Fast optimal energy-efficient resource allocation for downlink multi-user OFDM systems

In this paper, we propose an optimal resource allocation algorithm that maximizes the energy efficiency (EE) of a downlink (DL) multi-user orthogonal frequency division multiplexing (OFDM) system subject to a constraint on the total transmit power of the base station (BS). We show that the optimal EE is obtained by first allocating each subcarrier to the user equipment (UE) with the best channel power gain and then adapting the transmit power for each subcarrier using water-filling (WF) policy. Moreover, we propose a fast algorithm with linear complexity to compute the optimal WF level. We perform simulations to illustrate the effectiveness of the proposed algorithm and investigate the impact of maximum transmit power on the optimal EE. Simulation results indicate that an energy-efficient system behaves like a spectral-efficient system when the maximum transmit power of the BS is set to lower values.