  Caching popular contents at mobile devices, assisted by device-to-device
(D2D) communications, is considered as a promising technique for mobile content
delivery. It can effectively reduce backhaul traffic and service cost, as well
as improving the spectrum efficiency. However, due to the selfishness of mobile
users, incentive mechanisms will be needed to motivate device caching. In this
paper, we investigate incentive mechanism design in cache-assisted D2D
networks, taking advantage of the user mobility information. An inter-contact
model is adopted to capture the average time between two consecutive contacts
of each device pair. A Stackelberg game is formulated, where each user plays as
a follower aiming at maximizing its own utility and the mobile network operator
(MNO) plays as a leader aiming at minimizing the cost. Assuming that user
responses can be predicted by the MNO, a cost minimization problem is
formulated. Since this problem is NP-hard, we reformulate it as a non-negative
submodular maximization problem and develop
$(\frac{1}{4+\epsilon})$-approximation local search algorithm to solve it. In
the simulation, we demonstrate that the local search algorithm provides near
optimal performance. By comparing with other caching strategies, we validate
the effectiveness of the proposed incentive-based mobility-aware caching
strategy.
