## 1 Introduction  
Diabetes is an incurable chronic disease that occurs either when the pancreas is no longer able to produce insulin, or when the body is unable to utilize insulin properly [1,2]. This results in poor regulation of blood glucose level, which can lead to severe health complications such as chronic heart and kidney disease if blood glucose levels are not monitored carefully. According to the IDF (International Diabetes Federation) Diabetes Atlas [3] published in 2019, 463 million people in the 20-79 demographic have diabetes and is projected to reach 700 million by 2045. More than 4.2 million people died due to diabetes-related disorders in 2019. The conventional approach to glucose level monitoring requires several apparatuses such as a glucometer, a one-time test strip, and a single-use lancet or lancing device to draw blood [4]. It also requires alcohol pads, gloves, and a band-aid to reduce the risk of infection for the patient. It is an uncomfortable process and one that people with diabetes need to use to monitor their blood glucose level regularly. There is a demand for wearable, noninvasive, and smart health monitoring systems [5,6]. Previously, there have been studies conducted on noninvasive [7] or minimally invasive wearable devices based on different types of technologies for blood glucose level monitoring [8]. Some of the researchers used sensors built using carbon nanotubes [9,10], fluorescence [11], and plasmonic materials [12] to determine the glucose level. These tech niques had some issues such as long calibration processes, size, and susceptibility to autofluorescence. Several electrical techniques have been proposed for blood glucose level monitoring. [PERSON], [PERSON] et al. experimented with a millimeter-wave radar system to track glucose level changes [13]. Similarly, [PERSON], [PERSON] et al. experimented with glucose monitoring using electromagnetic devices in a noninvasive manner [14]. Other researchers utilized changes in bioimpedance [15] and ultrasound signal [16; 17] to determine a subject's blood glucose level. In general, electrical techniques have room for improvement due to poor selectivity, temperature sensitivity, and skin irritation. A handful of previous studies have explored the potential of optical techniques in tracking changes in blood glucose level. [PERSON], [PERSON] [18] studied a polarimetric optical measurement method for noninvasive glucose determination. Optical Coherence Tomography (OCT) [19], near-infrared spectroscopy [20], and mid-infrared (MIR) spectroscopy [21] have also been utilized under laboratory conditions to gauge changes in blood glucose level in diabetic subjects. Unfortunately, optical techniques suffer from a significant lag time of up to 30 min as well as high temperature sensitivity. It is noteworthy that these proposed techniques require intricately organized and expensive laboratory equipment, making it unlikely for them to be used for daily measurement of blood glucose by millions of diabetics without significant reduction in price and complexity. Sensor-based noninvasive techniques have also been proposed for this purpose. Studies have shown the possibility of using GSR (Galvanic Skin Response) sensors [22], saliva [23; 24], tear-based sensors [25], and sweat-based sensors [26] in order to measure blood glucose level. These techniques are generally inexpensive and require biological fluid samples other than blood to be collected, thereby reducing the invasiveness of the procedure and the subject's discomfort. Photoplethysmography (PPG) is a low-cost, noninvasive technique that measures the volumetric change of blood in the arteries. Previously researchers have investigated the estimation of heart rate using PPG signals acquired by various methods such as endo-cavity sensors [27] and pressure sensors [28]. Some researchers determined the glucose level using smartphone-based on the colorimetric and electrochemical methods [29]--this required an external apparatus to collect the sample before analyzing the sample using a smartphone.