Bitrate maximizing equalizers for adsl modems
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Bitrate Maximizing Equalizers for ADSL Modems. Koen Vanbleu Promotor: Marc Moonen Collaborators: Geert Ysebaert, Gert Cuypers KULeuven, ESAT SCD-SISTA, Belgium. February 6, 2003. Overview. ADSL Basics What? Transmitter/Receiver ADSL Equalizer Design Problem Description

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Bitrate Maximizing Equalizers for ADSL Modems

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Bitrate maximizing equalizers for adsl modems

Bitrate Maximizing Equalizers for ADSL Modems

Koen Vanbleu

Promotor: Marc Moonen

Collaborators: Geert Ysebaert, Gert Cuypers

KULeuven, ESAT SCD-SISTA, Belgium

February 6, 2003


Overview

Overview

  • ADSL Basics

    • What?

    • Transmitter/Receiver

  • ADSL Equalizer Design

    • Problem Description

    • Current Equalizers

    • Bitrate Maximizing Equalizers

  • Conclusions


Introduction

Introduction

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

  • 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


Introduction1

Introduction

Down

Up

Line length

ADSL

6 Mbps

640 Kbps

3 km

VDSL

52 Mbps

6.4 Mbps

300 m

Downstream

Central

Customer

Upstream

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

  • 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


Modulation and duplexing

Modulation and Duplexing

  • Multicarrier modulation scheme: Discrete Multitone (DMT)

e.g. ADSL

UP

&DOWN

POTS

UP

DOWN

POTS

DOWN

4

25

138

1104

f (kHz)

4

25

138

1104

f (kHz)

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

  • Assign different frequency bins to up- and downstream directions

    • Frequency Division Duplexing (FDD)

    • Overlap: Echo Cancellation (EC)

  • Traditional telephony (POTS) still available over the same wire.


Discrete multi tone transmitter

Discrete Multi Tone: Transmitter

bits

Data symbols (QAM)

Cyclic Prefix

CP

Im

0

10

00

...

...

Re

11

01

2 bits

N-point

P/S

Im

IFFT

Re

...

...

4 bits

IFFT modulation

(Inverse Fast Fourier Transform)

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions


Why equalization

Why Equalization?

CP

noise

...

...

channel

N-point

P/S

IFFT

...

...

Transmitter

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

Why equalization?

“Invert” channel distortion while not boosting noise


Discrete multi tone receiver

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

Discrete Multi Tone: Receiver

Unbiased Frequency Domain Equalizer

1 tap / tone

Data symbols

Im

bits

10

00

CP

...

noise

Re

11

01

...

2 bits

...

...

Im

FEQ

channel h

TEQ w

N-point

S/P

FFT

Re

Time Domain Equalizer

taps

4 bits

FFT demodulation

CP length + 1


Dmt equalization problem description

DMT Equalization: Problem Description

TEQ w

CP

where

is hard with time-domain equalizer w

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

Im{X}

10

00

Re{X}

T taps

1 tap/tone

N-point

11

01

...

...

...

S/P

FEQ Dn

FEQ

FFT

2 bits

To maximize bitrate:


Current adsl equalizers 1

Current ADSL Equalizers (1)

noise

Channel h

TEQ w

delay

TIR b

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

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

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

MMSE criterion  ADSL bitrate maximization


Current adsl equalizers 2

Current ADSL Equalizers (2)

T taps

1 tap/tone

TEQ w

N-point

...

...

...

S/P

FEQ Dn

FFT

CP

Approximations!

based on SNR at FFT output

residual ISI/ICI

noise

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

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

Maximize


Bitrate maximizing equalizers for adsl modems

Current ADSL Equalizers (3)

  • not only depending on walled shortened impulse response

  • : do not forget DFT leakage!

  • Approximate Bitrate Maximizing TEQs (continued)

residual ISI/ICI

noise

Examples of approximations :


Bitrate maximizing equalizers 1

Bitrate Maximizing Equalizers (1)

TEQ w

CP

AND

then

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

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

T taps

N-point

...

...

...

S/P

FEQ Dn

FFT

Maximize

where


Bitrate maximizing equalizers 2

Bitrate Maximizing Equalizers (2)

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

Exact bitrate maximizing (BM-)TEQ cost function

  • Nonlinear cost function in wonly with Anand Bn

    • tone dependent matrices

    • function of signal statistics

  • Recursive Gauss-Newton updating algorithm:

    • attains good local optimum

    • adaptivity (to track channel/noise changes)

    • however: high complexity


Bitrate maximizing equalizers 3

Bitrate Maximizing Equalizers (3)

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • 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]

HIGHER

BITRATE


Bitrate maximizing equalizers 4

Bitrate Maximizing Equalizers (4)

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

Simulations

Per-Tone Equalizer

Bitrate Maximizing-TEQ

MMSE-based TEQs

Approx. Bitrate Max. TEQs


Conclusions

Conclusions

  • ADSL Basics

  • - Intro

  • - DMT Transmitter

  • - Why Equalization?

  • - DMT Receiver

  • ADSL Equalizer Design

  • - Problem Description

  • - Current Equalizers

  • - Bitrate Maximizing

  • Equalizers

  • Conclusions

  • ADSL Equalizer Design

  • 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


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