  This correspondence paper investigates joint design of small base station
(SBS) clustering, multicast beamforming for access and backhaul links, as well
as frequency allocation in backhaul transmission to minimize the total power
consumption for wireless backhaul based ultra-dense cache-enabled cloud radio
access network (C-RAN). To solve this nontrivial problem, we develop a
low-complexity algorithm, which is a combination of smoothed ${\ell
_0}{\text{-norm}}$ approximation and convex-concave procedure. Simulation
results show that the proposed algorithm converges fast and greatly reduces the
backhaul traffic.
