Echo cancellation for an ADSL modem

• US 6,480,532 B1
• Filed: 07/13/1999
• Issued: 11/12/2002
• Est. Priority Date: 07/13/1999
• Status: Expired due to Term

• Current Assignee: STMicroelectronics Incorporated (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Incorporated (STMicroelectronics NV)
• Inventors: Vareljian, Albert
• Primary Examiner(s): Pham, Chi
• Assistant Examiner(s): Kumar, Pankaj
• Application Number:
  US09/352,813
• Time in Patent Office: 1,218 Days
• Field of Search: 375/216, 375/217, 375/219, 375/220, 375/221, 375/222, 375/224, 375/286, 375/288, 375/295, 375/296, 375/316, 375/328, 375/343, 375/346, 375/349, 375/350, 375/351, 375/257
• US Class Current: 375/222
• CPC Class Codes: H04B 3/23 using a replica of transmitted signal in the time domain, e.g. echo cancellers

Abstract

An echo cancellation functionality taps a digital transmit signal from a transmit channel for processing through an adaptive filter of an echo channel to generate an echo cancellation signal. The adaptive filter has a transfer function substantially matching an echo transfer function which defines a relationship between the transmit signal and an unwanted echo component corrupting an analog receive signal. The echo cancellation signal is digital-to-analog converted to an analog signal and then subtracted from the analog receive signal to substantially cancel out the unwanted echo component. The echo cancellation functionality may be configured in a training mode to generate an error signal used to adaptively configure the adaptive filter transfer function to substantially match the echo transfer function. When in training mode, certain components of an adaptation loop which contribute to a feedback loop transfer function are selectively by-passed. The adaptive filter may comprise a least mean square finite impulse response filter implementing a predistortion function to account for the feedback loop transfer function.

Claims

1. A bi-directional communications device including both an analog processing domain and a digital processing domain, comprising:

a transmit channel spanning the digital and analog processing domains for processing a transmit signal;

a receive channel spanning the analog and digital processing domains for processing a receive signal;

a line interface in the analog processing domain for transmitting the transmit signal over a communications line, and for receiving the receive signal as corrupted by an unwanted echo component of the transmit signal from the communications line; and

an echo channel spanning the digital and analog processing domains and including;

a tap in the digital processing domain for tapping the transmit signal from the transmit channel;

an adaptive filter in the digital processing domain for processing the tapped transmit signal to generate an echo cancellation signal, the adaptive filter having a transfer function substantially matching an echo transfer function which defines a relationship at the interface between the transmit signal and the unwanted echo component; and

a converter for converting the echo cancellation signal to an analog echo cancellation signal; and

wherein the receive channel further includes a summer in the analog processing domain for subtracting the analog echo cancellation signal from the echo component corrupted receive signal and outputting a corrected receive signal for further processing in the receive channel.

2. The communications device of claim 1 wherein the communications line comprises a twisted pair telephone line.

3. The communications device of claim 2 wherein the device comprises a modem.

4. The communications device of claim 3 wherein the modem comprises an Asymmetric Digital Subscriber Line (ADSL) modem.

5. The communications device of claim 1 wherein the transmit channel comprises:

a first interpolator;

a first digital-to-analog converter having an input connected to an output of the first interpolator;

a first low pass filter having an input connected to an output of the first digital-to-analog converter; and

a first amplifier driver having an input connected to an output of the first low pass filter and an output connected to the line interface.

6. The communications device of claim 5 wherein the echo channel comprises:

the adaptive filter;

a second interpolator having an input connected to an output of the adaptive filter;

a second digital-to-analog converter having an input connected to an output of the second interpolator;

a second low pass filter having an input connected to an output of the second digital-to-analog converter; and

a second amplifier having an input connected to an output of the second low pass filter and an output connected to the summer.

7. The communications device of claim 6 wherein:

the first and second interpolators have substantially matching transfer functions; and

the first and second low pass filters have substantially matching transfer functions.

8. The communications device of claim 6 wherein the receive channel comprises:

a line receiver having an input connected to an output of the summer;

filter means having an input connected to an output of the line receiver;

an analog-to-digital converter having an input connected to an output of the filter means; and

a decimator having an input connected to an output of the analog to digital converter.

9. The communications device of claim 8 further comprising:

a first switch moveable between an operating position connecting the transmit channel to receive the transmit signal and a training position connecting the transmit channel to receive a training signal;

a second switch moveable between an operating position wherein the first interpolator processes the transmit signal and a training position wherein the training signal is by-passed around the first interpolator;

a third switch moveable between an operating position wherein the first low pass filter processes the transmit signal and a training position wherein the training signal is by-passed around the first low pass filter.

10. The communications device of claim 8 further comprising:

a fourth switch moveable between an operating position wherein the second interpolator processes the echo cancellation signal output from the adaptive filter and a training position wherein the echo cancellation signal is by-passed around the second interpolator;

a fifth switch moveable between an operating position wherein the second low pass filter processes the echo cancellation signal and a training position wherein the echo cancellation signal is by-passed around the second low pass filter;

a sixth switch moveable between an operating position wherein the corrected receive signal is processed through the line receiver and filter means and a training position wherein the corrected receive signal comprising an error correction signal is by-passed around the line receiver and filter means; and

a seventh switch moveable between an operating wherein the corrected receive signal is processed by the decimator and a training position wherein the error correction signal is routed to the adaptive filter.

