## 4 Discussion  
### Advantage  
The original PPG based method for the continuous estimation of systolic blood pressure using ADC with a 60 Hz sampling rate and 8-bit resolution is proposed in this paper. The main advantage of the method is its high accuracy, which is achieved using lower sampling rates and less capable hardware than other methods. Since the resolution of SBP estimation is not linear (because the dependency between DPW and SBP is not linear, as explained in \"Systolic Blood Pressure Estimation Model\" section), the mean difference MD and standard deviation SD is used to evaluate the accuracy of the proposed method. The mean difference MD = \(-0.043\) and standard deviation SD = \(6.79\) matches, or even improve, the results of the known methods (see Table 2). Though the proposed method does not meet the accuracy requirements of the AAMI standard (due to protocol violations), the MD and SD values are within the requirements of this standard (MD = \(\pm 5\) mmHg and SD = \(\pm 8\) mmHg). This shows the great potential of the method and additional experiments are planned in the future work to evaluate full compliance with the AAMI standard. There are many authors performing state-of-the-art research activities in the field of continuous and cuffless blood pressure monitoring. They admit that continuous blood pressure monitoring is a very important activity and propose various non-invasive PPG based methods to solve this problem. However, the main attention is given to the verification of the accuracy of the proposed methods and comparison with related works, while very little or no attention is paid to the possibilities to implement these methods in wearable systems [10; 13; 15; 19; 20; 22; 25]. There are known wearable (portable) blood pressure measurement systems based on smartphones [26; 27], allowing to make measurements at any selected moment. But it is not continuous monitoring, since the idea of the continuous monitoring of physiological parameters (including blood pressure) is that the user does not need to pay any attention to registering those parameters. Therefore, the main distinction of the proposed method in comparison with other state-of-the-art approaches is that this method can be successfully applied for the implementation of a real life wearable continuous SBP monitoring systems. The method is successfully tested using a prototype system based on the widely available and quite cheap CMS50 FW Pulse Oximeter, which is capable of transferring PPG values via Bluetooth to a mobile phone or any other remote device. In order to reduce the number \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Methods**} & \multicolumn{3}{c}{**Systolic Blood Pressure Method Performance**} \\ \cline{2-4}  & **MD** & **SD** & **SPS** \\ \hline Proposed method & \(-0.043\) & \(6.79\) & \(60\) Hz \\ [PERSON] [15] & \(0.37\) & \(5.21\) & \(1\) KHz \\ [PERSON] [13] & \(3.22\) & \(8.02\) & \(1\) KHz \\ [PERSON] [25] & \(-0.37\) & \(4.3\) & \(200\) Hz \\ [PERSON] [22] & \(3.24\) & \(3.47\) & \(125\) Hz \\ [PERSON] [20] & \(0.78\) & \(13.1\) & \(100\) Hz \\ [PERSON] [19] & \(-1.1\) & \(5.7\) & \(100\) Hz \\ [PERSON] [21] & \(-1.67\) & \(2.46\) & \(100\) Hz \\ [PERSON] [10] & Method 1 & \(0.25\) & \(6.7\) & \(125\) Hz \\  & Method 2 & \(0.66\) & \(7.5\) & \(125\) Hz \\  & Method 3 & \(0.19\) & \(4.17\) & \(125\) Hz \\  & Method 4 & \(0.2\) & \(4.73\) & \(125\) Hz \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Evaluation of the performance of the systolic blood pressure estimation methods (MD—mean difference, SD—standard deviation, SPS—samples per second). and complexity of calculations (and energy consumption), the proposed method analyzes the rising edge of the PPG pulse wave only. The rising edge is less than 1/3 of the duration of the PPG pulse wave; therefore, this significantly reduces the calculations compared to the methods which process the entire PPG pulse wave. This is relevant for the development of wearable monitoring systems which have very limited processing and energy resources.