  Caching popular content at mobile devices, accompanied by device-to-device
(D2D) communications, is one promising technology for effective mobile content
delivery. User mobility is an important factor when investigating such
networks, which unfortunately was largely ignored in most previous works.
Preliminary studies have been carried out, but the effect of mobility on the
caching performance has not been fully understood. In this paper, by explicitly
considering users' contact and inter-contact durations via an alternating
renewal process, we first investigate the effect of mobility with a given cache
placement. A tractable expression of the data offloading ratio, i.e., the
proportion of requested data that can be delivered via D2D links, is derived,
which is proved to be increasing with the user moving speed. The analytical
results are then used to develop an effective mobility-aware caching strategy
to maximize the data offloading ratio. Simulation results are provided to
confirm the accuracy of the analytical results and also validate the effect of
user mobility. Performance gains of the proposed mobility-aware caching
strategy are demonstrated with both stochastic models and real-life data sets.
It is observed that the information of the contact durations is critical to
design cache placement, especially when they are relatively short or comparable
to the inter-contact durations.
