802.11ax EVM Measurement and IEEE 802.11 EVM Specification

This page describes IEEE 802.11ax EVM Measurement as defined in the IEEE 802.11 ax standard. We’ll also cover 802.11ax EVM test and measurement solutions, along with 802.11 EVM specifications.

What is EVM (Error Vector Magnitude)?

Introduction:

• EVM, or Error Vector Magnitude, gives us insight into the quality of a modulated signal/symbol.
• This modulated signal comes about when bits are mapped to symbols in complex modulation systems like QPSK, 16-QAM, 64-QAM, and so on.
• It’s also known as RCE (Relative Constellation Error).
• EVM can be expressed in “dB” or “rms.”
• For more information, refer to Error Vector Magnitude basics and EVM unit converter.

Figure 1 illustrates the concept of EVM measurement. The basic EVM equation is also shown. The EVM is calculated by:

$EVM = \sqrt{\frac{P_{error}}{P_{reference}}}$

Where $P_{error}$ is the power of the error vector and $P_{reference}$ is the power of the ideal signal.

IEEE 802.11ax EVM Specifications

To enable higher data rates and capacity, 802.11ax utilizes 1024-QAM modulation in addition to 16-QAM, 64-QAM, and 256-QAM. The following table details the EVM specifications for an 802.11ax compliant DUT (Device Under Test).

Source: Keysight Technologies & 11ax Standard

IEEE 802.11ax EVM Test and Measurement Solutions

To accurately test or measure the EVM of an 802.11x DUT, the test equipment must have a tighter EVM noise floor. Several companies provide test and measurement solutions for 802.11ax compliant devices:

• National Instruments (NI): Offers an RF vector signal transceiver (VST) with the NI WLAN Measurement Suite.
• Lite Point: Provides the zSeries-PA/FEM test solution for EVM testing and measurement. Litepoint offers instruments such as:
  • z8653 6 GHz Vector Signal Analyzer (VSA), 1 GHz analysis bandwidth
  • z8751 6 GHz Vector Signal Generator (VSG), 500 MHz modulation bandwidth
• Keysight Technologies: Offers an 802.11ax EVM Test solution with specific requirements. Keysight provides 802.11ax solutions including N7617B WLAN Signal Studio, N5182B MXG Signal Generator (SISO test), M9421A PXI VXT Vector Transceiver (SISO test), N5182B MXG signal generator (MIMO Test), M9421A PXI VXT (MIMO Test), 89600 VSA Software for 802.11ax signal analysis, and more.
  • Requirements include:
    • Better than -50dB EVM performance for chipset & PA characterization
    • Better than -48dB EVM performance for 8x8 device validation

802.11 EVM Specification

The table below specifies the different RMS EVM limits for each modulation scheme according to the 802.11 specification. It covers 802.11 a/g, 802.11n, and 802.11ac EVM limits.

References:

• IEEE 802.11ax draft standard and 802.11 EVM specification section