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The Road to 6G

By Keith Kirkpatrick

Communications of the ACM, Vol. 65 No. 9, Pages 14-16
10.1145/3546959

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Although 5G technology is still in its relative infancy, top technology companies from wireless carriers to chipset manufacturers to meta technology vendors are actively working on the development of the next milestone for wireless communications, known as the sixth generation, or 6G.

The demand for even faster networks with greater capacity is being driven by a desire to support more complex and data-intensive applications, connect an even greater number of devices and data sources, and enjoy persistent, latency-free data connections.

When fully developed, 6G technology may one day support data transfer rates of 1 terabit per second (100 times faster than the 10 gigabits-per-second hypothetical top speed offered by 5G), and network capacities of 50 to 100 times that of 5G networks, thereby allowing a much larger ecosystem of connected devices, allowing consumer, industrial, and infrastructure-based devices to operate on the same network with no adverse performance impact. Further, where 5G networks generally support latency rates of about 4 milliseconds (ms), 6G could reduce that latency to near zero, and each access point likely will be able to support multiple clients simultaneously.

However, the vision for 6G and its technical underpinnings is still being formed, as a wide range of technology companies, governments, and industry groups each work on the technology that could enable a persistent, reliable, and speedy communications infrastructure that would support mobile applications, smart cities, V2x communications, virtual and augmented reality technology, and even personal biologic-data systems.

"I think the key thing with 6G is, and I think this is quite refreshing, is that it's going to be a network of networks, an amalgam of complementary technologies," says Stephen Douglas, head of market strategy for Spirent, a U.K.-based provider of automated testing and assurance solutions. "In addition to having a macro terrestrial network, you're potentially going to have these body area networks where humans are part of it as well." Douglas adds it is likely 6G will allow the interlinking of wireless networks with satellite, drone, maritime, and fiber-linked networks, resulting in a fully connected ecosystem.

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No Standards, but Lots of Market Activity

At present, no standards for 6G have been developed or published, simply because there is still significant work that needs to be done, particularly with respect to managing the technology's power consumption and signal propagation for the transmission of wireless signals. The U.S. Federal Communications Commission (FCC) has allocated 95 GHz and 3 THz for early research, development, and testing, and in October 2020, the Alliance for Telecommunications Industry Solutions (ATIS), a Washington, D.C.-based standards group that develops technical and operational standards for mobile technologies, formed the Next G Alliance. Including all four major telecom companies (AT&T, T-Mobile, U.S. Cellular, and Verizon), as well as companies such as Ericsson, Facebook, Intel, LG, Microsoft, and Qualcomm, ATIS was formed to advance U.S. leadership in the rollout of 6G, focusing on the alignment of the technology's market participants across the life cycle of 6G deployment, from R&D and manufacturing to standardization and market readiness.


"It's going to be a network of networks, an amalgam of complementary technologies."


Of course, other countries and companies also are working to develop technologies that could be used in the development of 6G networks. News reports indicate Finland's University of Oulu launched the 6Genesis research project to develop a 6G vision for 2030, and has signed a collaboration agreement with Japan's Beyond 5G Promotion Consortium to coordinate the work of Finnish 6G Flagship research on 6G technologies. Meanwhile, South Korea's Electronics and Telecommunications Research Institute is conducting research on the terahertz frequency band for 6G, and Samsung announced plans to invest more than $200 billion into areas such as chipset manufacturing to support the development of 6G's infrastructure and devices.

Within the U.S., there's a concerted effort to consider not just the technological challenges of sending data at very high frequencies, but to identify how a 6G network or networks can best serve a range of new data-intensive applications.

"What we're trying to do with 6G is to jump into this much sooner in the process and get industry, government, and academia together from North American/U.S. perspective, and think about it not just in terms of technology, but think about the applications, the societal drivers, the future spectrum needs, and all the big market [issues] that we think are going to take many years to solve," says Mike Nawrocki, vice president, technology and solutions, at ATIS. "We have to think across all these different dimensions beyond just the technology domain."


In the U.S., there's a concerted effort to identify how future 6G networks can best serve a new range of data-intensive applications.


