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
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