HT 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:

HT to specify a high-throughput (HT) PHY implementation.
- HT refers to preamble fields formatted for association with 802.11n™ data. Section 19 of [1] defines and describes the HT PHY layer and PPDU. The standard defines two HT formats:
  - HT_MF indicates the HT-mixed format and contains a preamble compatible with HT and non-HT receivers. Support for HT-mixed format is mandatory.
  - HT_GF indicates the HT-greenfield format and does not contain a non-HT compatible part. WLAN Toolbox™ does not support HT_GF format.

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 HT PPDUs and waveforms, use the wlanHTConfig object.

HT 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 an HT-mixed 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
HT-SIG	HT SIGNAL field
HT-STF	HT Short Training field
HT-LTF	HT Long Training field, multiple HT-LTFs are transmitted as indicated by the modulation and coding scheme (MCS)
Data	The HT Data field includes the service bits, PSDU, tail bits, and pad bits

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

HT Preamble Fields

The high throughput signal (HT-SIG) field is located between the L-SIG field and HT-STF and is part of the HT-mixed format preamble. It is composed of two symbols, HT-SIG₁ and HT-SIG₂.

The HT-SIG field in the HT-mixed preamble

HT-SIG carries information used to decode the HT packet, including the MCS, packet length, FEC coding type, guard interval, number of extension spatial streams, and whether there is payload aggregation. The HT-SIG symbols are also used for auto-detection between HT-mixed format and legacy OFDM packets.

Packet structure of HT-SIG1 and HT-SIG2

For a detailed description of the HT-SIG field, see section 19.3.9.4.3 of IEEE Std 802.11-2020.

The high throughput short training field (HT-STF) is located between the HT-SIG and HT-LTF fields of an HT-mixed packet. The HT-STF is 4 μs in length and is used to improve automatic gain control estimation for a MIMO system. For a 20 MHz transmission, the frequency sequence used to construct the HT-STF is identical to that of the L-STF. For a 40 MHz transmission, the upper subcarriers of the HT-STF are constructed from a frequency-shifted and phase-rotated version of the L-STF.

The HT-STF in an HT-mixed packet

The high throughput long training field (HT-LTF) is located between the HT-STF and data field of an HT-mixed packet.

As described in Section 19.3.9.4.6 of IEEE Std 802.11-2016, the receiver can use the HT-LTF to estimate the MIMO channel between the set of QAM mapper outputs (or, if STBC is applied, the STBC encoder outputs) and the receive chains. The HT-LTF portion has one or two parts. The first part consists of one, two, or four HT-LTFs that are necessary for demodulation of the HT-Data portion of the PPDU. These HT-LTFs are referred to as HT-DLTFs. The optional second part consists of zero, one, two, or four HT-LTFs that can be used to sound extra spatial dimensions of the MIMO channel not utilized by the HT-Data portion of the PPDU. These HT-LTFs are referred to as HT-ELTFs. Each HT long training symbol is 4 μs. The number of space-time streams and the number of extension streams determines the number of HT-LTF symbols transmitted.

Tables 19-12, 19-13 and 90-14 from IEEE Std 802.11-2012 are reproduced here.

N_STS Determination N_HTDLTF Determination N_HTELTF Determination

Table 19-12 defines the number of space-time streams (N_STS) based on the number of spatial streams (N_SS) from the MCS and the STBC field.

Table 19-13 defines the number of HT-DLTFs required for the N_STS.

Table 19-14 defines the number of HT-ELTFs required for the number of extension spatial streams (N_ESS). N_ESS is defined in HT-SIG₂.

N_SS from MCS	STBC field	N_STS
`1`	`0`	`1`
`1`	`1`	`2`
`2`	`0`	`2`
`2`	`1`	`3`
`2`	`2`	`4`
`3`	`0`	`3`
`3`	`1`	`4`
`4`	`0`	`4`

N_STS	N_HTDLTF
`1`	`1`
`2`	`2`
`3`	`4`
`4`	`4`

N_ESS	N_HTELTF
`0`	`0`
`1`	`1`
`2`	`2`
`3`	`4`

Additional constraints include:

N_HTLTF = N_HTDLTF + N_HTELTF ≤ 5.
N_STS + N_ESS ≤ 4.
- When N_STS = 3, N_ESS cannot exceed one.
- If N_ESS = 1 when N_STS = 3 then N_HTLTF = 5.

HT Data Field

The HT-Data field follows the last HT-long training field (HT-LTF) of an HT-mixed packet.

The HT-Data field in an HT-mixed packet

The HT-Data field carries one or more frames from the medium access control (MAC) layer and consists of four subfields.

Service — Contains 16 zeros to initialize the data scrambler
PSDU — Variable-length field containing a PLCP service data unit (PSDU)
Tail — Contains six zeros for each encoding stream, required to terminate a convolutional code
Pad Bits — Variable-length field required to ensure that the HT-Data field consists of an integer number of symbols

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.