Signal Processing in Communications I:

xDSL

Samuel Sheng, Ph.D.

DataPath Systems, Inc.ssheng@datapathsystems.com

408-366-339115 August 99

1800

0feet: ~ 95% of cus tomers

9000

feet: ~ 50% of cus tomers1200

0feet: ~ 80% of cus tomers

xDSL: Broadband over Twisted Pair

• xDSL: the delivery of high-speed digital data over twisted-pair local loop infrastructure

• An “Alphabet Soup” of xDSL services

HDSLHDSL2

VDSL

SDSL

MDSLADSL

CentralOffice

Twisted PairInfrastructure

Goals of xDSL Signal Processing

• As in voiceband modems, “evolution” in signal processing

• Performance

Highest data rate

Longest loop possible (reach)

• Cost

Two-pair vs. one-pair

Repeatered

Line conditioning

xTU-C

PSTN

RemoteNetwork

POTS

Splitter

xTU-R

DigitalNetwork

RemoteInstallationTwisted

Pair

CentralOffice

(ADSL)

Splitter

(ADSL)

Outline

• The Twisted-Pair Transmission Environment

• High-Bit-Rate Digital Subscriber Line (HDSL)

• Asymmetric Digital Subscriber Line (ADSL)

• Some Future xDSL Systems

• HDSL2

• Very High-Bit-Rate Digital Subscriber Line (VDSL)

• Conclusions

Line Attenuation vs. Frequency, ADSL

0.01 0.1 1 10

-140

-120

-100

-80

-60

-40

-20

0

Lin

e L

oss

, dB

5000 ft.

10000 ft.

15000 ft.

20000 ft.

MHz

Twisted Pair: Transmission Environment

• Channel Frequency Response

• Channel Background Thermal Noise: -140 dBm/Hz

Twisted Pair: Other Impairments

Bridged Taps

Near-End/Far-End Crosstalk (NEXT/FEXT)

RF Ingress/Impulse Noise

Twisted-Pair

AM Radio, HAM Radio, etc.Motors,Car Ignitions,Etc.

RX

“Other” transmitters coupling into receiver

Near-End Far-End

Twisted-Pair 1

Twisted-Pair 2

Twisted-Pair

Unterminated bridge tap

frequency

Channel Response

Spectral Nulls!

TX TX

TX

High-Speed Digital Subscriber Line (HDSL)

• “Successor” to T1/E1

• Data transmission

• DS1/T1: 1.544 Mbps

• 12 kft, 24 gauge

• 768 kbps duplex per pair, two pairs

• Repeatered beyond 12 kft

• Transceiver:

• Dual-duplex

• Echo-cancelled

• Linear 2B1Q modulation, 196 kHz bandwidth

Basic HDSL Transceiver

2B1QEncode

PulseShaping

FIRDAC

Anal

og

Front En

d +

Hyb

rid

ADC

EchoCancel.

FIR

+-

Twisted-PairLine

Forward FIR

+

Reverse FIR

2B1QSlicer

TimingRecovery

Bits In

Bits Out

AdaptiveDecision-FeedbackEqualizer

2B1QDecode

+3

+1

-1

-3

10

11

01

00

Bits QuaternarySymbol

(128 tap)

(128 tap)

(8-24 taps)

One implementation:7.1mm x 8.55mm181000 transistors

[PairGain93]

825 mW, 50 MHz

HDSL Echo Cancellation

• Fully-overlapped spectrum between TX and RX -need to cancel echo response (hybrid leakage)

• Echo cancellation FIR adapted at startup

• Echo path impulse response typically time-limited to ~280 usec

-> Typical FIR filter size: ~128 taps for baud sampling (400 kHz)

Echo lea

kage

Hybrid leakage - function of:Mismatch parasitics in hybridImpedance of loop vs. frequency

“Compromise” hybrid

Echo cancel FIR response includes:

Analog TX/RX filter responseTransformerHybrid leakage

PulseShaping

FIR

ADC

EchoCancel.

FIR

+-

Twisted-PairLine

DAC

An

alo

g Fr

on

t En

d +

Hyb

rid

• Need to minimize intersymbol interference at slicer

• Employ zero-forcing equalization:

• However, noise enhancement is a problem:use decision-feedback equalization

HDSL Equalization

Forward FIR

+

+-

Reverse FIR

2B1QSlicer

Bits Out

Precursor

Decision-Feedback(Postcursor cancellation)

ReceivedSamples

cancellation

Most ISI is postcursor:At baud rate:

8-tap forward FIR128-tap reverse FIR

ωω

Boost!TX Symbols

Channel Equalizer

ω

AggregateChannel (at Slicer)

