RFWiFi 6 (802.11ax) Spectral Flatness Test and Measurement
This page provides an overview of spectral flatness testing and measurement as defined in the IEEE 802.11ax (WiFi 6) standard. It details the 11ax spectral flatness testing procedures for various bandwidths, adhering to WiFi 6 conformance requirements.
What is Spectral Flatness in a WLAN Transmitter?
OFDM (Orthogonal Frequency Division Multiplexing) enables the use of multiple subcarriers for WLAN Access Points (APs) or Stations (STAs). This approach boosts the device’s data rate by allowing the simultaneous transmission and reception of more bits, leveraging higher-level modulation schemes like QPSK or QAM.
Spectral flatness measurement is a way to assess the individual power levels of these subcarriers.
Figure 1 illustrates the spectrum mask of an 802.11ac/ax device used for different bandwidths (20/40/80/160 MHz). The spectrum mask helps to determine if the WLAN channel meets the regulatory limits for power radiation. Spectral flatness is typically calculated by performing a Fast Fourier Transform (FFT) on the preamble symbol.
WiFi 6 (802.11ax) Spectral Flatness Test and Measurement Procedure
Spectral flatness measurement involves the following steps:
Step 1: Estimate the average energy of the range of subcarriers used in the OFDM symbol. Usually, a BPSK modulated OFDM symbol is used. The test signal should consist of at least 20 PPDUs (PLCP Protocol Data Units), and each PPDU should have at least 16 data symbols. Unoccupied subcarriers are ignored during the measurement. Furthermore, RU (Resource Unit) power boosting and beamforming are disabled during the test/measurement.
Step 2: Measure the power/energy of each individual subcarrier and compare it with the average value calculated in Step 1.
Step 3: Ensure that the deviation between the average value and the individual subcarrier’s value remains within the specified limits to meet conformance requirements.
Spectral Flatness for 20/40/80 MHz
Figure 2 illustrates the spectral flatness requirements for various bandwidths as a function of the subcarrier, with values in Table 1 specified as A, B, C, and D.
| Channel size | A | B | C | D | Subcarriers used for average ( ) calculation |
|---|---|---|---|---|---|
| 20 MHz | 2 | 84 | 85 | 122 | 2 to 84 and -2 to -84 |
| 40 MHz | 3 | 168 | 169 | 244 | 3 to 168 and -3 to -168 |
| 80 MHz | 3 | 344 | 345 | 500 | 3 to 344 and -3 to -384 |
For example, according to the table and figure above, subcarrier index 5 must have a level within +/-4 dB of the average energy.
Spectral Flatness for 160 MHz
Figure 3 depicts the spectral flatness requirements for 160 MHz.
As shown in the figure, the 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 the +/-4 dB requirements relative to the average energy. The remaining subcarriers should meet requirements of +4dB/-6dB from the average energy.
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
