Subcarrier Reference Points Selection (802.11n/ac/ax/be)
To compute EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format., the demodulator subtracts the reference subcarrier constellation points from the measured constellation points. The parameter allows you to choose the source of these reference points.
(default) The demodulator will choose the closest point to the measured subcarrier constellation point as the reference point.
When you click , the reference points from the current measurement sweep (IQ Ref traces for the Streams) will be copied into the measurement state. The Subcarrier Reference Points Selection mode will also be set to User-Specified Points.
The reference points are specified explicitly instead of using autodetection. The total number of points is shown for information.
You can click to import reference points from a TXT file. The format is as follows:
real{ Si[0, 0] }
imag{ Si[0, 0] }
real{ Si[1, 0] }
imag{ Si[1, 0] }
...
real{ Si[K-1, N-1] }
imag{ Si[K-1, N-1] }
where Si[k,n] is the complex value of the reference constellation point for subcarrier k and OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. symbol n. For signals with multiple streams, all the points for Stream 1 are listed, then Stream 2, etc.
Clicking will allow you to save a copy of the reference stored in the measurement state.
This feature is useful when using high order modulations with a signal that has enough impairments to cause incorrect autodetection of the reference points. You can measure an ideal version of the signal (i.e. recall the ideal signal as a recording and run the measurement) and use the reference symbols from the ideal signal as the reference symbols for the measurement made from the signal that has impairments.
Presetting the measurement will clear any stored reference points.
A saved .setx file will contain all the reference symbols so that when you recall the .setx, you don't need to reimport the TXT file.
See Also
