Why cmWave spectrum is expected to be a powerful enabler of 6G and future networks
- While 5G continues to revolutionize the way in which we interact with the world, the evolution to 6G is already in the early applied research phase.
- Spectrum will be crucial in supporting new capacity and coverage demands on the network, and cmWave bands are emerging as a particularly attractive frequency range.
6G services that need additional spectrum
There is no dispute that 5G is a revolutionary technology, enabling seamless communication from machine to machine, as well as machine to human. By 2030, it will have shaped both industry and society for a decade. We will have witnessed the emergence of many new applications and services, while learning invaluable lessons from every 5G deployment.
At the same time, a continued evolution to 6G will be taking place, one that will bring together the digital and physical worlds as we lay out in our 6G white paper. Predicted to be a crucial component in supporting current use cases such as mobile broadband, fixed wireless access and smart factories to name a few examples that will continue to grow exponentially according to the Ericsson Mobility Report, 6G will also address emerging use cases such as holographic communication, massive digital twinning, immersive communication and joint communication and sensing (JCAS).
This raises two important questions: how do we meet future traffic demands and make the 6G vision a reality? While the answer will of course involve many components, we believe that spectrum will be one of the most fundamental. By 2030, the available spectrum should be able to support the anticipated threefold increase in traffic due to current use cases, while also meeting the additional traffic generated by the new 6G use cases as illustrated in Figure 1.
To meet the networks’ needs in the timeframe 2030 and beyond, at least 1.5-2.2 GHz of additional spectrum for wide-area coverage will be needed. It is important to note that these figures are on top of wide-area spectrum bands allocated before 2030, as we reference in our white paper.
The layer cake: Traffic steering and aggregation
To fulfill the anticipated large range of use cases and deployments in 2030 and beyond, all available frequency bands must be utilized in the best way and traffic must be steered to the most suitable bands (see Figure 2).
- Lowband FDD will continue to provide the basic coverage layer. This spectrum will help to bridge the digital divide between urban and rural areas, while also providing connectivity everywhere while on the move.
- Midband TDD is today most widely allocated to 5G globally. Enough spectrum is essential to a vast array of applications, including mobile broadband, XR, AR/VR and bringing fixed connectivity to small towns and villages not reachable by fiber.
- mmWave (Millimeter wave) bands enable very high capacity but are limited to localized dense environments due to propagation characteristics.
- Sub-THz bands are suitable for some of the anticipated 6G use cases and applications – for example front/backhaul and extreme gaming – that require data rates in the order of at least several 100 Gb/s which in turn require bandwidths exceeding 10 GHz. Such wide bandwidths are only available above 92 GHz, e.g. in D and W bands. Due to propagation properties in these bands, access areas will be very small and front/backhaul applications will require very narrow beams. Fortunately, it is not expected that such extreme data rates will be required everywhere, and sub-THz (92-300 GHz) bands will complement lower bands in these extreme scenarios.
Finding 1.5-2.2 GHz of additional wide-area spectrum is not easy, but crucial in meeting the capacity needs of 2030 and beyond. The lower frequency bands are beneficial from a coverage perspective, but the amount of new spectrum, if any, is very small. Higher up in frequency (in the mmWave band), it might be possible to find a good amount of additional spectrum but, given the propagation conditions, it is less suitable for wide-area coverage.
This raises an important question: can we find a way to get the benefits of both these frequency bands?
Meet the cmWave (Centimeter wave) spectrum: a very attractive frequency range for future systems, for example 6G. The frequency range 7-15 GHz holds the promise of combining good coverage, especially at the lower edge, with reasonably large bandwidths. While the spectrum is currently in use for various purposes, proper coexistence mechanisms (when needed) can unleash fairly large amounts of spectrum for cellular usage. In addition to existing frequency bands and bands already under discussion, we believe the cmWave spectrum will become important for 6G networks. Due to large arrays, this range can be deployed at the same grid as “classical” midband while enabling higher capacity due to massive spatial reuse, also enabled by large arrays. This range will not only enable the continued growth of traditional mobile broadband and AR/VR, it will also cater for the introduction of new applications such as holographic communications and massive digital twinning. It is highly preferable to allocate frequencies in the lower part of the 7 – 15 GHz range keeping in mind the relative propagation characteristics.
To enable the best user experience, all available bands must be used together and in the best way, dynamically allocated to users and applications within the network where they bring the most benefit. With very high bands requiring denser deployments to compensate for propagation loss, the importance of a system design whereby nodes and bands can quickly be activated and deactivated becomes imperative to enable an energy efficient operation of the network.
cmWave would be possible on the current grid
The cmWave range can support wide-area coverage through the use of massive MIMO technologies, thereby allowing the current 3.5 GHz site grid to be reused. Compared to a 3.5 GHz antenna panel, the number of half-wavelength elements fitted in the same antenna housing can be quadrupled at 7 GHz. The net result is an increased antenna gain which compensates for the increase in propagation loss.
As an example, consider the London scenario in Figure 3 where the average inter-site distance is 450 m. 80 percent of the users are indoor and the population density is in the range of 10,000 persons/km2.
Figure 4 depicts the capacity that is possible to provide on this grid using multiple frequency bands. In this scenario, the downlink capacity possible to support using 150 MHz in the current frequency bands is around 5 Gbit/s/km2, a number that can be increased to more than 13 Gbit/s/km2 by adding 400 MHz of spectrum in the cmWave range (200 MHz at 7 GHz and 200 MHz at 15 GHz). The increased downlink capacity could also be used to offload more downlink traffic from lower frequency bands such as 3.5 GHz, freeing up these bands for uplink traffic which could be a benefit from a coverage perspective.