RX Symbols

Asymmetric Digital Subscriber Line (ADSL)

• Intended for consumer deployment

• Asymmetric data transmission

Rate Adaptive: “Best effort” on a given loop:

(up to 8 Mbps downstream, 800 kbps upstream)

Single-pair, repeaterless

• Transceiver:

Full-duplex

Echo-cancelled or frequency-division

Discrete Multitone Modulation

Coexistence with POTS

• Two flavors of ADSL

g.dmt

(

g

.992.1): “full” ADSL

g.lite

(

g

.992.2): “splitterless, low-cost, rate-limited”

(1.544 Mbps peak downstream)

• Idea: separate information across narrow channels (or “bins”), with 256 (downstream) or 32 (upstream) possible bins.

• Optimize QAM constellation size for each bin based on SNR(“automatically” rate/margin adaptive)

• Symbol time is very slow: (~ 250 usec for ADSL);bits/symbol is

huge

Can theoretically deliver payload near the Shannon capacity!

Discrete Multitone Modulation (DMT)

frequency

. . .

4.3125 kHz spacing

1.104 MHzLow frequency bins unused

“Prototype” DMT Transceiver

• Leverage off of massive DSP capability

Signal synthesis is done entirely via FFT/IFFT engines

Serial-ParallelConvert

IFFT

FEQ TEQFFTParallel-Serial

Convert

To D

AC

/

Fro

m A

DC

/

Inter-polator

High-OrderFilter

High-OrderFilter

An

alo

gFr

on

t-En

d

An

alo

gFr

on

t-En

d

Group delay/amplitude equalization

Per-bin

equalization(1 complex tap per bin,

Time-domain smoothing

(time domain)256 bins)

Transmitter

Receiver

Some Key Issues....

• Timing recovery / Frequency accuracy

Intersymbol interference in the

frequency

domain

Expend downstream bin 64 (276 kHz) as pilot timing tone

• Cyclic prefix

Append L samples of the IFFT TX output

Serves as guard time against intersymbol interference

Equalization via FFT requires

circular

convolution

IFFT output

N samples

First L samples

Copy and append To DACN+L samples

“Looks” periodic!

. . .

Frequency error in receiverresults in “shift” of bins

Some Key Issues.... (cont)

• Splitterless vs. Splittered

Full rate

g.dmt

requires splitter at remote to minimize interference between ADSL and POTS bands.

g.lite

is “splitterless” for cost reasons (avoid rewiring).Proving to be difficult to deploy; customer installed “microfilters” are required.

Home/PCNetwork

POTS

Splitter

ATU-R

High-frequency ADSL signal

Low-frequency POTS signal

Without

the splitter:

ADSL intermodulates into POTS band

POTS on/off hook affects ADSL signal

An ADSL Transceiver

Serial-ParallelConvert

IFFT

UpdateEcho

FEQ

TimingRecovery

TEQ

EchoCancel

+

FFTParallel-Serial

Convert

CyclicPrefix

-

DAC

An

alo

g Fr

on

t En

d +

Hyb

rid

ADC

Twis

ted

-Pai

rLi

ne

Echo cancellationis system

•

Processing Load

([Macq98])

FFT/IFFT/TEQ: 70-90 MIPS (multiply-accumulate)

Other DSP functions: 20-50 MIPS

Total: 100-150 MIPS

Decimation/Baud Sampling

Mapper

DeMapper

ViterbiDecode

TrellisEncode

Bits In

Bits Out

Trellis Codingis systemdependent dependent

ADSL: DSP vs. ASIC?

• DSP Approach

• Extremely flexible, lower development costs

• Time to market

• Modem can evolve with standard via code change

• ASIC Approach

• Reduced flexibility, longer development cycle

• Power consumption: critical in central office

200+ MHz DSP’s vs. custom ASIC?

“Low power” DSP techniques applicable in custom

• Component may be cheaper (?)

• Both approaches seen in marketplace

ADSL: Analog Signal Processing Issues

• Goal of DMT: achieve as close to Shannon capacity as possible. Equivalently, maximize SNR in each bin

• Analog front-end performance (AFE) is critical

Operating in frequencies of the line with tremendous attenuation

Want AFE input referred noise significantly lower thanline/crosstalk noise

Intermodulation distortion in AFE will cause spillover from bin-to-bin: impacts noise floor

Analog front-end performance should not limit the system!

Dynamic Range Requirements in ADSL

• Key issue on longest lines: small in-band received signal alongwith huge out-of-band echo signal coming through the hybrid.

