Method and apparatus for filtering interference in a modem receiver

• US 6,240,150 B1
• Filed: 05/12/1998
• Issued: 05/29/2001
• Est. Priority Date: 05/12/1998
• Status: Expired due to Term

• Current Assignee: RPX Clearinghouse LLC (RPX Corporation)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Velez, Edgar, Darveau, Michel
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Fan, Chieh M.
• Application Number:
  US09/076,633
• Time in Patent Office: 1,113 Days
• Field of Search: 375/222, 375/229, 375/230, 375/231, 375/232, 375/233, 375/234, 375/235, 375/236, 375/346, 375/350, 375/261, 375/316, 455/307, 455/60
• US Class Current: 375/350
• CPC Class Codes: H04B 1/10 Means associated with receiver for limiting or suppressing noise or interference
• CPC Class Codes: H04B 15/02 Reducing interference from electric apparatus by means located at or near the interfering apparatus
• CPC Class Codes: H04L 27/38 Demodulator circuits; Receiver circuits

Abstract

A method and apparatus for filtering interference (e.g. transmitter feed-through and RF) in a modem receiver. The method of reducing transmitter feed-through (Tx-FT) interference at a receiver of a modem to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator includes attenuating the Tx-FT interference and passing the Rx signal at the carrier frequency using an analog filter. The output signal from the analog filter is converted to a digital representation by a analog-to-digital (ADC) converter. A digital band pass filter is used to pass the Rx signal at the carrier frequency and to attenuate the Tx-FT interference further. A mixer is then used to shift the carrier frequency of the spectrum of the Rx signal to an intermediate frequency. The mixer introduces at least one spectral transmitter feed-through image Tx-FT(I) and a receive signal image Rx(I). A digital low pass filter is used to pass the Rx signal at the intermediate frequency and attenuating the image spectrums Tx-FT(I) and Rx(I). A final decimation function is performed to downsample the Rx signal at a downsampling rate from the second digital filter to match the sampling rate of the QAM demodulator. Interference filtering is improved by dividing the functions between an analog filter prior to conversion and a pair of digital filters after conversion.

Claims

1. An apparatus for reducing transmitter feed-through (Tx-FT) interference at a receiver of a modem operating at a first sampling rate to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator operating at a second sampling rate, said apparatus comprising:

(a) an analog filter having a filter mask for attenuating the Tx-FT interference to a first prescribed level and for passing the Rx signal at the carrier frequency;

(b) a converter for converting output from the analog filter to a digital representation;

(c) a first digital filter having a mask for passing the Rx signal at the carrier frequency and for attenuating the Tx-FT interference to a second prescribed level;

(d) a mixer for shifting the carrier frequency of the spectrum of the Rx signal to an intermediate frequency, said mixer introducing at least one spectral transmitter feed-through image Tx-FT(I) and a receive signal image Rx(I);

(e) a second digital filter having a mask for passing the Rx signal at the intermediate frequency and attenuating the image spectrums Tx-FT(I) and Rx(I); and

(f) a decimator for downsampling the Rx signal at a downsampling rate from the second digital filter to match the second sampling rate of the QAM demodulator.

2. The apparatus of claim 1, wherein the second sampling rate of the QAM demodulator is four times the first symbol rate of the receiver.

3. The apparatus of claim 2, wherein the first prescribed level of attenuation of the Tx-FT interference is approximately 50 dB.

4. The apparatus of claim 3, wherein the second prescribed level of attenuation of the Tx-FT interference is approximately 30 dB.

5. The apparatus of claim 4, wherein the second prescribed level of attenuation of the Tx-FT interference is reduction to effectively zero.

6. The apparatus of claim 3, wherein the intermediate frequency is less than fs/2 divided by the downsampling rate.

7. The apparatus of claim 1, wherein the converter operates at a sampling rate fs to preserve received spectrums less than or equal to fs/2.