This simulation example clearly illustrates the benefits of the cmWave spectrum: improved capacity without requiring the site grid to be densified. Keep in mind this scenario took a modest approach to simulation assumptions where, for example, the 7 GHz antenna did not even exploit the possibility of a larger number of antenna elements when compared to 3.5 GHz. Adding mmWave spectrum, on the other hand, does not improve the traffic capacity in this example unless the site grid is densified. This is also schematically seen in Figure 4. The site grid in the London scenario would correspond to a coverage distance somewhere to the right of the mmWave layer, but inside the cmWave layer of Figure 2.
Future transmission technologies: what needs to be considered
As stated above, the aim is to initially deploy cmWave band on the same grid as “classical” midband. Also, according to the above, to compensate for the omni-directional pathloss increase that occurs between 3.5 and 7 GHz, at least four times as many antenna elements are needed.
5G radios often use 32 or 64 TRX ports. Each port is connected to a vertical subarray which contains three antenna elements. This results in a total of 96 or 192 antenna elements. Quadrupling antenna elements, starting from 64 TRX to compensate the increased omni-directional pathloss, results in 768 antenna elements. To gain additional coverage, the number of antenna elements can be further increased, see Figure 5.
Different architectural choices are possible to implement larger antenna arrays with many available options:
- Increase the subarray size. Using subarrays with 12 or even 24 elements could be arranged into one subarray, translating into a four or eight-fold increase in the total number of antenna elements, respectively. The advantages of this solution is simplicity and, due to the number of TRX ports not changing, required changes to the digital beamforming architecture and CSI acquisition are either minimized or eliminated altogether. However, a clear drawback of this solution is a much narrower vertical beamwidth.
- Increase the number of TRX ports. The other extreme case is to leave the subarray size as is, instead increasing the number of TRX ports to, for example, 256 or 512. In this setup, all beamforming (except for the subarray) is done digitally and in frequency-domain, enabling largest flexibility in MIMO precoder design and even frequency-selective precoders. Drawbacks include the need for a vastly increased number of analog-to-digital (uplink) and digital-to-analog (downlink) converters, resulting in increased complexity and energy consumption.
- In-between the two extremes. Multiple options exist between the two abovementioned extremes. One is to modestly increase the number of TRX ports and then rely on time-domain port expansion (downlink) / reduction (uplink) to interface the subarrays. This time-domain port expansion / reduction can either be done in digital domain, analog domain or split between the two.
Careful consideration is needed to strike a good balance between increased complexity and flexibility. Energy consumption also varies greatly from one option to the next, typically increasing with the number of TRX ports. To keep energy consumption low it must be possible to hibernate / switch off unused components, both in the radio and baseband. In the past, radio power consumption clearly dominated. As the number of antennas continues to increase, however, a corresponding spike in baseband power consumption takes place, now necessitating power consumption to be considered in both domains.
Different architectures also have different radiation characteristics. For example, a large vertical subarray would decrease the vertical beamwidth. Further research and consideration are needed to explore how the above solutions can improve coexistence with other co-primary services in these bands, as well as adjacent bands such as radar, fixed services and satellite services.
Increasing the number of TRX ports beyond 64 also requires improvements to CSI acquisition. For example, additional CSI-RS antenna ports are needed. Analyses of the benefits and tradeoffs for each option is ongoing and decisions are yet to be made on the best design.
Once base stations with increased array sizes are deployed, they can also be used to improve services beyond communication. Positioning is already an important service and JCAS is often mentioned as a promising beyond-communication service of 6G.
Both positioning and JCAS benefit from large bandwidths and large antenna arrays, making the cmWave band an interesting one for these two services. Assuming a per-operator contiguous bandwidth of 200 MHz in the cmWave band enables a range resolution of less than a meter for sensing and time-of-flight measurements in the same order for positioning. Several positioning and sensing methods rely on accurate angle-of-arrival estimation which improves with array size. By combining wide bandwidths likely available in cmWave bands with increased array sizes needed to maintain coverage, cmWave band deployments become a very promising candidate for high-accuracy positioning and sensing.
How do we get the spectrum? “Coexistence aspects”
In section 1, the amount of cmWave spectrum required was estimated to be in the order of 1.5-2.2 GHz. Assuming three network operators in a geographic area, this implies 500-750 MHz spectrum per operator. To avoid the high complexity of non-contiguous carrier aggregation, while also enabling positioning and sensing services with a range resolution in the order of a meter, spectrum should be allocated in contiguous blocks of no less than 100-200 MHz.
According to the International Telecommunication Union Radio Regulations (ITU-RR), a significant number of bands within the 7-15 GHz spectrum range are already allocated on co-primary basis to mobile and fixed, as well as other services. This necessitates careful considerations and studies on coexistence between mobile, and other co-primary services in these and adjacent bands.
The difficulty of finding a clean spectrum, combined with the determined goal of efficiently using scarce spectrum resources, make spectrum sharing and coexistence capabilities more important than ever before. Ericsson recognizes the challenge and is committed to exploring this area. Successfully addressing the challenges opens the possibility for a significant amount of spectrum in the cmWave bands to be unleashed for cellular communication. In essence, mmWave-like bandwidths combined with midband coverage – a very attractive way to meet the capacity demands of 2030 and beyond.
Read the recent proposal of the Electronic Communications Committee for 6G spectrum needs and candidate bands
Read our 6G white paper: connecting a cyber-physical world
Learn more about 6G and find other articles on Ericsson’s 6G pages.
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