  Caching the popular multimedia content is a promising way to unleash the
ultimate potential of wireless networks. In this paper, we contribute to
proposing and analyzing the cache-based content delivery in a three-tier
heterogeneous network (HetNet), where base stations (BSs), relays and
device-to-device (D2D) pairs are included. We advocate to proactively cache the
popular contents in the relays and parts of the users with caching ability when
the network is off-peak. The cached contents can be reused for frequent access
to offload the cellular network traffic. The node locations are first modeled
as mutually independent Poisson Point Processes (PPPs) and the corresponding
content access protocol is developed. The average ergodic rate and outage
probability in the downlink are then analyzed theoretically. We further derive
the throughput and the delay based on the \emph{multiclass processor-sharing
queue} model and the continuous-time Markov process. According to the critical
condition of the steady state in the HetNet, the maximum traffic load and the
global throughput gain are investigated. Moreover, impacts of some key network
characteristics, e.g., the heterogeneity of multimedia contents, node densities
and the limited caching capacities, on the system performance are elaborated to
provide a valuable insight.
