  The sixth-generation (6G) wireless networks are envisioned to provide a
global coverage for the intelligent digital society of the near future, ranging
from traditional terrestrial to non-terrestrial networks, where reliable
communications in high-mobility scenarios at high carrier frequencies would
play a vital role. In such scenarios, the conventional orthogonal frequency
division multiplexing (OFDM) modulation, that has been widely used in both the
fourth-generation (4G) and the emerging fifth-generation (5G) cellular systems
as well as in WiFi networks, is vulnerable to severe Doppler spread. In this
context, this article aims to introduce a recently proposed two-dimensional
modulation scheme referred to as orthogonal time-frequency space (OTFS)
modulation, which conveniently accommodates the channel dynamics via modulating
information in the delay-Doppler domain. This article provides an easy-reading
overview of OTFS, highlighting its underlying motivation and specific features.
The critical challenges of OTFS and our preliminary results are presented. We
also discuss a range of promising research opportunities and potential
applications of OTFS in 6G wireless networks.
