# Potential of Sub-GHz Wireless for Future IoT Wearables and Design of Compact 915 MHz Antenna
## 7 Conclusions  
In this paper, the potential of the Sub-GHz wireless communications is considered for future IoT wearable applications. This analysis has been performed to evaluate potential operational frequency bands for a wrist-worn wireless SpO2 sensor. The potential of a wrist-worn wireless SpO2 sensor for future IoT wearable medical applications has also been discussed. From the comparison between the two ISM bands under test, it has been shown that the 915 MHz ISM band has an 8.6 dB smaller free-space path loss when compared with 2.45 GHz. When the antenna gain limitation due to form factor restrictions is considered for the wrist-worn wireless SpO2 sensor, the 915 MHz ISM band offers a comparable link budget when compared to the 2.45 GHz ISM band. When evaluating various wireless protocols and radio transceivers, the results showed that average current consumption is a strong function of sleep current and sampling rate. The calculated results show that 2.45 GHz BLE protocol presents the lowest average current of 0.56 \(\upmu\)A when sampling at 1 sample per second. On the other hand, when the sampling rate is reduced to 1 sample per minute, then Zigbee, BLE and Z-Star RFIC current consumption are approximately the same. Finally, when the sampling rate is reduced to 1 sample per hour or greater, the Sub-GHz Z-Star protocol leads to significantly less current consumption and this sampling rate is suitable for SpO2 measurement. This work also presents an antenna design for the wrist-worn wireless SpO2 sensor. Its dimensions are 44 \(\times\) 28 \(\times\) 1.6 mm\({}^{3}\). It is designed to work both in free-space and on-human wrist scenarios in the 915 MHz ISM band. It presents a measured \(-\)10 dB impedance bandwidth of 55 MHz on the IXB-063 phantom column (used to mimic the effect of the human wrist on the antenna) and of 60 MHz in free space. The antenna is also characterized by a measured peak realized gain at 915 MHz of \(-\)6.1 dBi on IXB-063 phantom column and of \(-\)2.37 dBi in free space. This work also compares the developed antenna design with several other 2.45 GHz wrist-worn antennas from the literature. The results concluded that the designed 915 MHz antenna provides similar performance when compared with 2.45 GHz under the assumption that the reader gain and received power remain the same. The authors have also identified a number of challenges for the use of 915 MHz ISM band in a wrist-worn wireless SpO2 sensor. In fact, the gain figure and the \(-\)10 dB impedance bandwidth of an antenna at 915 MHz are limited by the form factor requirements. However a method to overcome the \(-\)10 dB impedance bandwidth have been proposed and implemented. Moreover, even though the gain figure of a 915 MHz is limited by the antenna dimensions, this frequency offers a lower free-space loss. In conclusion, the use of Sub-GHz frequencies for a wrist-worn SpO2 sensor is challenging due to physical size constraints that effects the antenna performance. However, the use of the 915 MHz Z-Star protocol shows that low power consumption can be achieved when compared with other wireless protocols under the constraint of requiring a maximum sampling rate of approximately 1 sample per minute. Future work regarding characterization of the final system implementation will be concerned with a detailed analysis of measured performance with respect to current consumption as well as a detailed investigation of human body effects on antenna performance. This publication has emanated from research funding provided by Sanmina Corporation and in part by a research grant from Science Foundation Ireland (SFI), co-funded under the European Regional Development Fund under Grant Number 13/RC/2077. We would like to acknowledge also [PERSON] from Microsemi Corporation for technical support.