  As the capacity demand of mobile applications keeps increasing, the backhaul
network is becoming a bottleneck to support high quality of experience (QoE) in
next-generation wireless networks. Content caching at base stations (BSs) is a
promising approach to alleviate the backhaul burden and reduce user-perceived
latency. In this paper, we consider a wireless caching network where all the
BSs are connected to a central controller via backhaul links. In such a
network, users can obtain the required data from candidate BSs if the data are
pre-cached. Otherwise, the user data need to be first retrieved from the
central controller to local BSs, which introduces extra delay over the
backhaul. In order to reduce the download delay, the caching placement strategy
needs to be optimized. We formulate such a design problem as the minimization
of the average download delay over user requests, subject to the caching
capacity constraint of each BS. Different from existing works, our model takes
BS cooperation in the radio access into consideration and is fully aware of the
propagation delay on the backhaul links. The design problem is a mixed integer
programming problem and is highly complicated, and thus we relax the problem
and propose a low-complexity algorithm. Simulation results will show that the
proposed algorithm can effectively determine the near-optimal caching placement
and provide significant performance gains over conventional caching placement
strategies.
