  The susceptibility of millimeter-wave (mmWave) signals to physical blockage
and abrupt signal strength variations presents a challenge to reliable 5G
communication. This work proposes and examines the feasibility of utilizing
lower-frequency signals as early-warning indicators of mobile mmWave signal
blockage or recovery. A physics-based channel simulation tool incorporating
Fresnel diffraction and image sources is employed to demonstrate that sub-6 GHz
signals "lead" mmWave signals in reaching a specific signal-strength threshold
by several to tens of milliseconds at mobile speeds, suggesting early-warning
systems are viable. This predictive approach stems from frequency-dependent
properties of diffraction and does not assume a specific topology or mobile and
obstacle speeds. Realistic simulations that include transitions from line of
sight (LoS) to non-line of sight (NLoS) and reflection scenarios are employed
to verify the proposed prediction capabilities. Moreover, prediction of the
strongest multipath component and its angle of arrival (AoA) using sub-6 GHz
observations is investigated.
