Wi-Fi 6

Wi-Fi 6 (802.11ax) marks a fundamental shift in wireless networking design, prioritizing efficiency and scalability rather than solely increasing raw speed. Where previous standards focused on maximum data rates, Wi-Fi 6 implements sophisticated mechanisms that enhance performance in dense, multi-client environments.

Building upon the 802.11ac foundation, Wi-Fi 6 delivers several key technical advancements:

• Enhanced modulation schemes with 1024-QAM, increase data rates by 25% in high SNR environments.
• OFDMA technology enables concurrent multi-user transmissions for both downlink and uplink traffic.
• Expanded MU-MIMO capabilities support up to 8 simultaneous users and adds support for UL MU-MIMO.
• Advanced power-saving mechanisms including individual Target Wake Time (i-TWT) extend battery life substantially.

The 802.11ax amendment continues to improve sophistication of 802.11 modulation techniques.

These innovations address critical challenges in modern wireless networks: high client density, diverse application requirements from IoT to video streaming, and deterministic performance needs in mission-critical deployments.

Subsequent sections detail each feature’s technical specifications, implementation considerations, and performance benefits across various use cases.

Increased modulation complexity

Quadrature amplitutde modulation (QAM) encodes data by manipulating both the amplitude and phase of carrier waves. Each point in the QAM constellation represents a distinct symbol, with each symbol encoding multiple bits of information. Higher-order QAM schemes allow more bits to be encoded per symbol, increasing spectral efficiency.

Building on digital modulation schemes in 802.11ac of up to 256-QAM, 802.11ax supports up to 1024-QAM. This means that each RF symbol represents one of 1024 possible combinations of amplitude and phase as illustrated in figure below. The move from 256-QAM to 1024-QAM increases number of bits carried per OFDM symbol from 8 to 10. This can result in up to a 25% increase in PHY data rates and can be attained in clean environments with a high signal to noise ratio (SNR).

The key determinants of PHY data rate are:

• Channel width. Available channel widths are 20 MHz, 40 MHz, 80 MHz, 80 + 80 MHz, and 160 MHz. Wider bandwidths allow use of more subcarriers, for example there are 242 subcarriers in a 20 MHz channel and 996 subcarriers in an 80 MHz channel (hence OFDMA terms RU242 and RU996).

• Modulation and coding. 802.11ax extends the modulation and coding scheme adding 1024-QAM options with coding rates of 3/4 and 5/6. All earlier options are still available, and are used if SNR is too low to sustain highest achievable rate.

• Subcarrier changes. The fast fourier transform (FFT) length was increased from 64 in 802.11ac to 256 in 802.11ax for 20 MHz. This results in a decrease in subcarrier spacing and an increase in number of data subcarriers. These factors contribute to a 10% increase in efficiency over the previous generation.

• Symbol duration. Symbol duration was increased to 13.6 ns, 14.4 ns, and 16 ns. Extended symbol durations result in increased efficiency due to availability of more data tones compared to older standards.

• Guard interval. Guard intervals are necessary to avoid multipath reflections of one symbol from arriving late and interfering with next symbol. Extended guard interval durations of 1600 ns and 3200 ns have been introduced in addition to 800 ns from 802.11ac. Longer guard intervals allow for better protection against signal delay spread in outdoor environments. These could also potentially increase effective range of wireless outdoors.

All above factors contribute to increased PHY data rate and efficiency. Following is a table with 802.11ax data rates for a single spatial stream full-bandwidth client. Note the rates double with 2 spatial streams, triple with 3, and so on until maximum number of spatial streams supported by specification. The included guard intervals (GI) for rates listed below are 1600 ns (1.6 μs) and 800 ns (0.8 μs).

