# IEEE 802.11ax

## Summary


IEEE 802.11ax standard is an evolution of 802.11ac. Unlike previous standards that focused mainly on increasing raw data rates, 802.11ax focuses on better efficiency, capacity and performance. This translates to a 4x improvement in average throughput per user and better user experience. This is true even for dense indoor/outdoor deployments. Changes that enable these include higher modulation, more OFDM sub-carriers, and longer OFDM symbol; multiplexing users via MU-MIMO, beamforming and OFDMA; scheduling uplink instead of contention; and mitigating co-channel interference via BSS colour codes. 

IEEE 802.11ax is also called *Wi-Fi 6* or *High Efficiency WLAN (HEW)*. While 802.11ac used only the 5 GHz band, 802.11ax addresses both 2.4 and 5 GHz bands, thus staying backward compatible and becoming the migration path for 802.11n and 802.11ac devices. With the extension *Wi-Fi 6E*, the standard supports 6 GHz bands.

## Discussion

### Why do we need 802.11ax?

IEEE 802.11ac increased raw data rates but left some problems unsolved. Uplink access is mainly based on contention. When there are many devices in a dense network, or multiple access points closely deployed, there can be collisions, backoffs and therefore reduced effective throughput. User experience is affected for all devices. 

The common use cases where this happens is at crowded public hotspots (airports) or event venues (football stadiums). It could also happen in apartment complexes, schools and educational campuses. Also, it's expected that by 2022 number as many as 50 Wi-Fi devices may be present in a smart home. This growth is mainly due to appliances and gadgets becoming IoT-enabled. IEEE 802.11ax aims to solves these problems from the perspective of overall network capacity utilization, efficiency, performance, user experience and reduced latency. 


### What are the main PHY changes 802.11ax brings compared to earlier 802.11 amendments?

802.11ax supports both 2.4 GHz and 5GHz bands. Therefore it's backward compatible with both 802.11n and 802.11ac, meaning that legacy clients can also connect to an 802.11ax AP, and vice versa. 802.11ax uses 4x larger FFT by increasing the number of subcarriers but also narrowing the subcarrier spacing, thus preserving channel bandwidth. OFDM symbol duration and cyclic prefix are increased for better performance in outdoor environments. For higher data rate for indoor environments, 1024-QAM and shorter cyclic prefix are introduced. 

While in 802.11ac Wave 2 only 4 simultaneous MU-MIMO streams were possible, this is increased to 8. MU-MIMO in the uplink is introduced while in 802.11ac Wave 2 this was possible only in the downlink. AP will send trigger frames to coordinate uplink MU-MIMO.

OFDMA is introduced for the first time in Wi-Fi, similar to how it's done in 4G cellular. With OFDMA, multiple users can be transmitting at the same time, each using its allocated set of OFDM subcarriers. Subcarriers are grouped in *Resource Units (RU)*, which are allocated.


### What are the main MAC changes 802.11ax brings compared to earlier 802.11 amendments?

Since 802.11ax is meant to solve the use case of high density networks, uplink access is scheduled and not based on contention. A new feature named *Target Wake Time (TWT)* is used to let stations sleep, save power and wake up at scheduled times. AP can thus do scheduling a way that minimizes contention among stations. This can also be seen as a load balancing technique to ease congestion. 

In dense networks, a neighbouring AP can cause cochannel interference. Stations on the overlapping areas will backoff excessively. This is alleviated in 802.11ax by a feature called *BSS Color*. This helps stations to identify if transmission is from another network and thereby take the right action. 


### For multiplexing users on the uplink path, we have both MU-MIMO and OFDMA. How are they different?

MU-MIMO increases overall capacity since multiple streams are transmitted at the same time. This is ideal for bandwidth-demanding applications. Transmission to each user is targeted via beamforming. OFDMA does not increase capacity but uses it more efficiently by allocating subcarriers to users based on their needs. With OFDM, a user would occupy all subcarriers for a given time even if that user doesn't have much to send. With OFDMA, multiple users can be multiplexed at the same time, each using different sets of subcarriers. This implies that OFDMA is suited for low-bandwidth applications. Users will also experience less latency with OFDMA. With OFDMA, multiple users with varying bandwidth needs can be scheduled at the same time. 

Thus, MU-MIMO and OFDMA complement each other. In typical deployments, performance of MU-MIMO in 802.11ac Wave2 was found to depend on distance between AP and clients, channel selection, antenna performance, presence and capability of other clients, etc. Some have noted that MU-MIMO may even result in lower throughput. 


### What is Wi-Fi 6E?

Wi-Fi 6 is the commercial term for IEEE 802.11ax in 2.4 and 5 GHz spectra. Operation in 6 GHz, first announced in 2020, is known as Wi-Fi 6E. 

Wi-Fi 6E brings the main features of Wi-Fi 6 to 6 GHz including OFDMA, Target Wake Time and WPA3. While Wi-Fi 6 can deliver a maximum data rate of 9.6 Gbps (1.5 Gbps/device), Wi-Fi 6E can deliver 9.6 Gbps (2.3 Gbps/device). Latency with Wi-Fi 6E can be below 1 ms. 