• Need 14-bit linear analog signal path (or better!) in theanalog front end (AFE)

DSP 14bADC

Integrated Low-Noise

Analog Front End RX Path

2-4 WireHybrid

Analog Front End TX Path

14bDAC

BIG TX SIGNAL

SMALL RX SIGNAL

-10dB

LowpassFilters

Amp

2 4 6 8 10 12 14 16 180

1

2

3

4

5

6

7

8

9

Loop Length (kFt)

Mb

ps

Ach

ieva

ble

Dat

a R

ate

Impact of the AFE on ADSL System Performance

A Few Words about SDSL...

• Symmetric (or Single-Pair) Digital Subscriber Line

Equal upstream/downstream data rates

Full duplex, single-pair

• Tends to be a single-pair variant of HDSL

• Early 1990’s:

Marginally achieved 1.544 Mbps over 6 kft of 26 AWG wire

• Late 1990’s:

Wanted: symmetric system achieving 1.544 Mbps with the same reach and robustness as HDSL.

Utilization of advanced signal processing is key!

HDSL2

• Single-pair 1.544 Mbps transmission

9000 ft. AWG 26, 12000 ft. AWG 24

Cannot adversely impact other xDSL systems present in binder

• Proposed Systems

Symmetric Echo-Cancelled Transmissions (SET)

Frequency-Division

Partial Overlap Echo-Cancelled Transmission (POET)

• End-to-end latency comparable to HDSL (< 500 usec)

• Still in committee; ratification likely later this year

• Modulation strategy:

16-level PAM transmission with Trellis-Coding

Transmit Spectra, xDSL

-100.00

-90.00

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

0

20

0

40

0

60

0

80

0

10

00

Frequency

Pow

er S

pec

tral

Den

sity

HDSL

DMT ADSL DS

DMT ADSL US

ISDN

T1

dBm/Hz

kHz

HDSL2 Spectral Compatibility

• Critical issue: NEXT interaction in binder groups

(figure cf. [Zimmer])

• Overlapped PAM Transmission with Interlocking Spectra

Slightly asymmetric upstream/downstream spectrum

Has superior spectral folding performance at baud sampling ([Zimmer])

-100.00

-90.00

-80.00

-70.00

-60.00

-50.00

-40.00

-30.000

200

400

600

800

1000

Frequency

Po

wer

Sp

ectr

al D

ensi

ty

OPTIS Upstream

OPTIS Downstream

dBm/Hz

kHz

HDSL2: OPTIS Transmit Mask

HDSL2: Signal Processing Complexity I

• Tomlinson-Harashima Precoding

Moves feedback equalizer in DFE into the transmitter

Eliminates DFE error propagation

Allows use of soft-decision error correction decoding

• Large complexity increase in transmitter (12-16 bits required)

2B1QEncode

PulseShaping

FIRForward FIR +

Reverse FIR

2B1QSlicer

Bits In Bits Out2B1Q

Decode

HDSL Transmitter HDSL Receiver

TwistedPair

Channel

TrellisEncode Forward FIR+

Reverse FIR

ModuloBits In Bits Out

ViterbiDecode

HDSL2 Transmitter HDSL2 Receiver

TwistedPair

Channel

PulseShaping

FIR

EchoCancel

FIR

Precoder

Modulo

HDSL2: Signal Processing Complexity II

• Use of Fractionally-Spaced Forward Equalizers

Doubles complexity in receiver forward FIR

• Use of Trellis Coding

Error Correction needs to provide > 4 dB (5.1 dB) SNR gain

Needs to be low latency

32 to 512 state Viterbi detectors needed: HUGE complexity!

• Estimated Processing Requirements: 200-250 MIPS

Forward FIR

Bits OutViterbiDecode

HDSL2 Receiver

Modulo

• “Next generation” in data rate to customers:

Symmetric or nonsymmetric data services

26 Mbps downstream, 3-26 Mbps upstream

Transmit bandwidth: ~ 3 - 20 MHz

• Short range: < 5000 ft. in loop

Depends having fiber-to-the-curb

Communicates to local ONU (optical network unit)

• Standard remains in question: DMT vs. QAM/CAP

Very High-Bit-Rate Digital Subscriber Line (VDSL)

ONU

< 5000 ft. oftwisted pair

FiberBackbone

VDSL: RF Ingress/Egress Issues

• RF Ingress

AM Radio: Interference PSD on line~ -80 to -120 dBm/Hz

0.56 - 1.6 MHz

HAM Radio: Interference PSD on line ~ -35 to -80 dBm/Hz

1.8 - 2.0 MHz3.5 - 4.0 MHz7.0 - 7.1 MHz10.1 - 10.15 MHz14.0 - 14.35 MHz...

• VDSL Signals

-60 dBm/Hz nominal transmit power spectrum

Typical receive power spectrum (AWG 26, 2 kft)

- 90 dBm/Hz @ 2 MHz- 140 dBm/Hz @ 10 MHz

RFI can be significantly larger than the received signal!

