Error Vector Spectrum (MB-OFDM)

Menu Path: Trace > Data > Composite > Error Vector Spectrum

The Comp Error Vector Spectrum trace shows the error vector spectrum for all occupied bands. The Error Vector Magnitude (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.) measurement for each subcarrier symbol displays the error between the measured (IQ Meas) vector and an ideal (IQ Ref) vector.

The trace is organized with the EVM on the y-axis and subcarrier numbers -61 to -1, and +1 to +61 on the x-axis. Each vertical subcarrier line displays a quantity of OFDM Symbols where the number of symbols corresponds to the settings for Result Length, Measurement Offset, and Measurement Interval where the settings appears as TimeLen on the trace.

The companion RMS Error Vector Spectrum trace displays the average error vector magnitude of all the EVMs at a particular subcarrier for all symbol-times within the burst. For individual band traces, see Low | Mid | High Band Error Vector Spectrum. For trace identification, the pilot and guard symbols are colored differently than the data symbols.

Setup Example

Let Result Length = 12 | Measurement Offset = 0 | Measurement Interval = 6 | TFC = 1. With these parameters, the first (6) hop symbol-times, 0 to 5, are being measured in sequence Low-Mid-High-Low-Mid-High (see below). Each of the (6) symbol-times is a burst of 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 -61 to -1 and +1 to +61. For this example, look at the EVMs for pilot subcarrier = 5, where each subcarrier displays (6) OFDM symbols 0 to 5. Using the Select Area tool and Markers = Show Marker, expand the trace for subcarrier 5 and note the EVM calculations.