QAM receiver with improved immunity to crosstalk noise

• US 6,289,063 B1
• Filed: 09/02/1998
• Issued: 09/11/2001
• Est. Priority Date: 09/02/1998
• Status: Expired due to Term

• Current Assignee: RPX Clearinghouse LLC (RPX Corporation)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Duxbury, Guy M. A. F.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): DEPPE, BETSY LEE
• Application Number:
  US09/146,341
• Time in Patent Office: 1,105 Days
• Field of Search: 375/346, 375/348, 375/350, 375/229, 375/230, 375/231, 375/232, 375/233, 375/235, 375/261, 375/332, 375/329, 375/322, 375/279, 375/280, 375/281, 333/28.R, 333/18, 455/307, 455/63, 455/295, 455/296, 455/67.3, 455/501, 708/300, 708/322, 708/323, 329/304, 370/206, 370/201, 370/286, 370/290, 370/291
• US Class Current: 375/348
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence
• CPC Class Codes: H04L 27/3809 Amplitude regulation arrangements

Abstract

The invention discloses a novel equalization system for reducing the deleterious effects of crosstalk on signals received at a modem, with particular regard to QAM signals transmitted over copper twisted pairs. The equalization system employs the common combination of a linear equalizer followed by a decision-feedback equalizer (DFE). However, instead of aiming for perfect equalization of channel distortion, the overall frequency response of the channel plus the linear equalizer is skewed such that higher frequencies are attenuated relative to lower ones. More generally, the spectral regions where crosstalk is strongest are attenuated, which reduces the crosstalk noise present at the input to the DFE at the expense of introducing inter-symbol interference (ISI). Fortunately, most DFEs are capable of handling the added ISI, leading to data decisions that are relatively unaffected by crosstalk noise.

Claims

1. An equalization system for equalizing an input signal corrupted by inter-symbol interference caused by channel distortion and crosstalk noise, the channel distortion and crosstalk noise having respective frequency characteristics, 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, wherein said linear equalizer has a frequency response made to resemble the difference between the inverse of the frequency characteristic of the channel distortion and the frequency characteristic of the crosstalk noise; and

a decision-feedback equalizer connected to the linear equalizer, for filtering the intermediate signal and further reducing the inter-symbol interference contained therein.

2. 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.

3. An equalization system according to claim 2, wherein the offset frequency response has an upwards-sloping curve.

4. An equalization system in accordance with claim 2, wherein the decision-feedback equalizer comprises means for evaluating a function of the difference between the output data signal and a delayed version of a signal formed by adding the intermediate signal to a filtered version of the output data signal, thereby to form a second error signal, wherein the frequency response of the adaptive linear equalizer is further controllable by the second error signal.

5. An equalization system according to claim 4, wherein the offset filter is adaptive and wherein the offset frequency response is controllable by the second error signal.

6. An equalization system according to claim 2, wherein the offset filter is a finite-impulse-response filter.

7. An equalization system according to claim 2, wherein the offset filter is a infinite-impulse-response filter.

8. An equalization system in accordance with claim 2, wherein the decision-feedback equalizer comprises means for evaluating a function of the difference between the output data signal and a delayed version of a signal formed by adding the intermediate signal to a filtered version of the output data signal, thereby to form a second error signal, wherein the offset filter is adaptive and wherein the offset frequency response is controllable by the second error signal.

9. 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 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.

10. 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

an adaptive equalization means placed at one or more points between the bandpass filter and the at least one decision-feedback equalizer, the adaptive equalization means having a selectable overall frequency response;

wherein the overall frequency response of the adaptive equalization means is selected to resemble a difference between the inverse of the frequency characteristic of the channel distortion and the frequency characteristic of the crosstalk noise.

11. A receiver according to claim 10, wherein the bandpass filter has a frequency range of 1 MHz to 30 MHz.

12. A receiver according to claim 10, wherein the demodulator is a QAM demodulator.

13. A receiver according to claim 10, further comprising a variable-gain amplifier placed at one point between the bandpass filter and the analog-to-digital converter.

14. A receiver according to claim 10, further comprising an offset filter connected to the adaptive equalization means for filtering the intermediate digital signal in accordance with a selectable offset frequency response and producing an offset-filtered signal.

15. A receiver according to claim 14, wherein the offset frequency response has an upwards-sloping curve.

16. A receiver in accordance with claim 14, wherein only one of the at least one decision-feedback equalizer comprises means for evaluating a function of the difference between the output data signal and a delayed version of a signal formed by adding the intermediate signal to a filtered version of the output data signal, thereby to form a second error signal, wherein the frequency response of the adaptive equalization means is further controllable by the second error signal.

17. A receiver according to claim 16, wherein the receiver has an initial mode of operation, during which adaptation of the adaptive equalization means is controlled by a first error signal, and a steady-state mode of operation, during which adaptation of the adaptive equalization means is controlled partly by the first error signal and partly by the second error signal.

18. A receiver according to claim 16, wherein the offset filter is adaptive and wherein the offset frequency response is controllable by the second error signal.

19. A receiver according to claim 14, wherein the offset filter us a finite-impulse-response filter.

20. A receiver according to claim 14, wherein only one of the at least one decision-feedback equalizers comprises means for evaluating a function of the difference between the output data signal and a delayed version of a signal formed by adding the intermediate signal to a filtered version of the output data signal, thereby to form a second error signal, wherein the offset filter is adaptive and wherein the offset frequency response is controllable by the second error signal.