  Caching popular content in the storage of small cells is being considered as
an efficient technique to complement limited backhaul of small cells in
ultra-dense heterogeneous cellular networks. Limited storage capacity of the
small cells renders it important to determine the optimal set of files (cache)
to be placed on each small cell. In contrast to prior works on optimal caching,
in this work we study the effect of retransmissions on the optimal cache
placement policy for both static and mobile user scenarios. With the popularity
of files modeled as a Zipf distribution and a maximum n transmissions, i.e.,
n-1 retransmissions, allowed to receive each file, we determine the optimal
caching probability of the files that maximizes the hit probability. Our
closed-form optimal solutions concretely demonstrate that the optimal caching
probabilities are very sensitive to the number of retransmissions.
