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TRANSCRIPT
Santa Clara Valley Chapter
Vectored DSLHow Crosstalk Cancellation Leads to
100 Mbps Broadband Access
George Ginis
September 8th, 2010
2
Basics of DSL Transmission
Signal always present on active lines
Uses frequencies between DC and up to 30 MHz
Impulse noise mitigated by FEC/interleaving or retransmission
Frequency division separates downstream from upstream
Crosstalk interference limits transmission data rates
Far-End Crosstalk (FEXT)
Backbone Network
Downstream signal
Upstream signal
Access Node
Impulse noiseOther interference
3
The Crosstalk Problem
Far-End Crosstalk impairs performance in short loops
25 VDSL2 lines Band plan: 998, 17a profile Limit PSD mask: D-32 Gap: 11.6dB Bit cap: 15 Background noise: -140dBm/Hz
500 1000 1500 2000 2500 3000 350020
40
60
80
100
120
140
160
180
Loop length (ft)
Dat
a ra
te (M
bps)
No crosstalkStrong crosstalk
4
Access Node
Downstream Vectoring Concept
Pre-compensates for FEXT
Downstream received signal is free of crosstalk
Requires joint signal processing by transmitters
Analog FEXTModulationEncoding
Encoding Modulation Analog
Line 1
Line 2
Customer PremisesEquipment
5
Simplification with Discrete Multi-Tone
Rx signal of line 1, tone n:
Rx signal of line 2, tone n:
Assumes synchronization among lines using “Zippering” technique
Precoding can be applied independently on each tone
Frequency Frequency
Direct Channel Crosstalk Coupling
)(1
)(2
)(12
)(1
)(11
)(1
nnnnnn NXHXHY ++=)(
2)(
2)(
22)(
1)(
21)(
2nnnnnn NXHXHY ++=
6
Transmitter Architecture
Precoding performed in the frequency domain
Precoding applied independently on each tone
Access Node
Symbol Encoder
Gain Scaling IFFT
Line 1 Cyclic Extension Insertion
Time-Domain
Proc.
Symbol Encoder
Gain Scaling IFFT
Cyclic Extension Insertion
Time-Domain
Proc.
Pre-Coder
Line 2
7
Near-optimality of Linear Precoding
DSL MIMO channel has special property:● “Row-wise diagonal dominance”
Linear techniques perform near-optimally
h11
h22
h12h21
2221
1112
hh
hh
<<
<<
8
Performance Gain from Linear Precoding
Linear precoding achieves near-crosstalk-free performance
500 1000 1500 2000 2500 3000 350020
40
60
80
100
120
140
160
180
Loop length (ft)
Dat
a ra
te (M
bps)
No crosstalkStrong crosstalkWith vectoring
25 VDSL2 lines Band plan: 998, 17a profile Limit PSD mask: D-32 Gap: 11.6dB Bit cap: 15 Background noise: -140dBm/Hz
9
Channel Estimation
Transmitter must know MIMO channel
Receivers must feed-back information
Information about channel
h12h21
Information about channel
10
Channel Estimation at the Transmitter
Known-sequence-based estimation at the transmitter:● Transmitters periodically send known sequences (“sync” symbols)● Known sequence sent by each transmitter is unique (e.g. WH codes)● Receivers compute error samples from “sync” symbols● Error samples sent to the transmitters● Transmitters update the pre-coder by correlation of:
○ Received error samples ○ Known sequences
Error samples of Rx 1
h12h21
Error samples of Rx 2 Error samples
Pre-Coder
Error samples
Received samples
Expected samples
11
Introduction of New Line
Problems with new line● Can appear at any time:
○ Service activation○ Modem re-initialization
● Causes crosstalk to vectored lines● Is impaired by crosstalk from the vectored lines
New line must join the vectored group● No disruption of already operating lines
New line
VectoredGroup
12
Sync Symbol Transmission for New Line
Transmit only sync symbols● No impact on data symbols
Learn crosstalk coupling● From vectored lines● Into vectored lines
Initialize pre-coder Start normal transmission
New line
VectoredGroup
Vectored line 1
Time
Vectored line 2
New line
Sync symbols
13
Management of Vectored DSL
Line 1
Line 2
Line N
Access Node
AFEModulation/ Demodulation
Encoding/ Decoding
…… … … …
Limited resources for vectoring operations
Performance trade-offs
among customers
Management system needed
to realize full benefits
Dynamic Spectrum Management (DSM)
Collect • management data
from all lines;• store for long
period.
Analyze • recent data and
history;• detect problems;• project
performance.
Act• reconfigure lines
to improve speed/stability;
• report faults.
