Proportional–Integral Synchronization for Nonidentical Wireless Packet-Coupled Oscillators With Delays

Abstract

Precise timing among wireless sensor nodes is a key enabling technology for time-sensitive industrial Wireless Sensor Networks (WSNs). However, the accuracy of timing is degraded by manufacturing tolerance, ageing of crystal oscillators, and communication delays. This paper develops a framework of Packet-Coupled Oscillators (PkCOs) to characterise the dynamics of communication and time synchronisation of clocks in WSNs. The nonidentical clock is derived to describe the embedded clock's behaviour accurately. A Proportional-Integral (PI) packet coupling scheme is proposed for synchronising networked embedded clocks, while, scheduling wireless Sync packets to different slots for transmission. It also possesses the feature of automatically eliminating the effects of unknown processing delay, which further improves the synchronisation performance. The rigorous theoretical analysis of PI-based PkCOs is presented via studying a closed-loop time synchronisation system. The performance of PI-based PkCOs is evaluated on a hardware testbed of IEEE 802.15.4 WSN. The experimental results show that the precision of the proportional-integral PkCOs protocol is as high as 60µs (i.e. 2 ticks) for 32.768kHz crystal oscillator-based clocks. Index Terms-Time synchronisation, packet-coupled oscillators, wireless sensor networks. I. INTRODUCTIONN Owadays, with the ever-growing developments in microelectro-mechanical systems (MEMS) technology, wireless communication and digital electronics, the manufacturing of low-cost and low-power tiny sensor nodes becomes feasible. These sensor nodes are usually deployed in an area to form a network, which is called the Wireless Sensor Network (WSN), in order to monitor and collect environmental information, and realise different applications. In WSNs, thanks to its tradeoff between high-quality signal accuracy and cost, the crystal oscillator is widely chosen as the clock source [1]. However, it fails to produce the same frequency, owing to internal factors (e.g. manufacturing tolerance [2]) and external environmental conditions, such as temperature [3] and supply voltage, and a common sense of timing does not exist in the network without a management protocol. As a result, a Time Synchronisation Manuscript

PkCOs: Synchronization of Packet-Coupled Oscillators in Blast Wave Monitoring Networks

“…Due to the limitation of RF communication, the packet exchange delay occurs between the transmission and reception of wireless packets. There exists a varying processing delay for calculation and register access, because of the processor's architecture and the impossibility of real-time computing [21]. However, the clock correction strategy in the PCO model fails to compensate for the effects of these two delays.…”

“…However, the time t[n] fails to model the clock's resetting feature. From [21], the clock state P 0 [n] is adopted to describe the behaviour of clock resetting, and P 0 [n] at the n-th event is given as…”

“…Owing to internal factors and environmental conditions, the i-th clock state P i [n] cannot be the same as the master clock state P 0 [n]. According to [21], the state P i [n] of the i-th sensor node is modelled as…”

“…Based on [21], the value of clock state increment within the delay duration κ i [k] is the summation of the value of delay κ i [k] and the extra value δ κi [k] dependent on the length of packet exchange delay. Thus, the measurement equation (10) [see Fig.…”

“…1b) [7]. Thus, the PCO model needs to be improved, and the PkCOs framework was proposed in earlier work [21]. For the PkCOs algorithm, we model each clock as an oscillator moving clockwise on the unit circle, as indicated in Fig.…”