40 mW5.280 GHz 200 mW5.805 GHz 800 mW

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Page 21

MS ORFS Due to Modulation (5180 MHz, 40mW) T&V Result: Passed

5.15E9 5.16E9 5.17E9 5.18E9 5.19E9 5.20E9 5.21E9-80

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Frequency (Hz)

Spe

ctru

m (

dB

r)

MS ORFS due to Modulation (5180MHz, 40 mW output)

The output RF spectrum due to modulation is the relationship between the frequencyoffset from the carrier and the power, measured in a specified bandwidth and time,produced by the mobile station due to the effects of modulation. The measurementprovides information about distribution of the transmitter's channel spectral energydue to modulation.

In the ORFS test, channel 36 corresponding to a channel center frequency 5180 MHzis used.

This test is shows that the RF spectrum (red line) does not exceed the Mask (blueline), indicating a successful test result.

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Page 22

MS ORFS Due to Modulation (5280 MHz, 200mW) T&V Result: Marginal

5.25E9 5.26E9 5.27E9 5.28E9 5.29E9 5.30E9 5.31E9-80

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Frequency (Hz)

Spe

ctru

m (

dBr)


In this case the configuration for channel 56 corresponds to a channel centerfrequency of 5280 MHz.

Test results are marginal because a few spectrum lines (red lines) exceed the Mask(blue line).

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Page 23

MS ORFS Due to Modulation (5805 MHz, 800mW) T&V Result: Failed

5.775E9 5.785E9 5.795E9 5.805E9 5.815E9 5.825E9 5.835E9-80

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Frequency (Hz)

Spe

ctru

m (

dBr)


In this test, the configuration for channel 161 corresponds to a channel centerfrequency of 5805 MHz.

The test failed because the RF spectrum (red line) exceeds the Mask (blue line).

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Page 24

EVM Test results for MGA-82563

Channel Number 36 Channel Number 56 Channel Number 161

Carrier Offset is zero EVM (%)

Carrier Offset is 50KHz EVM (%)

Carrier Offset is 100KHz EVM (%)

7.492

7.492

7.492

28.253

28.253

28.253

56.448

56.448

56.448

WLAN Error Vector Magnitude (EVM)

Spe c ific ation re quire me nts(unde r normal c onditions )The RMS EVM shall not exceed 11.2%.

Te s t Re s ults

WLAN Specification: IEEE Std 802.11a-1999

Passed Failed Failed

The EVM is very important for measuring the modulation accuracy.The 802.11a standard lists rates of 6,12 and 24 Mbit/sec as mandatory. In aproduction environment, it should be necessary to measure the EVM only at thehighest rate supported. This would be 15.8% for all modems. 54Mbit/sec modemswill need to achieve 5.6% EVM. 36Mbit/sec modems will need to achieve 11.2%EVM. Outside of slightly different power statistics, there are very few errorinducing mechanisms that would cause a transmitter to have a significantly differentmeasured EVM for each rate (given the normalized constellations).The EVM values are compared to the required EVM by the IEEE 802.11a standardautomatically and the most important final result is shown. The EVM values arebounded by the 11.2% which is the requirement by IEEE 802.11A for Channel 36 witha center frequency 5180 MHz, which means the EVM results are satisfactory.However, for channel 56 and 161, the EVM values exceed the requirements, whichmeans they fail the test.Agilent WSD will release new PA for WLAN system to cover channel 56 and 161.

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Page 25

Test and Measurement- ADS and VSA

Signal Source RFUPconversion Power

AmplifierVector Signal

Analyzer

Typical measurement setup schematic with Vector Signal Analyzer is shown in FigureSignal Source and RF section and the non-linear PA are circled. The VSA model is thesink model to the right and lower part of the schematic.The result of this co-simulation with VSA software is captured next.This simulation is channel 161 that was shown previously with ADS sinks andmeasurements.

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PA simulation measurement with VSAFirst measurement

As shown in Figure, the EVM measurement in the lower right quad is about 6.5 %.This agrees with the previous measurement in slide 23-left most table( 7.5%)measured from EVM model of ADSThe running EVM from the VSA measurement is just one the many features of theVSA software. Constellation and RF spectrum are also shown in the same figure

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Page 27

PA simulation measurement with VSASecond measurement

As shown in Figure, the EVM measurement in the lower right quad is about 54 %.This agrees with the previous measurement in slide 23-right most table(56.4%)measured with EVM model of ADSThe running EVM from the VSA measurement is just one the many features of theVSA software. Constellation and RF spectrum are also shown in the same figure.

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Summary

• 5-GHz WLAN is one of the fastest growing systems for low mobilitywireless standard.

• OFDM is the standard modulation for 5 GHz WLAN systems.• The ADS WLAN DL provides simulation capabilities for WLAN

system, and gives some key measurements such as EVM, CCDF,ORFS to test and verify designed components.

• ADS WLAN DL was used for test and verification of a WLAN PA tosee if the PA can meet the IEEE802.11a spec. The tested PA wasMGA-82563 from Agilent.

• Other Power Amps are also being tested with complete circuitavailable in ADS.

www.agilent.com/fi nd/emailupdatesGet the latest information on the products and applications you select.

www.agilent.com/fi nd/agilentdirectQuickly choose and use your test equipment solutions with confi dence.

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