Terahertz Integrated Sensing and Communication: Sensing Comes Before and For Communication

The Terahertz (THz) band offers vast spectrum resources that enable ultra-high-rate communication and high-precision sensing, positioning THz integrated sensing and communication (THz-ISAC) as a key technology for future wireless systems. This paper provides a sensing-centric investigation of THz-ISAC, reviewing representative THz sensing applications and clarifying why sensing naturally precedes and supports THz communication. Two complementary THz measurement platforms are introduced, from which sensing-channel data and a large-scale material database are obtained. Building on these datasets, the paper summarizes core signal-processing techniques for THz sensing, including high-resolution parameter estimation, environment reconstruction, and material identification. Leveraging empirical measurements, a hybrid THz sensing channel model is developed, and environment reconstruction results are demonstrated, achieving millimeter-level accuracy. Finally, open challenges and future research directions are discussed.