OFDMA

Orthogonal Frequency Division Multiple-Access (OFDMA) is a transmission technique which enables multiple devices to share same Wi-Fi channel at same time through use of subchannels. Wi-Fi was first major consumer technology to adopt OFDM in 1999, and was subsequently used by 3GPP community when designing LTE and now 5G. In turn, Wi-Fi 6 and 6E adopted OFDMA technology from other wireless technologies like WiMAX and LTE.

With OFDMA, multiple clients can simultaneously share a Wi-Fi channel during same transmit opportunity (TxOP) instead of having to take turns. OFDMA enables transmission on a 20 MHz channel up to nine clients at once, versus four as in 802.11ac (with MU-MIMO). This scales linearly as channel width increases i.e. 18 clients for 40 MHz and 37 clients for 80 MHz channels. And when needed, a single client can also use entire channel ensuring better client density does not come at a cost of peak performance. Notably, OFDMA is bidirectional, bringing uplink multi-user capability to Wi-Fi for first time.

Take into consideration a scenario where AP must send data to 3 clients. In 802.11ac Single User (SU) operation, AP would contend for medium and then send three packets consecutively as shown in figure below. Whereas in 802.11ax with OFDMA, transmissions for these 3 client devices are assigned a fractional channel and then sent to all 3 clients simultaneously (during same TxOP).

To summarize, downlink OFDMA (DL OFDMA) allows access point to bundle several frames together with a single preamble, in different sub-channels in a single transmit opportunity (TxOP). Clients can then tune their radios to respective sub-channels to receive their transmissions.

Benefits of OFDMA:

• OFDMA helps in reducing latency between client and Access Point (AP).

• OFDMA helps in reducing contention overhead which means that there is very little deterioration in capacity as number of clients increases. This is helpful in high density environments because OFDMA improves network capacity and efficiency.

• 802.11ax also improves performance for legacy generations. As more Wi-Fi 6 devices enter market and uses OFDMA to reduce airtime consumption, there is more usable airtime leftover for earlier Wi-Fi generations.

Applications and use cases

Voice over Wi-Fi

One of top use cases for OFDMA is Voice over Wi-Fi (VoWi-Fi). In high density environments, where a lot of users are contending for medium access, can result in increased latency and jitter. These factors can cause gaps in re-creation and playback of voice leading to an undesirable user experience. OFDMA also enables strong QoS mechanism by enabling AP to control medium access for both DL and UL. This can eliminate need for individual medium contention and enables the AP to schedule grouped transmissions in one transmit opportunity (TXOP). That is how OFDMA allows AP to better control latency and jitter. With 802.11ax, AP can assign frequent, short transmission opportunities so AP can transmit and receive packets without need to buffer them. OFDMA can be helpful in low-bandwidth streams like Voice over Wi-Fi (VoWi-Fi) by reducing latency and jitter thus improving call quality.

Internet of Things

Important metrics for an Internet of Things (IoT) device include data rate, scalability, range, power consumption, security and ease of configuration. OFDMA divides transmissions across frequency domain and smallest unit of allocated bandwidth can be as small as 2 MHz. This allows more individual devices to be reliably supported on an AP and thus fulfills a critical IoT requirement, scaling. Unlike most of mainstream Wi-Fi applications, IoT devices usually use lower speed connections, often in sub-Megabit range. OFDMA addresses first three requirements of this sector - data rate, scale and range. OFDMA allows sub-channelization which reduces data rates to ~2 Mbps. This inherently helps in improving range of IoT devices.

In addition to this, Wi-Fi 6 also offers various power-save features like TWT, Receive Operating Mode Indication, Transmit Operating Mode Indication, etc. along with 20 MHz-only clients to addresses most of basic requirements for encroaching into IoT Markets.

Video and factory automation applications

OFDMA is ideal for latency sensitive applications like video and factory automation applications. OFDMA enables several to many low-bandwidth streams to be transmitted in parallel. This can aid in reducing latency and jitter.

