  Caching at the base stations (BSs) has been widely adopted to reduce the
delivery delay and alleviate the backhaul traffic between BSs and the core
network. In this paper, we consider a collaborative content caching scheme
among BSs in cache-enabled multi-cell cooperative networks, where the requested
contents can be obtained from the associated BS, the other collaborative BSs or
the core network. Novelly, we model the stochastic request traffic and derive a
closed form expression for the average delay per request based on multi-class
processor sharing queuing theory. We then formulate a cooperative caching
optimization problem of minimizing the average delay under the finite cache
size constraint at BSs and show it to be at least NP-complete. Furthermore, we
prove it equivalent to the maximization of a monotone submodular function
subject to matroid constraints, allowing us to adopt the common greedy
algorithm with 1/2 performance guarantee. A heuristic greedy caching strategy
is also developed, achieving a better performance than the conventional greedy
solution. Simulation results verify the accuracy of the analytical results and
demonstrate the performance gains obtained by our proposed caching scheme.
