WLAN 802.11ax Frame Structure | 802.11ax PPDU formats

This page describesWLAN 802.11ax frame structure和PPDU formatsas defined in IEEE 802.11ax standard. This 11ax frame structure consists of various fields viz. preamble,header and data.

Introduction:
802.11ax standard is latest in the series of IEEE 802.11 WLAN standards. The 11ax devices are backward compatible with legacy wifi devices as per 11a/g/n/ac standards operating in the same band. Unlike 802.11ac, 802.11ax operates in both 2.4GHz and 5GHz bands. Moreover many new features have been introduced in this WLAN standard viz. MU-OFDMA for high efficiency (HE), 8x8 MU-MIMO for high capacity and contentionless uplink scheduling for power savings, BSS coloring to avoid CCI and so on. Higher modulation schemes such as 1024-QAM have been introduced to improve throughput.

WLAN 802.11ax frame structure

Let us understand 802.11ax frame. Its structure is similar to 802.11n and 802.11ac. It consists of preamble, header and data. The figure-1 depicts both single user and multi user frame structures. Let us understand various WLAN 802.11ax frame fields.

➨It starts with the preamble. The first part of the preamble consists of legacy (non-HE) training fields, and 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 the legacy wifi devices. RL-SIG field is used as repeated legacy (non-HT) Signal field. The HE preamble can be decoded by 802.11ax devices only.
➨HE preamble contains HE-STF and HE-LTF patterns.
➨The HE header usually consists of HE SIG-A and HE SIG-B fields. HE SIG-A contains information about 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 field carries PSDU(s).
➨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: It is used when transmitting to a single user.
•HE extended range SU PPDU format: It 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: It is used when transmitting to one or more users. It is similar to SU format, except that an HE-SIG-B field is present.
•HE trigger-based PPDU format: It 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