“…This task is challenging, and no consistent criteria have been established yet, which often needs to be determined empirically or experimentally. First, the poor signals can be removed by some empirical values, such as limiting systolic blood pressure (SBP) and diastolic blood pressure (DBP) to 80-180 mmHg and 60-130 mmHg, respectively [ 26 , 27 ], deleting the PPG signals with heart rate below 60 beats per minute [ 28 ], and rejecting outliers by using quartile [ 29 ] or 3-sigma criterion [ 30 ]. Moreover, considering the periodicity of PPG, some researches adopted autocorrelation between PPG periodic signals [ 31 ], dynamic time warping [ 32 ], or high-order statistics, such as skewness and kurtosis [ 33 , 34 ], to further standardize the quality of PPG signals - Normalization: converting the amplitude of the PPG signal to the range of [0-1] can simplify and enhance the analysis process of the PPG signal and ensure that the extracted values are meaningful and fair to the subsequent characterization process.
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