ADSL System Enhancement ADSL System Enhancement with Multiuser Detectionwith Multiuser Detection

Liang C. ChuLiang C. Chu

School of Electrical EngineeringSchool of Electrical Engineering

Georgia Institute of TechnologyGeorgia Institute of Technology

Atlanta, GA 30332Atlanta, GA 30332

Table of ContentsTable of Contents

Introduction Background: History of the Problem.

Crosstalk ADSL and SDSL in a binder.

DMT-ADSL Channel Characteristics DMT DMT-ADSL Standard

Multiuser Transmission Telephone Channel Multiuser Transmission Systems

ADSL System Enhancement Multiuser Detection on DMT-ADSL Channel Capacity Studies Joint MLSE Performance Studies

Low Complexity Enhancement on ADSL Receiver Tone-zeroing Multi-stage JMLSE

Simulation Studies and Results Conclusions Recommendations

IntroductionIntroduction An enhancement approach on the DMT-ADSL system.

Goal: spectral compatibility; better capacity utilization; support fast Internet services.

Core method: either increasing signal constellation sizes / per sub-channel, or extending the deployment ranges with a fixed transmission rate, or compensating on a poor BER channel in achieving better throughput.

ADSL service Telephone channel, high-bandwidth services. New infrastructure for multimedia service. Economical and less time to launch service.

Physical channel medium: unshielded twisted pair line.

Co-channel interference (crosstalk). TPC model and proposed multiuser model. Sub-optimal approach on receiver

enhancement.

Background Problems Background Problems

Major threat: spectral compatibility. Signals coupling in same binder crosstalk

NEXT Near-end crosstalk

FEXT Far-end crosstalk

Crosstalk Comes FromCrosstalk Comes From

Environmental Physical media: unshielded twisted pair. Bandwidth-efficient digital transmission system. Different kinds of DSL services in same binder.

Near-End CrosstalkNear-End Crosstalk

NEXT

Same Binder Group

Transmit

Receive

Far-End CrossatlkFar-End Crossatlk

FEXT

Same Binder Group

Transmit

Receive

Crosstalk CharacteristicsCrosstalk Characteristics

NEXT: dependent on frequency.

FEXT: dependent on frequency, but

attenuated by twisted cable length.

2/3fH nNEXT

22|)(| flkfHH channelFEXT

Example on NEXT and FEXTExample on NEXT and FEXT

Results:maximum theoretical data rate. NEXT and FEXT limited operation on ADSL.

ANSI ADSL, 256 channels from DC to 1.104MHz Tones #7 to #255 for data transmission. Each tone: QAM at 0 to 15 bits/Hz based on SNR AWGN at –140dbm/Hz, no ISI assumed

NEXT is the dominated crosstalk.

NEXT Coupling CharacteristicsNEXT Coupling Characteristics

NEXT POWER SUM LOSS(dB)

1000 FT, 24 AWG PIC

0

10

20

30

40

50

60

70

0.1 1 10 100

FREQUENCY(MHz) 1% Case

DiscussionsDiscussions

NEXT increases as f1.5 with frequency, but with significant variation in coupling function.

Any given frequency, only few other pairs may contribute significantly to crosstalk, but over all frequencies, many wire lines contribute randomly.

Challenge: hard to detect in single-user detection. Solution: modify receiver.

Current Crosstalk DistributionCurrent Crosstalk Distribution

Gaussian Distribution. Random interferes, central limit theorem.

Practical interests and only accurate on single type of

crosstalk.

Drawback: dependent on error size of Gaussian and true

distribution.

Pessimistic on channel capacity especially on multiple DSL

services.

New area on multiple DSL services crosstalk

models.

SDSL to ADSL (Multiple DSLs)SDSL to ADSL (Multiple DSLs)

SDSL: symmetric DSL 2B1Q modulation - 4-level baseband pulse amplitude

modulation signals same data rate in the upstream and downstream

directions same transmit PSD in the upstream and downstream

directions

Focus studies on SDSL crosstalk to ADSL SDSL services in high demand, together exiting with

ADSL service.

