Cross-Correlated EVM (Custom OFDM)
This parameter is shown in the Frame Definition tab and the Format tab. When Frame Definition > Apply is clicked, the value in the Frame Definition tab is applied to the Format tab (and the signal).
Cross-Correlated 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. (ccEVM) computes EVM by combining the results of two different measurements of the same signal so as to reduce the noise contributed by the acquisition hardware. This procedure causes the ccEVM value to primarily contain the noise coming from the DUT Device under Test: An acronym used to describe some type of electrical apparatus connected to test instrumentation. The apparatus can range from a single component to a complex subsystem such as a mobile phone, base station or MSC. (or source and DUT, in the case of an amplifier).
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Each resource element (subcarrier x symbol) is weighted equally when computing the ccEVM average. |
If the channels are not completely independent (e.g. shared LO), this technique will not be able to remove the common noise between channels.
All channels must make a measurement at the same point in time. The signals don't need to be phase-aligned, but both Custom 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. measurements must demodulate the same frame. If there is a difference between the frames, the Cross-Correlated EVM Summary trace will show a DATA? trace LED. This won't necessarily catch the case where two different frames in time are analyzed by each measurement. To avoid this case, make sure that no more than one frame is captured (adjust the parameters on the Time tab and verify with the Search Time trace).
Refer to the appropriate hardware topic to configure your hardware for a multichannel acquisition.
Cross-Correlated EVM is only available when the number of input channels is greater than one, and even (2, 4, 6, etc.). Two measurements will be performed on half of the measurement channels. Each measurement will be assigned a contiguous block of channels (channels 1 to N/2 assigned to Meas1, channels N/2+1 to N assigned to Meas2).
To perform a successful measurement, make sure the following are true:
Each DUT channel (call it channel k) is connected to appropriate hardware channels through a splitter (hardware channels k and k+N/2). See Making Cross-Correlated EVM Measurements for MIMO Multiple Input, Multiple Output: A physical layer (PHY) configuration in which both transmitter and receiver use multiple antennas. CcEVM configuration examples.
- Input > Channel configuration is set to an even number of channels.
- Sequential acquisition is not in use.
(default) Disables Cross-correlated EVM calculation.
Enables Cross-correlated EVM calculation and enables the Cross-Correlated EVM Summary trace for viewing results.
See Also
