# 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
#### 4.2.4 Waveform Design of Satellite-Terrestrial ISAC  
ISAC waveforms have been extensively proven in terrestrial environments, but work in ISTN environments has not yet begun. For the future ISAC waveform within ISTNs, we put forward relevant development suggestions based on the sensing characteristics of satellite and ground communication. According to the propagation characteristics of satellite-to-ground channels, the signal transmission process will experience atmospheric absorption attenuation, attenuation caused by rainfall and clouds, Doppler frequency offset, and multipath fading near the ground. Therefore, the new waveform requires good performance in high Doppler and low signal-to-noise ratio environments, and the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ref** & **Frame Structure Design** & **Contribution** & **Standard** \\ \hline
[115] & Based on the 5G NR frame structure, four optional DMRS-PDSCH modes are proposed. & PDSCH is used for communication and sensing functions. & 5G NR standard [122] \\ \hline
[116] & \(\bullet\) Covers all targets. & Optimizes the coordination gain effect of combining UAV ISAC. & — \\  & \(\bullet\) Covers only one target. & \(\bullet\) A combination of the above ideas. & Optimizes the coordination gain effect of combining UAV ISAC. & — \\ \hline
[35] & The system bandwidth is 800 MHz. Eight component carriers, each containing 1200 subcarriers. Each wireless frame contains 50 subframes in 10 ms. & The algorithm for flexible time-frequency resource allocation is proposed to achieve a performance compromise between the DFRC. & 5G NR standard \\ \hline
[118] & For every signal, the initial radar frame is utilized to estimate timing deviation and channel states, while the subsequent hops are dedicated to data transmission. & Enables radar data communication and reduce overhead. & — \\ \hline
[119] & The radar pulse repetition period is 1 ms, and there are 14 packets every 0.5 ms. Each packet is composed of a cyclic prefix and a data part, the cyclic prefix contains 288 symbol periods, the data part contains 4096 symbol periods, and the duration of a single symbol period is 8.146 ns. & Channel estimation is performed by transmitting known training symbols. & 5G NR standard \\ \hline
[120] & RadChat is a combination of FDM, Radar TDMA, and Communications CSMA. & The probability of interference in automotive radars is significantly reduced, reducing the frame time of each radar by about an order of magnitude. & — \\ \hline
[121] & The optimization strategy is designed as a combined optimization problem of PSLR and ISLR constraints. & 40\% reduction in sensing costs. & 5G NR standard \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of frame structure design in satellite–terrestrial ISAC systems. existing OFDM waveform is considered to be modified to alleviate its instability in high-mobility environments. The applicability of waveforms in satellite scenarios has been studied based on the cyclic prefix OFDM and discrete Fourier transform-spread OFDM waveforms [9; 123]. Principle-based simulation validation studies on OFDM and other communication waveforms have been conducted within ISAC waveforms. In the future scenario of satellite-terrestrial ISAC, NOMA can also be taken into consideration [124]. Considering the high latency of satellite-terrestrial transmission in the ISTNs, NOMA can effectively decrease the connection latency and communication latency of satellite-terrestrial transmission. In [125], the authors treated NOMA as an ISAC waveform. By solving the optimization equation, this method achieved radar waveform matching and the assurance of communication performance. In [42], the authors combined orthogonal multiple access with NOMA techniques to provide flexible sensing and communication between resource management schemes.