System Model of OTFS Over Joint Random and Deterministic Doppler Spread Channels

In high-mobility environments, Orthogonal Time Frequency Space (OTFS) modulation, by operating in the delay-Doppler domain, effectively converts time-variant channels into near-uniform and slowly evolving sparse channels, significantly enhancing the reliability of information transmission. However, most of the research on Doppler effect within OTFS system only consider adding a overall Doppler frequency shift to the signal, while ignoring that the frequency shift differs considerably for each frequency bin under large frequency spacing for a wideband signal. To accurately model the Doppler effect in wideband OTFS systems, it is necessary to calculate the Doppler frequency shift according to every frequency bin, thereby the overall frequency offset and spectral spread are included. Thus, this paper extends existing research about random Doppler spread (RDS) caused by multipath effects in narrowband time-varying channels, and proposes a new joint random and deterministic Doppler spread (DRDS) model for wideband signal. With these wideband Doppler effect models, we further conducted simulations for OTFS and OFDM systems under different velocities. Simulation results show that the OTFS can resist high Doppler frequency shift in high mobility scenarios than OFDM systems. In the case of large subcarrier spacing, OTFS is less affected by the Doppler frequency shift of each subcarrier than OFDM.