  mm-Wave communication employs directional beams to overcome high path loss.
High data rate communication is typically along line-of-sight (LoS). In outdoor
environments, such communication is susceptible to temporary blockage by
pedestrians interposed between the transmitter and receiver. It results in
outages in which the user is lost, and has to be reacquired as a new user,
severely disrupting interactive and high throughput applications. It has been
presumed that the solution is to have a densely deployed set of base stations
that will allow the mobile to perform a handover to a different non-blocked
base station every time a current base station is blocked. This is however a
very costly solution for outdoor environments. Through extensive experiments we
show that it is possible to exploit a strong ground reflection with a received
signal strength (RSS) about 4dB less than the LoS path in outdoor built
environments with concrete or gravel surfaces, for beams that are narrow in
azimuth but wide in zenith. While such reflected paths cannot support the high
data rates of LoS paths, they can support control channel communication, and,
importantly, sustain time synchronization between the mobile and the base
station. This allows a mobile to quickly recover to the LoS path upon the
cessation of the temporary blockage, which typically lasts a few hundred
milliseconds. We present a simple in-band protocol that quickly discovers
ground reflected radiation and uses it to recover the LoS link when the
temporary blockage disappears.
