# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 2 IS in Wireless Communication
### A Historical Perspective
The model fitted THz sensing application since the peak absorption was maintained while tuning the peak frequencies. Based on the modeling and design of tunable reflect-array elements in [76], [PERSON] et al. [77] constructed smart reflect-arrays that were hung on the walls of a room to enhance the indoor spectrum sharing capacity by changing the phases of the reflected signals and changing the spatial distribution of the signal strength. The experiment showed that the interference was canceled to \(-73\) dBm, and the signal-to-interference-plus-noise ratio (SINR) was increased to about 30 dB. A programmable metasurface capable of modeling incident electromagnetic waves polarization, scattering, and focusing was proposed in [78]. Using a digital metasurface similar to the one in [74], the authors fabricated a prototype with 1600 metamaterial particles, obtaining an aperture size greater than 20 wavelengths. With coding matrices being used to control the PIN diode states of each particle, they obtained reconfigurable polarization conversion, scattering, planar focusing, beam steering, and beam forming. In [79], the authors advanced the previously performed study in [80], using a 3D wireless modeling to simulate signal propagation in an indoor environment and an iterative stochastic optimization process to search for a 3D reflector shape to be put around an AP. The low-cost reflector, about US$30, provided a cheaper way to strengthen the AP signal in desired directions than using directional antennas. Their findings are 55.1% more throughput and a signal strength gain of 6 dB. Despite being a non-reconfigurablesurface, the presented reflectors could optimize wireless coverage. The same authors of [74] proposed space-time coding digital metasurfaces in [81]. Based on the time modulation of the digital metamaterial particle's reflection coefficient, they could control the metasurface scattering pattern and its harmonic power distribution. Switching coding sequences cyclically permitted redistributing the arriving radio wave and its odd harmonic frequencies in different directions. LIS was proposed in [82] as an extension of mMIMO systems. Assuming that an entire surface is used as an intelligent receiving antenna array, the area is sufficiently large, and matched-filtering is employed, the authors derived the channel capacity for three different positioning scenarios of the terminals communicating with the LIS: along a line, a plane, and a cube. The conclusion was that the inter-user interference of two users at the LIS is close to a sinc-function. The inter-user interference is negligible as long as the distance between two users is larger than half of the wavelength. The equivalence between the LIS continuous surface model in [82] and the discrete dense grid of antennas model was assessed in [83] and corroborated in [84]. Indeed, the authors in [84] compared SINR theoretical and simulation results using a \(128\times 128\) LIS implementation with a \(\lambda/2\) spacing at 5 GHz and observed that the mismatch could be reduced by decreasing the distance between the LIS points or augmenting the distance between the LIS and the user. Metasurfaces incorporating networked hardware control elements were extensively detailed in [85]. They are called hypersurfaces tile-coat environmental objects and communicate with existing systems, such as localization services and cloud computing, to create the optimum wave scattering and combat path loss and multipath phenomena. The researchers elaborated on the first model to describe programmable wireless indoor environments, focusing on hardware, networking, and software components. They also evaluated the potential of programmable environments via a full 3D ray tracing in 2.4 and 60 GHz cases. The smart wallpaper, called RFocus in [86], comprises 3200 antennas, each costing a few cents at scale, arranged on a 6 m\({}^{2}\) surface. The surface operated passively, in the sense that it did not amplify the receiving signal but directed it only.