Joint Beamformer Design for Wireless Fronthaul and Access Links in C-RANs

This paper presents a joint design framework of fronthaul and access links in cloud radio access networks, wherein the fronthaul data delivery between the central processor (CP) and small-cell base stations (SBSs) is carried over wireless links, which is more cost effective and flexible than the conventional wired fronthaul solutions. In this framework, the coordinated beamforming scheme is adopted by the SBSs to serve the users cooperatively, which not only reduces the bandwidth requirement over the fronthaul links, but also eliminates the inter-cell interference in the access links. To further exploit the spatial diversity over the fronthaul links, the multi-user beamforming scheme is adopted by the CP to deliver the data of all users to their serving SBSs simultaneously. This non-convex and combinatorial optimization problem is reformulated to a unified beamformer design problem using the l(0)/l(1)-norm framework, which is still difficult to obtain the optimal solutions. Therefore, we first propose an algorithm based on the difference of convex (DC) programming scheme to find the suboptimal solutions, whereby the formulated problem is transformed to the standard DC programming problem and solved iteratively using the convex-concave procedure algorithm. We then propose another algorithm based on the successive convex approximation and weighted minimum-mean-squarederror approaches, and solve the transformed problem using the block coordinate update scheme. The pros and cons of these two algorithms are discussed, and simulation results are provided to demonstrate the performance gain of our schemes over other benchmark schemes.