DL MU-MIMO improvements in Wi-Fi 6 and 6E

Multi-user Multiple-Input, Multiple-Output (MU MIMO) is a multi-user capability, originally introduced in 802.11ac for downlink traffic. MU-MIMO technology improves network capacity by allowing multiple devices to transmit simultaneously, making use of multipath spatial channels.

802.11ax introduces some new enhancements to existing 802.11ac downlink (DL) MU-MIMO. The number of users in a group is expanded up to eight users for MU-MIMO operation. Due to this advancement, now even with devices in single stream mode, MU-MIMO throughput can be doubled over single user operation. The improvement of increasing size of downlink multi-user MIMO groups can result in more efficient operation.

In addition to these improvements, 802.11ax allows uplink (UL) OFDMA as part of sounding protocol, which is more efficient than using single user transmission of feedback used for sounding protocol in 802.11ac.

All these factors can lead to increased capacity and efficiency aimed to be useful for high-bandwidth applications like mission-critical voice calls and video streaming.

Power-save enhancements

Wi-Fi 6 introduces various enhancements to already existing power-save modes. These new and improved power-save mechanisms allow longer sleep intervals and scheduled wake times for client devices. These enhancements were adopted to address power consumption issue mainly for handheld and battery powered devices and are targeted towards emerging IoT markets.

Target Wake Time (TWT)

Target Wake Time (TWT) is a power saving mechanism that was introduced in 802.11ah. A schedule (service period) can be negotiated between each client (station) and corresponding AP, which allows client to sleep for long periods of time and wake up at pre-scheduled (target) times to exchange information with the associated AP. TWT also significantly reduces small and inefficient control frame traffic that clients are required to use regularly to poll AP for buffered frames (PS-Poll or U-APSD). A client can greatly improve power savings with TWT if the client has a known deterministic traffic pattern like an Internet of Things (IoT) environmental sensor that only report sensor data every so often.

There are two types of TWT agreements.

• Individual TWT (i-TWT) is negotiated between an AP and each client to agree on parameters such as wake time, interval, and duration. This enables clients to specify when and how often to wake up.

• Broadcast TWT (b-TWT) is negotiated between an AP and a group of clients where target wake time parameters such as beacon and listen intervals are advertised in beacon frames. Clients will wake during the service period (SP) and contend for medium access. b-TWT may be used over i-TWT for the purpose of simplicity and less management frame overhead.

In addition to reducing contention between clients, use of TWT may also contribute to taking full advantage of other novel mechanisms in IEEE 802.11 universe, such as multi-user transmissions and coexistence in high-density WLAN scenarios. TWT operation is ideal for IoT devices which communicate infrequently. TWT can improve client power savings and reduces airtime contention with other clients.

20 MHz-only operation

20 MHz-only operation feature was specifically introduced for IoT markets. The feature seeks to reduce complexity, leading to low-power, lower-cost chips. Such devices are capable of operating in both 2.4 GHz and 5 GHz bands and also support nearly all Wi-Fi 6 mandatory features.

Receive operating mode indication

Receive operating mode indication enables client (station) to adapt to number of active receive chains and channel width for reception of subsequent PPDUs by using a field in MAC header of a data frame. This mechanism reduces overhead compared to 802.11ac as there is no additional Operating Mode Notification management frame exchange as there is in 802.11ac.

Transmit operating mode indication

This allows client devices (stations) to dynamically adapt their transmit capabilities like channel width and maximum number of spatial streams.

Uplink/downlink (UL/DL) flag

UL/DL flag in every preamble allows to identify frames as transmitted by an AP or client device. This helps client devices to switch off their radio circuitry as soon as they see an uplink bit in preamble.

Backward compatibility

Like previous generations of Wi-Fi, 802.11ax (Wi-Fi 6) is backward compatible and fully supports legacy 802.11a/b/g/n/ac clients. An 802.11ax access point (AP) communicates with each client device using only the protocols supported by that client. Legacy clients capable of 802.11a/b/g/n/ac can associate with an 802.11ax AP but cannot utilize 802.11ax specific features.

