## 3 Principle of Operation of RIS  
RIS surfaces are flat structures made of metamaterials and electromagnetically discontinuous. They do not adhere to the conventional laws of reflection and diffraction; instead, the phase and wavefront of the radio waves that affect them can be controlled. If deployed to cover building objects, walls, or facades, they could allow real-time personalization of the electromagnetic response of environments. Further, the RISs are reconfigurable EM material sheets that intensely monitor the propagation in the atmosphere to improve the efficiency of the received signal.  
The RISs are composed of many low-cost components (such as a PIN Diode capable of manipulating the electromagnetic waves) that influence them in ways that are not capable of naturally occurring materials. In addition, these \"intelligent\" surfaces can be controlled in phase with complexity and precision while requiring a very low consumption. The objective is to improve communication performance. Moreover, the benefit of RISs compared to traditional reflect array/transmit array antennas is that it is possible to dynamically adjust the parameters of each functionality, such as the direction of reflection/refraction, the location of the focal point, etc.  
When integrated into the physical environment (e.g., on walls), these surfaces create reflection anomalies, making it possible to control the radio propagation channel at a lower cost and with a lower energy footprint. By doing so, the RIS will reduce the transmission power required at a constant rate, serve a user located in an uncovered area, limit radio interference, or even reinforce the security of radio links concerning passive eavesdropping. In addition, the RIS was proposed to improve efficiency in terms of link quality and coverage.  
The basic principle of RIS is depicted in Figure 1; the EM wave is tuned to any other angle instead of the symmetric reflective wave, based on [PERSON]'s law when the surface is divided into a large number of closely spaced components (supercells of scattering particles). Each metasurface element is rendered to have suitable phase shifts. Ideally, if each metasurface element phase shift can be calibrated to any value, a reflected beam can be generated at any angle [2]. The RIS consists of meta-surfaces due to its huge number of advantages, and it also serves as programmable reflectors. However, the phase shift of the surface element must be set appropriately or smartly to shape the reflected beam with incident EM wave. A signal processing design technique or machine learning-based approaches can be applied, resulting in the so-called reconfigurable intelligent surface. Continuous or analog phase changes can be challenging to accomplish in practice. Recently, as stated in [56], quantized phase shift configurable RISs have been built. When the input or output signal is a plane, the basic features of RIS can be summarized in the following steps:  
1. reflection/refraction, where a plane wave is redirected from its original propagation path to another direction;
2. absorption, where the amplitude of a plane wave is greatly reduced;
3. focusing/collimation, where a plane wave is condensed to a single point or a spherical wave is transformed to a single plane wave from a point source;
4. alteration of polarization, where procedure (1) or (3) often includes modifying the incoming wave polarization from linear polarization to circular polarization. We have presented RIS performance parameters comparison in Table 3.