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Orthogonal time frequency space (OTFS) modulation has recently emerged as aneffective waveform to tackle the linear time-varying channels. In OTFSliterature, approximately constant channel gains for every group of sampleswithin each OTFS block are assumed. This leads to limitations for OTFS on themaximum Doppler frequency that it can tolerate. Additionally, presence ofcyclic prefix (CP) in OTFS signal limits the flexibility in adjusting itsparameters to improve its robustness against channel time variations.Therefore, in this paper, we study the possibility of removing the CP overheadfrom OTFS and breaking its Doppler limitations through multiple antennaprocessing in the large antenna regime. We asymptotically analyze theperformance of time-reversal maximum ratio combining (TR-MRC) for OTFS withoutCP. We show that doubly dispersive channel effects average out in the largeantenna regime when the maximum Doppler shift is within OTFS limitations.However, for considerably large Doppler shifts exceeding OTFS limitations, aresidual Doppler effect remains. Our asymptotic derivations reveal that thiseffect converges to scaling of the received symbols in delay dimension with thesamples of a Bessel function that depends on the maximum Doppler shift. Hence,we propose a novel residual Doppler correction (RDC) windowing technique thatcan break the Doppler limitations of OTFS and lead to a performance close tothat of the linear time-invariant channels. Finally, we confirm the validity ofour claims through simulations.

OTFS Without CP in Massive MIMO: Breaking Doppler Limitations with TR-MRC and Windowing