# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 1 Introduction  
Near-field communication (NFC) is a radio-frequency identification system (RFID) that enables fast communication between devices over a short range using the 13.56-MHz RFID band [1]. Although near-field communication has existed for over a decade [2], this technology did not become widespread until its extensive use in payment systems. NFC technology enables simple and safe two-way interactions between electronic devices, enabling consumers to perform contactless transactions, access digital content, and connect electronic devices with a single tap. Most current smartphones also incorporate an NFC reader. NFC systems are, therefore, gaining importance in the Internet of Things (IoT) scenario [3, 4]. NFC is also interesting for the development of low-cost sensors since it provides a quick and easy way of obtaining data from them simply by approaching the reader to the tag without having to pair the devices. The upcoming fifth generation (5G) of communication technology is expected to unleash a massive IoT ecosystem where networks can serve the communication needs for billions of connected devices, with the right trade-offs between speed, latency, and cost. RFID is one of the most important technologies for the massive deployment of IoT. It can bring IoT to unpowered objects with its ability to connect the unconnected. In addition, NFC can put IoT devices under a user's control and is easy to use with its \"tap-and-go\" nature. In particular, green NFC sensors based on energy harvesting can help in the design of a new generation of low-cost smart wearables and in the simplification of the man-machine interface, which opens the door to cooperative IoT for smart cities and Industry 4.0 applications. Batteries in many electronic devices should be managed as hazardous waste because of their toxic contents or reactive properties [5]. In this context, green electronics technology provides solutions that are well suited to the broad needs of an energy-efficient society. Ambient energy harvesting is the process whereby energy is converted from the environment and stored for use in electronicapplications [6]. This technology is usually applied to energy harvesting for low-power and small autonomous devices such as wireless sensor networks. Numerous sources, including solar power, ocean waves, piezoelectricity, thermoelectricity, and physical motion (active/passive human power), are available for energy scavenging [6]. Energy can be harvested from existing ambient radio-frequency sources (cellular networks, etc.). However, as available power is low and depends on distance and source availability [7], a long time is needed to charge the storage device (batteries or supercapacitors) before the device becomes operational. A review of the literature shows that no single power source is sufficient for all applications and that the energy sources must be selected according to the characteristics of the application [6]. From the energy-harvesting point of view, passive RFID systems can be considered devices that scavenge energy from an intentional RF source (the reader). Another technology related to energy harvesting is wireless power transfer (WPT) for battery charging using inductive links at high frequency (HF). Wireless battery charging was recently standardized by the Wireless Power Consortium (WPC) [8]. It became popular in modern smartphones [9] and was proposed for electric-vehicle charging [10]. In this context, interest in batteryless (passive) RFID sensors grew in the last few years. Ultra-high frequency (UHF) RFID technology is now commercially available and is used in logistics, inventory, and supply-chain applications. Although UHF readers are expensive ($1000-$2000), the inlays are cheap, and return on investment (ROI) is achieved thanks to a large number of units and the benefits associated with traceability [11]. Improvements in chip integrated circuit (IC) sensitivity (e.g., \(-22\) dBm for Impinj Monza R6) allows reaching several meters (about 15 m considering free space and read mode).