IEEE 802.11ax-2021 or 802.11ax is an IEEE standard for wireless local-area networks (WLANs) and the successor of 802.11ac. It is marketed as Wi-Fi 6 (2.4 GHz and 5 GHz) and Wi-Fi 6E (6 GHz) by the Wi-Fi Alliance. It is also known as High Efficiency Wi-Fi, for the overall improvements to Wi-Fi 6 clients under dense environments. It is designed to operate in license-exempt bands between 1 and 7.125 GHz, including the 2.4 and 5 GHz bands already in common use as well as the much wider 6 GHz band (5.925–7.125 GHz in the US). The main goal of this standard is enhancing throughput-per-area[lower-alpha 1] in high-density scenarios, such as corporate offices, shopping malls and dense residential apartments. While the nominal data rate improvement against 802.11ac is only 37%,:qt the overall throughput improvement (over an entire network) is 400% (hence High Efficiency).:qt This also translates to 75% lower latency. The quadruplication of overall throughput is made possible by a higher spectral efficiency. The key feature underpinning 802.11ax is orthogonal frequency-division multiple access (OFDMA), which is equivalent to cellular technology applied into Wi-Fi.:qt Other improvements on spectrum utilization are better power-control methods to avoid interference with neighboring networks, higher order 1024‑QAM, up-link direction added with the down-link of MIMO and MU-MIMO to further increase throughput, as well as dependability improvements of power consumption and security protocols such as Target Wake Time and WPA3. The IEEE 802.11ax-2021 standard was approved in February 9th, 2021.
|Data rate (in Mbit/s)|
|20 MHz channels||40 MHz channels||80 MHz channels||160 MHz channels|
|1600 ns GI||800 ns GI||1600 ns GI||800 ns GI||1600 ns GI||800 ns GI||1600 ns GI||800 ns GI|
In the previous amendment of 802.11 (namely 802.11ac), Multi-user MIMO has been introduced, which is a spatial multiplexing technique. MU-MIMO allows the Access Point to form beams towards each Client, while transmitting information simultaneously. By doing so, the interference between Clients is reduced, and the overall throughput is increased, since multiple Clients can receive data at the same time. With 802.11ax, a similar multiplexing is introduced in the frequency domain, namely OFDMA. With this technique, multiple Clients are assigned with different Resource Units in the available spectrum. By doing so, an 80 MHz channel can be split into multiple Resource Units, so that multiple Clients receive different type of data over the same spectrum, simultaneously. In order to have enough subcarriers to support the requirements of OFDMA, four times as many subcarriers are needed than by the 802.11ac standard. In other words, for 20, 40, 80 and 160 MHz channels, there are 64, 128, 256 and 512 subcarriers in the 802.11ac standard, but 256, 512, 1024 and 2048 subcarriers in the 802.11ax standard. Since the available bandwidths have not changed and the number of subcarriers increases by a factor of 4, the subcarrier spacing is reduced by the same factor, which introduces 4 times longer OFDM symbols: for 802.11ac the duration of an OFDM symbol is 3.2 microseconds, and for 802.11ax it is 12.8 microseconds (both without guard intervals).
The 802.11ax amendment will bring several key improvements over 802.11ac. 802.11ax addresses frequency bands between 1 GHz and 6 GHz. Therefore, unlike 802.11ac, 802.11ax will also operate in the unlicensed 2.4 GHz band. To meet the goal of supporting dense 802.11 deployments, the following features have been approved.
|OFDMA||Not available||Centrally controlled medium access with dynamic assignment of 26, 52, 106, 242(?), 484(?), or 996(?) tones per station. Each tone consists of a single subcarrier of 78.125 kHz bandwidth. Therefore, bandwidth occupied by a single OFDMA transmission is between 2.03125 MHz and ca. 80 MHz bandwidth.||OFDMA segregates the spectrum in time-frequency resource units (RUs). A central coordinating entity (the AP in 802.11ax) assigns RUs for reception or transmission to associated stations. Through the central scheduling of the RUs contention overhead can be avoided, which increases efficiency in scenarios of dense deployments.|
|Multi-user MIMO (MU-MIMO)||Available in Downlink direction||Available in Downlink and Uplink direction||With Downlink MU MIMO an AP may transmit concurrently to multiple stations and with Uplink MU MIMO an AP may simultaneously receive from multiple stations. Whereas OFDMA separates receivers to different RUs, with MU MIMO the devices are separated to different spatial streams. In 802.11ax, MU MIMO and OFDMA technologies can be used simultaneously. To enable uplink MU transmissions, the AP transmits a new control frame (Trigger) which contains scheduling information (RUs allocations for stations, modulation and coding scheme (MCS) that shall be used for each station). Furthermore, Trigger also provides synchronization for an uplink transmission, since the transmission starts SIFS after the end of Trigger.|
|Trigger-based Random Access||Not available||Allows performing UL OFDMA transmissions by stations which are not allocated RUs directly.||In Trigger frame, the AP specifies scheduling information about subsequent UL MU transmission. However, several RUs can be assigned for random access. Stations which are not assigned RUs directly can perform transmissions within RUs assigned for random access. To reduce collision probability (i.e. situation when two or more stations select the same RU for transmission), the 802.11ax amendment specifies special OFDMA back-off procedure. Random access is favorable for transmitting buffer status reports when the AP has no information about pending UL traffic at a station.|
|Spatial frequency reuse||Not available||Coloring enables devices to differentiate transmissions in their own network from transmissions in neighboring networks.
Adaptive Power and Sensitivity Thresholds allows dynamically adjusting transmit power and signal detection threshold to increase spatial reuse.
|Without spatial reuse capabilities devices refuse transmitting concurrently to transmissions ongoing in other, neighboring networks. With coloring, a wireless transmission is marked at its very beginning helping surrounding devices to decide if a simultaneous use of the wireless medium is permissible or not. A station is allowed to consider the wireless medium as idle and start a new transmission even if the detected signal level from a neighboring network exceeds legacy signal detection threshold, provided that the transmit power for the new transmission is appropriately decreased.|
|NAV||Single NAV||Two NAVs||In dense deployment scenarios, NAV value set by a frame originated from one network may be easily reset by a frame originated from another network, which leads to misbehavior and collisions. To avoid this, each 802.11ax station will maintain two separate NAVs — one NAV is modified by frames originated from a network the station is associated with, the other NAV is modified by frames originated from overlapped networks.|
|Target Wake Time (TWT)||Not available||TWT reduces power consumption and medium access contention.||TWT is a concept developed in 802.11ah. It allows devices to wake up at other periods than the beacon transmission period. Furthermore, the AP may group device to different TWT period thereby reducing the number of devices contending simultaneously for the wireless medium.|
|Fragmentation||Static fragmentation||Dynamic fragmentation||With static fragmentation all fragments of a data packet are of equal size except for the last fragment. With dynamic fragmentation a device may fill available RUs of other opportunities to transmit up to the available maximum duration. Thus, dynamic fragmentation helps reduce overhead.|
|Guard interval duration||0.4 µs or 0.8 µs||0.8 µs, 1.6 µs or 3.2 µs||Extended guard interval durations allow for better protection against signal delay spread as it occurs in outdoor environments.|
|Symbol duration||3.2 µs||12.8 µs||Since the subcarrier spacing is reduced by a factor of 4, the OFDM symbol duration is increased by a factor of 4 as well. Extended symbol durations allow for increased efficiency.|