Optimal File Dissemination and Beamforming for Cache-Enabled C-RANs

In this paper, we investigate the joint design of transmit beamforming over physical layer and file dissemination strategy at base stations (BSS) over transport layer in the cache-enabled cloud radio access networks. Our objective is to minimize the total user file download delay while satisfying each BS's power constraint and file dissemination proportion constraints. This problem is very challenging because of the highly coupled optimization variables in the objective function. To address the untractable non-convex problem, the original problem is decomposed into two subproblems. The first subproblem is a non-convex fractional program problem, and by transforming it into a second-order cone program, an iterative algorithm based on an inner approximation method is proposed to achieve the optimal beamforming vectors. For the other subproblem, we propose a sparse optimization algorithm and prove that there exists a closed-form solution in terms of l-0-norm. Then, the original non-convex problem is addressed in an iterative manner by employing the two proposed sub-Algorithms. Numerical results demonstrate that the proposed cross-layer optimization algorithms can efficiently reduce the total user download delay.