Optimization of energy efficiency in the downlink LTE transmission

In this paper, we investigate the energy-efficient resource allocation problem in the downlink of an Orthogonal Frequency Division Multiple Access (OFDMA) system with multiple users. A iterative algorithm is developed to maximize the overall energy efficiency of the downlink transmission by allocating the resource blocks (RBs) and the transmit power, which is constant for all RBs assigned to a given user. Moreover, we assume that all RBs assigned to the same user must use the same modulation and coding scheme (MCS), as in the LTE specification. In this form our optimization problem belongs to a broad class of problems called Mixed-Integer Nonlinear Fractional Problem (MINLFP), which is very difficult to solve analytically in the original form. Hence, in order to solve it, we propose the iterative algorithm with fast convergence based on the Dinkelbach method. In each iteration, the solution for the power and resource blocks allocation is derived by the numerical method using the Karush-Kuhn-Tucker (KKT) multipliers and KKT conditions, while modulation and coding scheme, for each user, is obtained by the iterative algorithm. Simulation results show that despite the computational simplicity of the proposed solution, it achieves better results than the solutions known from the literature.