VDSL: RFI Cancellation (DMT systems)

• Problem: Sidelobes in FFT from RFI corrupt other bins

• Partial solution: Apply windowing before receiver FFT

• May lead to spectral broadening of received DMT tones

Intersymbol interference in frequency domain (between bins)

FEQ block now needs to consider ISI: decision feedback is needed

• Need to estimate and eliminate RFI interference

dB

Frequency Bin

--50.00

-40.00

-30.00

-20.00

-10.00

0.00

25.0 50.0

Unwindowed

Windowed

• Modified DMT receiver

• Implementation Complexity

Much higher sample rates

Interference cancellation

One recent VLSI solution [Alcatel99]:680k gates, 0.35u CMOS150 mm2, 2.7W @ 3.3V

VDSL: RFI Cancellation (cont.)

TEQ+ FFTParallel-Serial

Convert-

ADCWindow

RFIEstimation

Error SignalSynthesis

FEQ

Conclusion

• xDSL has evolved on many fronts

Data rate

Reach

One pair vs. Two pair

• With each evolution, commensurate change in signal processing technology

HDSL: Adaptive DFE, echo cancellation

ADSL: DMT

VDSL: RFI cancellation, Very high-speed DSP

HDSL2: Tomlinson-Harashima precoding, Better error correction

Future xDSL’s will continue this trend: leveraging signal processing toward higher speed and longer reach, over existing twisted pair installation!

Bibliography

[Alcatel99] D. Veithen, P. Spruyt, T. Pollet, et al. “A 70 MB/s Variable-Rate DMT-Based Modem for VDSL.” IEEE 1999 International Solid-State Circuits Conference, San Francisco, CA Feb. 1999.

[Chen98] W. Chen. DSL: Simulation Techniques and Standards Development for Digital Subscriber Line Systems. New York: Macmillan Technical Publishing, 1998.

[Chow95] P. Chow, J. Cioffi, J. Bingham. “A Practical Discrete Multitone Transceiver Loading Algorithm for Data Transmission over Spectrally Shaped Channels.” IEEE Transactions on Communications, Vol. 43, No. 2/3/4, pp. 773-775. Feb/Mar/April 1995.

[Cioffi99] J. Cioffi, V. Oksman, J. Werner, et al. “Very-High-Speed Digital Subscriber Lines.” IEEE Communications Magazine, Vol. 27, No. 4, pp. 72-79. April 1999.

[Conroy99] C. Conroy, S. Sheng, A. Feldman, et al. “A CMOS Analog Front-End IC for DMT ADSL.” IEEE 1999 International Solid-State Circuits Conference, San Francisco, CA Feb. 1999.

[Gdmt] S. Palm, ed. ITU - G.992.1, Asymmetrical Digital Subscriber Line (ADSL) Transceiver. ITU - Telecommunication Standardization Sector. Geneva, 12-23 October 1998.

[Glite] C. Hansen, ed. ITU - G.992.2, Splitterless Asymmetrical Digital Subscriber Line (ADSL) Transceiver. ITU - Telecommunication Standardization Sector. 12 October 1998.

[Ho96] M. Ho, J. Cioffi, J. Bingham. “Discrete Multitone Echo Cancellation.” IEEE Transactions on Communications, Vol. 44, No. 7, pp. 817-825. July 1996.

[Lee88] E.A. Lee and D.G. Messerschmitt. Digital Communication. New York:Kluwer Academic Publishers, 1988.

[Macq98] D. Macq. “Short Course: xDSL Broadband Interactive Communications via POTS.” IEEE 1998 International Solid-State Circuits Conference, San Francisco, CA Feb. 1998.

[PairGain93] M. Kuczynski, W. Lao, A. Dong, et al. “A 1 Mb/s Digital Subscriber Line Transceiver Signal Processor.” IEEE 1993 International Solid-State Circuits Conference, San Francisco, CA Feb. 1993.

[ProakB83] J.G. Proakis. Digital Communications. New York: McGraw-Hill Book Co., 1983

[Rausch98] D. Rauschmayer. ADSL/VDSL Principles: A Practical and Precise Study of Asymmetric Digital Subscriber Lines and Very High Speed Digital Subscriber Lines. New York: Macmillan Technical Publishing, 1999.

[Starr99] T. Starr, J. Cioffi, P. Silverman. Understanding Digital Subscriber Line Technology. New Jersey: Prentice-Hall PTR, 1999.

[Zimmer] G. Zimmerman. “HDSL2 Tutorial: Spectral Compatibility and Real-World Performance.” PairGain Technologies, June 1998.