Improving the dynamics of off-line time synchronization in wireless sensor networks

Abstract

The time synchronization in wireless sensor networks (WSN) is a well-known issue. One of the approaches to the problem is by using off-line algorithms, meaning that the exchange of signaling messages for synchronization of individual clocks does not take place. Such an approach is typically based on a linear regression, which serves as an estimate of the actual ratio of the clock rates in the network. Due to the ever changing conditions in the network, where the wireless channel reliability and clock rates vary over time, the method needs to adapt dynamically. In the process of adaptation, discontinuities of determined linear regression lines may appear, which causes sudden time offsets. In this article, new procedures are proposed to cope with the varying conditions in such a way as to achieve as seamless transitions as possible. It is shown that unless such procedures are used, time offsets in the order of up to half a second may appear as peaks observed in the transition from one state to another. The proposed method reduces the offsets by an order of magnitude. The effectiveness of proposed solution is demonstrated on a specific case of wireless sensor ECG measurements.