Spectral Emission Mask (802.11n/ac/ax/be/bn)

Spectral Emission Mask shows the Spectrum of the detected WLAN WLAN - Wireless Local Area Network: A system that includes the distribution system (DS), access points (APs), and portal entities. It is also the logical location of distribution and integration service functions of an extended service set (ESS). A WLAN system contains one or more APs and zero or more portals in addition to the DS. burst, with SEM limit lines overlaid. It is a frequency-domain display (power vs. frequency) that allows the user to visually see out-of-band emissions relative to the spectral mask defined by the IEEE Institute of Electrical and Electronics Engineers. A US-based membership organisation that includes engineers, scientists, and students in electronics and related fields. The IEEE developed the 802 series wired and wireless LAN standards. Visit the IEEE at http://www.ieee.org 802.11 standard.

The following limit lines are displayed on this trace:

SEM Relative (dBr) — Relative mask referenced to the signal's peak power. Shows how far out-of-band emissions are below the signal peak.
SEM Combined (dBm deciBels referenced to a milliWatt: dB relative to 1 milliwatt dissipated in the nominal input impedance of the analyzer) — The less restrictive (max) of the relative mask and the absolute mask. This is the actual limit used for pass/fail determination.

The absolute power mask may be band-dependent (depending on the standard). In this case, the absolute limit chosen depends on the VSA measurement Center frequency.

How Limit Lines Are Determined

Limit lines are generated automatically based on the detected packet standard and channel bandwidth of the demodulated WLAN burst, per the IEEE 802.11 specifications.

Standard	Supported Bandwidths
802.11n	20 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second., 40 MHz
802.11ac	20 MHz, 40 MHz, 80 MHz, 160 MHz
802.11ax	20 MHz, 40 MHz, 80 MHz, 160 MHz
802.11be	20 MHz, 40 MHz, 80 MHz, 160 MHz, 320 MHz
802.11bn	20 MHz, 40 MHz, 80 MHz, 160 MHz, 320 MHz

Measurement Parameters (RBW / VBW)

The SEM computation uses standard-specific resolution and video bandwidth targets:

Standard	RBW Resolution Band Width (RBW or ResBW): specifies the minimum frequency bandwith that two separate frequency spectra can be resolved and viewed seperately. For FFT (digital) based VSA's the process is equivalent to passing a time-domain signal through a bank of bandpass filters, whose center frequencies correspond to the frequencies of the FFT bins. For a traditional swept-tuned (non-digital) spectrum analyzer, the resolution bandwidth is the bandwidth of the IF filter which determines the selectivity.	RBW
802.11a / 11n / 11ac	100 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second).	30 kHz
802.11ax / 11be / 11bn	100 kHz	7.5 kHz

Since the SEM is computed from the same acquisition used for demodulation (rather than a dedicated SEM acquisition), the actual achievable RBW/VBW depends on the burst length. If the burst is too short, the measurement still runs but a DATA? indicator warns the user that the RBW/VBW requirement was not met. The solution is to increase the number of data symbols in the transmitted PPDU PLCP Protocol data unit.

The RBW is the -3 dB bandwidth for the Gaussian Top window.