Synchronization of time in wireless ECG measurement
Andrej Vilhar, Matjaž Depolli · 2017
Wireless devices for ambulatory ECG monitoring are becoming increasingly popular. One of the challenges that wireless monitoring has to deal with is clock synchronization between the wireless sensor device and the controller device. The monitoring device is usually kept as simple as possible to maximize its autonomy, and is thus limited in its clock accuracy. In this article, we describe a method of off-line synchronization of data from a completely asynchronous sensor device. The sensor device sampling frequency is allowed to deviate from its declared value by more than 100 ppm, while the controlling device time is assumed to be absolutely accurate. Data synchronization is based on using two sources of timestamps for the data - the sensor device oscillator and the controller device local time. In our case the controlling device is an Android mobile phone, and the wireless technology used is Bluetooth LE. The challenges are: limited precision of time reported by the sensor device, clustering of messages on Android, and lost messages due to lossy transmission mode. The presented synchronization technique is able to achieve several orders of magnitude improvement in estimation of sample time.