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BLIND CROSSTALK CANCELLATION FOR DMT SYSTEMS Nadeem Ahmed Nirmal Warke ECE Dept. DSPS R&D Center Rice University Texas Instruments

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Motivation New multimedia and networking applications => increasing demand for bandwidth DSL is cost effective broadband solution 100 MHz10 MHz1 MHz100 kHz10 kHz POTS ADSL VDSL ISDN HDSL

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Motivation Increasing density of DSL deployment => Increased crosstalk Crosstalk typically increases with frequency => significant impairment for high speed DSL Binder ADSL lines HDSL lines POTS

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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 DFE’s –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 => main band 1 => excess band Demodulate DMT signal in excess band and subtract to estimate crosstalk signal

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Cancellation Algorithm M Project 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,

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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

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Dependence on crosstalk symbol delay Relative crosstalk symbol delay varies with DMT frame => 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_N=192kHz) and 1 T1 (f_N=772kHz) 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 2From 11 into 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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HDSL+T1 Crosstalk Cancellation require 2x oversampled receiver 12/7dB average crosstalk energy reduction for Q(T/4)/Q(T/2)

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HDSL+T1 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

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