Method and apparatus for pre-compensation of an XDSL modem
View Patent ↗An apparatus and method is provided for minimizing frequency distortion in the transmit path of an XDSL modem implementing digital multi-tone (DMT) line code. The current invention provides a means for both determining and correcting for distortion in the frequency domain. The apparatus may be incorporated in an existing X-DSL architecture without additional circuitry. In an embodiment of the invention the apparatus may include a calibration phase which may be implemented using the existing analog-to-digital (ADC) conversion and demodulation capabilities on the receive path of the modem. This calibration phase takes place before the training phase associated with establishing communications with a remote site. During the calibration phase a calibration sequence with known spectral characteristics in the frequency domain is injected digitally at the beginning of the transmit path into each of the tone bins of the inverse Fourier Transform engine (IFFT). The receive path is configured to receive feedback of a resultant analog output signal from the transmit path. A frequency analyzer is used to determine the spectral properties of the feedback from the analog output signal and a normalizer is used to compute a local gain table with gain factors for each tone bin which effect the required normalization.
1. An apparatus for equalizing multi-tone modulated transmissions of a modem with a transmit path and a receive path both configured to couple across a communication medium with a remote site, and the apparatus comprising:
a first set of scale factors to equalize the frequency response of the transmit path of the modem and the first set of scale factors obtained before initializing communication with a remote site, and the first set of scale factors obtained by;
a calibration signal source coupled to a digital portion of the transmit path to inject a calibration signal into selected tones;
a frequency analyzer coupled to an analog portion of the transmit path to analyze spectral components of analog signals resulting from an injection of the calibration signal by the calibration signal source;
a normalizer coupled to the frequency analyzer to determine the first set of scale factors from the spectral components analyzed by the frequency analyzer;
a gain scaler for scaling each of a plurality of tones associated with initializing communications with the remote site with a corresponding one of the first set of scale factors to equalize the frequency response of the transmit path of the modem.
2. The apparatus of claim 1 , wherein the frequency analyzer further comprises:
an analog to digital conversion (ADC) component of the receive path of the modem switchably coupled to the analog portion of the transmit path to analyze spectral components of the analog signals resulting from the injection of the calibration signal by the calibration signal source.
3. The apparatus of claim 1 , wherein the frequency analyzer further comprises:
a calibration analog to digital conversion (ADC) component switchably coupled to the analog portion of the transmit path to analyze spectral components of the analog signals resulting from the injection of the calibration signal by the calibration signal source.
4. The apparatus of claim 1 , wherein the communication medium comprises one of a wired and a wireless medium.
5. The apparatus of claim 1 , wherein the multi-tone modulated transmissions comprise at least one of the following XDSL protocols: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL and HDSL.
6. A multi-tone modem with a transmit path and a receive path configured to couple across a communication medium with a remote site, and the modem comprising:
a first set of scale factors to equalize the frequency response of the transmit path of the modem and the first set of scale factors obtained before initializing communication with a remote site, and the first set of scale factors obtained by;
a calibration signal source coupled to the digital portion of the transmit path to inject a calibration signal into selected ones of tones;
a frequency analyzer coupled to the analog portion of the transmit path to analyze spectral components of analog signals resulting from an injection of the calibration signal by the calibration signal source;
a normalizer coupled to the frequency analyzer to determine the first set of scale factors from the spectral components analyzed by the frequency analyzer;
a gain scaler for scaling each of a plurality of tones associated with initializing communications with the remote site with a corresponding one of the first set of scale factors to equalize the frequency response of the transmit path of the modem.
7. The multi-tone modem of claim 6 , wherein the frequency analyzer further comprises:
an analog to digital conversion (ADC) component of the receive path of the modem switchably coupled to the analog portion of the transmit path to
analyze spectral components of the analog signals resulting from the injection of the calibration signal by the calibration signal source.
8. The multi-tone modem of claim 6 , wherein the frequency analyzer further comprises:
a calibration analog to digital conversion (ADC) component switchably coupled to the analog portion of the transmit path to analyze spectral components of the analog signals resulting from the injection of the calibration signal by the calibration signal source.
9. The multi-tone modem of claim 6 , wherein the communication medium comprises one of a wired and a wireless medium.
10. The multi-tone modem of claim 6 , wherein communications between the multi-tone modem and the remote site exhibit at least one of the following XDSL protocols: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL and HDSL.
11. A method for operating a multi-tone modem with a transmit path and a receive path configured to couple across a communication medium with a remote site, and the method comprising:
determining a first set of scale factors to equalize the frequency response of the transmit path of the multi-tone modem, including:
injecting a calibration signal into selected ones of tones in a digital portion of the transmit path;
analyzing spectral components of analog signals on an analog portion of the transmit path responsive to the injecting act;
determining the first set of scale factors from the spectral components analyzed in the analyzing act; and
initializing communications between the multi-tone modem and the remote site across the communication medium with a plurality of tones each scaled with a corresponding one of the first set of scale factors to equalize the frequency response of the transmit path of the multi-tone modem.
12. The method of claim 11 , wherein the analyzing act further comprises the act of:
switchably coupling an analog portion of the receive path to the analog portion of the transmit path to analyze the spectral components of the analog signals.
13. The method of claim 11 , wherein the communication medium comprises one of a wired and a wireless medium.
14. The method of claim 11 , wherein the initializing act further comprises:
initializing communications between the multi-tone modem and the remote site in at least one of the following XDSL protocols: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL and HDSL.