# Portable Sensor System for Registration, Processing and Mathematical Analysis of PPG Signals
## 1 Introduction
### Research Background  
Different types of PPG devices have been developed according to the type of sensors used, according to the location of the device, according to the method of transmission of the recorded data [6]. The use of PPG data for cardiac activity analysis can be very useful in cases where the ECG cannot be taken or when the obtained ECG has large electrical artifacts [14]. The location of the sensor is one of the main factors for recording correct and quality cardiac signals. It has been found that, when a finger sensor is used [6; 15], high amplitude signals are obtained as compared to the amplitude of signals received from other locations of the human body. Sufficiently good quality of the registered PPG signals can be obtained by locating the sensors on the palms, ears, and face [16] of the subject under study. The improvement of PPG technology enables the entry of cardiac recording devices that can be located on the wrist of the hand [17], the ear mussel [18], the forehead [19], the esophagus [20]. The authors of [21] present a PPG device for measuring the patient's blood-oxygen saturation level (SpO2) and pulse rate. The device uses a sensor that is positioned on the patient's finger. The software has been created to process and investigate the received signals in a LabView environment. Different types of sensors and software implementation of linear and nonlinear methods for analyzing PPG signals are presented in the work [22]. A wearable photoplethysmography sensor based device with a microcontroller that is built into the device and its application for assessing mental distress presents the study's authors [23]. The conducted studies are based on an analysis of the pulse rate variability of the obtained PPG data (taken with a sampling rate of 60 Hz and a 22-bit resolution). The algorithms created are built into the microcontroller and can work in real time. In their work, [24] present an experimental model of a PPG based portable device that is designed for the individual observation of patients with cardiovascular disease. The device includes an infrared optical sensor, an analog filter, an operational amplifier, a user interaction module, a power management module, a flash memory, and a microcontroller that performs the analog-to-digital conversion and data transmission. The created device can transmit the received data to the server through a mobile phone to which it is connected. The authors of [25] investigate the use of PPG data for Pulse Rate Variability (PRV) analysis as an alternative to Heart rate variability (HRV [26] for ECG data) under non-stationary conditions. A software system for PPG and HRV analysis has been created in the time domain and frequency domain. Statistical, HRV indices and PRV indices, spectral capacities in LF and HF domain, TF coherence, etc. were determined, analyzed, and compared. The authors present results that are close to each other and only differences in the HF domain are observed. In the study [27], the authors analyze PPG data that were obtained from the right forefinger of healthy people and propose a method for improving the accuracy of blood pressure measurement. The authors of article [28] create a multi-site measurement system for analyzing right-left simultaneous differences in PPG pulse waveforms from the three segmental levels at the ears, thumbs, and toes (a total of six signals registered simultaneously). The system is validated by determining the RMSE coefficient for these signals and showing a high degree of correlation of the studied input signals. The authors investigate the similarities in the timing of signals from the left and right half of the human body and find a high degree of similarity in healthy individuals. In diseased individuals (especially in the presence of pathologies), differences are found that are explained by the presence of the disease in one half of the body (eg in an organ). An established system can be very useful in making clinical evaluations.