# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 1 Introduction
### Related Works  
Due to their attractive ultra-wideband capability, THz communications have recently been widely accepted as a promising technology for next-generation optical wireless communication systems. In addition, numerous fields of application have been studied in the literature, such as agriculture [20], IoT communication [21], and space communication [22], mainly because of the progressive development of devices that support this band. In this context, THz channel modeling is paramount for meeting future network demand. One of the difficulties for THz communication that has been found in the literature is the short range of the THz wave, leading to a limited coverage area of this band [23]. In addition, it suffers from high attenuation in the path, and its waves are highly directive, which hinders the reception of the transmitted signal [12]. This communication may be unfeasible if it adds to the ease of blockage by interior obstacles such as walls and doors [24] for this band. To overcome these limitations, different techniques using large-scale antenna arrays [25] and cooperative communication [26] have been proposed. These techniques have been widely studied to improve throughput and achieve multiple diversity. However, despite their exciting results, these techniques are complex and require costly hardware. For this reason, intelligent surfaces (ISs) have been proposed as a low-cost technique to extend signal coverage in THz communications [12; 14; 27]. Several surveys, magazines, and review works concern IS systems. They mainly discuss structure designs or address specific application areas. Remarkably, within the context of IS-aided THz communication channels, there have been developments in the modeling of IS unit elements and IS boosting wireless signal propagation. Table 1 summarizes the survey and review papers involving aspects of the IS-aided THz channel, as well as the recent papers on the THz communication and IS areas, which focus on design and feasibility. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**Summary and Focus**} & \multicolumn{3}{c}{**Approach Taken on the Subject:**} \\ \cline{3-4}  & & **References, Details and Future Directions** \\ \cline{3-4}  & & **Channel Discussion** & **IS Discussion** \\ \hline
[1] & Comparison between 5G and 6G, use cases for 6G enabling techniques, and beyond 6G technologies. & Deep & Intermediary \\ \hline
[2] & A survey of 6G wireless channel with future research challenges and IRS models. & Deep & Low \\ \hline
[3] & Application scenarios for THz RIS, enabling technologies and open issues. & Intermediary & Deep \\ \hline
[4] & THz technical challenges and performance issue solutions. & Intermediary & None \\ \hline
[14] & Overview of RIS technology and the key challenges in implementing a RIS-aided hybrid network. & Specific & Specific \\ \hline
[28] & A survey of RIS-empowered wireless networks. & Deep & Deep \\ \hline
[29] & Performance evaluation of RIS assisted D-band wireless communication. & Specific & Specific \\ \hline
[30] & TE-GD scheme is developed during the iterative process by a dynamic update of the step size in IRS assisted THz MIMO system. & Specific & Specific \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of surveys, magazines, and review papers related to IS in THz communication channel modeling and the classification according to discussion level of the approach taken on both the communication channel and IS. It is possible to realize that different terminologies have been interchangeably used to identify an IS in the literature, namely, large intelligent surface (LIS), RIS, and intelligent reflecting surface (IRS). Each terminology emphasizes one particular feature of the intelligent surface [27]. These terms have similar meanings for different authors. In this