Some of the world's biggest 5G equipment suppliers have begun putting the so-called "centimetric" spectrum bands – those between 7 GHz and 20 GHz – at the center of their pitches for 6G.

That's different from the early 5G noise that centered on the millimeter wave (mmWave) spectrum bands, which run from 24GHz to 40GHz. And it's a far cry from early 6G buzz that focused on the terahertz (THz) spectrum bands sitting between 90GHz and 300GHz (or 0.3THz for those paying attention).

"Spectrum from within the 7-20 GHz range is essential to realize the capacity-demanding use cases in future 6G networks," Eliane Semaan, director of spectrum and technology regulation for infrastructure vendor Ericsson, wrote earlier this year.

The issue is important to Ericsson: The company just announced a major investment into 6G research and development in the UK.

Figure 1: (Source: Kirill Ivanov/Alamy Stock Photo)

Nokia is also talking about centimetric spectrum for 6G.

"Ten years from now we expect new spectrum bands between 7 GHz and 20 GHz to open up for 6G use, which will provide the necessary bandwidth to create these new high-capacity carriers," wrote Harri Holma and Harish Viswanathan, two top Nokia researchers, on the company's website last year.

The focus on centimetric bands is serendipitous. Jessica Rosenworcel, chairwoman of the FCC – the agency in charge of managing spectrum in the US – recently said she's eyeing the 12.7GHz to 13.25GHz spectrum band as a possible location for the agency's next big spectrum push.

Terahertz as 'complementary'

As noted, the focus on spectrum bands between 7GHz and 20GHz for 6G represents a shift from earlier 6G speculation that focused on the terahertz bands above 90GHz. For example, an early Samsung white paper on 6G argued that it's "inevitable" that next-generation mobile networks will utilize the terahertz bands, from 100GHz to 10THz. And Wind River at one time promised that "6G will operate on terahertz bands from 100 GHz to 10 THz."

But the terahertz bands suffer from propagation challenges that are even more acute than those facing the mmWave spectrum bands. Meaning, transmissions in the THz bands may only travel a few feet, whereas transmissions in the mmWave band can sometimes travel several hundred feet.

That kind of performance in the THz bands isn't a surprise, though. Signals travel farther in lower bands due to physics. However, higher bands – like the mmWave bands or the THz bands – are alluring because they are relatively vacant and can handle massive amounts of data.

Because of limited coverage, the big 5G vendors now see the THz bands playing a supportive role rather than a leading one in 6G.

"We have thus identified new potential spectrum ranges for 6G, notably in the centimetric range from 7-15 GHz, which we believe will be an essential range, and in the sub-THz range from 92-300 GHz, which will have a complementary role serving niche scenarios," Ericsson's chief researcher, Magnus Frodigh, wrote earlier this month.

He continued: "Our learnings from 5G are that the mmWave range is a powerful spectrum range which allows operators to provide value to industry and enterprise through high data rates. However, due to limited coverage, it serves as a complement to other ranges that can be used in wider areas but with limited data rates (i.e. mid bands)."

Frodigh concluded: "We believe that in order to benefit society, the majority of 6G use cases should be enabled for wide-area coverage, both indoors and outdoors, and not limited to confined areas. This means that – whilst we ought to explore the sub-THz region for entirely novel 6G capabilities – the main value will be in the centimetric 7-15 GHz."

Learning from history

In the US, the early days of 5G were dominated by discussions about mmWave and the capabilities of highband spectrum. Verizon, a loud and early supporter of mmWave, based much of its initial 5G story on the short-range wireless technology.

But mmWave 5G failed to deliver substantive changes. Verizon backed off a plan to charge extra for its mmWave network. And when midband spectrum in the 3GHz range became available in the US, all the mobile providers in the country immediately pivoted to a 5G effort centered on midband.

The full extent of the difficulties around mmWave 5G is only now becoming apparent. For example, South Korean regulators just disclosed that operators in that country had been tasked in 2018 with building 7,500 mmWave basestations by 2021, but they managed to hit only 10% of that target.

"As a policymaker I express very strong regret for this outcome," Vice Minister Park Yun-kyu said about the operators' limited achievements.

South Korea has long been viewed as a top market for 5G development.

Thus, it's clear that wireless network operators simply cannot quickly construct extensive 5G networks in highband spectrum. Instead, they must start with spectrum that can cover broad geographic areas – in the low- and midband – which allows them to fill in high-traffic hotspots with highband networks as necessary.

The same scenarios, it seems, are expected to play out in 6G.

Holma and Viswanathan, of Nokia, explained that the bands between 7GHz and 20GHz should be considered the "midband" of 6G. It will serve as the "workhorse frequencies of mobile networks," they explained.

"Lower bands do not have sufficiently large bandwidth allocations to create the 400 MHz carriers needed for 6G," Holma and Viswanathan wrote. "Meanwhile the harsh signal propagation conditions in the ultra-high frequency bands make it expensive to use them for wide area coverage. Midband spectrum [between 7 GHz and 20 GHz] is the sweet spot for simultaneously providing extreme capacity and competitive coverage."

Related posts:

— Mike Dano, Editorial Director, 5G & Mobile Strategies, Light Reading | @mikeddano