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wlanHTLTF

Generate HT-LTF waveform

Syntax

y = wlanHTLTF(cfg)

Description

example

y = wlanHTLTF(cfg) generates an HT-LTF[1] time-domain waveform for HT-mixed format transmissions given the parameters specified in cfg.

Examples

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Create a wlanHTConfig object having a channel bandwidth of 40 MHz.

cfg = wlanHTConfig('ChannelBandwidth','CBW40');

Generate the corresponding HT-LTF.

hltfOut = wlanHTLTF(cfg);
size(hltfOut)
ans =

   160     1

The 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.

Create a 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).

size(hltfOut)
ans =

   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.

Input Arguments

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Format configuration, specified as a wlanHTConfig object. The wlanHTLTF function uses these properties:

Channel bandwidth in MHz, specified as 'CBW20' or 'CBW40'.

Data Types: char | string

Number of transmit antennas, specified as 1, 2, 3, or 4.

Data Types: double

Number of space-time streams in the transmission, specified as 1, 2, 3, or 4.

Data Types: double

Number of extension spatial streams in the transmission, specified as 0, 1, 2, or 3. When NumExtensionStreams is greater than 0, SpatialMapping must be 'Custom'.

Data Types: double

Spatial mapping scheme, specified as 'Direct', 'Hadamard', 'Fourier', or 'Custom'. The default value 'Direct', applies when NumTransmitAntennas and NumSpaceTimeStreams are equal.

Data Types: char | string

Spatial mapping matrix, specified as a scalar, matrix, or 3-D array. Use this property to rotate and scale the constellation mapper output vector. This property applies when the SpatialMapping property is set to 'Custom'. The spatial mapping matrix is used for beamforming and mixing space-time streams over the transmit antennas.

  • When specified as a scalar, NumTransmitAntennas = NumSpaceTimeStreams = 1 and a constant value applies to all the subcarriers.

  • When specified as a matrix, the size must be (NSTS + NESS)-by-NT. NSTS is the number of space-time streams. NESS is the number of extension spatial streams. NT is the number of transmit antennas. The spatial mapping matrix applies to all the subcarriers. The first NSTS and last NESS rows apply to the space-time streams and extension spatial streams respectively.

  • When specified as a 3-D array, the size must be NST-by-(NSTS + NESS)-by-NT. NST is the sum of the data and pilot subcarriers, as determined by ChannelBandwidth. NSTS is the number of space-time streams. NESS is the number of extension spatial streams. NT is the number of transmit antennas. In this case, each data and pilot subcarrier can have its own spatial mapping matrix.

    The table shows the ChannelBandwidth setting and the corresponding NST.

    ChannelBandwidthNST
    'CBW20'56
    'CBW40'114

The calling function normalizes the spatial mapping matrix for each subcarrier.

Example: [0.5 0.3; 0.4 0.4; 0.5 0.8] represents a spatial mapping matrix having three space-time streams and two transmit antennas.

Data Types: double
Complex Number Support: Yes

Output Arguments

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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.

ChannelBandwidthNS
'CBW20'80
'CBW40'160

Determination of the number of NHTLTF is described in HT-LTF.

Data Types: double
Complex Number Support: Yes

More About

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HT-LTF

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 20.3.9.4.6, 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.

NSTS DeterminationNHTDLTF DeterminationNHTELTF Determination

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.

NSS from MCSSTBC fieldNSTS
101
112
202
213
224
303
314
404

NSTSNHTDLTF
11
22
34
44

NESSNHTELTF
00
11
22
34

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.

HT-mixed

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 20.3.9.4. Support for the HT-mixed format is mandatory.

PPDU

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.

References

[1] 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.

Extended Capabilities

Introduced in R2015b


[1] IEEE Std 802.11-2012 Adapted and reprinted with permission from IEEE. Copyright IEEE 2012. All rights reserved.

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