  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.
