# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 2 IS in Wireless Communication
### IS Architecture  
An IS is a flat structure made of specific materials, which are the base of the IS classification. Copper and graphene have been used. Researchers in wireless communications have focused on two main implementations: antenna-array-based and metasurface-based structures. The radio waves impinging on these electromagnetic material surfaces can be reconfigured to adjust the wireless propagation environment. By controlling the phase shifts and wavefront, the waves are reflected to propagate toward their desired directions. By making the wireless environment programmable and controllable, these surfaces monitor the propagation to improve the efficiency of the received signal.  
The different types of IS found in the literature include active and passive surfaces, denominated according to the capacity to perform operations on the incident radio waves. In addition, ISs whose functions can or cannot be modified after manufacturing or deployment are denominated dynamic and static surfaces, respectively.  
Figure 1 depicts the IS architecture shared among most technical papers in the literature. A top-down IS perspective view can be seen as a thin composite material sheet controlled electronically. Composite means that the planar structure is typically made of three layers. The reflecting elements constitute the outer layer, and the inner layer is a control circuit board responsible for adjusting the reflection amplitude and phase shift of the IS through feeding lines and via holes. The middle layer is a conductor between the outer and inner layers to avoid signal energy leakage. The IS controller is used for both programming the surface response and interacting with other network elements, such as base stations (BSs), APs, and user devices. Most prototypes have used field programmable gate array (FPGA) as the IS controller.  
From a bottom-up perspective, the IS outer layer comprises multiple reflecting elements, or meta-atoms, of electric thickness. Electric thick means this reflecting element dimension is in the order of the sub-wavelength of the operating frequency. The meta-atom signal phase response is modified according to the electronic device states embedded in it. Devices used in practice include PIN diodes, field-effect transistors (FETs), micro-electromechanical systems (MEMSs) switches, and microcontrollers. Applying a biasing voltage to the feeding line energizes the reflecting element and the electronic devices through the via hole. The combination of the electronic device states defines the phase shift pattern. The meta-atom signal reflection amplitude can be controlled via a variable resistor in the element design. Varying the element resistance dissipates distinct portions of the arriving signal energy.  
Finally, prototypes have combined iterative algorithms, integrated with the IS controller, to pursue the optimum configuration of each reflecting element, therefore achieving an efficient operation of the IS in accordance with the application. Such algorithms have also been used to alter the IS element states cyclically so that the reflected signal power is distributed harmonically.  
Figure 1: Example of IS architecture.