A Perspective on Terahertz Next-Generation Wireless Communications: History Edit
Subjects: Engineering, Electrical & Electronic
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 its fundamental 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.

2. The Terahertz Channel

The successful deployment of terahertz systems requires a solid understanding and accurate modeling of wireless channel conditions (propagation characteristics) between the transmitter (Tx) and receiver (Rx). Research work has now begun in earnest to understand terahertz wireless channels, which have many unique characteristics that distinguish them significantly from microwave work. The usual concern of most interest is wave extinction, either by spreading loss or absorption. For absorption, molecular oxygen is important at and below 120 GHz, however, water vapor absorption dominates by far in the remaining bands. Figure 1 shows plots of atmospheric power attenuation for the 0–1 THz range.
 
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Figure 1. Terahertz wave atmospheric power attenuation for temperature T= 20 °C and water vapor density ρWV=7 g/m3 at sea level. Left and right plots shows linear and logarithmic scale, respectively. Left plot indicates two predominant molecular oxygen (O2) absorption lines at 60 GHz and 120 GHz.
 
Various studies since 2011 described and accurately modeled the absorption of the atmosphere [22,25,26] from the perspective of a wireless communication channel. Using time-domain spectroscopy (THz-TDS), some of this work [26] revealed that the atmosphere can be very accurately modeled by careful use of Molecular Response Theory taking individual resonant absorption lines from the HITRAN database [26,27]. Importantly, this work showed how continuum absorption is often handled inconsistently, leading to discrepancies in total absorption. Unlike microwaves and other lower frequency RF, continuum absorption is an important contributor to absorption in the terahertz regime, which means it can no longer be ignored. It is of practical interest that recently (in 2018), O’Hara and Grischkowsky showed that the standard tool provided by the Radiocommunication Sector of the International Telecommunication Union (ITU-R) for calculating absorption (recommendation ITU-R P.676-11, 2016) was not accurate above 400 GHz [28]. This was due to both the way the continuum is accounted for in ITU-R, as well as the shape of resonant absorption lines selected. It is thus apparent that even though modeling of the terahertz channel may be advanced, it has not yet fully translated into widely-utilized engineering tools.
While earlier studies underpinned very accurate descriptions of absorption in isolation, the nature of the channel is considerably more complicated. For one, the Friis transmission formula does not adequately capture several propagation characteristics of practical channels such as multipath, non-line of sight (NLOS), and Doppler effects. Further, because of the dynamic nature of the atmosphere and mobile communication links, it is necessary to measure the channel and extract key characteristics of the propagation as a function of time so that a reliable channel model can be developed and used in wireless communication system design emulators/simulators. Empirical data is therefore required to determine accurate analytic and stochastic models of terahertz wireless channels. In terms of multi-path and NLOS systems, the nature of terahertz scattering is an important concern. Measurements of terahertz scattering for communication purposes began as early as 2007 [29], where terahertz scattering off of building materials, such as plaster and wallpaper, was studied. More recently, such work advanced to actual measurements of bit error rates (BER) in NLOS indoor and multipath outdoor terahertz communication links, ranging from 100–400 GHz carrier frequencies [30]. The work showed that surprisingly good performance can be obtained in spite of NLOS and multi-path effects with ranges of between 10–60 m and modest transmitter power (6 dBm) assuming the carrier frequency can be tuned and the Tx and Rx can be optimally aligned.
This last research is an example of system-level channel testing, rather than piecewise measurements of scattering and absorption, for example. Ultimately, terahertz communications will require a large and dedicated effort in system-wide channel sounding. This is the experimental technique of measuring a wireless communication channel, with all of its various complications. Results of channel sounding will be married to channel modeling [15] for future communication systems to be predictably engineered. Summarily, the goal of channel sounding is to determine the complex channel impulse response (CIR) (or frequency response due to duality) of a wireless communication channel (Figure 2).
 
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Figure 2. Illustration of wireless channel, where the channel impulse response (CIR) is indicated by h(t).
 

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