LitePoint IQxel-M8 Wireless Connectivity Test System
Wireless Connectivity Test Systema
IQxel-M8 ™ is the ideal solution for flexible, high volume production environments for 802.11ac mobile, computing, network, or IoT products.
EVM (Error Vector Magnitude) is the key metric used to evaluate RF transmitter performance, because it provides a consistent “yardstick” to characterize the transmitter regardless of the receiver implementation and it encapsulates a wide range of possible impairments on the transmitter chain into a single measurement. New Wi-Fi generations have increased the modulation constellation size with 1024-QAM and 4096-QAM, placing an even higher requirement into transmitter accuracy. In this blog, we will look at what EVM measures in Wi-Fi and how it is measured.
Quick Facts about QAM Constellation
With each new Wi-Fi generation, higher data rates are achieved by encoding more data bits per symbol. Higher-order QAM enables to transmit more data bits while maintaining the same spectrum footprint and therefore achieve better efficiency.
With a higher-order modulation, the constellation points are closer together and are therefore more susceptible to noise and non-linearities. The digital communication channel requires a better Signal-to-Noise ratio (SNR) to operate error free compared to using lower modulation rates, because the separation between constellation points is reduced and so is the decision distance in the receiver. The transmitter also needs to perform with significantly better accuracy, the Error Vector Magnitude (EVM) is the metric used to quantify the accuracy.
What is EVM?
Error Vector Magnitude is the most commonly used modulation quality metric in digital communications, it is a measure of the deviation of the actual constellation points from their ideal locations in the constellation diagram. The Root-Mean-Square (RMS) error is averaged over subcarriers, frequency segments, OFDM frames and spatial streams and measured at the symbol clock transitions. It is expressed in %Root-Mean-Square or dB.
EVM is a comprehensive measure of the transmit quality because it reflects signal defects that affect the magnitude or phase of the transmitted symbol. It captures the sum of imperfections in the device implementation that impact the transmit symbol’s accuracy. Possible impairments can arise at the baseband, IF or RF elements of the transmit chain.
Some common types of corruption are:
IEEE EVM Requirements
The IEEE defines the maximum allowed transmitter constellation error as part of the standard. The maximum allowed depends on the data rate i.e. constellation size, since higher-order constellation requires a tighter modulation accuracy.
How is EVM Measured?
The test is performed over at least 20 frames. For 802.11ax HE-MU PPDU and HE-TB PPDU, if the occupied RU has 26 tones, the PPDUs under test shall be at least 32 data OFDM symbols long. For occupied RUs that have more than 26 tones, the PPDUs under test shall be at least 16 data OFDM symbols long. The frames should contain random data.
For an HE-TB PPDU with an RU smaller than a 2×996-tone RU, the test shall also include transmit modulation accuracy for the unoccupied subcarriers of the PPDU.
Because Wi-Fi operates on 3 frequency bands (2.4 GHz, 5 GHz and 6 GHz) the transmitter EVM performance should be verified at all transmit power levels and frequencies where the transmitter will operate.
Test equipment used for EVM measurements should support converting the transmitted signals into a stream of complex samples at 160 MHz or more, with sufficient accuracy in terms of I/Q amplitude and phase balance, dc offsets, phase noise, and analog-to- digital quantization noise to ensure low error margin in the measurements.
EVM Correction Items
EVM shows a dependency on the analysis options in the test equipment (such as Phase Tracking, Channel Estimate, Symbol Timing Tracking, Frequency Sync, and Amplitude Tracking). Because analysis parameters can improve EVM, they should be chosen carefully. Addition of non-standard EVM correction methods can artificially improve the DUT’s EVM measurement and hide defects that would have otherwise been detected. It is recommended to only apply IEEE standard defined EVM analysis methods to ensure an objective characterization of the DUT’s transmitter performance.
The following lists the EVM measurement and correction methods:
IEEE defined EVM analysis:
IEEE optional EVM analysis:
Non-standard EVM corrections:
Using non-standard EVM correction can artificially improve measurements and hide underlying EVM impairments, therefore care should be taken in the selection of these parameters.
Test Equipment and Error Margin
As the requirements for the transmitter modulation accuracy rise, so do the requirements of the equipment necessary to test it. The DUT transmitter’s EVM measurement requires the test equipment’s own EVM floor to perform even better in order to provide a small measurement error.
For measuring the same DUT performance, a larger margin between DUT performance and the tester’s EVM floor will result in a smaller measurement error.
As shown in the chart above:
For high order modulations 1024-QAM or 4096-QAM, that require stringent transmitter accuracy, selecting test equipment with low EVM floor is critical, otherwise the error uncertainty contributed by the test equipment reduces the confidence in the final measurement. In extreme cases, where the tester’s EVM floor equals that of the DUT, the measurement error is too large to determine if DUT meets the IEEE EVM requirements.
Key Takeaways
EVM provides a concise “one number” summary of the transmitter quality as it encapsulates a wide range of possible impairments on the transmitter chain. EVM is used during the design phase to characterize devices and uncover underlying sources of distortions. Because of its simplicity, it is also used in manufacturing to guarantee that transmitters will operate properly in real-world environments. However, it is important to understand that EVM is a calculated metric and numerous correction terms are possible that modify the measurement. The test equipment’s EVM floor is an equally important factor that affects the accuracy of the measurement. LitePoint’s IQxel family of testers provide best-in-class EVM performance to ensure high confidence in EVM measurements for the latest Wi-Fi generation.
To learn more about EVM: Read Litepoint's Application Notes.
MCS Test are the approved UK partner for Litepoint
Content Source: EVM Error Vector Magnitude Measure in Wi-Fi | LitePoint
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