Will Terahertz Communications Become the Next Battlefront for 6G?

The most transformative among the coveted 6G use cases are Internet of Everything (IoE), remote surgery, and holographic calls. The IoE transforms the Internet of Things (IoT) to include people, processes, and data, along with the connected devices, which is the hallmark of the IoT. Remote surgery is the crown jewel of 6G use cases that can revolutionize the healthcare sector by leveraging robots to operate on patients with surgeons giving instructions to the robots from thousands of miles away. Holographic calls will empower remote users with a local presence. But all of these use cases can only be supported if the underlying mobile network has achieved the desired network requirements. It should be noted that these use cases are not new; for example, remote surgery has been discussed in 5G, but current medical and computing technology does not make it either practical or cost-effective to enable. However, in 2030, when 6G is expected, these verticals may be ready for what 6G aims to offer.

6G network requirements currently being discussed in the market are the following: maximum bandwidth of 100 Gigahertz (GHz), peak data rate of >=1 Terabit per Second (Tbps), peak spectral efficiency of 60 Bits per Second (bps)/Hertz (Hz), network energy efficiency of 1 Picojoule per bit, area traffic capacity of 1 Gigabit per Second (Gbps)/square meter, mobility of >= 1,000 Kilometer (km)/hour (enabling the Airborne Wireless Network, making airspace available to personal air vehicles), and latency of 1 microsecond are some of the Key Performance Indicators (KPIs) that define a 6G network that can support the transformative use cases. Although these are visionary KPIs, which the industry will attempt to match, experience has shown us that realistic networks cater to lower requirements, which is a step ahead of previous networks nonetheless.

Terahertz (THz) radiation has very high molecular absorption (e.g., it is strongly absorbed by gases and it is massively attenuated/weakened in air reducing to zero within a few meters) and very high molecular noise generated by water vapor in response to the rapid attenuation of electromagnetic radiation. As a result of this massive attenuation of electromagnetic radiation, signal transmission distance and coverage range are severely limited (limited to a couple of meters in THz, whereas the transmission distance of 5G mmWave is about 500 meters). Overcoming the limitations of transmission distance and coverage range is absolutely central to THz becoming a candidate technology for the 6G network.

The combination of ISAC with THz communication delivers ultra-high data rates, opening up new application possibilities, such as the IoE and Augmented Reality (AR)/Virtual Reality (VR)/holographic video calls enabled by sensing and gesture/action recognition (critical for the success of remote surgery). Innovators leading this ISAC Transceiver bandwagon, apart from HiSilicon, are CamGraPhIC (a leading innovator of graphene transceivers, with close ties to the University of Cambridge), InnoLight, Hisense Broadband, Eoptolink, Intel, Accelink, Molex, Source Photonics, and Lumentum. Most of these innovators are combining THz communications with THz sensing functionalities, which is leading to future applications, such as Extended Reality (XR)/VR, delivering remote surgery/holographic video calls.

Delivering Ultra-Reliable Low Latency Communications (URLLC) is another major milestone for the 6G network. Low latency is achieved through the reduction of transmission delay by several kinds of channel coding schemes with short block length. Also, network coding, or Random Linear Network Coding (RLNC), delivers low latency by enabling intermediate nodes of the network to participate in the coding process, increasing the reliability and capacity of the network, while reducing the latency. Both THz and mmWave networks have embraced this path of using RLNC to reduce latency and increase reliability. In this context of achieving URLLC, both THz and mmWave networks will adopt RLNC, among other strategies, in order to win the race of making it to the 6G standards.

Extending the transmission range is, by far, the biggest challenge faced by THz networks, particularly in indoor environments with several obstacles, where line-of-sight cannot be achieved. A reflectarray close to the THz source can be used to bounce off the signal toward the user equipment and extend the transmission range. TICRA, a Denmark-based company developing next-generation reflectarray antennas, has also developed a software suite to analyze and design a complex reflectarray. However, it will not be financially feasible to place reflectors everywhere to cater to the most random communication scenarios that involve mobile users. So, there is considerable work to be done to improve the efficiency of reflective surfaces, something that started both in The 3rd Generation Partnership Project (3GPP) and in ETSI with a technology currently named Reflective Intelligent Surfaces (RIS).

Major 6G innovators are aiming to overcome these apparent impossibilities, such as tackling the critical problem of distance limitation through two types of technology innovations:

• A Distance-Aware Physical Layer Design: Physical layer parameters are manipulated to explore what achievable distance gains could be accomplished. This distance-aware-physical layer design technology solves the transmission distance part of THz puzzle by increasing the transmission distance manyfold.
• Ultra-Massive Multiple Input Multiple Output (MIMO) Communication: Massive MIMO aims to increase the spectral efficiency by loading several hundred antennas made of graphene (to avoid the large loss of THz frequency range for Gallium Arsenide (GaN) and indium phosphide antennas) delivering Tbps links over a communication distance of several tens of meters. This ultra-massive MIMO communication technology, particularly the beamforming part, solves the problem of large losses of THz frequency range by loading several hundred graphene-based antennas, leading to increased coverage range.

Many 6G aspirants have adopted technologies to achieve high data rates (achieved by wireless graphene transceivers) and extremely low latency (achieved by network/channel coding). The key network problems that remain unresolved are limitation of transmission distance and coverage range. Leading 6G aspirants are toying with the ideas of a distance-aware physical layer design and ultra-massive MIMO to solve these two obstacles. This war for THz communication’s inclusion in 6G standards will be decided by this pursuit of overcoming severely limited transmission distance and coverage range. In the current geopolitical environment, 6G is a big priority for many governments aiming to make their country the leader in 6G technology.