System-Level Analysis of Far-Field Radio Frequency Power Delivery for mm-Sized Sensor Nodes

Millimeter-sized and low-cost sensor nodes can enable future applications of the Internet of Things (IoT), for which the number of sensors is projected to grow to a trillion within the next decades. RF far-field power transfer is a potential technique for wirelessly powering these sensors since it offers flexible configuration of sensor networks, beamforming capability and a large power transfer range compared to near-field approaches. However, system design for RF power transfer needs to be completely rethought to enable this new paradigm of a trillion IoT sensors. This paper, therefore, presents a comprehensive, system-level analysis strategy and a modular framework for investigating the fundamental efficiency components in an RF power transfer chain. Through this detailed analysis, it is demonstrated that the optimal frequency is primarily determined by the antenna size and the quality factors $(Q)$ of components in the matching network. Millimeter-wave frequencies are shown to be optimal for powering mm-sized sensors for practical matching component $Q$ values. An intuitive explanation of our results is also provided, along with insights for the design and practical implementation of RF power transfer systems for the IoT space.