Coded Caching for Hierarchical Networks with a Different Number of Layers

Coded caching is a technique to reduce network loads. Recent works have discussed single-layer caching systems, but many networks consist of multiple layers of cache. In this paper, we discuss coded caching problem for a hierarchical network that has a different number of layers of cache. The network has users who connect to an origin server via a mirror server and users who directly connect to the origin server. We provide lower bounds on this problem setting based on cut-set bound. In addition, we propose three basic caching schemes and characterize these schemes. Also, we propose a novel coded caching scheme by combining two basic schemes and provide achievable rates of the combination coded caching scheme. Finally, we show that proposed combination scheme demonstrates good performance by a numerical result.