# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 1 Introduction  
The fifth generation (5G) wireless network technology is already commercialized worldwide. The study groups and partnership projects have been designing the 5G standard since the last decade. About the key performance indicators (KPIs) set for 5G, it can be mentioned peak data rate of 20 Gbps, peak spectral efficiency of 30 bps/Hz, maximum channel bandwidth of 1 GHz, area traffic capacity of 10 Mbps/m\({}^{2}\), end-to-end latency of 1 ms, a packet error rate of \(10^{-5}\), and mobility of 500 km/h [1]. This set has been chosen to support the following new applications: virtual and augmented reality (VAR), e-commerce, Internet of things (IoT), and enhanced mobile broadband. Compared to the fourth generation (4G), 5G has expanded the mobile broadband and added support to IoT [2]. As a continuous evolution, the next step in wireless network technology, the sixth generation (6G), aims to at least double 5G KPIs on its deployment after 2030 [1, 2]. In order to achieve 6G requirements, research groups have pointed out some solutions: terahertz (THz) band communications, intelligent communication environments, ubiquitous artificial intelligence (AI), extensive network automation, dynamic spectrum access, cell-free (CF) massive multiple-input multiple-output (mMIMO), Internet of space things (IoST), millimeter wave ( mmWave) band communications, ambient backscatter communications, space-air-ground-sea integrated networks, and built-in network security [1, 2]. THz band communications deserve special attention among the other solutions due to itsoverarching support. THz communication possesses enough spectrum bandwidth from 0.1 THz to 10 THz. It can achieve data rates from hundreds of Gbps to several Tbps, enabling new spectrum access, connecting IoST, and aiding network automation [3; 4]. In order to understand better the contributions of THz communications, it can be important to mention some 6G envisioned applications that THz communications will support. Home wireless networks will exhibit Super Hi-Vision digital video format with a \(7680\times 4320\) resolution by sending data at a rate higher than 24 Gbps [5]. The wireless cognition function is able to control the transmission over a communication link between a device and a base station with enough bandwidth and data rate to work as an edge server to carry complex tasks in real-time computations that encompass contextual awareness, vision, and perception [4]. THz over fiber system is proposed to connect central stations and distributed THz wireless antennas to support high-resolution mobile multimedia services, wireless video distribution systems, and wireless local area networks (WLANs) [6]. THz will also contribute to ultra-high-speed small cell systems, create secure wireless communication for military and defense applications, and enable advanced health monitoring and chemical attack prevention systems [7]. Applications other than telecommunications can also be addressed using the following THz characteristics: lower single-photon energy than X-rays, high spectral resolution, and power penetration through non-metallic or non-polar materials [8]. However, the use of THz communication is still challenging in terms of hardware and environment. Electromagnetic waves are more or less attenuated at traversing the environment according to their frequency and the atmosphere temperature, pressure, and humidity. Experiments have shown that THz waves can suffer an attenuation from \(10\) to\(100,000\) dB/km in standard atmospheric conditions [9]. Water vapor absorbs and refracts THz waves significantly to separate a single THz pulse into two overlapping pulses after 1000 m of propagation [10]. Human-made structures also impose an excess loss on THz communications. Wireless communication systems seek the most negligible absorption of electromagnetic waves as they reach an obstacle. Two waves originate from the incident wave at the occasion: reflected and refracted. The thicker the obstacle is regarding the wavelength, the more the incident wave is absorbed.