I. Introduction
Blood pressure (BP) is an essential diagnostic parameter for many diseases, e.g. hypertension, and simultaneously a risk factor for many diseases [1], [2]. In turn, photoplethysmogra-phy (PPG) is considered a promising method for non-invasive and continuous BP measurement. It is a relatively a simple, non-invasive, and moreover an unobtrusive method. It allows for the continuous estimation of BP, which is invaluable. It is based on a relation between pulse transit time (PTT) and BP. In some application instead of PTT the pulse arrival time (PAT) is measured (Fig. 1). However, in this case a pre-ejection period (PEP) variability is influencing the analysis and results. In practical application, for example when considering a wearable system for noninvasive BP monitoring, a crucial part is the relation between BP and PTT.