  The proliferation of highly capable mobile devices such as smartphones and
tablets has significantly increased the demand for wireless access. Software
defined network (SDN) at edge is viewed as one promising technology to simplify
the traffic offloading process for current wireless networks. In this paper, we
investigate the incentive problem in SDN-at-edge of how to motivate a third
party access points (APs) such as WiFi and smallcells to offload traffic for
the central base stations (BSs). The APs will only admit the traffic from the
BS under the precondition that their own traffic demand is satisfied. Under the
information asymmetry that the APs know more about own traffic demands, the BS
needs to distribute the payment in accordance with the APs' idle capacity to
maintain a compatible incentive. First, we apply a contract-theoretic approach
to model and analyze the service trading between the BS and APs. Furthermore,
other two incentive mechanisms: optimal discrimination contract and linear
pricing contract are introduced to serve as the comparisons of the anti adverse
selection contract. Finally, the simulation results show that the contract can
effectively incentivize APs' participation and offload the cellular network
traffic. Furthermore, the anti adverse selection contract achieves the optimal
outcome under the information asymmetry scenario.
