  Terahertz (THz) integrated sensing and communication (ISAC) enables
simultaneous data transmission with Terabit-per-second (Tbps) rate and
millimeter-level accurate sensing. To realize such a blueprint, ultra-massive
antenna arrays with directional beamforming are used to compensate for severe
path loss in the THz band. In this paper, the time-frequency-space transmit
design is investigated for THz ISAC to generate time-varying scanning sensing
beams and stable communication beams. Specifically, with the dynamic
array-of-subarray (DAoSA) hybrid beamforming architecture and multi-carrier
modulation, two ISAC hybrid precoding algorithms are proposed, namely, a
vectorization (VEC) based algorithm that outperforms existing ISAC hybrid
precoding methods and a low-complexity sensing codebook assisted (SCA)
approach. Meanwhile, coupled with the transmit design, parameter estimation
algorithms are proposed to realize high-accuracy sensing, including a wideband
DAoSA MUSIC method for angle estimation and a sum-DFT-GSS approach for range
and velocity estimation. Numerical results indicate that the proposed
algorithms can realize centi-degree-level angle estimation accuracy and
millimeter-level range estimation accuracy, which are one or two orders of
magnitudes better than the methods in the millimeter-wave band. In addition, to
overcome the cyclic prefix limitation and Doppler effects, an inter-symbol
interference- and inter-carrier interference-tackled sensing algorithm is
developed to refine sensing capabilities for THz ISAC.
