Fog Radio Access Networks With Hierarchical Content Delivery.

We investigate a novel hierarchical content delivery strategy for F-RAN in this paper to accommodate the acquisition of popular multimedia files. Particularly, the studied F-RAN is assumed to be formed by base stations (BSs), cache-enabled fog user equipments (F-UEs), and conventional cellular user equipments (C-UEs), where the BSs, F-UEs, and C-UEs are spatially distributed as independent HPPPs. Furthermore, we assume the presence of a multimedia content library F := {1, 2, ..., F}. To facilitate the repetitive downloading of multimedia files in F for C-UEs, a novel hierarchical content delivery strategy, which comprises of the BS transmission phase and the F-UE transmission phase, is proposed. Particularly, under the proposed hierarchical content delivery strategy, a C-UE with a random file request is jointly and hierarchically served by its associated BS and the F-UEs within a distance of R-d. Furthermore, to resolve the contentions among the F-UEs, a carrier sensing-based collision avoidance scheme is adopted in the F-UE transmission phase, and only the F-UEs which have the minimal back-off timer among their respective competitors are selected to deliver the cached file. As such, the proposed hierarchical content delivery strategy can substantially enhance the quality of experience of the F-RAN. In order to analyze the performance of the proposed hierarchical content delivery strategy, we derive the network successful content delivery probability of the F-RAN based on the HPPP approximation of active F-UEs. The simulation examples are presented to verify our analytical results.