Detecting CPD in HFC network with OFDM signals

1. An apparatus for detecting a common path distortion (CPD) source in a hybrid fiber coax (HFC) network of a network system including a cable modem termination system (CMTS) that transmits a multiplicity of downstream signals at a radio frequency (RF) downstream frequency band in the HFC network and a plurality of cable modems (CMs) that transmit upstream signals at an RF upstream frequency band in the HFC network, the multiplicity of downstream signals being transmitted during a multiplicity of downstream periods, respectively, and the upstream signals not being transmitted during a multiplicity of quiet periods, the CPD source generating from the multiplicity of downstream signals a multiplicity of CPD echo signals, respectively, at the RF upstream frequency band, the multiplicity of CPD echo signals being received by the CMTS during the multiplicity of quiet periods, respectively, said apparatus comprising:

(a) downstream signal processing means, associated with the CMTS of the network system, for acquiring the multiplicity of downstream signals in synchronism with the multiplicity of downstream periods, respectively, and for producing a set of RF downstream samples of each downstream signal at the RF downstream frequency band;

(b) CPD emulation means, associated with said downstream signal processing means, for generating a set of even order intermodulation (IM) product samples and a set of odd order IM product samples from each set of RF downstream samples, to produce a multiplicity of sets of even order IM product samples and a multiplicity of sets of odd order IM product samples;

(c) upstream signal processing means, associated with the CMTS of the network system, for acquiring the multiplicity of CPD echo signals in synchronism with the multiplicity of quiet periods, respectively, and for producing a set of CPD echo signal samples of each CPD echo signal at the RF upstream frequency band, such that a multiplicity of sets of CPD echo signal samples are provided;

(d) cross-correlation means, associated with said CPD generation means and said upstream signal processing means, for cross-correlating—

(i) the multiplicity of sets of even order IM product samples with the multiplicity of sets of CPD echo signal samples, respectively, to produce a multiplicity of even order cross-correlation functions, and

(ii) the multiplicity of sets of odd order IM product samples with the multiplicity of sets of CPD echo signal samples, respectively, to produce a multiplicity of odd order cross-correlation functions;

(e) accumulation means, associated with said cross-correlation means, for coherently accumulating—

(i) the multiplicity of even order cross-correlation functions, to produce an accumulated even order cross-correlation function characterized by an even order envelope, and

(ii) the multiplicity of odd order cross-correlation functions, to produce an accumulated odd order cross-correlation function characterized by an odd order envelope; and

(f) detection means, associated with said accumulation means, for detecting the CPD source from either or both of the even and odd order envelopes by the presence of a peak in either or both of the envelopes.

2. The apparatus of claim 1 , further comprising:

(g) time delay means, associated with said detection means, for measuring a time delay value associated with the CPD source by examining the position of the peak in either or both of the envelopes; and

(h) means, associated with said time delay means, for determining a location or a plurality of candidate locations of the CPD source in the HFC network based on the time delay value.

3. The apparatus of claim 1 , wherein the multiplicity of downstream signals are a multiplicity of downstream orthogonal frequency division multiplexing (OFDM) symbols, respectively, said downstream signal processing means—

(i) acquiring the multiplicity of downstream OFDM symbols at a first baseband frequency band,

(ii) up-converting the multiplicity of downstream OFDM symbols from the first baseband frequency band to the RF downstream frequency band, and

(iii) producing a set of RF downstream samples from each downstream OFDM symbol at the RF downstream frequency band.

4. The apparatus of claim 3 , wherein said upstream signal processing means acquires the multiplicity of CPD echo signals at a second baseband frequency band and up-converts the multiplicity of CPD echo signals from the second baseband frequency band to the RF upstream frequency band.

5. The apparatus of claim 3 , wherein said upstream signal processing means acquires the multiplicity of CPD echo signals at the RF upstream frequency band.

6. The apparatus of claim 1 , wherein said downstream signal processing means acquires the multiplicity of downstream signals at the RF downstream frequency band.

7. The apparatus of claim 6 , wherein said upstream signal processing means acquires the multiplicity of CPD echo signals at the RF upstream frequency band.

