The upper margin is the difference between the upper spectral flatness mask and the relative magnitude of the channel frequency response. The lower margin is the difference between the relative magnitude of the channel frequency response and the lower spectral flatness mask. The relative magnitude of the channel frequency response is relative to the mean power of a few subcarriers around the DC subcarrier. The spectral flatness mask is as defined in section 17.4.7.4, section 18.3.9.7.3, section 20.3.20.2 of IEEE Standard 802.11-2012, section 22.3.18.2 of IEEE Standard 802.11ac-2013, section 24.3.16.2 of IEEE P802.11ah/D1.3, section 23.3.18.2 of IEEE Standard 802.11af-2013, and section 27.3.18.2 of IEEE P802.11ax/D6.0.

The following figure is a graphical representation of the spectral flatness margin for a 20 MHz transmission. The OFDM Spectral Flatness Margin Average property or the NIWLANA_RESULT_OFDM_DEMOD_SPECTRAL_FLATNESS_MARGIN_AVERAGE attribute returns the smaller of the two margins shown in the figure.