Features and benefits

In 2020, the Wi-Fi Alliance announced Wi-Fi 6E, which extends Wi-Fi 6 into the 6 GHz spectrum. Adding Wi-Fi 6 to the 6 GHz band offers several advantages over Wi-Fi in the 2.4 GHz and 5 GHz bands. The Federal Communications Commission (FCC) then made history by opening up 1200 MHz for unlicensed use by technologies like Wi-Fi.

The iterations of Wi-Fi have done a remarkable job with backwards-compatibility over the years, but come at a cost of carrying and accommodating legacy protocols and reduced bandwidth efficiency. The 6 GHz band does not accommodate Wi-Fi technologies older than Wi-Fi 6. The band allows Wi-Fi 6E to operate without the constraints of legacy protocols, enabling improved bandwidth and latency. Crucial to understand and note, Wi-Fi 6E shares unlicensed spectrum with other technologies which might be operating at the same time. In certain scenarios, Wi-Fi 6E APs must perform frequency coordination through a frequency coordinator prior to medium access for minimized interference with licensed incumbent users of the spectrum.

Regulations in the 6 GHz band

Spectrum allocation is a regulatory function, and rules can be different at varying national levels. This page covers regulations in the FCC and ETSI regulatory domains; most countries will follow one or the other of these models, potentially with minor national variations.

Channels

The FCC unlicensed grant in 6 GHz is 1200 MHz of spectrum, compared to 83.8 MHz in the 2.4 GHz band and 570 MHz in sections of 5 GHz (FCC). After a 20 MHz guard band at the low end, usable 6 GHz spectrum starts at 5.945 GHz and continues up to 7.125 MHz. This allows for 59 channels at 20 MHz width, 29 at 40 MHz, 14 at 80 MHz, or 7 at 160 MHz (the maximum channel width supported by Wi-Fi 6E).

The number of wide channels in 6 GHz is especially significant, as gaps in allocated spectrum in the 5 GHz band limit 80 MHz channels to 6 (seven if we include U-NII-4) and 160 MHz channels to 2 (3 if we include U-NII-4), and wide channels are necessary for the highest data rates.

The FCC designates four sub-bands across the 1200 MHz of 6 GHz: U-NII-5, U-NII-6, U-NII-7, and U-NII-8. These sub-bands are significant because they contain different incumbent types (covered in a section below), so while indoor APs will have uniform power limits across the entire 1200 MHz, there are differing restrictions for higher power and outdoor use across the sub-bands.

The rules from European regulators are not the same as from the FCC. They have taken a more cautious approach, allowing operation in just the lower 500 MHz of the band (480 MHz after the 20 MHz guard band), the equivalent to U-NII-5. This is primarily due to caution over spectrum sharing approaches like the FCC’s Automated Frequency Coordination (AFC) which is necessary to protect incumbents from interference from 6 GHz unlicensed transmitters.

A consequence of this caution is that European countries will restrict 6 GHz operation to indoor WLANs initially. Decisions on extending into the upper part of the 6 GHz band and allowing outdoor and higher-power operation have been deferred, for now.

Many other countries worldwide are in the process of adopting the 6 GHz band for unlicensed use; most of them are expected to follow either the FCC (1200 MHz) or ETSI (lower 500 MHz) approaches, with minor deviations possible for national regulation.

Incumbents

While the 6 GHz band is continuous and channelized across the entire 1200 MHz, existing non-Wi-Fi technologies are active in all sub-bands of 6 GHz. To allow new users into the bands without disrupting the operation of incumbents, spectrum-sharing models are required; new users can be allowed to transmit only when they will not cause interference to incumbents.

However, there is no requirement in the 6 GHz band for special in-band sensing of incumbent transmissions, similar to the Dynamic Frequency Selection (DFS) mechanism used in parts of the 5 GHz band for radar avoidance. Instead, the 6 GHz band uses two spectrum-sharing techniques. Where possible to identify incumbent users in the vicinity of an unlicensed transmitter, power and frequency options are restricted to stay clear of the incumbents. Alternatively, where specific knowledge of nearby incumbents is unavailable, power levels must be kept low enough to ensure they will never cause interference, even in the worst case.

