Error Vector Time (802.11n)

Error Vector Time trace data 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. Time. It provides the individual EVMs for all symbol-times within the Measurement Interval. Each symbol-time location on the x-axis shows the symbol EVMs for all subcarriers in that symbol. The trace data includes data results for the pilot and data subcarriers. This display helps to easily attain information about the signal modulation quality as a function of the symbol-times.

There is also a plot (white line) of the RMS average EVM at each symbol-time (includes all data and pilot subcarriers at that symbol-time). This is same data and plot used in the RMS Error Vector Time trace data display.

EVM is the error vector magnitude between a measured (IQ Meas) constellation point and an ideal (IQ Ref) constellation point. RMS Error Vector Time trace data is the average of the symbol EVMs of all the subcarriers at each symbol-time (see RMS Error Vector Time trace data).

The number of symbol-times included and displayed in the data results can be decreased (or limited) with the Maximum Result Length format parameter.

Interpreting the Display

The y-axis annotates the EVM magnitude and the x-axis annotates the 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 number (which range from 0 through last symbol-time analyzed). The z-axis annotates the OFDM subcarrier number (which includes all data and pilot subcarriers). Each x-axis symbol-time on the display shows the individual symbol EVMs for all subcarriers at that particular symbol-time. Placing a marker on a symbol provides the symbol number, the subcarrier number, the symbol EVM and the RMS average of symbol EVMs at that particular symbol-time.

Carrier phase drift will show up in the Error Vector Time trace, and not in the Common Pilot Error trace when Track Phase is cleared. Carrier phase drift will show up in the Common Pilot Error trace, but will be mostly removed from the Error Vector Time trace when Track Phase is selected.

The Error Vector Time trace data uses a multi-color display that assigns a unique color to the pilot subcarriers, the data subcarriers, and the RMS Error Vector 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 Time trace data display.

To change the Data subcarrier symbol EVM color, click Utilities > Display Preferences > Color tab and select a new color for the active Mod Type Element.