A Brief History of Network Densification

This article aims at providing some clarity in the understanding of ultra-dense networks (UDNs), discussing the reasons why sparse small cell networks are fundamentally different from ultra-dense ones, and why the well-known linear scale of capacity with the base station (BS) density in the former does not apply to the latter. In more detail, we review the impact of closed-access operation and line-of-sight conditions, as well as the near-field effect, the antenna height difference between BSs and user equipment (UE), and the surplus of idle-modeequipped BSs with respect to UEs, on UDNs. Combining all these network characteristics and features, we present a new capacity scaling law for UDNs, which indicates the existence of an optimum BS density to maximise the area spectral efficiency for a given finite UE density. For a given finite BS density, this scaling law also indicates that there is an optimum number of UEs that can be simultaneously scheduled across the network to maximise capacity.