Error Vector Time (LTE-Advanced)

Error Vector Time shows the difference between the measured symbols and the reference symbols for each symbol in the Measurement Interval. The EVM values for subcarriers are plotted above their corresponding symbol-time on this trace. The values of the error vectors are usually plotted as a magnitude, but other formats can be chosen as well.

A marker placed on this trace which reads as follows

indicates that the marker is

on symbol 80
in the User03 channel allocation shown in the Composite Include list
on subcarrier 103
and has a value of ~0.33%, the RMS average error over all the subcarriers in this symbol being ~0.49% (see EVM for more information on how EVMs are calculated)

For uplink PRACH analysis, this trace's x-axis will be marked in subframes. The PRACH EVM data points are located at the subframe that the PRACH preamble begins in.

MBMS mixed-mode CP

MBMS signals are transmitted during the MBSFN region of MBSFN subframes using Extended CP mode (6 symbols per slot). The non-MBSFN region of these subframes (and the rest of the frame) can be transmitted in Normal or Extended CP mode.

For mixed-CP-mode LTE frames, traces that show time on the x-axis in units of syms (OFDM symbols) would need unevenly spaced x-axis points to correctly represent the time locations of the symbols (since extended CP symbols are longer than normal CP symbols).

Instead of using non-uniform x-axis spacing of symbol points, the LTE demodulator shows the time axis as if all the slots used Normal CP (with 7 symbols per slot), and for MBSFN slots, the demodulator fills the first 6 symbols of the slot with the extended CP symbols, leaving the 7th symbol blank.

Downlink MIMO, Layers, and EVM Calculations

EVM values are calculated using physical channels' layer data. The 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.