Uplink time scheduling with power level modulation scheme in wireless powered communication networks

In this paper, we propose an uplink scheduling scheme via downlink signal design for wireless powered communication networks (WPCNs). Although harvest-then-transmit protocols and related optimal resource allocation problems were studied, explicit methods of transmitting the scheduling information have not been considered in prior works. For uplink scheduling, we propose a design of the downlink energy signal with a power level modulation, which conveys the scheduling information to users. Hybrid-access point allocates different power levels to the subslots of the downlink energy signal, and the users recognize their uplink subslot lengths from their corresponding downlink subslots' power levels. The scheduling can be optimized based on the user channel state with respect to the sum rate. We formulate the sum throughput maximization problem for the proposed scheme, which is shown to be a convex optimization problem. We also study the proposed scheme in a noisy environment. The solution to the problem provides the optimal downlink and uplink slot lengths. The numerical results confirm that the throughput of the proposed WPCN scheme outperforms that of the conventional schemes. The improvement is shown even in imperfect synchronization scenario.