wlanLLTFDemodulate

Demodulate L-LTF waveform

Syntax

y = wlanLLTFDemodulate(x,cbw)

y = wlanLLTFDemodulate(x,cfg)

y = wlanLLTFDemodulate(___,symOffset)

Description

y = wlanLLTFDemodulate(x,cbw) returns the demodulated L-LTF¹ waveform given time-domain input signal x and channel bandwidth cbw.

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

Note

When you specify cfg as a wlanNonHTConfig object, the function supports demodulation only of OFDM-modulated waveforms.

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

Examples

Demodulate L-LTF for Non-HT Format Transmission

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 L-LTF for VHT Format Transmission

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 L-LTF with OFDM Symbol Offset

Set the channel bandwidth to 40 MHz and the OFDM symbol offset to 1. This 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

x — Time-domain input signal

vector | matrix

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

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

ChannelBandwidth	NS
'CBW5', 'CBW10', 'CBW20'	160
'CBW40'	320
'CBW80'	640
'CBW160'	1280
'CBW320'	2560

Data Types: single | double
Complex Number Support: Yes

cbw — Channel bandwidth

'CBW5' | 'CBW10' | 'CBW20' | 'CBW40' | 'CBW80' | 'CBW160' | 'CBW320'

Channel bandwidth, specified as one of these values.

• 'CBW5' — Channel bandwidth of 5 MHz

• 'CBW10' — Channel bandwidth of 10 MHz

• 'CBW20' — Channel bandwidth of 20 MHz

• 'CBW40' — Channel bandwidth of 40 MHz

• 'CBW80' — Channel bandwidth of 80 MHz

• 'CBW160' — Channel bandwidth of 160 MHz

• 'CBW320' — Channel bandwidth of 320 MHz

Data Types: char | string

cfg — Format information

wlanNonHTConfig | wlanHTConfig | wlanVHTConfig

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

symOffset — OFDM symbol sampling offset

0.75 (default) | scalar in the interval [0, 1]

OFDM symbol sampling offset, as a fraction of the cyclic prefix length, specified as a scalar in the interval [0, 1].

The value that you specify indicates the start location for OFDM demodulation relative to the beginning of the cyclic prefix.

Example: 0.45

Data Types: single | double

Output Arguments

y — Demodulated L-LTF signal

3-D OFDM symbol array

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

N_ST varies with channel bandwidth.

ChannelBandwidth	Number of Occupied Subcarriers (NST)
'CBW20', 'CBW10', 'CBW5'	52
'CBW40'	104
'CBW80'	208
'CBW160'	416
'CBW320'	832

More About

L-LTF

The L-LTF is the second field in the 802.11™ OFDM PLCP legacy preamble. The L-LTF is a component of EHT, HE, 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, 160, and 320	312.5	3.2 μs	1.6 μs	8 μs
10	156.25	6.4 μs	3.2 μs	16 μs
5	78.125	12.8 μs	6.4 μs	32 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™-2016 (Revision of IEEE Std 802.11-2012). “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” IEEE Standard for Information technology — Telecommunications and information exchange between systems — Local and metropolitan area networks — Specific requirements.

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

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2015b

R2022b: Single precision support

You can specify the function's numeric inputs as single precision values.