# 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
#### 4.2.3 Unified Frame Structure Design  
The ISTNs will include a variety of scenarios. To support the requirements of these diverse scenarios, a unified frame structure design will be adopted. The adaptive wireless frames and subframes will meet communication needs in different domains and scenarios. At the same time, for ISAC signals, a unified and flexible frame structure is beneficial for scenario adaptability and signal processing. By designing flexible frame structures and employing various scheduling techniques, it can adapt to different communication scenarios, thus improving the efficiency and reliability of communication and enhancing detection accuracy. Aiming at the characteristics of high latency and high Doppler shift in satellite-terrestrial transmission, compensation techniques for frames will be designed to achieve reliable access and efficient transmission, improving the transmission quality and reliability. In [115], the authors utilized the 5G New Radio (NR) standard to propose four optional mapping patterns for the physical downlink shared channel (PDSCH) based on demodulation reference signal (DMRS). Simulations unveiled the relationship between sensing and communication performance. In [116], the authors proposed three principles for the design of ISAC frame structures, thus coordinating with UAVs to achieve cooperative gain. In [35], the authors introduced an algorithm in their research, which is based on the 5G NR communication protocol. This algorithm not only integrates the advanced waveform capabilities but also empowers the efficient utilization of time and frequency resources, which has high localization accuracy and data transmission rate. In [118], the author introduced frequency hopping MIMO radar as the foundational system for implementing dual-function radar communications. They developed a downlink communication scheme for frequency hopping DFRC systems based on multi-antenna receivers, addressing estimation and synchronization challenges in MIMO scenarios. An integrated frame structure for radar communication based on the 3 GPP standard is designed in [119], which can be used to transmit training symbols and conduct channel estimation. In [120], the authors combined frequency division multiplexing (FDM), time division multiple access (TDMA), and communications carrier sense multiple access (CSMA) to form the RadChat scheduling scheme, thus greatly reducing the level of radar interference. In [121], the authors designed waveforms by optimizing cost functions based on the constrained combination of the peak side lobe ratio (PSLR) and integrated side lobe ratio (ISLR) of velocity ambiguity functions. By solving this optimization equation, the system can have high detection and estimation performance in a high SCNR scenario, thereby reducing the cost of the system. Table 6 summarizes the design and contribution of the frame structure. In general, for the integrated frame structure of ISTNs in the future, the terrestrial-based ISAC frame structure optimization scheme can still be used, and the frame multiplexing technology can be used to improve efficiency. However, it still needs to be optimized according to the characteristics of ISTNs. In the ISTNs, we recommend that the 5G NR frame structure can still be used in order to reduce changes to the existing frame structure. However, it \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ref** & **Networking Elements** & **Cooperative Approach** \\ \hline
[113] & UAV, BS, and UE & • Space cooperation and space–time cooperation \\ \hline
[114] & UAV and BS & • Centralized and distributed collaboration \\ \hline
[115] & Satellite, UAV, BS, and UE & • Centralized control unit unified management \\  & • Adopt multi-source data symbols \\ \hline
[116] & UAV and BS & • Beamforming, power control technology \\  & • UAVs communicate with each other to share detection \\  & • results \\ \hline
[117] & Satellite, BS, and UE & • Introduce satellite communication sensing technology \\ \hline \hline \end{tabular}
\end{table}