  Caching the content closer to the user equipments (UEs) in heterogenous
cellular networks (HetNets) improves user-perceived Quality-of-Service (QoS)
while lowering the operators backhaul usage/costs. Nevertheless, under the
current networking strategy that promotes aggressive densification, it is
unclear whether cache-enabled HetNets preserve the claimed cost-effectiveness
and the potential benefits. This is due to 1) the collective cost of caching
which may inevitably exceed the expensive cost of backhaul in a dense HetNet,
and 2) the excessive interference which affects the signal reception
irrespective of content placement. We analyze these significant, yet
overlooked, issues, showing that while densification reduces backhaul load and
increases spectral efficiency in cache-enabled dense networks, it
simultaneously reduces cache-hit probability and increases the network cost. We
then introduce a caching efficiency metric, area spectral efficiency per unit
spent cost, and find it enough to cache only about 3% of the content library
size in the cache of smallcell base stations. Furthermore, we show that range
expansion, which is known to be of substantial value in wireless networks, is
almost impotent to curb the caching inefficiency. Surprisingly, unlike the
conventional wisdom recommending traffic offloading from macro cells to small
cells, in cache-enabled HetNets, it is generally more beneficial to exclude
offloading altogether or to do the opposite.
