  Internet-of-Things (IoT) networks are expected to support the wireless
connection of massive energy limited IoT nodes. The emerging wireless powered
backscatter communications (WPBC) enable IoT nodes to harvest energy from the
incident radio frequency signals transmitted by a power beacon (PB) to support
their circuit operation, but the energy consumption of the PB (a potentially
high cost borne by the network operator) has not been sufficiently studied for
WPBC. In this paper, we aim to minimize the energy consumption of the PB while
satisfying the throughput requirement per IoT node by jointly optimizing the
time division multiple access (TDMA) time slot duration and backscatter
reflection coefficient of each IoT node and the PB transmit power per time
slot. As the formulated joint optimization problem is non-convex, we transform
it into a convex problem by using auxiliary variables, then employ the Lagrange
dual method to obtain the optimal solutions. To reduce the implementation
complexity required for adjusting the PB's transmit power every time slot, we
keep the PB transmit power constant in each time block and solve the
corresponding PB energy consumption minimization problem by using auxiliary
variables, the block coordinated decent method and the successive convex
approximation technique. Based on the above solutions, two iterative algorithms
are proposed for the dynamic PB transmit power scheme and the static PB
transmit power scheme. The simulation results show that the dynamic PB transmit
power scheme and the static PB transmit power scheme both achieve a lower PB
energy consumption than the benchmark schemes, and the former achieves the
lowest PB energy consumption.
