How Much Can D2D Communication Reduce Content Delivery Latency in Fog Networks with Edge Caching?

A Fog-Radio Access Network (F-RAN) is studied in which cache-enabled Edge Nodes (ENs) with dedicated fronthaul connections to the cloud aim at delivering contents to mobile users. Using an information-Theoretic approach, this work tackles the problem of quantifying the potential latency reduction that can be obtained by enabling Device-To-Device (D2D) communication over out-of-band broadcast links. Following prior work, the Normalized Delivery Time (NDT)-a metric that captures the high signal-To-noise ratio worst-case latency-is adopted as the performance criterion of interest. Joint edge caching, downlink transmission, and D2D communication policies based on compress-And-forward are proposed that are shown to be information-Theoretically optimal to within a constant multiplicative factor of two for all values of the problem parameters, and to achieve the minimum NDT for a number of special cases. The analysis provides insights on the role of D2D cooperation in improving the delivery latency.