Generate VHT-LTF waveform
Create a VHT configuration object with an 80 MHz channel bandwidth.
cfgVHT = wlanVHTConfig; cfgVHT.ChannelBandwidth = 'CBW80';
Generate a VHT-LTF waveform.
vltfOut = wlanVHTLTF(cfgVHT); size(vltfOut)
ans = 1×2 320 1
The 80 MHz waveform is a single OFDM symbol with 320 complex output samples.
cfg— Transmission parameters
Transmission parameters, specified as a
osf— Oversampling factor
1(default) | scalar greater than or equal to 1
Oversampling factor, specified as a scalar greater than or equal to 1. The oversampled cyclic prefix length must be an integer number of samples.
y— VHT-LTF time-domain waveform
VHT-LTF time-domain waveform, returned as an (NS × NVHTLTF)-by-NT matrix. NS is the number of time-domain samples per NVHTLTF, where NVHTLTF is the number of OFDM symbols in the VHT-LTF. NT is the number of transmit antennas.
NS is proportional to the channel bandwidth.
See VHT-LTF Processing for waveform generation details.
Complex Number Support: Yes
The very high throughput long training field (VHT-LTF) is located between the VHT-STF and VHT-SIG-B portion of the VHT packet.
It is used for MIMO channel estimation and pilot subcarrier tracking. The VHT-LTF includes one VHT long training symbol for each spatial stream indicated by the selected MCS. Each symbol is 4 μs long. A maximum of eight symbols are permitted in the VHT-LTF.
For a detailed description of the VHT-LTF, see section 126.96.36.199.5 of IEEE® Std 802.11™-2016.
The VHT-LTF is used for MIMO channel estimation and pilot subcarrier tracking. The number of OFDM symbols in the VHT-LTF (NVHTLTF) is derived from the total number of space-time streams (NSTS_Total). NSTS_Total = ΣNSTS(u) for user u, u = 0,…, NUsers–1 and NSTS(u) is the number of space-time streams per user.
For algorithm details refer to IEEE Std 802.11ac™-2013 , Section 188.8.131.52.
An oversampled signal is a signal sampled at a frequency that is higher than the Nyquist rate. WLAN signals maximize occupied bandwidth by using small guardbands, which can pose problems for anti-imaging and anti-aliasing filters. Oversampling increases guardband width relative to the total signal bandwidth, thereby increasing the number of samples in the signal.
This function performs oversampling by using a larger IFFT and zero pad when generating an OFDM waveform. This diagram shows the oversampling process for an OFDM waveform with NFFT subcarriers comprising Ng guardband subcarriers on either side of Nst occupied bandwidth subcarriers.
 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 802.11ac-2013 Adapted and reprinted with permission from IEEE. Copyright IEEE 2013. All rights reserved.