PLC Decoding Information (DOCSIS 4.0 Downstream)

The PLC Decoding Information trace the shows the data results of the decoded Physical-layer Link Channel (PLC). PLC demodulation is configured in the Initial Settings and PLC Settings within the Format tab of the DOCSIS 4.0 Downstream Demod Properties dialog.

A displayed value of "***", "-780dB" or "-999" means no data is available.

Initial Settings

Initial Settings: Detected or Not Detected when Initial Settings in the Format tab = Auto; shows Disabled when Initial Settings in the Format tab = Manual.
FFT Length: 4096 or 8192 Tsd (Downstream Elementary Period: Tsd = 1 / System Sample Frequency); used to perform PLC detection, but OCD FFT Length (below) is the one used by the demodulator.
Cyclic Prefix: Expressed as a fraction of the FFT length; allows PLC detection, but OCD Cyclic Prefix (below) is sent to the full-band demod.

PLC

PLC Location: Detected or Not Detected if PLC Settings = Auto in the Format tab; otherwise shows Disabled.
PLC Start: Frequency location of first PLC subcarrier. This doesn’t have to be an integer multiple of the subcarrier spacing from the VSA center frequency; however, the subcarrier index calculation assumes the VSA center frequency is spot-on. If it isn’t, you can get a different value here from OCD PLC Start Index (below).
PLC Info: Detected or Not Detected if PLC Settings = Auto in Format tab, otherwise shows Disabled.
Timestamp: Integer value that aligns with the first sample of the FFT.
Timestamp CRC Status:the Pass or Fail check of timestamp.
LDPC Decode Status: Pass or Fail check of the LDPC decode.
HCS CRC Status: Pass or Fail check of the HCS.
Payload CRC Status: Pass or Fail check of the payload.

OCD

OCD: Valid or Invalid. When Invalid, none of the other OCD settings will be populated (will show ***); DPD Downstream Profile Descriptor and OCD OFDM Channel Descriptor are messages that can exist in the PLC Data. They are usually both present together, but it’s possible a signal has only one or the other. DPD and OCD may not be sent in the same frame, or in separate frames.
Downstream Channel ID: Field in the OCD message. Not used by the demod.
Configuration Change Count: Field in the OCD message. Not used by the demod.
FFT Length: Will be either 4096 or 8192 like FFT Length (Initial Settings), except this is the value from the OCD message, not the measured value. This is the value sent on to the full-band demod when PLC settings = Auto.
Cyclic Prefix: same as Cyclic Prefix (above), except this is the value from the OCD message, not the measured value. This is the value used by the demod when PLC settings = Auto. Cyclic Prefix affects OFDM symbol length (= FFT Length + Cyclic Prefix). Cyclic Prefix is also known as Guard Interval.
Roll-off: OFDM symbol filter coefficient; doesn’t affect OFDM symbol length. Roll-off is also known as Window Beta.
OFDM Spectrum Location: Location of the lowest subcarrier (S0) in the OFDM band. This subcarrier may be outside the VSA’s measurement span. This location is only used for reporting. The expected S0 location used by the demod is calculated as (VSA Center – System Sample Frequency / 2). Subcarriers are numbered from 0 at the lowest frequency to (FFT size-1) at the highest frequency. The subcarrier at the center (FFT size/2) is also called the DC subcarrier. Typically the same number of subcarriers are reserved on the lower and upper ends of the spectrum as GUARD carriers – ideally, no energy is transmitted in the guard carriers.
PLC Start Index: subcarrier index of the first subcarrier in the PLC.
Time Interleaving Depth: Number of symbols used for time interleaving.
Exclusion Bands: Pairs of band edges in subcarrier indexes that indicate which subcarriers are not active from the total number of subcarriers (4096 or 8192)
Continuous Pilots: Subcarrier indexes of continuous pilots (which are transmitted on every symbol on the same subcarrier)

DPD NCP

DPD NCP: Valid or Invalid. When Invalid, none of the other NCP settings will be populated (will show ***). Indicates whether the PLC contains a DPD message that gives information about the NCP. NCP (next codeword pointer) subcarriers can be QPSK, 16-, or 64-QAM. NCP is a profile like A-P, except it has a different format (contains pointers to the codewords in the other profiles).
NCP Profile ID: Always 255
NCP DS Channel ID: Field in DPD message.
NCP Modulation: Modulation format found in Modulation Assignment vector; if more than one modulation format is found, “Mixed” is shown.
NCP Config Change Count: Field in DPD message.
NCP Modulation Assignment: Vector of (FFT Length) entries; each is a power of 2 and indicates the QAM order for each subcarrier index. *** means no assignment for the subcarrier.

DPD Profile A

DPD Profile A: Valid or Invalid. When Invalid, none of the other Profile A settings will be populated (will show ***). Indicates whether the PLC contains a DPD message that gives information about Profile A.
Profile A ID: 0
Profile A DS Channel ID: Field in DPD message.
Profile A Modulation: Modulation format found in Modulation Assignment vector; if more than one modulation format is found, “Mixed” is shown.
Profile A Config Change Count: Field in DPD message.

DPD Profile B-P

(Only available when PLC Settings are set to Automatic)

DPD Profile B-P: Valid or Invalid. When Invalid, none of the other Profile B-P settings will be populated (will show ***). Indicates whether the PLC contains a DPD message that gives information about Profile B-P.
Profile B-P ID: Profiles B-P have IDs 1-15.
Profile B-P DS Channel ID: Data not available (always ***)
Profile B-P Modulation: Auto-detected odulation format; if more than one modulation format is found, “Mixed” is shown.
Profile B-P Config Change Count: Data not available (always ***)