bitrate maximizing time-domain equalizer design for dmt-based systems koen vanbleu promotor: marc...

Bitrate Maximizing Time-Bitrate Maximizing Time-Domain Equalizer Design for Domain Equalizer Design for

DMT-based SystemsDMT-based SystemsKoen Vanbleu

Promotor: Marc MoonenCoauthors: Geert Ysebaert, Gert Cuypers, Katleen Van Acker

KULeuven, ESAT SCD-SISTA, BelgiumKULeuven, ESAT SCD-SISTA, BelgiumICC 2003

2

OverviewOverview

• ADSL Basics� What?� Transmitter/Receiver

• ADSL Equalizer Design� Problem Description� Current Equalizers� Bitrate Maximizing Equalizers

• Conclusions

3

IntroductionIntroduction

• Communication at high rates towards customer� telephone wire, cable, fiber, wireless

• Communication over telephone wire� Evolution: ever increasing bitrates� E.g. Time to download 10 Mbyte file

Modem Time56 Kbps

Voice band modem

24 minutes

128 Kbps ISDN 10 minutes6 Mbps ADSL 13 seconds52 Mbps VDSL 1.5 seconds

• ADSL Basics - Intro - DMT Transmitter - Why

Equalization? - DMT Receiver

• ADSL Equalizer Design

- Problem Description

- Current Equalizers

- Bitrate Maximizing

Equalizers

• Conclusions

4

IntroductionIntroduction

• Broadband communication over telephone line� ADSL (Asymmetric Digital Subscriber Line)� VDSL (Very high bit rate Digital Subscriber Line)� Bitrate is function of the line length

Upstream

Downstream

CustomerCentral

300 m6.4 Mbps52 MbpsVDSL

3 km640 Kbps6 MbpsADSL

Line lengthUpDown Frequency band1.1 MHz8.8 MHz







Equalizers

• Conclusions

5• Traditional telephony (POTS) still available

over the same wire.

Modulation and DuplexingModulation and Duplexing

• Assign different frequency bins to up- and downstream directions� Frequency Division Duplexing (FDD)� Overlap: Echo Cancellation (EC)

f (kHz)

POTS UP DOWN POTS UP&DOWN

DOWN

4 25 138 1104 f (kHz)4 25 138 1104

e.g. ADSL

• Multicarrier modulation scheme: Discrete Multitone (DMT)







Equalizers

• Conclusions

6

Discrete Multi Tone: TransmitterDiscrete Multi Tone: Transmitter

00

11

10

01Re

Im

2 bits

Re

Im

4 bits

bits Data symbols (QAM)

...

P/S

CP

kx

Cyclic Prefix

0

IFFTN-point

.

.

.

.

.

.

...

IFFT modulation(Inverse Fast Fourier Transform)

N12/ N







Equalizers

• Conclusions

7

Why Equalization?Why Equalization?

channel

ky

knnoise

...

P/S

CP

kxIFFTN-point

.

.

.

.

.

.

...

N

12/ N

nX

Transmitter

Why equalization?“Invert” channel

distortion while not boosting noise







Equalizers

• Conclusions

8

Discrete Multi Tone: ReceiverDiscrete Multi Tone: Receiver

00

11

10

01Re

Im

2 bits

Re

Im

4 bits

bits

Data symbols

...S/P

CP

.

.

.FFT

N-point

FFT demodulation

.

.

. FEQ

Unbiased Frequency

Domain Equalizer

1 tap / tone

.

.

.channel h

knnoise

kykxTEQ w

tapsT

Time Domain

Equalizer







Equalizers

• Conclusions

k

kh kk wh

CP length + 1k

9

DMT Equalization: DMT Equalization: Problem DescriptionProblem Description

S/P

CP... FFT FE

QTEQ w

ky

T taps

...

...

