Latency Limits for Content Delivery in a Fog-RAN with D2D Communication

A Fog-Radio Access Network (F-RAN) with arbitrary number of edge nodes and users is studied in which the users are able to cooperate by communicating over out-of-band broadcast Device-to-Device (D2D) links. Placement and delivery strategies are proposed with the aim of minimizing the Normalized Delivery Time (NDT) — a metric that captures the high signal-to-noise ratio worst-case latency for delivering any subset of requested contents to the users. The proposed strategies, based on compress-and-forward, are shown to be optimal to within a constant multiplicative factor of two for all values of the problem parameters. The analysis provides insights on the role of D2D cooperation in improving the delivery latency.