Generate L-LTF waveform
Generate the L-LTF for a 40 MHz single antenna VHT packet.
cfgVHT = wlanVHTConfig('ChannelBandwidth', 'CBW40'); y = wlanLLTF(cfgVHT); size(y)
ans = 1×2 320 1
plot(abs(y)) xlabel('Samples') ylabel('Amplitude')
The output L-LTF waveform contains 320 time-domain samples for a 40 MHz channel bandwidth.
cfg— Format configuration
|Transmission Format||Configuration Object||Applicable Object Properties|
y— L-LTF time-domain waveform
L-LTF time-domain waveform, returned as an NS-by-NT matrix. NS is the number of time-domain samples, and NT is the number of transmit antennas.
NS is proportional to the channel bandwidth. The time-domain waveform consists of two symbols.
Complex Number Support: Yes
The legacy long training field (L-LTF) is the second field in the 802.11™ OFDM PLCP legacy preamble. The L-LTF is a component of 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, and 160||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|
The L-LTF is two OFDM symbols long and follows the L-STF of the preamble in the packet structure for the VHT, HT, and non-HT formats. For algorithm details, refer to IEEE Std 802.11ac™-2013 , Section 18.104.22.168.3 and IEEE Std 802.11-2012 , Section 22.214.171.124.4.
 IEEE Std 802.11ac™-2013 IEEE Standard for Information technology — Telecommunications and information exchange between systems — Local and metropolitan area networks — Specific requirements — Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications — Amendment 4: Enhancements for Very High Throughput for Operation in Bands below 6 GHz.
 IEEE Std 802.11™-2012 IEEE Standard for Information technology — Telecommunications and information exchange between systems — Local and metropolitan area networks — Specific requirements — Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.
 IEEE® Std 802.11-2012 Adapted and reprinted with permission from IEEE. Copyright IEEE 2012. All rights reserved.