OFDM Impairments

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Wireless Networking

Design Seminar

Simulation of OFDM Impairments

using ADS WLAN 802.11a DesignLibrary and DesignGuide

Agilent Technologies



Page 2Wireless Networking Design Seminar DesignGuide

November, 2001

Page 2Agilent Technologies

Contents

OFDM Modulation Review

Using ADS to Evaluate Effects of Link Impairments

on OFDM Modulation

WLAN 802.11a ADS DesignGuide

(ADS 2002 software release)




November, 2001


OFDM Modulation Review




November, 2001


Concept of OFDM

• A type of multi-carrier modulation

• Single high-rate bit stream is

converted to low-rate N parallel bit

streams

• Each parallel bit stream is modulated

on one of N sub-carriers

• Each sub-carrier can be modulated

differently. For example, BPSK,QPSK or QAM

• To achieve high bandwidth efficiency,

the spectrum of the sub-carriers are

closely spaced and overlapped• Nulls in each sub-carrier’s spectrum

land at the center of all other sub-

carriers (orthogonal)

• OFDM symbols are generated using

IFFT

OFDM Spectrum




November, 2001


Advantages of OFDM

Robustness in muti-path propagation environment

More tolerant of delay spread:

Due to the use of many sub-carriers, the symbol duration on

the sub-carriers is increased, relative to delay spread.

Inter-symbol interference is avoided through the use of

guard interval.

Simplified or eliminate equalisation needs, as compared tosingle carrier modulation.

More resistant to fading. FEC is used to correct for sub-carriers

that suffer from deep fade.




November, 2001


Design Challenges of OFDM Modulation

Sensitive to frequency offset – need frequency offset correction

in the receiver.

Sensitive to oscillator phase noise -- “clean” and stable

oscillator required. Large peak to average ratio – amplifier back-off, reduced power

efficiency.

IFFT/FFT complexity -- fixed point implementation to optimize

latency and performance.

Intersymbol Interference (ISI) due to multipath-use guard

interval.




November, 2001


Intersymbol Interference(ISI) Due to Multipath

ISI Symbol interferes with

the delayed version of itself

Multipath

delayed signal

ofdm symbol 1 ofdm symbol 2

This part will

destroy

orthogonality of

OFDM symbol

Direct path

signal




November, 2001


Guard Interval

Multipath delays up to the guard time do not cause Inter-Symbol Interference

Sub-carriers remain orthogonal for multipath delays up to guard time (no Inter-

Carrier Interference)




November, 2001


Windowing

To reduce spectrum splatter, the OFDM symbol is multiplied by a raised-cosine window,

w(t) before transmission to more quickly reduce the power of out-of-band sub-carriers.

Figure above shows spectra for 64 sub-carriers with different values of the rolloff factor, βof the raised cosine window.

Larger β, better spectral roll-off.

However, a roll-off factor of β reduces delay spread tolerance by a factor of βTS.




November, 2001


OFDM Transceiver Block Diagram

IQ

Modulator

IQ

Modulator

QAM

Mapping

QAM

MappingPilot

Insertion

Pilot

Insertion

IFFT(TX)

FFT(RX)

IFFT(TX)

FFT(RX)

G.I.

Addition

&

Windowing

G.I.

Addition

&

Windowing

DACDAC

HPA

Remove

G.I.

Remove

G.I.Channel

Correction

Channel

Correction

De-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

Synchronisation

Timing &

Frequency

Synchronisation

QAM

Demapping

QAM

Demapping

DataOut

Frequencycorrectedsignal

Symbol Timing




November, 2001


OFDM Transceiver Block Diagram: Sources of

Link Impairments

IQ

Modulator

IQ

Modulator

QAM

Mapping

QAM

MappingPilot

Insertion

Pilot

Insertion

IFFT(TX)

FFT(RX)

IFFT(TX)

FFT(RX)

G.I.

Addition

&

Windowing

G.I.

Addition

&

Windowing

DACDAC

HPA

Remove

G.I.

Remove

G.I.Channel

Correction

Channel

Correction

LNA

AGC Amp

Rx Lev. Det.

DataIn

ADCADC

Timing &

Frequency

Synchronization

Timing &

Frequency

Synchronization

QAM

Demapping

QAM

Demapping

DataOut


Symbol Timing

FixedPointeffects

Oscillator Phase Noise

Oscillator Phase Noise,Frequency Offset

Power AmplifierNon-linearity

Multipath

De-interleaving/

FEC Decoding/

De-Scrambling

De-interleaving/

FEC Decoding/

De-Scrambling

Scrambling/

FEC Coding/

Interleaving

Scrambling/

FEC Coding/

Interleaving

Note: other impairments:

-AGC response time

-ADC/DAC quantization noise

-Modem impairments....






