Brought to you by Keysight Technologies
By Sarah LaSelva
New spectrum has been key to delivering new services, higher capacity, and faster data throughput rates in each new generation of cellular communications.
The new spectrum that will be available for 6G is unclear, but there are three frequency ranges under discussion: the upper mid-band (sometimes referred to as mid-band or unofficially as FR3) between 7-24 GHz, sub-THz bands between roughly 90 GHz to 300 GHz, and maximizing spectrum below 7 GHz through re-farming, new band allocation, and increased spectral efficiency.
Figure 1 shows spectrum allocation by generation, including potential bands for 6G. Each proposed band has benefits and drawbacks examined in detail below.
The Upper Mid-Band (7-24 GHz)
The spectrum between 7-24 GHz is the most appealing new spectrum for early 6G systems. A simplified chart of frequencies allocated for mobile and fixed wireless access between 7-24 GHz is shown in Figure 2.
The 7-15 GHz range is attractive for its propagation characteristics that are like the bands immediately below 7 GHz. At these frequencies, signals have less propagation loss than with FR2 and have a better chance of penetrating buildings and other structures allowing for indoor coverage.
This would give operators a way to increase network capacity without the need to add significant cell-site density, as required to expand mmWave FR2 coverage.
The main challenge for using this spectrum in 6G is regulatory. The spectrum is fraught with both civilian and government incumbents and used for applications other than fixed and mobile wireless access like meteorology, radio astronomy, and maritime radio navigation.
Many of the incumbents will be difficult or impossible to relocate given the number that are based on government or satellite communications – which are difficult, if not impossible, to change after the satellite is in orbit.
Even if regulators can come to an agreement on spectrum availability and licensing schemes, the most challenging technical aspect to overcome for this spectrum is how to share the spectrum without disrupting other users.
The Sub-THz Bands (90-300 GHz)
Sub-THz frequencies offer large, contiguous chunks of spectrum that could be allocated for 6G. With bandwidths up to 20 GHz, they must be considered for 6G even if they present profound technical challenges. It is not difficult to envision applications that require extremely high data throughput, greater than 100 Gbps.
Given the state of the art in spectral efficiency, it will require larger contiguous bandwidths than those available in lower frequency bands. Despite the associated technical challenges, the potential to solve difficult problems, like space to Earth links, multi-dimensional visual and audio communications, and advanced communications and sensing applications, makes these bands worth further research.
The frequencies used in sub-THz remains an open question. The 90-110 GHz range (W band) has multiple segments with reasonable contiguous bandwidth allocated for mobile or fixed wireless, and there are larger contiguous bandwidths available in the 110-170 GHz D band, as shown in Figure 3.
There are other bands allocated for mobile and fixed services above 200 GHz in the G and H band, as shown in Figure 4, but use of these frequencies for commercial communications will be further in the future than in W or D band.
The W and D band are the most likely sub-THz bands for initial use because of the relative maturity of the ecosystem at those frequencies, and because of their more favorable propagation characteristics.
The higher the frequency, the greater the attenuation in free space. This is aggravated by molecular absorption peaks in our atmosphere as shown below in Figure 5.
Technologies like beam steering, reconfigurable intelligent surfaces (RIS), and novel antennas could be used to overcome the path loss in sub-THz. Even with these technologies and techniques, the propagation loss is enough that it will be some time before sub-THz is used for traditional mobile applications.
The Lower Bands (Below 7 GHz)
The 600 MHz through 900 MHz bands will continue to be the mainstays of wide area coverage. These frequencies can cover the most distance and have high outdoor-indoor penetration. They will continue to be important in rural deployments and the best when reaching the cell edge.
Wider bandwidths in these bands are not feasible, even if more adjacent spectrum is allocated. Even without new spectrum in this band for 6G, these low bands will continue to play a key role.
Obtaining new spectrum between 1-7 GHz for mobile and fixed wireless will happen over the next 5 to 10 years to help meet the throughput demands of 5G. Any new spectrum allocated in this band will be leveraged by 6G as well.
There are several new bands for consideration during WRC-23—3.3-3.4 GHz, 3.6-3.8 GHz, 6.425-7.025 GHz, and 7.025-7.125 GHz, with the 7.025-7.125 GHz being the only band identified for use globally.
Reexamining how spectrum in this band is used and re-farming it for new generations, as seen for the C band in 5G, is an example of how wider bandwidths can be obtained below 7 GHz. Spectrum in this frequency range is a finite and scarce resource that must be utilized in the smartest and most efficient ways possible.
The exact frequencies that 6G will use are unknown. The ITU allocates spectrum for International Mobile Telecommunications (IMT) at World Radio Conferences, WRCs, which occur once every four years.
During the WRC, attendees work to identify frequency bands that could be used internationally for IMT in attempt to have global harmonisation of spectrum while balancing the need to protect incumbents in bands of interest. Global spectrum harmonisation is desirable because it enables an economy of scale for components and limits the number of bands that must be supported by user equipment.
After IMT bands are identified, national regulators must allocate the bands for mobile service in their region. This may require specific bands to be reserved or reallocated. Bands are assigned through a variety of mechanisms – including, but not limited to – auctions, bidding, and direct license.
At the end of each WRC, the agenda for the next WRC is set, including the list of frequencies covered for potential allocation. After the agenda is set for the WRC-27, there will be more clarity, but no guarantees the bands proposed there will be the bands implemented when 6G rolls out. Figure 6 shows when these meetings will occur in relation to the 3GPP release cycle and roll out of 6G.
Based on current industry trends, 6G will leverage many different bands. The 7-15 GHz band is the most likely candidate for initial 6G deployments, but 6G must leverage the already-allocated spectrum below 7 GHz. Improvements in dynamic spectrum sharing are necessary to leverage all bands with the highest efficiency.
The sub-THz bands, while not a target for 2030 deployments, still hold promise. These bands are seen as a target for a later phase of 6G which will deploy closer to 2035-2040. Their wide bandwidths are needed to enable some of the new applications and use cases targeted for 6G.
By leveraging different spectrum, like using different tools to provide various kinds of performance, 6G will be able to meet the ever-growing demands and expectations for cellular communications.
About the author
Sarah LaSelva, director of 6G marketing at Keysight Technologies
Sarah leads the marketing efforts for Keysight in 6G. She has over a decade of experience in test and measurement concentrating on wireless communications, both studying and promoting the latest wireless technologies. Throughout her career she has spent time in marketing, test engineering, and applications engineering.
Prior to joining Keysight, Sarah worked at NI (National Instruments) as a product marketing manager for the software defined radio team where she gained a deep knowledge of SDR hardware, software, and wireless communications.
Sarah’s background is in microwave and millimeter wave technology. She has a BS in electrical engineering from Texas Tech University.