Towards Terahertz: Part Of The 6G Picture
By Prof. Sana Salous, Durham University, Vice Chair of ETSI ISG THz
Higher frequencies support higher bitrates and exciting new use cases. A dedicated group in ETSI is coordinating pre-standardization efforts on Terahertz wireless communications technology that holds significant promise for tomorrow’s 6G networks.
Well ahead of its predicted launch around 2030, 6G is already the subject of intense work in research labs and universities worldwide.
From GSM to 5G, successive iterations of mobile technology have exploited the use of higher frequencies to enable wireless data transfers at progressively greater rates. The potential of the higher frequency bands from 24-86 GHz was identified by the World Radiocommunication Conference 2015 (WRC-15). Subsequently, WRC-19 assigned 17.25 GHz for IMT across multiple bands from 24-71 GHz in contrast to the previous 1.9 GHz, with 14.75 GHz harmonized worldwide, ~ 85% of global harmonization. The millimeter waves in the 50-300 GHz frequency range to support high data rates have been a pillar of 5G standardization efforts in 3GPP for some time. And now with 6G on the horizon, the global research community is examining how radio frequencies in the Terahertz range can realize industry ambitions to deliver point-to-point wireless data transmission rates of the order of hundreds of Gigabits, ultimately targeting 1 Tbps.
Despite the term ‘Terahertz,’ much research activity is currently focused on sub-THz frequencies between 100 GHz and 300 GHz. The spectrum for Terahertz communications is ready and waiting since the WRC-19 which identified 137 GHz at frequencies between 275 and 450 GHz to be used for the technology. This offers plenty of ‘real estate’ to attain extremely high data rates and help ease spectrum scarcity problems.
The sub-THz frequency band has been the subject of discussion and basic research for two decades, with early standardization efforts as long ago as 2008 with IEEE’s development of the first worldwide standard for wireless communications in the range 252 - 321 GHz. The technology’s feasibility was successfully demonstrated in 2013 when a German group successfully achieved 100 Gbps wireless data rates over 20 m in lab conditions. Recently EU Horizon 2020-funded projects such as Terranova developed 300 GHz technology and succeeded in transmitting 56 GBit/s wirelessly over a free space distance of 1 km. Last year (2022), the international project ThoR realized bidirectional transmission of ‘real’ data, achieving a net 2x20 Gbps throughput over 160 m in a bandwidth of 2×8.64 GHz.
But why do we need such high data rates over wireless connections? From the physical layer perspective for 6G, THz technology offers an attractive alternative to laying fiber optic links in urban or hard-to-reach environments where a cabled connection is not always practical. One obvious example is mobile backhaul, where a radio cell on the roof of a building can be linked to the network over kilometer-scale distances without digging up city streets.
Aside from fixed point-to-point communications, THz suggests some interesting mobile and integrated sensing use cases that demand extremely high bandwidth real-time wireless links. These range from virtual and augmented reality to V2X (vehicle-to-everything) communications, cooperative robotics, and immersive telepresence.
Part of the technology’s appeal lies in the very small (sub-millimeter) wavelength of THz signals, enabling the design of compact antennas in highly miniaturized devices. Highly directional RF propagation characteristics at these frequencies dictate very precise alignment to within a degree or two of pencil-thin beams between the transmitter and receiver antennas. In contrast with fixed applications such as mobile front/backhaul or in data centers, incorporating mobility will require technologies including beam steering and the use of phased arrays.
THz waves are also highly susceptible to atmospheric refraction and absorption by water vapor and oxygen molecules, exacerbated by rain and precipitation, placing practical limitations on transmission distances. In parallel, researchers are investigating possible solutions – including the use of photonics – to generate adequate transmitter power. This is a particular issue where output levels of conventional electronic methods used for microwave and millimeter wave links fall off sharply with increasing frequencies. These are just some of the considerable practical challenges currently being addressed.
Ensuring the timely success of THz communications represents a major collaborative effort, involving the orchestration of numerous European and global research initiatives. Created to coordinate pre-standardization efforts, ETSI’s Industry Specification Group (ISG) on Terahertz communications was launched formally in September 2022. Supporting future THz standardization work as a candidate 6G technology in 3GPP, the group’s activities are initially focused on several key areas.
In parallel with our examination of potential application scenarios and use cases, we are identifying suitable frequency bands for THz communications. Complementing this work, we are looking closely at THz radio propagation aspects. These include molecular absorption, the effects of micro-mobility with reference to scattering, reflection, and diffraction, as well as specific considerations for near-field propagation. This requires the mapping of specific use cases onto relevant channel measurement scenarios, drawing on an extensive library of previously published studies as well as measurement work by our members. We expect to conduct measurement campaigns in both indoor and outdoor environments, considering scenarios with and without mobility. Drawing on Machine Learning methods to generate and analyze radio channels, this exercise will lead to the development of detailed channel models for our selected use cases and chosen frequency bands above 100 GHz. This in turn will serve as the foundation for further pre-standardization work.
The development of affordable mass-market THz solutions also demands the design and fabrication of energy-efficient RF circuits. Here we are exploring how suitable components can be readily integrated with baseband, networking elements, and other hardware elements.
The successful development of THz communications systems shares some of the goals and challenges as millimeter wave technology, reflected in cooperation between ISG THz and ETSI’s Industry Specification Group on millimeter Wave Transmission (ISG mWT). Similarly, we are liaising with another ETSI Industry Specification Group which is conducting its pre-standardization work on the design and applications for reconfigurable intelligent surfaces (RIS). Harnessing potentially thousands of small antennas or metamaterial elements to dynamically shape and control radio signals in a goal-oriented manner. RIS is a complementary enabler for the incorporation of THz communications in 6G.
About The Author
Professor Sana Salous started her academic career as an Assistant Professor at Yarmouk University, Jordan. In 1989 she joined the Department of Electrical Engineering & Electronics at UMIST. In 2003 she joined Durham University where she currently holds the Chair in Communication Engineering and is the Director of the Centre for Communication Systems. Professor Salous’s radio propagation research covers HF for sky wave propagation for long-range communication and UHF to the millimeter wave band for 5G mobile communications. In this area, she introduced the digital frequency sweep technique for high bandwidth channel sounders for radio propagation studies. She is an active member of URSI, various COST actions, and a member of the U.K. delegation to the International Union of Telecommunications.