# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 1 Introduction
### Related Works
The proposed solution was analyzed in terms of the OP. RIS can generate a significant performance gain for the THz communication system. The large number of reflecting elements at the IRS and its passive operation represent essential challenges in beamforming design for THz due to its dependence on the accuracy of the estimated CSI, considering various degrees of CSI availability, namely imperfect CSI, partial CSI, and statistical CSI [36]. The beamforming design must be varied to minimize interference and enable robust communication [37]. However, the beamforming solution depends upon the application. For instance, when designing the beamforming for vehicular communication, the goal is to reduce latency and not to increase throughput [60]. In [38], the authors exploited a secure communication for IRS-assisted THz systems. One method is based on semidefinite programming (SDP) and another on block coordinate descent (BCD). They were proposed to design the reflective matrix to maximize the secrecy rate of the system. The secrecy rate is the maximum transmission rate at which the eavesdropper cannot decode any information [61]. No matter the spy's location, this scheme can improve the performance of the secrecy rate. Additionally, the authors in [39] have the beamforming concept and the IRS with improving the security of THz communication in a multi-input single-output (MISO) scheme. This proposal achieved better secrecy rate performance than the existing IRS-assisted design and the traditional non-IRS-assisted secure design. Furthermore, the attributes of RIS that differentiate them from other technologies have been collected in [40], outlining security and privacy attacks in RIS-assisted 6G applications. Recently, research works attempted to clarify the modeling of the path loss of RISs, with the development of sufficiently accurate models for the power received at a given location in space when waves illuminating a RIS have been transmitted. In [41], the authors employed antenna theory to study the path loss of RISs in the far-field and proved that a RIS is capable of acting as an anomalous mirror under far-field propagation. The authors in [62] proved that the performance of each configuration can be different and needs to be judged based on the complexity and the amount of environmental information needed for a proper operation. They used an analytical approach based on the diffraction theory and the Huygens-Fresnel principle to model the path loss in both the near and far fields of RISs. Using the stationary phase method, the authors found the regimes under which the path loss depends on the sum and the product of the distances between the transmitter and the RIS, and the RIS and the receiver. In [42], the authors suggested the use of the principle of spatial modulation for LIS data by modulating the information onto the on/off states of the LIS reflecting elements. In addition, passive beamforming was achieved by adjusting the phase shifts of the activated reflecting elements. In passive beamforming, the authors formulated an optimization problem of maximizing the average receiver SNR and solved it using the semi-definite relaxation technique. To retrieve the information data from the transmitter and the LIS, they developed a two-step detection algorithm based on compressed sensing and matrix factorization. At last, the trade-off between the passive beamforming gain and the information rate provided by RISs was investigated. ISs are progressively impacting the evolution of wireless system architecture, access technologies, and networking protocols by transforming the wireless communication environment from naturally passive to intelligent. While extensive research efforts continue towards IS, the European Telecommunications Standards Institute (ETSI), which produces globally applicable standards for information and communications technology (ICT)-enabled systems and services, has created a new industry specification group (ISG) on RIS (ISG RIS), to review and establish global standardization for RIS technology [63].