## 2 Research Background  
### RIS Relationship with 5G and Internet of Things (IoT)  
A RIS-assisted downlink transmission scenario to help multiple users is presented in [12]. However, a high-gain line-of-sight (LOS) link will almost certainly suffer from a spatially sparse low-rank channel, limiting the number of spatial streams and, as a result, the possible rate. Furthermore, there is only one suitable propagation vector in several LOS cases, and spatial multiplexing is not feasible. Therefore, to avoid these drawbacks, the authors focused on the system model in which a multi-antenna BS supports K single-antenna users through intelligent reflections without a direct LOS path between the users and the BS. The fundamental studies and state-of-the-art schemes on intelligent surfaces and initial technologies are summarized in Tables 1 and 2 with their architectures and functionalities, merits, and demerits. Energy consumption has become a severe concern to 5G and beyond wireless networks to achieve high data rate requirements. To reduce energy consumption, multiple approaches were proposed: renewable energy sources, relevant deployment techniques, energy-efficient hardware components with green resource allocation, and transceiver signal processing algorithms, to name a few, [13; 14]. RIS has emerged as a new hardware technology with increased potential for significant energy consumption reductions. RIS technology is advantageous in energy consumption because it allows for the amplification and forwarding of incoming signals without using a power amplifier. Each reflected signal can also be constructively combined by adequately engineering the phase shifts applied by each reflecting element. A RIS consumes significantly less energy because no power amplifier is employed [15; 16]. To maximize energy efficiency by using RIS reflectors with finite resolution phases, authors performed computer simulations and showed that even 1-bit phase (0 and \(\pi\)) resolution reflecting elements improved the system's energy efficiency compared to traditional AF relaying systems [17]. In reference [4], researchers proposed the RIS-SM and RIS-SSK schemes to understand the concept of intelligent surfaces-assisted index modulation (IM) techniques. The phase optimization and the principle of index modulation are combined in these schemes to improve spectral performance and signal quality. As a result, RISs are a promising software-defined architecture that can be realized at lower cost, size, weight, and power (C-SWaP design) and are seen as a key enabler for implementing the growing notion smart radio environments (SREs). IoT devices in the future will be compact and have a restricted power supply. Reference [18] proposed the ambient backscattering (ABSc) approach, together with spatial \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Technology** & **Functionality** & **Structure** \\ \hline \(\left[\text{21-23}\right]\) & Reconfigurable Intelligent wall & Complete transparent/reflecting base & Frequency selective active base  utilizing PIN diodes \\ \hline \(\left[\text{24}\right]\) & Spatial microwave modulators & Complex microwave fields shaping & Tunable meta-surfaces with binary  phase state capabilities \\ \hline \(\left[\text{25}\right]\) & Coding meta-materials & Scattering patterns (reconfigurable) & Binary elements based  Meta-surfaces \\ \hline \(\left[\text{26}\right]\) & Meta-materials (Programmable) & Reconfigurable polarization, phase  and scattering & PIN diode-equipped cells based  Meta-surfaces \\ \hline \(\left[\text{27}\right]\) & Reconfigurable reflect-arrays & Adjustable reflection phase & Reflect-arrays with tunable (varcator-tuned) resonators \\ \hline \(\left[\text{28-31}\right]\) & Large intelligent surface & Gains compared to massive MIMO & Active contiguous surface for \(\text{Tx}/\text{Rx}\) \\ \hline \(\left[\text{32,33}\right]\) & Hypersurface (software-controlled) & Wave absorption, steering  and polarization & IoT gateways based Meta-surfaces \\ \hline \hline \end{tabular}
\end{table}