  Terahertz (THz)-band communications are a key enabler for future-generation
wireless communications systems that promise to integrate a wide range of
data-demanding applications. Recent advances in photonic, electronic, and
plasmonic technologies are closing the gap in THz transceiver design.
Consequently, prospect THz signal generation, modulation, and radiation methods
are converging, and corresponding channel model, noise, and hardware-impairment
notions are emerging. Such progress establishes a foundation for well-grounded
research into THz-specific signal processing techniques for wireless
communications. This tutorial overviews these techniques, emphasizing
ultra-massive multiple-input multiple-output (UM-MIMO) systems and
reconfigurable intelligent surfaces, vital for overcoming the distance problem
at very high frequencies. We focus on the classical problems of waveform design
and modulation, beamforming and precoding, index modulation, channel
estimation, channel coding, and data detection. We also motivate signal
processing techniques for THz sensing and localization.
