Spectrally constrained impulse shortening filter for a discrete multi-tone receiver

• US 6,631,175 B2
• Filed: 04/03/1998
• Issued: 10/07/2003
• Est. Priority Date: 04/03/1998
• Status: Expired due to Term

• Current Assignee: Tellabs Operations Incorporated (Infinera Corp.)
• Original Assignee: Tellabs Operations Incorporated (Infinera Corp.)
• Inventors: Harikumar, Gopal, Marchok, Daniel
• Primary Examiner(s): Liu, Shuwang
• Application Number:
  US09/054,468
• Publication Number:
  US 20020106035A1
• Time in Patent Office: 2,013 Days
• Field of Search: 375/350, 375/231, 375/296, 375/232, 375/222, 375/346, 375/340, 375/260, 375/229, 375/230, 375/233, 370/286
• US Class Current: 375/350
• CPC Class Codes: H04L 2025/03414 Multicarrier
• CPC Class Codes: H04L 25/03012 operating in the time domain H04L25/03165, H04L25/03178 take precedence

Abstract

A channel in a multiple carrier communication system is equalized by computing a target spectral response, shortening the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length and filtering the signal based on the target spectral response.

Claims

1. A method for equalizing a channel in a multiple carrier communication system, the channel being configured to receive a signal and including a spectrally constrained impulse shortening filter, the method comprising:

measuring received noise power spectral density;

computing a target spectral response having a magnitude constraint that is based on the measured noise power spectral density;

selecting a frequency response of the spectrally constrained impulse shortening filter based on the target spectral response to reduce noise bleeding from a particular channel to adjacent channels; and

filtering the communication signal with the spectrally constrained impulse shortening filter.

2. The method of claim 1, wherein the communication system includes a discrete Fourier transform connected to receive an output of the spectrally constrained impulse shortening filter and the noise power spectral density is measured at an output of the discrete Fourier transform.

3. The method of claim 1, wherein the communication system includes a discrete cosine transform and the noise power spectral density is measured at an output of the discrete cosine transform.

4. The method of claim 1, wherein the spectrally constrained impulse shortening filter comprises a time domain digital filter.

23. The method of claim 1 wherein the selected frequency response of the spectrally constrained impulse shortening filter attenuates noise and signal components in the particular channel substantially equally.

5. A method for equalizing a channel in a multiple carrier communication system, the channel having an impulse response and being configured to receive a signal having a cyclic prefix, the method comprising:

computing a target spectral response having a magnitude constraint that is based on measured noise power spectral density;

shortening the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length; and

filtering the signal based on the target spectral response.

6. The method of claim 5, wherein the target length is a length of the cyclic prefix.

7. The method of claim 5, wherein the communication system includes a discrete Fourier transform and the noise power spectral density is measured at an output of the discrete Fourier transform.

8. The method of claim 5, wherein the filtering step is performed with a filter having a frequency response selected to match the target spectral response.

9. The method of claim 5, wherein the shortening of the impulse response is performed by a time domain digital filter.

10. The method of claim 5, wherein the filtering is performed by a time domain digital filter.

11. The method of claim 5, wherein the shortening of the impulse response and the filtering are performed by a time domain digital filter.

12. A spectrally constrained impulse shortening filter for a multiple carrier communication system, the system being configured to receive a signal and including a channel that has an impulse response, the filter comprising:

an input connected to receive the signal;

a digital filter structure configured to apply a frequency characteristic to the signal, the frequency characteristic being determined by filter coefficients; and

taps connected to receive the filter coefficients,

wherein the coefficients are selected to shorten the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length and to apply a frequency characteristic to the signal based on a target spectral response having a magnitude constraint that is based on measured noise power spectral density.

13. The filter of claim 12, wherein the target length is a length of the cyclic prefix.

14. The filter of claim 12, wherein the noise power spectral density is measured at an output of a discrete Fourier transform connected to receive an output of the spectrally constrained impulse shortening filter.

15. A receiver for receiving a multiple carrier signal from a communication channel having an impulse response, the receiver comprising:

an analog-to-digital converter connected to receive the signal from the communication channel;

a spectrally constrained impulse shortening filter connected to receive the signal from the analog-to-digital converter and configured to shorten the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length and to apply a frequency characteristic to the signal based on a target spectral response having a magnitude constraint that is based on measured noise power spectral density to reduce noise bleeding from the channel to adjacent channels;

a discrete Fourier transform connected to receive the output of the spectrally constrained impulse shortening filter; and

a decoder connected to receive outputs of the discrete Fourier transform.

16. The receiver of claim 15, wherein the target spectral response is computed from measured noise power spectral density.

17. The receiver of claim 16, wherein the noise power spectral density is measured at an output of a discrete Fourier transform.

18. The receiver of claim 16, wherein the target spectral response is approximated by the inverse of the measured noise power spectral density.

19. The receiver of claim 15, wherein the target length is a length of the cyclic prefix.

24. The receiver of claim 15 wherein the applied frequency characteristic causes attenuation of noise and signal components in the channel substantially equally.

20. A modem comprising:

an encoder connected to receive digital data and configured to output a constellation of complex values;

an inverse discrete Fourier transform connected to receive the constellation from the encoder;

a digital-to-analog converter connected to the inverse discrete Fourier transform and configured to output a signal to a communication channel;

an analog-to-digital converter configured to receive the signal from the communication channel;

a spectrally constrained impulse shortening filter configured to shorten an impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length and filter the signal based on a target spectral response having a magnitude constraint that is based on measured noise power spectral density;

a discrete Fourier transform connected to receive an output of the spectrally constrained impulse shortening filter; and

a decoder connected to the discrete Fourier transform and configured to output digital data.

21. The modem of claim 20, wherein the noise power spectral density is measured at an output of the discrete Fourier transform.

22. The modem of claim 20, wherein the target length is a length of the cyclic prefix.