  The millimeter wave (mmWave) frequencies offer the potential of orders of
magnitude increases in capacity for next-generation cellular wireless systems.
However, links in mmWave networks are highly susceptible to blocking and may
suffer from rapid variations in quality. Connectivity to multiple cells - both
in the mmWave and in the traditional lower frequencies - is thus considered
essential for robust connectivity. However, one of the challenges in supporting
multi-connectivity in the mmWave space is the requirement for the network to
track the direction of each link in addition to its power and timing. With
highly directional beams and fast varying channels, this directional tracking
may be the main bottleneck in realizing robust mmWave networks. To address this
challenge, this paper proposes a novel measurement system based on (i) the UE
transmitting sounding signals in directions that sweep the angular space, (ii)
the mmWave cells measuring the instantaneous received signal strength along
with its variance to better capture the dynamics and, consequently, the
reliability of a channel/direction and, finally, (iii) a centralized controller
making handover and scheduling decisions based on the mmWave cell reports and
transmitting the decisions either via a mmWave cell or conventional microwave
cell (when control signaling paths are not available). We argue that the
proposed scheme enables efficient and highly adaptive cell selection in the
presence of the channel variability expected at mmWave frequencies.
