Even before regulators outside the USA have decided on their timescales and policies for millimeter wave spectrum, researchers are already casting their eyes upwards to the airwaves above 95 GHz. Last week, the USA’s FCC approved experiments in these very high bands.
Just as mmWave spectrum up to 90 GHz is synonymous, for many, with 5G, so vendors and academics often link even higher bands with ‘6G’. Bluesky projects looking at post-5G generations of wireless technology are heavily focused on new spectrum options, and the technologies to address their implementation challenges.
None of this will become real for a decade at least, but the FCC’s decision does take very high frequencies out of the realm of fantasy and put them on the roadmap. “Prior to this decision, the Commission had no rules for authorizing communications above 95 GHz, other than by amateur operators or through experiments of limited duration and scope,” the regulator noted in announcing a unanimous vote, by its five members, to create “a new category of experimental licenses for use of frequencies between 95 GHz and 3 THz. These licenses will give innovators the flexibility to conduct experiments lasting up to 10 years, and to more easily market equipment during the experimental period.”
Professor Ted Rappaport of NYU Wireless, perhaps the most influential research outfit in mmWave, says terahertz spectrum could support “wireless cognition” – or data rates that are high enough to carry human cognition, as well as supporting enhanced versions of the emerging tactile Internet experiences.
He returned to the role of director of NYU Wireless last August, partly to drive the center’s work into even higher bands. He believes recent technological developments such as those in quantum computing and nanotechnology will make the terahertz spectrum more usable in future.
“While we have pioneered the use and understanding of mmWave frequencies for 5G, it is clear that new knowledge will be needed to bridge the gap between the fundamentals of these new areas with the design and fabrication of devices,” he said.
NYU Wireless is studying techniques in areas like beamforming that could help deal with the propagation challenges of high frequencies. Partners include Qualcomm, AT&T, Sprint, Ericsson, Huawei and InterDigital. Another group, in which Nokia is prominent, is the mmWave Coalition, which also includes NYU Wireless, plus Global Foundries, Keysight, Nuvotronics, Qorvo and others.
This group was set up in late 2017, and Nokia’s Paul Norkus greeted it with a mission statement. “The mmWave Coalition member companies are united in the objective of removing regulatory barriers to technologies and using frequencies ranging from 95 GHz to 450 GHz,” he wrote in a blog post. “While 5G and possibly even 6G(!) might look at these as potential frequency bands to use, the Coalition is not limiting itself to supporting any particular use or technology. Instead, it is working to create a regulatory structure for these frequencies that would encompass all technologies and all possible uses, limited only by the constraints of physics, innovation and the imagination.”
At last month’s Mobile World Congress, Nokia’s CTO Marcus Weldon said that any 6G technology – which he associated with spectrum above 95 GHz – could be included in 3GPP Releases 18 or 19 – which would give 5G just two more releases after the current Rel15. Despite that, Weldon does not expect to see ‘6G’ trials unti 2030. The biggest challenge for spectrum above 95 GHz, he said, would be the need for new chip technology, as CMOS would not be usable. The biggest breakthrough in making mmWave practical for mainstream, mass market equipment came when IBM and Toshiba succeeded in making 60 GHz chips in CMOS, but it is unclear that the same could be done at even higher frequencies.
Not all the terahertz R&D is taking place in the USA. The European Union is supporting, under the auspices of its Horizon 2020 program, a project called iBROW (innovative ultra-broadband ubiquitous wireless communications through terahertz transceivers), led by the University of Glasgow, UK. The project’s aim is to develop an energy-efficient and compact ultra-broadband short-range wireless transceiver technology, initially for 60 GHz, but later for bands as high as 1 THz.
And one of ETSI’s Industry Specification Groups, called mWT, was set up to promote the use of spectrum from 50 GHz up to 300 GHz for present and future critical transmission use cases. The IEEE, home of the standards which underpin WiFi and Bluetooth, is also active in this area, via its 802.15 WPAN Terahertz Interest Group (IGTHz).
In Asia, Japan and South Korea have been hotbeds of development in the very high frequency spectrum area. In early 2017, researchers in Japan said they had developed a transmitter which could achieve data rates of over 100Gbps over a single channel in the 300 GHz band, creating true ‘wireless fiber’.
The team of scientists from two academic institutions (Hiroshima University and the National Institute of Information and Communications Technology) plus Panasonic, said the technology enables data rates “10 times or more faster” than 5G. The group claims that terahertz wireless links to satellites could make gigabit speeds available around the world, even on planes in flight.
“We usually talk about wireless data rates in megabits per second or gigabits per second,” said Minori Fujishima of Hiroshima University in a statement. “But we are now approaching terabits per second using a plain simple single communication channel. Fiber optics could offer ultra-high speed links to satellites as well, which can only be wireless. That could, in turn, significantly boost in-flight network connection speeds, for example. Other possible applications include fast download from content servers to mobile devices and ultrafast wireless links between base stations.”
The researchers said they used QAM modulation to increase data rates, harnessing a frequency range from 290 GHz to 315 GHz. This band is unallocated at present, but falls within one of the ranges (275 MHz to 450 MHz) which is scheduled to be discussed at World Radiocommunication Conference in 2019, where global allocations for 5G will top the agenda.
What is the terahertz band?
Despite its name, the terahertz band is generally restricted, for potential commercial mobile usage, to the area between 100 GHz (where mmWave leaves off) and 540 GHz. Rappaport explained in an interview: “Technically, the terahertz band is 300 GHz to 3000 GHz, but due to the molecular make-up of air, the spectrum that is implied by ‘THz’ in the circuits and sensing and communications realm is, practically speaking, in the range of from 100 GHz to 540 GHz or so, and the term ‘THz’ is now being loosely used to describe frequencies ‘above 100 GHz’.”
Similarly, mmWave is technically between 30 GHz and 300 GHz, but for practical purposes, in the cellular industry, is deemed to be above 10 GHz and up to 100 GHz.