Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-05-12
2001-05-29
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S222000, C375S261000
Reexamination Certificate
active
06240150
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of interference filtering in modem receivers.
BACKGROUND OF THE INVENTION
In communication systems a modem is used to convert (modulate) digital signals generated by a computer into analog signals suitable for transmission over telephone lines. Another modem, located at the receiving end of the transmission, converts (demodulates) the analog signals back into digital form. The ability of modems to operate in the presence of interference is important in modern digital communication systems.
Traditional telephone voiceband modems use the same band (frequency) of transmission for upstream and downstream. However, in digital subscriber line (DSL) based systems there is a need to move to higher frequencies, and at those frequencies coupling between adjacent lines and attenuation of transmit signals at the receiver makes it preferable to use separate bands for upstream (US) and downstream (DS).
There are many potential types of interference including: (1) transmitter feed-through (Tx-FT) which is the result of the transmission signal Tx feeding back into the receive (Rx) path caused by imperfect transhybrid cancellation in 2 to 4 wire interfaces; and (2) radio frequency interference (RFI) which includes ingress from external sources using amplitude modulation (AM) for commercial radio broadcasting.
Tx-FT and RF interference can be very large and must be eliminated in order recover the desired signal with an appropriate signal-to-noise ratio (SNR). Further, due to the large attenuation in the twisted pair, Tx-FT and RF interference can be larger than the received signal, which generally requires additional filtering.
Making a communication system tolerant to interference (Tx-FT and RF) provides several benefits. In particular, an interference tolerant system can make more efficient use of spectrum by being able to share that spectrum with multiple users. Such a system also provides more reliable communications supporting higher transfer rates while utilizing the same bandwidth and power.
Analog filters are usually employed to reduce interference before digitizing the signal at the receiver. Although this provides a theoretical optimal solution, it requires very expensive analog filtering schemes with high-order filters that usually produce other associated problems such as distortion noise and the introduction of non-linearities in the signal.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and an apparatus to improve interference filtering at the receiver of a modem.
In accordance with an aspect of the present invention there is provided an apparatus for filtering interference in a receiver of a modem to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency, said method comprising: (a) analog filtering means for attenuating interference signals to first prescribed levels and for passing the Rx signal at the carrier frequency; (b) conversion means for converting output of the analog filtering means to a digital representation; (c) means for shifting the carrier frequency of the spectrum of the Rx signal to an intermediate frequency; and (d) digital filtering means for attenuating the interference signals to second prescribed levels and for passing the Rx signal at the intermediate frequency.
In accordance with another aspect of the present invention there is provided 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.
In accordance with an aspect of the present invention there is provided 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.
In accordance with an aspect of the present invention there is provided a method of filtering interference in a receiver of a modem to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency, said method comprising: (a) attenuating interference to a first prescribed level and passing the Rx signal at the carrier frequency; (b) converting output of the analog filtering means to a digital representation; (c) shifting the carrier frequency of the spectrum of the Rx signal to an intermediate frequency; and (d) attenuating the interference at levels above the intermediate frequency.
In accordance with an aspect of the present invention there is provided 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 the 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 the second digital filter to match the sampling rate of the QAM demodulator.
In accordance with an aspect of the present invention there is provided a method of 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
Darveau Michel
Velez Edgar
Chin Stephen
Fan Chieh M.
Nortel Networks Limited
Renault Swabey Ogilvy
Wood Max R.
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