  This paper aims at investigating the achievable performance and the issues
that arise in ultra-dense networks (UDNs), when the signal propagation includes
both the Line-of-Sight (LOS) and Non-Line-Of-Sight (NLOS) components. Backed by
an analytical stochastic geometry-based model, we study the coverage, the Area
Spectral Efficiency (ASE) and the energy efficiency of UDNs with LOS/NLOS
propagation. We show that when LOS/NLOS propagation components are accounted
for, the network suffers from low coverage and the ASE gain is lower than
linear at high base station densities. However, this performance drop can
partially be attenuated by means of frequency reuse, which is shown to improve
the ASE vs coverage trade-off of cell densification, provided that we have a
degree of freedom on the density of cells. In addition, from an energy
efficiency standpoint, cell densification is shown to be inefficient when both
LOS and NLOS components are taken into account. Overall, based on the findings
of our work that assumes a more advanced system model compared to the current
state-of-the-art, we claim that highly crowded environments of users represent
the worst case scenario for ultra-dense networks. Namely, these are likely to
face serious issues in terms of limited coverage.
