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wlanLLTFDemodulate

Demodulate L-LTF waveform

Syntax

y = wlanLLTFDemodulate(x,cbw)
y = wlanLLTFDemodulate(x,cfg)
y = wlanLLTFDemodulate(___,symOffset)

Description

example

y = wlanLLTFDemodulate(x,cbw) returns the demodulated L-LTF[1] waveform given time-domain input signal x and channel bandwidth cbw.

example

y = wlanLLTFDemodulate(x,cfg) returns the demodulated L-LTF given the format configuration object, cfg.

example

y = wlanLLTFDemodulate(___,symOffset) specifies the OFDM symbol offset, symOffset, using any of the arguments from the previous syntaxes.

Examples

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Demodulate the L-LTF used in a non-HT OFDM transmission, after passing the L-LTF through an AWGN channel.

Create a non-HT configuration object and use it to generate an L-LTF signal.

cfg = wlanNonHTConfig;
txSig = wlanLLTF(cfg);

Pass the L-LTF signal through an AWGN channel. Demodulate the received signal.

rxSig = awgn(txSig,15,'measured');
y = wlanLLTFDemodulate(rxSig,'CBW20');

Demodulate the L-LTF used in a VHT transmission, after passing the L-LTF through an AWGN channel.

Create a VHT configuration object and use it to generate an L-LTF signal.

cfg = wlanVHTConfig;
txSig = wlanLLTF(cfg);

Pass the L-LTF signal through an AWGN channel.

rxSig = awgn(txSig,5);

Demodulate the received L-LTF using the information from the wlanVHTConfig object.

y = wlanLLTFDemodulate(rxSig,cfg);

Demodulate the L-LTF for the HT-mixed transmission format, given a custom OFDM symbol offset.

Set the channel bandwidth to 40 MHz and the OFDM symbol offset to 1. That way, the FFT takes place after the guard interval.

cbw = 'CBW40';
ofdmSymOffset = 1;

Create an HT configuration object and use it to generate an L-LTF signal.

cfg = wlanHTConfig('ChannelBandwidth',cbw);
txSig = wlanLLTF(cfg);

Pass the L-LTF signal through an AWGN channel.

rxSig = awgn(txSig,10);

Demodulate the received L-LTF using a custom OFDM symbol offset.

y = wlanLLTFDemodulate(rxSig,'CBW40',ofdmSymOffset);

Input Arguments

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Time-domain input signal corresponding to the L-LTF of the PPDU, specified as an NS-by-NR vector or matrix. NS is the number of samples and NR is the number of receive antennas.

NS is proportional to the channel bandwidth. The time-domain waveform consists of two symbols.

ChannelBandwidthNS
'CBW5', 'CBW10', 'CBW20'160
'CBW40'320
'CBW80'640
'CBW160'1280

Data Types: double
Complex Number Support: Yes

Channel bandwidth in MHz, specified as 'CBW5', 'CBW10', 'CBW20', 'CBW40', 'CBW80', or 'CBW160'.

Data Types: char | string

Format information, specified as a WLAN configuration object. To create these objects, see wlanNonHTConfig, wlanHTConfig, or wlanVHTConfig.

OFDM symbol offset as a fraction of the cyclic prefix length, specified as a real scalar from 0 to 1.

Data Types: double

Output Arguments

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Demodulated L-LTF signal, returned as an NST-by-NSYM-by-NR array. NST is the number of occupied subcarriers, NSYM is the number of OFDM symbols, and NR is the number of receive antennas. For the L-LTF, NSYM is always 2.

NST varies with channel bandwidth.

ChannelBandwidthNumber of Occupied Subcarriers (NST)
'CBW20', 'CBW10', 'CBW5'52
'CBW40'104
'CBW80'208
'CBW160'416

More About

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L-LTF

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 160312.53.2 μs1.6 μs8 μs
10156.256.4 μs3.2 μs16 μs
578.12512.8 μs6.4 μs32 μs

PPDU

The 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 [2].

PLCP

The physical layer convergence procedure (PLCP) is the upper component of the physical layer in 802.11 networks. Each physical layer has its own PLCP, which provides auxiliary framing to the MAC [2].

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.

[2] Gast, Matthew S. 802.11n: A Survival Guide. Sebastopol, CA: O’Reilly Media Inc., 2012, p. 120.

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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