1. Main features
  2. PHY improvements
  3. Quick Score Card
  4. MU Frame Format
  5. Operating in the 6Ghz Frequency
The IEEE 802.11ax standard scope defines standardized modifications to both the IEEE 802.11 physical layers (PHY) and the IEEE 802.11 Medium Access Control layer (MAC) that enable at least one mode of operation capable of supporting at least four times improvement in the average throughput per station (measured at the MAC data service access point) in a dense deployment scenario, while maintaining or improving the power efficiency per station.
This amendment defines operations in frequency bands between 1 GHz and 7.125 GHz. The new amendment shall enable backward compatibility and coexistence with legacy IEEE 802.11 devices operating in the same band.
In December 2017 the IEEE-SA NesCom approved 802.11ax PAR Modification to include operation in the 6 GHz band

802.11ax Main Features

  • The use of Orthogonal Frequency Division Multiple Access (OFDMA)
    • Allows the multiplexing of multiple users in the frequency domain.
    • A departure from the use of the OFDM where all resources are assigned to a single user as in previous IEEE 802.11 amendments.
  • Support of OFDMA for the Uplink (UL) and the Downlink (DL)
    • Supporting Triggered UL MU MIMO
    • DL MU MIMO support is already in IEEE 802.11ac.
    • Allows multiplexing of multiple users in the spatial domain
  • The use of 256 FFT (20 MHz Channel) for the data portion of the 802.11ax PPDU.
    • A departure from the 64 FFT used in previous IEEE 802.11 amendments.
  • Pre-defined resource unit (RU) sizes
  • Four frame formats
  • Allows Spatial Reuse
  • MCS 10 and MCS 11 introducing 1024 QAM

802.11ax PHY improvements

802.11ax frame format
802.11ax frame format
  • As in IEEE 802.11n/ac, HEW PPDU starts with a legacy preamble for backward compatibility
    • Legacy preamble is duplicated on every 20 MHz channel.
    • L-Preamble consists of L-STF, L-LTF, and L-SIG.
  • Repeated L-SIG (RL-SIG) is included for auto-detection.
  • HE-SIG-A is two-symbol long and is duplicated on every 20 MHz channel.
    • HE-SIG-A is available in every PPDU.
  • HE-SIG-B is of variable length. It includes resource allocation information.
    • HE-SIG-B is only present in the MU PPDU.
  • HE-Data uses DFT period of 12.8 msec and subcarrier spacing of 78.125 KHz.
  • Tone plan allowing 26-tone, 52-tone, 106-tone, 242-tone for OFDMA. 484-tone and 996-tone for non-OFDMA cases.
  • Mandatory support for LDPC coding in HE PPDU Data field for allocation sizes of 484 tones, 996 tones and 996*2 tones.
  • 1024-QAM is an optional feature for SU and MU using resource units equal to or larger than 242 tones in 11ax.
  • Dual sub-carrier modulation (DCM) is an optional modulation scheme for the HE-SIG-B and Data fields. DCM is only applied to BPSK, QPSK and 16-QAM modulations

802.11ax Score-Card

Channel Bandwidth20, 40, 80, and 160 MHz20, 40, 80, and 160 MHz20, 40 MHzSame Channel Bandwidth as in Wi-Fi 5
WaveformOFDMAOFDMOFDMAchieve multiplexing gain and per User focus
Band2.4, 5, and 6 GHz5 GHz2.4 and 5 GHzMake use of the large spectrum available in the 6 GHz band
Number of Antennas884Same as in Wi-Fi 5
Power Save
Target Wake up Time (TWT)NoNoEfficient support of devices with power constraints
Aggregate Data Rate9.6 Gbps6.9 Gbps600 MbpsModest rate increase compared to Wi-Fi 5
User Experience4x improvement??Focus is on user experience and per user throughput.
20 MHz-only operationYesNoYesAllows support of IoT applications and eventual replacement of Wi-Fi 4
Spatial ReuseYesNoNoEfficient use of available spectrum
MU MIMODL MU MIMO and Triggered UL MIMODL MU MIMONoAccess point schedules users based on their traffic requirements, e.g. buffer size and delay requirements.
OutdoorImproved supportNoNoImproved outdoor performance  for open stadiums and hot spots.
QAM1024 QAM256 QAM64 QAMImproved throughput by packing more signals in the same space.

MU Frame Format

SIG-A-1 details

BitField Name
B5-B10BSS Color
B11-B14Spatial Reuse
B18-B21Number of Sig-B symbols or MU-MIMO Users
B22SIG-B Compression
B23-B24GI+LTF Size

SIG-A-2 details

BitField Name
B0-B6TXOP Duration
B8-B10Number of LTF Symbols
B11LDPC Extra Symbol
B13-B14Pre-FEC padding Factor
B15PE Disambiguity

802.11ax works in the 6Ghz Frequency

•Starting frequency of 5940 MHz
• Only 10 MHz of Guard band for U-NII-5
•Challenging filter design
•Channels can cross U-NII boundaries
•In case U-NII-5 and 6 work under different regulatory rules
•No 80 MHz channel in U-NII-6
•Only one 40 MHz channel in U-NII-6
•An HE STA indicated its capability to operate in the 6 GHz band
•An HE AP operating in the 6 GHz band shall indicate support for at least 80 MHz channel width 
•A STA shall not transmit an HT PPDU (802.11n) in the 6 GHz band. A STA shall not transmit a VHT PPDU (802.11ac) in the 6 GHz band. A STA shall not transmit a DSSS, HR/DSSS (802.11b), or ERP-OFDM (802.11g) PPDU in the 6 GHz band. 
•An HE AP may transmit an HE SU beacon in the 6 GHz band. 
•Rules are defined for passive and active scanning and out-of-band discovery (for APs in the 2.4 and 5 Ghz and collocated with AP in the 6 GHz).

Other resources

Roaming with 802.11k/v/r
Listen Interval and DTIM
CWAP Study Guide

More info about the 802.11 Protocol Analysis here

More info about the 802.11 standard here

More info about the God of the Bible here


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Fernando Rivasplata

Saved by grace, husband, father and very passionate about WIFI and Network automation technologies.