Multi-carrier transmission systems

Details Multi-carrier transmission systems Multi-carrier transmission systems

1999-06-07

2001-11-20

Le, Amanda T. (Department: 2634)

Pulse or digital communications

Equalizers

Automatic

C375S260000, C375S350000

Reexamination Certificate

active

06320903

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of adaptively updating frequency domain equalizer parameters in a multi-carrier system, a channel estimation and equalization system, employing the method, a modem and a transceiver incorporating such a channel estimation and equalization system and a multi-carrier transmission system incorporating such transceivers.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DENT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
Modern multi-carrier techniques using orthogonal carriers with high order QAM constellations for the transmission of a plurality of bits per carrier and symbol, use some method for the determination of an inverse channel model to be used in an equalization process. In the frequency domain, this is normally based on an estimate of the channel attenuation and phase for each carrier.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a simplified method of equalisation that uses data at the symbol detector input and output to adaptively estimate an inverse channel model.
It is a further object of the present invention to provide a simplified channel estimation and equalisation system that uses data at the symbol detector input and output to adaptively estimate an inverse channel model.
It is a yet further object of the present invention to provide a multi-carrier transceiver which includes a simplified equaliser that uses data at the symbol detector input and output to adaptively estimate an inverse channel model.
According to a first aspect of the present invention, there is provided a channel estimation and equalization system, for use in a multi-carrier system, arranged to adaptively update frequency domain equalization parameters for an -inverse channel model, characterised in that modifying means are provided for modifying said equalization parameters in small steps so that an equalization means converges on said inverse channel model.
In use, an output signal, U, from said equalization means may be fed to a detector/quantizer means, an output signal, Y, from said detector quantizer means may be fed to a first input of said modifying means and an output signal, EQ, from said modifying means may be fed to said equalization means.
In use, said output signal, Y, may be fed to a symbol decoder means adapted to produce a decoded data stream.
Said modifying means may be operatively connected to variance estimation means adapted to produce a signal, W, indicative of a variance of interference on each carrier of said multi-carrier system.
Said modifying means may operate on an algorithm defined by:
EQ
k
+
1
=
EQ
k
+
μ
&LeftBracketingBar;
U
k
&RightBracketingBar;
2
·
EQ
k
·
U
k
*
·
(
Y
k
-
U
k
)
where &mgr;, is a positive constant (&mgr;<<1), to determine successive values of said equalization parameters.
The expression &mgr;/|U
k
|
2
, in the algorithm may be replaced by:
2
−integer 2.log
2
|U
k
|)+integer(log
2
&mgr;)
The exponent:
−integer (2.log
2
|U
k
|)+integer(log
2
&mgr;)
may be produced by using an absolute value of U
k
as an input to a binary priority encoder and negating an output thereof.
Multiplicative operations in said algorithm may be implemented by means of a barrel shifter.
A value for variance of interference on each carrier of said multi-carrier system may be estimated using a standard method of integrating squared deviations from a mean, in which each quantized value, Y, is used as a mean for a range of data values, U, that are quantized to Y.
A value for variance of interference on each carrier of said multi-carrier system may be estimated using an algorithm defined by:
W
k+1
=(1−&egr;)·W
k
+&egr;·|Y
k
−U
k
|
2
in which integration is simulated by use of an exponentially weighted averaging filter and E is a small positive constant (&egr;<<1).
According to a second aspect of the present invention, there is provided a modem, for use with a multi-carrier system, characterised in that said modem includes a channel estimation and equalisation system, as set out in the preceding paragraphs.
According to a third aspect of the present invention, there is provided a transceiver, for use with a multi-carrier system, characterised in that said transceiver includes a modem as set out in the previous paragraph.
Said multi-carrier system may be a DMT VDSL transmission system employing Discrete Fourier Transforms to create and demodulate individual carriers.
According to a fourth aspect of the present invention, there is provided a DMT VDSL transmission system employing Discrete Fourier Transforms to create and demodulate individual carriers, characterised in that said DMT VDSL transmission system includes at least two tra

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