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BLIND CROSSTALK CANCELLATION FOR DMT SYSTEMS. Nadeem Ahmed Nirmal Warke ECE Dept. DSPS R&D Center Rice University Texas Instruments. Motivation. ADSL. POTS. VDSL. ISDN. HDSL. 10 kHz. 100 kHz. 1 MHz. 10 MHz. 100 MHz.

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blind crosstalk cancellation for dmt systems

Nadeem Ahmed Nirmal Warke

ECE Dept. DSPS R&D Center

Rice University Texas Instruments







10 kHz

100 kHz

1 MHz

10 MHz

100 MHz

  • New multimedia and networking applications => increasing demand for bandwidth
  • DSL is cost effective broadband solution


ADSL lines

HDSL lines


  • Increasing density of DSL deployment => Increased crosstalk
  • Crosstalk typically increases with frequency => significant impairment for high speed DSL
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

Types of Crosstalk
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.

DSL System Model
combating crosstalk
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
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
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 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
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
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
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 cancellation1
Excess Band Crosstalk Cancellation
  • Paper by Zeng et al on Crosstalk Cancellation for DMT Systems
excess band crosstalk cancellation2
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

Excess Band Crosstalk Cancellation
mathematical formulation
Mathematical Formulation
  • DMT Modulation
  • System Impaiments- crosstalk and noise
  • DMT Demodulation
mathematical formulation1
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
cancellation algorithm
Cancellation Algorithm
  • Let x = M.r be a linear estimate of crosstalk signal component x
  • MMSE Estimate:
  • Hence crosstalk signal in main band is,

Project onto

main band


blind cancellation
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
dependence on crosstalk symbol delay
Dependence on crosstalk symbol delay
  • Relative crosstalk symbol delay varies with DMT frame => varies with DMT frame
  • where,
blind cancellation practical solution
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
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
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
Channel Measurements
  • 4000 ft, 24AWG, 21 pair wire binder
next coupling functions
NEXT Coupling Functions

From 1 into 2

From 11 into 5

  • 4000 ft, 24AWG, 21 pair wire binder
hdsl crosstalk cancellation
HDSL Crosstalk Cancellation
  • 15/12dB average crosstalk energy reduction for Q(T/4)/Q(T/2)
hdsl crosstalk cancellation1
HDSL Crosstalk Cancellation
  • 1500/1000ft average reach improvement at 1Mbps for Q(T/4)/Q(T/2)
hdsl t1 crosstalk cancellation
HDSL+T1 Crosstalk Cancellation
  • require 2x oversampled receiver
  • 12/7dB average crosstalk energy reduction for Q(T/4)/Q(T/2)
hdsl t1 crosstalk cancellation1
HDSL+T1 Crosstalk Cancellation
  • 2000/1500ft average reach improvement at 1Mbps for Q(T/4)/Q(T/2)
  • 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
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