Timing synchronization of low power wireless sensor nodes with largely differing clock frequencies and variable synchronization intervals

In this paper a novel synchronization method for wireless sensor networks with star topology is presented. We address timing synchronization using low frequency real-time clocks in all nodes. A beacon-driven TDMA-protocol for bidirectional node/base communication is used. Between the beacons, which are sent by the base station, lie the superframe time intervals to handle data transmission from node to base. We discuss the protocol and its energy saving advantages including the challenges of synchronization. We reduce the required communication for synchronization based on long term synchronicity of the node to save energy. Due to the individual node clock, the accurate superframe time interval usually will consist of a rational number of clock ticks. We propose to use a ΔΣ-converter to generate a sequence of superframes with different time durations, but each consisting of integer multiples of clock ticks, which - on average - achieve the accurate superframe duration for any rational number of clock ticks. We show by theory and measurements that our novel approach leads to a variance of the synchronization error which is constant at a value of 0.25 clock cycles. The variance is independent of the rate at which the nodes listen to the beacon of the base station.