Experimental Evaluation of Nonidentical PulseCoupled Oscillators Synchronisation in IEEE 802.15.4 Wireless Sensor Networks In this paper, a concise twostate clock model is proposed to describe the behaviour of drifting internal oscillator clocks with nonidentical and timevarying frequencies, which provides a simple analytical solution of different coupling schemesxe2x80x99 effects on synchronisation performance. The improved PulseCoupled Oscillators (PCO) scheme presented in this paper guarantees the synchronisation on nonidentical and timevarying clocks. Meanwhile, it differs from classical PCO by scheduling the transmission of Syncs (i.e., the Pulses of clock) at different time slots. This collisionfree pulse coupling scheme makes the concept of PCO applicable to wireless sensor networks, as it is practically impossible in realistic wireless networks for multiple oscillators to transmit Pulses at the same time. Furthermore, a proportional controller is adopted to correct the clock, and convergence and stability of the proposed algorithm are theoretically analysed, proved and evaluated by hardware experiments. The experimental results show that the improved PCO scheme can achieve synchronisation with the precision of around 400 xc2xb5s on internal oscillator clocks of about 350, 000 ppm.

Exploiting External Interference for Clock Synchronization Many wireless sensor network (WSN) applications require the synchronization of the networkxe2x80x99s independent node clocks. That synchronization is a challenging problem that often requires energyconstrained devices to exchange many messages.This paper introduces the Interferencebased Clock Synchronization (ICS) protocol, an approach suitable for dense WSNs located in urban environments. ICS uses external interference to synchronize the networkxe2x80x99s nodes to either a networklocal internal or global (e.g., UTC) external time. All of the networkxe2x80x99s nodes observe interference at roughly the same time, and the node possessing the common time transmits its observed pattern to the network. The network nodes then compare and align the received and locallyobserved patterns, and subsequently, they update their clocks to the common time.The protocol is evaluated using both a simulator and hardware. Using this approach, the number of transmitted messages scales linearly with the network size. In the simulator, ICSsynchronized clocks had a mean time difference of 0.3635 ms when compared with the common time. Using hardware, the difference was larger at 4.9388 ms, but much of that difference can likely be attributed to the experimental setup.