H.263
Tutorial

Low bit rate
video case study

Reed-Solomon
Coding Tutorial

ADSL Tutorial

ADSL Tutorial

Background

Asymmetric Digital
Subscriber Line offers the benefits of the Information Superhighway
without the expense or wait. ADSL enables the delivery of switched
digital video services now over the existing telephone
infrastructure, composed primarily of copper wire pair, instead of
waiting for the implementation of costly new cabling systems.

An ADSL circuit connects an ADSL modem on
each end of a twisted-pair telephone line, creating three
information channels -a high speed downstream channel, a medium
speed duplex channel, and a plain old telephone service (POTS)
channel. The POTS channel is split off from the digital modems by
filters, thus guaranteeing uninterrupted POTS. The high speed
channel ranges from 1.5 to 10 Mbps, while the low speed channel
rates range from 16 Kbps to 1 Mbps. Each channel can be
submultiplexed to form multiple, lower rate channels.

General
Description

4i2i offers a
complete tested in silicon ADSL IP
core and the associated embedded training
software.

System
Overview

Figure 1 ADSL system block
diagram

The "Remote" block models the MODEM at the
customer’s home (ATU-R) and the "Central" block models the MODEM at the
telecommunication provider’s central office (ATU-C).

The main functional blocks of the MODEM models
include the following:

Block

Description

Reed-Solomon

Variable overhead
Reed-Solomon Forward Error Correction

Scrambler

Data
scrambling/unscrambling

Interleaver

Convolutional
interleaving with variable depth

Tone
ordering

Adaptive bit
allocation to a series of "tones" or channels

Trellis-Coded
Modulation

Combined
Convolutional Encoding and Quadrature Amplitude
Modulation

IFFT/FFT

Fast Fourier
Transform-based frequency multiplexing

Equalization

Time- and
Frequency-domain signal equalization

Echo
cancellation

Time- and
Frequency-domain echo suppression

Clock
recovery

Frequency-domain
recovery based on pilot tone

Protocols

Training and
initialization, adaptive protocols

Channel

Supports all ADSL
reference lines.

A selection of some performance results
are shown below:

Figure 2
Performance of QAM

Figure 3
Performance of TCM

Figure 4
Constellation: before FEQ training

Figure 5
Constellation: with FEQ training

Figure 6 Adaptive
estimation of channel transfer function

Figure 7
Constellation: before echo cancellation

Figure 8
Constellation: after echo cancellation

Figure 9 Channel
Response

You can contact 4i2i at:

4i2i Communications
Ltd

Doig Scott Building, Craibstone Estate, Bucksburn,
Aberdeen, Scotland, UK
Telephone +44 1224 712844,

Fax +44 1224 714413,

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