For U-NII-5 running from 5.925 to 6.425 GHz and U-NII-7 from 6.525 to 6.875 MHz, the most important incumbents are point-to-point, licensed radio links used by service providers and private communications for utility companies and others. These links are licensed by the FCC and included in a database known as the Universal Licensing System (ULS). As they are point-to-point, they have narrow-beam antennas, often on tall masts, and can reach for tens of kilometers. The ULS database contains tens of thousands of these links, along with their location (transmitter and receiver, giving direction), frequency, and other characteristics. The FCC allows two spectrum sharing models for these sub-bands. At a low power threshold, and indoors only, unlicensed radios can transmit anywhere. Meanwhile, outdoor locations and higher-power transmitters are allowed where a calculation based on the ULS database shows they will not interfere with incumbents.

The spectrum sharing protocol for high-powered or outdoor unlicensed transmitters requires them to contact a central AFC server, which uses their location and transmit power to calculate whether any of the licensed incumbents in the ULS database might be affected and identifies safe power and frequency parameters.

In other sub-bands, U-NII-6 and U-NII-8, the incumbents are more difficult to coordinate, as they include mobile transmitters, and also temporary links used by local TV stations for outside broadcasts. These bands are less suited for spectrum sharing because the usage patterns are more dynamic, so outdoor, high-power transmitters are not allowed.

The protection of incumbents, and differing views of how to solve this problem, shapes regulators’ approach to opening the 6 GHz band to unlicensed transmitters. Some national regulators are following the FCC approach and introducing spectrum-sharing mechanisms to ensure new unlicensed services can use the spectrum without causing interference, while others are shelving the question by addressing only the lower part of the 6 GHz band, without more complex spectrum sharing arrangements like the FCC’s AFC.

Equipment classes for 6 GHz unlicensed operation

The FCC wants to allow 6 GHz networks to have the best possible performance while ensuring that licensed incumbents are not adversely affected. The FCC policy achieves their goal by defining four separate operating classes for 6 GHz equipment: Low Power Indoor (LPI), Standard Power (SP), Very Low Power (VLP) and Client Devices. VLP is not in scope for this page.

Low power indoor

The most common class for Wi-Fi 6E access points (APs) is low power indoor (LPI). These will be the familiar home or enterprise campus APs. By definition, these APs are shielded by buildings, to some extent, so the signal ’leaking’ outside will be attenuated, which allows safe operation across the band at a power level similar to today’s indoor Wi-Fi APs.

Low power indoor (LPI) AP characteristics:

• Fixed indoor only
• PSD 5 dBm/MHz
• No antenna connectors
• No weatherproofing
• Wired power

The limiting power level for LPI APs is not defined in absolute dBm, as for the lower bands, but at 5 dBm/MHz spectral density. This method of limiting allows an increase of 3 dB for every doubling of channel bandwidth, which gives approximately 18 dBm EIRP for a 20 MHz channel, and up to 27 dBm for a 160 MHz channel in Wi-Fi 6E. Note that spectrum is never perfectly flat and actual maximum EIRP may be further restricted as to not exceed regulatory limits or by hardware limits.

The FCC can apply this rule because incumbent links are generally narrow band compared to Wi-Fi channels. This is advantageous to Wi-Fi network because noise increases proportionally with channel bandwidth. SNR will remain similar for different channel widths, given maximum transmit power levels, hardware capability, and environmentals are the same.

Low power indoor (LPI) APs can operate across the whole 1200 MHz band, as their transmit power is considered safe for all incumbents after building exit loss is subtracted and attenuated. To ensure that these indoor-only units are not used outdoors, or with external high-gain antennas (which have the potential to cause interference), the FCC provides a list of physical requirements for certifying a LPI AP:

• No connectors for external antennas
• No battery-powered operation
• Not weatherized

The current rules from European regulators allow only Low Power Indoor (LPI) APs; outdoor mounting is not allowed and transmit power (EIRP) is limited to 23 dBm. Some countries deviate from this value, but only by one or two dBm.

