Cache-induced Hierarchical Cooperation in Wireless Device-to-Device Caching Networks

We consider a wireless device-to-device (D2D) caching network where n nodes are placed on a regular grid of area A (n). Each node caches LCF (coded) bits from a library of size LF bits, where L is the number of files and F is the size of each file. Each node requests a file from the library independently according to a popularity distribution. Under a commonly used “physical model” and Zipf popularity distribution, we characterize the optimal per-node capacity scaling law for extended networks (i.e., A (n) = n). Moreover, we propose a cache-induced hierarchical cooperation scheme and associated cache content placement optimization algorithm to achieve the optimal per-node capacity scaling law. When the path loss exponent α < 3, the optimal per-node capacity scaling law achieved by the cacheinduced hierarchical cooperation can be significantly better than that achieved by the existing state-of-theart schemes. To the best of our knowledge, this is the first work that completely characterizes the per-node capacity scaling law for wireless caching networks under the physical model and Zipf distribution with an arbitrary skewness parameter τ . While scaling law analysis yields clean results, it may not accurately reflect the throughput performance of a large network with a finite number of nodes. Therefore, we also analyze the throughput of the proposed cache-induced hierarchical cooperation for networks of practical size. The analysis and simulations verify that cache-induced hierarchical cooperation can also achieve a large throughput gain over the cache-assisted multihop scheme for networks of practical size.