  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 L_C*F (coded) bits
from a library of size L*F 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). 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 \alpha<3, the optimal per-node
capacity scaling law achieved by the cache-induced hierarchical cooperation can
be significantly better than that achieved by the existing state-of-the-art
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 \tau. 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.
