# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 1 Introduction  
In recent years, with the large-scale commercialization of fifth-generation (5G) wireless networks, sixth-generation (6G) wireless networks have become the focus of a new round of technological competition [1]. It is expected that society will step into a new era of intelligent 6G in 2030 [2]. The visions of 6G include further enhanced mobile broadband, massive machine-type communication, ultra-massive machine-type communication, long-distance and high-mobility communications, and extremely low-power communications [3, 4]. It is anticipated that 6G will achieve deep integration between intelligent applications and networks to enable appealing use cases such as virtual reality and environment sensing based on artificial intelligence and the Internet of Things (IoT) [5]. Therefore, 6G is considered an essential driver for many emerging applications, which require high-quality wireless connectivity as well as high-precision and robust sensing capability [6]. Existing terrestrial communication mainly focuses on terminals in urban areas, but the coverage performance is poor in remote mountainous areas, deserts, and oceans due to the lack of base stations (BSs), while satellites can be used to extend the coverage of terrestrial communication networks [7, 8]. The integrated satellite-terrestrial networks (ISTNs) will combine satellite communication and terrestrial communication to pave the way for wide-area coverage and seamless connectivity [9]. Meanwhile, with the rapid growth of the wireless communication industry and the ever-increasing demand for connection services, spectrum resources are becoming increasingly scarce [10]. Conventional sensing andcommunication systems are typically designed separately and operate within distinct frequency bands. However, due to the evolution of technologies such as millimeter wave, terahertz, and massive multiple-input multiple-output (MIMO), communication signals in high-frequency bands typically exhibit high resolution in both the time and angular domains, allowing for high-precision sensing based on communication signals. Integrated sensing and communication (ISAC) enables communication systems and radar systems to utilize the same frequency band and hardware for enhancing energy efficiency, boosting spectrum efficiency, and lowering hardware expenses. In recent years, extensive research has been conducted on terrestrial communication and sensing. However, considering the ubiquitous connectivity of 6G, the research on satellite-terrestrial ISAC is meaningful. In the future, ISAC technology will be further integrated into the ISTNs to realize a more efficient and intelligent integrated architecture and achieve global cooperative sensing. At the current stage, ISAC technology research towards ISTNs faces the following challenges. (1) The application scenarios of satellite-terrestrial ISAC have not been clearly defined, and it is necessary to explore potential scenarios ISAC towards ISTNs. (2) For the new type of integrated system, the original technology may no longer be applicable. Therefore, new key technologies derived from the integrated system need to be explored to provide key support for the integrated system. The primary goal of this paper is to provide an overview and outlook on the development of ISAC towards ISTNs. These mainly include potential scenarios, key technologies, and future research directions. Through this paper, we hope it can be an opportunity to outline a way forward to realize ISAC towards ISTNs in the future. The main contributions of this paper are summarized as follows:  
* We studied the development of existing ISAC in various frequency bands and analyzed the characteristics of existing technologies. * We summarize the development status of ISTNs, key technologies, and networking architectures. * We propose three use cases for the future ISAC towards 6G ISTNs. Using the characteristics of ISTNs, we discuss the road for the existing ISAC technology towards ISTNs. The remainder of this paper is organized as follows.