  In this paper, a salary-plus-bonus incentive mechanism is proposed to
motivate WiFi Access Points (APs) to provide data offloading service for mobile
network operators (MNOs). Under the proposed salary-plus-bonus scheme, WiFi APs
are rewarded not only based on offloaded data volume but also based on the
quality of their offloading service. The interactions between WiFi APs and the
MNO under this incentive mechanism are then studied using Stackelberg game. By
differentiating whether WiFi APs are of the same type (e.g. offloading cost and
quality), two cases (homogeneous and heterogeneous) are studied. For both
cases, we derive the best response functions for WiFi APs (i.e. the optimal
amount of data to offload), and show that the Nash Equilibrium (NE) always
exists for the subgame. Then, given WiFi APs' strategies, we investigate the
optimal strategy (i.e. the optimal salary and bonus) for the MNO to maximize
its utility. Then, two simple incentive mechanisms, referred to as the
salary-only scheme and the bonus-only scheme, are presented and studied using
Stackelberg game. For both of them, it is shown that the Stackelberg
Equilibrium (SE) exists and is unique. We also show that the salary-only scheme
is more effective in offloading more data, and the bonus-only scheme is more
effective in selecting premium APs (i.e. providing high-quality offloading
service at low cost), while the salary-plus-bonus scheme can strike a well
balance between the offloaded data volume and the offloading quality.
