Error Vector Spectrum (802.11n/ac/ax/be)
shows the signal 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. vs. frequency. It provides both the RMS average Error Vector Magnitude (shown by 2D Average Line) and the individual Error Vector Magnitudes (EVM) for each subcarrier for all 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. symbols within the Measurement Interval. The individual symbol EVMs are displayed by common subcarrier number--that is, for a particular subcarrier, all the symbol EVMs for that subcarrier are shown on a vertical line. This provides a convenient way to view all the subcarrier EVMs at a particular subcarrier for all symbol-times in the burst. The trace data includes data results for both the pilot and data subcarriers. Unused subcarriers are blanked out.
EVM is the error vector magnitude between a measured (IQ Meas) constellation point and an ideal (IQ Ref) constellation point. trace data is the average error vector magnitude of all the symbol EVMs at a particular subcarrier for all symbol-times within the burst (see RMS Error Vector Spectrum trace data).
The number of symbol-times included and displayed in the data results can be decreased (or limited) with the Maximum Result Length parameter. The TimeLen value below the trace grid shows the number of symbol-times included in the trace.
For 802.11n/ac, when the Subcarrier Selection is Pilots Only, the OFDM Err Vect Spectrum trace may display evenly spaced pilot subcarriers even if the pilot subcarriers have uneven spacing.
Interpreting the Display
The y-axis annotates the EVM magnitude and the x-axis annotates the OFDM Subcarrier number. The z-axis annotates the symbol position. The display format shows all the symbols at a particular subcarrier, for all the symbol-times in the Measurement Interval. Placing a marker on a subcarrier shows the subcarrier number, the symbol number, the subcarrier EVM (for that particular symbol), and the RMS average EVM (for that particular subcarrier).
802.11n/ac Formats
The trace data uses a multi-color display that assigns a unique color to the pilot subcarriers, the data subcarriers, and the trace data. The colors are also user definable (see Display Preferences' Color tab).
Trace data display symbols
- Colored squares:
Identifies Data subcarrier symbol EVMs.
- Magenta for BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros. (Mod Type 1)
- Red for QPSK Quadrature phase shift keying (Mod Type 2)
- Blue for 16 QAM Quadrature Amplitude Modulation (Mod Type 3)
- Green for 64 QAM (Mod Type 4)
- Yellow for 256 QAM (Mod Type 5)
- White squares: Identifies Pilot subcarrier symbol EVMs.
- Vertical lines: Identifies common subcarrier positions.
- White line: Plot of the RMS average symbol EVM for each subcarrier. This is the same as the OFDM RMS Error Vector Spectrum trace data display.
To change the Data subcarrier symbol EVM color, click tab and select a new color for the active Element.
802.11ax Format
The trace data uses a multi-color display that assigns a unique color to the pilot subcarriers and data subcarriers. The display for 802.11ax is different from 802.11n/ac in that EVM is displayed by Pilot and Data subcarrier instead of by modulation type. The colors are also user definable (see Display Preferences' Color tab).
Trace data display symbols
- White squares: Identifies Pilot subcarrier symbol EVMs.
- Red squares: Identifies Data subcarrier symbol EVMs.
See Also
