  In this paper, we propose downlink signal design and optimal uplink
scheduling for the wireless powered communication networks (WPCNs). Prior works
give attention to resource allocation in a static channel because users are
equipped with only energy receiver and users cannot update varying uplink
schedulling. For uplink scheduling, we propose a downlink signal design scheme,
called a power level modulation, which conveys uplink scheduling information to
users. First, we design a downlink energy signal using power level modulation.
Hybrid-access point (H-AP) allocates different power level in each subslot of
the downlink energy signal according to channel condition and users optimize
their uplink time subslots for signal transmission based on the power levels of
their received signals. Further, we formulate the sum throughput maximization
problem for the proposed scheme by determining the uplink and downlink time
allocation using convex optimization problem. Numerical results confirm that
the throughput of the proposed scheme outperforms that of the conventional
schemes.
