Transmit pre-equalization in a modem environment

• US 20030231716A1
• Filed: 06/13/2002
• Published: 12/18/2003
• Est. Priority Date: 06/13/2002
• Status: Active Grant

• Current Assignee: ARRIS Enterprises LLC (CommScope Holding Company, Inc.)
• Original Assignee: ADC Broadband Access Systems, Inc.
• Inventors: Unger, J. David
• Granted Patent: US 6,879,641 B2
• Time in Patent Office: Days
• US Class Current: 375/296
• CPC Class Codes: H04L 27/367 using predistortion

Abstract

The signal distortion present in a transmitted signal is measured and used to generate filter coefficients. The coefficients are stored in memory. A modulator generates an intermediate frequency signal. The modulator inputs the signal to a programmable predistortion filter. The characteristics of the filter are altered by the filter coefficients that have been programmed into the filter. The resulting filtered signal is upconverted to a frequency in a transmit frequency band.

Claims

1. A method for removing distortion in an information signal generated in a cable modem termination system, the method comprising:

retrieving filter coefficients from memory;

loading a predistortion filter with the filter coefficients; and

filtering the information signal with the predistortion filter in response to the filter coefficients.

2. The method of claim 1 and further including modulating the information signal prior to filtering.

3. The method of claim 2 wherein the modulating comprises quadrature amplitude modulating.

4. The method of claim 2 and further including upconverting the filtered information signal prior to transmission over a cable network.

5. The method of claim 1 wherein the information signal is a baseband data stream originating from a Transmission Control Protocol/Internet Protocol compliant network.

6. The method of claim 1 wherein the filter coefficients are generated from an error between an ideal modulated information signal and a distorted modulated information signal.

7. The method of claim 6 and further including:

generating filter coefficients in response to the error; and

saving the filter coefficients.

8. The method of claim 6 and further including:

generating a modulated data pattern with a modulation method in the cable modem termination system;

causing an activated equalizer to converge in response to the modulated data pattern, the activated equalizer having a substantially similar quantity of taps and substantially similar sample rate as the predistortion filter;

extracting real and imaginary tap values from the activated equalizer after convergence; and

storing the real and imaginary tap values for use as the filter coefficients.

9. The method of claim 6 and further including:

generating a continuous wave signal through a cable modem termination system modulator;

measuring a frequency response of the cable modem termination system;

determining an inverse frequency response in response to the measured frequency response; and

generating the filter coefficients in response to an inverse Fast Fourier Transform on the inverse frequency response.

10. A method for transmit pre-equalization of a modulated information signal with a digital filter, the method comprising:

reading filter coefficients from memory;

loading the digital filter with the filter coefficients; and

filtering the modulated information signal with the digital filter by modifying the digital filter'"'"'s characteristics in response to the filter coefficients.

11. The method of claim 10 wherein the modulated signal is a quadrature amplitude modulated signal.

12. The method of claim 10 and further including:

measuring distortion in the modulated information signal without filtering;

generating the filter coefficients in response to the measured distortion; and

saving the filter coefficients in memory.

13. A method for transmit pre-equalization of an information signal with a digital filter in a cable modem termination system, the method comprising:

modulating the information signal with a quadrature amplitude modulation (QAM) technique such that a QAM signal is generated;

channel filtering the QAM signal;

reading filter coefficients from memory;

loading the digital filter with the filter coefficients;

filtering the QAM signal with the digital filter by altering the digital filter'"'"'s characteristics in response to the filter coefficients to generate a filtered signal;

converting the filtered signal from a digital signal to an analog information signal; and

upconverting the analog information signal to a transmit frequency.

14. The method of claim 13 wherein upconverting comprises:

mixing the filtered signal with a first local oscillator signal to generate a first mixed signal; and

mixing the first mixed signal with a second local oscillator signal to generate the information signal at the transmit frequency.

15. The method of claim 14 wherein the transmit frequency falls in a frequency band of 88 MHz to 860 MHz.

16. A method for transmit pre-equalization of a modulated information signal in a cable modem termination system having a predistortion filter, the method comprising:

measuring a distortion level of the modulated information signal;

generating predistortion filter coefficients in response to the measured distortion level; and

saving the predistortion filter coefficients in memory.

17. The method of claim 16 and further including:

reading the predistortion filter coefficients from memory;

loading the predistortion filter with the predistortion filter coefficients; and

filtering the modulated information signal with the predistortion filter by altering the predistortion filter'"'"'s characteristics in response to the predistortion filter coefficients.

18. A cable modem termination system comprising:

a modulator for generating a modulated information signal from a baseband signal;

a predistortion filter coupled to the modulator, the predistortion filter generating a filtered signal in response to programmable filter coefficients that have been generated in response to a measured distortion of a signal transmitted from the cable modem termination system; and

an upconverter, coupled to the predistortion filter, for mixing the filtered signal to a frequency in a transmit frequency band.

19. The cable modem termination system of claim 18 and further including a cable media access controller, coupled to the modulator, that provides the baseband signal.

20. A cable modem termination system comprising:

a cable media access controller capable of providing a baseband data stream signal from a network data signal having a first network protocol;

a quadrature amplitude modulation mapper, coupled to the cable media access controller, for generating a modulated signal from the baseband data stream signal;

a channel filter that generates a filtered signal from the modulated signal;

memory that stores programmable filter coefficients that have been generated in response to a measured distortion of a signal transmitted from the cable modem termination system;

a digital predistortion filter coupled to the channel filter, the digital predistortion filter generating a predistortion filtered signal in response to the programmable filter coefficients;

an intermediate frequency converter that generates a digital intermediate frequency signal from the predistortion filtered signal;

a digital to analog converter that generates an analog intermediate frequency signal from the digital intermediate frequency signal; and

an upconverter that converts the analog intermediate frequency signal to a signal having a frequency in a transmit frequency band.

21. The cable modem termination system of claim 20 and further including forward error correction

22. A method for transmit pre-equalization of an information signal with a predistortion filter in a cable modem termination system, the method comprising:

modulating the information signal with a quadrature amplitude modulation (QAM) technique such that a QAM signal is generated;

channel filtering the QAM signal to produce a filtered signal;

converting the filtered signal from a digital signal to an analog intermediate frequency signal;

reading filter coefficients from memory;

loading the predistortion filter with the filter coefficients;

filtering the filtered signal with the predistortion filter by altering the predistortion filter'"'"'s characteristics in response to the filter coefficients to generate an analog information signal; and

upconverting the analog information signal to a transmit frequency.