# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 3 Development Status and Key Technologies of ISTNs
### Key Technologies of ISTNs
#### 3.2.3 Network Architecture of the ISTNs  
The ISTN architecture shows great promise for future wireless networks, attracting the attention of numerous researchers. In this section, we will summarize the ISTN architectures in existing studies and analyze their reliabilities.  
(1) Hybrid satellite-terrestrial relay network  
The signals between satellites and terrestrial terminals suffer from severe attenuation caused by clouds, fog, beam tilt during transmission, and obstacle shielding, which will result in a significant decline in communication quality. To enhance the quality of satellite-terrestrial communication, ground relay nodes are utilized to improve the reliability and stability of communication. [PERSON] et al. [82] proposed the concept of a hybrid satellite terrestrial relay network (HSTRN), where multiple antenna relay nodes forward the satellite signals to the target users based on beamforming techniques. There are two types of relay networks: basic relay structure [83] and cooperative relay structure [84; 85].  
The basic relay structure is shown in Figure 3a. In HSTRN, the masking effect hinders the user from establishing a stable communication link with the satellite. However, through the implementation of terrestrial relays, satellite users can maintain communication when the direct link experiences masking. This contributes significantly to enhancing the stability of the overall system. The cooperative relay structure shown in Figure 3b does not incorporate the masking effect in the direct link between the satellite and the user. Therefore, signals from the direct link can be merged into the signals obtained by the transmission of relay nodes.  
(2) Cognitive satellite-terrestrial network  
With the growing demand for wide-band communications and a large number of connectivity devices, the spectrum scarcity of satellite communications becomes an imminent issue. It is a feasible method for rational sharing of spectrum resources to optimize the efficiency of spectrum resource utilization [86]. Cognitive radio (CR) is considered as a spectrum management approach in conventional wireless communication systems It allocates the same spectrum resources to both major and minor networks, which enhances spectrum utilization rate [87]. In this way, we can classify them into cognitive relay networks and cognitive satellite-terrestrial networks.  
The fundamental structure of the cognitive satellite-terrestrial network, as shown in Figure 4a, comprises satellite networks and terrestrial networks. The satellite networks possess licensed spectrum resources, thereby enabling the transmission of signals on demand. The terrestrial networks reuse the same spectrum with the satellite networks, the mechanism of which operates on the premise of ensuring no interference with satellite networks. Through the reuse of spectrum resources between satellites and terrestrial systems, enhanced spectrum efficiency can be attained, thereby mitigating the strain on spectrum resources. In [87], the authors studied the performance of cognitive satellite-terrestrial networks and compared the outage performance of three secondary transmission schemes, which verified the superiority of the directional beamforming technique.  
The cognitive relay architecture is shown in Figure 4b, where relay nodes are introduced into the cognitive satellite-terrestrial network to construct the novel relay network architecture [88]. The satellite network, which is the main network in the architecture, transmits signals to the satellite users within the licensed frequency bands with the aid of the terrestrial relay. When the terrestrial BS serves as a relay to facilitate satellite network transmission, the terrestrial network is granted permission to reuse the licensed frequency bands of the primary satellite network.  
Figure 3: Architecture of HSTRNs. (**a**) Basic relay architecture. (**b**) Cooperative relay architecture.