8. The apparatus of claim 6 , wherein the multiplicity of downstream signals at the RF downstream frequency band are captured in one of the plurality of CMs and then transmitted as a CM-captured signal to the CMTS, said downstream signal processing means acquiring the CM-captured signal from the CMTS.

9. The apparatus of claim 1 , further comprising:

(g) means, associated with said accumulation means, for calculating the even order envelope from the accumulated even order cross-correlation function and the odd order envelope from the accumulated odd order cross-correlation function.

10. The apparatus of claim 1 , wherein a multiplicity of quiet probe symbols at the RF upstream frequency band are received by the CMTS during the multiplicity of quiet periods, respectively, the multiplicity of CPD echo signals being received together with the multiplicity of quiet probe symbols, respectively, said upstream signal processing means acquiring from the CMTS, for each quiet period, a set of samples of a combined signal containing one of the quiet probe symbols and one of the CPD echo signals.

11. A method of detecting a common path distortion (CPD) source in a hybrid fiber coax (HFC) network of a network system including a cable modem termination system (CMTS) that transmits a multiplicity of downstream signals at a radio frequency (RF) downstream frequency band in the HFC network and a plurality of cable modems (CMs) that transmit upstream signals at an RF upstream frequency band in the HFC network, the multiplicity of downstream signals being transmitted during a multiplicity of downstream periods, respectively, and the upstream signals not being transmitted during a multiplicity of quiet periods, the CPD source generating from the multiplicity of downstream signals a multiplicity of CPD echo signals, respectively, at the RF upstream frequency band, the multiplicity of CPD echo signals being received by the CMTS during the multiplicity of quiet periods, respectively, said method comprising the steps of:

(a) acquiring the multiplicity of downstream signals in synchronism with the multiplicity of downstream periods, respectively, and producing a set of RF downstream samples of each downstream signal at the RF downstream frequency band;

(b) generating a set of even order intermodulation (IM) product samples and a set of odd order IM product samples from each set of RF downstream samples, to produce a multiplicity of sets of even order IM product samples and a multiplicity of sets of odd order IM product samples;

(c) acquiring the multiplicity of CPD echo signals in synchronism with the multiplicity of quiet periods, respectively, and producing a set of CPD echo signal samples of each CPD echo signal at the RF upstream frequency band, such that a multiplicity of sets of CPD echo signal samples are provided;

(d) cross-correlating—

(i) the multiplicity of sets of even order IM product samples with the multiplicity of sets of CPD echo signal samples, respectively, to produce a multiplicity of even order cross-correlation functions, and

(ii) the multiplicity of sets of odd order IM product samples with the multiplicity of sets of CPD echo signal samples, respectively, to produce a multiplicity of odd order cross-correlation functions;

(e) coherently accumulating—

(i) the multiplicity of even order cross-correlation functions, to produce an accumulated even order cross-correlation function characterized by an even order envelope, and

(ii) the multiplicity of odd order cross-correlation functions, to produce an accumulated odd order cross-correlation function characterized by an odd order envelope; and

(f) detecting the CPD source from either or both of the even and odd order envelopes by the presence of a peak in either or both of the envelopes.

12. The method of claim 11 , further comprising the steps of:

(g) measuring a time delay value associated with the CPD source by examining the position of the peak in either or both of the envelopes; and

(h) determining a location or a plurality of candidate locations of the CPD source in the HFC network based on the time delay value.

13. The method of claim 11 , wherein the multiplicity of downstream signals are a multiplicity of downstream orthogonal frequency division multiplexing (OFDM) symbols, respectively, and wherein step (a) includes—

(i) acquiring the multiplicity of downstream OFDM symbols at a first baseband frequency band,

(ii) up-converting the multiplicity of downstream OFDM symbols from the first baseband frequency band to the RF downstream frequency band, and

(iii) producing a set of RF downstream samples from each downstream OFDM symbol at the RF downstream frequency band.

14. The method of claim 13 , wherein step (c) includes—

(i) acquiring the multiplicity of CPD echo signals at a second baseband frequency band, and

(ii) up-converting the multiplicity of CPD echo signals from the second baseband frequency band to the RF upstream frequency band.

15. The method of claim 11 , wherein step (c) includes acquiring the multiplicity of CPD echo signals at the RF upstream frequency band.

