OFDM Common Pilot Error (DOCSIS 4.0 Downstream)
When DOCSIS 4.0 Downstream Demodulation is selected, the trace data shows the difference between the measured and ideal pilot subcarrier symbols for the specified measurement hardware.
The error data result (OFDM Error Summary table) provides the RMS level of the Common Pilot Error trace data as a percentage of the ideal signal.
Residual phase and frequency settling that occurs following the preamble is measured via the Common Pilot Error trace. At each symbol in the burst, the measured symbol values of the pilot subcarriers are compared with the ideal values. The differences are averaged together, producing a single complex value for each symbols.
The OFDM Common Pilot Error trace only includes the errors that are tracked. When Track Phase is selected, the CPE trace has phase data. When Track Amplitude is selected, the CPE trace has amplitude data. Track Timing applies pilot subcarrier timing error correction (frequency offset correction) to the pilot and data subcarriers. This is in addition to Track Amplitude or Track Phase. Track Amplitude or Track Phase must be selected in order for the CPE trace to show useful information.
With Track Phase selected, the CPE trace shows the average phase error of the OFDM pilots as a function of time (versus data symbol number). The CPE is shown as phase deviation when the trace format is Unwrap Phase or Wrap Phase. Select Group Delay as the Trace Format to trace the CPE as frequency deviation. With Track Amplitude selected, select LinMag or LogMag as the trace format to trace the CPE as magnitude changes.
Carrier phase drift will show up in the Error Vector Time trace, and not in the CPE trace when Track Phase is cleared. Carrier phase drift will show up in the CPE trace, but will be mostly removed from the Error Vector Time trace when Track Phase is selected.
See Also
Available Trace Data (DOCSIS 4.0 Downstream)
