  This letter provides a comparison of indoor radio propagation measurements
and corresponding channel statistics at 28, 73, and 140 GHz, based on extensive
measurements from 2014-2020 in an indoor office environment. The empirical
investigations used a wideband sliding correlation-based channel sounder with
steerable horn antennas at both the transmitter and receiver to emulate
futuristic electronic beam steering at both the mobile device and the access
point. Side-by-side comparisons of propagation characteristics (e.g.,
large-scale path loss and multipath time dispersion) across a wide range of
frequencies from the low millimeter wave band of 28 GHz to the sub-THz band of
140 GHz illustrate the key similarities and differences in indoor wireless
channels. The measurements and models show remarkably similar path loss
exponents over frequencies in both LOS and NLOS scenarios, when using a one
meter free space reference distance, while the multipath time dispersion
becomes smaller at higher frequencies. Results here differ from the 3GPP indoor
channel model.