Communication remains limited to the most recent Wi-Fi standard supported by the client device. For example, an 802.11ac client associating with a Wi-Fi 6 AP uses only features defined in the 802.11ac Wi-Fi standard. Mixed environments with both Wi-Fi 6 and legacy clients do allow Wi-Fi 6 clients to leverage advanced features such as OFDMA and 1024-QAM.

While 802.11ax includes newer higher-efficiency techniques and frame formats that can only be decoded by other 802.11ax devices, continued support of VHT, HT and older 802.11 equipment is an integral part of standard.

802.11ax radios will communicate with other 802.11ax radios using High Efficiency (HE) OFDM symbols and Physical Protocol Data Unit (PPDU) formats. As far as compatibility with clients is concerned, they can communicate with 802.11a/g, 802.11n (HT) and 802.11ac (VHT) clients using 802.11a/g/n/ac formatted PPDUs. When 802.11ax-only OFDMA conversations are occurring, RTS/CTS mechanism may be used to protect legacy receivers during period when HE transmissions are underway.

This ensures that an 802.11ax AP is a good neighbor to adjacent to older APs while fully embracing all of generations of client devices that exist in environment. 802.11ax has a number of features for co-existence, but main one is extension of an 802.11n/ac technique: The first 20 μs of 11ax preamble uses 802.11a preamble. Non-802.11ax equipment can read first 20 μs and identify that channel will be occupied for a given time, and therefore can avoid transmitting simultaneously with high efficiency frame. Non-802.11ax devices communicating with an 802.11ax AP will still require the entire channel for transmission rather than sharing channel resources through OFDMA subcarrier allocation.

Protection, dynamic bandwidth, and channelization

802.11ax inherits dynamic bandwidth operation and protection mechanisms from 802.11ac standard. There have not been any new modifications to these mechanisms in 802.11ax. 6 GHz aside which is covered in Wi-Fi 6E, channelization for 802.11ax has changed little since introduction of 802.11ac.

When an 802.11ac or 802.11ax AP using an 80 MHz channel is operating in neighborhood of an older AP, or a network that is only using a 20 MHz or 40 MHz channel, newer generation AP must avoid transmitting simultaneously with a station in neighboring network. The question is how this can be achieved without permanently reducing channel bandwidth from 80 MHz.

The solution can be found by answering following questions.

1. How can a station (AP or client) wanting to operate at 80 MHz, warn older stations to stay off air while transmitting in 802.11ac or 802.11ax mode, which 802.11n and older cannot decode?
2. How will 802.11ac or 802.11ax station know that full channel is clear of other stations’ transmissions?
3. How can bandwidth usage be optimized if, for instance, an older station is transmitting in just 20 MHz of 80 MHz channel?

Sending a warning to other stations to stay off air is achieved by request to send (RTS) frames. The 802.11ac station sends out multiple parallel RTS protection frames in each 20 MHz channel in the bonded set, at rates an 802.11a or 802.11n clients can understand. The multiple RTS frames use duplicate, quadruplicate, or octuplicate transmission. Before sending RTS, clear channel assessment (CCA) is performed to ensure no transmissions in progress are heard. On receiving RTS frame, older stations know how long to wait for 802.11ac transmission.

Next, the recipient runs a CCA in each of 20 MHz channels. The RTS frame format is extended so that the originator can indicate channel options and replies with a clear to send (CTS) response to indicate whether transmissions in progress are heard from any neighboring network. If not, originator transmits data frame using full bandwidth (80 MHz in this example).

However, if the recipient does find transmissions in progress on any secondary channel, recipient can still continue responding with CTS, while indicating which primary channels are clear (20 MHz or 40 MHz). Then, originator can transmit using only the usable part of 80 MHz channel. This may force a reduction in channel width from 80 MHz to 40 MHz or even 20 MHz, but frame will be transmitted using airtime that would otherwise be unused. This feature is called dynamic bandwidth operation.

