WiFi 6 802.11ax spectral flatness test and measurement

This page describes spectral flatness test and measurement as per IEEE 802.11ax (WiFi 6). The 11ax spectral flatness testing for various bandwidths is described as per wifi 6 conformance requirements.

What is Spectral flatness in WLAN transmitter ?

OFDM allows use of multiple subcarriers for WLAN AP (Access Point) or STA (Station). It increases data rate of the device as it allows simultaneous transmission and reception of more number of bits based on higher level modulation scheme used such as QPSK, QAM etc. Spectral flatness measurement offers way to check individual power of these subcarriers.

The figure-1 depicts spectrum mask of 802.11ac/ax device used for different bandwidths (20/40/80/160 MHz). Spectrum mask helps in determining whether the WLAN channel meets spectrum regulatory limit for power radiations or not. Usually spectral flatness is calculated by taking FFT (Fast Fourier Transform) of the preamble symbol.

WiFi 6 802.11ax Spectral flatness test and measurement

Spectral flatness measurement is carried out by following two steps.
➨Step-1 : Estimate average energy of range of sub-carriers used in OFDM symbol. Usually BPSK modulated OFDM symbol is used. The test signal should have at least 20 PPDUs and each PPDU should have at least 16 data symbols. Unoccupied subcarriers are ignored during measurement. Moreover RU (Resource Unit) power boosting and beamforming is not used during test/measurement.
➨Step-2 : Measure power/energy of all the individual subcarriers and compare with average value calculated in step-1.
➨Step-3 : The deviation between average value and individual subcarrier's value should be within the limit specified to meet the conformance requirements.

Spectral flatness for 20/40/80 MHz

Following figure-2 depicts spectral flatness requirements for various bandwidths as a function of subcarrier with values in table-1 specified as A, B, C and D.

For example, as mentioned in the table and figure above, subcarrier index 5 requires level to be within +/-4 dB of average energy.

Spectral flatness for 160 MHz

The figure-3 depicts spectral flatness requirement for 160 MHz.

As shown in the figure, average energy is calculated using subcarriers from -696 to -515, from -509 to -166, from 166 to 509 and from 515 to 696. These subcarriers should meet +/-4 dB requirements as shown from average energy. Rest of the subcarriers should meet requirements of +4dB/-6dB from average energy.

The other 11ax transmitter tests include Transmit spectrum mask, transmitter modulation accuracy, LO leakage, EVM etc.

References:
•  IEEE 802.11ax standard and WLAN 11ax conformance specifications

RF and Baseband Measurement Links

WiFi 6 Resources as per IEEE 802.11ax

Useful Links to Legacy WLAN Standards

IoT Wireless Technologies

RF Wireless Test and Measurement Solutions