  This letter is the first part of a three-part tutorial on orthogonal time
frequency space (OTFS) modulation, which is a promising candidate waveform for
future wireless networks. This letter introduces and compares two popular
implementations of OTFS modulation, namely the symplectic finite Fourier
transform (SFFT)- and discrete Zak transform (DZT)-based architectures. Based
on these transceiver architectures, fundamental concepts of OTFS modulation,
including the delay-Doppler (DD) domain, DD domain information multiplexing,
and its potential benefits, are discussed. Finally, the challenges ahead for
OTFS modulation are highlighted. Parts II and III of this tutorial on OTFS
modulation focus on transceiver designs and integrated sensing and
communication (ISAC), respectively.
