## 1 Introduction  
It is very important to be able to determine the moment when physiological parameters deviate from normal levels in order to prevent chronic diseases. The dynamics of observed physiological parameters are more important over the long-run than accurate instant values, which are used in an emergency or cases of critical health disorders. It is important to detect the degree of a physiological parameter's deviation from normal during a day, week, month or even a year, as well as the length of such a deviation. If deviation persists for a long time, this can suggest the risk of irreversible changes. Timely detection of such dangerous deviations from normal and an immediate start of treatment can help to avoid negative and irreversible health changes. Therefore, the long-term constant observation of physiological parameters, such as blood pressure allows us to reduce the healthcare costs, as discussed in Reference [1], minimizes the risk of visiting secondary healthcare institutions, and ensures a better quality of life for a longer period. Blood pressure measurements made at home indicate the state of the cardiovascular system better than those obtained during regular visits to a doctor, as discussed in Reference [2]. A very significant proportion of chronic diseases are cardiovascular diseases. Atherosclerosis and endothelial function changes are accompanied by a rise in blood pressure. As suggested in Reference [3], increasing blood pressure values, measured at home, can be considered as a prognostic sign of atherosclerosis or endothelial dysfunction. Long-term blood pressure variability, obtained during daily home blood pressure measurement (HBP), can show internal organ damage or risk of cardiovascular events, as reported in Reference [4]. The authors of [5] show that increased blood pressure is a potential dementia illness risk factor; thus, registering the dynamics of blood pressure for elderly people can serve as a preventive mental health disorder measure. Hence, the continuous monitoring of blood pressure is critical for the prevention and diagnosis of hypertension and cardiovascular diseases. Both SBP and diastolic blood pressure (DBP) are important to assess the state of the cardiovascular system, but according to References [6; 7; 8], systolic hypertension is much more common than diastolic hypertension, and systolic blood pressure contributes more to the huge global disease burden attributable to hypertension than diastolic pressure. Systolic blood pressure is much more important for elderly people and their health state evaluation, as discussed in References [9; 10; 11; 12]. Since elderly people are the major group of geriatrics patients, which are subjects of this study, the estimation of systolic blood pressure is the main focus of this paper. The traditional auscultatory blood pressure measurement method is not continuous and is quite uncomfortable, because it uses a cuff wrapped around the arm which is inflated repeatedly. To address this problem, continuous blood pressure measurement methods have been developed recently. The most popular methods for continuous and cuffless blood pressure monitoring are based on pulse transit time (PTT) [13; 14; 15; 16; 17; 18]. However, PTT is calculated using two signals (usually electrocardiogram and pulse wave), which is quite difficult to implement in wearable devices for daily use. Blood pressure estimation methods which are based on one-channel pulse wave measurements are less complex and allow us to create wearable devices which are more comfortable for the end users. A pulse wave is often obtained using the photoplethysmogram (PPG) optical measurement method. Blood pressure measurement methods based on one-channel PPG are implemented using stationary medical devices [10; 19; 20; 21; 22], original PPG measurement devices, such as those in References [23; 24; 25], or even smartphones [26; 27]. The smartphone approach is not truly continuous, since the user does not wear a PPG sensor.