PSD of 2B1Q SDSLPSD of 2B1Q SDSL

Spectral compatibility problem with ADSL overlap in psd

1168, 1552 and 2320 kbps SDSL

-110

-100

-90

-80

-70

-60

-50

-40

-30

0 400000 800000 1200000 1600000 2000000

Frequency (Hz)

PS

D (

dB

m/H

z)

1168 kbps

1552 kbps

2320 kbps

SDSL with T1.413 ADSLSDSL with T1.413 ADSL

Results are calculated for same-binder NEXT with the standard Unger 1% NEXT model.

T1.413 full-rate DMT ADSL in the presence of NEXT from SDSL (1552 kbps and 2320 kbps). DMT tones separated by 4.3125 kHz. each tone carries with a 6dB SNR margin. Downstream ADSL transmits from 160 kHz to 1104

kHz.

SDSL Crosstalk to ADSLSDSL Crosstalk to ADSL

6 8 10 12 14 16 18

1000

2000

3000

4000

5000

6000

7000

8000

26-AWG Loop Length in kft

Dow

nst

ream

Bit

Rat

e in

kb

ps

DMT-ADSL System with 24-SDSL Crosstalk

1552 kbps SDSL crosstalk

2320 kbps SDSL Crosstalk

Simulation based on ANSI T1E1.4/99-261

SDSL self-NEXT loop limits:

1552 kbps to 7.5 kft

2320 kbps to 6 kft

Current Mitigation PlanCurrent Mitigation Plan

Loop plan Testing & estimating deployment loops. Limiting coverage area and customers. Limiting on deployed data rate.

Drawback: Inconvenience. Capacity waste.

Observation and PlanObservation and Plan

Crosstalk channel characteristics change very slowly over the time. Modeled as static and time invariant.

Types of crosstalk on practical loops does not change. Normally fixed DSL services in the same binder from

the CO to CPE sides. Plan on mitigate crosstalk

Enhance the ADSL receiver, “filters” the crosstalk noise.

Multiaccess ADSL channel model

Multiaccess ADSL Channel ModelMultiaccess ADSL Channel Model

Noise, 2

+

+ ADSL ReceiverADSL Channel

Crosstalk Filtering

Crosstalk Filtering

Transmit1 (x1)

Transmit2 (x2)

Transmitk (xk)

r

x

y

)()()()(1

inixihir k

K

kk

hk is the channel impulse response when

k=1, and sum together with crosstalk coupling function when k>1

Discussions Discussions

Background noise is Gaussian. DSL: Gaussian channel

Crosstalk is not Gaussian distribution. Sum of several filtered discrete data signals:

ADSL (desired) and SDSL (crosstalk).

Channel model: multiple input and single

(vector) output.

Brief on DMTBrief on DMT

Basic Principle: Split available BW into a large number of subchannels.

Motivation: Make BW of each the sub-channel sufficiently narrow,

then no ISI occurs on any sub-channel.

Technique method: Transmits many parallel data-streams concurrently over

the transmission channel.

DMT-ADSL (ANSI)DMT-ADSL (ANSI)

Two traffic channels downstream transmission:

sampling rate of 2.208 MHz, a block size of 512 (FFT), meaning 256 tones from 0 to 1.104MHz.

symbol rate is 4 kHz and the width of a tone is 4.3125 kHz. Average downstream PSD is –40 dBm/Hz.

Upstream transmission:sampling rate of 276 kHz, a block size 64, meaning 32 tones

from 0 to 138 kHz. symbol rate is 4 kHz and the width of the tone remains 4.3125

kHz. Average upstream PSD is –38 dBm/Hz

DMT-ADSL SpectrumDMT-ADSL Spectrum

Frequency in kHz

1104240138304

Upstream Channel Downstream Channel

POTS

14

# of Bits

0

Loading AlgorithmLoading Algorithm

Bits/channel

Frequency Frequency Frequency

Crosstalk

AMAttenuation

Physical ChannelPhysical Channel

Unshielded twisted pairs does not change its physical behavior significantly with

time and considered a stationary channel. The transfer function:

The sources of noise in the telephone channel:digital quantization noise, thermal noise in detectors,

impulse noise and crosstalk.

