Table 1.

An overview of survey papers dealing with 6G and SAGIN.

Paper	Main Contribution	Focus on
Arum et al., 2020 [45]	Review of the role played by High-Altitude Platforms (HAPs) in exploiting cellular radio spectrum for wireless communications service in remote areas.	Overview on aerial platforms, network topology, coverage and hap-terrestrial joint exploitation. Mostly devoted to cellular networks.
Ye et al., 2020 [46]	SAGIN system from the perspective of cooperative communication point of view. The approximated and asymptotic closed-form expressions for outage probabilities of each link as well as the outage probability of the SAGIN system have been derived.	Cooperation between high-altitude platforms (HAPs) and terrestrial base stations (BSs) for serving communication from geostationary (GEO) satellites and the user. System outage performance are analyzed in detail. Mostly dealing with SAGIN.
Yaacoub et al., 2020 [47]	Thorough survey on fronthaul and backhaul technologies that offer the 6G connectivity in rural areas.	Mostly devoted to 6G
Guo et al., 2021 [48]	Overview of quasi-optical techniques employed in multi-beam antennas for B5G and 6G mmWave and THz networks.	Antennas for terrestrial and non-terrestrial wireless communications networks for Beyond 5G (B5G) and 6G with emphasis on mmWave and Terahertz frequency range.
Ray, 2021 [26]	Basics behind the SAGIN and 6G and their convergence into 6G-SAGIN, with particular attention devoted to the role of unmanned aerial vehicles (UAVs).	Enabling technologies for 6G, SAGIN and their synergic use. Research challenges and future directions on these topics.
Jiang et al., 2021 [49]	Survey on UAV communications for 6G and analysis of their energy consumption.	Mostly devoted to 6G.
Cheng et al., 2021 [50]	Service-oriented SAGINs management architecture.	Two categories of enabling key technologies, heterogeneous resource orchestration technologies and cloud-edge synergy technologies are addressed and discussed. Mostly devoted to 6G SAGIN.
Zhao et al., 2021 [51]	Overview of some promising technologies in 6G networks with focus on AI, intelligent surfaces, terahertz and cell-free massive MIMO.	Mostly devoted to 6G. Security and privacy techniques that can be applied to protect 6G data.
Wang et al., 2022 [52]	Survey of the integration of blockchain technologies for securing Space–Air–Ground Internet of Things (SAG-IoT) applications.	Analysis of architecture, characteristics, and security threats of SAG-IoT systems. Challenges in blockchain integration and artificial intelligence exploitation in the SAG-IoT framework.
Wei et al., 2022 [53]	SAGIN architecture exploitation for enabling Immersive Media (IM) services.	Architectural challenges for SAGIN in supporting low-latency and high reliability services.
This work	Review of technological solutions and advances in the framework of a Vertical Heterogeneous Network (VHetNet) integrating satellite, airborne and terrestrial networks.	Strong emphasis on the available antenna systems in satellite, airborne and ground layers. SAGIN and 6G are both considered. Overview on applications exploiting these frameworks.