WLAN PPDU Structure

Physical Layer Protocol Data Unit

IEEE^® 802.11™¹ is a packet-based protocol. Each physical layer protocol data unit (PPDU) contains preamble and data fields. The preamble field contains 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:

• EHT to specify an extremely high-throughput (EHT) physical layer (PHY) implementation.

  • EHT refers to fields formatted for association with 802.11be™ data. Reference [4] defines and describes the EHT PHY layer and PPDU.

  • For EHT, the TXVECTOR parameters, as defined in Table 36-1 of [4], determine the structure of PPDUs transmitted by an EHT STA.

• WUR to specify a wake-up radio (WUR) PHY implementation.

  • WUR refers to fields formatted for association with 802.11ba™ data. Reference [5] defines and describes the WUR PHY layer and PPDU.

  • For WUR, the TXVECTOR parameters, as defined in Table 30-1 of [5], determine the structure of PPDUs transmitted by a WUR STA.

• HE to specify a high-efficiency (HE) PHY implementation.

  • HE refers to fields formatted for association with 802.11ax™ data. Reference [1] defines and describes the HE PHY layer and PPDU.

  • For HE, the TXVECTOR parameters, as defined in Table 27-1 of [1], determine the structure of PPDUs transmitted by an HE STA.

• DMG to specify a directional multi-gigabit (DMG) PHY implementation.

  • DMG refers to preamble fields formatted for association with 802.11ad™ data. Section 20 of [2] define and describe the DMG PHY layer and PPDU.

  • For DMG, the TXVECTOR parameters, as defined in Table 20-1 of [2], determines the structure of PPDUs transmitted by a DMG STA. For a DMG STA, the MCS parameter determines the overall structure of the DMG PPDU.

• S1G to specify a sub-1-GHz (S1G) PHY implementation.

  • S1G refers to preamble fields formatted for association with 802.11ah™ data. Section 23 of [2] defines and describes the S1G PHY layer and PPDU.

  • For S1G, the TXVECTOR parameters, as defined in Table 23-1 of [2], determines the structure of PPDUs transmitted by an S1G STA. For an S1G STA, the FORMAT parameter determines the overall structure of the S1G PPDU.

• 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 [2] defines and describes the VHT PHY layer and PPDU.

  • For VHT, the TXVECTOR parameters, as defined in Table 21-1 of [2], determine the structure of PPDUs transmitted by a VHT STA. For a VHT STA, the FORMAT parameter determines the overall structure of the PPDU and enables:

    • Non-HT format (NON_HT), based on Section 17 and including non-HT duplicate format.

    • HT-mixed format (HT_MF), as specified in Section 19.

    • HT-greenfield format (HT_GF), as specified in Section 19. WLAN Toolbox™ does not support HT_GF format.

    • VHT format (VHT), as specified in Section 21. The VHT format PPDUs contain a preamble compatible with Section 17 and Section 19 STAs. The non-VHT portions of the VHT preamble (the parts that precede the VHT-SIG-A field) are defined to enable decoding of the PPDU by VHT STAs.

• HT to specify a high-throughput (HT) PHY implementation.

  • HT refers to preamble fields formatted for association with 802.11n™ data. Section 19 of [2] 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.

• non-HT to specify a PHY implementation that is not HT and is not VHT.

  • Non-HT refers to preamble fields formatted for association with pre-802.11n data. Section 17 of [2] defines and describes the OFDM PHY layer and PPDU for non-HT transmission. In addition to supporting non-HT synchronization, the non-HT preamble fields are used in support of HT and VHT synchronization.

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

802.11 Version	Transmission Vector Format	Modulation Format	Bandwidths/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
	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

WLAN Toolbox configuration objects define the properties that enable creation of PPDUs and waveforms for the specified 802.11 transmission format. See wlanEHTTBConfig, wlanEHTMUConfig, wlanWURConfig, wlanHEMUConfig, wlanHESUConfig, wlanDMGConfig, wlanS1GConfig, wlanVHTConfig, wlanHTConfig, and wlanNonHTConfig.

EHT PPDU Field Structure

For EHT, there are two transmission modes: EHT multi-user (EHT MU) and EHT trigger-based (EHT TB). WLAN Toolbox supports both.

The field structure for EHT PPDUs consists of preamble and data portions. The legacy preamble fields (L-STF, L-LTF, and L-SIG) are present in EHT, WUR, HE, VHT, and non-HT PPDUs. The RL-SIG and PE fields are common to EHT and HE PPDUs. The structure of an EHT MU PPDU is summarized in this table.

