## 2 RISs and Their Integration with NTNs
### Recent Advances in RIS-Integrated NTNs: Unveiling Progress  
Recent advances in RIS-integrated NTNs highlight resource optimization and performance analysis developments. Researchers are delving into strategies for power consumption minimization, energy efficiency enhancements, and sum-rate maximization, utilizing optimization techniques. These studies span from exploring joint beamforming strategies to designing optimal phase shifts, showcasing the adaptability and efficiency of RISs across various scenarios [50]. Researchers actively explore secure cooperative communications and employ alternating optimization strategies in RIS-integrated NTNs. These multifaceted contributions extend to comprehensive performance analysis, with studies deriving expressions for outage probability, symbol error rate, ergodic capacity, secrecy, and connection probability [51; 52]. This collective body of work illuminates the versatility and robustness of RISs in enhancing the efficiency, security, and overall performance of NTN landscapes [53; 54; 35]. The simulation results in [7] revealed that our proposed robust scheme significantly outperforms the RIS with random phase, the non-robust scheme, and the secure transmission scheme without the RIS, achieving improvements of approximately 8%, 17%, and 41%, respectively, in terms of friendly interference performance. From addressing fundamental link challenges to enhancing security and performance, RIS stands as a cornerstone in shaping the future of NTNs. We critically examined beamforming methods customized for HAP systems equipped with RIS technology. Some of the core advantages found are as follows: \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Attribute** & **RIS** & **Massive MIMO** & **Relay** & **Backscatter** \\ \hline Operating & Passive/Active, & Active, & Active, & reception & \\ Mechanism & reflection & transmission/ & and & \\  & & reception & transmission & \\ \hline Duplex & Full & Half/full & Half/full & Full \\ \hline No. of RF & & & & \\ Chains & 0 & N & N & 0 \\ \hline Hardware Cost & Low & Very high & High & Very low \\ \hline Energy & & & & \\ Consumption & Low & Very high & High & Very low \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparative analysis of various technologies with the RIS. * **Enhanced Network Coverage:** In scenarios where direct line-of-sight (LoS) links are blocked or absent, RISs can create virtual LoS links between ground users and HAPs in air-ground networks, significantly improving communication quality and extending network coverage [8; 55; 56; 57]. * **Dynamic and Smart Control:** Mounting the RISs on a HAP allows for the dynamic control of the RIS's location, leveraging the HAP's mobility. This mobility introduces a new degree of freedom in the RIS's design compared to terrestrial RISs (TRISs) installed at fixed locations as shown in Figure 1[58; 59]. * **Expanded Wireless Coverage:** When installed on an HAP, the RIS will likely establish an LoS path for transmitting and receiving components. Unlike TRISs, which support communication between two nodes in half the space, an RIS on an HAP can expand wireless coverage and achieve full-angle reflection [60]. * **Ultra-wide Bandwidth in THz or mmWave Band:** An HAP and RIS can operate in the terahertz (THz) or mmWave band, offering an ultra-wide bandwidth to meet high-data-rate requirements [61]. The mobility of HAPs and reconfigurability of RISs efficiently compensate for the high path loss and blockages typical in THz or mmWave links [60]. * **Enhancing Communication Links:** An imperative motive for incorporating RISs into NTNs is their ability to surmount the intricate challenge of closing communication links. Designed explicitly for scenarios involving mobile ground users with low-directional antennas, RISs strategically compensate for path losses. This intervention significantly enhances signal reception, resulting in elevated data rates and an overall boost in system performance [59].