Telephone channel is normally treated as a Gaussian channel.

dRCfatt

efdH 1010),(

Multiuser TransmissionsMultiuser Transmissions

The fundamental limit of multiuser detection: mitigate the interference among different modulated

signals.

Basic model:

(4.2.1.1)

multiuserchannel

.

.

.

x1

x2

xL

XY

NXY H

Multiuser channel is described by the conditional probability distribution :

Normally, many channels fit in the linear AWGN model, shown in Eq. (4.2.1.1).

Optimum multiuser detection: a generalization form of the optimum single-user

channel detector - maximum likelihood multiuser detector.

Y

Xp

Linear multiuser detection in AWGN channel As Eq. (4.2.1.1) detection of desired input user xl, it may be that the overall

minimum distance is too small: a single fixed value for xl may corresponding to the two multiuser

codewords that determine the overall dmin.

defined as : (4.2.2.1.1)

Results: (4.2.2.1.2)

it is possible for a detector extracting a single user to have better performance on one that extracts all other users.

)(min 'min, ''

XXXX

Hdll xx

l

minmin, dd l

Channel CapacityChannel Capacity

Conventional single-user ADSL receiver Sum all the crosstalk signals and background noise

together as AWGN.

(5.1.2.5)

Enhanced multiuser ADSL receiver JMLSE – selects all possible inputs, min. distance on

output.

(5.1.2.8)

dffPfHfN

fPfHC

erferenceNEXTo

desiredc

PP

usergle

erferencedesired

])(|)(|)(

)(|)(|1[log

int2

2

0

2sin supint,

dffN

fPfHC

o

desiredc

P

multiuser

desired

])(

)(|)(|1[log

2

0

2sup

Two UsersTwo Users

Consider the two user case:where, N is AWGN,

, the desired signal and ,an interfered signal. Capacity for user 1: (5.1.2.10)

Capacity for user 2: (5.1.2.11)

NXXY 21

1X 2X

úû

ùêë

é

nBP

PBC

2

1*1 1log

úû

ùêë

é

nBP

PBC

1

2*2 1log

Jointly detect, then the achievable capacity:

(5.1.2.12)

Considerable capacity improvement when the interference structure is taken into account.

(5.1.2.13)

úûù

êëé £

nB

PBR i

i 1log úûù

êëé

£nB

PPBRR 21

21 1log

úûù

êëé

nB

PBC i

i 1log

Single vs. Multiuser Channels

Rate(User 1)

Multiuser

SingleUser

C1

C1*

C2C2*

Rate(User 2)

Alternative ViewpointAlternative Viewpoint

Multiple input: x.

Mutual information: I(x,y), and I(r,y).

Data rate: individual input.

Aggregate data rate: .

Shannon theorem: upbounded by

I(r,y).

i

ii T

bR

k

iisum RR

1

sumR

Achievable data rate on desired channel:

(5.1.2.15) Discussion:

Limit on (5.1.2.15) can be much larger than the data rate based on Gaussian crosstalk assumptions.

The sum on right can be much smaller number, due to frequency-selective crosstalk coupling function.

k

iidesired RyrIR

2

),(

Analysis and ExamplesAnalysis and Examples

SDSL Crosstalk

ADSL Channel

Example#1 Example#2

Example #1:crosstalk mutual informationExample #1:crosstalk mutual information

1552 kbps SDSL coupling to ADSL

Mutual information of crosstalk on each DMT-ADSL tone

.5.78),( kbpsyxI adslsdsl

If silence near 20 tones, fully detected

Unger 1% model, 5.1910 f

)101(log3125.4 52.80.142

kHz

kbpskbps 1552205.78

Example #2:Throughput ComparisonExample #2:Throughput Comparison

Conclusion: still having enough room for ADSL. too pessimistic on current model.

Theoretic ADSL capacity:

.21),( MbpsyxI adsladsl

SDSL crosstalk

ADSL Channel

Gaussian model:

.330kbpsRadsl

Joint MLSEJoint MLSE

Principle search all possible transmitted signals, find a

best match signal set on the received signal.

The best detector, with upper bound on

multiuser system.

Drawback: large computational complexity.

