EVM Measurements Using NI RFmx

Updated Sep 6, 2023

Environment

Hardware

  • PXIe-5644
  • PXIe-5646
  • PXIe-5831
  • PXIe-5841

Software

  • RFmx NR
  • RFmx WLAN
  • RFmx LTE/LTE-Advanced

This article provides some general information about EVM measurements using some of the RFmx personalities where it is available and includes a step by step instructions on how to take an EVM measurement with different waveforms.
Requirements:
  • NI-RFSG driver
  • NI-RFSA driver
  • RFmx
Error Vector Magnitude (EVM) is a measurement of demodulator performance in the presence of impairments on the signal path. After decimating the recovered waveform at the demodulator output, the acquired symbols are compared against the ideal symbol locations. The RMS error vector magnitude and phase error are then used in determining the EVM measurement over a window of N demodulated symbols.
A more in-depth explanation about the variables and a graphical example can be found in the following online help topic: EVM
 

For the steps, we will focus on an NR EVM measurement using the RFmx Soft Front Panel within InstrumentStudio.
First, we will open the RFmx Soft Front Panel by clicking on the InstrumentsStudio button from the instrument window within NI MAX:
1 -NI MAX Instrument Studio.png
Then click on Personalities and select the desired personality (for this case we will select NR):
2-Select the Personality.png
By default, the NR layout will include the ACP and ModAcc measurement panels with the RMS EVM per Subcarrier Mean graph and the PUSCH Constellation, but in this article, we will focus on the EVM measurement.
For configuring the settings for the EVM measurement we should pay attention to the waveform that we are expecting. In this case, it is a 2x10MHz Uplink waveform with a center frequency of 3.5 GHz and an output power level of -10 dBm which we created following the instructions in the Creating and Generating Signals Using NI RFmx Waveform Creator Knowledge Base article  and we are generating it as shown below:
3 - generating NR waveform.png
Based on the characteristics of the waveform we are measuring (we have connected the RF OUT and RF IN ports of the VST in loopback for this example) we should configure the center frequency to 3.5 GHz, the Bandwidth to 10 MHz, the Reference level to 4.5 dBm, the Subcarrier spacing to 15 kHz, and add an additional subblock with the Component Carrier 0 settings (this is also illustrated on the above Knowledge Base article).
       
 
 
 

After properly configuring the Soft Front Panel it will show the resulting EVM (and ACP) measurements:
4 - resulting measurement.png
Note: Depending on the desired BW for the ACP or EVM measurement you may need to remove one of the measurements, as explained in the following Knowledge Base article: Invalid IQ Rate in RFmx Soft Front Panel Measuring a 5G NR Signal
 
 

In this demonstration, we will utilize one of the shipping examples from the RFmx software, specifically for the WLAN personality, called RFmxWLAN OFDMModAcc with EVM based Auto Level, which can be found in the following location: C:\Program Files\National Instruments\LabVIEW 202x\examples\RFmx\WLAN.
For this example, we are generating a WLAN signal using the RFmx Waveform Creator for WLAN which can be launched from the RFmx Waveform Creator by selecting WLAN as the Modulation type. 
We are using the default setting for the waveform: 802.11 a/g OFDM Standard with a Channel Bandwidth of 20 MHz and we are generating it using a PXIe-5644 at a center frequency of 2.412 GHz and a Power Level of -10 dB. 
Since the RF In and Out ports on the VST are connected in a loopback test, we will need to configure the front panel of the LabVIEW example to match those settings (Standard, Channel Bandwidth, Center Frequency, and Reference Level) and the result is shown below:
5 - Using API with WLAN EVM measurement.png
After making these changes on the VI you will be able to see the symbols clearly on the constellation and the composite EVM values. 
The EVM measurement is also available on other digitally modulated signals on other personalities like LTE and GSM.
 
 

Next Steps

If you want to learn more about the other functionalities of the RFmx Soft Front Panel (now integrated with Instrument Studio as of version 2022 Q3) you can check the instructor lessons on the Introduction to RF Signal Generation Acquisition and Analysis with NI RF Technologies online training.

Related Links

Other Support Options

Ask the NI Community

Collaborate with other users in our discussion forums



Request Support from an Engineer

A valid service agreement or active software subscription may be required, and support options vary by country.

Was this information helpful?

Yes

No