The L-LTF is the second field in the 802.11™ OFDM PLCP legacy preamble. The L-LTF is a component of EHT, HE, VHT, HT, and
non-HT PPDUs.
Channel estimation, fine frequency offset estimation, and fine symbol
timing offset estimation rely on the L-LTF.
The L-LTF is composed of a cyclic prefix (CP) followed by two
identical long training symbols (C1 and C2). The CP consists of the
second half of the long training symbol.
The L-LTF duration varies with channel bandwidth.
Channel Bandwidth (MHz) | Subcarrier Frequency Spacing ΔF
(kHz) | Fast Fourier Transform (FFT) Period (TFFT = 1 / ΔF) | Cyclic Prefix or Training Symbol Guard Interval
(GI2) Duration (TGI2 = TFFT / 2) | L-LTF
Duration (TLONG = TGI2 + 2 × TFFT) |
---|
20, 40, 80, 160, and 320 | 312.5 | 3.2 μs | 1.6 μs | 8 μs |
10 | 156.25 | 6.4 μs | 3.2 μs | 16 μs |
5 | 78.125 | 12.8 μs | 6.4 μs | 32 μs |