## 2 SRE Empowered by RISs  
RISs represent a revolutionary approach to wireless communication that transforms traditionally passive wireless environments into programmable spaces. They are programmable structures consisting of numerous sub-wavelength elements that can dynamically manipulate electromagnetic waves. These elements may be configured independently, leading to the formulation of a surface that can alter in real-time the amplitude, phase, and polarization of the impinging wave, effectively creating customized propagation environments by reflecting, refracting, or absorbing waves in controlled directions [12; 15; 17]. The fundamental principle behind a RIS is to create favorable propagation conditions by intelligently reflecting or refracting incident signals towards desired directions while suppressing interference in others. This capability enables the RIS to establish virtual line-of-sight paths in non-line-of-sight scenarios, effectively bypassing obstacles that would otherwise block mmWave and terahertz frequencies. In other words, RIS has the potential to modify the environment in such a way that the environmental objects, which are typically considered as obstacles, are now used to form an EM response, thereby contributing to the communications channel and improving communication. By controlling the electromagnetic response of the environment itself, RIS can extend coverage, enhance signal quality, improve spectral efficiency, and reduce power consumption across the network. The implementation of a RIS includes arrays to reflect signals, liquid crystal surfaces, and meta-surfaces that are software controlled. The use of such surfaces may be complementary to other techniques, such as spatial modulation, by working together for signal improvement. Unlike conventional relays or repeaters that actively process and amplify signals, RISs operate in a nearly passive manner, requiring minimal power only for reconfiguration control. However, RISs are not necessarily passive. There are three different classifications in respect of their way of functioning, i.e., passive, dynamic and active RISs. The former are programmable once, before installation, and they cannot be reprogrammed easily. The dynamic RISs are surfaces that can be easily reprogrammed to fit different scenarios; thus, they can provide a true SRE. The third category consists of active RISs that do not just reflect but also amplify the signal and can take the place of relays, with their own advantages and disadvantages. In this work, we focus on dynamic RISs and their use for creating the SRE. RISs are not the only way to create an SRE. Alternative methods/techniques include relay-aided transmission and backscatter communications. In general, an SRE can be represented as shown in Figure 1. According to the latter, various network assets (such as base stations, mobile stations, satellite service stations, aircraft, and aeronautical communications) may be reconfigured by a control system. Data are being collected from a network of sensors or by feedback from the wireless network itself. These data are processed to extract the Channel Status Information that is used by the control system. SREs do not necessarily require a RIS to operate, although the RIS-approach is one of the most promising [15; 17]. Its primary advantages over other solutions include low deployment cost, as it is relatively cost-effective, as well as its passive nature, which means that it does not require any power to operate. Other advantages are full band compatibility, as it can be programmed to work at various frequencies, and ease of deployment as it is a thin surface that can be mounted anywhere, i.e., from buildings to even clothes. The aforementioned advantages offer the possibility of creating complex solutions at minimal cost. New challenges are also set by these surfaces regarding their deployment, as there are still challenging aspects as regards their modeling.