  Wireless networks are becoming extremely pervasive while traffic demand is
ever increasing. In order to cope with the forecast increase in traffic volume
for the upcoming years, as well as the number of connected devices, new
technologies, practices and spectrum rearrangements are required. In this
context, a key question arises: how to provide extensive backhaul connectivity
and capacity for pervasive ultra dense networks? The answer is rather complex,
if feasible. To shed some light into this issue we overview potential
technologies, either wired or wireless, and identify technical challenges.
Moreover, we evaluate an illustrative scenario of a ultra-dense network that
operates with hybrid wired-wireless backhaul. We assume multiple radio access
technologies at small and macro base stations (BSs), and we discuss optimal
traffic splitting and routing solutions for different topologies and traffic
profiles.
