  Todays heterogeneous networks comprised of mostly macrocells and indoor small
cells will not be able to meet the upcoming traffic demands. Indeed, it is
forecasted that at least a 100x network capacity increase will be required to
meet the traffic demands in 2020. As a result, vendors and operators are now
looking at using every tool at hand to improve network capacity. In this epic
campaign, three paradigms are noteworthy, i.e., network densification, the use
of higher frequency bands and spectral efficiency enhancement techniques. This
paper aims at bringing further common understanding and analysing the potential
gains and limitations of these three paradigms, together with the impact of
idle mode capabilities at the small cells as well as the user equipment density
and distribution in outdoor scenarios. Special attention is paid to network
densification and its implications when transitioning to ultra-dense small cell
deployments. Simulation results show that network densification with an average
inter site distance of 35 m can increase the cell- edge UE throughput by up to
48x, while the use of the 10GHz band with a 500MHz bandwidth can increase the
network capacity up to 5x. The use of beamforming with up to 4 antennas per
small cell base station lacks behind with cell-edge throughput gains of up to
1.49x. Our study also shows how network densifications reduces multi-user
diversity, and thus proportional fair alike schedulers start losing their
advantages with respect to round robin ones. The energy efficiency of these
ultra-dense small cell deployments is also analysed, indicating the need for
energy harvesting approaches to make these deployments energy- efficient.
Finally, the top ten challenges to be addressed to bring ultra-dense small cell
deployments to reality are also discussed.
