## 4 Features Selection and Analysis
### Features Description  
The Bio-Radar signal features were selected taking into account the ones typically used in the literature [21; 26; 36; 37; 38; 39; 40], resulting in the following list:  
1. **L:** Amplitude of the respiratory signal;
2. **med ibi:** Interprethath Interval (IBI) median;
3. **mean d1 signal resp:** Mean of the respiratory signal first derivative;
4. **mean Nd1 signal resp:** Mean of the normalized respiratory signal first derivative;
5. **mean d2 signal resp:** Mean of the respiratory signal second derivative;
6. **mean Nd2 signal resp:** Mean of the normalized respiratory signal second derivative;
7. **skewness T inhale:** Skewness of inhalation time;
8. **med T inhale:** Median of inhalation time;
9. **iqr T inhale:** Inter-Quartile Range (IQR) of inhalation time;
10. **mean T inhale:** Mean of inhalation time;
11. **skewness T exhale:** Skewness of exhalation time;
12. **med T exhale:** Median of exhalation time;
13. **iqr T exhale:** IQR of exhalation time;
14. **mean T inhale:** Mean of exhalation time;
15. **RR:** Respiratory Rate;
16. **p01-p06:** Average Power Spectral Density (PSD) energy in the band 0-0.1 Hz;
17. **p02:** PSD energy in the band 0.1-0.2 Hz;
18. **p03:** PSD energy in the band 0.2-0.3 Hz;
19. **p04:** PSD energy in the band 0.3-0.4 Hz;
20. **p05:** PSD energy in the band 0.4-0.9 Hz;
21. **p02:** PSD energy in the band 0.9-1.5 Hz;
22. **med sign:** Signal median;
23. **mean sign:** Signal mean;
24. **iqr sign:** Signal IQR;
25. **iqr ibi:** Interprethath Interval Inter-Quartile Range;
26. **mean ibi:** IBI mean;
27. **skewness ibi:** Skewness of the IBI;
28. **AppEn max:** Signal maximum approximate entropy;
29. **kurtosis:** Respiratory signal kurtosis;
30. **SDNN Resp:** Standard Deviation of IBI (SDNN) of the respiratory signal;
31. **SAM:** Number of samples above the mean value of the waveform;
32. **RMSSD Resp:** Root Mean Square of Successive Differences (RMSSD) between IBI of the respiratory signal;
33. **skewness sign:** Skewness of the signal;
34. **RRV:** Respiratory Rate Variability (RRV);
35. **RMS:** Quadratic mean value of the signal;
36. **freq resp:** Respiratory frequency;
37. **mean peak valley width:** Average of the peaks and valleys width of the respiratory signal in the time domain;  
Figure 6: Sliding window of the feature extraction. 38. **Ratio PSD:** PSD of the ratio between the PSD of the Low Frequency (0.1-0.4 Hz) and High Frequency (0.4-1.5 Hz);
39. **var sign:** Variance of the respiratory signal;
40. **AppEn resp:** Approximate entropy of the respiratory signal. More details regarding the feature extraction are now provided: Feature 1 is determined through the calculation of arc length. Features 2, 25-27, 30, and 34 are derived from the Interbreath Interval. The Interbreath Interval (IBI) refers to the time interval between individual breaths. The Inter-Quartile Range (IQR) is a statistical measure that represents the spread of the middle 50% of the IBI data, specifically the difference between the 75 th and 25 th percentiles. This metric is important in our analysis as it provides insight into the variability and regularity of the breathing pattern. The features related to inspiration and expiration, numbers 7-14, are statistical measures extracted from the peaks and troughs of the respiratory signal. The determination of RR involves calculating the ratio between the number of signal peaks and the time period in which they occur, measured in minutes. This calculation yields the RR in Breaths Per Minute (BrPM). Features 16-21 were computed using the Welch method. The Welch method is a spectral estimation technique that divides the signal into overlapping segments, computes a periodogram for each, and averages them to yield a more accurate and less noisy estimation of the PSD (Figure 7). This method was applied to the signal after removing its average. Similarly to the latter ones, features 3-6, 17-19, 22-24, 28-33, 37, 39, and 40 are also statistical measures concerning the entire respiratory signal.