Charging and transmission time minimization for wireless powered communication networks

In this paper, we consider a wireless communication network with a hybrid access point (HAP) and a set of wireless users with energy harvesting capabilities. The HAP is assumed to have two antennas: one for transferring wireless energy in the downlink and one for receiving wireless information in the uplink. Without fixed energy sources, users first have to harvest energy from the wireless signals broadcast by the HAP, and then using the harvested energy to transmit their individual collected data to the HAP through dynamic-time-division-multiple-access (D-TDMA). All users can harvest the wireless energy prior to its transmission, and thus latter users can harvest more energy. We investigate the optimal time allocation to minimize the completion time of charging and transmitting all data subject to the constraints that all users have to send back some minimum amount of data. A high-efficient algorithm is then proposed to obtain an optimal time allocation of the formulated problem by exploring the properties of the constraints. It is also shown by simulations that the users with higher SNR should be scheduled to transmit first. Then, a suboptimal algorithm is also proposed based on the optimal time allocation. It is then shown by simulations that the suboptimal time allocation can achieve a close-to-optimal performance.