Even before millimeter wave spectrum is deployed on a mainstream basis, researchers are starting to cast their eyes even higher, to the so-called terahertz spectrum (in cellular terms, from 100 GHz to 540 GHz).
Technologies to make these very high bands usable by mobile devices are the foundation of the initial R&D projects referred to as ‘6G’. The latest development comes from the University of California, Irvine (UCI), whose Nanoscale Communication Integrated Circuits lab has demonstrated a wireless transceiver chip which can send signals in frequencies above 100 GHz, claiming lower cost and power consumption than existing designs.
These are, of course, the two major challenges facing chips in any new spectrum band, especially at very high frequency. Now that mass market consumer devices run with WiGig in 60 GHz, it is hard to remember that, only 15 years ago, mmWave circuits were still in the labs or confined to specialist markets like defense, and were manufactured with expensive materials like gallium arsenide. The industry was still to implement a 60 GHz radio in CMOS – without that breakthrough, achieved initially by IBM and some advanced start-ups, mass market economics and low power consumption for high frequencies would have remained elusive.
Now the R&D labs of vendors and universities are looking at reducing cost and energy in even higher frequencies, with a view to going ‘beyond 5G’ in a decade or so from now, and creating a wireless standard that really could match the performance of fiber.
The UCI team has built a 4.4-square millimeter receiver chip that claims to achieve new levels of energy efficiency as well as speed, compared to other prototypes that have been demonstrated. It does this by using a new digital-analog architecture that significantly relaxes digital processing requirements. Traditionally, changing the frequencies of signals through modulation and demodulation is done via digital processing, but this is running up against the limitations of Moore’s Law because transistors are reaching the stage where they cannot be made any smaller. “You cannot break electrons in two, so we have approached the levels that are governed by the physics of semiconductor devices,” said the Labs team.
Instead, they carry out modulation of digital bits in the analog and RF domains, avoiding the need for energy-guzzling high resolution data converters, and reducing cost. The prototype consumes total DC power of 200.25mW.
The prototype chip operates on one channel in spectrum between 115 GHz and 135 GHz and is made in a 55-nanometer silicon germanium BiCMOS process. It was demonstrated delivering speeds of 36Gbps over a 30-centimeter (one foot) distance, highlighting the way that terahertz spectrum will intensify the trade-offs already seen in mmWave, between very high capacity but increasingly limited range when working within the power limitations set for mainstream wireless communications.
The full details of the design are set out in a paper, published in the IEEE Journal of Solid State Circuits, called ‘A 115-135-GHz 8PSK Receiver Using Multi-Phase RF-Correlation-Based Direct-Demodulation Method’.
Payam Heydari, director of UCI NCIC Labs and lead author of the paper, said that if wireless systems could achieve the performance of fiber optics, “it would transform the telecommunications industry, because wireless infrastructure brings about many advantages over wired systems”. He also said it was the first transceiver to support end-to-end capabilities in terahertz spectrum, at a time when R&D in bands above 100 GHz is becoming less bluesky – for instance, the FCC recently issued proposals to open up new frequency bands above 100 GHz, and actions like that will drive investment in getting real world products close to market readiness over the coming few years.
TowerJazz and STMicroelectronics provided semiconductor fabrication services to support the research project.
Over the past year, there has been a rising tide of projects focused on future applications above 100 GHz. One important team is based at New York University’s (NYU’s) Wireless Research Center, which has been a significant driver of mmWave development in bands between 26 GHz and 95 GHz, and is now moving on to terahertz. Professor Ted Rappaport, the founding director of NYU Wireless, 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 recently, announcing a series of free sessions run by his institution which are exploring the “vast unknown” between the optical spectrum and the mmWave frequencies of 5G.
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. While the current focus is on 60 GHz, which is already in commercial use, in future there is the potential to move higher up the spectrum, as far as 1 THz. T
At the end of last year, Nokia announced it was one of the founders of a new mmWave Coalition, which despite its name, has been established to lobby US agencies such as the FCC, international regulators and the International Telecommunications Union (ITU), to open up bands above 95 GHz. Other founders include testing firm Keysight Technologies and Virginia Diodes.
“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,” wrote Nokia’s Paul Norkus 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.”
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.
Ultrafast point-to-point terrestrial wireless links are one possible application, but as Norkus said, these do not have to use 3GPP technologies. 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).
Terahertz R&D is not confined to the USA. Japan and South Korea have been hotbeds of development in this area, with Samsung, NEC and NTT Docomo among those pushing the boundaries in their labs. 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 spectrum?
Professor Ted Rappaport, director of NYU Wireless, explains that, 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. “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.