Dynamic bandwidth optimization is constrained by 802.11ac and 802.11ax definitions of primary and secondary channels. For each channel, such as an 80 MHz channel, one 20 MHz channel (subchannel) is designated as primary. This is carried through from 802.11n, and in networks with a mix of 802.11ac or 802.11ax, and older clients, all management frames are transmitted in this channel so that all clients can receive them.

The second part of 40 MHz channel is called secondary 20 MHz channel. And 40 MHz of wide channel that does not contain primary 20 MHz channel is secondary 40 MHz channel. Data transmissions can be in primary 20 MHz channel, 40 MHz channel including primary 20 MHz channel, or full 80 MHz channel, but not in other channel combinations.

Additionally, introduction of wide band channels, especially 80 + 80 MHz channels, requires some changes to channel switch announcement (CSA) frame. CSA is used by an AP to inform associated clients when the AP is about to switch channels after radar has been detected in current channel. CSA was first introduced in 802.11h as part of dynamic frequency selection (DFS). Otherwise, operation of DFS remains unchanged with 802.11ac and 802.11ax.

Wi-Fi 6 (802.11ax) represents an advancement in Wi-Fi aimed at delivering improved efficiency and performance in dense environments with many connected devices. This page describes considerations and deployment strategies for Wi-Fi 6 networks, focusing on key features that enhance efficiency, speed, and battery life.

Unlike the previous 802.11ac standard that primarily focused on increasing data rates, Wi-Fi 6 introduces technologies borrowed from other wireless technology that fundamentally improve how wireless networks handle multiple simultaneous connections. By implementing features like Orthogonal Frequency Division Multiple Access (OFDMA), enhanced Multi-User MIMO (MU-MIMO), and Target Wake Time (TWT), network administrators can enhance both network capacity and client device performance.

Configuration

In general, the recommendation is to use default settings for 802.11ax (HE) unless directed to change per TAC.

High efficiency (HE)

All 11ax specific features fall under High Efficiency (HE) profile in AOS-8 or the SSID profile in AOS-10. Enabling the High Efficiency parameter enables all the 802.11ax features on the radio. Users can utilize Wi-Fi 6 features like TxBF, HE supported higher MCS rates (10 and 11), HE OFDMA, MU-MIMO, TWT, etc.

High Efficiency is enabled by default and is recommended to keep enabled to reap the benefits of 802.11ax.

HE OFDMA

HE OFDMA is enabled by default and is recommended to keep enabled for increased efficiency and reduced latency. OFDMA is best for applications leveraging smaller packet sizes (IoT, Voice applications, etc.) and is suitable for low bandwidth applications.

Transmit beamforming (TxBF)

802.11ax employs an explicit beamforming procedure, similar to that of 802.11ac. Under this procedure, the beamformer (AP) initiates a channel sounding procedure with a Null Data Packet. The beamformee (client) measures the channel and responds with a beamforming feedback frame, containing a compressed feedback matrix. The beamformer uses this information to compute the antenna weights in order to focus the RF energy toward each user which can result in an increased MCS rate.

TxBF is enabled by default. In most deployments, we recommend to keep TxBF enabled for performance benefits in applicable scenarios. In certain very high density deployments, you may consider disabling to minimize sounding overhead.

Downlink MU-MIMO

Downlink MU-MIMO is enabled by default and is recommended to keep enabled in configuration as DL MU-MIMO can lead to increased capacity and enable higher speeds per user in applicable scenarios. DL MU-MIMO for applications that larger packets (video, streaming, etc.) and is suitable for high bandwidth applications.

TWT

A new power saving features offered by Wi-Fi 6 is Target Wake Time (TWT). This feature allows for clients to “wake up” at negotiated times rather than waking up for every beacon.

TWT is enabled by default and is recommended to keep enabled as i-TWT allows capable clients to request specific wake up time to AP, so that clients can go to sleep for longer time than normally and save power.