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wlanLSTF

Generate L-STF waveform

Description

example

y = wlanLSTF(cfg) generates an L-STF[1] time-domain waveform using the specified transmission parameters.

y = wlanLSTF(cfg,OversamplingFactor=osf) generates an L-STF waveform for the specified oversampling factor. For more information about oversampling, see FFT-Based Oversampling.

Examples

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Generate the L-STF waveform for a 40 MHz single antenna VHT packet.

Create a VHT configuration object. Use this object to generate the L-STF waveform.

cfgVHT = wlanVHTConfig('ChannelBandwidth','CBW40');
y = wlanLSTF(cfgVHT);
size(y)
ans = 1×2

   320     1

plot(abs(y))
xlabel('Samples')
ylabel('Amplitude')

Figure contains an axes object. The axes object contains an object of type line.

The output L-STF waveform contains 320 samples for a 40 MHz channel bandwidth.

Input Arguments

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Transmission parameters, specified as a wlanVHTConfig, wlanHTConfig, or wlanNonHTConfig object.

Example: wlanVHTConfig

Oversampling factor, specified as a scalar greater than or equal to 1. The oversampled cyclic prefix length must be an integer number of samples. The resultant inverse fast Fourier transform (IFFT) length must be even.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Output Arguments

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(L-STF) time-domain waveform, returned as an NS-by-NT matrix. NS is the number of time-domain samples, and NT is the number of transmit 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
'CBW320'2560

Data Types: double
Complex Number Support: Yes

More About

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L-STF

The legacy short training field (L-STF) is the first field of the 802.11™ OFDM PLCP legacy preamble. The L-STF is a component of VHT, HT, and non-HT PPDUs.

L-STF location within legacy preamble.

The L-STF duration varies with channel bandwidth.

Channel Bandwidth (MHz)Subcarrier Frequency Spacing, ΔF (kHz)Fast Fourier Transform (FFT) Period (TFFT = 1 / ΔF)L-STF Duration (TSHORT = 10 × TFFT / 4)
20, 40, 80, 160, and 320312.53.2 μs8 μs
10156.256.4 μs16 μs
578.12512.8 μs32 μs

Because the sequence has good correlation properties, it is used for start-of-packet detection, for coarse frequency correction, and for setting the AGC. The sequence uses 12 of the 52 subcarriers that are available per 20 MHz channel bandwidth segment. For 5 MHz, 10 MHz, and 20 MHz bandwidths, the number of channel bandwidths segments is 1.

Algorithms

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L-STF Processing

The L-STF is two OFDM symbols long and is the first field in the packet structure for the VHT, HT, and non-HT OFDM formats. For algorithm details, see IEEE Std 802.11ac™-2013 [1], Section 22.3.8.2.2.

FFT-Based Oversampling

An oversampled signal is a signal sampled at a frequency that is higher than the Nyquist rate. WLAN signals maximize occupied bandwidth by using small guardbands, which can pose problems for anti-imaging and anti-aliasing filters. Oversampling increases guardband width relative to the total signal bandwidth, thereby increasing the number of samples in the signal.

This function performs oversampling by using a larger IFFT and zero pad when generating an OFDM waveform. This diagram shows the oversampling process for an OFDM waveform with NFFT subcarriers comprising Ng guardband subcarriers on either side of Nst occupied bandwidth subcarriers.

FFT-based oversampling.

References

[1] IEEE Std 802.11ac™-2013 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 — Amendment 4: Enhancements for Very High Throughput for Operation in Bands below 6 GHz.

Extended Capabilities

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

Introduced in R2015b

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