WiFi基本概念（十三）射频（一）（EVM）

This page describes IEEE 802.11ax EVM Measurement as defined in IEEE 802.11 ax standard. The 802.11ax EVM test and measurement solutions are also described along with 802.11 EVM specifications.

What is EVM (Error Vector Magnitude)?

Introduction: 
• EVM or Error vector magnitude provides insight into quality of the modulated signal/symbol.
• This modulated signal originates when bits are mapped to symbols in a complex modulation systems such as QPSK, 16-QAM, 64-QAM etc.
• It is also referred as RCE (Relative Constellation Error).
• It can be expressed in units of "dB" or "rms". 
• Refer Error Vector Magnitude basics >> and EVM unit converter >> for more information.

The figure-1 depicts concept of EVM measurement. The basic EVM equation is also mentioned.

IEEE 802.11ax EVM specifications

In order to support higher data rates and capacity, 802.11ax uses 1024-QAM modulation in addition to 16-QAM, 64-QAM and 256-QAM. Following table mentions EVM specifications of 802.11ax compliant DUT (Device Under Test).

 

Source: Keysight Technologies & 11ax Standard
 

IEEE 802.11ax EVM Test and Measurement Solutions

To test or measure EVM of 802.11x DUT, test equipments should have tighter EVM noise floor. Following are the companies offering test and measurement solutions for 802.11ax compliant devices. 
➤National Instruments (NI) offers RF vector signal transceiver (VST) with the NI WLAN Measurement Suite. 
➤Lite Point offers zSeries-PA/FEM test solution for EVM test and measurement. The Litepoint offers following instruments among the other which are used for the test. 
• 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 802.11ax EVM Test requirement which requires following specifications. Keysight offers 802.11ax solutions which include 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 so on. 
• Better than -50dB EVM performance for chipset & PA characterization
• Better than -48dB EVM performance for 8x8 device validation

802.11 EVM specification

Following table mentions different RMS EVM limits for each modulation scheme as per 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 

802.11ax EVM Measurement | IEEE 802.11 EVM specification

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