# A Perspective on Terahertz Next-Generation Wireless Communications
## 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. 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  
Figure 1: Terahertz wave atmospheric power attenuation for temperature \(T=20^{\circ}\)C and water vapor density \(\rho_{WV}=7\) g/m\({}^{3}\) at sea level. Left and right plots shows linear and logarithmic scale, respectively. Left plot indicates two predominant molecular oxygen (O\({}_{2}\)) absorption lines at 60 GHz and 120 GHz. 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), [PERSON] and [PERSON] 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.