The Evolution of Applications, Hardware Design, and Channel Modeling for Terahertz (THz) Band Communications and Sensing: Ready for 6G?

For decades, the terahertz (THz) frequency band had been primarily explored in the context of radar, imaging, and spectroscopy, where multi-gigahertz (GHz) and even THz-wide channels and the properties of THz photons offered attractive target accuracy, resolution, and classification capabilities. Meanwhile, the exploitation of the THz band for wireless communication had originally been limited due to several reasons:1) no immediate need for such high data rates available via THz bands and 2) challenges in designing sufficiently high-power THz systems at reasonable cost and efficiency, leading to what was often referred to as "the THz gap." Over the recent decade, advances on many fronts have drastically changed the THz landscape. First, the evolution from 5G-to 6G-grade wireless systems dictates the need to support novel bandwidth-hungry applications and services for both data transfer i.e., eXtended Reality (XR), the Metaverse, and vast modeling needs of artificial intelligence (AI) and machine learning (ML), as well as centimeter-precision sensing and classification (i.e., for standalone position location, vehicle-to-everything (V2X), or unmanned aerial vehicle (UAV) tracking). Second, substantial progress in THz hardware has been achieved, offering promise that the THz technology gap will be closed. Hence, THz-band wireless communication seems inevitably an essential part of the future networking technology landscape in the coming decades. To design efficient THz systems, the peculiarities of THz hardware and THz channels need to be understood and accounted for. This roadmap paper first reviews the evolution of the hardware design approaches for THz systems, including electronic, photonic, and plasmonic approaches, and the understanding of the THz channel itself, in diverse scenarios, ranging from common indoors and outdoors scenarios to intrabody and outer space environments. This article then summarizes the lessons learned during this multidecade process and the cutting-edge state-of-the-art findings, including novel methods to quantify power efficiency, which will become more important in making design choices. Finally, this article presents the authors' perspective and insights on how the evolution of THz systems design will continue toward enabling efficient THz communications and sensing solutions as an integral part of next-generation wireless systems.