PPDU Field Abbreviation	Description	Duration (µs)
L-STF	Non-HT Short Training field	8
L-LTF	Non-HT Long Training field	8
L-SIG	Non-HT Signal field	4
RL-SIG	Repeated Non-HT Signal field	4
U-SIG	Universal Signal field	8
EHT-SIG	EHT Signal field	4
EHT-STF	EHT Short Training field	4
EHT-LTF	EHT Long Training field	Variable
EHT-Data	Data field carrying the PSDUs	Variable
PE	Packet Extension field	Variable

The structure of an EHT TB PPDU is the same, except that the EHT-SIG field is not present and the EHT-STF has double the duration. For more information, see Section 36.3.4 of [4].

WUR PPDU Field Structure

For WUR, there are two transmission modes: WUR Basic and WUR FDMA. WUR Basic transmissions must have a bandwidth of 20 MHz. WUR FDMA transmissions must have a bandwidth of 40 or 80 MHz. The field structure for WUR PPDUs consists of preamble and data portions.

The structure of a WUR Basic PPDU is summarized in this table.

PPDU Field Abbreviation	Description	Duration (µs)
L-STF	Non-HT Short Training field	8
L-LTF	Non-HT Long Training field	8
L-SIG	Non-HT Signal field	4
BPSK-Mark1	A BPSK modulated OFDM symbol	4
BPSK-Mark2	A BPSK modulated OFDM symbol	4
WUR-Sync	WUR Synchronization field	64 or 128
WUR-Data	WUR Data field	Variable

The preamble consists of all the fields up to and including the BPSK-Mark2 field.

WUR FDMA PPDUs are divided into 20 MHz subchannels. Each subchannel has its own preamble. The preamble of each subchannel has the same structure as the preamble of a WUR Basic PPDU. Within a PPDU, each 20 MHz subchannel has the same values in its preamble fields. The values of the two non-preamble fields can differ between subchannels.

For more information, see Sections 30.3.2 and 30.3.3 of [5].

HE PPDU Field Structure

In HE, four transmission modes are supported. The field structure for HE PPDUs consists of preamble and data portions. The legacy preamble fields (L-STF, L-LTF, and L-SIG) are common to all four HE transmission modes and the VHT, HT, and non-HT format preambles.

HE preamble fields include additional format-specific signaling fields. Each format defines a data field for transmission of user payload data.

PPDU Field Abbreviation	Description
L-STF	Non-HT Short Training field
L-LTF	Non-HT Long Training field
L-SIG	Non-HT Signal field
RL-SIG	Repeated Non-HT Signal field
HE-SIG-A	HE Signal A field
HE-SIG-B	HE Signal B field
HE-STF	HE Short Training field
HE-LTF	HE Long Training field
HE-Data	Data field carrying the PSDUs
PE	Packet Extension field

The RL-SIG, HE-SIG-A, HE-STF, HE-LTF, and PE fields are present in all HE PPDU formats. The HE-SIG-B field is present only in the HE MU PPDU. For more information, see Section 27.3.4 of [1].

DMG Format PPDU Field Structure

In DMG, there are three physical layer (PHY) modulation schemes supported: control, single carrier, and OFDM.

The single-carrier chip timing, T_C = 1/F_C = 0.57 ns. For more information, see Waveform Sampling Rate on the wlanWaveformGenerator function reference page.

The supported DMG format PPDU field structures each contain these fields:

• The preamble contains a short training field (STF) and channel estimation field (CEF). The preamble is used for packet detection, AGC, frequency offset estimation, synchronization, indication of modulation type (Control, SC, or OFDM), and channel estimation. The format of the preamble is common to the Control, SC, and OFDM PHY packets.

  • The STF is composed of Golay Ga sequences as specified in Section 20.3.6.2 of [2].

  • The CEF is composed of Golay Gu and Gv sequences as specified in Section 20.3.6.2 of [2].

    • When the header and data fields of the packet are modulated using a single carrier (control PHY and SC PHY), the Golay sequencing for the CEF waveform is shown in Figure 20-6 of [2].

    • When the header and data fields of the packet are modulated using OFDM (OFDM PHY), the Golay sequencing for the CEF waveform is shown in Figure 20-7 of [2].

• The header field is decoded by the receiver to determine transmission parameters.

• The data field is variable in length. It carries the user data payload.

• The training fields (AGC and TRN-R/T subfields) are optional. They can be included to refine beamforming.

Section 20.3 of [2] specifies the common aspects of the DMG PPDU packet structure. The PHY modulation-specific aspects of the packet structure are specified in these sections:

• The DMG control PHY packet structure is specified in Section 20.4.

• The DMG OFDM PHY packet structure is specified in Section 20.5.

• The DMG SC PHY packet structure is specified in Section 20.6.

S1G Format PPDU Field Structure

In S1G, there are three transmission modes:

• ≥2 MHz long preamble mode

• ≥2 MHz short preamble mode

• 1 MHz mode

Each transmission mode has a specific PPDU preamble structure:

• An S1G ≥2 MHz long preamble mode PPDU supports single-user and multi-user transmissions. The long preamble PPDU consists of two portions; the omni-directional portion and the beam-changeable portion.

