Adding Active Elements to Reconfigurable Intelligent Surfaces to Enhance Energy Harvesting for IoT Devices

Reconfigurable intelligent surface (RIS) panels with passive and active elements significantly enhance Internet of Things (IoT) systems performance by, respectively, reflecting and amplifying incident signals to receiving entities. However, RIS panel active elements consume more energy than passive elements due to the signals reflection property of passive elements and the signals reflection and amplification properties of active elements. In addition, IoT devices may require harvesting energy from radio frequency (RF) signals from a nearby base station (BS) when they do not have enough operational energy. This paper investigates a trade-off between RIS panels containing active and passive elements energy consumption and energy harvested from RF signals of a nearby BS by a power-hungry IoT device. We consider all possible links via the RIS panel between transmitting and receiving nodes. In our model, the RIS panel is powered by harvesting energy from BS RF signals. We consider a fixed-length time frame that is divided into two optimal time slots. In the first time slot, the IoT device harvests energy from the BS RF signals with the help of the RIS. Using harvested energy from the BS RF signal, the IoT device transmits bits to the BS in the second time slot, also with the help of the RIS. We achieve the optimal number of RIS active and passive elements, therefore, reducing the RIS energy consumption for both time slots subject to RF energy harvesting and bits transmission. An optimization problem is formulated as a non-convex mixed-integer nonlinear problem. We propose a robust iterative algorithm to solve the problem. Finally, we present results to show the improved performance of our proposed model.