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Multicast beamforming for wireless backhaul with user-centric clustering in Cloud-RANs

The cloud radio access network (Cloud-RAN) is an emerging network architecture for the next generation mobile wireless networks. However, connecting a large number of small base stations (SBSs) through wired backhaul links is inconvenient and cost-prohibitive. Therefore, wireless backhaul has become a promising solution for Cloud-RAN. In this paper, we propose to apply the multicast beamforming technique in wireless backhaul, which is efficient to share each user's message to a cluster of SBSs for collaboratively serving the user through joint beamforming. The joint optimization of multicast beamforming in backhaul links and user-centric clustering in access links is formulated as a weighted sum rate maximization problem. To tackle this intractable problem, several approximation and transformation techniques are introduced based on the semidefinite relaxation method and successive convex approximation approach. A block coordinate descent algorithm is developed to solve the approximate optimization problem iteratively by exploiting its special structure. Simulation results show that the proposed algorithm is guaranteed to converge to the stationary point, and the performance outperforms other wireless backhaul schemes under various network scenes.