# A Perspective on Terahertz Next-Generation Wireless Communications
## 2 The Terahertz Channel
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). The CIR is ideal for characterizing the channel as it is comprehensive in the information it provides. Of course it contains both magnitude and phase information from the signal. However, it also permits determination of some other important channel parameters including angle of arrival; angle of departure; Doppler shift; and power delay profile. Part of the importance of channel sounding  
Figure 2: Illustration of wireless channel, where the channel impulse response (CIR) is indicated by \(h(t)\). is that it must be used for developing a _stochastic_ channel model, taking into account the general channel _fading_, which describes the attenuation of the wireless signal due to a large combination of variables. Fading may occur from atmospheric attenuation, obstructions, time-domain variations (e.g., due to receiver motion), multi-path interference effects, and more. In RF engineering practice, systems can be designed by simplified models where large-scale fading is summarized by a single equation [31],  
\[\frac{P_{r}}{P_{t}}[\mathrm{dB}]=10\log_{10}K-10\gamma\log_{10} \frac{d}{d_{0}}-\psi_{\mathrm{dB}}. \tag{1}\]  
Here, \(K\) and \(d_{0}\) are reference constants associated with the antenna characteristics and range, respectively. NLOS fading requires even more complication due to the (typically) Rician statistical variations of the received signal [31]. For an ideal LOS channel, it would be found that \(\gamma=2\) corresponding to normal space loss (\(1/r^{2}\)) in propagation, and \(\psi_{\mathrm{dB}}=0\), corresponding to zero shadowing effects. Many of the extant terahertz measurements are of this variety (or nearly so) since it makes isolation of physical mechanisms like absorption possible [25; 32; 33; 34; 35]. In real communication system channels, however, neither of these is true. Instead \(\gamma\) may range from 1.6-6.5 and \(\psi_{\mathrm{dB}}\) is a zero-mean Gaussian variable with some non-zero variance [31]. Moreover, NLOS channels are unavoidable in realistic wireless channels, adding further complication. Accurate stochastic descriptions for terahertz wireless systems require full-scale, real-world channel sounding to be validated. However, we note that first steps are underway. For example, empirical bit error rate determinations have been made for terahertz systems operating in environmental conditions with dust [36], fog [33; 37], and rain [38]. The key requirements for accurate terahertz channel sounding are: high dynamic range (within a narrow frequency range) while maintaining wide tunability, fast detection (to capture real-time dynamics), and a long enough measurement window to capture power delay profiles. More generally, long measurement windows and real-time dynamics are conflicting requirements that can only be met with a compromise [15]. Terahertz researchers have therefore adopted different channel sounding techniques with complementary benefits to handle all cases: i) THz-TDS. The TDS method is widely used in basic material spectroscopy research and offers extremely large bandwidth and excellent time resolution. In addition, since it is a pulsed method, time-gating is possible to isolate scattering mechanisms within the channel, a decided advantage over frequency-domain systems for multi-path studies. However, these systems generally suffer from lower dynamic range, low spectral power density, and limited measurement windows (1-2 ns).