Generate HT-LTF waveform
wlanHTConfig object having a channel bandwidth of 40 MHz.
cfg = wlanHTConfig('ChannelBandwidth','CBW40');
Generate the corresponding HT-LTF.
hltfOut = wlanHTLTF(cfg); size(hltfOut)
ans = 1×2 160 1
cfg parameters result in a 160-sample waveform having only one column corresponding to a single stream transmission.
Generate an HT-LTF having four transmit antennas and four space-time streams.
wlanHTConfig object having an MCS of 31, four transmit antennas, and four space-time streams.
cfg = wlanHTConfig('MCS',31,'NumTransmitAntennas',4,'NumSpaceTimeStreams',4)
cfg = wlanHTConfig with properties: ChannelBandwidth: 'CBW20' NumTransmitAntennas: 4 NumSpaceTimeStreams: 4 SpatialMapping: 'Direct' MCS: 31 GuardInterval: 'Long' ChannelCoding: 'BCC' PSDULength: 1024 AggregatedMPDU: 0 RecommendSmoothing: 1
Generate the corresponding HT-LTF.
hltfOut = wlanHTLTF(cfg);
Verify that the HT-LTF output consists of four streams (one for each antenna).
ans = 1×2 320 4
Because the channel bandwidth is 20 MHz and has four space-time streams, the output waveform has four HT-LTF and 320 time-domain samples.
y— HT-LTF waveform
HT-LTF waveform, returned as an (NS × NHTLTF)-by-NT matrix. NS is the number of time domain samples per NHTLTF, where NHTLTF is the number of OFDM symbols in the HT-LTF. NT is the number of transmit antennas.
NS is proportional to the channel bandwidth. Each symbol contains 80 time samples per 20 MHz channel.
Determination of the number of NHTLTF is described in HT-LTF.
Complex Number Support: Yes
The high throughput long training field (HT-LTF) is located between the HT-STF and data field of an HT-mixed packet.
As described in IEEE® Std 802.11™-2012, Section 18.104.22.168.6, the receiver can use the HT-LTF to estimate the MIMO channel between the set of QAM mapper outputs (or, if STBC is applied, the STBC encoder outputs) and the receive chains. The HT-LTF portion has one or two parts. The first part consists of one, two, or four HT-LTFs that are necessary for demodulation of the HT-Data portion of the PPDU. These HT-LTFs are referred to as HT-DLTFs. The optional second part consists of zero, one, two, or four HT-LTFs that can be used to sound extra spatial dimensions of the MIMO channel not utilized by the HT-Data portion of the PPDU. These HT-LTFs are referred to as HT-ELTFs. Each HT long training symbol is 4 μs. The number of space-time streams and the number of extension streams determines the number of HT-LTF symbols transmitted.
Tables 20-12, 20-13 and 20-14 from IEEE Std 802.11-2012 are reproduced here.
Table 20-12 defines the number of space-time streams (NSTS) based on the number of spatial streams (NSS) from the MCS and the STBC field.
Table 20-13 defines the number of HT-DLTFs required for the NSTS.
Table 20-14 defines the number of HT-ELTFs required for the number of extension spatial streams (NESS). NESS is defined in HT-SIG2.
Additional constraints include:
NHTLTF = NHTDLTF + NHTELTF ≤ 5.
NSTS + NESS ≤ 4.
When NSTS = 3, NESS cannot exceed one.
If NESS = 1 when NSTS = 3 then NHTLTF = 5.
As described in IEEE Std 802.11-2012, Section 20.1.4, high throughput mixed (HT-mixed) format packets contain a preamble compatible with IEEE Std 802.11-2012, Section 18 and Section 19 receivers. Non-HT (Section 18 and Section19) STAs can decode the non-HT fields (L-STF, L-LTF, and L-SIG). The remaining preamble fields (HT-SIG, HT-STF, and HT-LTF) are for HT transmission, so the Section 18 and Section 19 STAs cannot decode them. The HT portion of the packet is described in IEEE Std 802.11-2012, Section 22.214.171.124. Support for the HT-mixed format is mandatory.
The physical layer convergence procedure (PLCP) protocol data unit (PPDU) is the complete PLCP frame, including PLCP headers, MAC headers, the MAC data field, and the MAC and PLCP trailers.
 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.