# OTFS: A Potential Waveform for Space-Air-Ground Integrated Networks in 6G and Beyond
## 2 Related Work  
Researchers in the past few years have focussed on developing techniques like the OFDM to mitigate the effects of interference in frequency-selective and time-varying channels. In [7], they introduced a space-frequency transmitter diversity technique for frequency-selective fading channels. This technique employs OFDM to transform frequency-selective fading channels into multiple flat fading subchannels, thereby mitigating interference effects. [PERSON] et al., in [8], presented an ICI mitigation method for OFDM downlinks in high-speed railway (HSR) communication systems with distributed antennas. Multiple carrier frequency offsets cause the ICI due to the number of transmit antennas and the fast time-varying channel in the HSR. Their proposed low-complexity ICI mitigation method achieved almost the same quality of service (QoS) as systems without ICI when the train's velocity was 300 km/h. Furthermore, researchers in [9] have explored index-modulated OFDM (IM-OFDM) with ICI self-cancellation techniques in various communication scenarios, such as underwater acoustic communications and V2X communications. Reference [4] also investigated the performance of IM-OFDM in scenarios with ICI and found that IM-OFDM can exhibit worse performance than conventional OFDM. They proposed a novel ICI self-cancellation technique integrated into the IM-OFDM framework to address this issue. The technique achieves a better trade-off between the ICI cancellation and spectral efficiency of the system [4]. Some researchers have also shown how (Multiple-Input Multiple-Output) MIMO-OFDM can combat interference. In [10; 11], MIMO-OFDM channel estimation techniques were proposed to address the interference in time-varying channels. Researchers in [12], discussed the impact of using a cyclic prefix shorter than the channel delay spread in MIMO-OFDM systems. This approach can enhance bandwidth utilization or range extension. However, it can also lead to increased ISI and ICI, which is undesirable. Overall, these researchers have aimed to improve the performance and reliability of OFDM systems in challenging communication scenarios. However, much improvement still needs to be achieved, and further research is required, especially in channels with very high mobility, as expected in SAGIN. There has been an increasing research interest in OTFS since 2017, when it was introduced by [PERSON] et al. [13]. This is because it has shown better BER performance than OFDM in different scenarios, especially in channels with high mobility and Doppler effects. It transforms information carried in the DD domain to the time-frequency domain and exploits the full channel diversity over both time and frequency. The comparative performance of OFDM and OTFS has been the subject of extensive research. Studies have indicated that while OFDM is highly effective in low-mobility environments, OTFS provides significant gains in high-mobility scenarios. Researchers in [14], highlighted the benefits of OTFS over OFDM. They also derived the explicit input-output relationship, which describes OTFS modulation and demodulation for delay-Doppler channels. Comparisons between OTFS and OFDM have also been conducted to evaluate their pragmatic capacity in scenarios involving non-terrestrial networks (NTN), such as low Earth orbit (LEO) satellite constellations [15]. The application of OTFS modulation in radar systems has also been demonstrated in [16]. An efficient OTFS-based matched filter algorithm was proposed for the estimation of target range and velocity, leveraging the diversity gains for radar processing. OTFS has also been recommended as a modulation technique for millimetre-wave communication systems, offering significant advantages over OFDM in channels with high Doppler effects [17].