8. An apparatus for reducing radio frequency (RF) interference at a receiver of a modem to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator operating at a sampling rate, said apparatus comprising:

(a) an analog filter having a filter mask for attenuating the RF interference to a first prescribed level and for passing the Rx signal at the carrier frequency;

(b) a converter for converting output from the analog filter to a digital representation;

(c) a first digital filter having a mask for passing the Rx signal at the carrier frequency and attenuating the RF interference to a second prescribed level;

(d) a mixer for shifting the carrier frequency of the spectrum of the Rx signal to an intermediate frequency, said mixer introducing a receive signal image Rx(I);

(e) a second digital filter having a mask for passing the Rx signal at the intermediate frequency and attenuating the image spectrum Rx(I) and the RF interference to a third prescribed level; and

(f) a decimator for downsampling the Rx signal at a downsampling rate from the second digital filter to match the sampling rate of the QAM demodulator.

9. The apparatus of claim 8, wherein the third prescribed level of attenuation of the RF interference is reduction to a negligible level.

10. The apparatus of claim 8, wherein the converter operates at a sampling rate fs to preserve received spectrums less than or equal to fs/2.

11. The apparatus of claim 10, wherein the intermediate frequency is less than fs/2 divided by the downsampling rate.

12. A method of reducing transmitter feed-through (Tx-FT) interference at a receiver of a modem operating at a symbol rate to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator operating at a sampling rate, said method comprising the steps of:

(a) attenuating the Tx-FT interference to a first prescribed level and passing the Rx signal at the carrier frequency;

(b) converting output from an analog filter to a digital representation;

(c) passing the Rx signal at the carrier frequency and for attenuating the Tx-FT interference to a second prescribed level;

(d) shifting the carrier frequency of the spectrum of the Rx signal to an intermediate frequency, said mixer introducing at least one spectral transmitter feed-through image Tx-FT(I) and a receive signal image Rx(I);

(e) passing the Rx signal at the intermediate frequency and attenuating the image spectrums Tx-FT(I) and Rx(I); and

(f) downsampling the Rx signal at a downsampling rate from a second digital filter to match the sampling rate of the QAM demodulator.

13. The method of claim 12, wherein the sampling rate of the QAM demodulator is four times the symbol rate of the receiver.

14. The method of claim 13, wherein the first prescribed level of attenuation of the Tx-FT interference is approximately 50 dB.

15. The method of claim 14, wherein the second prescribed level of attenuation of the Tx-FT interference is approximately 30 dB.

16. The method of claim 15, wherein step (e) attenuates the Tx-FT interference to effectively zero.

17. The method of claim 13, wherein the step of converting comprises operating a converter at a sampling rate fs to preserve received spectrums less than or equal to fs/2.

18. The method of claim 17, wherein the intermediate frequency is less than fs/2 divided by the downsampling rate.

19. A method reducing radio frequency (RF) interference at a receiver of a modem to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator operating at a sampling rate, said method comprising the steps of:

(a) attenuating the RF interference to a first prescribed level and for passing the Rx signal at the carrier frequency;

(b) converting output from an analog filter to a digital representation;

(c) passing the Rx signal at the carrier frequency and attenuating the RF interference to a second prescribed level;

(d) shifting the carrier frequency of the spectrum of the Rx signal to an intermediate frequency, said mixer introducing a receive signal image Rx(I);

(e) passing the Rx signal at the intermediate frequency and attenuating the image spectrum Rx(I); and

(f) downsampling the Rx signal at a downsampling rate from a second digital filter to match the sampling rate of the QAM demodulator.

20. The method of claim 19, wherein step (e) attenuates the Rx(I) interference to a negligible level.

21. The method of claim 20, wherein the step of converting comprises operating a converter at a sampling rate fs to preserve received spectrum less than or equal to fs/2.

22. The method of claim 21, wherein the intermediate frequency is less than fs/2 divided by the downsampling rate.