Millimeter-Wave Spectrum Still Underperforms: Former FCC CTO
Former FCC Chief Technology Officer Monisha Ghosh warned Tuesday hat high-band spectrum isn’t living up to the hype. One of the biggest 6G challenges is “where does the new spectrum come from,” she said during the Fierce Wireless 6G Evolution Summit: “Every G has required new spectrum because the G’s are not backward compatible.”
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Ghosh, now an engineering professor at Notre Dame, said she has done extensive testing of millimeter-wave spectrum in Chicago and the performance “is still lacking.” Problems “due to propagation, obstruction, body blockage, still exist,” she said: “There are a lot of signal processing approaches to overcome these challenges -- they’re not easy to implement. There is almost no outdoor-to-indoor coverage that we have been able to measure.”
Even outdoors, the coverage is limited, Ghosh said. There are also “huge handset constraints,” she said. When a user is connected to millimeter-wave and is downloading at 2 Gbps speeds “your phone heats up tremendously and you’re not able to sustain that throughput for very long,” she said.
One question regulators have to ask is whether to focus on higher data rates or on “making sure that everybody is connected at the minimum speed,” Ghosh said. “That requires a very, very different architecture, very, very different frequency planning,” she said. Ghosh suggested 6G should be the last G, designed in a way that it will evolve similar to Wi-Fi. Wi-Fi uses the same spectrum in all its generations, and all are backward compatible, she said. “6G should be designed to be both future-proof and backward compatible,” she said.
With initial comments now in on 12.7 GHz (see 2212130047), Ghosh said 6G should emphasize the use of shared spectrum. “As we look at future bands, 7-24 [GHz], it is highly likely that most, if not all, of that frequency range, will end up being shared in some way,” she said.
6G Unknowns
Open radio access networks will be part of 6G and will help address backward compatibility, predicted Qammer Abbasi, senior lecturer at the University of Glasgow. 6G will mean advances in technologies like massive multiple-input and multiple-output and beam forming “to increase capacity and speed,” he said. “6G will have inherited integrated mobile edge computing” from 5G and lead to “seamless connectivity around the globe” using satellite connections, he said.
Abbasi said he's doing research on virtual reality that will involve all the senses, not just hearing and sight. Now we can’t communicate touch, smell and taste, and doing so will require “ultra-, ultra-high reliability, ultra-, ultra-low latency … which 5G cannot do,” he said.
“For us to see a generational change in 6G we need to first see what new things 6G is going to bring that 5G could not deliver,” said Rikin Thakker, Wireless Infrastructure Association chief technology officer. With 5G, big changes include the use of high-band spectrum, general acceptance of small cells and densification, and lower latency, he said. “We need to ask ... will 6G bring another round of firsts,” he said. “I think it’s going to, but [that’s] yet to be seen at this point,” he said.
5G-advanced will bring more use of AI and machine learning, with more support for extended reality, Thakkar said. Thakker advised keeping a close watch on 3rd Generation Partnership Project Release 18, the next big 5G release, as “a stepping stone” to 6G. “It’s too early speculate on the use cases for 6G,” he said.
The biggest promise offered by 6G is immersive experiences -- “how the digital world comes together and replicates what you can see in the physical world,” said Renuka Bhalerao, Meta Connectivity lead-radio access network ecosystems and technologies. Meta believes people should enjoy virtual experiences as much as they do the in-person world, she said: “That will be the best possible use case for 6G.”
U.S. networks are built based on Release 15, which means commercial networks are about two years behind the latest standards work, said Abhimanyu Gosain, senior director-technical programs at the Institute of Wireless IoT at Northeastern University. The biggest change in 6G will be in ultra-reliable, low-latency communications, he predicted. “Here, we really need to evolve positioning and localization technology, leverage spectrum in the higher bands, in millimeter-wave and THz, and enhance those so joint communications and sensing can happen,” he said. That means that for the first time, the same band will be used at the same time for communications and sensing, he explained. Integrating backhaul and access “is pretty much where the promise of rural broadband and bridging the digital divide is going to happen,” Gosain said.
“Mid-band spectrum is a fleeting resource, heavily utilized” and divided in the U.S. between DOD and operators, Gosain said. “Dynamic spectrum sharing is going to be a key construct that needs to be explored and standardized,” he said: “Non-terrestrial networks are going to be a key part of future G, or 6G, and we need to integrate those technologies.”