# Potential of Sub-GHz Wireless for Future IoT Wearables and Design of Compact 915 MHz Antenna
## 1 Introduction  
The continuous growth of the number of wireless sensor-actuator nodes has led to the creation of the Internet of Things (IoT) [1]. Among them, wearable devices represent a significant proportion. In fact, according to the forecasts, in 2021 there will be 928 million connected wearable units worldwide [2], and the revenue worldwide in the same year is expected to be valued at 95,270.0 million USD [3]. Specifically, medical wearables represent a rapidly growing market [4]. At the present time, a large number of commercial wearable health and fitness monitors incorporate heart rate monitoring (HRM) [5; 6; 7; 8; 9]. The addition of SpO2 can provide new functionality to predict activities such as human kinetic and sleep patterns for medical and sports applications. The main principle behind an optical SpO2 sensor uses a photoplethysmography (PPG) technique to detect blood volume changes in the micro-vascular bed of tissue. Pulse oximetry is a noninvasive method for accurately estimating blood oxygen saturation (SpO2) in the human body. In terms of realizing wearable sensors, wireless communications capability is a key enabling technology. This is especially true in IoT applications as it enables information exchange with thecloud for data collection, analysis and provision of near real-time responses [10] and addressing the future increasing demands of IoT-based health-care services [11]. In this paper, the design of a wrist-worn wireless SpO2 sensor is proposed. Figure 1 presents a system block diagram composed of an SpO2 sensor, a processing block, a radio transceiver (XCVR) and an antenna as well as a battery and power management block. The key focus of this work concerns the radio XCVR and antenna. A key challenge concerns the antenna, which is constrained in size by the dimensions of the overall system, and which can accommodate a board of dimensions approximately 44 mm \(\times\) 28 mm. At the present time, the 2.45  GHz ISM band [12] is invariably used for the provision of IoT wireless connectivity for medical wearable applications as it offers advantages such as small antenna size and availability of a large number of wireless standards e.g. Bluetooth [13], Zigbee [14] and Wi-Fi [15]. Other wireless bands for medical applications include the Medical Device Radiocommunications Service (MedRadio) band [16] and 915 MHz ISM band [12] in United States, the 433 MHz ISM band and the 869.5 MHz Short Range Device band in Europe [17]. These frequency bands can offer potential performance advantages in terms of reduced path loss, band congestion and channel stability that we investigate in this work. In the literature, several works have been published that proposes integrated antenna designs for wrist-worn devices such as smart watches, where a large number are proposed for a working frequency of 2.45  GHz, e.g., [18; 19; 20]. In particular, in these works, the antenna dimensions are comparable to the dimensions available for the radio XCVR plus antenna board in the wrist-worn wireless SpO2 sensor proposed. Several examples of antenna designs for wrist-worn devices operating also in Sub-GHz frequencies are reported in [21]. These designs use watch Printed Circuit Board (PCB) of dimensions 40 \(\times\) 40 mm\({}^{2}\) with a metal belt of dimensions \(\sim\)100 \(\times\) 20 mm\({}^{2}\) as an integral part of the antenna. Figure 1: Wrist-worn wireless SpO2 sensor in final configuration with corresponding system block diagram. The interaction between human body and antenna is a key issue with wearable wireless devices. In fact, the human body is characterized by a specific conductivity and by several layers of material with different thicknesses and different dielectric permittivity. These parameters are also frequency dependent and when the antenna is placed in proximity of the human body it can experience a variation of the impedance characteristics, namely frequency detuning, with respect to a free-space scenario [22].