  In low latency applications and in general, for overspread channels, channel
delay spread is a large percentage of the transmission frame duration. In this
paper, we consider OTFS in an overspread channel exhibiting a delay spread that
exceeds the block duration in a frame, where traditional channel estimation
(CE) fails. We propose a two-stage CE method based on a delay-Doppler (DD)
training frame, consisting of a dual chirp converted from time domain and a
higher power pilot. The first stage employs a DD domain embedded pilot CE to
estimate the aliased delays (due to modulo operation) and Doppler shifts,
followed by identifying all the underspread paths not coinciding with any
overspread path. The second stage utilizes time domain dual chirp correlation
to estimate the actual delays and Doppler shifts of the remaining paths. This
stage also resolves ambiguity in estimating delays and Doppler shifts for paths
sharing same aliased delay. Furthermore, we present a modified low-complexity
maximum ratio combining (MRC) detection algorithm for OTFS in overspread
channels. Finally, we evaluate performance of OTFS using the proposed CE and
the modified MRC detection in terms of normalized mean square error (NMSE) and
bit error rate (BER).
