  Future 5G cellular networks are expected to exploit the abundant spectrum
resources of the millimeter-wave (mm-wave) bands to satisfy demand for
multi-Gbps mobile links anticipated by exponential data traffic growth.
However, given the directional nature of mm-wave links, the feasibility of
mm-wave mobile networks is critically dependent on efficient antenna
beamsteering and a rich inventory of strong LOS (line-of-sight) and NLOS (non
line-of-sight) paths from effective reflectors in the urban environment. In
this paper we report results from detailed angular measurements of 60 GHz links
at an example outdoor pico-cellular site in a mixed-use urban environment
typical of European cities. Our work is the first to systematically analyze the
beamsteering requirements of future mm-wave cellular networks based on real
measurements. Our results reveal that the urban environment provides
substantial opportunities for multi-Gbps mm-wave connectivity, but that the
availability of strong LOS/NLOS links is highly location and
orientation-specific. Our results also show that high speed mm-wave links are
very sensitive to beam misalignment. This has important implications for
practical mm-wave cellular network design: (i) high-precision beamsteering is
required to maintain stable data rates even for quasi-stationary users; and
(ii) providing seamless high speed service in mobility scenarios will be
extremely challenging. Our results thus cast doubt on whether outdoor mm-wave
cellular deployments will be feasible in practice, given the high network
control overhead of meeting such stringent beamsteering requirements.
