# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 2 IS in Wireless Communication
### A Historical Perspective  
Ancient people have already succeeded in controlling electromagnetic wave propagation. For example, the concepts of linear optics were stated more than 2400 years ago in China [64]. With the growth of studies and dissemination of knowledge over the globe, the beginning of the twenty-first century brought breakthrough discoveries on shaping electromagnetic waves as desired. Initially, simultaneous negative electric permittivity and magnetic permeability responses were achieved [65]. In addition, the negative index of refraction was verified experimentally [66]. Furthermore, they fabricated an electromagnetic invisible cloak [67]. Afterward, frequency-selective surface (FSS) were presented in [68], and the design for an absorbing metamaterial element was provided in [69]. Finally, metasurfaces and telecommunication systems were combined by introducing subwavelength thickness planar antenna arrays and generalizing the laws of reflection and refraction in [70]. In [71], the authors presented an experimental high-gain reconfigurable sectoral antenna that uses an active cylindrical FSS structure to obtain a directive radiation pattern. The structure was made of metallic discontinuous strips with positive-intrinsic-negative (PIN) diodes in their discontinuities and placed cylindrically around a dipole. Switching the diodes on the structure allowed a 360\({}^{\circ}\) radiation pattern sweep. In other words, the transmission and reflection coefficients of the FSS structure could be controlled by the state of the diodes. An intelligent wall was proposed in [72], consisting of a wall equipped with an active FSS. These surfaces had PIN diodes fixed on the metal connection parts of each surface element that allowed the wall to behave as totally transparent or reflective in response to the incident radio waves. As reported in [72], the system performance of 80% was incrementally obtained using the proposed intelligent wall and a wireless cognitive network. [PERSON] et al. [73] used an electronically tunable metasurface as a spatial microwave modulator. To be placed on the wall, the metasurface was divided into 102 unit cells, defined as rectangular patches on a ground plane, spaced by half of the wavelength at the working frequency of 2.47 GHz. The resulting metasurface thickness was 1.5 mm. Each unit cell was a two states resonator that could reflect incident waves positively or negatively. The goal of the experiment conducted in a 36 m\({}^{3}\) office room was to maximize the transmission between two antennas, focusing the waves onto well-designed \(\lambda/2\) isotropic focal spots or to minimize the electromagnetic field on an antenna placed at any location in the room. Digital metamaterials, containing \(30\times 30\) particles, were proposed in [74]. Their metamaterial particles were two planar symmetrical metallic structures printed on the top surface of the F4B commercial dielectric board substrate and joined by a PIN diode. The total dimension of the particle was \(0.172\times 0.172\times 0.057\lambda^{3}\) at the central frequency of 8.6 GHz. Applying a biased voltage of 3.3 V to the diodes, the particle behaved as a \"1\" digitally coded material. When there was no biased voltage, it behaved as a \"0\" digitally coded material. The tests showed that the phase response difference between those states was exactly 180\({}^{\circ}\) at 8.6 GHz. The authors in [74] were able to alter the scattering pattern of normally incident beams on the digital metamaterial from being directly reflected to being reflected in two different directions, forming an angle of approximately 60\({}^{\circ}\). [PERSON] et al. [75] proposed a method to model, analyze, and design graphene metasurface-based THz absorbers. They used the equivalent circuit model approach and designed a broadband and tunable absorber with an absorption over 70% fraction bandwidth. The results were verified with full-wave electromagnetic simulation.