6GHz and spectrum-sensing

A new and dynamic wireless landscape is evolving in the 6GHz spectrum range. Around the world, regulators are creating different sharing and coexistence models in the band between 5.925-7.125GHz. This necessitates new ways to monitor and control spectrum, as it becomes home to mix of:  

• Existing incumbent users, such as fixed microwave links and satellite systems 
• Unlicensed wireless systems such as Wi-Fi, fixed wireless access (FWA), Bluetooth, and other technologies. 
• Cellular 5G and future 6G networks, for both mobile and FWA applications. 
• Scientific sensing, for instance, of sea-surface temperatures 

In much of the discussion, the treatment of the overall 1200MHz-wide band is divided between: 

• Upper 6GHz from 6.425-7.125GHz (U6GHz) 
• Lower 6GHz from 5.925-6.425GHz (L6GHz) 

Originally, this band received little attention from commercial wireless providers, either from Wi-Fi or cellular camps. The original definition of 5G mid-band was from 1-6GHz, while IEEE and the Wi-Fi Alliance only started serious discussions about the 6GHz band from about 2018. In 2020, the US opened up the entire 1200 MHz for unlicensed use, ushering in Wi-Fi 6E and more recent Wi-Fi 7 standards. The mobile industry belatedly realised that it had overlooked the potential of the band and ramped up its industry lobbying and standards efforts. 

For unlicensed use of 6GHz, there are three categories to consider: 

• Low power indoor (LPI) use, suitable for Wi-Fi in homes, industrial settings or offices. This is usually “true” unlicensed, where no individual permits or access point registration is required. Equipment is ensured as being indoor-only by denying certification to any access points with weatherproofing. 

• Standard power for outdoor or indoor use, typically using an AFC to ensure no conflicts with incumbent users. 

Very low-power (VLP) use, intended for short-range communications, such as between a smartphone and accessories, in a similar fashion to Bluetooth and other IoT-centric wireless technologies. 

Now, there is a complex debate between different stakeholders around sharing vs. exclusivity, different power levels, protection rights of incumbents, indoor vs. outdoor usage, and the use of techniques such as databases, cross-technology signalling, and various forms of sensing. While the mobile industry would prefer exclusive licensing for 5G / 6G, this is only likely in some countries or some portion of the band. For the rest, some form of sharing will be the baseline case. 

Where is 6GHz allocation today? 

This article is not the place for a full analysis of the potential use cases for 6GHz, the believability of different forecasts for spectrum demand, or implied economic value.  

In general, the Lower 6GHz has been less contentious and has already been opened for unlicensed LPI (low-power indoor) use in much of the world outside China and central Asia. Remaining countries, such as India, seem likely to follow suit.  

The upper part of the band has seen much more debate, including at WRC-27, where it was identified for IMT (mobile) use regionally, but with a footnote also noting that local network use (essentially Wi-Fi and unlicensed) was possible, too.  

As well as the US, various other countries have also released the whole 1200MHz band for unlicensed use, including Canada, South Korea, Kazakhstan, and a number of Latin American nations. 

China has been the main driver behind assigning the band for 5G / 6G operators. Along with support from mobile industry associations, it has led to an expectation of clearing incumbents and issuing full-power exclusive licenses to MNOs for wide-area deployment. That said, it hasn’t yet been allocated to any Chinese operators yet, except for an auction in Hong Kong which demonstrated fairly lacklustre enthusiasm. 

Other places in Europe, the UK, and Australia are contemplating a band split—for instance, making a chunk at the bottom of U6GHz (maybe an extra 160MHz) unlicensed and putting the remainder in the hands of mobile. Some are also considering an opportunistic sharing model on top of that, for instance, allowing Wi-Fi in the upper part if there is no prioritised cellular use nearby. 

As the U6GHz controversy rumbles on, a secondary trend is occurring in L6GHz. In places that have already made it available for LPI use, there is growing interest in using a dynamic mechanism to allow an increase to standard power in locations and sub-bands where it will not cause interference to incumbents. This involves querying a database of incumbents’ locations and requirements called an AFC (automatic frequency coordination) system.  

Before a standard-power access point can transmit, it must transmit its geolocation (with GPS or similar) and device information to the AFC and obtain a list of available channels/power levels. If an access point is moved, or else every 24 hours, it must re-check the database for any new constraints. 

That model is already in use in Canada and the US (for the whole 6GHz band) and is being consulted on in countries including the UK, South Africa, Mexico, and a few others. 
 

