Making a DOCSIS 4.0 Downstream Measurement

Overview

This topic describes how to configure the VSA to make a DOCSIS 4.0 Downstream measurement including instructions to manually configure the Physical-layer Link Channel.

Signal Setup

Before selecting DOCSIS 4.0 Downstream demodulation, make sure that the input signal is correctly captured by the analyzer. Specify the VSA basic signal measurement setup parameters so that the input signal is fully captured including the center frequency, span, input range, etc.. You can use the Spectrum and Time traces to verify that the signal of interest is properly captured.

Set the Center frequency to subcarrier (FFT Length / 2) and set the Span wide enough so that all active subcarriers are captured by the Span.

When using a UXA or PXA to make DOCSIS 4.0 Downstream measurements, select the BestWideOffset Input Extension setting for the best MER result (this could improve MER by 1-2dB).

Tutorial Demo Signal

This measurement tutorial uses a demo signal based on a DOCSIS 3.1 Downstream signal, although the configuration is the same for DOCSIS 4.0. The DOCSIS Downstream configuration developed below can be used with the supplied VSA Demo Signal "docsis3.1-1024qam-ber" which uses the "docsis3.1-1024qam-ber.sdf" recorded signal.

To use the demo signal, recall the "docsis3.1-1024qam-ber" Demo Signal, click File > Recall > Recall Demo > navigate to the "%PROGRAMFILES%\Keysight\89600 Software <ReleaseVersion>\89600 VSA Software\Help\Signals\DOCSIS3.1" folder and click the "docsis3.1-1024qam-ber.htm" Demo Signal File.

Configuring the Demodulator

Setup for the demodulator can be quite automated such that the user needs only to set the appropriate center frequency, span, and amplitude. In this mode, the VSA will attempt to automatically configure the settings based on the ability to find the PLC channel, demodulate the PLC channel, and extract valid DPD and OCD messages.

For highest measurement speed, the CMTS can transmit the DPD and OCD messages every frame. If the CMTS transmits the DPD and OCD messages less frequently then the VSA can continue to scan the signal until these messages are found.

Until the DPD and OCD messages are found, the VSA cannot provide accurate modulation quality measurements because the modulation format of the various subcarriers is not known.

If necessary, the automatic mode can be disabled, and the user may elect to manually enter the parameters necessary to demodulate the signal.

These parameters are available on the DOCSIS 4.0 Downstream Demod Properties dialog.

Format Tab:

  1. Set Initial Settings to Manual mode
  2. Set FFT length (4096 or 8192)
  3. Select the appropriate Cyclic Prefix Length
  4. Enter the PLC start index – which is a single integer subcarrier number
  5. Set PLC Setting to Manual
  6. Enter the time interleaving parameter (1..16 for 4096 FFT, 1..32 for 8192 FFT)
  7. Select the appropriate roll-off factor
  8. Enter the locations of the continuous pilots. This is a comma separated list of integer subcarrier numbers. Note: This includes the PLC mandated continuous pilot locations
  9. Enter the exclusion bands. This is a comma separated list of integer pairs. The first number of each pair indicates the first subcarrier of an exclusion band and the second number of each pair indicates the last subcarrier of a contiguous band of excluded subcarriers. If the exclusion band contains only one subcarrier, then the first and last subcarrier would be the same number. Typically there will be a minimum of four numbers here, the first pair indicates the lower guard band starting from subcarrier 0, and the second pair indicates the upper guard band ending at the last subcarrier (FFT size – 1)
  10. Enter the profile ID information
    1. Profile IDs – this is a comma separated list integers of profile IDs. Use the integer 255 to indicate the profile of the NCP subcarriers. Use the integers 0..15 to indicate the profiles A .. P. These don’t have to be in any particular order, but the order must match the data put into the Modulation Assignments vector below.
    2. Modulation assignments: This is a comma separated list of integers showing the modulation type associated with each subcarrier of a symbol for each modulation type. The size of this list is (FFT size) time the number of profile IDs defined. For example, if the FFT size is 4096 and the Profile IDs is 0,1,255, then there are three profiles defined, and the list will need to have 4096 integers for each profile for a total list size of 3*4096.
    3. Use the following integers for modulation types:
      4. QPSK -2
      16QAM -4
      32QAM -5
      64QAM -6
      128QAM -7
      256QAM -8
      512QAM -9
      1024QAM -10
      2048QAM -11
      4096QAM -12
    4. For example, if profile A is 256 QAM, profile B is 64 QAM, and the NCP is 16 QAM you would have:
      1. Profile ID: 0,1,255
      2. iModulation Assignments: 8 repeated 4096 times (256QAM Profile A-0), 6 repeated 4096 times (64QAM Profile B-1) and 4 repeated 4096 times (16QAM Profile NCP-255)

Advanced Tab:

If the settings above are in Manual mode, you can also adjust the system sampling frequency from it’s default of 204.8 MHz as defined in the specification. This can be useful if a system is running in some bread-board implementation which can not run at full speed.

See Also

Selecting the DOCSIS 4.0 Downstream Demodulator

Troubleshooting DOCSIS 4.0 Downstream Measurements