DSL lines Modems
Vectored DSL Access Nodes
DSL management
Backbone Network
14
…
New Management Challenges (DSM Level 3)
Manage trade-offs among vectored lines
Achieve co-existence of non-vectored lines and vectored lines
Manage for noise sources that become dominant after eliminating the crosstalk
15
Data Parameters
16
…
Error samples
Received samples
Expected samples
Rx PSD crosstalkCrosstalk coupling (XLOG)• Received crosstalk
PSD divided by received signal PSD
Noise correlation• Correlation of error
samples of different receivers
Frequency
Rx PSD direct
Crosstalk Diagnosis
Identify source pair for high crosstalk (“rogue” pair).
Noise correlation can indicate common outside source.
Pair likely causing disruption/degradation on non-vectored lines as well.
17
Cable cross-section
Vectored lines
Non-vectored lines
Vectored/non-vectored line creating strong crosstalk
…
DSL management
Performance Prediction
18
Line Current Service Service with vectoring – low priority
Service with vectoring – high priority
123-456-7890 50 Mbps 60 Mbps 65 Mbps
650-654-3400 30 Mbps 35 Mbps 42 Mbps
… … … …
Use data parameters (XLOG, noise correlation) to
estimate data rates
Understand performance trade-offs and make decisions on
priorities
Control Parameters (1)
19
Vectoring enable/disable• Choose on which
lines to allocate vectoring resources
Frequency controls• Choose on which
frequencies to allocate vectoring resources
Access Node
AFEMod/ Demod
Encoding/ Decoding
…… … … …
Frequency
Vectoring enabled Vectoring disabled
Control Parameters (2)
20
Target Data Rate
Max Data Rate
Min Data Rate
Not allowed
Not allowed
Allocate vectoring resources to achieve Target Data Rate. If resources are insufficient, sacrifice data rate up to Min Data Rate.
Allocate vectoring resources according to Line Priority (HIGH/LOW). Do not exceed Max Data Rate.
Controlling the Lines
21
Enable vectoring for lines that benefit the most
Enable vectoring for customer with high-end services
Disable vectoring for malfunctioning equipment…
DSL management
Line Current Service Benefit from vectoring
Purchasedservice
Enable vectoring?
123-456-7890 50 Mbps 15 Mbps Premium YES
650-654-3400 30 Mbps 2 Mbps Basic NO
Controlling the Frequencies
22
Frequency
Rx PSD crosstalk
RFI PSD
Disable vectoring in frequencies dominated by RFI, AM, or other time-varying interference.
Disable vectoring in frequencies where crosstalk from non-vectored systems dominates.
Controlling Rates and Priorities
23
Achievable data rates depend on the allocation of vectoring resources
Must choose rate controls and line priorities based on achievable data rates and customer’s purchased service
Co-existence of Vectored and Non-vectored lines (DSM Level 2+3)
24
Network Operations
Legacy VDSL Access Node
Spectrum Management
Center
Vectored DSL Access Node
Loop length
2 vectored lines
1 non-vectored line
Non-vectored lines in the same binder increase the
crosstalk level
Reducing power for non-vectored lines
restores the rates of the vectored lines
500 1000 1500 2000 2500 3000 35000
20
40
60
80
100
120
140
160
180
Loop length for both vectored and non-vectored lines (ft)Rat
es o
f the
vec
tore
d (m
inim
um o
f the
two)
and
non
-vec
tore
d lin
es (M
bps) Data rates of a group two vectored lines and one non-vectored lines
FEXT free vectored lines rateMin of vectored lines rate with DSM Level 2, non-vectored capped at 25MbpsNon-vectored line rate with DSM Level 2Min of vectored lines rate, non-vectored line rate is uncappedNon-vectored line rate, uncapped
Data Rate Comparison (DSM Level 2+3)
Adaptive Spectrum and Signal Alignment, Inc 25
Crosstalk-free
No DSMVectored line
No DSM Non-vectored line
Vectored line with DSM Level 2
Non-vectored line with DSM Level 2
Managing for External Noise (DSM Level 1+3)
Crosstalk often hides other
noise sources
With crosstalk eliminated, lines
are more sensitive to noise effects
Management for impulse and other time-varying noise
becomes even more important
26
…
Frequency
Rx PSD crosstalk
Rx PSD with crosstalk cancellation
0 1 2 3 4 5 6-100
-90
-80
-70
-60
-50
msec
Diff
-mod
e
Diff-mode (Data),x0A Capture-xxx2
0 1 2 3 4 5 6
-1000
-500
0
500
1000
msec
Com
mon
-mod
e
Common-mode (Reference)
Conclusions
27
Vectored DSL brings copper pairs to the 100
Mbps performance region
Proper management brings out the full
benefits of the technology