Coordinated multipoint (CoMP) transmission has been proposed as an important strategy to improve the signal quality for cell-edge users in future mobile cellular standards. However, when the joint transmission technique is applied in the CoMP downlink, the user data transferred from the data center to the base stations (BSs) can lead to a huge backhaul signaling overhead. Subject to quality-of-service (QoS) and per-BS power constraints, minimizing the user data transfer in the backhaul needs to jointly optimize the beamformer design and the BS assignment for each user data. We show that this problem can be cast into a sparsity-maximization problem, which is NP-hard. A semi-distributed algorithm is proposed for that problem to obtain suboptimal solutions. Such an algorithm is based on the uplink-downlink duality in multicellular networks. It utilizes the projected subgradient method and iterative function evaluation. Simulations show that the proposed method can reduce the backhaul user data transfer by 10%-30% compared to full cooperation for moderate number of users and QoS requirement.
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