Table 6.

Sixteen frequency-domain features.

Feature	Definition
76. Peak-1	The amplitude of the first peak from the fast Fourier transform of the PPG signal
77. Peak-2	The amplitude of the second peak from the fast Fourier transform of the PPG signal
78. Peak-3	The amplitude of the third peak from the fast Fourier transform of the PPG signal
79. Freq-1	The frequency at which the first peak from the fast Fourier transform of the PPG signal occurred
80. Freq-2	The frequency at which the second peak from the fast Fourier transform of the PPG signal occurred
81. Freq-3	The frequency at which the third peak from the fast Fourier transform of the PPG signal occurred
82. A0–2	Area under the curve from 0 to 2 Hz for the fast Fourier transform of the PPG signal
83. A2–5	Area under the curve from 2 to 5 Hz for the fast Fourier transform of the PPG signal
84. A0–2/A2–5	The ratio of the area under the curve from 0 to 2 Hz to the area under the curve from 2 to 5 Hz
85. Peak-1/peak-2	The ratio of the first peak to the second peak from the fast Fourier transform of the PPG signal
86. Peak-1/peak-3	The ratio of the first peak to the third peak from the fast Fourier transform of the PPG signal
87. Freq-1/freq-2	The ratio of the frequency at first peak to the frequency at second peak from the fast Fourier transform of the PPG signal
88. Freq-1/freq-3	The ratio of the frequency at first peak to the frequency at third peak from the fast Fourier transform of the PPG signal
89. Maximum Frequency	The value of highest frequency in the signal spectrum $f_{max}$
90. Magnitude at Fmax	Signal magnitude at highest frequency X$(f_{max})$
91. Ratio of signal energy	Ratio of signal energy between $(f_{max}\pm \mathsf{\Delta }f$) and the whole spectrum X$(f_{max}\pm \mathsf{\Delta }f)/\sum _{i=0}^{N-1}{X}_i\left(f\right)$