BLIND CROSSTALK CANCELLATION FOR DMT

SYSTEMS

Nadeem Ahmed Nirmal Warke ECE Dept. DSPS R&D Center Rice University Texas Instruments

Motivation

• New multimedia and networking applications => increasing demand for bandwidth

• DSL is cost effective broadband solution

100 MHz10 MHz1 MHz100 kHz10 kHz

POTS ADSL

VDSLISDN

HDSL

Motivation

• Increasing density of DSL deployment => Increased crosstalk

• Crosstalk typically increases with frequency => significant impairment for high speed DSL

Binder

ADSL linesHDSL linesPOTS

Types of Crosstalk

Near-End Crosstalk (NEXT):

• Interference that arises when signals are transmitted in opposite directions

Far-End Crosstalk (FEXT):

• Interference that arises when signals are transmitted in the same direction

DSL System Model

• FEXT signals travel the entire length of the channel

• FDD modems virtually eliminate self-NEXT.

• Main source of crosstalk comes from other services (i.e. HDSL, T1, etc), which are much stronger than self-FEXT.

Crosstalk Power on Line

Combating Crosstalk

• Crosstalk Avoidance– Varying transmit spectra – Modified bit-loading algorithm – Block coding across modems at CO

• Crosstalk Cancellation– Treat as multiuser detection problem – Using DFE’s – Exploit symbol rate differences

Varying Transmit Spectra

• Design optimal transmit spectra which vary with channel, noise and interference

• Designed to reject self-NEXT in a manner which maximizes overall data rate

• Maintains spectral compatibility with other services

Modified Bit-Loading Algorithm

• Modify the bit-loading algorithm

– Change order of placing power in bins

– Factor NEXT into algorithm

– Minimizes NEXT within cable binder and extend reach of service

Block Coding Across COs

• Block coding to eliminate NEXT

• If code blocks are greater than a minimum length, NEXT can be completely eliminated

• Need control of a service i.e., all DSL modems– only useful for self-NEXT rejection

Multi-User Detection

• Use multiuser detection techniques to cancel crosstalk

• Jointly detect desired and crosstalk signals

• Published results for Home LAN interference cancellation from VDSL

DFE For Self-NEXT/FEXT

• Use DFE to remove cyclo-stationary crosstalk

• Assumes crosstalk has same sampling rate as source

• Useful for self-NEXT and self-FEXT cancellation

Excess Band Crosstalk Cancellation

• Crosstalkers like ISDN, HDSL, T1 have large excess band

• Algorithm– Exploits lower symbol rate of crosstalker

relative to the sampling rate of DSL– Crosstalker estimated in excess band and

cancelled in main band

Practical Issues

• Most methods require knowledge of crosstalk coupling function

• How do you reliably estimate the coupling function-– Use models?– Based on training data?

• Very difficult problem

Excess Band Crosstalk Cancellation

• Paper by Zeng et al on Crosstalk Cancellation for DMT Systems

Excess Band Crosstalk Cancellation

• Brick wall filters cannot be realized

• After D/A conversion, filter cannot remove all of image energy

• If crosstalk signal is oversampled with respect to xDSL, excess band can be observed

• Estimate crosstalk signal in excess band and predict crosstalk in main band

Mathematical Formulation• DMT Modulation

• System Impaiments- crosstalk and noise

• DMT Demodulation

*Qa

nCbPay

nHx

Qyz

Mathematical Formulation

• Partition into 2 freq. Bands:

2 => main band 1 => excess band

• Demodulate DMT signal in excess band and subtract to estimate crosstalk signal

1

2

1

2

11

22

1

2

n

n

xH

xH

z

z

11

111

nxH

zr

Cancellation Algorithm

xMrxMrEM

*minarg

rRHRHRHHX nxnc1*

1

111

1*12

MProject onto main band

• Let x = M.r be a linear estimate of crosstalk signal component x

• MMSE Estimate:

• Hence crosstalk signal in main band is,

Blind Cancellation

• If = .C and x = b, channel is assumed to be known => Zeng’s solution

• Instead, let = and x = C.b => Blind Approach

• Solution uses crosstalk statistics i.e. autocorrelation information – Estimate coupling function and crosstalk data

simultaneously

rRHRHRHHX nxnc1*

1

111

1*12

2H 2Q

2H 2Q

Dependence on crosstalk symbol delay

rDQQDX c*

1,*1022,

1121

*10 ))0(( Cbn RQQ

*** )0( jjCbiijCbi DQRQDQRQ

• Relative crosstalk symbol delay varies with DMT frame => varies with DMT frame

•

•

where,

CbR

Blind Cancellation- Practical Solution

• Autocorrelation can be easily estimated during training and/or quiet periods

• Crosstalk cancellation matrix can be pre-computed and stored

• Steady state operation involves product of cancellation matrix with vector r

• Practical to implement

Crosstalk Simulations• Consider an ADSL system:

– Transmission bandwidth: (25.875, 1104) kHz– 256 tones over 1104 kHz bandwidth– AWGN at –140 dBm/Hz– Crosstalk: 1 HDSL (f_N=192kHz) and 1 T1

(f_N=772kHz)

• Assumption– Assume crosstalk symbol delay is known to

within some finite precision

Crosstalk Measurements

• Used vector signal analyzer

• 12 wire twisted pair cable binder (4000 ft)

• Used periodic chirp as input signal

• Captured magnitude and phase of transfer function

Channel Measurements

• 4000 ft, 24AWG, 21 pair wire binder

NEXT Coupling Functions

From 1 into 2 From 11 into 5

• 4000 ft, 24AWG, 21 pair wire binder

HDSL Crosstalk Cancellation

• 15/12dB average crosstalk energy reduction for Q(T/4)/Q(T/2)

HDSL Crosstalk Cancellation

• 1500/1000ft average reach improvement at 1Mbps for Q(T/4)/Q(T/2)

HDSL+T1 Crosstalk Cancellation

• require 2x oversampled receiver

• 12/7dB average crosstalk energy reduction for Q(T/4)/Q(T/2)

HDSL+T1 Crosstalk Cancellation

• 2000/1500ft average reach improvement at 1Mbps for Q(T/4)/Q(T/2)

Conclusions• Blind crosstalk cancellation method uses statistical

properties of received signal

• Signal cancellation matrix can be pre-computed (steady state operation involves inner products)

• Simulations show significant gain for realistic ADSL system

• Performance is robust to jitter in crosstalk symbol timing estimate

Future Work

• Investigate methods for estimating crosstalk symbol timing

• Study effect of incorrect DMT decisions in excess band on cancellation performance (multiple crosstalkers)

• Investigate alternative crosstalk cancellation methods