Digital Quality Index For QAM Digital Signals

• US 20080089402A1
• Filed: 09/18/2007
• Published: 04/17/2008
• Est. Priority Date: 10/12/2006
• Status: Active Grant

• Current Assignee: Viavi Solutions, Inc. (Lumentum Holdings Inc.)
• Original Assignee: Acterna LLC (Lumentum Holdings Inc.)
• Inventors: Oliver, Kevin J., Massey, Gregory W., Ramname, Reena
• Granted Patent: US 7,792,183 B2
• Time in Patent Office: Days
• US Class Current: 375/227
• CPC Class Codes: H04B 17/21 for calibration; for correcting measurements
• CPC Class Codes: H04B 17/309 Measuring or estimating channel quality parameters

Abstract

The present invention relates to a system for calculating a digital quality index (DQI), which provides a rating on a predetermined scale, indicative of the impairments in a received quadrature amplitude modulated (QAM) signal in a CATV cable system. The DQI system utilizes bit error rates prior to and after forward error correction to calculate the DQI value. To increase the calculation rate, the bit error rates can be estimated using voltage errors, signal to noise ratios and/or average corner error metrics.

Claims

1. A method of determining an indication of the quality on a QAM digital signal on a cable network with forward error correction comprising the steps of:

a) determining a first bit error rate (BER) of the digital signal prior to forward error correction (FEC);

b) determining a second bit error rate after forward error correction (FEC);

c) determining a digital quality index (DQI) on a predetermined scale based on the first and second bit error rates; and

d) displaying the DQI to provide an indication of the quality of the QAM digital signal on the cable network.

2. The method according to claim 1, wherein step a) comprises:

determining an error voltage V_ERR of the digital signal; and

determining the first bit error rate based on a predetermined statistical relationship between the V_ERR and the first BER.

3. The method according to claim 2, wherein the first bit error rate f(x) is computed from a scaled V_ERR value x using a modified exponential equation

4. The method according to claim 2, wherein step b) comprises determining the second bit error rate based on a predetermined statistical relationship between the first BER and the second BER

5. The method according to claim 4, wherein the second BER g(x) is computed from the first BER x using the equation

g(x)=xe⁻

p/x^q

wherein p and q>

0, and based on decoding hardware.

6. The method according to claim 4, wherein the FEC includes interleaving bits in the digital signal utilizing an interleaver;

wherein M equals a sample burst correction time divided by a sample period, and N equals interleaver latency time divided by the sample period; and

wherein step b) includes taking an average of a lowest M values of a most recent N values of the second BER;

whereby burst protection effects of interleaving are approximated.

7. The method according to claim 2, wherein step a) also comprises:

determining a corner average error metric of the digital signal; and

determining the first bit error rate based on a predetermined statistical relationship between the corner average error metric and the first BER.

8. The method according to claim 7, wherein p(x)=ax^b

wherein x is the corner average error metric, p is the pre-FEC BER estimate, and a and b are based on decoding hardware.

9. The method according to claim 4, wherein the second BER is also determined based on a predetermined statistical relationship between the first BER and the second BER;

wherein

10. The method according to claim 1, wherein step c) includes:

i) determining whether the post-FEC BER is equal to or greater than a post minimum threshold;

if the post-FEC BER is equal to or greater than a post minimum threshold, then determining whether the post-FEC BER is equal to or greater than a post maximum threshold, whereby if so, then the DQI score is determined to be a lowest rating on the predetermined scale;

if the post-FEC BER is not equal to or greater than the post maximum threshold, then determining whether the post-FEC BER is between the post minimum and post maximum thresholds, whereby if so, then the DQI score varies between the minimum rating and a middle rating; and

ii) if the post-FEC BER is not equal to or greater than a post minimum threshold;

determining whether pre-FEC BER is equal to or greater than an prior maximum threshold, whereby if so then the DQI score is the middle rating on the predetermined scale;

if the pre-FEC BER is not equal to or greater than an prior maximum threshold, then determining whether the pre-FEC BER is between the prior minimum and prior maximum thresholds, whereby if so then the DQI score varies between the middle rating and the maximum rating on the predetermined scale;

whereby if the pre-FEC BER is less than an prior minimum threshold, then the DQI score is a highest rating on the predetermined scale.

11. The method according to claim 10, wherein the post minimum threshold is approximately 1×

e<sup>−

8</sup>, and the post maximum threshold is approximately 1×

e<sup>−

4</sup>, whereby if the post-FEC BER is between the post minimum and post maximum thresholds the DQI score equals int(−

12.5 log(second BER)−

50)/10; and

wherein the prior maximum value is approximately 1×

e<sup>−

4</sup>, and the prior minimum value is approximately 1×

e<sup>−

8</sup>, whereby if the pre-FEC BER is between the prior minimum and prior maximum values, the DQI score is int(−

12.5 log(first BER)/10.