November, 2001


Effects of Frequency Offset

IQ

Modulator

IQ

Modulator

QAM

Mapping

QAM

MappingPilot

Insertion

Pilot

Insertion

IFFT(TX)

FFT(RX)

IFFT(TX)

FFT(RX)

G.I.

Addition

&

Windowing

G.I.

Addition

&

Windowing

DACDAC

HPA

Remove

G.I.

Remove

G.I.Channel

Correction

Channel

Correction

LNA

AGC Amp

Rx Lev. Det.

DataIn

ADCADC

Timing &

Frequency

Synchronization

Timing &

Frequency

Synchronization

QAM

Demapping

QAM

Demapping

DataOut


Symbol Timing

Frequency Offset

De-interleaving/

FEC Decoding/

De-Scrambling

De-interleaving/

FEC Decoding/

De-Scrambling

Scrambling/

FEC Coding/

Interleaving

Scrambling/

FEC Coding/

Interleaving

Note: In OFDM link, the sub-carriers are perfectly

orthogonal only if transmitter and receiver use exactly

the same frequencies, any frequency offset results in

Inter-Carrier Interference (ICI).




November, 2001


Inter-Carrier Interference(ICI) Due to Frequency

Offset

1/T

FFT

T

T

FFT

No frequency offset

With frequency offset

FFT window

Integer number of cycles of the sub-carrier ensures that the nulls of the

spectrum lands on the FFT bin,

condition to avoid inter-carrier

interference (ICI)




November, 2001


OFDM Operation (ICI problem)

10 20 30 40 50 600 70

0.5

1.0

0.0

1.5

Index

m a g ( F 1 )

10 20 30 40 50 600 70

-0.02

-0.01

0.00

0.01

0.02

-0.03

0.03

Index

r e a l ( T 1 )

Symbol to be transmitted(Magnitude spectrum)

I-channel signal(after IFFT)

Recovered symbolcontains ICI

DemodulatedI-channel signal

10 20 30 40 50 600 70

-0.02

-0.01

0.00

0.01

0.02

-0.03

0.03

Index

r e a l ( R T 1 )

10 20 30 40 50 600 70

0.5

1.0

0.0

1.5

Index

m a g ( R F 1 )

Frequency offset = 60 kHz

FFT

I-Q Demodulator

IFFT

I-Q Modulator

N_Tones

N2

RandomPhase=No

Phase1=0.0

Power1=.010 W

Frequency1=(FCarrier-0.06) MHz

TStep=tstep sec

N_Tones

N1

Phase1=0.0

Power1=.010 W

Frequency1=FCarrier MHz

TStep=tstep sec

RectToCx

R1

NumericSink

RF1

FFT_Cx

F17

Direction=Forward

Size=64

Order=6

TimedToFloat

T5

TimedToFloat

T4

QAM_DemodExtOsc

Q2

PhaseImbalance=0

GainImbalance=0

Sensitivity=0.5

FloatToTimed

F16

TStep=0.0 sec

FloatToTimed

F15

TStep=0.0 sec

FFT_Cx

F11

Direction=Inverse

Size=64

Order=6

WaveFormCx

W1

Period=64

Periodic=YES

ControlSimulation=NO

Val ue="(0) (0) (0) (1+j) "

CxToRect

C1

QAM_ModExtOsc

Q1

PhaseImbalance=0

GainImbalance=0

VRef=1 V

Power=0.01 W




November, 2001


Effects of Frequency Offset – Without

Frequency Correction

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

real(iq[1,::])

i m a g ( i q [ 1

, : : ] )

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

-2

-1

0

1

2

real(iq[2,::])

i m a g ( i q [ 2

, : : ] )

-15 -10 -5 0 5 10 15

-10

-5

0

5

10

15

real(iq[3,::])

i m a g ( i q [ 3

, : : ] )

AA=0Freq. Offset = 0 HzFreq. Offset / Freq. Spacing = 0%

AA=1Freq. Offset = 3.125 kHzFreq. Offset / Freq. Spacing = 1%



-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.0

-0.5

0.0

0.5

1.0

real(iq[0,::])

i m a g ( i q [ 0

, : : ] )

Freq. Offset / Freq. Spacing

0%

1%

10%

50%

IndexAA=0.000

40

AA=1.000

40AA=2.000

40

AA=3.00040

ReceiverEVM

1.198E-5

0.025

0.268

2.840

Frequency offset expressed as a percentage of sub-carriers frequency spacing (∆f=312.5kHz):

0% 1% 10% 50%

Why you need

frequency offset correction:

assume freq. offset=3kHz (1%),

requires:

oscillator stability: 3k/5.8G=0.5ppm!




