  Proactive wireless caching and D2D communication have emerged as promising
techniques for enhancing users' quality of service and network performance. In
this paper, we propose a new architecture for D2D caching with inter-cluster
cooperation. We study a cellular network in which users cache popular files and
share them with other users either in their proximity via D2D communication or
with remote users using cellular transmission. We characterize the network
average delay per request from a queuing perspective. Specifically, we
formulate the delay minimization problem and show that it is NP-hard.
Furthermore, we prove that the delay minimization problem is equivalent to
minimization of a non-increasing monotone super modular function subject to a
partition matroid constraint. A computationally efficient greedy algorithm is
proposed which is proven to be locally optimal within a factor 2 of the
optimum. Simulation results show more than 45% delay reduction compared to a
D2D caching system without inter-cluster cooperation.
