  This article provides dives into the fundamentals of dense and ultra-dense
small cell wireless networks, discussing the reasons why dense and ultra-dense
small cell networks are fundamentally different from sparse ones, and why the
well-known linear scaling law of capacity with the base station (BS) density in
the latter does not apply to the former. In more detail, we review the impact
of the following factors on ultradense networks (UDNs), (i) closed-access
operations and line-of-sight conditions, (ii) the near-field effect, (iii) the
antenna height difference between small cell BSs and user equipments (UEs), and
(iv) the surplus of idle-mode-enabled small cell BSs with respect to UEs.
Combining all these network characteristics and features, we present a more
realistic capacity scaling law for UDNs, which indicates (i) the existence of
an optimum BS density to maximise the area spectral efficiency (ASE) for a
given finite UE density, and (ii) the existence of an optimum density of UEs
that can be simultaneously scheduled across the network to maximise the ASE for
a given finite BS density.
