Introduction to ADSL ModemsIntroduction to ADSL Modems

Prof. Brian L. EvansDept. of Electrical and Comp. Eng.The University of Texas at Austin

http://signal.ece.utexas.edu

UT graduate students: Mr. Zukang Shen, Mr. Daifeng Wang, Mr. Ian Wong

UT Ph.D. graduates: Dr. Güner Arslan (Silicon Labs), Dr. Biao Lu (Schlumberger), Dr. Ming Ding (Bandspeed), Dr. Milos Milosevic (Schlumberger)

UT senior design students: Wade Berglund, Jerel Canales, David J. Love,Ketan Mandke, Scott Margo, Esther Resendiz, Jeff Wu

Other collaborators: Dr. Lloyd D. Clark (Schlumberger), Prof. C. Richard Johnson, Jr. (Cornell), Prof. Sayfe Kiaei (ASU), Prof. Rick Martin (AFIT),

Prof. Marc Moonen (KU Leuven), Dr. Lucio F. C. Pessoa (Motorola),Dr. Arthur J. Redfern (Texas Instruments)

Last modified August 27, 2005

25-2

OutlineOutline• Broadband Access

– Applications– Digital Subscriber Line (DSL) Standards

• ADSL Modulation Methods– ADSL Transceiver Block Diagram– Quadrature Amplitude Modulation– Multicarrier Modulation

• ADSL Transceiver Design– Inter-symbol Interference– Time-Domain Equalization– Frequency-Domain Equalization

• Conclusion

25-3

Residential Application Downstream rate (kb/s)

Upstream rate (kb/s)

Willing to pay Demand Potential

Database Access 384 9 High Medium On-line directory; yellow pages 384 9 Low High Video Phone 1,500 1,500 High Medium Home Shopping 1,500 64 Low Medium Video Games 1,500 1,500 Medium Medium Internet 3,000 384 High Medium Broadcast Video 6,000 0 Low High High definition TV 24,000 0 High Medium

Business Application Downstream rate (kb/s)

Upstream rate (kb/s)

Willing to pay Demand Potential

On-line directory; yellow pages 384 9 Medium High Financial news 1,500 9 Medium Low Video phone 1,500 1,500 High Low Internet 3,000 384 High High Video conference 3,000 3,000 High Low Remote office 6,000 1,500 High Medium LAN interconnection 10,000 10,000 Medium Medium Supercomputing, CAD 45,000 45,000 High Low

Applications of Broadband AccessApplications of Broadband Access

25-4

DSL Broadband AccessDSL Broadband Access

Customer Premises

downstream

upstreamVoice

Switch

Central Office

DSLAM

ADSL modem

ADSL modem

LPFLPF

PSTN

Internet

25-5

DSL Broadband Access StandardsDSL Broadband Access Standards

Courtesy of Mr. Shawn McCaslin (National Instruments, Austin, TX)

xDSL Meaning Data Rate Mode Applications ISDN Integrated Services

Digital Network 144 kbps Symmetric Internet Access, Voice,

Pair Gain (2 channels) T1 T-Carrier One

(requires two pairs) 1.544 Mbps Symmetric Business, Internet

Service HDSL High-Speed Digital

Subscriber Line (requires two pairs)

1.544 Mbps Symmetric

Pair Gain (12 channels), Internet Access, T1/E1 replacement

SHDSL Single Line HDSL 1.544 Mbps Symmetric Same as HDSL except pair gain is 24 channels

Splitterless ADSL

Splitterless Asymmetric DSL (G.Lite)

Up to 1.5 Mbps Up to 512 kbps

Downstream Upstream

Internet Access, Video Phone

Full-Rate ADSL

Asymmetric DSL (G.DMT)

Up to 10 Mbps Up to 1 Mbps

Downstream Upstream

Internet Access, Video Conferencing, Remote LAN Access

VDSL Very High-Speed Digital Subscriber Line (proposed)

Up to 22 Mbps Up to 3 Mbps Up to 6 Mbps

Downstream Upstream Symmetric

Internet Access, Video-on-demand, ATM, Fiber to the Hood

25-6

Spectral Compatibility of xDSLSpectral Compatibility of xDSL

10k 100k 1M 10M 100M

POTS

ISDN

ADSL - USA

ADSL - Europe

HDSL/SHDSL

HomePNA

VDSL - FDD

Upstream Downstream Mixed

Frequency (Hz)

optional

1.1 MHz

12 MHz

25-7

P/S

QAM demod

decoder

invert channel

=frequency

domainequalizer

S/P

quadrature amplitude

modulation (QAM) encoder

mirrordataand

N-IFFT

add cyclic prefix

P/SD/A +

transmit filter

N-FFTand

removemirrored

data

S/Premove

cyclic prefix

TRANSMITTER

RECEIVER

N/2 subchannels N real samples

N real samplesN/2 subchannels

time domain

equalizer (FIR filter)

