THz wireless communications: challenges and paradigm shift

Information and Communication Technologies (ICT) have a significant impact in several aspects of modern society, facilitating the vision of ubiquitous communication and the internet of things. ICT has and will continue to have a substantial influence on the way we communicate, how information is disseminated, the way we work and conduct business, social life, and ultimately on the macroeconomic growth. All these factors, in turn, affect society enabling improvements in infrastructure and standard of living. The challenges of the growing need for bandwidth, and the evolution of several generations of communication systems – 3G to 5G – have been achieved as a result of the interplay between technologies based on optical systems (high speeds at a global level on the planet) and ubiquitous communication facilitated by wireless radio-frequency communication systems and more recently mobile communications between vehicles and via satellite.

Whereas 5G is focused on facilitating real-time video streaming, building on the efforts of 4G to provide low-cost data capacity (and free voice), 6G will target augment intelligence and human experience requiring even wider bandwidths. We see a trend that is the increase of bandwidth to serve new use case scenarios. It is envisaged that mobile service data rates will be comparable to wireless LAN by 2035 (see Figure 1).

To meet this need for higher communication speeds, one approach would be to increase the spectral efficiency by increasing the order of the modulation format being used, which demands higher output powers and larger signal-to-noise ratios, which is difficult to achieve, hence not desirable. An alternative solution is to increase the available bandwidth by exploring other frequencies of the electromagnetic spectrum. This is where the THz band comes into play.

THz communication is a general designation of wireless communications using the band of the electromagnetic spectrum between 100 GHz to 10 THz. This band is not restricted to communications. In effect, the THz band opens up room for tremendous potential applications, including high-precision radars, non-destructive testing and material characterization, biomedical imaging and medical diagnosis. Remember that high frequency means lower wavelength, hence higher accuracy, so the listed applications should be of no surprise. The emphasis here will be on the THz waves for wireless communications, which are expected to play a major role in the 6th generation of wireless standard (6G), enabling high-speed wireless links with data rates above 100 Gb/s with low latency, besides the high accuracy localization and sensing.

The IEEE standardization effort 802.15.3d recently assigned the frequency band between 252.72 GHz and 321.84 GHz to wireless communications, which proves the growing interest in THz communications. The standard includes 69 overlapping channels and 8 supported channel bandwidths from 2.16 GHz to a single 69.12 GHz channel. However, to be able to access this still relatively “unchartered territory”, some significant technological challenges still need to be overcome:

However, to be able to access this still relatively “unchartered territory”, some significant technological challenges still need to be overcome:

- High level of free space loss;

– The availability of high-power electronic THz sources suitable for modulation;

– Beam steering capability to establish point to multipoint links;

– Signal conditioning and processing;

– High-speed electronics;

– Low cost and small form factor produced in a scalable technology.

In the microwave/mm-wave range, even for a distance of 100 m the loss can be as high as 130 dB at 1 THz, which is approximately 100 000 times higher than at 2.4 GHz. When combined with the lack of high-power emitters (known as the “THz gap”), the loss factor is a major issue due to the inefficiency of photonic and electronic devices in the THz band operating at room temperature. That could be addressed, for example, by using multiple sources coupled to antenna arrays with additional signal conditioning to control the direction of the beam (either electronic or photonic) and establish point to multipoint communications. In this instance, package integrated antenna design is a research topic by itself, with its own challenges.

Still, high-speed electronics are required. Current efforts point to the use of low-cost and high-level integration of CMOS SiGe-HBT⁽¹⁾ (Silicon-Germanium heterojunction bipolar transistor) that is already capable of operating at maximum cut-off frequencies, fmax, of 750 GHz [Heinemann, 2016]. SiGe combined with III-V of semiconductor technology will enable higher output powers and low noise-figures. It is not possible to mention all the research directions, which are many and include, among others, the use of new materials, e.g., graphite and metamaterials to enhance the device or antenna performance. The combination of photonic and electronic technologies exploiting the benefits of each family of devices will ultimately lead to THz systems with the highest performance possible with wide bandwidth, high spectral purity, extensive dynamic range and tuning range, power amplifying capability and ease of functional on-chip integration [Nagatsuma, 2016]. Also, photonics is directly compatible with the existing fibre optic network allowing seamless integration. As an illustration Figure 2 depicts the use of resonant-tunneling diodes with integrated photodetectors (RTD-PD), operating as optoelectronic transceivers in high-speed wireless systems [Zhang, 2019].

It remains a technological challenge to access the entire bandwidth available in this band. The success of THz communications depends on viable technical solutions to the challenges that face the research community today. To meet this objective, a new paradigm shift is required from the traditional single-discipline approach, be it electronic, photonic or physical/material science, to a multi-disciplinary one, and a high level of integration of multiple technologies is needed.

Novel techniques that are already being addressed are pushing the THz wireless communications to a new level.