# Power Autonomy Estimation of Low-Power Sensor for Long-Term ECG Monitoring
## 5 Validation of the Models and Power Consumption Analysis
### Analysis of Sensor Autonomy
The optimal connection interval for the selected sampling interval \(tp_{sample}\) is therefore in our case \(tp_{comm}=14\times tp_{sample}\), where 14 samples are packed in a single ECG report packet. Before providing guidelines for further development of the ECG sensor, additionally to taking into consideration the power anatomy analysis, we also consider the experience gathered from users who bought/reuted the device for themselves [12] or used the device in a renting scheme with weekly turn-around [13]. While most of the experience is beyond the scope of this article, the most important findings are summarized below. The first finding refers to the current autonomy of the sensor as more than sufficient. While the sensor has been mostly used with maximum transmission power and a very low sampling frequency of 128 Hz, its autonomy far surpassed both the autonomy of smartphones, which it was used in combination with, and the period in which people felt comfortable wearing it continuously. While the latter varies from person to person, most people are inclined to remove it after 2 days of use or less. This is not due to the weight of the device worn on one's chest--most people find it completely unobtrusive--but because self-adhesive electrodes are required, which are prone to irritating the skin. Therefore, the autonomy could be reduced to just above 2 days under some usage scenarios. The second finding was that the sensor's sampling frequency of 128 samples/s was often too low for clinical use. The medical personnel examining measurements expressed their wish for more details, which would broaden the specter of potential use of the sensor. Since the sensor is an excellent platform for long-term heart rate variability monitoring, the rate of 250-500 samples/s was taken as a design goal, based on research which excluded lower frequencies as insufficient [33]. Lastly, the processing requirements of the sensor have not increased, so neither the MCU replacement nor a change in its usage pattern was planned. Having the power consumption model available, as well as the above experience, both user-subjective and medical, we are able to make recommendations regarding the sensor redesign. Our recommendations are to increase the sampling frequency to 512 Hz and leave radio transmission power at 0 dB, since its effect on autonomy is insignificant. The sampling frequency increase is required to increase the sensor's capabilities, often required by the medical personnel, and is enabled by the continuous development of more capable smartphone hardware, which no longer struggle with receiving more than 128 samples per second. Furthermore, we envision two scenarios regarding the battery capacity, which can be used to tailor the next version of the sensor. The first scenario is to decrease the battery capacity to 160 mA h, provided that it would also lead to sensor size, weight, and price reduction. While a smaller battery would primarily increase comfort, it might also help reduce the motion-induced measurement artifacts due to reduced sensor weight. Under this scenario, the users would be required to recharge the sensor every 2 days or less, which would still be acceptable since a large portion of users already require a break from the irritable self-adhesive electrodes in nearly the same time. In the second scenario, the battery could be increased to 400 mA h, extending the autonomy to over 5 days at 512 Hz sampling rate, which might be seen as beneficial for health care services where the sensor is lent to a different user every week. The users would therefore not even need a charger, which would make this scenario more user-friendly. The battery would have to be larger and heavier though, which might create some unforeseen adverse effects for the engineers designing the device and for the users wearing it. Given that, so far, no user has complained about the device weight, this might be a path worth exploring.