Details on ReceiverDetails on Receiver

Viterbi decoding: engine for MLSE receiver. Select the state having the smallest accumulated error

metric and save the state number of that state. Iteratively perform the following step until the

beginning of the trellis is reached: working backward through the state history table, for the selected state, select a new state, which is listed in the state history table as being the predecessor to that state. Save the state number of each selected state. This second step is called traceback.

work forward through the list of selected states saved in the previous steps. Look up what best estimated input bit corresponds to a transition from each predecessor state to its successor state.

Use T/2-spaced MLSE Receiver eliminate implementation for whitening matched filters

- with fixed analog filters, not depend on unknown channel (pulse shaping filter).

nearly insensitive to sampling time off-set, capable of recoving non synchronized cochannel signals more easily.

JMLSE ADSL Receiver (Optimal)JMLSE ADSL Receiver (Optimal)

Multiple input and single output model. Detect desired ADSL and filtered coupling crosstalk signals.

JMLSE ADSL Receiver extension of the single channel MLSE. assume Gaussian channel. Ex: co-channel pairs case:JMLSE selects the ith joint symbol

sequence { } that maximizes the metric

(5.2.6.1)

meaning: select a signal set with minimized distance from the received signals.

Method:Joint Viterbi algorithm.

ki

ki xx 2,1, ,

),|(),|( ,2,1,2,1k

jk

jkk

ik

ik xxrpxxrp

Joint Viterbi AlgorithmJoint Viterbi Algorithm

Objective: determine the pair of sequence

that minimizes the sum of squared errors

defined by the error sequence: .

},{ 2,1,kj

ki xx

kjie ,

+

rk

Primary Channel Estimate

Secondary Channel Estimate

+

kix 1,

kjir,ˆ -

kjie ,

kjx ,2

f1(k)

f2(k)

Joint VA (JVA) for JMLSE is very similar to the standard VA. Joint state: number of states required to implement JVA: Each joint state at time k-1:

Transition to states at time k.Be reached by same number of states from time k-2.

},{ 1,1,2

1,1,1

1,1 21 Lki

Lki

Lki ssS

21 LLM

2M

Prototype on Modification of ReceiverPrototype on Modification of Receiver

Noise, 2

+Transmit1 (x1)

Transmit2 (x2)

Transmitk (xk)

r

+

+ LE

JMLSD

Feedback

Section

H

Performance Study (Optimal)Performance Study (Optimal)

17 17.5 18 18.5 19 19.5 20 20.5 21 21.5 2210

-16

10-14

10-12

10-10

10-8

10-6

10-4

10-2

SNR in dB

BE

RBit Error Rate for ADSL

single-user detector

multiuser via JMLSE

one SDSL disturber NEXT into one T1.413 full rate

DMT-ADSL system

gap of 4 dB ,FIR channel with 256

memory states

4 6 8 10 12 14 16 18-30

-25

-20

-15

-10

-5

0

5

10

15

20

ADSL Service Length in kft

Mar

gin

s in

dB

JMLSD

Single-user Detector with SDSL Crosstalk

Low Complexity EnhancementLow Complexity Enhancement

JMLSE is an optimal solution. drawback: high computational complexity.

Goal: Reduce computational complexity Multistage JMLSE: multiple MLSE “like”. Tone zeroing: use DMT loading algorithm, and

“adaptive decision feedback” or “echo cancellation like”.

Tone Zeroing MethodTone Zeroing Method

Principle: Use loading algorithm to silence some selected BW tones with low SNR, then building a adaptive cancellation table.

y FFT

CrosstalkDetector

CrosstalkTable

+ DMTDecoder

Y

Ci-

SDSL Coupling to ADSL SDSL Coupling to ADSL ExampleExample

Adjacent pairs: SDSL to ADSL. assume ADSL channel is static. relative constant on crosstalk profile table using LMS

algorithm. zeroing about 20 tones to build up a NEXT cancellation

table. Result: up to 6 dB in margin. Discussions:

advantage of mitigate the NEXT and complexity reduction (comparing with JMLSE) with asymmetric and symmetric services coexist.

key issue for the tone zeroing is necessity of accurate modeling of noise (crosstalk).

feedback section is using some kind of adaptive filter technique, and adaptive filter coefficient is largely depends on frequency components with high power.

