## 2 Space Segment
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Paper** & **Main Contribution** & **Focus on** \\ \hline \multirow{4}{*}{[PERSON] et al., 2020 [45]} & Review of the role played by High-Altitude & Overview on aerial platforms, network \\  & Platforms (HAPs) in exploiting cellular radio & topology, coverage and hap-terrestrial joint \\  & spectrum for wireless communications service in & exploitation. Mostly devoted to \\  & remote areas. & cellular networks. \\ \hline \multirow{4}{*}{[PERSON] et al., 2020 [46]} & SAGIN system from the perspective of & Cooperation between high-altitude platforms (HAPs) and terrestrial base stations (BSs) for serving communication from geostationary \\  & cooperative communication point of view. The & (GEO) satellites and the user. System outage \\  & expressions for outage probabilities of each link & performance are analyzed in detail. Mostly \\  & system have been derived. & dealing with SAGIN. \\ \hline \multirow{4}{*}{[PERSON] et al., 2020 [47]} & Thorough survey on fronthaul and backhaul & \\  & technologies that offer the 6G connectivity in & Mostly devoted to 6G \\  & rural areas. & \\ \hline \multirow{4}{*}{[PERSON] et al., 2021 [48]} & Overview of quasi-optical techniques employed & Antennas for terrestrial and non-terrestrial wireless communications networks for \\  & in multi-beam antennas for B5G and 6G & Beyond 5G (B5G) and 6G with emphasis on \\  & mmWave and THz networks. & mmWave and Terahertz frequency range. \\ \hline \multirow{4}{*}{[PERSON], 2021 [26]} & Basics behind the SAGIN and 6G and their & Enabling technologies for 6G, SAGIN and \\  & convergence into 6G-SAGIN, with particular & their synergic use. Research challenges and \\  & attention devoted to the role of unmanned aerial & future directions on these topics. \\ \hline \multirow{2}{*}{[PERSON] et al., 2021 [49]} & Survey on UAV communications for 6G and & Mostly devoted to 6G. \\  & analysis of their energy consumption. & \\ \hline \multirow{4}{*}{[PERSON] et al., 2021 [50]} & Service-oriented SAGINs & Two categories of enabling key technologies, \\  & management architecture. & heterogeneous resource orchestration \\  & management architecture. & technologies and cloud-edge synergy \\  & & technologies are addressed and discussed. \\  & & Mostly devoted to 6G SAGIN. \\ \hline \multirow{2}{*}{[PERSON] et al., 2021 [51]} & Overview of some promising technologies in 6G & Mostly devoted to 6G. Security and privacy \\  & networks with focus on AI, intelligent surfaces, & techniques that can be applied to protect \\  & terahertz and cell-free massive MIMO. & 6G data. \\ \hline \multirow{4}{*}{[PERSON] et al., 2022 [52]} & Survey of the integration of blockchain & Analysis of architecture, characteristics, and \\  & technologies for securing Space–Air–Ground & security threats of SAG-IoT systems. \\  & Internet of Things (SAG-IoT) applications. & Challenges in blockchain integration and \\  & Internet of Things (SAG-IoT) applications. & artificial intelligence exploitation in the \\  & SAGIN architecture exploitation for enabling & Architectural challenges for SAGIN in \\  & Immersive Media (IM) services. & supporting low-latency and high \\  & & reliability services. \\ \hline \multirow{4}{*}{This work} & Review of technological solutions and advances & Strong emphasis on the available antenna \\  & in the framework of a Vertical Heterogeneous & systems in satellite, airborne and ground \\  & Network (VHetNet) integrating satellite, & layers. SAGIN and 6G are both considered. \\  & airborne and terrestrial networks. & Overview on applications exploiting \\  & & these frameworks. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: An overview of survey papers dealing with 6G and SAGIN. institutional and governmental entities [54]. This resulted in a more dynamic space segment industrial landscape, but it was also more competitive, as cheaper access to space provided opportunities for new entrants.