# A Perspective on Terahertz Next-Generation Wireless Communications
## 1 Introduction  
The recent deployment of fifth-generation (5G) wireless networks opens a new chapter in mobile communications, spurred onward by the continuing push for ultra-reliable, low-lag, high bandwidth communications for applications such as smart homes, e-health, Internet-of-Things (IoT), connected vehicles, and virtual reality [1; 2]. The roll-out is moving rapidly forward, having started in 2018 with fixed network deployments in major cities, now moving to mobile networks, hot-spot devices, and basic network infrastructure on a much wider scale [3; 4]. Mobile phones are still in development, but are also expected to deploy in 2019 [4].  
5G networks promise to offer important connectivity advantages: energy savings, higher system capacity, reduced latency, and of course higher data rates. Counting on the implementation of concepts such as massive MIMO and smart antenna technology, the International Telecommunication Union (ITU) IMT-2020 specification targets peak download/upload speeds at 20/10 Gbps, with channel bandwidths between 100-1000 MHz. Such high data rates and wide channels are obviously not possible except by the use of millimeter-wave (mm-wave) carriers. Indeed, the United States Federal Communication Commission has opened unlicensed bands extending all the way to 71 GHz for 5G [5]. This already overlaps what many scientists and engineers consider to be \"terahertz\". However, for purposes of this review we adopt a more common convention in the literature, where \"terahertz\" is defined as 0.1-10 THz.  
The terahertz (THz) region has been actively investigated in the literature, enabling a variety of new applications including spectroscopy, sensing, and communications [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. There is a wide variety of publications focusing on the frequency range approximately from 300 GHz up to 3 THz, and itsfundamental and promising capability as a next technology platform [10; 11; 23]. Potential use cases of wireless terahertz communication are: ultrafast wireless local-area-networks, intra-chip connectivity, kiosk downloads [20; 21], and server farm connectivity [24].  
Before the next-generation wireless technology can utilize waves in the >0.1 THz frequency range, there are many scientific and engineering challenges, as well as opportunities, that will need to be addressed. In this review we focus on three broad classes. First, there is the terahertz channel. This encompasses the new challenges of working with terahertz frequency waves as they propagate from transmitter to receiver, in predominately terrestrial communication links. While terahertz waves are like radio-frequency (RF) waves in many respects, their shorter wavelength affects beam directivity, diffraction, and antenna properties. In addition, the reflectivity, transmissivity, and absorption of materials, especially the atmosphere, are quite different. Second, we consider terahertz devices. The generation, reception, and conversion of terahertz waves in mobile devices require cutting-edge electronic, photonic, or hybrid approaches that push the limits of material properties and device capabilities, while simultaneously enabling cost-effective fabrication and device integration. Third, we consider space-based terahertz opportunities, since these applications may be uniquely suited to terahertz communications, and since they demand system-level solutions that will be common to terrestrial systems and space systems alike, with probably greater restrictions arising from the latter. In other words, space-based terahertz communications may be excellent development surrogates for future terrestrial systems. The relevant engineering challenges involve topics such as beam pointing controls, vibration, link stability, radiation-hardening, and power consumption. As we will show, all of these topics have been the target of recent terahertz research.