Characterizing Network Problems Using DOCSIS® 3.1 OFDM RxMER Per Subcarrier Data A Technical Paper prepared for SCTE•ISBE by Ron Hranac Technical Marketing Engineer Cisco Systems 9155 E. Nichols Ave., Ste. 400, Centennial, CO 80112 720-875-1338 [email protected] James Medlock Founder and CEO Akleza,Inc. 18695 Pony Express Dr. 2350, Parker, CO 80134 303-670-7951 [email protected] and Bruce Currivan, JJP Development Roger Fish, Broadcom Tom Kolze, Broadcom Jason Rupe, CableLabs® Tom Williams, CableLabs® Larry Wolcott, Comcast © 2019 SCTE•ISBE and NCTA. All rights reserved. Table of Contents Title Page Number Table of Contents .................................................................................................................................... 2 Introduction ............................................................................................................................................. 6 Content ................................................................................................................................................... 6 1. What is MER? ................................................................................................................................ 6 1.1. SC-QAM RxMER ............................................................................................................... 8 1.2. OFDM RxMER Per Subcarrier ........................................................................................... 9 1.2.1. Description of RxMER per Subcarrier Measurement ......................................... 11 2. Impairment Identification .............................................................................................................. 13 2.1.1. Simulated Ingress ............................................................................................ 14 2.1.2. Inverted Plot and Equalized Noise Floor ........................................................... 14 2.1.3. Impairment Examples ...................................................................................... 15 3. Lab Testing .................................................................................................................................. 17 3.1. Amplitude Ripple ............................................................................................................. 18 3.1.1. Amplitude Ripple Test Results Discussion ........................................................ 20 3.2. RxMER Edge Rolloff ........................................................................................................ 23 3.2.1. RxMER Edge Rolloff Discussion ...................................................................... 25 4. Using RxMER per Subcarrier Data for Maintenance and Troubleshooting .................................... 26 5. Areas for Further Investigation ..................................................................................................... 30 Conclusion ............................................................................................................................................ 31 Abbreviations......................................................................................................................................... 31 Bibliography & References .................................................................................................................... 32 Appendix ............................................................................................................................................... 33 6. Appendix I – Lab Test Results ...................................................................................................... 33 6.1. Test Case 1A................................................................................................................... 33 6.2. Test Case 1B................................................................................................................... 35 6.3. Test Case 1C .................................................................................................................. 36 6.4. Test Case 2A................................................................................................................... 37 6.5. Test Case 2B................................................................................................................... 39 6.6. Test Case 2C .................................................................................................................. 40 6.7. Test Case 3A................................................................................................................... 41 6.8. Test Case 3B................................................................................................................... 43 6.9. Test Case 3C .................................................................................................................. 44 6.10. Test Case 4A................................................................................................................... 45 6.11. Test Case 4B................................................................................................................... 47 6.12. Test Case 4C .................................................................................................................. 48 6.13. Test Case 5A................................................................................................................... 49 6.14. Test Case 5B................................................................................................................... 51 6.15. Test Case 5C .................................................................................................................. 52 7. Appendix II – Acknowledgements ................................................................................................. 53 List of Figures Title Page Number Figure 1. MER is the ratio of average symbol power to average error power. ........................................... 7 © 2019 SCTE•ISBE and NCTA. All rights reserved. 2 Figure 2. The symbol points in the left constellation are tightly grouped, indicating higher MER (27 dB). The symbol points in the right constellation are diffuse (spread out), indicating lower MER (17.5 dB). .................................................................................................................................................. 8 Figure 3. Example test equipment screen shot showing 38.3 dB RxMER for an SC-QAM signal on CTA channel 19. .............................................................................................................................. 9 Figure 4. RxMER per subcarrier plot for a 96 MHz-wide OFDM signal. ................................................... 11 Figure 5. OFDM RxMER computation. ................................................................................................... 13 Figure 6. RxMER per subcarrier graph for a 96 MHz-wide OFDM signal. ............................................... 14 Figure 7. OFDM RxMER per subcarrier plot showing simulated ingress and its impact on the affected subcarriers. .................................................................................................................................... 14 Figure 8. Inverted plot of RxMER per subcarrier. .................................................................................... 15 Figure 9. OFDM RxMER per subcarrier graph showing ingress interference at about 890 MHz. ............. 16 Figure 10. "Edge rolloff" visible at the left edge of the RxMER per subcarrier plot. .................................. 16 Figure 11. Multiple impairments are evident in this RxMER per subcarrier graph (see text), along with an exclusion band from 759.85 MHz to 763.55 MHz. ...................................................................... 17 Figure 12. RxMER per subcarrier graph with amplitude ripple caused by an impedance mismatch- related reflection. ........................................................................................................................... 17 Figure 13. Equipment configurations for RxMER per subcarrier amplitude ripple testing. ........................ 18 Figure 14. Test Case 4B spectrum analyzer capture (the frequency domain ripple in the channel was the same for Test Case 3B). ........................................................................................................... 19 Figure 15. Test Case 3B RxMER per subcarrier graph showing little or no amplitude ripple. ................... 20 Figure 16. Test Case 4B RxMER per subcarrier graph showing amplitude ripple. ................................... 20 Figure 17. Test Case 4B OFDM channel power (the OFDM channel power was the same for Test Case 3B). ....................................................................................................................................... 20 Figure 18. OFDM signal test system block diagram. ............................................................................... 21 Figure 19. Test conditions with Noise 1 injected before the source of an echo (reflection). As seen at observation point (B) both Noise 1 and the OFDM signal have amplitude ripple, but the modem’s adaptive equalizer removes the channel effect so that at observation point (C) the RxMER-per- subcarrier plot does not show ripple. In this figure, the horizontal axis is frequency, and the vertical axis is relative amplitude in decibels. .............................................................................................