Unlike at 2.4/5 GHz spectra, there are less interfering devices at 6 GHz. The new spectrum brings greater capacity. It opens up wide contiguous spectrum to enable high-bandwidth ultra-low-latency applications. These include online gaming, virtual reality, high-definition video calls, IoT device management, gigabit connectivity at event venues, and more.  Wi-Fi 6E is expected to power 5G services. Indoors, where GPS is not accurate, Wi-Fi 6E can enable more precise location tracking for applications such as asset tracking, emergency services, and keyless entry. 


### What bands are supported by Wi-Fi 6 and Wi-Fi 6E?

IEEE 802.11ax operates in bands 2.4/5/6 GHz. At 2.4/5 GHz bands, the channels are the same as in IEEE 802.11n/ac. However, 802.11ax uses the spectrum more efficiently due to OFDMA. 

At 6 GHz spectrum (5925-7125 MHz), 1.2 GHz of contiguous unlicensed spectrum is available. Wi-Fi 6E adds capacity with fourteen 80 MHz channels or seven 160 MHz channels. The spectrum is composed of four channel allocations in the US, termed as U-NNI-5 to U-NNI-8, given Unlicensed National Information Infrastructure (U-NII). U-NNI-5 (5925-6425 GHz) is also allocated in the EU. 

Deployments at 6 GHz can be at very low power, low power or standard power with maximum EIRP of 14, 30 and 36 dBm respectively. Low power operation is only for indoor coverage. Standard power operation is at U-NNI-5 and U-NNI-7 and must be controlled by an Automated Frequency Coordination (AFC) system. This is to mitigate interference to and from incumbent devices in the vicinity. AFC is not required for very low power and low power operations.  


### What are possible implementation challenges for 802.11ax?

OFDM subcarrier spacing narrower at 78.125 kHz, which implies that oscillators must have better phase noise performance and RF frontends must have better linearity. Since 1024-QAM is possible, EVM requirements are tighter. Good performance requires tight frequency synchronization and clock offset correction. Stations must also maintain frame timing based on their clocks since their transmissions must be in coordinated precisely as noted in trigger frames. 

## Milestones

May  
2013

Within the IEEE, High Efficiency WLAN (HEW) Study Group is formed. A year later, HEW Task Group is formed to start the development of 802.11ax standard. 

Oct  
2016

Quantenna announces world's first 802.11ax chipset for APs. Named QSR10G-AX, it supports 8 streams at 5 GHz and 4 streams at 2.4 GHz. 

Feb  
2017

Qualcomm announces chips IPQ8074 (for routers/APs) and QCA6290 (for clients). It's expected that router devices will come out first (end 2017) followed by client devices (2018). 

Aug  
2017

Broadcom announces three chips as part of its 802.11ax family without support for MU-MIMO in the uplink. 

Sep  
2017

**Draft 2.0** of 802.11ax is released. This may be incompatible with Draft 1.0 released in 2016 and mixing these two implementations within a Wi-Fi network might result in sub-optimal performance. By now, chipsets are already available from Broadcom, Qualcomm, and Quantenna. Meanwhile, Asus shows off an 802.11ax router that can deliver 4.8 Gbps but this is not yet in commercial availability. 

Jan  
2018

Intel announces that it will release 802.11ax chipsets some time this year. The chipsets will be for 2x2 and 4x4 home routers and gateways, supporting as many as 256 devices sharing bandwidth simultaneously. 

Jun  
2018

Asus launches two 802.11ax routers, one of them capable of aggregate peak throughput of 11 Gbps to cater to high-bandwidth multiplayer gaming. 

Feb  
2019

Samsung Galaxy S10 becomes the first smartphone to support Wi-Fi 6. 

Sep  
2019

To certify devices for Wi-Fi 6, Wi-Fi Alliances launches **Wi-Fi Certified 6** program. Subsquently, Samsung Galaxy S10 becomes the first smartphone to be certified under this program. Also in September, Apple iPhone 11 starts supporting Wi-Fi 6. 

Nov  
2019

**Draft 6.0** is released. 

Dec  
2019

IEEE announces that 802.11ax meets requirements for 5G Indoor Hotspot and Dense Urban test environments of the enhanced Mobile Broadband (eMBB) usage scenario. This includes minimum acceptable peak performance, average and cell-edge user experience, mobility performance and latency performance. 

2020

This year sees the first developments of **Wi-Fi 6E**. Wi-Fi Alliance announces the branding early in the year. Spectrum at 6 GHz is opened up in America, Europe and Asia. First chips are demonstrated. Actual products appear on the market in 2021, with Wi-Fi Alliance launching a certification program in January 2021. In 2022, we expect 350 million Wi-Fi 6E devices to enter the market and 15% of all 802.11ax shipments will be Wi-Fi 6E. 

May  
2021

IEEE publishes the standard **IEEE 802.11ax-2021** as Amendment 1 of the IEEE 802.11-2020 standard. The amendment was formally approved towards the end of 2020 and early 2021.