# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 6 Conclusions  
Combining the sensing capability of radar with the original information transmission capability of communication, ISAC can enhance the high-precision positioning, imaging, and environment reconstruction capabilities of devices to enable sensory enhancement and the connection of everything. In addition, ISTNs will integrate satellites, various types of vehicles, land, and the ocean, aiming to provide seamless global network coverage and resolve the challenge of poor connectivity in remote regions.  
Based on the above visions, we provided a comprehensive survey of ISAC technology towards 6G ISTNs in this paper. Firstly, we introduced the historical development process, evaluation indicators, and the key technologies of ISAC. Then, the development status of ISTNs was elaborated, especially the integrated network architectures. Finally, we proposedthe potential scenarios and the key technologies for future satellite-terrestrial ISAC. ISAC technology and the construction of ISTNs are in a state of rapid development. It is believed that ISAC technology will be deeply integrated into the ISTNs and ultimately realize the interconnection of everything.  
Conceptualization, Y.G. and T.X.; methodology, Y.G.; software, T.X.; validation, Y.O., W.D., X.T. and T.L.; writing--original draft preparation, Y.G.; writing--review and editing, T.X. and K.F.; supervision, T.X. All authors have read and agreed to the published version of the manuscript.  
This research was funded in part by the Science and Technology Commission Foundation of Shanghai (No. 22511100600), the International Partnership Program of Chinese Academy of Sciences for Future Network (No. 307 GJHZ2023070 FN), the Pudong Industry, Education and Research Cooperation Program (No. PKX2022-D02), and the Shanghai Special Fundation of Promoting High-Quality Industrial Development (No. 2211106).  
The data that support the findings of this study are available from the corresponding author upon reasonable request.  
Authors [PERSON] and [PERSON] were employed by the company DS Information Technology Co., Ltd. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.  
The following abbreviations are used in this manuscript:  
3 GPP The 3 rd Generation Partner Project  
5G Fifth generation  
6G Sixth generation  
AI Artificial intelligence  
BER Bit error ratio  
BS Base station  
CR Cognitive radio  
CRB Cramer-Rao bound  
CSMA Carrier sense multiple access  
DFRC Dual-functional radar-communication  
DMRS Demodulation reference signal  
EE Energy efficiency  
FDM Frequency division multiplexing  
FMCW Frequency-modulated continuous wave  
FSO Free-space optical  
HSTRN Hybrid satellite-terrestrial relay network  
ICI Inter-carrier interference  
IEEE The Institute of Electrical and Electronics Engineers  
IMT International Mobile Telecommunications  
IoT Internet of Things  
ISAC Integrated sensing and communication  
ISI Inter-symbol interference  
ISLR Integrated side lobe ratio  
ISTNs Integrated satellite-terrestrial networks  
ITU International Telecommunication Union  
LEO Low earth orbit  
LFM Linear frequency modulation  
MIMO Multiple-input multiple-output  
MSE Mean squared error  
MU-MIMO Multi-user MIMO  
NOMA Non-orthogonal multiple access  
NR New radio