  Beam management is central in the operation of dense 5G cellular networks.
Focusing the energy radiated to mobile terminals (MTs) by increasing the number
of beams per cell increases signal power and decreases interference, and has
hence the potential to bring major improvements on area spectral efficiency
(ASE). This benefit, however, comes with unavoidable overheads that increase
with the number of beams and the MT speed. This paper proposes a first
system-level stochastic geometry model encompassing major aspects of the beam
management problem: frequencies, antennas, and propagation; physical layer,
wireless links, and coding; network geometry, interference, and resource
sharing; sensing, signaling, and mobility management. This model leads to a
simple analytical expression for the effective ASE that the typical user gets
in this context. This in turn allows one to find, for a wide variety of 5G
network scenarios including millimeter wave (mmWave) and sub-6 GHz, the number
of beams per cell that offers the best global trade-off between these benefits
and costs. We finally provide numerical results that discuss the effects of
different systemic trade-offs and performances of mmWave and sub-6 GHz 5G
deployments.
