Smoothed -Minimization for Green Cloud-RAN With User Admission Control

Abstract

The cloud radio access network (Cloud-RAN) has recently been proposed as one of the cost-effective and energy-efficient techniques for 5G wireless networks. By moving the signal processing functionality to a single baseband unit (BBU) pool, centralized signal processing and resource allocation are enabled in cloud-RAN, thereby providing the promise of improving the energy efficiency via effective network adaptation and interference management. In this paper, we propose a holistic sparse optimization framework to design green cloud-RAN by taking into consideration the power consumption of the fronthaul links, multicast services, as well as user admission control. Specifically, we first identify the sparsity structures in the solutions of both the network power minimization and user admission control problems, which call for adaptive remote radio head (RRH) selection and user admission. However, finding the optimal sparsity structures turns out to be NP-hard, with the coupled challenges of the $\ell_0$ -norm-based objective functions and the nonconvex quadratic QoS constraints due to multicast beamforming. In contrast to the previous works on convex but nonsmooth sparsity inducing approaches, e.g., the group sparse beamforming algorithm based on the mixed $\ell_1/\ell_2$ -norm relaxation, we adopt the nonconvex but smoothed $\ell_p$ -minimization ( $0 ) approach to promote sparsity in the multicast setting, thereby enabling efficient algorithm design based on the principle of the majorization–minimization (MM) algorithm and the semidefinite relaxation (SDR) technique. In particular, an iterative reweighted- $\ell_2$ algorithm is developed, which will converge to a Karush–Kuhn–Tucker (KKT) point of the relaxed smoothed $\ell_p$ -minimization problem from the SDR technique. We illustrate the effectiveness of the proposed algorithms with extensive simulations for network power minimization and user admission control in multicast cloud-RAN.