# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 5 Potential Research for ISAC towards 6G ISTNs
### Cognitive Radio  
Using CR technology is a good solution for efficiently utilizing spectrum resources. However, in the existing research on ISAC, it has not been given much attention. On the one hand, ISAC as an emerging technology is still immature, and at this stage, more experiments are needed on basic waveforms instead of the scenario with many ISAC terminals. On the other hand, ISAC achieves efficient spectrum utilization by sharing frequency bands between communication and radar sensing. In early radar and communication coexistence systems, the two subsystems operated separately, sharing a frequency band, thus requiring the use of CR technology [6]. However, with the development of 6G ISAC technology, communication and radar signals will be integrated into a single waveform. Although the new waveform will have certain impacts on the performance of existing communication and radar systems, the issue of mutual interference between radar and communication waveforms is no longer considered for the shared waveform.  
In summary, in the existing research on ISAC, the application of CR technology is rarely considered. However, in the future, as ISAC technology gradually matures and is deployed for ISTNs, each ISAC terminal will become an independent entity. Furthermore, multiple ISAC terminals will enable cooperative networking, thereby achieving high-precision environmental sensing and localization. In this environment, the spectrum utilization between multiple ISAC terminals will become an important research topic. Therefore, in the future architecture of ISAC networking within ISTNs, CR technology can be adopted to maximize spectrum resource utilization.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Key Technologies** & **Pros** & **Cons** \\ \hline Interference cancellation technology & \(\bullet\) Improves system reliability & \(\bullet\) Increases system complexity \(\bullet\) Raises the cost \\ \hline Distributed cooperative sensing technology & \(\bullet\) Improves sensing and coverage capability & \(\bullet\) More complex co-scheduling algorithms are required \(\bullet\) Leads to new information security issues \\ \hline Unified frame structure design & \(\bullet\) Simplifies system design \(\bullet\) Reduces system overhead & \(\bullet\) Increases the complexity of design and implementation \\ \hline Waveform design of satellite–terrestrial ISAC & \(\bullet\) Multi-level information fusion \(\bullet\) Multi-scenario adaptation & \(\bullet\) Complex system design and implementation \(\bullet\) High costs and resource requirements \\ \hline Beam enhancement technology for ISTN & \(\bullet\) Efficient energy utilization \(\bullet\) Increases the detection range of the system & \(\bullet\) High algorithm complexity \(\bullet\) Requires target location information in advance \\ \hline \hline \end{tabular}
\end{table}
Table 7: Summary of key technologies in satellite–terrestrial ISAC systems and their pros and cons.  
Additionally, CR-based ISAC technology can be effectively combined with MIMO-NOMA technology. On the one hand, MIMO-NOMA allows multiple users to share a frequency band, which optimizes spectrum utilization efficiency and increases information transmission rates. On the other hand, MIMO-NOMA employs multiple antenna technology, which enhances spatial reuse, enabling spatial resource allocation. There have been many studies on CR-NOMA technology, which have achieved good results [130; 131]. It is believed that the reasonable combination of this technology with ISAC can further improve spectrum efficiency and optimize energy consumption. However, the combination of these technologies also poses challenges, including complex system design, trade-offs in various technical performances, intelligent scheduling in CR environments, and the need for algorithm design for global optimization. Furthermore, hardware feasibility must be considered to mitigate hardware complexity in system implementation.