The Institute of Electrical and Electronic Engineers (IEEE)'s International Network Generations Roadmap, 2022 Edition, echoes this view, noting that "The evolution and deployment of network generations is influenced and impacted not only by emerging, evolving, and potential convergence of technologies, but also by local and world socioeconomic and health conditions (and politics)."

Commercialization of 6G is likely 8 to 10 years away, as significant work must be completed with respect to the development of suitable applications and use cases, the setting of 6G standards and metrics, and the building and testing of the network technology and infrastructure. The technological hurdles involved with delivering even more speed, reliability, resiliency, capacity, and lower latencies than fully functional 5G networks represent a significant challenge.

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High-Frequency Spectrum Propagation Limitations

A key challenge with the development of 6G technology is identifying the technological approach to transmitting faster data rates. Several approaches are under consideration, but it is likely signal multiplexing techniques that support improved spectral efficiency within the area they are deployed will be used, including techniques such as Non-Orthogonal Multiple Access (NOMA) and Massive Multiple-Input-Multiple-Output (mMIMO). While these approaches provide greater capacity (in terms of the number of users that can be served in an area), they do not improve spectral efficiency per device or user, meaning each device served would not see a higher data capacity. Further, these approaches can introduce greater system latency and can feature low energy utilization efficiency. That said, orthogonal frequency division multiplexing (OFDM)-based NOMA systems have been proposed and found to achieve reasonable gains in spectral efficiency. OFDM is used in 5G systems, and is also used in the Wi-Fi 802.11 wireless LAN standard.

Another approach to supporting higher device data throughout is to pair a conventional OFDM waveform with an additional modulation technique that can create another dimension for conveying extra data per OFDM symbol. Techniques such as spatial modulation OFDM (SM-OFDM), subcarrier-index modulation OFDM (SIM-OFDM), OFDM with index modulation (OFDM-IM), and OFDM with subcarrier number modulation (OFDM-SNM) have been reported in the literature, and are likely being considered as potential techniques for delivering higher data throughput within a 6G system.

In addition to needing to support higher data rates, sending data via air interfaces generally requires higher frequencies than are used for 4G or 5G networks. Whereas today's 5G signals tend to operate in the 3.4Ghz to 3.8Ghz range, with future 5G implementations operating up to about 5Ghz, wireless 6G networks likely will use frequencies located in the terahertz or sub-terahertz range, roughly 95Ghz to 3Thz. The challenge is signal propagation; the distance radio signals are able to propagate or travel decreases as the transmission frequency rises. The shorter transmission range of projected 6G networks likely will require a denser network of base stations and repeaters to provide adequate coverage, given the relatively short (10-meter) radiation range of high-frequency 6G signals.

One potential solution to the challenges involved with using high-frequency spectrum to deliver radio waves without installing hundreds or thousands of power-hungry antennas or signal repeaters is to use reconfigurable intelligent surfaces, which can made from materials with specialized properties that can be used to redirect 6G signals and serve as amplifiers without requiring a dedicated power source. One such material is graphene, a single-layer, hexagonal matrix-based material that can be configured to sense and reflect electromagnetic waves in a specific direction, boosting and reflecting wireless signals.

Progress is being made, according to reports on successful tests of potential 6G wireless approaches. LG Electronics and European research lab Fraunhofer-Gesellschaft used adaptive beamforming and high gain antenna switching technology to send a 15 -decibel-milliwatt (dBm) transmission in the 155GHz-175GHz band about 300 feet. China's Ministry of Industry and Information Technology is investing in and monitoring 6G research and development in that country. A government-backed lab called Purple Mountain Laboratories announced in January 2022 that a research team had achieved wireless transmission speeds of up to 206.25 gigabits per second, albeit in a controlled environment.

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6G: A Network of Networks?

Still, industry watchers say 6G is not just about air interfaces; a 6G standard likely will include multiple interface types to account for the "network of networks" approach that seems to make the most sense, given the wide variety of applications that will require network access, and the expected convergence of certain technologies.

"A lot of the [future] applications get tied via marketing to 5G or 6G, but in reality, they may run mostly over Wi-Fi, because with Wi-Fi 6, the [technology] went from OFDM to OFDMA, which is more like 6G," says Phil Solis, research director for IDC's Enabling Technologies team for wireless and mobile connectivity technologies and semiconductors. OFDMA (orthogonal frequency-division multiple access) is a technology in Wi-Fi 6 enabling concurrent uplink and downlink communication with multiple clients by assigning subsets of subcarriers called Resource Units (RUs) to the individual clients, supporting larger data transmission channels and greater security. "So the point is that Wi-Fi is getting better and better, too," Solis adds.

