VHT PPDU Structure

IEEE^® 802.11™¹ is a packet-based protocol. Each packet, also called a physical layer protocol data unit (PPDU), contains preamble and data fields. The preamble fields contain the transmission vector format information. The data field contains the user payload and higher layer headers, such as medium access control (MAC) fields and cyclic redundancy check (CRC). The transmission vector format and the PPDU structure vary between 802.11 versions. The transmission vector (TXVECTOR) format parameter is classified as:

VHT to specify a very-high-throughput (VHT) PHY implementation.
- VHT refers to preamble fields formatted for association with 802.11ac™ data. Section 21 of [1] defines and describes the VHT PHY layer and PPDU.

The table shows 802.11 versions that WLAN Toolbox™ supports, along with the supported TXVECTOR options and associated modulation formats.

802.11 Version	Transmission Vector Format	Modulation Format	Bandwidth/MHz
802.11b™	non-HT	DSSS/CCK	11
802.11a™	non-HT	OFDM only	5, 10, 20
802.11j™	non-HT	OFDM only	10
802.11p™	non-HT	OFDM only	5, 10
802.11g™	non-HT	OFDM	20
802.11g™	non-HT	DSSS/CCK	11
802.11n™ (Wi-Fi 4)	HT_MF, Non-HT	OFDM only	20, 40
802.11ac (Wi-Fi 5)	VHT, HT_MF, Non-HT	OFDM only	20, 40, 80, 160
802.11ah™	S1G	OFDM only	1, 2, 4, 8, 16
802.11ad™	DMG	Single Carrier and OFDM	2640
802.11ax™ (Wi-Fi 6)	HE	OFDMA	20, 40, 80, 160
802.11ba™	WUR	MC-OOK	20, 40, 80
802.11be™ (Wi-Fi 7)	EHT	OFDMA	20, 40, 80, 160, 320

To create VHT PPDUs and waveforms, use the wlanVHTConfig object.

VHT PPDUs are made up of preamble and data portions. The legacy preamble fields (L-STF, L-LTF, and L-SIG) are common to non-HT, HT, VHT, HE, and EHT preambles.

The structure of a VHT PPDU

PPDU Field Abbreviation	Description
L-STF	Non-HT Short Training field
L-LTF	Non-HT Long Training field
L-SIG	Non-HT SIGNAL field
VHT-SIG-A	VHT Signal A field
VHT-STF	VHT Short Training field
VHT-LTF	VHT Long Training field
VHT-SIG-B	VHT Signal B field
Data	The VHT Data field includes the service bits, PSDU, tail bits, and pad bits

For information about the legacy fields, see Non-HT (Legacy) Preamble Fields.

VHT Preamble Fields

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.
N_STS — 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.

The very high throughput short training field (VHT-STF) is a single OFDM symbol (4 μs in length) that is used to improve automatic gain control estimation in a MIMO transmission. It is located between the VHT-SIG-A and VHT-LTF portions of a VHT packet.

The VHT-STF in a VHT packet

The frequency domain sequence used to construct the VHT-STF for a 20 MHz transmission is identical to the L-STF sequence. Duplicate L-STF sequences are frequency shifted and phase rotated to support VHT transmissions for the 40 MHz, 80 MHz, and 160 MHz channel bandwidths. As such, the L-STF and HT-STF are subsets of the VHT-STF.

For a detailed description of the VHT-STF, see Section 21.3.8.3.4 of IEEE Std 802.11-2016.

The very high throughput long training field (VHT-LTF) is between the VHT-STF and VHT-SIG-B portion of the VHT packet.

The VHT-LTF in the VHT packet

It is used for MIMO channel estimation and pilot subcarrier tracking. The VHT-LTF includes one VHT long training symbol for each spatial stream indicated by the selected modulation and coding scheme (MCS). Each symbol is 4 μs long. A maximum of eight symbols are permitted in the VHT-LTF.

For a detailed description of the VHT-LTF, see Section 21.3.8.3.5 of IEEE Std 802.11-2016.

Receivers use the very high throughput signal B field (VHT-SIG-B) in multi-user scenarios to set up the data rate and to fine-tune MIMO reception. The field uses MCS 0 and consists of a single OFDM symbol.

The VHT-SIG-B field is located between the VHT-LTF and the data portion of the VHT format PPDU.

The VHT-SIG-B field in a VHT packet

The VHT-SIG-B field contains the actual rate and A-MPDU length value per user. For a detailed description of the VHT-SIG-B field, see Section 21.3.8.3.6 of IEEE Std 802.11-2016. The number of bits in the VHT-SIG-B field varies with the channel bandwidth. The assignment of the bits depends on whether there is a single user or multiple users. For single user configurations, the same information is available in the L-SIG field, but the VHT-SIG-B field is included for continuity purposes.

Field	VHT MU PPDU Allocation (bits)			VHT SU PPDU Allocation (bits)			Description
	20 MHz	40 MHz	80 MHz, 160 MHz	20 MHz	40 MHz	80 MHz, 160 MHz
VHT-SIG-B	B0-15 (16)	B0-16 (17)	B0-18 (19)	B0-16 (17)	B0-18 (19)	B0-20 (21)	A variable-length field that indicates the size of the data payload in four-byte units. The length of the field depends on the channel bandwidth.
VHT-MCS	B16-19 (4)	B17-20 (4)	B19-22 (4)	N/A	N/A	N/A	A four-bit field that is included for multiuser scenarios only.
Reserved	N/A	N/A	N/A	B17–19 (3)	B19-20 (2)	B21-22 (2)	All ones
Tail	B20-25 (6)	B21-26 (6)	B23-28 (6)	B20-25 (6)	B21-26 (6)	B23-28 (6)	Six zero-bits used to terminate the convolutional code.
Total # bits	26	27	29	26	27	29
Bit field repetition	1	2	4 For 160 MHz, the 80 MHz channel is repeated twice.	1	2	4 For 160 MHz, the 80 MHz channel is repeated twice.

For a null data packet (NDP), the VHT-SIG-B bits are set according to Table 21-15 of IEEE Std 802.11-2016.

VHT Data Field

The VHT-Data field carries one or more frames from the medium access control (MAC) layer. This field follows the VHT-SIG-B field in a VHT PPDU.

The VHT-Data field in a VHT PPDU

For a detailed description of the VHT-Data field, see section 21.3.10 of IEEE Std 802.11-2016. The VHT Data field consists of four subfields.

The four subfields of the VHT-Data field

Service field — Contains a seven-bit scrambler initialization state, one bit reserved for future considerations, and eight bits for the VHT-SIG-B cyclic redundancy check (CRC) field
PSDU — Variable-length field containing a PLCP service data unit
PHY Pad — Variable number of bits passed to the transmitter to create a complete OFDM symbol
Tail — Bits required to terminate a convolutional code (not required when the transmission uses LDPC channel coding)

References

[1] IEEE Std 802.11-2020 (Revision of IEEE Std 802.11-2016). “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.