  The concept of Ultra Dense Networks (UDN) is often seen as a key enabler of
the next generation mobile networks. However, existing analysis of UDNs,
including Stochastic Geometry, has not been able to fully determine the
potential gains and limits of densification. In this paper we study performance
of UDNs in downlink and provide new insights on the impact of antenna height
and careful site selection on the network performance. We focus our
investigation on the probability of coverage, average cell rate and average
area spectral efficiency for networks with regular and random BS deployments.
We show that under a path-loss model which considers antenna height there
exists an upper limit on network performance which is dependent on the
path-loss model parameters. Our analysis shows an interesting finding that even
for over-densified networks a non-negligible system performance can be
achieved.