  

  • The omni-directional portion is transmitted to all users without beamforming. It consists of three fields:

    • The short training field (STF) is used for coarse synchronization.

    • The long training field (LTF1) is used for fine synchronization and initial channel estimation.

    • The signal A field (SIG-A) is decoded by the receiver to determine transmission parameters relevant to all users.

  • The data portion can be beamformed to each user. It consists of four fields:

    • The beamformed short training field (D-STF) is used by the receiver for automatic gain control.

    • The beamformed long training fields (D-LTF-N) are used for MIMO channel estimation.

    • The signal B field (SIG-B) in a multi-user transmission, signals the MCS for each user. In a single-user transmission, the MCS is signaled in the SIG-A field of the omni-directional portion of the preamble. Therefore, in a single-user transmission the SIG-B symbol transmitted is an exact repetition of the first D-LTF. This repetition allows for improved channel estimation.

    • The data field is variable in length. It carries the user data payload.

• An S1G ≥2 MHz short preamble mode PPDU supports single-user transmissions. All fields in the PPDU can be beamformed.

  

  The PPDU consists of five fields:

  • The short training field (STF) is used for coarse synchronization.

  • The first long training field (LTF1) is used for fine synchronization and initial channel estimation.

  • The signaling field (SIG) is decoded by the receiver to determine transmission parameters.

  • The subsequent long training fields (LTF2-N) are used for MIMO channel estimation. N_SYMBOLS = 1 per subsequent LTF

  • The data field is variable in length. It carries the user data payload.

• An S1G 1 MHz mode PPDU supports single-user transmissions. It is composed of the same five fields as the S1G ≥2 MHz short preamble mode PPDU and all fields can be beamformed. An S1G 1 MHz mode PPDU has longer STF, LTF1, and SIG fields, so this mode can achieve sensitivity that is similar to the S1G ≥2 MHz short-preamble mode transmissions.

  

VHT, HT-Mixed, and Non-HT Format PPDU Field Structures

The field structure for VHT, HT, and non-HT PPDUs consist of preamble and data portions. The legacy preamble fields (L-STF, L-LTF, and L-SIG) are common to VHT, HT, and non-HT format preambles. VHT and HT format preamble fields include additional format-specific training and signaling fields. Each format defines a data field for transmission of user payload data.

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 MCS
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	VHT, HT, and non-HT Data fields include the service bits, PSDU, tail bits, and pad bits

For more information, see Section 19.3.2 of [2].

Non-HT (Legacy) Short Training Field

The legacy short training field (L-STF) is the first field of the 802.11 OFDM PLCP legacy preamble. The L-STF is a component of EHT, HE, VHT, HT, and non-HT PPDUs.

The L-STF duration varies with channel bandwidth.

Channel Bandwidth (MHz)	Subcarrier Frequency Spacing, ΔF (kHz)	Fast Fourier Transform (FFT) Period (TFFT = 1 / ΔF)	L-STF Duration (TSHORT = 10 × TFFT / 4)
20, 40, 80, 160, and 320	312.5	3.2 μs	8 μs
10	156.25	6.4 μs	16 μs
5	78.125	12.8 μs	32 μs

Because the sequence has good correlation properties, it is used for start-of-packet detection, for coarse frequency correction, and for setting the AGC. The sequence uses 12 of the 52 subcarriers that are available per 20 MHz channel bandwidth segment. For 5 MHz, 10 MHz, and 20 MHz bandwidths, the number of channel bandwidths segments is 1.

Non-HT (Legacy) Long Training Field

The L-LTF is the second field in the 802.11 OFDM PLCP legacy preamble. The L-LTF is a component of EHT, HE, 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 320	312.5	3.2 μs	1.6 μs	8 μs
10	156.25	6.4 μs	3.2 μs	16 μs
5	78.125	12.8 μs	6.4 μs	32 μs

Non-HT (Legacy) Signal Field

The L-SIG is the third field of the 802.11 OFDM PLCP legacy preamble. This field is a component of EHT, HE, VHT, HT, and non-HT PPDUs. It consists of 24 bits that contain rate, length, and parity information. The L-SIG field 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 160	312.5	3.2 μs	0.8 μs	4 μs
10	156.25	6.4 μs	1.6 μs	8 μs
5	78.125	12.8 μs	3.2 μs	16 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 Bandwidth	10 MHz Channel Bandwidth	5 MHz Channel Bandwidth
  1101	BPSK	1/2	6	3	1.5
  1111	BPSK	3/4	9	4.5	2.25
  0101	QPSK	1/2	12	6	3
  0111	QPSK	3/4	18	9	4.5
  1001	16-QAM	1/2	24	12	6
  1011	16-QAM	3/4	36	18	9
  0001	64-QAM	2/3	48	24	12
  0011	64-QAM	3/4	54	27	13.5

  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 formats, specify the data length (amount of data transmitted in octets) as described in Table 17-1 and Section 10.27.4 IEEE Std 802.11-2020.

