# Potential of Sub-GHz Wireless for Future IoT Wearables and Design of Compact 915 MHz Antenna
## 6 Discussion  
To evaluate the potential of the 915 MHz ISM band with respect to the 2.45 GHz ISM band, in Section 2.1 a calculation of the link budget at 915 MHz and 2.45 GHz considering the theoretical gain defined by the size constraints of the wrist-worn wireless SpO2 sensor has been performed. In this Section, the same calculation will be proposed but considering measured gain in free-space of the proposed antenna for the 915 MHz case compared with three antennas proposed in the literature, i.e., in [18; 19; 20], for the 2.45 GHz case. Moreover, the comparison is extended also to the on-body case. Note also that the antennas taken into account for comparison are characterized by a form factor slightly bigger than the proposed antenna. Specifically, in [18] [PERSON] and [PERSON] demonstrate a 2.45 GHz loop antenna of dimensions \(40\,\times\,50\,\times\,5\) mm\({}^{3}\) and Peak Gain of approximately 3.8 dBi at the operating frequency \(f_{0}\) in free space. In this work is reported also simulated Peak Realized Gain value of \(-0.89\) dBi at \(f_{0}\) of the antenna on-body. In [19], [PERSON] and [PERSON] present a cavity-backed annular slot antenna working at 2.45 GHz with dimensions \(\pi\,\times\,21^{2}\,\times\,10\) mm\({}^{3}\) and peak realized gain of approximately 3.5 dBi and 2.5 dBi at \(f_{0}\) for free-space and on-body scenario respectively. Finally, in [20], [PERSON], [PERSON] et al., propose a 2.45 GHz internal shorted monopole antenna of dimensions \(25\,\times\,35\,\times\,4\) mm\({}^{3}\) and peak realized gain of 1.5 dBi at \(f_{0}\). The scenario and conditions adopted in the calculation of Section 2.1 has been considered, namely a distance between transmitting antenna and receiving antenna \(d=3\) m, a receiving antenna characterized by a specified gain \(G_{RX}\) of 0 dBi and a transmitted power \(P_{TX}=0\) dBm. A summary of the calculations performed using (3) is reported in Table 9. In particular, \(G_{TX-FS}\) is the peak realized gain of transmitting antenna at \(f_{0}\) in free-space scenario, \(G_{TX-OB}\) is the peak realized gain at \(f_{0}\) of the transmitting antenna on the body, \(P_{RX-FS}\) is the calculated received power in the in free-space scenario and \(P_{RX-OB}\) is the calculated received power in the on-body scenario. This comparison shows improved performance in terms of link budget at 915 MHz when compared with 2.45 GHz in the free-space. Specifically, the difference in received power ranges from 2.4 to 4.7 dB. As for the on-body scenario, the difference between the calculated received power in the 915 MHz case and in the 2.45 GHz case ranges from 0 to 3.4 dB. This shows that similar performance can be achieved at 915 MHz and 2.45 GHz under the assumption that the reader gain and received power remain the same. \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Measured** & **Simulated** & **\(\Delta\) (dB)** \\ \hline Peak Realized Gain (dBi) & \(-4.87\) & \(-6.09\) & \(1.22\) \\ Directivity (dBi) & 2.33 & 3.71 & \(1.38\) \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Comparison between measured and simulated radiation characteristics of the antenna on the IXB-063 phantom column. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Antenna** & **Dimensions** & **\(f_{0}\) (MHz)** & \(G_{TX-FS}\) & \(G_{TX-OB}\) & \(P_{RX-FS}\) & \(P_{RX-OB}\) \\  & **Description** & **(mm\({}^{3}\))** & **(dBi)** & **(dBi)** & **(dBi)** & **(dBm)** & **(dBm)** \\ \hline
[18] & Loop & \(40\,\times\,50\,\times\,5\) & \(2450\,\) & \(\sim\,3.8\) & \(-0.89\,^{1}\) & –46 & –50.7 \\ \hline
[19] & Cavity-backed annular slot & \(\pi\,\times\,21^{2}\,\times\,10\) & \(2450\,\) & \(\sim\,3.5\) & \(\sim\,2.5\) & –46.3 & –47.3 \\ \hline
[20] & Internal shorted monopole & \(25\,\times\,35\,\times\,4\) & \(2450\,\) & \(+1.5\) & – & –48.3 & – \\ \hline Proposed & Curved Inverted-F & \(44\,\times\,28\,\times\,1.6\) & \(915\,\) & \(-2.37\) & -6.1 & –43.6 & –47.3 \\ \hline \hline \end{tabular}
\end{table}