By: Thomas Kurner

The possibility of using frequencies between 100 GHz and 1 THz for wireless communications—the so-called THz communications—has been discussed for almost two decades. The first concepts were published in 2007. Especially the spectrum above 275 GHz allows the use of bandwidth in the order of several tens of GHz, which enable data rates of 100 Gbits and beyond with transmission schemes of moderate complexity and spectral efficiencies. In the context of the development of the sixth generation (6G) of wireless communications, THz communications are discussed as an enabler to realize wireless links with 1 Tbps.

Challenges of THz communications

To realize a radio system at ultra-high frequencies comes with challenges, which need to be met. These include path loss and generating output power levels at the transmitter.

Research has shown that path loss increases with frequency. The free space loss (in linear scale) is a factor of 1000 to 6000 higher compared to the “traditional” carrier frequencies below 6 GHz. In addition, atmospheric attenuation and the impact of weather reduces the link distance significantly. This has to be compensated by highly directive antennas, which makes it challenging for transceivers to find each other and to track the signal, especially when at least one of the transceivers is moving. On the other hand, the small wavelength enables the realization of antenna arrays with small form factors, which are in turn a prerequisite to master the device discovery and tracking problems.

At the frequency range of interest, generating enough output power levels at the transmitter is challenging. Two principal methods to generate THz signals exist. Electronic approaches transform the methods used at millimeter wave systems to higher frequencies. In this case, output power levels decrease with increasing frequencies. The other approach uses photonic waves, where the output power levels decrease with decreasing frequencies. Both trends meet between 0.1 and 1 THz and the effect is called “the THz gap.” In recent years, semiconductor technology has made significant progress both in III-V and silicon-based realization of integrated RF circuits, yielding numerous demonstrations of THz communications with moderate output power levels in the order of 10 to 20 dBm.

Applications of THz communications

The above-mentioned chances and limitations make it obvious that THz communications will be able to provide powerful solutions to a specific set of applications rather than enabling a full-fledged cellular communication system. In this sense, THz communications can be used as a data pipeline enabling ultra-high data rates. Early applications of THz communication probably will be fixed point-to-point applications, where the locations of the antenna positions at both ends of the link are completely known. Examples for such applications include:

• Wireless backhaul links connecting access points with the backbone network. These wireless links can complement fiber links when they are not available or too expensive to deploy.

• Wireless links in data centers complementing fiber links, increasing the flexibility of reconfiguring connectivity in data centers.

• Wireless links within devices like computers, cameras, and video projectors. Here, ultra-high data rates can be provided, avoiding a connector and increase the flexibility in terms of reconfiguration.

• Close-proximity communications between wireless devices to allow quick data exchange of a large amount of data without the need for cabling.