BLIND CROSSTALK CANCELLATION FOR DMT SYSTEMS

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BLIND CROSSTALK CANCELLATION FOR DMT SYSTEMS
Nadeem Ahmed Nirmal Warke ECE
Dept. DSPS RD Center Rice
University Texas Instruments
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Motivation

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

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Motivation

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

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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

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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.

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Crosstalk Power on Line
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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 DFEs
• Exploit symbol rate differences

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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

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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

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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

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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

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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

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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

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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

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Excess Band Crosstalk Cancellation

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

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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

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Mathematical Formulation

• DMT Modulation
• System Impaiments- crosstalk and noise
• DMT Demodulation

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Mathematical Formulation

• Partition into 2 freq. Bands
• 2 gt main band 1 gt excess band
• Demodulate DMT signal in excess band and subtract
  to estimate crosstalk signal

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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
M
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Blind Cancellation

• If .C and x b, channel is assumed to
  be known gt Zengs solution
• Instead, let and x C.b gt Blind
  Approach
• Solution uses crosstalk statistics i.e.
  autocorrelation information
• Estimate coupling function and crosstalk data
  simultaneously

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Dependence on crosstalk symbol delay

• Relative crosstalk symbol delay varies with DMT
  frame gt varies with DMT frame
• where,

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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

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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_N192kHz) and 1 T1
  (f_N772kHz)
• Assumption
• Assume crosstalk symbol delay is known to within
  some finite precision

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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

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Channel Measurements

• 4000 ft, 24AWG, 21 pair wire binder

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NEXT Coupling Functions
From 1 into 2
From 11 into 5

• 4000 ft, 24AWG, 21 pair wire binder

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HDSL Crosstalk Cancellation

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

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HDSL Crosstalk Cancellation

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

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HDSLT1 Crosstalk Cancellation

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

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HDSLT1 Crosstalk Cancellation

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

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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

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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