Standard power

APs mounted outdoors, or indoors operating at higher power than LPI, are subject to ‘Standard Power’ (SP) rules. This is because they may interfere with incumbents and, because they would not otherwise be aware of the risk of interference, they must check periodically with the AFC for channel availability.

Standard power (SP) AP characteristics:

• Fixed indoor/outdoor
• EIRP 36 dBm max
• Controlled by an AFC database
• Automated geolocation
• Pointing angle restriction

The AFC query protocol is defined by the Wi-Fi Alliance and consists of an inquiry message from the AP and a response from the AFC server. The most important information in the inquiry is the AP’s geolocation. In exchange for latitude, longitude, antenna height (above ground level) and some other information in an inquiry message, the AP or controller receives a response containing the set of channels or frequency ranges and the maximum power levels that will not cause interference.

The transmit power of a standard power AP can be as high as 36 dBm EIRP. Because of the increased risk of interference to incumbents, standard power APs are only allowed in the U-NII-5 and U-NII-7.

Number of channels available for standard power (FCC):

Client devices

As with the lower sub-bands, client devices are expected to be limited in geography by APs. If there is no AP signal, devices must not transmit or attempt connection. Therefore, we assume the AP is transmitting in an authorized manner, and the client can adjust client transmit power and channel with reference to the AP. Refer to the respective regulatory in your locale for rules regarding client devices.

6 GHz operation

One attractive feature of the 6 GHz band is that there are no older Wi-Fi devices that need to be accommodated. Wi-Fi has done a remarkable job with backwards-compatibility over the years, but these efforts come at a cost of carrying legacy protocols and reduce bandwidth efficiency. The initial Wi-Fi 6E release sets a high bar with the new baseline of Wi-Fi 6 (802.11ax).

Devices using the 6 GHz band must use Wi-Fi 6 standards, must conform to WPA3 or Enhanced Open security, and cannot offer older security options like WPA2 and Open modes. For WPA3-Enterprise, the differences are slight and exclude some combinations of WPA2-Enterprise security protocols that could expose vulnerabilities, but these combinations were never configurable on most enterprise class APs. The pre-shared key options for WPA2 are significantly changed and improved with WPA3-Personal, and a new ‘Enhanced Open’ standard replaces Open mode to ensure that over-the-air transmissions are encrypted even without authentication of the client device or the network. These mandates are in line with the Wi-Fi 6 standard, without allowance for backwards-compatibility for older client devices.

The Wi-Fi Alliance has introduced a limited number of new features in Wi-Fi 6E, many previously optional features are now mandatory, with two goals:

• Improved airtime efficiency. In crowded areas, much of the available airtime is taken up with AP discovery frames such as beacons, probe requests, and probe responses. This detracts from airtime efficiency, so several features allow client devices to discover target APs and channels with fewer frames on the air.

• Faster AP discovery. Although the large number of new channels is a significant advance, client devices would take a long time to step through each channel, transmitting probe requests and awaiting replies to find a suitable AP. Therefore, new features aim to improve the speed and efficiency of AP discovery

The features are classified as ‘in-band’ and ‘out of band’, meaning the client device discovers APs on the 6 GHz channel where they transmit, or on another channel in a lower band, 2.4 GHz or 5 GHz.

In-band AP discovery features

The traditional way for a client device to discover a suitable AP for connection is to tune its radio to a 20 MHz channel, transmit a number of probe requests, wait on-channel for ~20 msec for probe responses from APs operating on that channel, then tune to the next channel and repeat. This takes time, may result in jitter or data loss as the device is away from its serving AP, and reduces battery life through extra frame transmissions. In addition, the probe requests and responses on the air reduce throughput for other user traffic.

