U.S. wireless operators are set to receive access to new millimeter-wave (mmWave) spectrum resources in the 42 GHz band that could help boost 5G and potentially 6G network deployments, especially in more limited geographic settings like in-building and private networks. However, propagation challenges could continue to be an issue.

The Federal Communications Commission (FCC) last month issued a notice that it was starting to look at a potential spectrum-sharing model for up to 500 megahertz of spectrum in the 42 GHz band. The FCC noted spectrum in that band “is largely unused” and could be ideal for “an innovative, nonexclusive spectrum access model in this greenfield spectrum, which is ideal for experimentation due to the lack of incumbent licensees.”

“Our goal here is to come up with a new model to lower barriers, encourage competition and maximize the opportunities in millimeter-wave spectrum,” FCC Chairwoman Jessica Rosenworcel said in a statement about the 42 GHz plans. “In short, it’s time to be creative. I look forward to the record that develops – and then look forward to sharing our creativity with the world.”

The FCC’s efforts feed into repeated claims from the mobile telecommunications industry about the need for more spectrum resources.

A recent report commissioned by industry trade group CTIA stated the U.S. needs access to 400 megahertz of new licensed spectrum resources over the next five years, with that need swelling to 1,400 megahertz by 2032.

While the FCC has been able to free up some lower-band spectrum resources – most specifically the C-band in the 3.7 GHz to 3.95 GHz band that is currently the basis for Verizon and AT&T to boost their 5G capacity – the more than 1,000 megahertz of new spectrum being requested will have to come from the mmWave band. This is considered spectrum residing above the 20 GHz band, and can span all the way up to 100 GHz.

However, the challenge with mmWave spectrum is that it has very limited propagation characteristics compared to low- (sub-2 GHz) and mid-band (2 GHz to 10 GHz) spectrum. This includes issues with mmWave spectrum penetrating walls and being blocked by foliage.

“Millimeter wave has arguably gone down as one of the greatest failures in the wireless world in recent memory,” Dan Hays, partner at PwC, said in an interview with SDxCentral during the recent MWC Barcelona 2023 event. “There was a lot of wishful thinking. And, to be fair, there have been advances in radio technology and massive [multiple-input/multiple-output] that cures a lot of things. But it only goes so far with the laws of physics.”

Jeff Wang, global 5G and networks lead at Accenture, noted in an interview that those higher spectrum bands are also going to force a complete re-think of deployment models to take into account their more challenging propagation characteristics.

“Accenture is working constantly with clients on how do we deploy this because that density is not easy economically,” Wang said.

As an example, he noted that if a sub-1 GHz-based network required 50,000 cell sites to cover the U.S., using sub-6 GHz, which many operators are rolling out as part of their highly touted mid-band updates, or millimeter wave (mmWave) will need hundreds of thousands of sites.

Operators have already spent billions of dollars to gain licensed access to mmWave spectrum and have been working tirelessly to integrate those assets into commercial operations.

Jennifer Fritzsche, managing director at Greenhill & Company, noted in a blog post via analyst firm iGR that AT&T has an average of 1,040-megahertz of nationwide mmWave spectrum and Verizon controls 2,024-megahertz of mmWave spectrum nationwide.

Kyle Malady, EVP and president of global networks and technology at Verizon, recently stated the carrier has deployed more than 40,000 nodes beaming out mmWave spectrum.

“Now that millimeter-wave technology turns into a tool for RF engineers to use in hot spots that they have,” Malady said.

Verizon has also touted the use of its mmWave spectrum holdings to power private 5G deployments. The carrier last year also aggregated 20 megahertz of “LTE” spectrum and 400 megahertz of 28 GHz spectrum that resulted in upload speeds up to 1.26 Gb/s.

AT&T is less enamored with mmWave spectrum and has only used it modestly in deployments.

“What we found is propagation off of millimeter-wave towers is so short,” Chris Sambar, president for network at AT&T, recently explained. “That’s a very difficult value proposition to make work, so we are focused on serving it with our mid-band rollout spectrum.”

AT&T did use those assets as part of its initial 5G deployment, similar to Verizon, but also like Verizon those efforts have since been overshadowed by its better propagating C-band spectrum holdings.

Vendors have been aggressively working to make mmWave spectrum more useful.

Nokia recently claimed it was able to generate more than 2 Gb/s in average download speeds using mmWave spectrum over a fixed-wireless access network over a distance of nearly 7 miles.

“This demonstrates that mmWave solutions will be an essential building block for operators to efficiently deliver widespread, multi-gigabit 5G broadband coverage to their customers in urban, suburban and rural areas, complementing sub-6 GHz spectrum assets,” Ari Kynäslahti, head of strategy and technology at Nokia Mobile Networks, noted on the event. “This is a substantial achievement that reflects how we are constantly innovating and evolving our 5G services and solutions.”

Operators are also dabbling with so-called “sub-terahertz” (Thz) spectrum that resides above 100 GHz.

Japanese telecom giant NTT, its mobile arm DoCoMo and Nokia earlier this year integrated artificial intelligence (AI), machine learning (ML) and sub-terahertz spectrum to power potential 6G services. The spectrum part of the trial used 144 megahertz of spectrum, a single 256 quadrature amplitude modulation (QAM) transmission scheme and antenna beamforming to produce network speeds of 25 Gb/s.

The sub-THz spectrum bands are an even greater challenge for telecommunication networks due to their limited propagation characteristics. However, there is a lot of untapped spectrum in those bands and the limited propagation characteristics play into better re-use opportunities that can be advantageous in confined environments.

NTT and Nokia noted that the combination of smarter radio transmission capabilities and the THz spectrum are “well-suited for high-accuracy radio sensing, which will likely be another key feature of 6G.”

Accenture’s Jeff Wang added that this level of innovation and creativity is going to be critical to squeeze value from mmWave and sub-THz spectrum.

“If you put that much stress on any process you have to be able to build in a new level of automation, a new level of intelligence, a new level of rigor to this and if you even let your mind go toward sub-terahertz [spectrum], it’s very hard to imagine,” Wang said. “If we don’t figure this out now, how we bring intelligence in, how we automate this, how we self-heal this, it’s just an unmanageable, logistical nightmare.”