# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 2 The Development Status and Key Technology Of ISAC
### Key Technology  
#### 2.4.1 Waveform Design for ISAC  
The integrated waveform of ISAC is capable of simultaneously accomplishing the functions of radar detection and communication signal transmission. Among the existing integrated signals, they can be mainly divided into three types, which are the waveform centered on communication, the waveform centered on radar, and the new integrated waveform [40].  
The communication waveform is mainly based on OFDM. In [22], the authors summarized a series of integrated waveforms centered on OFDM, including the integration of OFDM and linear frequency modulation (LFM), the combination of OFDM and phase coding, and the union of OFDM and spread spectrum. They also compared the advantages and disadvantages of different waveform combinations. A dual-functional radar-communication (DFRC) system [41] utilized the proposed OFDM radar waveform design technique to maximize the communication capacity. It can achieve a performance trade-off between two subsystems. ISAC based on orthogonal multiple access (OMA) has achieved remarkable results. In recent years, academic researchers have also conducted certain research on ISAC with non-orthogonal multiple access (NOMA). NOMA superimposes the signals of multiple users in the same frequency band, improving the spectrum utilization efficiency and enhancing the system transmission rate. It can mitigate inter-user interference when the channels are highly correlated or the system is overloaded. In [42], the authors revealed the trend of ISAC access technology, transition from orthogonal to non-orthogonal; introduced the basic models of the downlink and uplink ISAC; and effectively eliminated interference between users through rational design and new sensing-to-communication interference.  
Within the radar waveform-based ISAC waveforms, the frequency-modulated continuous wave (FMCW) can measure the distance and velocity of objects. This is achieved by transmitting a continuous waveform with a continuously changing frequency and observing the frequency shift of the reflected signal. In [43], the authors embedded the communication signal into the radar pulse signal by using different beams and index modulation to achieve communication and detection. In [44], the authors conducted a waveform design based on FMCW and indexed modulation. It is demonstrated that the bit error ratio (BER) performance is superior to methods utilizing only phase modulation.  
Among the new waveforms, orthogonal time frequency space (OTFS) [45] has the advantage of adapting to the high Doppler frequency shift environment. An ISAC scheme based on OTFS [46] is proposed to mitigate the impact of communication signals on radar sensing in the time-frequency domain through the method of echo preprocessing. In [47], the authors proposed an efficient approximate maximum likelihood algorithm based on OTFS, which is superior for distance and velocity estimation. The authors of [48] used OTFS for ISAC-assisted receiver design and utilized the novel deep residual denoising network framework to achieve effective channel estimation, which has higher performance and system robustness. An ISAC framework based on OTFS modulation was proposed in [49], which adopts a matched filter radar sensing scheme based on fractional parameter estimation to achieve better BER and estimation performance. In addition, in [50], the authors proposed the novel universal frequency multi-carrier waveform and compared it with the conventional OFDM. They experimentally proved that it has performance improvement with respect to spectral efficiency and system complexity.  
In Table 2, the performance differences between two types of communication waveforms OFDM and OTFS are comparatively analyzed.