EVM Window Length (LTE-Advanced)

Default: 3GPP

Range: 3GPP, Custom: 1-512 samples

EVM Window Length specifies the length of the window used for EVM calculations. The EVM window is centered in the cyclic prefix.

A value of 3GPP will set EVM Window Length according to the LTE standard for EVM measurements. Appendix F.5 of 3GPP TS 36.101 defines the window lengths for each bandwidth.

A Custom EVM window length can also be specified in the range of 1-512 samples. A value of 512 samples corresponds to the entire CP length for Extended CP on a 20 MHz signal.

The standard states that the EVM for an LTE signal's subcarriers should be taken from the higher of the two EVM RMS averages calculated from the FFTs taken from the start and from the end of the EVM window. For example, an EVM Window Length of 3 samples means that two FFTs would be taken. The starting points of the FFTs would be on the samples adjacent to the center sample of the CP. The EVMs for the subcarriers would come from the FFT with the higher EVM RMS average. However, the location of the symbol FFT used for EVM calculations can be set specifically using the Symbol Timing Adjust parameter.

A value of 1 sample will cause the EVM to be measured from an FFT taken from the center of the cyclic prefix, since any further FFTs would have to be taken over the same sample points.

EVM Window Length does not apply when Symbol Timing Adjust is set to % of FFT Size or EVM Window Center since these settings cause only one FFT to be taken starting from the specified location within the cyclic prefix regardless of the EVM Window Length setting.

The EVM Window Length selection of 3GPP is not available for PRACH analysis since EVM window is not defined in the standard for PRACH.