# A Perspective on Terahertz Next-Generation Wireless Communications
## 2 The Terahertz Channel
Furthermore, an accurate _stochastic_ picture of time-variable, multi-path contributions cannot be built with a TDS system. This is because TDS is not \"real-time,\" but instead employs a relatively slow averaging measurement technique in order to increase the signal-to-noise ratio. That technique requires the measured channel remain constant over the whole measurement duration, which is typically several minutes. Therefore, time-variant effects from Doppler shifts and limited channel coherence time cannot be measured in time-domain systems. Furthermore, it is an ultrafast laser-driven technique, making it difficult and expensive to implement in practical situations. We will devote more discussion to the other two methods: (ii) the VNA-swept-frequency method, where the channel frequency response is measured by using a vector network analyzer (VNA), (iii) the sliding-correlator (SC) method. The combination of these approaches will be suitable for a wide range of channel propagation measurements with 1) short- and long-distances, 2) narrowband and wideband signals, 3) time- and frequency-domain, and 4) stationary and mobile scenarios. Historically, the VNA and SC based methods have been practically more important, being the common approaches in microwave systems where TDS is not commonly available. The development of these methods is now beginning in the terahertz realm [12; 13; 14; 15; 21]. Both time- and frequency-domain channel sounders operating between 3 to 110 GHz are summarized in [13], although most of these fall into the 60 GHz and lower frequency range. More recent results extend this range to 140 GHz [16]. In the VNA-swept-frequency method, the channel frequency response is measured by using a VNA (together with subharmonic mixers to operate at THz bands). The VNA steps through a range of discrete, narrowband frequency tones swept across the bandwidth of interest and, at each frequency, measures the channel's magnitude and phase response followed by an inverse discrete Fourier transform of the channel transfer function, which results in a complex CIR [13]. With this technique, very precise channel measurements can be achieved in static environments using a VNA with desired frequency resolution [12]. Vector network analyzers are typically utilized in static environments because of the long frequency sweep time across a broad spectrum, which can exceed the channel coherence time [13]. In addition, hard-wired connections are required for VNA two-port network synchronization that make it more suitable for indoor and short-range measurements [12; 14; 13]. Hence, it is restricted to one waveguide band at a time and cannot support wideband measurements with mobile transmitters or receivers. Two rubidium (Rb)- and/or cesium (Cs)-based high-stability clocks can be employed to replace the synchronization cable and act as frequency standards for VNA-based sounding over limited durations beyond which their phase coherence degrades. Phase stability, even with hard-wired synchronization connections, becomes increasingly difficult as terahertz frequencies are approached, due to the relatively small wavelengths. The sliding-correlator method is a time-domain channel measurement where, in the simplest method, a short electromagnetic pulse is transmitted in the time domain. The receiver can measure the different arrival times of multipath components of this pulse to determine the CIR. However, since synchronization is often not employed, this method can suffer the 3 dB penalty of non-coherent detection. It also requires a high peak-to-average-power (PAPR) ratio, and can have in-band interference and poor dynamic range [13]. Instead of transmitting a single impulse, it is also possible to transmit a carefully-timed train of impulses, also known as a direct sequence spread spectrum (DSSS) signal. The DSSS signal can be properly processed at the receiver to determine the CIR, and the DSSS signals also provide rejection of in-band interference components as well as improvements in dynamic range [13; 15; 21].