  Caching at mobile devices can facilitate device-to-device (D2D)
communications, which may significantly improve spectrum efficiency and
alleviate the heavy burden on backhaul links. However, most previous works
ignored user mobility, thus having limited practical applications. In this
paper, we take advantage of the user mobility pattern by the inter-contact
times between different users, and propose a mobility-aware caching placement
strategy to maximize the data offloading ratio, which is defined as the
percentage of the requested data that can be delivered via D2D links rather
than through base stations (BSs). Given the NP-hard caching placement problem,
we first propose an optimal dynamic programming (DP) algorithm to obtain a
performance benchmark, but with much lower complexity than exhaustive search.
We will then prove that the problem falls in the category of monotone
submodular maximization over a matroid constraint, and propose a time-efficient
greedy algorithm, which achieves an approximation ratio as 1/2. Simulation
results with real-life data-sets will validate the effectiveness of our
proposed mobility-aware caching placement strategy. We observe that users
moving at either a very low or very high speed should cache the most popular
files, while users moving at a medium speed should cache less popular files to
avoid duplication.
