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Non-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:

  • non-HT to specify a PHY implementation that predates the 802.11n™ standard. Non-HT means non-high-throughput.

    • Section 17 of [1] 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, VHT, HE, and EHT synchronization.

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

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™WURMC-OOK20, 40, 80
802.11be™ (Wi-Fi 7)

EHT

OFDMA

20, 40, 80, 160, 320

To create non-HT PPDUs and waveforms, use the wlanNonHTConfig object.

A non-HT PPDU is 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 non-HT format 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

Data

The non-HT Data fields includes the service bits, PSDU, tail bits, and pad bits

Non-HT (Legacy) Preamble Fields

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 320312.53.2 μs8 μs
10156.256.4 μs16 μs
578.12512.8 μs32 μs

Because the sequence has good correlation properties, receivers use it for start-of-packet detection, coarse frequency correction, and 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 bandwidth segments is one.

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 320312.53.2 μs1.6 μs8 μs
10156.256.4 μs3.2 μs16 μs
578.12512.8 μs6.4 μs32 μs

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 uses BPSK modulation with rate 1/2 binary convolutional coding (BCC).

The L-SIG consists of 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, 160, and 320312.53.2 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.

Packet information in the L-SIG

  • 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 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 [1].

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.

1 IEEE Std 802.11-2016 Adapted and reprinted with permission from IEEE. Copyright IEEE 2016. All rights reserved.