November, 2001


Frequency Offset Estimation Using Preambles

Make use of short preamble for coarse frequency offset estimationand long preamble for fine frequency offset estimation.

Short preamble symbol duration of 0.8µs allows frequency

correction up to 1/(2x0.8µs)=±625kHz Assume RF frequency=5.8GHz, the tolerable frequency offset

(worst case) =0.5x625k/5.8G=±53.8ppm > ±20ppm specified in802.11a.




November, 2001


Frequency Offset Estimation and Correction

Frequency offset compensation network:

WLAN_PhaseEst

PhaseDerot

Order=Order

Carriers=52

WLAN

O

WLAN_ChEstimator

ChannelEstimator

Order=Order Carriers=52

WLAN

CIR

WLAN_OFDMEqualizer

Equalizer

Carriers=52

WLAN

Equalizer

WLAN_PhaseTrack

PhaseDerotated

Phase=0

Order=Order

Carriers=52

Rate=Rate

Length=Length

WLAN

track

O

WLAN_MuxDataChEst

MuxData_and_ChEst

Order=Order

Rate=Rate

Length=Length

WLAN

x

h

xh Port

P2

Num=2FFT_Cx

F10

Direction=ForwardSize=FFTSize

Order=Order

FFT_Cx

F8

Direction=Forward

Size=FFTSize

Order=Order

FFT_Cx

F11

Direction=Forward

Size=FFTSize

Order=Order

WLAN_BurstReceiver

BurstRec

Order=Order

Rate=Rate

Length=Length

WLAN

burst

WLAN_BurstSync

FrameSync

Order=Order

Sync

WLAN

Fork3

F12

Fork3

F13

WLAN_FineFreqSync

W1

Order=Order

f

WLAN

O

WLAN_FreqSync

FreqOffsetDetect

Order=Order

WLAN

O

Fork2

F14

Add2

A1

WLAN_DemuxBurst

DemuxFrame

Order=Order

Rate=Rate

Length=Length

WLAN0

S L DATA

OFDM Sym

Port

P1

Num=1

Channel correction

Fine frequency offset correction

& channel estimation

LongPreamble1

LongPreamble2

ShortPreamble

Coarse frequency offset

correction




November, 2001


Effects of Frequency Offset – with Frequency

CorrectionFrequency offset expressed as a percentage of sub-carriers frequency spacing (∆f=312.5kHz):

0% 1% 10% 50%

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.0

-0.5

0.0

0.5

1.0

real(iq[2,::])

i m a g ( i q [ 2 , : : ] )

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.0

-0.5

0.0

0.5

1.0

real(iq[3,::])

i m a g ( i q [ 3 , : : ] )

AA=0

Freq. Offset = 0 HzFreq. Offset / Freq. Spacing = 0%

AA=1

Freq. Offset = 3.125 kHzFreq. Offset / Freq. Spacing = 1%

AA=2


AA=3


-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.0

-0.5

0.0

0.5

1.0

real(iq[0,::])

i m a g ( i q [ 0 , : : ] )

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.0

-0.5

0.0

0.5

1.0

real(iq[1,::])

i m a g ( i q [ 1 , : : ] )

Freq. Offset / Freq. Spacing

0%

1%

10%

50%

IndexAA=0.000

40

AA=1.00040

AA=2.00040

AA=3.00040

ReceiverEVM

1.199E-5

1.214E-5

1.211E-5

1.210E-5




November, 2001


Effects of Oscillator Phase Noise

IQ

Modulator

IQ

Modulator

QAM

Mapping

QAM

MappingPilot

Insertion

Pilot

Insertion

IFFT(TX)

FFT(RX)

IFFT(TX)

FFT(RX)

G.I.

Addition

&

Windowing

G.I.

Addition

&

Windowing

DACDAC

HPA

Remove

G.I.

Remove

G.I.Channel

Correction

Channel

Correction

LNA

AGC Amp

Rx Lev. Det.

DataIn

ADCADC

Timing &

Frequency

Synchronization

Timing &Frequency

Synchronization

QAM

Demapping

QAM

Demapping

DataOut


Symbol Timing

Oscillator Phase Noise

De-interleaving/

FEC Decoding/

De-Scrambling

De-interleaving/

FEC Decoding/

De-Scrambling

Scrambling/

FEC Coding/

Interleaving

Scrambling/

FEC Coding/

Interleaving

Note: A practical oscillator does not

produce a carrier at exactly one frequency,

but rather a carrier that is phase

modulated by random phase jitter, As a

result, the frequency is never perfectly

constant, thereby causing ICI.




