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This document is owned by Agilent Technologies, but is no longer kept current and may contain obsolete or
inaccurate references. We regret any inconvenience this may cause. For the latest information on Agilent’s
line of EEsof electronic design automation (EDA) products and services, please go to:
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Agilent EEsof EDA
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
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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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Simulation of WLAN SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
Page 4
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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Simulation of WLAN SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
Page 7
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 SystemNovember 2001
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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 SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
Page 11
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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Simulation of WLAN SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
Page 13
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.
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Simulation of WLAN SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
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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Simulation of WLAN SystemNovember 2001
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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Simulation of WLAN SystemNovember 2001
Page 18
CCDF in ADS
OFDM CCDF- Gaussian OFDM - clippednon-linear PA
Shows normal OFDM and saturated OFDM signal
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Simulation of WLAN SystemNovember 2001
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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Simulation of WLAN SystemNovember 2001
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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Simulation of WLAN SystemNovember 2001
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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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)
MS ORFS due to Modulation (5280MHz, 200 mW output)
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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Simulation of WLAN SystemNovember 2001
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)
MS ORFS due to Modulation (5805MHz, 800 mW output)
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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Simulation of WLAN SystemNovember 2001
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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Simulation of WLAN SystemNovember 2001
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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Simulation of WLAN SystemNovember 2001
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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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Simulation of WLAN SystemNovember 2001
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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Simulation of WLAN SystemNovember 2001
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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.
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