# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 4 IS-Aided THz Channels
All components on the right side of (8) are composed of amplitude and phase, and researchers consider the IS controller knows the channel, producing phase shifts that cancel out the phase response of the channels [16]. In addition, IS are assumed to work passively and have unitary amplitude. The different probabilistic channel models used to investigate IS-aided THz channels have been mixture Gamma [117], Rayleigh [118; 119], Rice [120; 121], Nakagami [120; 122; 123; 124], and shadowed \(\kappa\)-\(\mu\)[125] models. Due to the intelligent reflections, an IS-aided communication has dynamic control capability over the wireless propagation channel phenomenon. This IS capacity to change propagation characteristics of wireless channels promotes the development of the THz communication systems to meet the ever-increasing user demand for higher data rates. The benefits of the IS applications in the next-generation wireless technologies depend on the development of an accurate channel model, which is necessary for designing and deploying future wireless communication systems. However, only the study in [107] exploits this development in THz band. The authors proposed the first statistical model for channels in THz, where extensive ray tracing simulations were performed to obtain the statistical parameters of the channel. Technical challenges arise when an IS channel modeling is incorporated in a system project, such as mobility of users or ISs and optimization of the phase shift of the IS. From this perspective, the THz propagation channel modeling involving modification of the wireless propagation environment through the inclusion of IS has attracted the attention of researchers in recent years. To enable the efficient transmission of THz waves, which suffer because of high path loss, the IS operation in THz wireless communication links has been analyzed for the proposed path loss models. For instance, the study in [31] employs electromagnetic theory to deduce the expression that returns the phase shifting of each IS reflection unit. Also, [PERSON] [32; 41] proposed a path loss model at THz by using physical optics from electromagnetic theory to elaborate a path loss expression. The work in [41] assumes far-field path loss with a plane-wave approximation for the THz channel model, and [32] provides a near-field channel model for path loss calculation, taking into account the spherical wavefront of the radiated waves. To study realistic path loss models for IS-aided THz wireless networks, it is necessary to find the IS link budget potential as well as their optimal placement in space. The studies employing spherical wavefront for the THz waves show that when the source emits an electromagnetic wave reflected to the receiver through the IS, the reflected signal will present a narrow beam. As THz waves encounter high propagation losses and short wavelengths, many reflecting elements are expected to compensate for the severe impairments. Due to their beam width, a THz IS operates in its Fresnel zone, resulting in a radiating near-field that covers several meters. Near-field conditions bring the possibility of focusing the signal towards a specific location in space instead of a specific direction, as in the conventional far-field assumption. In such setups, ISs are expected to communicate with multiple users, possibly operating in the near-field condition. The authors in [51] used the spherical wave channel model to analytically determine the expression for the path loss and beam pattern of a holographic IS in the radiating near-field, i.e., in the Fresnel zone. They proposed a holographic control of THz IS with the capability of generating multiple predefined shaped beams in several directions to determine the field scattered. The holographic control of electromagnetic waves can create versatile radiated beams, eliminating the need for hardware with complex circuits. Another way to combat the THz drawbacks on the propagated waves is implementing the mMIMO antenna systems.