WLAN 802.11ax Frame Structure and PPDU Formats

This page describes the WLAN 802.11ax frame structure and PPDU formats as defined in the IEEE 802.11ax standard. The 802.11ax frame structure consists of various fields: preamble, header, and data.

Introduction

The 802.11ax standard is the latest in the IEEE 802.11 WLAN series. The 802.11ax devices are backward compatible with legacy Wi-Fi devices as per the 802.11a/g/n/ac standards operating in the same band.

Unlike 802.11ac, 802.11ax operates in both the 2.4 GHz and 5 GHz bands. Moreover, many new features have been introduced in this WLAN standard, such as:

• MU-OFDMA for high efficiency (HE)
• 8x8 MU-MIMO for high capacity
• Contention-less uplink scheduling for power savings
• BSS coloring to avoid CCI

Higher modulation schemes such as 1024-QAM have been introduced to improve throughput.

WLAN 802.11ax Frame Structure

802.11ax Frame Details

Let’s delve into the 802.11ax frame. Its structure is similar to 802.11n and 802.11ac, consisting of a preamble, a header, and data. The figure above depicts both single-user and multi-user frame structures. Let’s understand the various WLAN 802.11ax frame fields.

• Preamble: It starts with the preamble. The first part of the preamble consists of legacy (non-HE) training fields, and the second part consists of HE preamble fields. The legacy portion of the preamble contains L-STF (Legacy, i.e., non-HT short training field), L-LTF (legacy long training field), and L-SIG (legacy signal field). This legacy part is decoded by legacy devices. It is included for backward compatibility and coexistence with legacy Wi-Fi devices. The RL-SIG field is used as a repeated legacy (non-HT) Signal field.

  The HE preamble can be decoded by 802.11ax devices only.

• HE Preamble: The HE preamble contains HE-STF and HE-LTF patterns.

• HE Header: The HE header usually consists of HE SIG-A and HE SIG-B fields. HE SIG-A contains information about the packet to follow, both in downlink and uplink, MCS rate, modulation, BSS color, BW, spatial stream, remaining time in transmit opportunity, etc. HE SIG-B is included only for multi-user packets.

• HE-Data: The HE-Data field carries PSDU(s).

• Packet Extension: Max. packet extension modes of duration, either 8 µs or 16 µs, are used at the end of the 802.11ax frame.

802.11ax PPDU Formats

IEEE 802.11ax defines four different transmit modes, i.e., (PLCP) protocol data unit (PPDU) formats, as described below.

• HE SU PPDU format: This is used when transmitting to a single user.

• HE extended range SU PPDU format: This is used when transmitting to a single user, but further away from the Access Point (AP), such as in an outdoor scenario.

• HE MU PPDU format: This is used when transmitting to one or more users. It is similar to the SU format, except that an HE-SIG-B field is present.

• HE trigger-based PPDU format: This is used for uplink OFDMA and/or MU-MIMO transmission. It carries a single transmission and is sent as an immediate response to a Trigger frame sent by the AP.

Source: Keysight Technologies

11ac frame structure