# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 2 NFC Energy Harvesting  
In batteryless mode, the tag is fully passive. In this mode, the NFC-enabled sensor harvests energy from incoming RF emissions (from a reader) to power the sensor interface and RF transmissions. In battery-assisted (semi-passive) mode, the NFC-enabled sensor can operate stand-alone in \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Property** & **Chipless RFID** & **NFC** & **UHF RFID** & **Bluetooth BLE** \\ \hline \multirow{3}{*}{\begin{tabular}{c} Typical read \\ range \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} \textless{}50 cm frequency coded \\ 2-3 m, time-coded UWB \\ \end{tabular} } & 1-2 cm for proximity cards & \begin{tabular}{c} Up to 15 m with nilay tags with \\ \(-\)22 dBm read IC sensitivity. \\ with energy harvesting, \\ 0.5 m for vicinity cards \\ \end{tabular} & 10 m \\  & & 0.5 m for vicinity cards & \begin{tabular}{c} \textless{}9 dBm read IC sensitivity. \\ Up to 30 m UHF sensors (with \\ \(-\)9 dBm read IC sensitivity). \\ Up to 30 m BAP. \\ \end{tabular} & 10 m \\ \hline Power source & Passive & Passive or semi-passive & Passive or semi-passive & Active \\ \hline Tag price & Moderate & Low & Low & High \\ \hline Reader cost & High, no commercial & Low, smartphone & High, $1000-$2000 & Low, smartphone \\ \hline Standard & No & Yes & Yes & Yes \\ \hline Universal frequency regulation & No, often used UWB & Yes, ISM & No, by regions & Yes, ISM \\ \hline Tag size & Large & Medium & Medium & Small \\ \hline Memory capacity & \textless{}40 bits & \textless{}64 kilobits & \begin{tabular}{c} 96 bits EPC, typically 512 bits for \\ users (\textless{}64 kbytes) \\ \end{tabular} & \
\begin{tabular}{c} Several kilobytes \\ depending on the \\ microcontroller \\ \end{tabular} \\ \hline ID rewritable & No & Yes & Yes & Yes \\ \hline Energy harvesting & No & Approx. 10 mW &
\begin{tabular}{c} Few \\ \(\mu\)W \\ \end{tabular} & No \\ \hline Tag substrate & Low loss microwave & Low cost or FR4 & Low cost or FR4 & FR4 \\ \hline Tag flexibility & Depends on the substrate & Depends on the substrate & Depends on the substrate & No \\ \hline Tag robustness & High & Low (inlays) & Low (inlays) & Moderate \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of radio-frequency identification (RFID) sensor technologies. NFC—near-field communication; UHF—ultra-high frequency; BLE—Bluetooth low energy; UWB—ultra-wideband; IC—integrated circuit; BAP—battery-assisted passive; ISM—industrial, scientific, and medical. applications requiring autonomous and continuous monitoring working as data loggers. The life of a sensor tag may include operation in both modes: in semi-passive mode until the battery is exhausted, and thereafter, in passive mode. Data are stored in non-volatile memory and retained when the device is not powered. Figure 1 shows the block diagram of an NFC-based data logger assisted with a complementary energy source (e.g., solar cell). NFC employs electromagnetic induction between two loop antennas. It operates within the globally available unlicensed radio-frequency industrial, scientific, and medical (ISM) band of 13.56 MHz on the ISO/IEC 18000-3 air interface at rates ranging from 106 to 424 kbit/s. Communication in NFC systems is based on inductive coupling between the reader and the tag antennas. The receiver antenna is connected to the internal tag rectifier, which takes energy from the RF field that is used to power up the tag electronics. The internal logic demodulates the amplitude shift keyed (ASK) message from the reader. The tag transponder (which is assumed to be passive) responds, using the passive load modulation technique, by changing the antenna impedance of the tag [1; 31]. The passive load modulation spectrum consists of the RF carrier, two sidebands (at 12.712 MHz and 14.408 MHz), and modulated sidebands on these two subcarrier signals (Figure 2). All the transmitted data are carried in the two sidebands. The 13.56-MHz RF carrier, therefore, does not have to be transmitted by the transponder.