# 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
#### 4.2.2 Distributed Cooperative Sensing Technology  
More integration of satellite-terrestrial application scenarios will emerge in the 6G era, and ubiquitous terminals will further complicate the heterogeneous network [65]. By ISAC technology, the information parameters acquired by the ISAC system are processed in the cloud, and the cross-convergence of communication, sensing, and computing can be realized [111]. Meanwhile, 6G is expected to be highly integrated for networks of all \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Type** & **Ref.** & **Pros** & **Cons** \\ \hline \multirow{6}{*}{Radar communication coexistence system} & [51] & \(\bullet\) Low storage occupancy \(\bullet\) Low time complexity \(\bullet\) Higher radar and communication transmission rates \(\bullet\) Lower BER \(\bullet\) Low communication side lobes \(\bullet\) Higher SINR & Not considering power allocation \\ \cline{1-1} \cline{3-4}  & [105] & \(\bullet\) Lower BER \(\bullet\) Low communication side lobes \(\bullet\) Higher SINR & Only considering one radar beam and one communication beam \\ \cline{1-1} \cline{3-4}  & [106] & \(\bullet\) Higher SINR & Not considering robust design of communication system mismatches \\ \hline \multirow{6}{*}{Radar communication co-design system} & [107] & \(\bullet\) ISI and ICI are used to extend the maximum non-blurring range of sensing. Higher probability of detection \(\bullet\) Higher probability of detection \(\bullet\) Higher probability of detection \(\bullet\) Higher probability of detection \(\bullet\) Higher range performance in the middle and long distances \(\bullet\) Better SINR Item: lower average power limit \(\bullet\) Lower algorithm complexity & Not considering power allocation \(\bullet\) Not setting variable guard intervals \\ \cline{1-1} \cline{3-4}  & [109] & \(\bullet\) Better SINR Item: lower average power limit \(\bullet\) Lower algorithm complexity & Only considering one communication pair of the transmitter and receiver \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of key technologies for interference cancellation technique in ISAC towards ISTN systems. frequency bands. It can promote intelligent human-machine-thing interconnection and multi-system collaboration, thus optimizing the service quality of the full-frequency band. Satellites, aircraft, ground relays, and terminal devices are utilized to form a multi-level ISAC architecture for the ISTNs. In this network, the deployed ISAC intelligent nodes will work together with intelligent terminals, the ISAC access network, and the core network to construct an innovative distributed cooperative sensing system. Each node in the system collaborates to support the integration and interconnection of intelligent information. In the integrated space-air-ground network, UAVs can collect information and data from ground IoT devices while simultaneously communicating with multiple LEO satellites [112]. Within a swarm of UAVs, ISAC will realize the positioning and dynamic sensing of UAVs, stabilize the formation of the UAV swarm, and sense the surrounding environment with high accuracy, enhancing the communication effectiveness of the swarm. In [113], the authors introduced cooperative wireless positioning technologies in which UAVs, BS, and cars are viewed as sensing nodes from the perspectives of spatial cooperation and spatiotemporal cooperation. In [114], the authors discussed the potential challenges and development directions of ISAC systems where UAVs act as nodes. From the perspective of BS, UAV nodes will achieve environmental sensing, optimize the communication process, and further realize beam management. It is proposed that there are three types of cooperative sensing technologies, namely, multi-static cooperation, multi-band cooperation, and multi-source cooperation in [115].