  Device-to-Device (D2D) communications incorporated with content caching have
been regarded as a promising way to offload the cellular traffic data. In this
paper, the caching strategy is investigated to maximize the D2D offloading gain
with the comprehensive consideration of user collaborative characteristics as
well as the physical transmission conditions. Specifically, for a given
content, the number of interested users in different groups is different, and
users always ask the most trustworthy user in proximity for D2D transmissions.
An analytical expression of the D2D success probability is first derived, which
represents the probability that the received signal to interference ratio is no
less than a given threshold. As the formulated problem is non-convex, the
optimal caching strategy for the special unbiased case is derived in a closed
form, and a numerical searching algorithm is proposed to obtain the globally
optimal solution for the general case. To reduce the computational complexity,
an iterative algorithm based on the asymptotic approximation of the D2D success
probability is proposed to obtain the solution that satisfies the
Karush-Kuhn-Tucker conditions. The simulation results verify the effectiveness
of the analytical results and show that the proposed algorithm outperforms the
existing schemes in terms of offloading gain.
