Order-Aware Energy Recycling for Backscatter-Aided Wireless-Powered Cooperative Communications

In wireless-powered sensor networks, active uplink (UL) transmissions can support both sensed-data delivery and order-dependent RF energy recycling among energy-constrained devices. This paper studies a single hybrid access point (HAP) wireless-powered sensor network (WPSN), where one wireless device (WD) is selected as the relay, and the remaining WDs act as sources. The HAP first transfers wireless energy to all WDs in the downlink. Then, the source WDs reuse the incident wireless energy signal as a controllable carrier and deliver their information to the relay through passive backscatter communication. In the subsequent active uplink phase, the source WDs transmit according to an optimized order, while later scheduled sources recycle energy from the active signals transmitted by preceding sources. The source order therefore determines not only the transmission sequence but also the directed energy-recycling structure among source WDs, thereby reshaping the energy-causality constraints and the feasible transmit-energy region. The relay finally forwards the decoded source information to the HAP. To improve throughput fairness among sensor devices, a max-min throughput optimization problem is formulated by jointly designing the source transmission order, time allocation, and transmit-power allocation. For a given order, the continuous resource allocation problem is transformed into a convex problem through transmit-energy variables, and the optimal order is obtained by searching over all candidate orders. Numerical results show that the proposed scheme achieves higher minimum throughput than the three representative comparison schemes, demonstrating the benefit of jointly exploiting passive source-to-relay collection, relay-assisted forwarding, and order-aware energy recycling.