  Millimeter-wave multi-input multi-output (mm-Wave MIMO) systems are one of
the candidate schemes for 5G wireless standardization efforts. In this context,
the main contributions of this article are three-fold. 1) We describe parallel
sets of measurements at identical transmit-receive location pairs with 2.9, 29
and 61 GHz carrier frequencies in indoor office, shopping mall, and outdoor
settings. These measurements provide insights on propagation, blockage and
material penetration losses, and the key elements necessary in system design to
make mm-Wave systems viable in practice. 2) One of these elements is hybrid
beamforming necessary for better link margins by reaping the array gain with
large antenna dimensions. From the class of fully-flexible hybrid beamformers,
we describe a robust class of directional beamformers towards meeting the high
data-rate requirements of mm-Wave systems. 3) Leveraging these design insights,
we then describe an experimental prototype system at 28 GHz that realizes high
data-rates on both the downlink and uplink and robustly maintains these rates
in outdoor and indoor mobility scenarios. In addition to maintaining large
signal constellation sizes in spite of radio frequency challenges, this
prototype leverages the directional nature of the mm-Wave channel to perform
seamless beam switching and handover across mm-Wave base-stations thereby
overcoming the path losses in non-line-of-sight links and blockages encountered
at mm-Wave frequencies.