1 tap/tone

N-point

To maximize bitrate:

n

nbtones

toneonbits

00

11

10

01

Re{X}

Im{X}

2 bits







Equalizers

• Conclusions

nnn EXZ

FEQ Dn

n n

nn DSNRtones

2),(1log w

is hard with time-domain equalizer w

where 2

2

),(),(

nn

nnn

DEE

XEDSNR

ww

10

Current ADSL Equalizers (1)Current ADSL Equalizers (1)• ADSL Basics - Intro - DMT Transmitter - Why






Equalizers

• Conclusions

• Channel shorteners, e.g. MMSE-based TEQ [Al-Dhahir, Cioffi]

Channel h

knnoise

kxTEQ w

ky

delay TIR b

ke

bwbw,

oron constraint s.t.min 2keE

TIR = target impulse response of (CP-length+1)

MMSE criterion ADSL bitrate maximization

11

Current ADSL Equalizers (2)Current ADSL Equalizers (2)• ADSL Basics - Intro - DMT Transmitter - Why






Equalizers

• Conclusions

• Approximate Bitrate Maximizing TEQs [Al-Dhahir], [Evans]

S/P

CP... FFT FEQ Dn

TEQ wky

T taps

...

...

1 tap/tone

N-point nnn EXZ

n n

nSNRbtones

2 1logMaximize

based on SNR at FFT output

2

21

)(),(

),()(

whw

hww

nn

nnn

NIE

XDESNR

residual ISI/ICI noise

Approximations!

12

• Approximate Bitrate Maximizing TEQs (continued)

2

21

)(),(

),()(

whw

hww

nn

nnn

NIE

XDESNR

residual ISI/ICI noiseExamples of approximations :

•

• Signal component in SNR does not only depend on `windowed’ (shortened) channel impulse response

• : do not forget DFT leakage (ICI)!

nnn WHD 1

nN

• ADSL Basics - Intro - DMT Transmitter - Why Equalization? - DMT Receiver

• ADSL Equalizer Design - Problem Description - Current Equalizers - Bitrate Maximizing Equalizers

• Conclusions

Current ADSL Equalizers (3)Current ADSL Equalizers (3)

13nnnnnnn XYDXZDE )(),( wwthen

= residual ISI/ICI+noise sources (XT, RFI, …)

n n

nn DSNRbtones

2),(1log w

Bitrate Maximizing Equalizers Bitrate Maximizing Equalizers (1)(1)• ADSL Basics

- Intro - DMT Transmitter - Why






Equalizers

• Conclusions

S/P

CP... FFT FEQ Dn

TEQ wky

T taps

...

...

N-point nnn EXZ

where 2

2

),(),(

nn

nnn

DEE

XEDSNR

ww

Maximize

)(wnY

AND

*2

)( nn

nn XYE

XED

w

14

Bitrate Maximizing Equalizers Bitrate Maximizing Equalizers (2)(2)

Exact bitrate maximizing (BM-)TEQ cost function

• Nonlinear cost function in w only with An and Bn

• tone dependent matrices• function of signal statistics







Equalizers

• Conclusions • Recursive Gauss-Newton updating algorithm:• attains good local optimum• adaptivity (to track channel/noise changes)• however: high complexity

15








Equalizers

• Conclusions

Bitrate maximizing equalizers:

• Bitrate maximizing (single) TEQ: NG = all used tones nonlinear cost function

• “Per group” equalization: BM-TEQ w per group SG of NG tones nonlinear cost function

• “Per tone” equalization: NG = 1 tone (advantageous) linear MMSE problem

[Vanacker, Leus, Moonen]

HIGHERBITRATE

16








Equalizers

• Conclusions

Simulations

Bitrate Maximizing-TEQPer-Tone Equalizer

MMSE-based TEQsApprox. Bitrate Max. TEQs

17

ConclusionsConclusions• ADSL Basics - Intro - DMT Transmitter - Why






Equalizers

• Conclusions


• Truly Bitrate Maximizing Per-Group Equalizer• Time-Domain Equalizer (1 group of tones)• Per-Tone Equalizer (groups of 1 tone)

• Recursive Gauss-Newton algorithm • Good local optimum• Complex

bitrate maximizing time-domain equalizer design for dmt-based systems koen vanbleu promotor: marc...

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