  Base station cooperation in heterogeneous wireless networks (HetNets) is a
promising approach to improve the network performance, but it also imposes a
significant challenge on backhaul. On the other hand, caching at small base
stations (SBSs) is considered as an efficient way to reduce backhaul load in
HetNets. In this paper, we jointly consider SBS caching and cooperation in a
downlink largescale HetNet. We propose two SBS cooperative transmission schemes
under random caching at SBSs with the caching distribution as a design
parameter. Using tools from stochastic geometry and adopting appropriate
integral transformations, we first derive a tractable expression for the
successful transmission probability under each scheme. Then, under each scheme,
we consider the successful transmission probability maximization by optimizing
the caching distribution, which is a challenging optimization problem with a
non-convex objective function. By exploring optimality properties and using
optimization techniques, under each scheme, we obtain a local optimal solution
in the general case and global optimal solutions in some special cases.
Compared with some existing caching designs in the literature, e.g., the most
popular caching, the i.i.d. caching and the uniform caching, the optimal random
caching under each scheme achieves better successful transmission probability
performance. The analysis and optimization results provide valuable design
insights for practical HetNets.
