  To meet the ever-growing demand for a higher communicating rate and better
communication quality, more and more small cells are overlaid under the macro
base station (MBS) tier, thus forming the heterogeneous networks. Small cells
can ease the load pressure of MBS but lack of the guarantee of performance. On
the other hand, cooperation draws more and more attention because of the great
potential of small cell densification. Some technologies matured in wired
network can also be applied to cellular networks, such as Software-defined
networking (SDN). SDN helps simplify the structure of multi-tier networks. And
it's more reasonable for the SDN controller to implement cell coordination. In
this paper, we propose a method to offload users from MBSs through small cell
cooperation in heterogeneous networks. Association probability is the main
indicator of offloading. By using the tools from stochastic geometry, we then
obtain the coverage probabilities when users are associated with different
types of base stations (BSs). All the cell association and cooperation are
conducted by the SDN controller. Then on this basis, we compare the overall
coverage probabilities, achievable rate and energy efficiency with and without
cooperation. Numerical results show that small cell cooperation can offload
more users from MBS tier. It can also increase the system's coverage
performance. As small cells become denser, cooperation can bring more gains to
the energy efficiency of the network.