Protecting incumbent users 

Various organisations’ wireless systems already occupy 6GHz spectrum in many countries. While some incumbents may be easy to move to alternative bands, or perhaps “re-pack” to a sub-section of the 6GHz frequency range, regulators have largely adopted a conservative approach to protecting those existing users when allowing unlicensed use. 

Most typically, these incumbents include  

• Fixed point-to-point microwave links (used for telecom backhaul, utilities, public safety communications, and various other industries’ connectivity). Many of these links are important for critical infrastructure and services and require extreme reliability and protection from interference. 
• Satellite communications (earth stations and feeder links) 
• Broadcast auxiliary services such as wireless cameras or outdoor TV links 

All of these are highly sensitive to interference, especially from high-power macro 5G / 6G cell towers, or to a lesser extent, outdoor standard-power Wi-Fi. That said, almost all the incumbent microwave users are situated in static locations, which makes geographic sharing easier to achieve via the AFC. Broadcast and some others are more intermittent and semi-mobile, however, which makes them less suitable for this approach. 

While the mobile industry would prefer to clear incumbents from the 6GHz band and obtain exclusive rights, the reality is that this will face numerous challenges. Some parts of the band may ultimately use geographic sharing with protection or exclusion zones, while others may have more dynamic models. 

By starting with LPI, coupled with restrictions on devices (no weather-hardened or battery-powered access points), they have been able to convince incumbents to coexist with new users.  

The importance of sensing 

One thing is absolutely certain about 6GHz – there will be a messy patchwork of different licensing regimes and technologies, which will continue to evolve over time. There will also be many different mass-market products supporting the band, both in terms of infrastructure and end-user devices. 

This inevitably means that some equipment may be used in ways or places that is not permitted. This is made more complex because incumbents can’t “see” Wi-Fi signals or licensed 5G radios through their highly directional antennas, even if they create interference. 

That therefore means that incumbent protection will increase in importance, as well as enforcement of any hybrid / sharing models. A core element of this is the use of separate sensors. A co-located sensor near an incumbent’s site can scan a wider view of the band and can catch interference sources that the main receiver’s antenna sidelobes might pick up.  

In fact, there are multiple potential use-cases for spectrum-sensing in 6GHz, depending on the specific incumbents of the band, and the levels of dynamic sharing that are made available in a given market: 

• Monitoring and enforcement of AFC rules and permissions, such as which sub-bands are being used. 

• Detection of rogue or non-compliant devices, such as an imported Wi-Fi router or a badly-configured 5G / 6G smartphone or small-cell. High-end sensors with geolocation abilities allow quick pinpointing of violators, so that corrective action can be taken. 

• Protection of satellite uplink sites, either from unlicensed outdoor use by FWA, or to ensure that cellular 5G sites conform to specified exclusion areas. 

• Ensuring no illicit outdoor use of standard-power unlicensed technologies in spectrum bands allocated to mobile use 

• Sensing as part of dynamic sharing and cognitive-radio models for blending incumbents, Wi-Fi and cellular. (It should be noted that early ideas may instead use cross-technology signalling mechanisms – although again, monitoring and enforcement may be needed). 

• Ongoing monitoring of cellular 6GHz deployment and usage, to allow for fine-tuning of band allocation and enforcement of any coverage commitments 

• Measurement and trend analysis of cumulative interference from very low power outdoor, or low-power indoor unlicensed radios 

• Ensuring that opportunistic use of 6GHz by cellular networks is at appropriate power levels and does not cause interference to prioritised unlicensed users 

• Sensing allows regulators to collect and analyse spectrum usage data to see, for example, what percent of time certain channels are busy with cellular or Wi-Fi, to help guide future spectrum policy.  

Sensor network outputs can also be integrated with AFC databases or management systems to feed back information on actual interference events to improve the propagation models in the AFC. 

Conclusion 

In summary, the 6GHz band represents an enormous opportunity for wireless innovation, which will likely vary over time, and across different regions of the world. Unlocking that innovation fully – especially where sharing of spectrum between licensed, unlicensed and incumbent users occurs - requires trust and verification.  

Spectrum sensing equipment can provide the truth, as it can catch the real-world things that databases cannot know. The end architecture will likely involve multi-layer coordination, using databases for reliability and predictability, sensing for agility and real-time adaptation, and direct signaling across wireless technology domains to assist dynamic mechanisms. 

It can empower regulators and incumbents with confidence that new devices and networks are behaving, and thus help guide the continuous evolution of new spectrum management models, based on the experience with 6GHz.