  Multicast transmission and wireless caching are effective ways of reducing
air and backhaul traffic load in wireless networks. This paper proposes to
incorporate these two key ideas for content-centric multicast transmission in a
cloud radio access network (RAN) where multiple base stations (BSs) are
connected to a central processor (CP) via finite-capacity backhaul links. Each
BS has a cache with finite storage size and is equipped with multiple antennas.
The BSs cooperatively transmit contents, which are either stored in the local
cache or fetched from the CP, to multiple users in the network. Users
requesting a same content form a multicast group and are served by a same
cluster of BSs cooperatively using multicast beamforming. Assuming fixed cache
placement, this paper investigates the joint design of multicast beamforming
and content-centric BS clustering by formulating an optimization problem of
minimizing the total network cost under the quality-of-service (QoS)
constraints for each multicast group. The network cost involves both the
transmission power and the backhaul cost. We model the backhaul cost using the
mixed $\ell_0/\ell_2$-norm of beamforming vectors. To solve this non-convex
problem, we first approximate it using the semidefinite relaxation (SDR) method
and concave smooth functions. We then propose a difference of convex functions
(DC) programming algorithm to obtain suboptimal solutions and show the
connection of three smooth functions. Simulation results validate the advantage
of multicasting and show the effects of different cache size and caching
policies in cloud RAN.
