Total Transmission Time Minimization in Wireless Powered Hybrid Passive-Active Communications

Transmission delay is critical to time-sensitive and power-limited Internet of Things (IoT) systems. This work proposes a hybrid transmission scheme, which enables energy-constrained sensor nodes (SNs) to deliver data to an information fusion via a hybrid of backscatter communications (BackCom) and wireless powered active communications (WPAC). Considering a non-linear energy harvesting (EH) model for each SN, we formulate a non-convex optimization problem to minimize the total transmission time of all SNs while satisfying the minimum throughput requirement for each SN, by jointly optimizing the time allocation between BackCom and active communications (AC), the transmit power and the power reflection coefficients of each SN, and the transmit power of the energy source (ES). We first determine the optimal transmit power of the ES by contradiction and then transform the non-convex problem into a convex one by introducing a series of auxiliary variables. We theoretically prove that the minimum total transmission time is achieved when each SN exhausts all the harvested energy and does not work in the pure BackCom mode. Simulation results show that the proposed scheme achieves a much shorter total transmission time than the existing schemes, e.g., binary transmission scheme, WPAC, and pure BackCom.