  Orthogonal time frequency space (OTFS) modulation has recently emerged as an
effective waveform to tackle the linear time-varying channels. In OTFS
literature, approximately constant channel gains for every group of samples
within each OTFS block are assumed. This leads to limitations for OTFS on the
maximum Doppler frequency that it can tolerate. Additionally, presence of
cyclic prefix (CP) in OTFS signal limits the flexibility in adjusting its
parameters to improve its robustness against channel time variations.
Therefore, in this paper, we study the possibility of removing the CP overhead
from OTFS and breaking its Doppler limitations through multiple antenna
processing in the large antenna regime. We asymptotically analyze the
performance of time-reversal maximum ratio combining (TR-MRC) for OTFS without
CP. We show that doubly dispersive channel effects average out in the large
antenna regime when the maximum Doppler shift is within OTFS limitations.
However, for considerably large Doppler shifts exceeding OTFS limitations, a
residual Doppler effect remains. Our asymptotic derivations reveal that this
effect converges to scaling of the received symbols in delay dimension with the
samples of a Bessel function that depends on the maximum Doppler shift. Hence,
we propose a novel residual Doppler correction (RDC) windowing technique that
can break the Doppler limitations of OTFS and lead to a performance close to
that of the linear time-invariant channels. Finally, we confirm the validity of
our claims through simulations.
