Coordinated downlink and uplink user association and beamforming for energy minimizationincloud radio access network

Cloud radio access network (C-RAN) has been recently proposed, in which densely deployed access points (APs) are empowered by cloud computing, to achieve enormous mobile data rates. However, close proximity of many active APs results in more severe interference and also inefficient energy consumption. To tackle this problem, we propose a joint downlink (DL) and uplink (UL) user-AP association and beamforming design in this paper to coordinate interference in the C-RAN for energy minimization. The design problem is shown to be NP hard, but exhibits an interesting "group-sparse" property. By establishing a virtual DL transmission for the original UL transmission based on the celebrated UL-DL duality result, we convert the problem to an equivalent DL problem in C-RAN with two inter-related subproblems for the original and virtual DL transmissions, respectively, and obtain an efficient solution through "group-sparse" optimization.