# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 2 NFC Energy Harvesting
### Link Budget  
#### 2.1.1 Forward Link  
To establish communication between the reader and the tag (forward link) and to ensure enough power for the RF to direct current (DC) conversion to feed the electronic circuitry, the aim is to maximize power transfer from the reader to the tag. The power delivered to the tag IC (\(P_{d}\)) must, therefore, be above a threshold power (\(P_{th}\)):  
\[P_{d}=P_{s}G_{T}(f_{c})>P_{th}, \tag{1}\]  
where \(P_{s}\) is the power transmitted at the reader, and \(G_{T}\) is the available gain at the center frequency, \(f_{c}\). To this end, efficiency must be maximized. The system is modeled as shown in Figure 3. Maximum efficiency is obtained from the available gain under matching conditions, \(G_{Tmax}\). This efficiency can be computed from the S-parameter measurements and expressed as a function of the parameter, \(\chi=k^{2}Q_{1}Q_{2}\), where \(k\) is the magnetic coupling between the reader and tag coils (\(k=M/\sqrt{L_{1}L_{2}}\)) and \(Q_{1}\) and \(Q_{2}\) are the quality factors of the reader and tag coils, respectively [32].  
\[\frac{P_{dmax}}{P_{s}}=\eta_{max}=G_{Tmax}=\frac{\chi}{\left(1+\sqrt{1+\chi} \right)^{2}}. \tag{2}\]  
Power transfer can, therefore, be maximized by increasing the quality factor of the antennas or increasing the coupling that is a function of the distance between the antennas. However, a high \(Q\) factor would lead to limited bandwidth (see Figure 2) and long-time constants, causing severe distortion in the modulated signal (see Figure 4).  
To ensure that the communication works properly, the maximum value of the \(Q\) factor of the initiator antenna must be such that the bandwidth B (at \(-3\) dB), which is equal to \(f_{c}/Q\), is at least capable of channeling all the frequencies contained in the spectrum of the signal modulating the carrier frequency. The bandwidth of the forward link is the bandwidth of the modulation sidebands of the carrier and is dependent on the modulation scheme used by the reader. In the worst-case scenario (corresponding to the maximum value, \(Q_{1 max}\), of the initiator antenna circuit), the fundamental bit rate of the square-wave signals (with a cyclic ratio of 50% in the case of non-return-to-zero (NRZ) bit coding) of that digital data stream must, therefore, be at least equal to half of the bandwidth B of the tuned circuit. The result is that \(Q_{1 max}\) is limited to \(f_{c}/(2\times\) bit rate). Unfortunately, for NFC uplink ISO 18092 (or ISO 14443, type A), in order to ensure maximum energy transference, the carrier is modulated  
Figure 3: (a) Two-port transfer model between reader and tag; (b) tag model.  
using ASK (with 100% of the modulation index) and modified Miller coding. In this modified Miller bit coding, a pause on the carrier frequency of duration, \(T_{p}\), is made (see the top of Figure 4). This pause, \(T_{p}\), is equivalent to the transmission of a frequency whose period is \(2T_{p}\), which is an equivalent bit rate of \((1/2T_{p})\). Another interpretation in the time domain can be made. Figure 4 shows the amplitude envelope that decreases exponentially with a time constant, \(\tau=Q/\pi f_{c}\). Assuming that the envelope will vanish after a few time constants, the reader \(Q\) factor is limited by Equation (3) [31].  
\[Q_{1 max}=f_{c}\;\times\;T_{p}. \tag{3}\]  
According to NFC forum standard ISO 14443, the quality factor is limited to 40 (35 considering design tolerances) at 106 kbit/s bit-rate transfers [31]. For applications that use NFC IP2-ISO 21481 with the authorized use of ISO 15693 and NFC-V targets, whatever the bit rate, the shortest time present in the uplink communication protocol is a \"pause\" lasting \(T_{p}=9.44\)\(\upmu\)s. In this case, \(Q_{1 max}=128\), which generally can be reduced to \(Q_{1 max}\) usable = 100 assuming design tolerances. Generally, these values of \(Q_{1 max}\) usable are not difficult to obtain and are easy to reduce using serial resistors.