  TeraHertz (THz) communications are envisioned as a promising technology,
owing to its unprecedented multi-GHz bandwidth. In this paper, wideband channel
measurement campaigns at 140 GHz and 220 GHz are conducted in indoor scenarios
including a meeting room and an office room. Directional antennas are utilized
and rotated for resolving the multi-path components (MPCs) in the angular
domain. Comparable path loss values are achieved in the 140 and 220 GHz bands.
To investigate the large-scale fading characteristics for indoor THz
communications, single-band close-in path loss models are developed. To further
analyze the dependency on the frequency, two multi-band path loss models are
analyzed, i.e., alpha-beta-gamma (ABG) and multi-frequency CI model with a
frequency-weighted path loss exponent (CIF), between which the ABG model
demonstrates a better fit with the measured data. Moreover, a coherent beam
combination that constructively sums the signal amplitudes from various arrival
directions can significantly reduce the path loss, in contrast with a
non-coherent beam combination.
