## 3 Emerging and State-of-the-Art Technologies  
### Reconfigurable Intelligence Surface (RIS)  
A highly promising and rapidly emerging technology for enabling ultra-high-throughput communication in the mmWave spectrum and for decoupling wireless communication KPIs from strict dependence on Tx-Rx distance is the reconfigurable intelligent surface (RIS). RIS has gained recognition as a key enabler in enhancing wireless networks, especially in the domains of massive MIMO, beamforming, and integrated sensing. The recent literature underscores rapid advancements in RIS architectures, performance optimization techniques, and deployment strategies. RIS encompasses a broad class of passive or active two-dimensional microwave surfaces that can dynamically manipulate incident RF signals [59]. These surfaces are capable of reflecting, diffracting, amplifying, focusing, or steering electromagnetic waves. The resulting directive beams help address the fundamental limitations of mmWave bands, such as high free-space path loss and poor penetration through obstacles. What distinguishes RIS from comparable technologies such as transmitarrays, reflectarrays, or frequency-selective surfaces is its ability to electronically reconfigure its electromagnetic properties in real-time [59]. For example, an RIS installed on the exterior of a building and configured as a reflectarray can dynamically redirect incoming mmWave signals toward a targeted receiver at the optimal frequency and angle, thereby improving link quality and overall system performance. An example of such a scenario is illustrated in Figure 3. This transformative potential has sparked significant research interest and innovation in RIS development. A sharp rise in related publications highlights this growing attention [59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69]. For instance, in [70], the performance of a double-stacked intelligent metasurface integrated into massive MIMO systems is explored. This configuration, depicted in Figure 4, effectively enhances spatial diversity, thereby improving communication reliability and throughput. A variety of innovative RIS architectures and applications have been explored in the recent literature, further highlighting the versatility and potential of this technology in next-generation communication systems. In [71], a simple stacked metasurface architecture with reconfigurable amplitude and phase is utilized to optimize downlink beamforming, significantly improving signal quality in multiuser environments. Active RIS with discrete phase shifts is examined in [72], demonstrating enhanced spectral efficiency in communication systems. Hybrid RIS configurations designed for integrated sensing and communication are presented in [73], underscoring the multifunctionality of RIS in future network paradigms. A location-driven beamforming strategy tailored for RIS-enabled near-field communications is proposed in [74], where beam patterns are dynamically optimized based on user position data to improve connectivity in dense urban settings. Deployment challenges in complex cellular networks are addressed in [75], which introduces a frequency-dependent RIS model applicable to multiband, multicell MIMO networks. In [76], the integration of RIS with spatial modulation is explored to enhance antenna selection for vehicular communications, leading to improvements in link robustness and data rate performance. Furthermore, ref. [77] presents a transparent, graphene-based RIS operating in the THz band for 6G systems, an innovative design that demonstrates the promise of advanced materials for supporting ultra-high-frequency communications. Despite these advances, several key challenges must be overcome to enable commercially viable RIS deployment. These include optimizing energy efficiency and scalability, refining real-time electronic control mechanisms, and addressing practical concerns such as cost, power consumption, and seamless integration with existing communication infrastructure.