THz Precoding for 6G: Challenges, Solutions, and Opportunities

Benefiting from the ultra-wide bandwidth, terahertz (THz) communication is becoming a promising technology for future 6G networks. In order to support the desired coverage, precoding by using a large-scale antenna array is an essential technique for THz communication to overcome the severe path loss of THz signals. In this article, we systematically investigate the dominant THz precoding techniques for future 6G networks, with the highlight of its key challenges and opportunities. Specifically, we first clarify the major differences between milli-meter-wave and THz channels, based on which we reveal the key challenges of THz precoding, such as the distance-dependent path loss, the beam split effect, the high power consumption, and so on. To address these challenges, five representative THz precoding techniques: analog beamforming, hybrid precoding, delay-phase precoding, dynamic RF chains based precoding, and reconfigurable intelligent surface based precoding, are extensively investigated in terms of their different structures, designs, most recent results, and 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 potential research opportunities, such as the integrated sensing and communication, are pointed out and discussed.