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
- 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
| 802.11ax | 802.11ac | 802.11n | ||
| Channel Bandwidth | 20, 40, 80, and 160 MHz | 20, 40, 80, and 160 MHz | 20, 40 MHz | Same Channel Bandwidth as in Wi-Fi 5 |
| Waveform | OFDMA | OFDM | OFDM | Achieve multiplexing gain and per User focus |
| Band | 2.4, 5, and 6 GHz | 5 GHz | 2.4 and 5 GHz | Make use of the large spectrum available in the 6 GHz band |
| Number of Antennas | 8 | 8 | 4 | Same as in Wi-Fi 5 |
| Advanced Power Save | Target Wake up Time (TWT) | No | No | Efficient support of devices with power constraints |
| Aggregate Data Rate | 9.6 Gbps | 6.9 Gbps | 600 Mbps | Modest rate increase compared to Wi-Fi 5 |
| User Experience | 4x improvement | ? | ? | Focus is on user experience and per user throughput. |
| 20 MHz-only operation | Yes | No | Yes | Allows support of IoT applications and eventual replacement of Wi-Fi 4 |
| Spatial Reuse | Yes | No | No | Efficient use of available spectrum |
| MU MIMO | DL MU MIMO and Triggered UL MIMO | DL MU MIMO | No | Access point schedules users based on their traffic requirements, e.g. buffer size and delay requirements. |
| Outdoor | Improved support | No | No | Improved outdoor performance for open stadiums and hot spots. |
| QAM | 1024 QAM | 256 QAM | 64 QAM | Improved throughput by packing more signals in the same space. |
MU Frame Format
SIG-A-1 details
| Bit | Field Name |
| B0 | UL/DL |
| B1-B3 | SIG-B MCS |
| B4 | SIG-B DCM |
| B5-B10 | BSS Color |
| B7 | DCM |
| B11-B14 | Spatial Reuse |
| B15-B17 | Bandwidth |
| B18-B21 | Number of Sig-B symbols or MU-MIMO Users |
| B22 | SIG-B Compression |
| B23-B24 | GI+LTF Size |
| B25 | Doppler |
SIG-A-2 details
| Bit | Field Name |
| B0-B6 | TXOP Duration |
| B8-B10 | Number of LTF Symbols |
| B11 | LDPC Extra Symbol |
| B12 | STBC |
| B13-B14 | Pre-FEC padding Factor |
| B15 | PE Disambiguity |
| B16-B19 | CRC |
| B20-B25 | Tail |
802.11ax works in the 6Ghz Frequency
• 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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