receive filter

+A/D

channel

ADSL Modem ADSL Modem

Bits

00110

25-8

Bit ManipulationsBit Manipulations• Serial-to-parallel

converter

• Example of one input bit stream and two output words

• Parallel-to-serialconverter

• Example of two input words and one output bit stream

S/P

Bits

00110110

00

Words

S/P

Words

00110

110

00

Bits

25-9

00

0

2

1

cos

FY

ttfty

Amplitude Modulation by Cosine FunctionAmplitude Modulation by Cosine Function• Multiplication in time is convolution in Fourier

domain

• Sifting property of the Dirac delta functional

• Fourier transform property for modulation by a cosine

000 ttxdtxttttx

txdtxttx

00 2

1

2

1 FFY

25-10

Amplitude Modulation by Cosine FunctionAmplitude Modulation by Cosine Function

• Example: y(t) = f(t) cos(0 t)– f(t) is an ideal lowpass signal

– Assume 1 << 0

– Y() is real-valued if F() is real-valued

• Demodulation is modulation then lowpass filtering

• Similar derivation for modulation with sin(0t)

0

1

-

F()

0

½

- - - +

- +

½F½F

00 2

1

2

1 FFY

Y()

25-11

00

0

2

1

sin

jFY

ttfty

Amplitude Modulation by Sine FunctionAmplitude Modulation by Sine Function• Multiplication in time is convolution in Fourier

domain

• Sifting property of the Dirac delta functional

• Fourier transform property for modulation by a sine

000 ttxdtxttttx

txdtxttx

00 22 F

jF

jY

25-12

Amplitude Modulation by Sine FunctionAmplitude Modulation by Sine Function

• Example: y(t) = f(t) sin(0 t)– f(t) is an ideal lowpass signal

– Assume 1 << 0

– Y() is imaginary-valued ifF() is real-valued

• Demodulation is modulation then lowpass filtering

0

1

-

F()

j ½

- - - +

- +

-j ½Fj ½F

-j ½

00 22 F

jF

jY

Y()

25-13

– One carrier – Single signal, occupying the

whole available bandwidth– The symbol rate is the

bandwidth of the signal being centered on carrier frequency

Quadrature Amplitude Modulation (QAM)Quadrature Amplitude Modulation (QAM)

frequency

channel

mag

nitu

de

fc

Bits Constellation encoder Bandpass

Lowpassfilter

Lowpassfilter

I

Q

TX

cos(2fct)

sin(2fct)

-

Modulator

I

Q iX

00110

25-14

Multicarrier ModulationMulticarrier Modulation• Divide broadband channel into narrowband

subchannels• Discrete Multitone (DMT) modulation

– Based on fast Fourier transform (related to Fourier series)

– Standardized for ADSL

– Proposed for VDSL

subchannel

frequency

ma

gn

itude

carrier

channel

every subchannel behaves like QAM

Subchannels are 4.3 kHz wide in ADSL

25-15

Multicarrier Modulation by Inverse FFTMulticarrier Modulation by Inverse FFT

2/NX

x

tfje 12

1X

x

tfje 22

x

tfj Ne 2/2

+

g(t)

2X g(t)

g(t)

x

nN

je

12

1X

x

nN

je

22

x

nN

Nj

e2/

2

+2X

2/NX

Discretetime

g(t) : pulse shaping filter Xi : ith symbol from encoder

I

QiX

25-16

Multicarrier Modulation in ADSLMulticarrier Modulation in ADSL

N-pointInverse

FastFourier

Transform(IFFT)

X1

X2

X1*

x0

x1

x2

xN-1X2

*

XN/2

XN/2-1*

X0

N time

samples

N/2 subchannels

(carriers)

QAM00101

I

QiX

25-17

Multicarrier Modulation in ADSLMulticarrier Modulation in ADSL

CP: Cyclic Prefix

N samplesv samples

CP CPs y m b o l ( i ) s y m b o l ( i+1)

copy copy

D/A + transmit filter

ADSL downstream upstream

CP 32 4 N 512 64

Inverse FFT

25-18

Multicarrier Demodulation in ADSLMulticarrier Demodulation in ADSL

N-pointFast

FourierTransform

(FFT)

N time

samples

N/2 subchannels

(carriers)