If a frequency band making NEXT noise has small power, it can not be modeled correctly due to high power frequency component until sufficient number of coefficient are used.

tone zeroing modeling works well for high frequency power noise component.

telephone channel, many kinds of random noises often occur in any selected frequency band.

may make an error decision on the cancellation table and induce error propagation.

Proposed multi-stage joint MLSE for ADSL receiver (applied to both DMT and non-DMT DSL solutions).

Same Example w/Tone-ZeroingSame Example w/Tone-Zeroing

4 6 8 10 12 14 16 18-30

-25

-20

-15

-10

-5

0

5

10

15

20

ADSL Service Length in kft

Mar

gins

in d

B

JMLSE

Single-user Detector with SDSL Crosstalk

Tone-zeroing

Complexity Reduced JMLSEComplexity Reduced JMLSE

Multi-stage JVA very similar to conventional VA receiver. having multi-stage inputs and outputs. Method as adjacent pair-wise case:

the primary (strong) signal r1(k) is estimated using low delay decisions from a single-channel VA, and is forwarded to the second VA section to estimate the co-channel signal.

Advantage: this structure is largely reducing the complexity on optimal JVA (JMLSE). Complexity as a similar range of a conventional VA,

with just a scale-increasing factor by N.

)(ˆ)( 1 krkr

NN Co-channel Binder Co-channel Binder

Ratio:

Assume equal lengths, L,

obvious to us R is always (much) < 1.

N

N

LLL

LLL

M

MMMR

...21

21 ...

JMLSE

Multi-stage JMLSE

)1( NLLN

L

MNM

MNR

Two Methods (Pair-wise)Two Methods (Pair-wise)

VA 1LM states

VA 2LM states +

+

_

1L

r(k)

)(ˆ11 Lkd )(ˆ

22 Lkd

)(ˆ1 kd

)(ˆ)( 11 krkr

VA 1LM states

VA 2LM states

+ + +

_

)(ˆ2 kf

)1(1̂ kf

+

_ r(k)

)(ˆ11 Lkd )(ˆ

22 Lkd

)(ˆ 2,2 kd Lk

)(ˆ 1,1 kd Lk

)1(2̂ kr

)(1 ks )(2 ks

)(1̂ kr

Two-stage JVA ,without Feedback Section

Two-stage JVA ,with Feedback Section

only an additional L tap filter computational

increasing

Make DecisionMake Decision

4 6 8 10 12 14 16 18 20 2210

-4

10-3

10-2

10-1

100

Signal-to-Noise Ratio

Sym

bo

l Err

or

Ra

te

* : MS-JMLSE-W/FB

o : Ideal-JMLSE

+ : MS-JMLSE-WO/FB

Example on PAM channel, signal-

corsstalk-ratio=10 dB

T/2-spaced MS-JMLSE-W/FB

Performance SimulationsPerformance Simulations

Test Environment SDSL and other DSLs NEXT to ADSL.

