  Late visions and trends for the future sixth Generation (6G) of wireless
communications advocate, among other technologies, towards the deployment of
network nodes with extreme numbers of antennas and up to terahertz frequencies,
as means to enable various immersive applications. However, these technologies
impose several challenges in the design of radio-frequency front-ends and
beamforming architectures, as well as of ultra-wideband waveforms and
computationally efficient transceiver signal processing. In this article, we
revisit the Time Reversal (TR) technique, which was initially experimented in
acoustics, in the context of large-bandwidth 6G wireless communications,
capitalizing on its high resolution spatiotemporal focusing realized with low
complexity transceivers. We first overview representative state-of-the-art in
TR-based wireless communications, identifying the key competencies and
requirements of TR for efficient operation. Recent and novel experimental
setups and results for the spatiotemporal focusing capability of TR at the
carrier frequencies $2.5$, $36$, and $273$ GHz are then presented,
demonstrating in quantitative ways the technique's effectiveness in these very
different frequency bands, as well as the roles of the available bandwidth and
the number of transmit antennas. We also showcase the TR potential for
realizing low complexity multi-user communications. The opportunities arising
from TR-based wireless communications as well as the challenges for finding
their place in 6G networks, also in conjunction with other complementary
candidate technologies, are highlighted.
