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wlanLSIG

Generate L-SIG waveform

Description

example

[y,bits] = wlanLSIG(cfgFormat) generates an L-SIG[1] time-domain waveform using the specified transmission parameters.

example

[y,bits] = wlanLSIG(cfgFormat,OversamplingFactor=osf) generates an oversampled HT-LTF waveform with the specified oversampling factor. For more information about oversampling, see FFT-Based Oversampling.

Examples

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Create a non-HT configuration object. The default modulation and coding scheme (MCS) is 0.

cfg = wlanNonHTConfig;

Generate the L-SIG waveform and information bits. Extract the rate from the returned bits.

[~,bits] = wlanLSIG(cfg);

Display the first four bits, which contain the rate information. As defined in Table 18-6 of IEEE Std 802.11™-2012, a value of [1 1 0 1] corresponds to a rate of 6 Mbps for 20 MHz channel spacing.

disp(bits(1:4)')
   1   1   0   1

Change the MCS to 7 then generate the corresponding L-SIG waveform and information bits. Extract the rate from the returned bits and analyze. The rate information is contained in the first four bits.

cfg.MCS = 7;
[y,bits] = wlanLSIG(cfg);

Display the first four bits. As defined in IEEE Std 802.11-2012, Table 18-6, a value of [0 0 1 1] corresponds to a rate of 54 Mbps for 20 MHz channel spacing.

disp(bits(1:4)')
   0   0   1   1

Configure an 80 MHz VHT transmission.

cfgVHT = wlanVHTConfig(ChannelBandwidth="CBW80");

Specify an oversampling rate and generate the L-SIG waveform.

osf = 2;
[y,bits] = wlanLSIG(cfgVHT,OversamplingFactor=osf);
size(y)
ans = 1×2

   640     1

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-SIG 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.

ChannelBandwidthNS
'CBW5', 'CBW10', 'CBW20'80
'CBW40'160
'CBW80'320
'CBW160'640

Data Types: double
Complex Number Support: Yes

Signaling bits from the legacy signal field, returned as a 24-by-1 bit column vector. See L-SIG for the bit field description.

Data Types: int8

More About

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L-SIG

The legacy signal (L-SIG) field is the third field of the 802.11™ OFDM PLCP legacy preamble. It consists of 24 bits that contain rate, length, and parity information. The L-SIG is a component of HE, VHT, HT, and non-HT PPDUs. It is transmitted using BPSK modulation with rate 1/2 binary convolutional coding (BCC).

The L-SIG is one OFDM symbol with a duration that varies with channel bandwidth.

Channel Bandwidth (MHz)Subcarrier frequency spacing, ΔF (kHz)Fast Fourier Transform (FFT) period (TFFT = 1 / ΔF)Guard Interval (GI) Duration (TGI = TFFT / 4)L-SIG duration (TSIGNAL = TGI + TFFT)
20, 40, 80, and 160312.53.2 μs0.8 μs4 μs
10156.256.4 μs1.6 μs8 μs
578.12512.8 μs3.2 μs16 μs

The L-SIG contains packet information for the received configuration,

  • Bits 0 through 3 specify the data rate (modulation and coding rate) for the non-HT format.

    Rate (bits 0–3)Modulation

    Coding rate (R)

    Data Rate (Mb/s)
    20 MHz channel bandwidth10 MHz channel bandwidth5 MHz channel bandwidth
    1101BPSK1/2631.5
    1111BPSK3/494.52.25
    0101QPSK1/21263
    0111QPSK3/41894.5
    100116-QAM1/224126
    101116-QAM3/436189
    000164-QAM2/3482412
    001164-QAM3/4542713.5

    For HT and VHT formats, the L-SIG rate bits are set to '1 1 0 1'. Data rate information for HT and VHT formats is signaled in format-specific signaling fields.

  • Bit 4 is reserved for future use.

  • Bits 5 through 16:

    • For non-HT, specify the data length (amount of data transmitted in octets) as described in Table 17-1 and section 10.26.4 IEEE® Std 802.11-2016.

    • For HT-mixed, specify the transmission time as described in sections 19.3.9.3.5 and 10.26.4 of IEEE Std 802.11-2016.

    • For VHT, specify the transmission time as described in section 21.3.8.2.4 of IEEE Std 802.11-2016.

  • Bit 17 has the even parity of bits 0 through 16.

  • Bits 18 through 23 contain all zeros for the signal tail bits.

Note

Signaling fields added for HT (wlanHTSIG) and VHT (wlanVHTSIGA, wlanVHTSIGB) formats provide data rate and configuration information for those formats.

  • For the HT-mixed format, section 19.3.9.4.3 of IEEE Std 802.11-2016 describes HT-SIG bit settings.

  • For the VHT format, sections 21.3.8.3.3 and 21.3.8.3.6 of IEEE Std 802.11-2016 describe bit settings for the VHT-SIG-A and VHT-SIG-B fields, respectively.

Algorithms

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L-SIG Processing

The L-SIG follows the L-STF and L-LTF of the preamble in the packet structure.

For L-SIG transmission processing algorithm details, see:

  • VHT format – refer to IEEE Std 802.11ac™-2013 [1], Section 22.3.8.2.4

  • HT format – refer to IEEE Std 802.11-2012 [2], Sections 20.3.9.3.5

  • non-HT format – refer to IEEE Std 802.11-2012 [2], Sections 18.3.4

The wlanLSIG function performs transmitter processing on the L-SIG field and outputs the time-domain waveform.

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™-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.

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.