Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-09-02
2001-09-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S350000, C375S229000, C375S232000, C375S233000, C375S332000, C375S261000, C329S309000
Reexamination Certificate
active
06289063
ABSTRACT:
FIELD OF THE INVENTION
The present invention is concerned with the reception of electronic signals and particularly relates to a method and apparatus for reducing the effects of crosstalk on received subscriber loop signals.
BACKGROUND OF THE INVENTION
A signal travelling along an unshielded copper twisted pair undergoes frequency-dependent attenuation due to the transmission characteristics of the cable. This distortion can be modelled as a slowly time-varying filtering operation applied to the transmitted signal. The relative variations in the attenuation at different frequencies cause phase shifts in the transmitted waveform and, in digital transmission systems, ultimately lead to spreading of the transmitted symbols into adjacent symbol intervals, a phenomenon known as inter-symbol interference (ISI). This results in the receiver committing errors when data decisions are made based on the distorted received signal.
In order to compensate for the distortive effects of cable loss, it is customary to employ a linear equalizer, possibly even an adaptive one, followed by a decision-feedback equalizer (DFE). The linear equalizer usually consists of a filter, either digital or analog, which has a frequency response equal to (or close to) the inverse response of the twisted pair “channel”. Therefore, if the transmitted signal is attenuated by a downwards sloping frequency response as it travels along the twisted pair, then the frequency response of the linear equalizer will have an upwards sloping shape.
The purpose of linear equalization is to equalize, or “flatten”, the overall channel response affecting the transmitted signal as it arrives at the decision-feedback equalizer. This linear equalization process has the effect of removing much, but not all, of the ISI corrupting the transmitted signal. The DFE is then used for removing any remaining ISI and for making data decisions on the received digital signal. Much theory has been developed around the problems of channel equalization and decision-feedback equalization, and various implementations have proven successful in the case of a digital signal transmitted along isolated media.
However, when a telephone company considers delivering digital signals to a plurality of subscribers over a copper twisted pair infrastructure, it is often the case that multiple twisted pairs are bundled together for at least part of the journey between a central office and the subscribers. As a result of poor shielding provided by the thin layer of insulant surrounding each copper wire, electromagnetic fields may be induced by one wire into other wires in the bundle, creating an effect known as crosstalk.
In general, crosstalk couples more at higher frequencies and therefore after a long voyage along a twisted pair, the effect of crosstalk on a transmitted signal will be most noticeable at high frequencies. A serious problem then occurs if the linear equalizer in the receiver boosts the high frequencies in an attempt to equalize the channel: the effects of crosstalk noise, which is stronger at higher frequencies, are actually enhanced.
At the DFE, therefore, the received signal equalized in accordance with prior art techniques has reduced ISI but possibly increased crosstalk noise. When crosstalk noise becomes the dominant cause of distortion, as when many twisted pairs are bundled together, the decision-feedback equalizer is incapable of making correct data decisions, with obvious deleterious consequences.
SUMMARY OF THE INVENTION
It is an object of the present invention to mitigate or obviate one or more disadvantages of the prior art.
Therefore, the invention may be summarized according to a first broad aspect as an equalization system for equalizing an input signal corrupted by inter-symbol interference and crosstalk noise, the equalization system comprising: a linear equalizer for filtering the input signal and producing an intermediate signal having reduced crosstalk noise and reduced inter-symbol interference; and a decision-feedback equalizer connected to the linear equalizer, for filtering the intermediate signal and further reducing the inter-symbol interference contained therein.
According to a second broad aspect, the invention may be summarized as an equalization system for producing an output data signal from an input signal corrupted by channel distortion and crosstalk noise, the crosstalk noise having a frequency characteristic, the channel distortion causing inter-symbol interference, the equalization system comprising: an adaptive linear equalizer for filtering the input signal and producing an intermediate signal having reduced crosstalk noise and reduced inter-symbol interference, wherein the adaptive linear equalizer has a frequency response controllable by a first error signal; a decision-feedback equalizer connected to the adaptive linear equalizer, for filtering the intermediate signal and further reducing the inter-symbol interference contained therein, thereby to produce the output data signal; an offset filter connected to the adaptive linear equalizer, for filtering the intermediate signal in accordance with a selectable offset frequency response and producing an offset-filtered signal, wherein the offset frequency response is selected to resemble the crosstalk frequency characteristic; and an error calculation block connected to the decision-feedback equalizer, to the offset filter and to the adaptive linear equalizer, for measuring a characteristic of the difference between the offset-filtered signal and a delayed version of the output data signal, thereby to produce the first error signal.
The invention may be summarized according to a third broad aspect as a method of equalizing an input signal corrupted by inter-symbol interference and crosstalk noise, comprising the steps of: filtering the input signal with a linear equalizer, thereby to produce an intermediate signal having reduced crosstalk noise and reduced inter-symbol interference; and filtering the intermediate signal with a decision-feedback equalizer, thereby to further reduce the inter-symbol interference in the intermediate signal.
In a method of equalizing an input signal corrupted by inter-symbol interference caused by cable loss varying in the frequency domain, said method consisting of filtering the input signal with a linear equalizer having a frequency response which compensates for the inter-symbol interference, the invention may be summarized according to another broad aspect as the improvement wherein the frequency response of the linear equalizer is deliberately made different from the frequency response which fully compensates for the inter-symbol interference so as to compensate for crosstalk, the remaining inter-symbol interference being further reduced by filtering with a decision-feedback equalizer.
According to yet another broad aspect, the invention may be summarized as a receiver used for producing a plurality of streams of digital data from a received analog input signal containing an analog message signal carrying the digital data, the message signal being centered about a carrier frequency and corrupted by channel distortion and crosstalk noise having respective frequency characteristics, the channel distortion causing inter-symbol interference, the receiver comprising: a bandpass filter for receiving the analog input signal and extracting the analog message signal therefrom; an analog-to-digital converter connected to the bandpass filter for converting the analog message signal to an intermediate digital signal; a demodulator connected to the analog-to-digital converter for creating a plurality of digital output signals from the intermediate digital signal; at least one decision-feedback equalizer connected to the demodulator for filtering the digital output signals and creating the digital data streams therefrom; and adaptive equalization means placed at one or more points between the bandpass filter and the decision-feedback equalizer, the equalization means having a selectable overall frequency response; wherein the overall freq
Chin Stephen
Deppe Betsy L.
Nortel Networks Limited
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