  In Orthogonal Time Frequency Space (OTFS) modulation, information symbols are
embedded in the delay-Doppler (DD) domain instead of the time-frequency (TF)
domain. n order to ensure compatibility with existing OFDM systems (e.g. 4G
LTE), most prior work on OTFS receivers consider a two-step conversion, where
the received time-domain (TD) signal is firstly converted to a time-frequency
(TF) signal (using an OFDM demodulator) followed by post-processing of this TF
signal into a DD domain signal. In this paper, we show that the spectral
efficiency (SE) performance of a two-step conversion based receiver degrades in
very high mobility scenarios where the Doppler shift is a significant fraction
of the communication bandwidth (e.g., control and non-payload communication
(CNPC) in Unmanned Aircraft Systems (UAS)). We therefore consider an alternate
conversion, where the received TD signal is directly converted to the DD
domain. The resulting received DD domain signal is shown to be not the same as
that obtained in the two-step conversion considered in prior works. The
alternate conversion does not require an OFDM demodulator and is shown to have
lower complexity than the two-step conversion. Analysis and simulations reveal
that even in very high mobility scenarios, the SE achieved with the alternate
conversion is invariant of Doppler shift and is significantly higher than the
SE achieved with two-step conversion (which degrades with increasing Doppler
shift).
