  In a Fog Radio Access Network (F-RAN) architecture, edge nodes (ENs), such as
base stations, are equipped with limited-capacity caches, as well as with
fronthaul links that can support given transmission rates from a cloud
processor. Existing information-theoretic analyses of content delivery in
F-RANs have focused on offline caching with separate content placement and
delivery phases. In contrast, this work considers an online caching set-up, in
which the set of popular files is time-varying and both cache replenishment and
content delivery can take place in each time slot. The analysis is centered on
the characterization of the long-term Normalized Delivery Time (NDT), which
captures the temporal dependence of the coding latencies accrued across
multiple time slots in the high signal-to- noise ratio regime. Online caching
and delivery schemes based on reactive and proactive caching are investigated,
and their performance is compared to optimal offline caching schemes both
analytically and via numerical results.
