  This paper overviews the principles, theoretical analysis and numerical
simulation of STM media and their applications in communication systems. We
first present a global perspective on the principles of ST modulation,
naturally and artificially created STM media and their applications in
communication systems. Next, the interaction of artificial STM media with
electromagnetic waves is analyzed, and numerical simulation of electromagnetic
wave propagation and scattering from these media is provided. Then, the
dispersion diagram of the STM medium is studied for the characterization of
subluminal, luminal and superluminal ST modulations. In addition, we
investigate the effect of length, modulation velocity, modulation frequency,
and equilibrium on wave scattering from the STM medium. The extraordinary
interaction of the STM medium with electromagnetic waves leads to
unidirectional frequency generation and amplification, which are leveraged for
the realization of various enhanced-efficiency broadband microwave components
that are integrable with circuit technology, lightweight, tunable, and low
cost. These applications include, but are not limited to, magnet-free
isolators, magnet-free circulators, transceiver front-ends, and pure frequency
mixers.