12. The method according to claim 10, wherein the post minimum threshold is approximately 1×

e<sup>−

8</sup>, and the post maximum threshold is approximately 1×

e<sup>−

4</sup>, whereby if the post-FEC BER is between the post minimum and post maximum thresholds the DQI score equals int(−

12.5 log(second BER)−

50)/10; and

wherein the prior maximum value is approximately 1×

e<sup>−

4</sup>, and the prior minimum value is approximately 1×

e<sup>−

9</sup>, whereby if the pre-FEC BER is between the prior minimum and prior maximum values, the DQI score is int(−

10 log(first BER)+10)/10.

13. The method according to claim 10, wherein the post minimum threshold is approximately 1×

e<sup>−

8</sup>, and the post maximum threshold is approximately 1×

e<sup>−

6</sup>, whereby if the post-FEC BER is between the post minimum and post maximum thresholds the DQI score is between the lowest and middle ratings; and

wherein the prior maximum value is approximately 1×

e<sup>−

6</sup>, and the prior minimum value is approximately 1×

e<sup>−

8</sup>, whereby if the pre-FEC BER is between the prior minimum and prior maximum values, the DQI score is between the middle and highest ratings.

14. The method according to claim 1, wherein step d) includes displaying a plurality of DQI over time on a graph.

25. The method according to claim 2, further comprising determining the second bit error rate after FEC based on a predetermined statistical relationship between the first BER and the second BER;

wherein the FEC includes interleaving bits in the digital signal utilizing an interleaver;

wherein M equals a sample burst correction time divided by a sample period, and N equals interleaver latency time divided by the sample period; and

wherein step b) includes taking an average of a lowest M values of a most recent N values of the second BER;

whereby burst protection effects of interleaving are approximated.

15. A testing device for generating an indication of the quality on a QAM digital signal on a cable network with forward error correction (FEC) comprising:

an input port for receiving the QAM digital signal;

first bit error rate (BER) generating means for determining the BER of the QAM digital signal prior to FEC;

second first bit error rate (BER) generating means for determining the BER of the QAM digital signal after FEC;

digital quality index generating means for determining a digital quality index (DQI) of the QAM digital signal on a predetermined scale based on the first and second BER; and

a display for displaying the DQI to provide an indication of the quality of the QAM digital signal on the cable network.

16. The testing device according to claim 15, wherein the first BER generating means includes a voltage error (V_ERR) generator, and first logic means for determining the first bit error rate based on a predetermined statistical relationship between the V_ERR and the first BER.

17. The testing device according to claim 16, wherein the first BER f(x) is computed from a scaled V_ERR value x using a modified exponential equation

18. The testing device according to claim 16, wherein the second BER generating means includes second logic means for determining the second bit error rate based on a predetermined statistical relationship between the first BER and the second BER.

19. The testing device according to claim 18, wherein the second BER g(x) is computed from the first BER x using the equation

g(x)=xe⁻

p/x^q

wherein p and q>

0, and based on the first BER generating means.

20. The testing device according to claim 16, wherein the first BER generating means also includes a corner average metric generating means for determining a corner average error metric of the digital signal;

and

wherein the digital quality index generating means determines the first bit error rate based also on a predetermined statistical relationship between the corner average error metric and the first BER.

21. The testing device according to claim 18, wherein the FEC includes interleaving bits in the digital signal utilizing an interleaver;

wherein the second BER generating means includes taking an average of a lowest M values of a most recent N values of the second BER;

wherein M equals a sample burst correction time divided by a sample period, and N equals interleaver latency time divided by the sample period;

whereby burst protection effects of interleaving are approximated.

22. The testing device according to claim 15, further comprising signal level measuring means for measuring a power level of the QAM digital signal, whereby the digital quality index generating means adjusts the DQI based on the measured signal level.

23. The testing device according to claim 16, wherein the voltage error (V_ERR) generator comprises a QAM demodulator, which includes an equalizer containing a plurality of feed-forward equalization (FFE) and decision feedback equalization (DFE) taps indicative of impairments to the network;

whereby the digital quality index generating means adjusts the DQI based on the plurality of feed-forward equalization (FFE) and decision feedback equalization (DFE) taps.

24. A method of determining a bit error rate of a QAM digital signal on a cable network with forward error correction (FEC) comprising the steps of:

a) determining an error voltage V_ERR of the digital signal once every 5 to 50 microseconds;

b) determining a first bit error rate prior to FEC based on a predetermined statistical relationship between the V_ERR and the first BER once every 5 to 50 microseconds; and

c) averaging the first BER every 0.5 to 2 seconds.