Guard Interval (802.11n/ac/ax/be)

The Guard Interval is the ratio of the Cyclic Prefix "CP 1) Contention period, or 2) Cyclic prefix" time to the inverse FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. time "T(IFFT Inverse Fast Fourier Transform)." The Guard Interval is used to eliminate inter-symbol and inter-carrier interference. A copy of the last Guard Interval T(GI Guard interval) of the useful symbol period "T(IFFT)," termed Cyclic Prefix "CP," is used to collect multipath, while maintaining the orthogonality of the subcarriers. Each symbol is transmitted for a slightly longer time, extended symbol time T(s), than the active (or useful) symbol time T(IFFT). The extra time is the guard interval.

Automatic: When selected, the VSA software attempts to automatically set the Guard Interval.
From SIG Symbols: the VSA uses information detected within the HT-SIG High-Throughput SIGNAL field or VHT very high throughput (VHT) physical (PHY) layer protocol data unit (PPDU):A PPDU transmitted using the TXVECTOR FORMAT parameter equal to VHT.-SIG sections of the preamble to determine the Guard Interval.
Manual
- 1/16: Sets the Guard Interval to 1/16 (see Guard Interval Time Calculation below).
- 1/8: Sets the Guard Interval to 1/8 (see Guard Interval Time Calculation below).
- 1/4: Sets the Guard Interval to 1/4 (see Guard Interval Time Calculation below).
- Other: User entered Guard Interval values between 0 to 1.

The Guard Interval time period T(GI) is specified as a fraction (percentage) of the inverse FFT time period T(IFFT). For 802.11n/ac/ax/be, the Guard Interval selection is 1/4, 1/8, or 1/16. The Other selection inputs a non-standard Guard Interval value between 0 and 1.

Guard Interval (TGI)Time Calculation

The following table shows calculated Guard Interval values for 802.11n/ac:

Guard Interval	T(FFT)	T(GI)
1/4	3.2 µs	0.8 µs
1/8	3.2 µs	0.4 µs
1/16	3.2 µs	0.2 µs
where: T(FFT) = FFT time period for the OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. symbol T(GI) = Guard Interval time period = Guard Interval ´T(FFT)

The following table shows calculated Guard Interval values for 802.11ax:

Guard Interval	T(FFT)	T(GI)
1/4	12.8 µs	3.2 µs
1/8	12.8 µs	1.6 µs
1/16	12.8 µs	0.8 µs
where: T(FFT) = FFT time period for the OFDM symbol T(GI) = Guard Interval time period = Guard Interval ´T(FFT)

OFDM Frame and the Guard Interval

$image\OFDMGuardInterval_wmf.jpg$