Fog Radio Access Network (F-RAN) architectures can leverage both cloudprocessing and edge caching for content delivery to the users. To this end,F-RAN utilizes caches at the edge nodes (ENs) and fronthaul links connecting acloud processor to ENs. Assuming time-invariant content popularity, existinginformation-theoretic analyses of content delivery in F-RANs rely on offlinecaching with separate content placement and delivery phases. In contrast, thiswork focuses on the scenario in which the set of popular content istime-varying, hence necessitating the online replenishment of the ENs' cachesalong with the delivery of the requested files. The analysis is centered on thecharacterization of the long-term Normalized Delivery Time (NDT), whichcaptures the temporal dependence of the coding latencies accrued acrossmultiple time slots in the high signal-to-noise ratio regime. Online edgecaching and delivery schemes based on reactive and proactive caching principlesare investigated for both serial and pipelined transmission modes acrossfronthaul and edge segments. Analytical results demonstrate that, in thepresence of a time-varying content popularity, the rate of fronthaul links setsa fundamental limit to the long-term NDT of F-RAN system. Analytical resultsare further verified by numerical simulation, yielding important designinsights.
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