  Investigation of millimeter (mmWave) and Terahertz (THz) channels relies on
channel measurements and estimation of multi-path component (MPC) parameters.
As a common measurement technique in the mmWave and THz bands, direction-scan
sounding (DSS) resolves angular information and increases the measurable
distance. Through mechanical rotation, the DSS creates a virtual multi-antenna
sounding system, which however incurs signal phase instability and large data
sizes, which are not fully considered in existing estimation algorithms and
thus make them ineffective. To tackle this research gap, in this paper, a
DSS-oriented space-alternating generalized expectation-maximization
(DSS-o-SAGE) algorithm is proposed for channel parameter estimation in mmWave
and THz bands. To appropriately capture the measured data in mmWave and THz
DSS, the phase instability is modeled by the scanning-direction-dependent
signal phases. Furthermore, based on the signal model, the DSS-o-SAGE algorithm
is developed, which not only addresses the problems brought by phase
instability, but also achieves ultra-low computational complexity by exploiting
the narrow antenna beam property of DSS. Simulations in synthetic channels are
conducted to demonstrate the efficacy of the proposed algorithm and explore the
applicable region of the far-field approximation in DSS-o-SAGE. Last but not
least, the proposed DSS-o-SAGE algorithm is applied in real measurements in an
indoor corridor scenario at 300~GHz. Compared with results using the baseline
noise-elimination method, the channel is characterized more correctly and
reasonably based on the DSS-o-SAGE.
