## 4 Principle and Operation of the Metamaterials  
Artificial bulk and planar materials having subwavelength structural inclusions are known as metamaterials and metasurfaces. Metasurfaces have demonstrated significant powers in achieving various capabilities, such as perfect absorption, anomalous reflection, focusing, imaging, and so on, due to their customized unit cell architectures. The RIS is designed to use small unit cells of metamaterials, referred to as metasurfaces [65]. The idea is to use new materials with controllable electromagnetic properties to manipulate the propagation of radio waves in the environments where we live and work. Metamaterials have established themselves as a powerful technology with many applications, especially in wireless communications. Metamaterials consist of a class of artificial materials whose physical properties, such as permeability and permittivity, can be varied to desired features [66]. A brief survey of the existing state-of-the-art schemes for metamaterials is compiled in Table 4. In addition, metamaterial structures have recently been anticipated as a novel way to turn any naturally passive wireless transmission environment (the set of items between a transmitter and a receiver makes up the wireless environment) into an active one [67; 32; 65]. Figure 2: RIS assisted communication model for uplink cellular network. In the case of planar structures (e.g., metasurfaces), effective parameters of metamaterials can be tuned to achieve the desired transformation on transmitted, received, or impinging electromagnetic waves [76]. They are cost-effective, and their extremely low hardware footprint technology makes them usable in different components of the wireless propagation environment such as room walls/ceilings and building facades. This is beneficial in controlling artificial EM wave propagation and environmental AI. Due to \begin{table}
\begin{tabular}{l l} \hline \hline
**References** & **History of Metamaterials** \\ \hline
[68] & Concept of left-handed material \\  & \(\epsilon<0\), \(\mu<0\) \\  & Negative refraction \\ \hline
[69] & Proposal of meta-surface \\  & Two-dimensional \\  & Simplify design and manufacturing \\ \hline
[70] & Transformation optics \\  & Design metamaterial with any \(\epsilon\) and \(\mu\) \\  & Enabling flexible control of EM wave \\ \hline
[71] & Reconfigurable large reflectarray \\  & A reflecting element consisting of a microstrip patch with PIN diodes as a single-bit \\  & digital phase shifter was used. \\  & Easy to control \\  & Millimeter wave \\ \hline
[25] & Programmable/digital metasurface \\  & A digital metamaterials with unit cells is presented with a unique metamaterial \\  & particle with a 0 or 1 responses \\  & Simplify the design \\  & Digital coding \\ \hline
[72] & Realize -\(\epsilon\) and -\(\mu\) \\  & \(\leftarrow\): periodic array of metallic rods \\  & \(\leftarrow\): periodic array of split ring \\ \hline
[73] & Programmable metasurface \\  & A tunable metasurface absorber with an optically programmable capacitor as the \\  & tuning element. \\  & \(\bullet\) Varactor \\  & \(\bullet\) 360\({}^{\circ}\) reflection phase tuning \\ \hline
[65] & Proposal of reconfigurable intelligent surfaces \\  & \(\bullet\) Focus on reflection \\  & Extensive applications in wireless networks \\ \hline
[74] & Proposal of intelligent omni-surface \\  & RIS enabled metasurface provides dual function of transmission and reflection \\  & Enabling dual function of reflection and transmission \\  & Extends the coverage of wireless networks \\ \hline
[75] & Protype of metamaterial reflector \\  & \(\bullet\) 10\(\times\) increase in data rate \\ \hline \hline \end{tabular}
\end{table} 
Table 4: State of the art schemes for metamaterials. usages of metamaterials, RIS-enabled wireless communications have recently gained a lot of attention in 5G and the forthcoming 6G broadband network applications. Moreover, semiconductors and graphene are two preferred RIS materials.