Sparse Delay-Doppler Channel Estimation for OTFS Modulation Using 2D-Music

In this paper, we address the problem of estimating the delays and Doppler shifts introduced by a sparse wireless channel for orthogonal time frequency space (OTFS) modulation. We show that in the discrete time-frequency (TF) domain, the received signal resulting from an OTFS pilot signal is a superposition of two-dimensional (2D) complex exponentials, where the 2D frequencies of the complex exponentials are given by the delays and Doppler shifts of the scatterers. Thus, estimating the delays and Doppler shifts can be formulated as a 2D sinusoidal frequency estimation problem. We apply a 2D TF domain version of the well-known MUSIC algorithm to recover the delays and Doppler shifts. Since the reduced guard interval frame structure eliminates most of the interference between the pilot and data symbols, the data symbols can be filtered out from the received signal such that only the pilot signal component is used for the 2D MUSIC algorithm.