Wireless Powered Communications With Initial Energy: QoS Guaranteed Energy-Efficient Resource Allocation

This letter considers a wireless powered communication network (WPCN) where users first harvest energy in the downlink (DL) and then use the energy to transmit information signals in the uplink (UL). Furthermore, each user is also equipped with some initial energy, which generalizes the previous works on energy transfer and initial energy. Our goal is to maximize the system energy efficiency while guaranteeing the user's quality of service via joint time allocation and power control in both DL and UL. We find that the power station always transmits with the maximum power in DL while the transmit power of users in UL follows the waterfilling structure. In addition, the time allocation between DL and UL can be simplified into a linear programming problem. Finally, we propose an efficient algorithm to obtain the optimal solution. Simulation results demonstrate the system EE analysis for the WPCN as well as the special cases.