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wlanHTLTFDemodulate

Demodulate HT-LTF waveform

Syntax

y = wlanHTLTFDemodulate(x,cfg)
y = wlanHTLTFDemodulate(x,cfg,OFDMSymbolOffset)

Description

example

y = wlanHTLTFDemodulate(x,cfg) returns the demodulated HT-LTF[1] , y, given received HT-LTF x. The input signal is a component of the HT-mixed format PPDU. The function demodulates the signal using the information in the wlanHTConfig object, cfg.

example

y = wlanHTLTFDemodulate(x,cfg,OFDMSymbolOffset) specifies the OFDM symbol sampling offset.

Examples

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Create an HT configuration object.

cfg = wlanHTConfig;

Generate an HT-LTF signal based on the object.

x = wlanHTLTF(cfg);

Pass the HT-LTF signal through an AWGN channel.

y = awgn(x,20);

Demodulate the received signal.

z = wlanHTLTFDemodulate(y,cfg);

Display the scatter plot of the demodulated signal.

scatterplot(z)

Create an HT configuration object having two transmit antennas and two space-time streams.

cfg = wlanHTConfig('NumTransmitAntennas',2,'NumSpaceTimeStreams',2, ...
    'MCS',8);

Generate the HT-LTF based on the configuration object.

x = wlanHTLTF(cfg);

Pass the HT-LTF signal through an AWGN channel.

y = awgn(x,10);

Demodulate the received signal. Set the OFDM symbol offset to 0.5, which corresponds to 1/2 of the cyclic prefix length.

z = wlanHTLTFDemodulate(y,cfg,0.5);

Input Arguments

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Input signal comprising an HT-LTF, specified as an NS-by-NR matrix. NS is the number of samples and NR is the number of receive antennas. You can generate the signal by using the wlanHTLTF function.

Data Types: double
Complex Number Support: Yes

HT format configuration, specified as a wlanHTConfig object. The function uses the following wlanHTConfig object properties:

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

Data Types: char | string

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

OFDM symbol sampling offset represented as a fraction of the cyclic prefix (CP) length, specified as a scalar value from 0 to 1. This value indicates the start location for OFDM demodulation, relative to the beginning of the cyclic prefix. OFDMSymbolOffset = 0 represents the start of the cyclic prefix and OFDMSymbolOffset = 1 represents the end of the cyclic prefix.

Data Types: double

Output Arguments

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Demodulated HT-LTF signal for an HT-Mixed PPDU, returned as an NST-by-NSYM-by-NR matrix or array. NST is the number of data and pilot subcarriers. NSYM is the number of OFDM symbols in the HT-LTF. NR is the number of receive antennas.

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

High throughput mixed (HT-mixed) format devices support a mixed mode in which the PLCP header is compatible with HT and non-HT modes.

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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