Doppler Interference Analysis for OTFS-Based LEO Satellite System

Low Earth orbit (LEO) satellite system has revolutionized the way to provide wireless seamless access on a global scale. One of the primary limitations is the low data rates resulting from Doppler shifts induced by the high mobility of LEO satellites. Although orthogonal time frequency space (OTFS) modulation has been proposed to deal with the serious Doppler problem by converting a time-variant fading channel in the time-frequency (TF) domain into a time-invariant channel in the delay-Doppler (DD) domain, it needs to be reconsidered in the LEO satellite system due to the facts that the scale of Doppler axes is not big enough and the velocity of satellite is too fast. In this paper, we analyze two interferences caused by Doppler that will be produced in OTFS-based LEO satellite system. Specifically, we establish a system model of LEO satellite-to-ground communication, involving the fractional Doppler interference (FDI) from the non-integer Doppler tap, and the other is the squint Doppler interference (SDI) from the frequency-dependent Doppler. By deriving the closed-form expressions of FDI and SDI respectively, we find that the simplest but most practical solution to mitigate interference is to increase the value of DD plane bins. Finally, numerical results showcase the significant impact of Doppler on transmission signals by quantifying the signal-to-interference (SIR) ratio and bit error rate (BER) and highlight the dominance of an applicable number of bins on alleviating Doppler in OTFS-based LEO satellite system.