Coded Caching in a Multi-Server System with Random Topology

Cache-aided content delivery is studied in a multi-server system with K users, each equipped with a local cache memory. In the delivery phase, each user connects randomly to any ρ servers out of a total of P servers. Thanks to the availability of multiple servers, which model small base stations with limited storage capacity, user demands can be satisfied with reduced storage capacity at each server and reduced delivery rate per server; however, this also leads to reduced multicasting opportunities compared to a single server serving all the users simultaneously. A joint storage and proactive caching scheme is proposed, which exploits coded storage across the servers, uncoded cache placement at the users, and coded delivery; and it is shown that the achievable average sum delivery rate is comparable to that achieved by a single server, while the gap between the two depends on ρ, the available redundancy across servers. It is also shown that the gap can be reduced by increasing the storage capacity at the SBSs.