IEEE 802.11ax, marketed as Wi-Fi 6 by Wi-Fi Alliance, is one of two Wi-Fi specifications standards of IEEE 802.11 expecting full deployment in late 2019. The second is IEEE 802.11ay, and both of them together can be classified as High Efficiency Wireless.
802.11ax is designed to operate in all band spectrums between 1 and 7Ghz when they become available, this will be in addition to the existing 2.4 and 5Ghz. So in the scenario of dense deployments, the throughput speeds are expected to be 4x higher than IEEE 802.11ac, even though the nominal data rate is just 37% faster. Latency is also expected to be reduced by 75%.
So in order to improve spectrum efficient utilisation, there is now power control methods with the purpose of avoiding interference with neighbouring networks, additional OFDMA modulation and higher order 1024-QAM modulation. There is also uplink direction and downlink of MIMO and MU-MINO to further increases the throughput. In addition there are now dependency improvements of power consumption through security protocols such as Target Wake Time and WPA3.
It should be noted that unlike 802.11ac, 802.11ax will also operate in the unlicensed 2.4Ghz.
So in order to meet the goal of supporting dense 802.11 deployments, the below features have been approved:
- OFDMA which is a technology that seeks to segment the spectrum in time-frequency resource units (RUs), so that a central coordinating entity (the AP in 802.11ax) assigns RUs for reception or transmission to associated stations. Therefore using the central scheduling of the RUs, contention overhead can be avoided and ultimately increase efficiency in scenarios of dense deployments.
- Multi-user MIMO (MU-MIMO) with Downlink MU-MIMO is a technology that allows a device to transmit concurrently to multiple receivers, and with Uplink MU-MIMO, a device may simultaneously receive from multiple transmitters. Historically OFDMA would separate receivers to different RUs, but with MU-MIMO, the devices are now separated to different spatial streams, so in 802.11ax, both OFDMA and MU-MIMO technologies can now be used simultaneously. So in order to enable uplink MU transmissions, the AP will now transmit a new control frame (Trigger) that contains the scheduling information (RUs allocations for stations, modulation and coding scheme (MCS) that should be used for each station). Furthermore the use of Trigger will provides synchronisation for an uplink transmission, since the transmission starts SIFS after the end of Trigger.
- Trigger-based Random Access is a technology that allows performing UL OFDMA transmissions by stations that are not allocated RUs directly. So by using a Trigger frame, the AP can specify scheduling information about subsequently UL MU transmission. However, several RUs can now be assigned for random access, and stations that are not currently assigned RUs directly can still perform transmissions within RUs assigned for random access. Therefore to reduce collision probability (a situation in which two or more stations select the same RU for transmission), the 802.11ax amendment now specifies a special OFDMA back-off procedure. However it should be noted that random access is still favourable for transmitting buffer status reports when the AP has no information about pending UL traffic at a station.
- Spatial Frequency Reuse through Colouring enables devices to differentiate transmissions in their own network from transmissions in neighbouring networks. This is used in conjunction with Adaptive Power and Sensitivity Thresholds that dynamically adjustment of transmit power and signal detection threshold to increase spatial reuse. So without Spatial Reuse capabilities, devices will refuse transmitting concurrently to transmissions ongoing in other neighbouring networks. But with Colouring, a wireless transmission is marked at its very beginning that helps surrounding devices to decide if a simultaneous use of the wireless medium is permissible or not. This allows a station to consider the wireless medium as idle and instead start a new transmission even when the detected signal level from a neighbouring network exceeds the legacy signal detection threshold, and this takes into account that the transmit power for the new transmission is appropriately decreased.
- NAV has now increased from one to two, so in dense deployment scenarios, the NAV value set by a frame originated from one network may now easily reset by a frame originated from another network. Although this does lead to misbehaviour and collisions, therefore to avoid this, each 802.11ax station will now maintain two separate NAVs, one that is modified by frames that originated from a network that the station is associated with, and one that is modified by frames that originated from overlapped networks.
- Target Wake Time (TWT) is used to reduces power consumption and medium access contention. This was originally a concept that was developed in 802.11ah with the purpose of allowing devices to wake up at other periods other than the beacon transmission period. Furthermore, the AP may now group devices to different TWT periods, thus reducing the number of devices that are contending simultaneously to the wireless medium.
- Fragmentation (Dynamic), started originally with static fragmentation that means that all fragments of a data packet which are of equal size except for the last fragment. However with Dynamic Fragmentation, a device may fill all available RUs with other opportunities to transmit up to the available maximum duration. So as a result, Dynamic Fragmentation helps to reduce the overhead.
- Guard Interval Duration has now increased so it allows for better protection against signal delay spread as it occurs in outdoor environments.
- Symbol Duration has again been increased to allows for increased efficiency.