“…Nowadays, PPG sensors are widely used in various physiological applications, such as monitoring heart rate [ 7 , 8 , 9 ], heart rate variability [ 10 , 11 , 12 ], blood pressure [ 13 , 14 , 15 ], and blood oxygen level [ 16 , 17 , 18 ] etc. Moreover, since the hardware of PPG sensors can be manufactured to small sizes with low cost, PPG sensors can be implemented into various wearable applications, such as bracelets [ 19 ], watches [ 20 ], patches [ 21 ], earpieces [ 22 ], rings [ 23 ], etc. Despite many advantages, the ambient light interferences and the motion artifacts are the two great challenges for PPG sensors.…”