Content-Centric Multicast Beamforming in Cache-Enabled Cloud Radio Access Networks

摘要

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