S/P

*1

~X

*12

~NX

2

~NX

12

~NX

0

~X

0~x

1~x

2~x

1~

Nx

25-19

Inter-symbol Interference (ISI)Inter-symbol Interference (ISI)• Ideal channel

– Impulse response is an impulse

– Frequency response is flat

• Non-ideal channelcauses ISI– Channel memory– Magnitude and phase

variation

• Received symbol is weighted sum of neighboring symbols– Weights are determined by

channel impulse response

1 1 1

-1

1.7

.4 .1

1

1.7

2.1

11 1 1

Channelimpulseresponse

Received signal

Thresholdat zero

Detected signal

* =

1 .7

1

25-20

Channel Impulse ResponseChannel Impulse Response

25-21

Channel Impulse ResponseChannel Impulse Response

25-22

Cyclic Prefix Helps in Fighting ISI Cyclic Prefix Helps in Fighting ISI • Provide guard time between successive symbols

– No ISI if channel length is shorter than +1 samples

• Choose guard time samples to be a copy of the beginning of the symbol – cyclic prefix– Cyclic prefix converts linear convolution into circular

convolution

– Need circular convolution so that

symbol channel FFT(symbol) x FFT(channel)

– Then division by the FFT(channel) can undo channel distortionN samplesv samples

CP CPs y m b o l ( i ) s y m b o l ( i+1)

copy copy

25-23

Cyclic Prefix Helps in Fighting ISICyclic Prefix Helps in Fighting ISI

cyclic prefix

equal

to be removed

Repeated symbol

*

=

25-24

Combat ISI with Time-Domain EqualizerCombat ISI with Time-Domain Equalizer• Channel length is usually longer than cyclic prefix• Use finite impulse response (FIR) filter called a

time-domain equalizer to shorten channel impulse response to be no longer than cyclic prefix length

channel

Shortened channel

25-25

m

mkxmhky ][ ][ ][ dtxhty

h[k] y[k]x[k]Representedby its impulseresponse

h(t) y(t)x(t)Representedby its impulseresponse

Convolution ReviewConvolution Review• Discrete-time

convolution

• For every k, we compute a new summation

• Continuous-time convolution

• For every value of t, we compute a new integral

25-26

1

0

][ ][ ][N

m

mkxmhky

z-1 z-1 z-1…

…

x[k]

y[k]

h[0] h[1] h[2] h[N-1]

Finite Impulse Response (FIR) FilterFinite Impulse Response (FIR) Filter• Assuming that h[k] is causal and has finite

duration from k = 0, …, N-1

• Block diagram of an implementation (called a finite impulse response filter)

25-27

z-

h + w

b

-xk

yk ek

zk

rk

nk

+

• Minimize mean squared error

E{ek2} where ek=bk- - hk*wk

– Chose length of bk to shorten length of hk*wk

• Disadvantages– Does not consider channel capacity

– Deep notches in equalizer frequency response

Example Time-Domain EqualizerExample Time-Domain Equalizer

25-28

Frequency Domain Equalizer in ADSLFrequency Domain Equalizer in ADSL• Problem: FFT coefficients (constellation points)

have been distorted by the channel.

• Solution: Use Frequency-domain Equalizer (FEQ) to invert the channel.

• Implementation: N/2 single-tap filters with complex coefficients.

25-29

Frequency Domain Equalizer in ADSLFrequency Domain Equalizer in ADSL

YN/2-1

Y1

Y0

N/2 subchannels

(carriers)

QAM decoder

0101 Yi

1

~X

12

~NX

0

~X

FEQ

iii XcY~

iX~

Yi

Q

I

FEQ

25-30

P/S

QAM demod

decoder

invert channel

=frequency

domainequalizer

S/P

quadrature amplitude

modulation (QAM) encoder

mirrordataand

N-IFFT

add cyclic prefix

P/SD/A +

transmit filter

N-FFTand

removemirrored

data

S/Premove

cyclic prefix

TRANSMITTER

RECEIVER

N/2 subchannels N real samples

N real samplesN/2 subchannels

time domain

equalizer (FIR filter)

receive filter

+A/D

channel

ADSL Modem ADSL Modem

Bits

00110

25-31

Crosstalk and Near-End EchoCrosstalk and Near-End Echo

f

f f

TX

RX

H cable H

TX

RX

f f

TX

RX

H cable H

TX

RXfNEXT

FEXT

Near-End echo

Near-End echo

25-32

ADSL vs. FEXT, NEXT, Near-end EchoADSL vs. FEXT, NEXT, Near-end Echo• ADSL with Freq. Division Multiplexing - FDM

– Near-End Echo filtered out

– Self-NEXT (NEXT from another ADSL) mostly filtered out

– FEXT and NEXT (from another type of DSL) are problems

• ADSL with overlapped spectrum (Echo Cancelled)– Near-End Echo Eliminated using an echo canceller

– FEXT, NEXT and self-NEXT are a problem

– Larger Spectrum available for downstream – higher data rate

US DS DSUS

f f

FDM EC