# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 1 Introduction
Consequently, the tag is more tolerant to environmental conditions such as losses or detuning due to the proximity of metals and materials with high permittivity and high losses, such as the body, or multipath fading [12]. In these cases, special designs to improve the read range are required [13; 14]. The read range in write mode is noticeably lower than in read mode due to the higher power consumption needed during the memory writing process, and the write sensitivity is between 2 and 6 dB lower than in read mode, depending on the IC model. This not only permits a digital code to be associated with an object, but also allows RFID tags to be equipped with a large variety of sensors. Examples of UHF RFID sensors based on a modification of the antenna's electromagnetic response [15; 16] or integration of electronic pressure controller (EPC)-compatible integrated circuits with sensor functionalities [17; 18] can be found in the literature. These can operate in passive mode or battery-assisted passive (BAP) mode. The EM4325 from EM Microelectronics integrates a temperature sensor and a serial parallel interface (SPI) connection to an external microcontroller, while the SL900 from Austria Microsystems integrates a temperature sensor and an analog-to-digital converter (ADC). The SPS1M002 from On Semiconductor uses the Magnus\(-\)S2 Sensor IC from RF Micron, which integrates a moisture contact sensor. In this case, changes in antenna detuning due to moisture contact are digitized by the sensor, which can then be read by a standard EPC Gen 2 compliant reader. Like Impinj R6, this IC technology incorporates an input internal tuning capacitance in order to combat the effect of detuning due to materials [19]. Several sensors (temperature, strain, and pressure) based on UHF technology were commercialized by Farsens using a proprietary IC. In passive mode, these UHF tags have read sensitivities on the order of \(-9\) dBm (e.g., EM4325); therefore, the read range is considerably lower (about 2-3 m) than inlay tags without sensor capabilities. Another interesting technology is chipless RFID [20], where the identification code is encoded in the frequency or time domain. The read range is a few centimeters for the frequency-coded tags (\(<\)1 m) or up to 2-3 m for the time-domain-coded tags [21]. Several studies [22; 23; 20; 24] focused on increasing the number of bits to encode in order to compete in cost with chip-based competitive technologies (HF or UHF). The number of bits depends on the frequency bandwidth; thus, chipless RFID is usually designed at ultra-wideband (UWB, 3.1-10.6 GHz). Another important challenge for chipless technology is the lack of standardized commercial low-cost readers. In spite of these challenges for identification, chipless technology aroused much interest in sensor applications [25; 26; 27]. Some challenges can be softened; for example, narrowband readers may be used since the identification can be done by other methods and the number of sensors in the read range may be small. Concerning NFC, the most important NFC IC manufacturers, such as NXP, TI, ST Microelectronics, AMS, and Melexis, recently introduced advanced integrated circuits (IC) with energy-harvesting capabilities [28]. These chips collect part of the energy received by the magnetic field generated at the reader to provide an analog voltage output that can be used to power external electronics such as low-power microcontrollers or sensors. The progressive introduction of these ICs into the market enables the development of low-cost batteryless portable sensors [29; 30]. A comparison of RFID technologies is shown in Table 1. Bluetooth low energy (BLE) is also included as an example of low-power, short-range wireless technology. The availability of low-cost standardized technology and the custom of users to use NFC technology and wireless power transfer makes NFC technology one of the keys to the development of a new generation of green sensors for IoT applications. Recent advances in NFC-based sensor technologies are reviewed in this paper.