On the Capacity of Multihop Device-to-Device Caching Networks

We consider a wireless device-to-device (D2D) network where n nodes are uniformly distributed at random over the network area. We let each node with storage capacity M cache files from a library of size m. Each node in the network requests a file from the library independently at random, according to a popularity distribution, and is served by other nodes having the requested file in their local cache via (possibly) multihop transmissions. Under the classical "protocol model" of wireless networks, we characterize the optimal per-node capacity scaling law for a broad class of heavy-tailed popularity distributions including the Zipf distribution with Zipf exponent less than one. Surprisingly, in the parameter regimes of interest, we show that decentralized random caching uniformly across the library yields optimal per-node capacity scaling of Theta (root M/m), which outperforms the single-hop caching networks whose capacity scales as Theta (M/m) [1], [2].