For some time, the industry has been working towards passive scanning where devices learn about other APs and their serving channels through Neighbor Reports. This allows the device to switch to the new channel, reducing time off-channel and frames on the air, but can require up to 102 msec off-channel waiting for the next beacon transmission. This last issue is mitigated in the Neighbor Report by a beacon offset time value, allowing efficient passive scanning, but only in cases where an AP provides comprehensive Neighbor Reports in the beacon or probe response.

As a new, greenfield band, 6 GHz offers an opportunity to mandate features that reduce acquisition time, improve battery life, and avoid excess frames on the air. Several existing and new features improve 6 GHz AP discovery in all these areas.

Preferred scanning channels

Starting with channel 5, every fourth 20 MHz channel is designated for scanning, and APs should align their transmitting channels with Preferred Scanning Channels (PSCs) when using wider channel widths.

For wider channels, the ‘primary’ 20 MHz channel where the beacon is transmitted should align with a PSC where possible. This achieves two goals.

1. Client devices searching for a suitable AP must to scan at most 15 channels to find a beacon or other advertisement.
2. Ensures that non-PSC channels are not burdened by beacons, probe requests or responses, and thus can transfer the maximum possible user data.

To enforce good behavior, several rules are in place to reduce excessive probing and encourage client device designers to optimize their probing algorithms.

For example, a client device should not transmit (e.g. probe requests) in a non-PSC channel unless the client has learned that an AP is present, either by listening for 802.11 frames or through one of the mechanisms explained here. Even in PSC channels, wildcard probe requests are restricted, and the rate at which probes can be sent is limited.

Probe request rules:

Beacon changes

The beacon itself can be shortened in time and complexity because there are no existing Wi-Fi devices operating in the 6 GHz band. 6 GHz takes advantage of opportunities for house cleaning, forgoing transmissions that would only be of interest to older equipment. An example is the backwards-compatibility in the form of ‘capabilities’ and ‘operation’ information elements. Because, for example, a Wi-Fi 4 device is not programmed to understand Wi-Fi 5 parameters, beacons in the 5 GHz band must include the older Information Elements (IEs) in addition to newer ones added for subsequent generations.

The greenfield nature of 6 GHz allows these older elements to be dropped, saving time on the air and improving bandwidth efficiency. The example above shows that the HT and VHT operations and capabilities IEs are removed, and those values that are not superseded are added to the equivalent HE (Wi-Fi 6) IEs.

The ‘operations’ IEs are announcements from the AP about the transmission channel and are part of the beacon, probe response, association response, and re-association response frames. The ‘capabilities’ IEs list the options that APs and devices can use, and are transmitted in the information elements listed above and also the equivalent requests from client devices. Other modifications to the beacon include the rate at which the elements are transmitted: no pre-Wi-Fi 6 rates are allowed, forcing higher rates, shorter durations, and better bandwidth efficiency.

Multiple BSSIDs in one beacon

This feature was introduced as optional in 802.11v-2011 and was not initially implemented in clients or APs. For Wi-Fi 6E, clients are required to support multiple BSSIDs.

Where many SSID/BSSIDs are advertised on the same radio, as is commonplace in residential and enterprise WLANs, previously, each BSSID or virtual AP had to transmit its beacon separately. If an wanted to transmit four SSIDs, four separate beacons were transmitted in each 102.4 msec beacon interval, and many IEs were duplicated across beacons.

With the multiple BSSID feature, the common elements are transmitted once, and a separate information element is appended with the values unique to each virtual AP, now termed a ’non-transmitted’ BSSID.

The “new” beacon is considerably shorter, improving WLAN efficiency. However, this method may not be universally applied. For example, a limit on the beacon size may restrict the number of multiple BSSIDs to a maximum of 4, so when more than 5 SSIDs are used, multiple MBSSID beacons are still necessary to support the multiple BSSID feature. Thus, more than 4 SSIDs are split into multiple MBSSID sets.

