# Potential of Sub-GHz Wireless for Future IoT Wearables and Design of Compact 915 MHz Antenna
## 2 Comparison of Wireless Performance in the 915 MHz and 2.45 GHz ISM Bands
### Wireless Link Characteristics  
The application proposed in this work concerns IoT smart wearable sensor devices used for medical monitoring applications. This class of applications is referred to as Body Area Networks (BANs). Different possible wireless communication scenarios have been identified from the IEEE 802.15 WPAN Task Group 6 (TG6) Body Area Networks [25], such as the radio channel between implanted devices, implanted device to body surface, implanted device to external, body surface to body surface and body surface to external radio channel [26]. The latter represents the scenario of interest in the proposed system and it is depicted in Figure 2 where the transmitting device Tx represents the wrist-worn wireless SpO2 sensor. Tx communicates with a remote receiver Rx, placed at a given distance \(d\) by means of Tx Antenna and Rx Antenna. Rx is also connected to the Internet. The transmitter and receiver antenna gains are denoted \(G_{TX}\) and \(G_{RX}\), the transmitted and received power are denoted \(P_{TX}\) and \(P_{RX}\). Initially, the free-space path-loss is evaluated considering the scenario depicted in Figure 2 for the two frequency bands under investigation. To this aim, referring to Figure 2, it has been assumed that the communication between two antennas Tx and Rx take place in line-of-sight and in free-space scenarios, moreover the distance between Tx and Rx antenna is \(d\) = 3 m. The FSPL have been calculated using the following equation [27; 28],  
\[\mathrm{FSPL(dB)}\ =\ 10\ \log_{10}\biggl{(}\frac{4\pi d}{\lambda_{0}}\biggr{)}^{2}. \tag{1}\]  
From (1), the FSPL when the system operates at a frequency of 915 MHz, \(\mathrm{FSPL}_{915\mathrm{MHz}}\) is 41.2 dB, and for an operating frequency of 2.45  GHz, \(\mathrm{FSPL}_{2.45\mathrm{GHz}}\) is 49.8 dB and \(\lambda_{0}\) is the free-space wavelength. Therefore, under the same conditions the FSPL for an operating frequency of 2.45  GHz is 8.6 dB greater than the case of an operative frequency of 915 MHz. Hence, in the first case the transmitted signals experience a more severe attenuation than the latter. With respect to the form factor, in the proposed application the transmitting antenna needs to be integrated in a wrist-worn wireless SpO2 sensor device of finite dimensions. However, in [29] the fundamental performance limits of antennas with respect to electrical size have been investigated. This work relates the maximum theoretical gain achievable from an antenna given to its physical size and its working frequency. Referring to Figure 2, the maximum transmitting antenna dimension is \(2a\), where \(a\) represents the minimum radius of a sphere enclosing the antenna element [29]. In this specific case, dimension \(a\) is limited to 22 mm as this is a physical constraint of the wrist-worn wireless SpO2 sensor depicted in Figure 1. It follows an analysis of the impact of the form factor constraints on the antenna performance for the transmitting antenna gain \(G_{\mathrm{TX}}\) using [29]  
\[G_{TX}\ =\ (\beta a)^{2}\ +\ 2(\beta a),\ \mathrm{where}\ \beta\ =\ 2\pi/\lambda_{0}. \tag{2}\]  
In particular, the maximum theoretical gain using (2) for the Tx antenna at 915 MHz is \(G_{TX\_915 MHz}\ =\ +0.09\) dBi while the maximum theoretical gain at 2.45  GHz is \(G_{TX\_2.45 GHz}=+5.48\) dBi. These calculations show that theoretically, given the form factor constraints above, the maximum gain achievable by an antenna resonant at 915 MHz is around 5.4 dBi lower than the maximum theoretical gain of an antenna resonant at 2.45 GHz. Considering the Tx antenna gains \(G_{TX\_915 MHz}\) and \(G_{TX\_2.45 GHz}\) and the FSPL calculated above from (1) for both the frequencies under test it is possible to calculate the received power from Rx antenna using the Friis equation [30] reported below.