November, 2001


Effects of Oscillator Phase Noise

1/T

FFT Bin Spacing is 1/T




November, 2001


Effects of Oscillator Phase Noise (continued)

N_Tones model is used to model the phase noise

Based on Lorentzian spectrum

Characterized by –3dB-linewidth, -20dB per decade slope

N_Tones

N1

PN_Type=Random PN

PhaseNoiseData=PN

RandomPhase=No

AdditionalTones=""

Phase1=0.0

Power1=.010 W


TStep=50 nsec

Frequency offset[Hz]

PSD[Hz]

0

-3

-20dB/decade

-3dB linewidth

linewidthdB f

f f

f f S

l

l

l s

3:

/1

/2)(

22

−

+

=π

Phase noise profile based on Lorentzian spectrum




November, 2001



3 profiles of phase noise were simulated:

5.199 5.201

-120

-20

freq, GHz

d B m ( S 1 [ 0 , : : ] )

5.199 5.201

-120

-20

freq, GHz

d B m ( S 1 [ 2 , : : ] )

5.199 5.201

-120

-20

freq, GHz

d B m ( S 1 [ 1 , : : ] )

N_TonesN1

PN_Type=Random PN

PhaseNoiseData=PN

RandomPhase=No

AdditionalTones=""

Phase1=0.0

Power1=.010 W


TStep=50 nsec

VAR

VAR4

PN=if (AA==0) then PN0 elseif (AA==1) then PN1 else PN2 endif

PN2=" 120 -35 300 -43 3000 -63 30000 -83 300000 -103 1200000 -115"

PN1=" 120 -15 300 -23 3000 -43 30000 -63 300000 -83 1200000 -95"

PN0=" "

AA=1

EqnVar

Ideal -3dB linewidth=30Hz=0.01% of sub-carrier spacing

-3dB linewidth=3Hz=0.001% of sub-carrier spacing




November, 2001



-1 .2

-1 . 0

- 0 . 8

- 0 . 6

- 0 .4

- 0 .2

0 . 0

0 .2

0 .4

0 . 6

0 . 8

1 . 0

1 .2

-1.0

-0.5

0.0

0.5

1.0

real(iq[0,::])

i m a g ( i q [ 0 , : : ] )

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

real(iq[1,::])

m a g

q

, : :

-1 .2

-1 . 0

- 0 . 8

- 0 . 6

- 0 .4

- 0 .2

0 . 0

0 .2

0 .4

0 . 6

0 . 8

1 . 0

1 .2

-1.0

-0.5

0.0

0.5

1.0

real(iq[2,::])

i m

a g ( i q [ 2 , : : ] )

Index

40

ReceiverEVM_pn

AA=0.000 AA=1.000 AA=2.000

1.199E-5 0.143 0.014

8 10 12 14 16 18 20

1E-5

1E-4

1E-3

1E-2

1E-1

5E-1

BER_pn.DF.CN

B E R

_ p n -3dB linewidth=30Hz

-3dB linewidth=3HzIdeal

-3dB linewidth=30Hz -3dB linewidth=3HzIdeal




November, 2001


Effects of Fixed Point Implementation of IFFT/FFT

IQ

Modulator

IQ

Modulator

QAM

Mapping

QAM

MappingPilot

Insertion

Pilot

Insertion

IFFT(TX)

FFT(RX)

IFFT(TX)

FFT(RX)

G.I.

Addition

&

Windowing

G.I.

Addition

&

Windowing

DACDAC

HPA

Remove

G.I.

Remove

G.I.Channel

Correction

Channel

Correction

LNA

AGC Amp

Rx Lev. Det.

DataIn

ADCADC

Timing &

Frequency

Synchronization

Timing &Frequency

Synchronization

QAM

Demapping

QAM

Demapping

DataOut


Symbol Timing

FixedPointeffects

De-interleaving/

FEC Decoding/

De-Scrambling

De-interleaving/

FEC Decoding/

De-Scrambling

Scrambling/

FEC Coding/

Interleaving

Scrambling/

FEC Coding/

Interleaving








November, 2001


WLAN 802.11a ADS DesignGuide –

Schematic Menu




November, 2001


The Details

3 Parts:

Tutorial

Impairments

Pre-built test benches



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Documents

OFDM Impairments