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wlanVHTSIGA

Generate VHT-SIG-A waveform

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Syntax

y = wlanVHTSIGA(cfg)
[y,bits] = wlanVHTSIGA(cfg)
[___] = wlanVHTSIGA(cfg,OversamplingFactor=osf)

Description

y = wlanVHTSIGA(cfg) generates a VHT-SIG-A1 time-domain waveform for the specified transmission parameters. See VHT-SIG-A Processing for waveform generation details.

example

[y,bits] = wlanVHTSIGA(cfg) also outputs VHT-SIG-A information bits.

[___] = wlanVHTSIGA(cfg,OversamplingFactor=osf) generates an oversampled waveform with the specified oversampling factor. For more information about oversampling, see FFT-Based Oversampling.

Examples

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Open Live Script

Create a VHT configuration object, specifying an 80 MHz channel bandwidth. 

cfgVHT = wlanVHTConfig(ChannelBandwidth="CBW80");
cfgVHT.ChannelBandwidth = 'CBW80';

Generate a waveform for the VHT-SIG-A field.

y = wlanVHTSIGA(cfgVHT);
size(y)
ans = 1×2

   640     1

The 80 MHz waveform has two OFDM symbols and is a total of 640 samples long. Each symbol contains 320 samples.

Open Live Script

Create a VHT configuration object, specifying a 40 MHz channel bandwidth.

cfgVHT = wlanVHTConfig(ChannelBandwidth="CBW40");

Generate the VHT-SIG-A waveform and information bits.

[y,bits] = wlanVHTSIGA(cfgVHT);

Extract the bandwidth from the returned bits and analyze. The bandwidth information is contained in the first two bits.

bwBits = bits(1:2);
bit2int(bwBits,2,false)
ans = int8

1

As defined in IEEE Std 802.11ac-2013, Table 22-12, a value of '1' corresponds to a 40 MHz channel bandwidth.

Input Arguments

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Transmission parameters, specified as a wlanVHTConfig 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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VHT-SIG-A 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
'CBW20'160
'CBW40'320
'CBW80'640
'CBW160'1280

See VHT-SIG-A Processing for waveform generation details.

Data Types: double
Complex Number Support: Yes

Signaling bits used for the VHT-SIG-A, returned as a 48-bit column vector.

Data Types: int8

More About

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VHT-SIG-A

The very high throughput signal A (VHT-SIG-A) field contains information required to interpret VHT format packets. Similar to the non-HT signal (L-SIG) field for the non-HT OFDM format, this field stores the actual rate value, channel coding, guard interval, MIMO scheme, and other configuration details for the VHT format packet. Unlike the HT-SIG field, this field does not store the packet length information. Packet length information is derived from L-SIG and is captured in the VHT-SIG-B field for the VHT format.

For a detailed description of the VHT-SIG-A field, see Section 21.3.8.3.3 of IEEE® Std 802.11™-2016. The VHT-SIG-A field consists of two symbols: VHT-SIG-A1 and VHT-SIG-A2. These symbols are located between the L-SIG and the VHT-STF portion of the VHT format PPDU.

The VHT-SIG-A field in a VHT packet

The VHT-SIG-A field includes these components. The bit field structures for VHT-SIG-A1 and VHT-SIG-A2 vary for single user or multi-user transmissions.

  • BW — A two-bit field that indicates 0 for 20 MHz, 1 for 40 MHz, 2 for 80 MHz, or 3 for 160 MHz.

  • STBC — A bit that indicates the presence of space-time block coding.

  • Group ID — A six-bit field that indicates the group and user position assigned to a STA.

  • NSTS — A three-bit field for a single user or 4 three-bit fields for a multi-user scenario, that indicates the number of space-time streams per user.

  • Partial AID — An identifier that combines the association ID and the BSSID.

  • TXOP_PS_NOT_ALLOWED — An indicator bit that shows whether client devices are allowed to enter dose state. This bit is set to false when the VHT-SIG-A structure is populated, indicating that the client device is allowed to enter dose state.

  • Short GI — A bit that indicates use of the 400 ns guard interval.

  • Short GI NSYM Disambiguation — A bit that indicates if an extra symbol is required when the short GI is used.

  • SU/MU[0] Coding — A bit field that indicates if convolutional or LDPC coding is used for a single user or for user MU[0] in a multi-user scenario.

  • LDPC Extra OFDM Symbol — A bit that indicates if an extra OFDM symbol is required to transmit the data field.

  • MCS — A four-bit field.

    • For a single user scenario, it indicates the modulation and coding scheme used.

    • For a multi-user scenario, it indicates the use of convolutional or LDPC coding and the MCS setting is conveyed in the VHT-SIG-B field.

  • Beamformed — An indicator bit set to 1 when a beamforming matrix is applied to the transmission.

  • CRC — An eight-bit field used to detect errors in the VHT-SIG-A transmission.

  • Tail — A six-bit field used to terminate the convolutional code.

Algorithms

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VHT-SIG-A Processing

The VHT-SIG-A field includes information required to process VHT format packets.

For algorithm details, refer to IEEE Std 802.11ac™-2013 [1], Section 22.3.4.5. The wlanVHTSIGA function performs transmitter processing on the VHT-SIG-A field and outputs the time-domain waveform.

VHT-SIG-A processing steps and workflow at the transmitter

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 the guardband width relative to the total signal bandwidth, which increases 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 made up of 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™.

Version History

Introduced in R2015b

See Also

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1 IEEE Std 802.11ac-2013 Adapted and reprinted with permission from IEEE. Copyright IEEE 2013. All rights reserved.