Loop Characteristics

ATU - RATU - C

ATU - RATU - C

ATU - RATU - C

18000 ft

26 AWG

6000 ft

26 AWG

1500 ft

26 AWG

3000 ft

26 AWG

9000 ft

26 AWG

2000 ft

24 AWG

500 ft

24 AWG

500 ft

24 AWG

1500 ft26 AWG

1500 ft26 AWG

1500 ft26 AWG

1500 ft26 AWG

1500 ft26 AWG

Test Loop #3

Test Loop #1

Test Loop#2

ATU - RATU-C

ATU - RATU- C

ATU - RATU-C

26 AWG

6000 ft

26 AWG

1500 ft

26 AWG

3000 ft

9000 ft

26 AWG

2000 ft

24 AWG

500 ft

24 AWG

500 ft

24 AWG

1500 ft 1500 ft26 AWG

1500 ft26 AWG

1500 ft26 AWG

1500 ft26 AWG

Test Loop #1Test Loop #1

9 10 11 12 13 14 15 16 17 180

0.5

1

1.5

2

2.5

3

3.5

4

ADSL Service Length in kft

Ach

ieva

ble

Do

wns

tre

am

Da

ta R

ate

in M

bp

s square : ideal JMLSE

x : multi-stage JMLSE

o : conventional ADSL receiver

Test Loop #2Test Loop #2

9 10 11 12 13 14 15 16 17 180

0.5

1

1.5

2

2.5

3

3.5

ADSL Service Length in kft

Ach

ieva

ble

Do

wns

tre

am

Da

ta R

ate

in M

bp

s

square : ideal JMLSE

x : multi-stage JMLSE

o : conventional ADSL receiver

Test Loop #3Test Loop #3

4 6 8 10 12 14 16 18 200

1

2

3

4

5

6

7

8

9

10

ADSL Service Length in kft

Ach

ieva

ble

Do

wns

tre

am

Da

ta R

ate

in M

bp

s

square : ideal JMLSE

x : multi-stage JMLSE

o : conventional ADSL receiver

extensionprediction

Other works on xDSL CrosstalkOther works on xDSL Crosstalk Crosstalk with Gaussian Distribution for DSL:

(1) cook,1999; (2) zimmerman, 1998; (3) kerpez, 1995; (4) kerpez, 1993.

Multiuser Detection, but for wireless communications: (5) Verdu, 1998.

Multiuser detection in VDSL study: (6)Cioffi, 1998.

(1) “The noise and crosstalk environment for ADSL and VDSL systems “,Cook, J.W.; Kirkby, R.H.; Booth, M.G.; Foster, K.T.; Clarke, D.E.A.; Young, G.,IEEE Communications Magazine , Volume: 37 Issue: 5 , May 1999.

(2) “Achievable rates vs. operating characteristics of local loop transmission: HDSL, HDSL2, ADSL and VDSL “, Zimmerman, G.A. , Conference Record of the Thirty-First Asilomar Conference on , Volume: 1 , 1998.

(3) “High bit rate asymmetric digital communications over telephone loops “, Kerpez, K.J.; Sistanizadeh, K.,Communications, IEEE Transactions on , Volume: 43 Issue: 6 , June 1995.

(4)“Near End Crosstalk is Almost Gaussian”, K. J. Kerpez, IEEE Transactions on Communications, Vol. 41, No. 5, May 1993.

(5) “Multiuser Detection,” S. Verdu , Cambridge Press, 1998.

(6)“Mitigation of DSL Crosstalk via Multiuser Detection and CDMA”, J. Cioffi , ANSI, T1E1.4/98-253, August 1998.

Related DSL PublicationsRelated DSL Publications

“An Enhancement Study on the SDSL Upstream Receiver”, 2001 IEEE International Symposium on , Volume: 4 , 6-9 May 2001, Page(s): 442 –445.

“Mitigation of Crosstalk on the SDSL Upstream Transmission with Vector Equalization”, IEEE International Conference on Communications, Session AN5: Transmission Systems, Helsinki, Finland, June 11-14, 2001 .

“A Study on Multiuser DSL Channel Capacity with Crosstalk Environment”, 2001 IEEE Pacific Rim Conference on Communications, Computers, and Signal Processing, Session MP4: DSP for Communications, Victoria, BC, Canada, August 24-28, 2001.

“Performance Enhancement on a Multiuser Detection ADSL Modem”, In preparation to IEEE Transitions on Consumer Electronics.

“Complexity Reduced ADSL System with Multiuser Detection ”, Submitted to 2002 IEEE International Conference on Communications.

ConclusionsConclusions

Overview the problem on xDSL spectral compatibility problems.

Traditional Gaussian crosstalk under-project ADSL achievable capacity.

ADSL system enhancement with multiuser detection. a core method on improvements of either increasing transmission

data rate, or extending deployment areas, or compensating in poor BER DSL channels, based on different requirements.

Enhanced ADSL receiver has acceptable computational complexity for a chip realization.

Benefit on QoS for last-mile fats Internet transmission.

RecommendationsRecommendations

This approach can apply to DMT and non-DMT ADSL, HDSL, SDSL and future VDSL studies. may extensible to fiber and wireless.

Other complexity reduction methods for JVA decoding can be further studied (this thesis gives a kind of beginning point).

Possible dual-mode DSL transceivers.