Energy efficiency tradeoff in downlink and uplink TDD OFDMA with simultaneous wireless information and power transfer

Energy-efficient design in orthogonal frequency division multiple access (OFDMA) is becoming increasingly important, considering the booming energy consumption in wireless networks and the extensive deployment of OFDMA-based wireless infrastructures. On the other hand, wireless power transfer brings another promising approach for saving energy and prolonging lifetime in wireless networks by letting users scavenge energy from the received radio-frequency (RF) signals all over the wireless environments, including the desired information beams and undesired interference beams. In this paper, we study the downlink and uplink energy efficiency (EE) tradeoff in time-division duplexing (TDD) OFDMA with one access point (AP) and multiple users, where the users are allowed to split the received signals for decoding information and harvesting energy in the downlink, respectively. The established optimization problem turns to be an integer-mixed nonconvex program. Through relaxation and transformation, we develop a near-optimal resource allocation strategy that approaches the Pareto optimal tradeoff performance. Numerical results show that there may exist a tradeoff between the downlink and uplink EE and the AP can help improve the uplink EE by operating the downlink at the suboptimal EE regime.