  • For HT-mixed formats, specify the transmission time as described in Sections 19.3.9.3.5 and 10.27.4 of IEEE Std 802.11-2020.

  • For VHT formats, specify the transmission time as described in Section 21.3.8.2.4 of IEEE Std 802.11-2020.

• 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 802.11-2020 describes HT-SIG bit settings.

• For the VHT format, Sections 21.3.8.3.3 and 21.3.8.3.6 of IEEE Std 802.11-2020 describe bit settings for the VHT-SIG-A and VHT-SIG-B fields, respectively.

Non-HT Data Field

The non-high throughput Data (non-HT Data) field is used to transmit MAC frames and is composed of a service field, a PSDU, tail bits, and pad bits.

• Service field — Contains 16 zeros to initialize the data scrambler.

• PSDU — Variable-length field containing the PLCP service data unit (PSDU).

• Tail — Tail bits required to terminate a convolutional code. The field uses six zeros for the single encoding stream.

• Pad Bits — Variable-length field required to ensure that the non-HT data field contains an integer number of symbols.

Processing of an 802.11a data field is defined in section 17.3.5 of [2].

The six tail bits are set to zero after a 127-bit scrambling sequence has been applied to the full data field. The receiver uses the first seven bits of the service field to determine the initial state of the scrambler. Rate 1/2 BCC encoding is performed on the scrambled data. The zeroed tail bits cause the BCC encoder to return to a zero state. Puncturing is applied as needed for the selected rate.

The coded data is grouped into several bits per symbol, and two permutations of block interleaving are applied to each group of data. The groups of bits are then modulated to the selected rate (BPSK, QPSK, 16-QAM, or 64-QAM) and the complex symbols are then mapped onto corresponding subcarriers. For each symbol, the pilot subcarriers are inserted. An IFFT is used to transform each symbol group to the time domain and the cyclic prefix is prepended.

The final processing preceding DAC up-conversion to RF and the power amplifier is to apply a pulse shaping filter on the data to smooth transitions between symbols. The standard provides an example pulse shaping function but does not specifically require one.

High Throughput Signal Field

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.

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.

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

High Throughput Short Training Field

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.

High Throughput Long Training Fields

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.

NSTS Determination	NHTDLTF Determination	NHTELTF Determination
Table 19-12 defines the number of space-time streams (NSTS) based on the number of spatial streams (NSS) from the MCS and the STBC field.	Table 19-13 defines the number of HT-DLTFs required for the NSTS.	Table 19-14 defines the number of HT-ELTFs required for the number of extension spatial streams (NESS). NESS is defined in HT-SIG2.
NSS from MCS STBC field NSTS 1 0 1 1 1 2 2 0 2 2 1 3 2 2 4 3 0 3 3 1 4 4 0 4	NSTS NHTDLTF 1 1 2 2 3 4 4 4	NESS NHTELTF 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 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

Very High Throughput SIG-A Field

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 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 multiuser 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 if 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 multiuser 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.

Very High Throughput Short Training Field

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 the 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.

Very High Throughput Long Training Fields

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

Very High Throughput SIG-B Field

The very high throughput signal B field (VHT-SIG-B) is used for multiuser scenario to set up the data rate and to fine-tune MIMO reception. It is modulated using MCS 0 and is transmitted in a single OFDM symbol.

The VHT-SIG-B field consists of a single OFDM symbol located between the VHT-LTF and the data portion of the VHT format PPDU.

The very high throughput signal B (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 and the assignment depends on whether single user or multiuser scenario in allocated. 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.

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.

• 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 802.11ax-2021 (Amendment to IEEE Std 802.11-2020). “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 1: Enhancements for High Efficiency WLAN.” IEEE Standard for Information Technology — Telecommunications and Information Exchange between Systems. Local and Metropolitan Area Networks — Specific Requirements.

[2] 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.

[3] Perahia, E., and R. Stacey. Next Generation Wireless LANs: 802.11n and 802.11ac. 2nd Edition. United Kingdom: Cambridge University Press, 2013.

[4] IEEE P802.11be/D3.0. “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 8: Enhancements for Extremely High Throughput (EHT).” Draft Standard for Information Technology — Telecommunications and Information Exchange between Systems — Local and Metropolitan Area Networks — Specific Requirements.

[5] IEEE Std 802.11ba-2021. “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 3: Wake-Up Radio Operation.” IEEE Standard for Information Technology — Telecommunications and Information Exchange between Systems. Local and Metropolitan Area Networks — Specific Requirements.