# Hybrid Satellite-Terrestrial Networks toward 6G: Key Technologies and Open Issues
###### Abstract
Our motivation is driven by the potential of HSTNs as future wireless communication networks for providing broader coverage and higher data rates with reliable and efficient quality of service and the requirement for understanding their performances for optimal deployment. Several surveys have been published so far, covering various aspects of satellite, satellite-terrestrial, and satellite-air-terrestrial integrated communication networks. For example, [PERSON] et al. in 2006 published a survey of handover schemes in satellite communications [28]. [PERSON] et al. reviewed network architectures, operational systems, services, and research issues in mobile satellite systems in 2011 [29]. Research conducted by the small satellite community, including design parameters for inter-satellite communications and solutions that enable operations in small satellite systems, has focused on inter-satellite communications for small satellites [30]. [PERSON] et al. provided state-of-the-art satellite and terrestrial network convergence and focused on functionality to optimize traffic distribution, architectures, and related adaptations to provide support for converged satellite-terrestrial networks [31]. Challenges related to the performance of optical communications in integrated satellite-terrestrial networks and techniques for the mitigation of atmospheric side effects were reviewed by [PERSON] et al. in [32]. [PERSON] et al. reviewed space-air-ground integrated networks, focusing on network design, resource allocation, mobility management, open challenges, and future directions [33]. [PERSON] et al. discussed the convergence of satellite and terrestrial networks with a prime focus on recent developments in satellite terrestrial networks, system architectures, performance evaluation and simulation platforms [34]. [PERSON] et al. in 2020 reviewed non-terrestrial network (NTN) use cases and architectures, a satellite network roadmap and the role of NTN in cellular communications, and NTN open issues and research directions beyond 5G [35]. A survey of the state-of-the-art technologies for models X, L, and Y of cooperative HSTNs was provided in [36]. Future research directions toward establishing a cell-free, hierarchical, decoupled HSTN were presented, and open issues to envision an agile, smart, and secure HSTN were outlined. In [37], the authors reviewed state-of-the-art hybrid satellite-terrestrial MCNs for future maritime communications and proposed an environment-aware, service-driven, and integrated satellite-air-ground MCN that included external auxiliary information such as sea level and atmospheric conditions. The list of published surveys of HSTNs and their contributions are summarized in Table 2.  
Although all the surveys mentioned above have provided deep insights and diverse perspectives about satellite, satellite-terrestrial and satellite-air-terrestrial integrated networks, there is still the need for a comprehensive review of HSTNs, especially their integration architectures, such as cooperation and cognition; key technologies, such as non-orthogonal multiple access (NOMA), beamforming, software defined radio; and the state-of-the-art in physical-layer security of HSTNs. Our work will mainly focus on providing a review of the up-to-date work on HSTN integration for 5G and beyond and key technologies with respect to performance evaluation studies using key measures such as outage probability (OP), symbol-error probability (SEP), symbol error rate (SER), average symbol error ratio (ASER), ergodic capacity (EC), signal-to-interference-plus-noise ratio (SINR), and bit error rate (BER) for different cooperative and cognitive technologies used in HSTNs. The rest of this paper is structured as follows. Section 2 provides a comprehensive and up-to-date overview of HSTN cooperative networks. Section 3 provides an overview of cognitive HSTNs, including spectrum sharing, performance parameters, hardware impairment, and interference. Section 4 discusses various potential applications of HSTNs.