Unsolicited probe responses

Unsolicited probe responses (UPR) function like mini beacons. While the usual beacon interval is 102.4 msec, unsolicited probe responses can be transmitted every 20 msec. The increased rate enables clients to decide whether the AP is suitable for connection through fast passive scanning rather than active probing or passive discovery of neighbor APs through reduced neighbor reports. The unsolicited probe response allows a client, rather than tuning to a channel and transmitting some probe requests and waiting ~20 msec for responses, to listen passively for just 20 msec and be sure the client has heard all BSSIDs on that channel. Unsolicited probe responses can contain the same information elements as a ’normal’ probe response, but they are transmitted to the broadcast address.

FILS announcements

Fast initial link setup (FILS) is a complete protocol for AP discovery, authentication, and handover that was introduced in 802.11ai and the Wi-Fi Alliance optimized connectivity certification. FILS was aimed particularly at public networks but was not widely adopted. FILS announcements act as mini beacons, transmitted every 20 msec. Each announcement frame contains the information necessary for a client device to decide whether the AP is suitable for connection.

FILS announcements can also incorporate reduced neighbor reports to advertise the channels of other APs of the same network. The ‘short SSID’ can optionally be substituted for the SSID in the FILS announcement. This value is a (non-reversible) hash of a full SSID.

FILS announcements and unsolicited probe responses serve similar purpose. Only one or the other would normally be required. Starting in AOS-8.11 and AOS-10.5, FILS announcements are the default advertisement for 6 GHz channels when capable HPE Aruba Networking APs are operating as single-band 6 GHz only (without 2.4 GHz or 5 GHz operation enabled). When the capable AP is operating in a multi-band state, FILS advertisement in 6 GHz is automatically disabled to conserve airtime. This is not configurable.

Out-of-band (OOB) AP discovery

The techniques mentioned above allow a client to scan the 6 GHz band to discover APs in-band. But most 6 GHz APs will operate as multi-band, where non-6 GHz radios can be coordinated, enabling a client scanning 2.4 GHz or 5 GHz can learn about a 6 GHz AP without tuning the radio to, or scanning, 6 GHz.

Reduced neighbor report

The reduced neighbor report (RNR) was initially developed for use with the FILS discovery protocol, but use is expanded for 6 GHz operation. When associated at 2.4 GHz or 5 GHz, the client can listen for BSSIDs containing RNR information elements and discover the channels where 6 GHz neighbor APs, in the same physical unit or separate units, are transmitting. This allows clients to move directly to the target 6 GHz channel.

• List networks on adjacent radios or neighboring APs
• Supports faster 6 GHz BSS discovery
• Includes a relative offset for target beacon transmission time
• Found in existing passive/active scanning through beacon or probe response frames.
• No configuration and is automatically appended to all BSSes on 2.4 GHz and 5 GHz radios on capable APs.

The reduced neighbor report IE is included in the beacon and probe response frames of the lower-band (2.4 GHz or 5 GHz) BSSID. The report includes fields for each neighbor BSS, if transmitting the same SSID in the 6 GHz band, along with channel and beacon offset.

The operating class and channel number refer to tables in IEEE 802.11 listing all possible channel widths and center frequencies. For 6 GHz, this guides the client device to the correct channel out of the possible 59 at 20 MHz, 29 at 40 MHz, etc. With this information, the client device can parse the RNR from a 2.4 GHz or 5 GHz BSSID and go directly to the channel in the 6 GHz band to find an equivalent SSID/BSSID.

The target beacon transmission time (TBTT) information in the reduced neighbor report refers to the beacon offset in time. The TBTT is measured in time units of 1.024 msec, and allows the client device to schedule an accurate time to go off-channel from the current AP and passively scan the beacon of the 6 GHz BSSID, subsequently authenticating if desired.

The full neighbor report could be used in place of the RNR, as a superset of the RNR information is contained with-in, but the standard promotes the latter because the RNR is shorter and more efficient. Reduced neighbor reports are supported on 6 GHz capable HPE Aruba Networking APs starting in AOS-8.9.

