  Caching popular contents at the edge of cellular networks has been proposed
to reduce the load, and hence the cost of backhaul links. It is significant to
decide which files should be cached and where to cache them. In this paper, we
propose a distributed caching scheme considering the tradeoff between the
diversity and redundancy of base stations' cached contents. Whether it is
better to cache the same or different contents in different base stations? To
find out this, we formulate an optimal redundancy caching problem. Our goal is
to minimize the total transmission cost of the network, including cost within
the radio access network (RAN) and cost incurred by transmission to the core
network via backhaul links. The optimal redundancy ratio under given system
configuration is obtained with adapted particle swarm optimization (PSO)
algorithm. We analyze the impact of important system parameters through
Monte-Carlo simulation. Results show that the optimal redundancy ratio is
mainly influenced by two parameters, which are the backhaul to RAN unit cost
ratio and the steepness of file popularity distribution. The total cost can be
reduced by up to 54% at given unit cost ratio of backhaul to RAN when the
optimal redundancy ratio is selected. Under typical file request pattern, the
reduction amount can be up to 57%.
