# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 4 IS-Aided THz Channels  
IS has been considered a promising technology and an indispensable part of THz communication systems. As channels in the THz band are highly frequency-selective, IS applications in these frequencies can solve the limitations on propagation distance and coverage, which are easily affected in THz frequencies due to very high propagation attenuation and molecular absorption [101]. In addition, it can provide an additional propagation path where NLoS link exists and, consequently, assist the communication where the LoS path is blocked by obstacles [14]. Therefore, the IS ability to create alternative signal paths makes IS applications essential in THz wireless communication, where the environment introduces several challenges, as described in the preceding sections. In addition to creating additional links between the transmitter and receiver, the ISs also use wireless mm-wave intercell communication for tunability. The IS architecture is based on FPGAs, which are responsible for enabling the control and intelligence of IS. To transfer information among unit cells, wireless signals not leaked outside the IS, and no interference with the network, frequency and time domain and path propagation analysis have been performed [109; 110; 111]. The thorough analysis of the authors has provided that wireless communication to control the IS is an optimal choice over wired communication for large or dense IS employments. Besides that, out of three options of communication channels: the chip layer, a dedicated parallel-plate waveguide, and inside the IS dielectric; the first option yields the most considerable path loss (40-50 dB) while the third option shows the smallest one (20-40 dB). However, the IS design must consider its dispersion factor while enabling its work at the targeted frequency. Moving from the perspective of the IS controlling channel to the one used to enable communication, Figure 5 shows a typical application scheme of an IS for THz communication. As shown, User 1 desires to connect to the BS; thus, the system requires a LoS link between User 1 and the BS. Nonetheless, the wireless link is blocked by obstacles. In a traditional approach, the User 1 would try to find another BS with a weaker signal due to the longer distance from User 1. Also, high path loss bounds THz waves to a short propagation range [112]. However, there is an IS with a total \(m=m_{x}m_{y}\) elements, where \(m_{x}\) accounts for the number of elements in each row and \(m_{y}\) accounts for the number of elements in each column. Due to the IS installation position, the signals from the BS can be transmitted to User 1 by reflection from the IS. The changes in directions of the reflected beam are controllable, according to the generalized Snell's law, which is derived from [PERSON]'s principle to interpret the anomalous reflection and refraction phenomena [113]. This means that when the IS elements alter the phase and magnitude of the incident waves, occurs constructive or destructive interference of these incident waves with the reflected waves from IS in desired directions. Each element introduces a phase shift, ideally given by the generalized Snell's law to create a single beam. Still, the phase shift must be explicitly optimized since a superposition of multiple beams should be created. The information sent to the desired user in Figure 5 must be received only by User 1. Therefore, the information-bearing signal and the reflected signal constructively interfere, and consequently, a constructive reflection is sent toward the desired user (User 1). There is also an interfering user (User 2) that must not participate in the communication between User 1 and the BS. Therefore, destructive interference occurs between the waves that are incident on IS and those that are reflected in the direction of User 2. In this case, there is no communication path between User 2 and BS, so that information remains secure with the desired user.