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wlanLLTF

Generate L-LTF waveform

Description

example

y = wlanLLTF(cfg) generates an L-LTF[1] time-domain waveform with transmission parameters cfg.

y = wlanLLTF(cfg,OversamplingFactor=osf) generates an oversampled L-LTF waveform with the specified oversampling factor. For more information about oversampling, see FFT-Based Oversampling.

Examples

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

cfgVHT = wlanVHTConfig('ChannelBandwidth', 'CBW40');
y = wlanLLTF(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-LTF waveform contains 320 time-domain samples for a 40 MHz channel bandwidth.

Input Arguments

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

Oversampling factor, specified as a scalar greater than or equal to 1. The oversampled cyclic prefix length must be an integer number of samples.

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

Output Arguments

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L-LTF 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-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, 160, and 320312.53.2 μs1.6 μs8 μs
10156.256.4 μs3.2 μs16 μs
578.12512.8 μs6.4 μs32 μs

Algorithms

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L-LTF Processing

The L-LTF is two OFDM symbols long and follows the L-STF of the preamble in the packet structure for the VHT, HT, and non-HT formats. For algorithm details, refer to IEEE Std 802.11ac™-2013 [1], Section 22.3.8.2.3 and IEEE Std 802.11-2012 [2], Section 20.3.9.3.4.

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.

[2] 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

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.