# An NFC on Two-Coil WPT Link for Implantable Biomedical Sensors under Ultra-Weak Coupling
## 5 Experimental Results
### Measurement Results
Figure 15: (**a**) ADC samples of the uplink receiving filter output; and (**b**) the correlation results of the matched filter in the uplink demodulator. The coupling coefficient and the PTE of the inductive link are measured at 2 MHz by using a network analyzer (E5071, Keysight, Santa Rosa, CA, USA) through the method introduced in [24; 25]. When measuring the PTE, a 2 k\(\Omega\) resistor is substituted as the load. To measure the BER, a pseudo-random data sequence is transmitted continuously at a speed of 10 kbps in half an hour for each measurement at different coil separations. The uplink and the downlink are measured separately, as they work in half-duplex. When measuring the BER, the PDL is kept as constant as 5 mW by adjusting the transmitted power of the Class E PA.  
To keep the PDL constant, the transmitted power of the Class E PA is adjusted. Thus, the power carrier interference is stronger when the distance is large. The uplink filter of the out-body receiver was saturated by the power carrier when \(d\) approached 70 mm. However, as the transmitted power of the power carrier is increased according to the adaptive power adjustment, the downlink BER keeps below \(10^{-7}\) within our operating range. We measured the PTE with and without the NFC circuits (disconnect and disable the transceivers) separately to estimate the PTE loss caused by the NFC circuits. The measured results are listed in Figure 17. As seen, although the NFC shares the same coil pair with the WPT, the total PTE still maintains a sufficiently acceptable level without significant drop. The coupling coefficient and the power transfer efficiency are measured over different coil misalignments to evaluate the effects of eyeball movements and position displacement of glasses. We classify the coil misalignments into two types. One is the lateral displacement, which corresponds to the changes of the glasses position, and the other is the angular misalignment, which corresponds to the eyeball turning. The measured results are shown in Figure 18. As seen, the coupling coefficient  
Figure 16: The measured uplink BER and the coupling coefficient versus the coil separation, when the PDL is kept constant at 5 mW. Figure 17: The power transfer efficiency of WPT versus coil separation \(d\) with or without connecting the added NFC circuits. and the PTE are not very sensitive to the coil misalignments. This is benefited from the simple coil configuration and the large size of the out-body coil of our system. According to the statistics from our cooperative hospital, the normal turning of human eyeball is within \(\pm 30^{\circ}\), and the normal lateral displacement of the glasses is within 10 mm. Therefore, the normal turning of human eyeball could lead to about an 11% decrease of the coupling coefficient and a 16% decrease of the PTE, and the normal lateral displacement of the glasses could lead to about a 10% decrease of the coupling coefficient and a 14% decrease of the PTE. These effects of the coil misalignments are acceptable to the wireless power transfer and near-field communication of our system.