A Comprehensive Review of Battery‐Free Energy Efficient RF and Microwave Systems

Battery‐free radiofrequency (RF) and microwave systems have emerged as a promising solution for enabling long‐lifetime, maintenance‐free wireless sensing and communication platforms by eliminating conventional battery dependence. These systems operate by harvesting ambient or dedicated electromagnetic energy, converting it to usable electrical power, and utilizing ultralow‐power circuit and communication techniques to sustain sensing, processing, and wireless data transmission. This paper presents a comprehensive review of battery‐free energy‐efficient RF and microwave systems, covering the evolution of the field, energy harvesting and rectenna modeling techniques, power management strategies, ultralow‐power communication methods, and energy‐aware networking protocols. The review further examines key enabling circuit technologies, including subthreshold microwave integrated circuits, passive and backscatter‐based transmitters, and envelope‐detection receivers designed for nanowatt‐to‐microwatt operation. Recent experimental developments in battery‐free Internet of Things (IoT) nodes, wearable and implantable platforms, and smart‐environment monitoring systems are analyzed to highlight the transition from laboratory prototypes to real‐world deployments. Performance evaluation metrics such as RF‐to‐DC conversion efficiency, communication reliability, scalability, and system lifetime are discussed using representative experimental results from recent literature. The paper also identifies major open challenges, including energy intermittency, miniaturization constraints, and security considerations, and outlines future research directions such as integration with 5G/6G infrastructure, AI‐assisted energy management, and advanced materials for flexible and multiband harvesting structures. The review demonstrates that advances in rectenna codesign, ultralow‐power circuit architectures, adaptive communication techniques, and network‐level coordination are rapidly enabling practical battery‐free wireless systems, positioning RF and microwave energy harvesting as a key technology for sustainable massive IoT and long‐term autonomous sensing applications.