Other experts also point to the hybrid nature of tomorrow's applications. For example, data within a home may use the latest incarnation of Wi-Fi, which supports very high data rates. If data needs to be sent beyond the home, some type of fiber optic connection would be used to send that signal to a cell tower, because fiber optic technology may be less costly to install in densely populated areas, compared with building out a vast network of antennas and repeaters within a small geographic area, according to Paolo Gargini, chairman of the International Roadmap for Devices and Systems (IRDS) sponsored by IEEE.

"If you really want to do 6G 10 years from now, there is a lot of infrastructure that is missing and needs to be put in place," Gargini says. "The reality is that if you really want to carry this higher frequency, like for 6G, you have to go to fiber," noting the limitation of signal propagation when using very high sub-terahertz and terahertz spectrum to transmit data.


"A lot of the [future] applications get tied via marketing to 5G or 6G, when in reality, they may run mostly over Wi-Fi."


Another example of the convergence of formerly disparate networks is seen in Huawei's approach to 6G. The Chinese tech giant announced plans to integrate terrestrial and non-terrestrial networks by launching several low- or very low Earth orbit (LEO/VLEO) satellites to form a mega satellite constellation, which will expand the coverage of the terrestrial cellular infrastructure and empower new low-latency solutions for ultra-long-haul transmission. Both networks are expected to be deeply integrated as one system where the terrestrial and non-terrestrial network nodes can be treated as base stations in a similar way, enabling users to leverage the advantages of each type in different service conditions.

Within North America and beyond, satellite 6G is projected to provide KPIs and QoS at an unprecedented level for Non-Terrestrial Networks (NTNs), according to the IEEE's Satellite Working Group, which has identified use cases that combine several technologies, metrics, and approaches. A chapter in the association's International Network Generations Roadmap, 2022 Edition, describes the technological hurdles and solutions to using satellite technology to support 5G and 6G networks, and "contains an enriched description of use cases combining direct satellite access and satellite back-haul, satellite IoT, mmWave for satellite networks, network management aspects, QoS/QoE, security, and recent standardization activities by 3GPP, ETSI, ITU, and IEEE."

Another approach to improving data transfer speeds and reducing power consumption include Japan-based telecommunications company NTT's 6G work, which includes the development and testing of a wireless network that uses an end-to-end optical communications infrastructure called Innovative Optical and Wireless Network (IOWN). The network uses photonics, or beams of light, to transmit data without converting the signals to electrical signals. Because such signal conversion is not be required, NTT is targeting a 100-fold improvement to power consumption, end-to-end latency, and transmission capacity levels, compared to traditional networks.

However, it is important to note that the visions for 6G, as well as any potential standards approaches, are likely to change before its commercial rollout, given that there is a significant amount of technical and funding work that needs to be completed. From a technical standpoint, it is possible that the actual 6G architecture standard may not be that important in the future, so long as the interfaces between various networks are standardized to allow data to flow across networks.

"I would say 6G is the opportunity to really converge Wi-Fi and cellular together," Spirent's Douglas says. "Could we not just have sort of one universal wireless that's connecting that can connect them to any type of backend network required? What is underneath the hood of the 6G network could be radically different, as long as the interoperability between them is standardized." Douglas says.

* Further Reading

ATIS' Next G Alliance 6G Roadmap: https://nextgalliance.org/working_group/national-6g-roadmap/

IEEE International Network Generations Roadmap, https://bit.ly/3wojXG0

H. Tataria, M. Shafi, A. F. Molisch, M. Dohler, H. Sjöland and F. Tufvesson,
"6G Wireless Systems: Vision, Requirements, Challenges, Insights, and Opportunities," in Proceedings of the IEEE, vol. 109, no. 7, pp. 1166–1199, July 2021, doi: 10.1109/JPROC.2021.3061701.

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Author

Keith Kirkpatrick is principal of 4K Research & Consulting, LLC, based in New York, NY, USA.


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