  Heterogenous wireless networks (Hetnets) provide a powerful approach to meet
the massive growth in traffic demands, but also impose a significant challenge
on backhaul. Caching at small base stations (BSs) and wireless small cell
backhaul have been proposed as attractive solutions to address this new
challenge. In this paper, we consider the optimal caching and user association
to minimize the total time to satisfy the average demands in cached-enabled
Hetnets with wireless backhaul. We formulate this problem as a mixed
discrete-continuous optimization for given bandwidth and cache resources.
First, we characterize the structure of the optimal solution. Specifically, we
show that the optimal caching is to store the most popular files at each pico
BS, and the optimal user association has a threshold form. We also obtain the
closed-form optimal solution in the homogenous scenario of pico cells. Then, we
analyze the impact of bandwidth and cache resources on the minimum total time
to satisfy the average demands. Finally, using numerical simulations, we verify
the analytical results.
