Hybrid Beamforming with Orthogonal Delay-Doppler Division Multiplexing Modulation for Terahertz Sensing and Communication

The Terahertz band holds a promise to enable both super-accurate sensing and ultra-fast communication. However, challenges arise that severe Doppler effects call for a waveform with high Doppler robustness while severe propagation path loss urges for an ultra-massive multiple-input multiple-output (UM-MIMO) structure. To tackle these challenges, hybrid beamforming with orthogonal delay-Doppler multiplexing modulation (ODDM) is investigated in this paper. First, the integration of delay-Doppler waveform and MIMO is explored by establishing a hybrid beamforming-based UM-MIMO ODDM input-output relation. Then, a multi-dimension sensing algorithm on target azimuth angle, elevation angle, range, and velocity is proposed, which features low complexity and high accuracy. Finally, an innovative decoupling of optimization goals is achieved by prioritizing sensing performance: Cramér-Rao lower bounds (CRLB) are minimized through dynamic beam scanning at the sensing combiner, which in turn allows the spectral efficiency to be independently maximized with stable beams at the precoder. Numerical results show that the sensing accuracy of the proposed sensing algorithm is sufficiently close to CRLB. Moreover, the proposed hybrid beamforming design allows us to achieve maximal spectral efficiency, millimeter-level range estimation accuracy and millidegree-level angle estimation accuracy.