11. The communications device of claim 10 further including a switch controller operating responsive to the initiation of communication through the communications device for moving certain ones of the switches into the training position for a short period of time necessary to adaptively configure the transfer function of the adaptive filter to substantially match the echo transfer function.

12. The communications device of claim 1 wherein the adaptive filter comprises a least mean square finite impulse response filter.

13. The communications device of claim 12 wherein the least mean square filter comprises:

a first n-tap delay line;

a recursive tap weight update section connected to the first n-tap delay line;

a predistortion filter having a transfer function substantially matching a combined transfer function for the echo channel and the receive channel; and

a second n-tap delay line having an input connected to an output of the filter and an output generating a predistortion vector for input to the recursive tap weight update section.

24. The device of claim 1 further including an error signal generated by the receive channel in the digital domain and applied to the adaptive filter to tune the echo cancellation signal to substantially cancel the unwanted echo component.

25. The device of claim 24 wherein the error signal is generated during a training mode.

14. An echo cancellation functionality, comprising:

an adaptation loop, including;

a receive channel for processing a received analog receive signal; and

an echo channel, including;

a digital adaptive filter for processing a digital transmit signal to generate a digital echo cancellation signal, the adaptive filter having a transfer function substantially matching an echo transfer function which defines a relationship between the transmit signal and an unwanted echo component corrupting the analog receive signal; and

a converter for converting the digital echo cancellation signal to an analog echo cancellation signal; and

a summer operable to subtract the analog echo cancellation signal from the echo component corrupted analog receive signal and output the receive signal for further processing in the receive channel.

15. The echo cancellation functionality of claim 14 wherein adaptation loop has a combined transfer function and wherein the adaptive filter comprises:

an adaptation functionality responsive to an error correction signal for adaptively configuring an overall transfer function of the adaptive filter to substantially match the echo transfer functionality; and

a predistortion functionality for predistorting the overall transfer function of the adaptive filter to account for the combined transfer function of the adaptation loop.

16. The echo cancellation functionality of claim 15 further including switch means operable to place the echo cancellation functionality in a training mode wherein adaptation loop processed signals by-pass certain components of the adaptation loop which contribute to the combined transfer function and wherein the receive channel generates the error correction signal for application to the adaptation functionality of the adaptive filter.

17. The echo cancellation functionality of claim 16 further including a switch controller for placing certain switch means into the training mode for a short period of time necessary to adoptively configure the overall transfer function of the adaptive filter to substantially match the echo transfer function.

18. The echo cancellation functionality of claim 15 wherein the predistortion functionality comprises:

a predistortion filter having a transfer function substantially matching the combined transfer function for the adaptation loop; and

a first n-tap delay line having an input connected to an output of the filter and an output generating a predistortion vector; and

wherein the adaptation functionality comprises;

a second n-tap delay line; and

a recursive tap weight update section connected to the first n-tap delay line and responsive to the error correction signal and the predistortion vector to adaptively configure the overall transfer function to substantially match the echo transfer function.

19. The echo cancellation functionality of claim 14 wherein the adaptive filter comprises a least mean square finite impulse response filter.

26. The functionality of claim 14 including an error signal generated by the receive channel in a digital domain and applied to the adaptive filter to tune the echo cancellation signal to substantially cancel the unwanted echo component.

27. The functionality of claim 26 wherein the error signal is generated during a training mode.

20. A method, comprising the steps of:

processing a first signal in a transmit channel spanning a digital processing domain and an analog processing domain;

transmitting the first signal over a communications line;

receiving a second signal from the communications line, the second signal being corrupted by an unwanted echo component of the first signal;

processing the receive signal in a receive channel spanning the analog and digital processing domains;

tapping in the digital processing domain the first signal from the transmit channel;

adaptively filtering in the digital processing domain the tapped first signal to generate an echo cancellation signal, the adaptive filter having a transfer function substantially matching an echo transfer function which defines a relationship between the first signal and the unwanted echo component;

converting the echo cancellation signal to an analog echo cancellation signal; and

subtracting in the analog processing domain the analog echo cancellation signal from the echo component corrupted second signal and outputting a corrected second signal for further processing in the receive channel.

21. The method of claim 20 further including the step of training the adaptive filter transfer function to substantially match the echo transfer function.

22. The method of claim 21 wherein the step of training is performed at the beginning of each new communication.

23. The method of claim 21 wherein the step of training comprises the steps of:

applying a training signal to the transmit channel as the transmit signal;

tapping in the digital processing domain the first training from the transmit channel;

adaptively filtering in the digital processing domain the tapped training signal to generate an echo cancellation training signal;

converting the echo cancellation training signal to an analog echo cancellation signal;

subtracting in the analog processing domain the analog echo cancellation training signal from the echo component corrupted second signal and outputting a corrected training signal; and

adjusting the adaptive filter transfer function in response to the corrected training signal to drive the echo cancellation training signal closer to substantially zero.

28. The method of claim 20 further including the steps of:

generating an error signal by the receive channel in the digital processing domain; and

applying the error signal to tune the adaptive filtering step such that the echo cancellation signal substantially cancels the unwanted echo component.

29. The method of claim 28 wherein the steps of generating and applying are performed in connection with a training mode of operation.

30. The method of claim 20 wherein the step of adaptively filtering comprises the step of predistorting the transfer function for the adaptive filtering step to account for a combined transfer function of an adaptation loop formed by the steps of processing, tapping, adaptively filtering, converting and summing.