EVM (LTE)

EVM shows the Root Mean Square (RMS) of the Error Vector Magnitudes for all channels and signals selected in the Composite Include list of channels, except Non-Alloc, over all symbol-times in the Measurement Interval.

The location of the symbol FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. used in 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. calculations is displayed to the right of the EVM value. Possible values are EVM Window Start, EVM Window Center, EVM Window End, or % of FFT Size. See the Symbol Timing Adjust parameter for more information about these values.

EVM can also be shown in units of dB by selecting the Report EVM in dB parameter.

Calculation

The EVM for a single point is the distance between the tips of the measured IQ and reference IQ vectors. This distance is normalized to the average power of the signal and expressed as a percentage (%). The average power of the signal is calculated by averaging the powers for all resource elements in the Search Time trace (capture length determined by Result Length).

For signals with multiple C-RS Cell-specific RS antenna ports where precoding is used, measured IQ data comes from the data points after precoding has been removed. See the MIMO Decoding topic for more information.

For EVM calculations to be valid, the MIMO Decoding parameter must be set to 3GPP MIMO Decoding.

Downlink MIMO, Layers, and EVM Calculations

EVM values are calculated using physical channels' layer data. The LTE Long Term Evolution demodulator compares measured data (in IQ Meas) with the calculated ideal data (in IQ Ref) to compute an EVM. The data points for IQ Ref are always calculated and shown with precoding removed. For EVM calculations to work, IQ Meas data must also be shown with the precoding removed.

When the MIMO Decoding parameter is set to No Decoding, IQ Meas data will be shown with the precoding present. This will cause EVM calculations to be invalid. For EVM calculations to be valid, MIMO Decoding must be set to 3GPP MIMO Decoding.