16. The method of claim 11 , wherein step (a) includes acquiring the multiplicity of downstream signals at the RF downstream frequency band.

17. The method of claim 16 , wherein step (c) includes acquiring the multiplicity of CPD echo signals at the RF upstream frequency band.

18. The method of claim 16 , wherein, prior to step (a), the multiplicity of downstream signals at the RF downstream frequency band are captured in one of the plurality of CMs and then transmitted as a CM-captured signal to the CMTS, and wherein step (a) includes acquiring the CM-captured signal from the CMTS.

19. The method of claim 11 , further comprising the step of:

(g) calculating the even order envelope from the accumulated even order cross-correlation function and the odd order envelope from the accumulated odd order cross-correlation function.

20. The method of claim 11 , wherein a multiplicity of quiet probe symbols at the RF upstream frequency band are received by the CMTS during the multiplicity of quiet periods, respectively, the multiplicity of CPD echo signals being received together with the multiplicity of quiet probe symbols, respectively, and wherein step (c) includes acquiring from the CMTS, for each quiet period, a set of samples of a combined signal containing one of the quiet probe symbols and one of the CPD echo signals.

21. A method of detecting a common path distortion (CPD) source in a hybrid fiber coax (HFC) network of a network system including a cable modem termination system (CMTS) that transmits a downstream orthogonal frequency division multiplexing (OFDM) symbol at a radio frequency (RF) downstream frequency band in the HFC network and a plurality of cable modems (CMs) that transmit upstream signals at an RF upstream frequency band in the HFC network, the downstream OFDM symbol being transmitted during a downstream symbol period and the upstream signals not being transmitted during a quiet period, the CPD source generating from the downstream OFDM symbol a CPD echo signal at the RF upstream frequency band, the CPD echo signal being received by the CMTS during the quiet period, said method comprising the steps of:

(a) acquiring the downstream OFDM symbol at a first baseband frequency band and in synchronism with the downstream symbol period;

(b) up-converting the downstream OFDM symbol from the first baseband frequency band to the RF downstream frequency band, to produce a constructed RF downstream OFDM symbol that is coherent with the downstream OFDM symbol transmitted in the HFC network;

(c) generating even and odd order intermodulation distortion products from the constructed RF downstream OFDM symbol;

(d) acquiring the CPD echo signal at a second baseband frequency band and in synchronism with the quiet period;

(e) up-converting the CPD echo signal from the second baseband frequency band to the RF upstream frequency band, to produce a constructed RF CPD echo signal;

(f) cross-correlating the even and odd order intermodulation distortion products separately with the constructed RF CPD echo signal, to produce separate even and odd order cross-correlation functions;

(g) repeating steps (a) through (f) a multiplicity of times for a multiplicity of downstream OFDM symbols transmitted in the HFC network and for a multiplicity of CPD echo signals generated by the CPD source from the multiplicity of downstream OFDM symbols, respectfully, to produce a multiplicity of even order and odd order cross-correlation functions;

(h) coherently accumulating—

(i) the multiplicity of even order cross-correlation functions, to produce an accumulated even order cross-correlation function characterized by an even order envelope, and

(ii) the multiplicity of odd order cross-correlation functions, to produce an accumulated odd order cross-correlation function characterized by an odd order envelope; and

(i) detecting the CPD source from either or both of the even and odd order envelopes by the presence of a peak in either or both of the envelopes.

22. The method of claim 21 , further comprising the steps of:

(j) measuring a time delay value associated with the CPD source by examining the position of the peak in either or both of the envelopes; and

(k) determining a location or a plurality of candidate locations of the CPD source in the HFC network based on the time delay value.

