dsp2 project adsl equalization students: dung nguyen quoc- master student tudor tesu- erasmus...

DSP2 ProjectADSL Equalization

Students:

Dung Nguyen Quoc- Master Student Tudor Tesu- Erasmus Student

SupervisorJan Vangorp

12 November 2008 2

ADSL Equalization- General Topics

Asymmetric Digital Subscriber Line (ADSL) is a form of DSL (Digital Subscriber

Line), a data communication technology that enables faster data transmission over

copper telephone lines than a conventional voiceband modem.

Broadband communications over telephone lines is summarized in the next table:

A gateway is commonly used to make an ADSL connection; the application of ADSL is

on GSTN (General Switched Telephone Network), FDM (Frequency Division

Multiplexed) group-band and cellular radio.

ADSL has several similarities to ISDN (Integrated Services Digital Network or

Isolated Subscriber Digital Network).

DSP2 Project

12 November 2008 3


ADSL provides access to digital services at several Mb/s over the

ordinary wire-pair connection between customers and the local telephone

company central office.

Available transmission band is divided in a certain number of

subbands (sending data in interleaved bit streams), each one

transmitting different substreams over different channels.

DMT voiceband modems have been adopted for the Asymmetric

Digital Subscriber Line (ADSL), which provides digital communication

at several Mb/s from a telephone company central office to a subscriber,

and a lower rate in the reverse direction, over a normal twisted pair of

wires in the loop plant.

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12 November 2008 DSP2 Project 4


General Trasmission Channel for ADSL



Power Spectrum for ADSL Transmission Channel

Including examples for 3 schemes of QAM modulation


MCM Technique

MCM (Multicarrier modulation) is the general generic name for QAM

and Orthogonal FDM Modulation technique).

Parameters that have to be taken into consideration when talking about

MCM: mc=bits per symbol, the proportion, the total transmitted

power—all these refer to multicarrier signal.

MCM is good because in modern receivers the MCM signal has to be

processed (in digital domain) without enhancement of noise and

interference caused by linear equalization of a single carrier signal.

MCM in good because the symbols generated are immune to noise

and fast fades.

Multiplexing: needs the separation of carriers at the receiver (for

future demodulation).

The performance of sending/receiving data is evaluated using SNR (Signal

To Noise Ratio)


MCM Technique

Problems that can appear with the MCM: impulse noise, phase jitter,

nonlinear distortion, single frequency interference fades.

Reduction in bit rate caused by the use of general period can be

done by linearly equalizing the received signal.

TDL (Tapped Delay Line is the most suitable for Equalization)

FFT/IFFT MCM based methods: OFDM (Orthogonal Division Frequency

Multiplexing) and DMT( Discrete Multitone).

The key components of an OFDM system are the Inverse DFT(IFFT

Block) in the transmitter and the DFT(FFT Block) at the receiver.

ADSL Equalization - Why equalizing filters?

Compensate for the unequal frequency response of some other signal processing circuit or system (e.g. improve the fidelity of sound, to remove undesired noises)

When the channel consists of telephone lines, the channel transfer function changes from call to call and the equalizing filter needs to be an adaptive filter

812 November 2008 DSP2 Project


Equalizer configuration in training mode

ADSL Equalization- Types of equalizations in time domain

Time domain equalization is used to restrict Inter-symbol

Interference to a tolerable extent

- CF: Impulse shortening

filter

- CB: Reference filter with

reference tap values

- Estimator: Adaptive

filter to find the optimum

equalizer coefficients)

12 November 2008 10

Frequency domain equalization is used to compensate for channel complex

gain (correct for signal phase rotation and signal amplitude

attenuation) at each sub-carrier frequency.

DSP2 Project

ADSL Equalization- Types of equalizations in frequency domain

LMS adaptation of a frequency domain equalizer multiplier.

- Y: Tap Input

- D: Desired response

- Decision: Making desired

response

- X: Adaptive weight-

control filter

- A: Transversal filter with

optimum multiplier A (in

case of 1 tap filter)

11

ADSL Equalization- Conclusions

Conclusions: General transmission channel for ADSL lines/power spectrum for upstream/downstream data MCM Technique used in ADSL modems Equalization methods in time/frequency domainFurther Studies (Second presentation- 4th December 2008) Full block simulation in MATLAB/SIMULINK for the communication channel (both the transmitter and the receiver). Adaptive frequency-domain Equalization Filters (N 1-tap adaptive filters). Equalizer in time domain (The channel impulse response is modeled as ARMA system - autoregressive moving average model).Bibiliography John A.C Bingham- Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come, IEEE Communications Magazine,

May 1990 Koen Vanbleu, Geert Ysebaert, Gert Cuypers and Marc Moonen- On the relationship between time domain equalizers and PER Tone

Equalizers for DMT Based Systems IEEE Benelux Signal Processing Symposium , April 2004


dsp2 project adsl equalization students: dung nguyen quoc- master student tudor tesu- erasmus...

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