# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 1 Introduction
### Related Works
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**Summary and Focus**} & \multicolumn{2}{c}{**Approach Taken on the Subject:**} \\  & & **References, Details and Future Directions** \\ \cline{3-4}  & & **Channel Discussion** & **IS Discussion** \\ \hline
[31] & Analytical path loss model for THz RIS. & Specific & Specific \\ \hline
[32] & THz IRS in MIMO systems operating in the near-field for beamfocusing and power gain MIMO. & Specific & Specific \\ \hline
[33] & Channel estimation and transmission solutions for mMIMO-assisted THz system. & Specific & Specific \\ \hline
[34] & Channel acquisition for the IRS enabled THz mMIMO system. & Specific & Specific \\ \hline
[35] & CSI and optimal data rate for IRS THz multiple-input multiple-output (MIMO) Systems. & Specific & Specific \\ \hline
[36] & IRS with beamforming optimization based on statistical CSI knowledge. & Specific & Specific \\ \hline
[37] & Development of a downlink beam training/alignment method for IRS-assisted mmWave/THz systems. & Specific & Specific \\ \hline
[38] & Secure transmission for an IRS-assisted mmWave and THz system. & Specific & Specific \\ \hline
[39] & Secure THz communication with IRS. & Specific & Specific \\ \hline
[40] & Security challenges affecting RIS-empowered 6G wireless networks. & Negligible & Negligible \\ \hline
[41] & Far-field path loss using physical optics techniques, considering an IRS in propagation. & Specific & Specific \\ \hline
[42] & Beamforming and information transfer technique based on spatial modulation of the large intelligent surface (LIS) elements index. & Specific & Specific \\ \hline
[43] & THz RISs for 6G communication links. & Intermediary & Deep \\ \hline
[44] & RIS for wireless communications: an overview of hardware designs, channel models, and estimation techniques. & Deep & Deep \\ \hline
[45] & RIS for smart wireless environments: channel estimation, system design, and applications in 6G networks. & Deep & Deep \\ \hline
[46] & A survey of 6G wireless communications: emerging technologies. & Negligible & Negligible \\ \hline
[47] & Overview of THz-specific signal processing techniques for wireless communications, emphasizing ultra-mMIMO systems and RIS. & Intermediary & Deep \\ \hline
[48] & A tutorial on THz band localization for 6G communication systems. & Deep & Deep \\ \hline
[49] & Seven defining features of THz wireless systems: a fellowship of communication and sensing. & Deep & Intermediary \\ \hline
[50] & RIS-aided THz communications. & Specific & Specific \\ \hline
[51] & Near-field study of holographic IRS operating at THz frequency band. & Specific & Specific \\ \hline
[52] & Holographic RIS for application to mMIMO systems at THz band. & Specific & Specific \\ \hline
[53] & RIS-assisted THz systems. & Specific & Specific \\ \hline
[54] & THz communications employing supporting unmanned aerial vehicles (UAVs) and IRS. & Specific & Specific \\ \hline This work & A comprehensive survey on IS-aided THz wireless communication focusing on channel modeling. & Deep & Deep \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._subsection, the term that the authors adopted for their works is used. In the subsequent sections, they refer to intelligent surfaces generically by IS. In [55; 56], LIS is described as sizeable active antenna arrays, like a much larger mMIMO [57; 58] comprising hundreds of antennas, which can enlarge the coverage and improve the positioning performance, which is suitable for THz scenarios. The term RIS has been commonly used to highlight the surface reconfiguration property for the incident signal. In [28], RISs were applied as anomalous reflectors and single-RF transmitters to analyze and compare the bit error probability offered by both implementations. The authors in [14] considered IRS a planar surface of many low-cost passive reflecting elements able to reshape the wireless propagation environment via software-controlled reflection.