# Frequency Selective Surfaces: Design, Analysis, and Applications
## 2 Applications of FSSs
### Intelligent Reflecting Surfaces (IRSs)  
Intelligent Reflecting Surfaces (IRSs) have become a transformative technology in modern wireless communication systems, particularly for their ability to manipulate electromagnetic (EM) waves in real-time. These surfaces consist of large arrays of low-cost reflecting elements, which can be either passive or active, and are placed strategically to improve signal quality and coverage [95].  
**Passive IRS** utilizes reconfigurable, but non-powered, elements (e.g., metallic patches or dipoles) that reflect incoming signals based on their geometrical configuration and material properties. By adjusting the orientation and spatial configuration of these passive elements, signals can be directed toward specific areas, reducing the chances of coverage holes in areas traditionally plagued by poor reception (i.e., blind spots). This makes a passive IRS particularly effective in complex urban environments, where signals are often obstructed by buildings or other large structures [96].  
**Active IRS**, on the other hand, integrates tunable components such as varactor diodes, PIN diodes, or even micro-electro-mechanical systems (MEMS), allowing for the surface to dynamically adjust the phase and amplitude of reflected signals. An active IRS provides enhanced control over the EM wave properties, enabling real-time adjustments to signal paths. These surfaces can actively suppress interference or nullify jamming attempts by fine-tuning the reflection coefficients of each element. The reconfigurable nature of an active IRS also allows for these surfaces to respond to changing environmental conditions, such as user movement or fluctuating interference patterns [97].  
This combination of passive and active IRSs provides unprecedented flexibility and control in shaping wireless environments, ensuring more reliable communication even in the presence of obstacles or interference. IRS technology can be applied in various communication scenarios, including but not limited to cellular networks, satellite communications, and vehicular networks.  
\begin{table}
\begin{tabular}{c l l l} \hline \hline
**FSS Refs.** & **Application** & **FSS Refs.** & **Application** \\ \hline [33; 34; 35] & MMs FSS & [36; 37; 38] & FSS for mobile communication \\ \hline [39; 40; 41] & MM/MSs Lens & [33; 42; 43] & X band \\ \hline [44; 45; 46] & Textile FSS & [47; 48] & C band \\ \hline [49; 50; 51] & Optical FSS/MSs & [52] & 5G EMI reduction \\ \hline [53] & Near Infrared & [34] & RCS reduction \\ \hline [54; 55] & Infrared & [56; 57; 58] & Secure wireless network \\ \hline [59] & Filtering/Anti-reflecting coating & [60; 61; 62; 63; 64] & RF interference and harmonic suppression \\ \hline [65; 66; 67; 68] & Wearable FSS & [52; 69; 70; 71] & EMI shielding \\ \hline [72; 73; 74; 75] & FSS absorber & [76] & Radio Astronomy \\ \hline [77; 78] & Sensors & [79] & Dichroic sub-reflector \\ \hline [80; 81; 82] & RFID & [83; 84] & Terahertz band \\ \hline [82; 85; 86; 87; 88] & FSS antenna & [72; 89; 90; 91; 92] & Stealth radomes \\ \hline [93] & Reconfigurable mechanisms & [94] & Wireless Charging \\ \hline \hline \end{tabular}
\end{table}
Table 1: Practical applications of FSSs.