  Benefiting from the ultra-wide bandwidth, terahertz (THz) communication is
becoming a promising technology for future 6G networks. For THz communication,
precoding is an essential technique to overcome the severe path loss of THz
signals in order to support the desired coverage. In this article, we
systematically investigate the dominant THz precoding techniques for future 6G
networks, with the highlight on its key challenges and opportunities.
Specifically, we first illustrate three typical THz application scenarios
including indoor, mobile, and satellite communications. Then, the major
differences between millimeter-wave and THz channels are explicitly clarified,
based on which we reveal the key challenges of THz precoding, such as the
distance-dependent path loss, the beam split effect, and the high power
consumption. To address these challenges, three representative THz precoding
techniques, i.e., analog beamforming, hybrid precoding, and delay-phase
precoding, are extensively investigated in terms of their different structures,
designs, most recent results, pros and cons. We also provide simulation results
of spectrum and energy efficiencies to compare these typical THz precoding
schemes to draw some insights for their applications in future 6G networks.
Finally, several important open issues and the potential research
opportunities, such as the use of reconfigurable intelligent surface (RIS) to
solve the THz blockage problem, are pointed out and discussed.
