On the Characterizations of OTFS Modulation Over Multipath Rapid Fading Channel

Orthogonal time frequency space (OTFS) modulation has been confirmed to provide significant performance advantages against Doppler in high-mobility scenarios. The core feature of OTFS is that the time-variant channel is converted into a non-fading 2D channel in the delay-Doppler (DD) domain so that all symbols experience the same channel gain. In available literature, the channel is assumed to be quasi-static over an OTFS frame. As for more practical scenarios, the Doppler shift is dynamic and the channel gain could be time-variant as the environment or medium changes. In this paper, we analyze the characterizations of OTFS modulation over a more general multipath channel, where the signal in each propagation path has experienced a unique rapid fading. First, we derive the explicit input-output relationship of OTFS in the DD domain for the case of ideal pulse and rectangular pulse. It is shown that the rapid fading will produce extra Doppler dispersion without impact on delay domain. We next demonstrate that OTFS can be interpreted as an efficient time diversity technology that combines space-time encoding and interleaving against the rapid fading. Simulation results reveal that OTFS is insensitive to rapid fading and still outperforms conventional multicarrier systems in these channels.