# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 4 Development Trend and Key Technology of ISAC towards 6G ISTNs
### Critical Techniques in Integrated Design
#### 4.2.1 Interference Cancellation Technique  
In the ISTN system, communication will utilize spatial isolation, time isolation, space-division multiplexing, and spectrum management to achieve satellite-terrestrial spectrum sharing and frequency multiplexing [9]. Similarly, in the ISAC system, it is inevitable to experience interference from both internal signals and external clutter signals. Therefore, implementing appropriate interference cancellation strategies can improve both communication quality and the accuracy of object detection. In the ISAC system, the multiplexing of the spectrum by radar and communication signals can help to further improve spectrum efficiency and minimize costs. However, the transmission with same-frequency signals will lead to self-interference and interference from external sources, which requires specific anti-interference techniques during network deployment [40]. [PERSON] et al. [103] introduced the mutual interference problem from the coexistence and co-designed systems. In co-located systems, radar and communication transceivers regard each other as coexisting sources of interference, so it is necessary to analyze the signal interference existing among them. In the radar-communication co-designed system, the radar and the communication system will constitute a complete joint unit integrating signals, waveforms, and other relevant parameter structures. For the radar-communication coexistence system, radar and communication systems are deployed together, spectrum sharing technology can be used for better cooperation of them [104]. Their signal can be mapped into the zero domains of each other to minimize the interference. [PERSON] et al. [51] established optimization equations based on maximizing SINR and radar covariance matrix matching. It eliminates the mutual interference between radar and communication waveforms by setting up zero-domain mapping of mutual interference. [PERSON] et al. [105] proposed an ISAC solution based on a time-modulated antenna array. The technique is used to realize radar range target detection and directional communication and to avoid mutual interference in the DFRC system. In [106], the authors proposed that for cooperative scenarios, the time-sharing approach is a straightforward and practical implementation solution to avoid mutual interference and simplify the system. In this scheme, the system of MIMO radar and communication employs time division multiplexing for coexistence, and the solution to suppress the clutter interference is acquired by optimizing convex functions. For the radar-communication co-design system, [PERSON] et al. [107] utilized OTFS waveforms along with a low-complexity generalized likelihood ratio test detection algorithm. This algorithm increases the maximum detectable range and speed by embracing the inter-symbol interference (ISI) and the inter-carrier interference (ICI) effects. In [108], the authors eliminated signal interference by using OFDM with zero padding (ZP) and single carrier frequency division multiple access (SC-FDMA). In [109], the authors maximized the minimum SINR under the total energy constraints and communication QoS guarantee using the technique of forcing zeros. In [110], the authors used the remote interference management signal for target detection, which is placed on the third OFDM symbol of the second slot. Through a unified frame structure, it can avoid mutual interference between systems. The pros and cons of the above research are listed in Table 4. Among the interference cancellation techniques in ISAC systems, for the systems of radar-communication coexistence, the primary issue is the elimination of mutual interference. Based on the implementation of effective interference cancellation, it is desired to further optimize the time and space complexity of the algorithm and realize better communication and radar detection performance.