Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
2000-08-16
2004-08-17
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S350000, C375S285000
Reexamination Certificate
active
06778618
ABSTRACT:
BACKGROUND OF THE INVENTION
A “digital subscriber loop” (“DSL”) is a type of communications connection and/or service which is now being offered by many local exchange carriers (e.g., telephone companies) to consumers and businesses alike as a way of receiving faster Internet connections and downloads.
Digital, multi-tone transceivers (“DMTs”) are devices which are designed to send and receive DSL-compatible signals (hereinafter “DSL signals”).
The quality of a DSL signal received by a DMT depends on a number of factors. One such factor is the amount of interblock interference (“IBI”). In an attempt to avoid the undesirable effects of IBI, DMTs are designed to generate and insert a guard time sequence, called a cyclic prefix, between each data block. In order to completely eliminate IBI, the cycle prefix has to be at least as long as the impulse response of the channel through which DMTs transmit DSL signals. However, in practice this can rarely be achieved, necessitating the use of supplemental methods and devices to minimize IBI. One such device is a time domain equalizer (“TEQ”).
Generally speaking, a TEQ is an electronic filter which filters out the effects of IBI. More specifically, a TEQ is a type of “finite impulse response” (“FIR”) filter.
FIG. 1
depicts a graph of a typical, simplified impulse response, h(n), of a DSL channel. Traditionally, the graph shown in
FIG. 1
is described as having a “non-causal” portion, (“NC”), and a “causal” portion, (“C”). The non-causal section comprises, what will be referred to as a “lower tail,” while the causal portion comprises “main lobe” and “upper tail sections,” respectively.
Existing TEQs have been able to reduce IBI by reducing the power in the tails. However, existing TEQs have not been able to substantially minimize IBI.
FIG. 2
depicts a graph of an “ideal”, theoretical impulse response where the power within both tails has been reduced to zero. Simplified, eliminating the power within both tails means eliminating the tails altogether.
Comparing
FIG. 1
with
FIG. 2
, it can be seen that the signal levels of points
1
-
4
have been lowered to zero. Whereas the impulse response in
FIG. 1
contained power in each of its tails (represented by the area under each tail, P
L
and P
u
, respectively), such power has been eliminated within the ideal impulse response shown in FIG.
2
.
FIG. 3
depicts a simplified block diagram of a circuit
100
comprising a TEQ
104
. It should be understood that this “circuit” may comprise integrated circuits, discrete devices, or the like.
As shown, a signal S
1
is transmitted by transmitter
101
through a communications channel
102
. The channel
102
can be characterized by an impulse response
102
a. Signal S
2
represents a distorted version of signal S
1
. Some of the distortion is due to IBI. Before being received by a receiver
103
, the signal S
2
is input into a TEQ
104
in order to reduce IBI. It should be noted that transmitter
101
and receiver
103
may both comprise DMT/DSL transceivers or the like and that the TEQ
104
is typically a part of receiver
103
.
In an attempt to minimize IBI, existing TEQ's utilize “equalizer coefficients” (hereafter “coefficients”) which are adapted to reduce the power under the tails of a composite impulse response (i.e., combination of the TEQ and original channel). This approach reduces IBI, but it does not substantially minimize IBI.
To make the explanation which follows as clear as possible, these coefficients can be thought of as “weights.” That is, a TEQ is adapted to filter the impulse response with its weights so as to lower the power in the tails of the composite impulse response. There exists methods and devices which utilize such weights to so reduce IBI. Again, though IBI is reduced, it is not substantially minimized.
A major reason why IBI is not minimized is due to a fundamental flaw regarding the nature of how different points along a tail contribute to IBI.
To date, it has been assumed that different points along the tails of an impulse response contribute equally to IBI. Based on this erroneous assumption, existing TEQs are designed so that the coefficients uniformly reduce the power in each tail.
The present inventor discovered that each portion of a tail, for example, portions represented by points
1
-
4
in
FIG. 1
, contribute non-uniform amounts of power to IBI.
Accordingly, it is a desire of the present invention to provide for methods and devices which substantially minimize IBI in DMTs.
It is another desire of the invention to provide for methods and devices which substantially minimize IBI in DMTs by taking into consideration the fact that each portion of a tail of a channel's impulse response contributes a non-uniform amount of IBI.
Other desires will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings and claims.
SUMMARY OF THE INVENTION
In accordance with the present invention there are provided methods and devices for substantially minimizing interblock interference. One device envisioned by the present invention comprises a finite impulse response filter adapted to apply differential coefficients to a channel's impulse response. Such a device may comprise a DMT, or DSL transceiver to name just a few examples.
The differential coefficients envisioned by the present invention are derived from a novel weighting matrix of differential factors. The factors are selected by taking into account that each point along a tail of a channel's impulse response contributes a different amount of IBI. More specifically, points located the furthest from the time of reference of a channel's impulse response contribute more IBI than points located closer (e.g., point
1
in
FIG. 1
contributes more IBI than point
2
).
Novel filters making use of such differential coefficients are applied to tails of the impulse response to substantially minimize IBI.
Both devices and methods are envisioned by the present invention.
REFERENCES:
patent: 5117291 (1992-05-01), Fadavi-Ardekani et al.
patent: 5142551 (1992-08-01), Borth et al.
patent: 5471504 (1995-11-01), Lee et al.
patent: 5917855 (1999-06-01), Kim
patent: 6269131 (2001-07-01), Gothe et al.
patent: 6396886 (2002-05-01), Kapoor
patent: 6600794 (2003-07-01), Agarossi et al.
patent: 2001/0008543 (2001-07-01), Tanada
patent: 001028563 (2000-08-01), None
Agere Systems Inc.
Corrielus Jean B.
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