# A Perspective on Terahertz Next-Generation Wireless Communications
## 2 The Terahertz Channel
However, DSSS channel sounding performs best only for narrowband channels. As the required bandwidth increases, the sampling rate of the ADC must increase as well. This presents a possibly significant challenge for next-generation terahertz systems, where the goal is to utilize very wide bandwidth for high data rates. One solution to improve bandwidth is to introduce a swept time-delay in the sliding correlator, where DSSS signals are used with temporal-dilation to compress the wide bandwidth of the measured CIR [14; 15]. Temporal dilation approximates a true correlator receiver by multiplying the received signal with a \"slow\" copy of the transmitted signal and then filtering the product [15]. This process results in a time-dilated CIR that is narrowband filtered and digitized [12; 13]. The sliding correlator can be used to measure multipath components, time delays, and received power in a wide range of scenarios and can even be used to measure antenna patterns [13]. The bandwidth compression of swept-time delay channel sounding allows a more economical means of recording and processing CIR measurements. The sliding correlator channel sounder is the most popular implementation of swept time-delay channel sounding and has been used extensively for wideband wireless channel measurements and characterizations. It is also a natural solution for sounding the massive spectrum occupied by large bandwidth terahertz communication systems [15]. However, when time-dilation is employed, the sliding correlator sounding method requires a more complex system design, additional hardware, and a longer acquisition period than direct RF pulse sounders. Nevertheless, it is still much faster than VNA-based sounders, and can still record several hundred CIRs per second (depending on the selected slide factor) [13]. A recent example [12] of the SC method was implemented at 300 GHz, offering sounding of dynamic channels with a 444 ns measurement window, with 108.5 ps (3.25 cm) resolution. It also features up to 17,590 CIR/s measurements and can distinguish Doppler shifts of up to 8.8 kHz (equivalent max speed of 31.7 km/h). However, it requires a hard-wire synchronization signal, and the cyclic recording method removes absolute distance information. While this is an excellent step forward, much work remains in SC terahertz channel sounding. What is apparent from the discussion is that channel sounding in the terahertz is currently limited by phase synchronization challenges. This is true for both VNA and SC implementations. Some of the proposed channel sounders do not utilize synchronization and therefore cannot measure absolute delays [13]. That does not totally eliminate their usefulness. It just means they can measure only delays relative to the strongest or first arriving multipath component. For relatively short links, synchronization is generally not a problem since hard-wired connections between the Tx and Rx are acceptable. A synchronization cable is used as a phase reference for channel sounding methods only to enable measurements of absolute timing between transmitted and received signals. The channel itself is wireless. In many practical communication scenarios, no cable can be used for synchronization. References [12; 13; 14; 15; 16; 17; 18; 19; 20; 21] are also available for the interested reader to learn more detail about the terahertz channel sounding methods. Synchronized, high-precision Rb- or Cs-based clocks also suffice for brief intervals. So long as measurements do not require too much time, separate Tx- and Rx-connected clocks will maintain synchronization sufficient for absolute delay measurements. However, as terahertz links go to hundreds of meters or even kilometers, it becomes increasingly difficult to maintain sufficient synchronization between the clocks. At the same time, the high frequencies in terahertz signals only become more prone to small phase errors.