# Portable Sensor System for Registration, Processing and Mathematical Analysis of PPG Signals
## 4 Discussion  
The created experimental device is suitable for continuous observation of the subjects studied, is easy to use, can be placed in different places on the human body, and allows for measuring different parameters, such as: pulse, blood pressure, oxygen saturation, etc. The device created is in line with the desire of the scientific community around the world to use modern technologies at the service of humanity [66]. From Figure 4, it is observed that the peak R in the ECG signal does not coincide with the onset of pulse wave propagation at the three recorded photoplethysmographic signals. This is because the peak R of the ECG signal corresponds to the electrical excitation of the heart muscle, after which it takes some time for the muscle to respond to this excitation, so that the aortic valve opens and the pulse wave begins to propagate through the arteries. The delay in the pulse wave of the PPG signal that was recorded by the sensor placed in the left ear is less than the delay in the PPG signals recorded by the other two sensors due to the greater proximity to the heart. The graphs of the four studied signals (Figure 5) corresponding to the RR and PP interval series of a healthy individual while using the non-linear Poincare mathematical method are identical and well expressed ellipses, which is evidence that the studied data correspond to a healthy individual. The shape of the graphs has the appearance of a comet, which is further proof that the data examined relates to a healthy individual. Similar results have been reported in the publications [67; 68]. Based on the results that are shown in Table 3, it follows that the RR and PP intervals of the two signal types (ECG and PPG) are identical. PPG signals versus ECG signals have a very small mean square error (MSE). The MSE for the signal recorded by the sensor placed on the ear mussel is smaller than the MSE for the signals that are registered by the fingers (index finger and middle finger) of the test subject. The reason for this is due to the following physiological features:  
* the muscle of the ear is smaller than the finger of the hand;
* on the ear mussel no cartilage tissue; and,
* the pulse wave reaches the ear sensor faster than the sensor placed on the fingers due to the greater proximity to the heart. From the experimental studies that were performed on the operation of the experimental, portable PPG device by comparison with the second reference method--electrocardiogram, it follows that the device is applicable for recording PPG signals and that the mathematical data that were obtained from them correspond to RR intervals. Studies that were conducted by the authors on PPG data from healthy people and patients with arrhythmia show a decrease in the value of PRV in the presence of disease. It could be suggested that a significant or sudden decrease in these values is a sign of a sudden worsening of the disease and the need for emergency measures to protect the health and life of the patient. For this reason, an organized system can be especially valuable when monitoring patients in conditions that are close to critical. Arrhythmia is one of the serious diseases of the human cardiovascular system, leading to serious disorders of the heart rhythm. When there are major deviations from the heart rate, a pacemaker might need to be used to artificially regulate the heart's activity. Every year, millions of people worldwide are hospitalized as a result of the disease. For these reasons, the authors believe that the early detection of this disease, its constant monitoring, and the maintenance of optimally effective treatment of patients is essential. Wearing an individual, inexpensive, the lightweight biomedical device that, in an almost imperceptible way, to measure at regular intervals (or continuously as needed) cardiac activity could be of great benefit to patients, can guarantee a low risk to their health, and improve overall health. The linear methods, such as Time-Domain and Frequency-Domain, can be used to directly estimate PRV.