  This paper compares orthogonal time frequency space (OTFS) modulation and
single-carrier frequency division multiple access (SC-FDMA). It shows that
these are equivalent except for a set of linear phase shifts, applied to the
transmit/receive data symbols, which can be absorbed into the channel. Through
mathematical and numerical analysis, it is confirmed that SC-FDMA is in fact a
delay-Doppler domain multiplexing technique that can achieve the same
performance gains as those of OTFS in time-varying wireless environments. This
is a promising result as SC-FDMA is already a part of the current wireless
standards. The derivations in this paper also shed light on the time-frequency
resources used by the delay-Doppler domain data symbols with the fine
granularity of delay and Doppler spacings. While comparing the detection
performance of the two waveforms, a timing offset (TO) estimation technique
with orders of magnitude higher accuracy than the existing solutions in the
literature is proposed. From multiple access viewpoint, the underlying tile
structures in the time-frequency domain for OTFS and SC-FDMA are discussed.
Finally, multiuser input-output relationships for both waveforms in the uplink
are derived.