Access network query protocol

The access network query protocol (ANQP) is a pre-association exchange protocol initially added for Passpoint operation. ANQP allows a client device to query an AP about capabilities such as Passpoint, information about the venue, and identity providers that can be reached for authentication. When used for 6 GHz WLANs, ANQP conveys a full neighbor report for 6 GHz BSSIDs that may be in the same WLAN, whether using the same SSID or different SSIDs. Transmitting the neighbor report element over ANQP rather than directly in the beacon or probe responses is more efficient, as air time is not consumed unless the client device requests the neighbor report.

In a multi-band network, BSSIDs in the lower bands (2.4 GHz or 5 GHz) could advertise ANQP capability, and multi-band client devices could request neighbor reports via ANQP, allowing them to discover full SSID, BSSID, channel and beacon offset information without leaving the lower band to receive or transmit in the 6 GHz band.

Automatic frequency coordination

The 6 GHz bands contain incumbent users, and for higher power or outdoor operation with standard power APs, the FCC requires that incumbents are protected from interference from unlicensed 6 GHz users, including Wi-Fi. Some areas of the band are not allowed, even under automatic frequency coordination (AFC) control. The available channels for higher power or outdoor operation are from 5945-6425 MHz and 6525-6875 MHz, the U-NII-5 and U-NII-7 bands.

Fortunately, the majority of incumbent, such as point-to-point fixed service, users are known, as they are licensed and their details are listed in the ULS database, maintained and updated by the FCC.

The general architecture for automatic frequency coordination (AFC) centers on an AP, or group of managed APs, generating an inquiry message that includes respective location, elevation, and indications of the desired transmit power levels, channels or frequencies for transmission. The AFC uses this information as input to algorithms along with ULS data of licensed links and a terrain map. Some AFC operators may add building outlines to their terrain map to identify RF shadows. The response from the AFC to the AP inquiry will include a range of channels or frequencies that are available, and the AP, or RRM can pick from this list and transmit. The FCC will license a number of AFC service providers. APs must check with an AFC at least every 24 hours to receive fresh information.

The general concept of AFC calculations is as follows. Each licensed receiver’s location, antenna pattern, elevation above ground, and sensitivity are taken from the ULS database and used to plot contours defining zones of interference for given transmit power levels. The example above shows one contour calculated for interference to the antenna A; another plot would be generated for antenna B.

As the antennas for point-to-point links are usually very high-gain, these plots are shaped like a keyhole, with a short-distance circle around the back for sidelobe sensitivity and a long path down the antenna boresight.

The contour is significant because any Wi-Fi transmitter within the contour and above the power threshold for which the contour is calculated would cause interference at the fixed link’s receive antenna and should not be allowed at that location, frequency, and transmit level.

Many plots or contours must be calculated and plotted depending on the power of the interfering signal, and terrain is taken into account to determine line-of-sight conditions or whether high ground obscures the path. AFCs may pre-calculate the contours to reduce response delays. The diagram above shows a set of contours from an AFC, calculated for different power levels for a single fixed link receiver.

The response from an AFC to an AP inquiry message may be either a list of available channels, or available frequency ranges, along with the maximum power levels available, or both. The graphical example shows how such information might be displayed on an AFC client application. The top row is a frequency range response, giving power spectral density limits for the various ranges. The lower levels show channel availability by transmit power level (coloring is not part of the AFC response). The example clearly shows fixed links near the location, and that these effectively exclude a number of channels in the response. The AP would only be allowed channel assignment from channels allowing viable operation.

Because AFC operation is not permitted in the U-NII-6 and U-NII-8 bands, the results shown in the diagram for those frequencies are invalid.

Note that the AFC does not grant a particular frequency to the AP; the AFC just indicates those channels, frequencies, and power levels that will not cause interference to incumbents. There is no attempt to allocate specific channels or coordinate different channels for neighboring APs or WLANs from the AFC provider. Those tasks are left up to the AirMatch RRM service.