  In cloud radio access networks (C-RANs), a substantial amount of data must be
exchanged in both backhaul and fronthaul links, which causes high power
consumption and poor quality of service (QoS) experience for real-time
services. To solve this problem, a cluster content caching structure is
proposed in this paper, which takes full advantage of distributed caching and
centralized signal processing. In particular, redundant traffic on the backhaul
can be reduced because the cluster content cache provides a part of required
content objects for remote radio heads (RRHs) connected to a common edge cloud.
Tractable expressions for both effective capacity and energy efficiency
performance are derived, which show that the proposed structure can improve QoS
guarantees with a lower power cost of local storage. Furthermore, to fully
explore the potential of the proposed cluster content caching structure, the
joint design of resource allocation and RRH association is optimized, and two
distributed algorithms are accordingly proposed. Simulation results verify the
accuracy of the analytical results and show the performance gains achieved by
cluster content caching in C-RANs.
