  Proactive wireless caching and device to device (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 the minimization of a non-increasing monotone
supermodular function subject to a uniform partition matroid constraint. A
computationally efficient greedy algorithm is proposed which is proven to be
locally optimal within a factor 0.63 of the optimum. We analyze the average per
request throughput for different caching schemes and conduct the scaling
analysis for the average sum throughput. We show how throughput scaling depends
on video content popularity when the number of files grows asymptotically
large. Simulation results show a delay reduction of 45% to 80% compared to a
D2D caching system without inter-cluster cooperation.
