  Directional radio beams are used in the mm-Wave band to combat the high path
loss. The mm-Wave band also suffers from high penetration losses from drywall,
wood, glass, concrete, etc., and also the human body. Hence, as a mobile user
moves, the Line of Sight (LoS) path between the mobile and the Base Station
(BS) can be blocked by objects interposed in the path, causing loss of the
link. A mobile with a lost link will need to be re-acquired as a new user by
initial access, a process that can take up to a second, causing disruptions to
applications. UNBLOCK is a protocol that allows a mobile to recover from
transient blockages, such as those caused by a human hand or another human
walking into the line of path or other temporary occlusions by objects, which
typically disappear within the order of $100$ ms, without having to go through
re-acquisition. UNBLOCK is based on extensive experimentation in office type
environments which has shown that while a LoS path is blocked, there typically
exists a Non-LoS path, i.e., a reflected path through scatterers, with a loss
within about $10$ dB of the LoS path. UNBLOCK proactively keeps such a NLoS
path in reserve, to be used when blockage happens, typically without any
warning. UNBLOCK uses this NLoS path to maintain time-synchronization with the
BS until the blockage disappears, as well as to search for a better NLoS path
if available. When the transient blockage disappears, it reestablishes LoS
communication at the epochs that have been scheduled by the BS for
communication with the mobile.
