Coded Caching for Relay Networks: The Impact of Caching Memories

Relay is a traditional key technology to improve the communication reliability and enlarge the covering range of service. Recently, coded caching schemes that reduce traffic congestion through coding and injecting duplicate data among users have attracted wide interests. This paper studies a relay network where all nodes including the central server, relay nodes and users are equipped with cache memories. Each user demands a file from the server’s library, and is connected to the server through a specific relay node. We define the communication delay for this model and propose new coded caching schemes for the deterministic and random caching setups, respectively. The proposed schemes exploit the spared transmission time resource and can greatly reduce the transmission delay compared to the previously known caching schemes. Surprisingly, we show that even when relay nodes do not cooperate with each other, using a small amount of caching memories at each relay node is sufficient to achieve the same communication delay as if each relay had access to the full library. To our best knowledge, this is the first result showing that even the caching size is strictly smaller than the library’s size, increasing the caching size is wasteful in reducing the transmission latency.