Energy Efficient Resource Optimization in User-Centric UDNs with NOMA and Beamforming

In this paper, we address the problem of energy efficient resource optimization for downlink transmission in user-centric ultra-dense networks enabled by wireless access via non-orthogonal multiple access and wireless backhaul via beamforming. Our objective is to maximize the system energy efficiency by jointly optimizing user/access point scheduling, subchannel assignment, and power allocation. The problem is formulated as a non-convex mixed-integer nonlinear programming problem which is NP-hard. We then transform it into a convex subproblem using the sum-of-ratios decoupling and the iterative successive convex approximation method. An overall algorithm is further developed to solve the subproblem iteratively. Simulation results show that the proposed algorithm has improved the system-wide energy efficiency significantly when compared to the benchmark scheme.