  We show that a space-time-varying medium can function as a front-end
transceiver, i.e., an antenna-mixer-amplifier. Such a unique functionality is
endowed by space-time surface waves associated with complex space-time wave
vectors in a subluminal space-time medium. The proposed structure introduces
pure frequency up- and down-conversions and with very weak undesired time
harmonics. In contrast to other recently proposed space-time mixers, a large
frequency up-/down conversion ratio, associated with gain is achievable.
Furthermore, as the structure does not operate based on progressive energy
transition between the space-time modulation and the incident wave, it
possesses a subwavelength thickness (metasurface). Such a multi-functional,
highly efficient and compact medium is expected to find various applications in
modern wireless telecommunication systems.
