Optimal Caching in Content-Centric Mobile Hybrid Networks: A Variable Decoupling Approach

In this paper, large-scale content-centric mobile hybrid networks consisting of mobile nodes and static small base stations (SBSs) (or static helper nodes) are studied, where each node moves according to the random walk mobility model and requests a content object from the library independently at random according to a Zipf popularity distribution. Instead of allowing access to content objects at macro BSs via costly backhaul providing connection to the core network, we consider a more practical and challenging scenario where mobile nodes and SBSs, each having a finite-size cache space, are able to cache a subset of content objects so that each request is served by other mobile nodes or SBSs via multihop transmissions. We analyze the optimal throughput–delay trade-off by presenting a new cache allocation strategy using variable decoupling. In particular, under a given caching strategy, we first characterize a fundamental throughput–delay trade-off in terms of scaling laws by introducing a general content delivery multihop routing protocol. Then, the optimal throughput–delay trade-off is characterized by presenting the optimal cache allocation strategy, which jointly finds the replication sets at mobile nodes and SBSs via a novel variable decoupling approach. An interesting observation is that highly popular content objects are mainly served by mobile nodes while the rest of content objects are served by static SBSs. We perform numerical evaluation to validate our analytical results. We also show that the optimal strategy strictly outperforms a baseline approach, where the replication sets at mobile nodes and SBSs are optimized separately. Index Terms Caching, content-centric network, mobile hybrid network, throughput–delay trade-off, variable decoupling. ✦