Coded Caching in Presence of User Inactivity

We consider a one-server cache-enabled network under homogeneous file and network settings in presence of user inactivity, which is inherent to wireless mobile networks. Coded caching has been well studied for wired networks with static network properties. However, the lack of user inactivity information in the placement phase in the considered scenario requires redesigning coded caching against uncertainty. Unlike random or probabilistic caching studied in the literature, deterministic coded caching is considered to minimize the worst-case backhaul load by optimizing the file subpacketization and the caching strategy. First, a coded caching method is used, where each file is split into the same type of fragments labeled using sets with fixed cardinality, and the optimality of the selected cardinality is proved. Optimal file subpacketization by splitting the file into multiple types of fragments labeled with multiple cardinalities is then discussed. We show that the closed-form optimum turns out to be given by the same fixed cardinality as the one obtained without user inactivity-optimizing for user inactivity only affects file delivery, cache placement is not affected.