23. A method of detecting a common path distortion (CPD) source in a hybrid fiber coax (HFC) network of a network system including a cable modem termination system (CMTS) that transmits a downstream orthogonal frequency division multiplexing (OFDM) symbol at a radio frequency (RF) downstream frequency band in the HFC network and a plurality of cable modems (CMs) that transmit upstream signals at an RF upstream frequency band in the HFC network, the downstream OFDM symbol being transmitted during a downstream symbol period and the upstream signals not being transmitted during a quiet period, the CPD source generating from the downstream OFDM symbol a CPD echo signal at the RF upstream frequency band, the CPD echo signal being received by the CMTS during the quiet period, said method comprising the steps of:

(a) acquiring the downstream OFDM symbol at a first baseband frequency band and in synchronism with the downstream symbol period;

(b) up-converting the downstream OFDM symbol from the first baseband frequency band to the RF downstream frequency band, to produce a constructed RF downstream OFDM symbol that is coherent with the downstream OFDM symbol transmitted in the HFC network;

(c) generating even and odd order intermodulation distortion products from the constructed RF downstream OFDM symbol;

(d) acquiring the CPD echo signal at the RF upstream frequency band and in synchronism with the quiet period;

(e) cross-correlating the even and odd order intermodulation distortion products separately with the CPD echo signal at the RF upstream frequency band, to produce separate even and odd order cross-correlation functions;

(f) repeating steps (a) through (e) a multiplicity of times for a multiplicity of downstream OFDM symbols transmitted in the HFC network and for a multiplicity of CPD echo signals generated by the CPD source from the multiplicity of downstream OFDM symbols, respectfully, to produce a multiplicity of even order and odd order cross-correlation functions;

(g) coherently accumulating—

(i) the multiplicity of even order cross-correlation functions, to produce an accumulated even order cross-correlation function characterized by an even order envelope, and

(ii) the multiplicity of odd order cross-correlation functions, to produce an accumulated odd order cross-correlation function characterized by an odd order envelope; and

(h) detecting the CPD source from either or both of the even and odd order envelopes by the presence of a peak in either or both of the envelopes.

24. The method of claim 23 , further comprising the steps of:

(i) measuring a time delay value associated with the CPD source by examining the position of the peak in either or both of the envelopes; and

(j) determining a location or a plurality of candidate locations of the CPD source in the HFC network based on the time delay value.

25. A method of detecting a common path distortion (CPD) source in a hybrid fiber coax (HFC) network of a network system including a cable modem termination system (CMTS) that transmits a downstream signal at a radio frequency (RF) downstream frequency band in the HFC network and a plurality of cable modems (CMs) that transmit upstream signals at an RF upstream frequency band in the HFC network, the downstream signal being transmitted during a downstream period and the upstream signals not being transmitted during a quiet period, the CPD source generating from the downstream signal a CPD echo signal at the RF upstream frequency band, the CPD echo signal being received by the CMTS during the quiet period, said method comprising the steps of:

(a) acquiring the downstream signal at the RF downstream frequency band and in synchronism with the downstream period;

(b) generating even and odd order intermodulation distortion products from the downstream signal acquired in step (a);

(c) acquiring the CPD echo signal at the RF upstream frequency band and in synchronism with the quiet period;

(d) cross-correlating the even and odd order intermodulation distortion products separately with the CPD echo signal at the RF upstream frequency band, to produce separate even and odd order cross-correlation functions;

(e) repeating steps (a) through (d) a multiplicity of times for a multiplicity of downstream signals transmitted in the HFC network and for a multiplicity of CPD echo signals generated by the CPD source from the multiplicity of downstream signals, respectfully, to produce a multiplicity of even order and odd order cross-correlation functions;

(f) coherently accumulating—

(i) the multiplicity of even order cross-correlation functions, to produce an accumulated even order cross-correlation function characterized by an even order envelope, and

(ii) the multiplicity of odd order cross-correlation functions, to produce an accumulated odd order cross-correlation function characterized by an odd order envelope; and

(g) detecting the CPD source from either or both of the even and odd order envelopes by the presence of a peak in either or both of the envelopes.

26. The method of claim 25 , further comprising the steps of:

(h) measuring a time delay value associated with the CPD source by examining the position of the peak in either or both of the envelopes; and

(i) determining a location or a plurality of candidate locations of the CPD source in the HFC network based on the time delay value.

27. The method of claim 25 , wherein the downstream signal is an orthogonal frequency division multiplexing (OFDM) signal, and wherein

step (a) includes acquiring the downstream OFDM signal at the RF downstream frequency band and in synchronism with the downstream period, and

step (b) includes generating even and odd order intermodulation distortion products from the downstream OFDM signal acquired in step (a).