Abstract

Recently, wireless sensor networks (WSNs) have drawn great interest due to their outstanding monitoring and management potential in medical, environmental and industrial applications. Most of the applications that employ WSNs demand all of the sensor nodes to run on a common time scale, a requirement that highlights the importance of clock synchronization. The clock synchronization problem in WSNs is inherently related to parameter estimation. The accuracy of clock synchronization algorithms depends essentially on the statistical properties of the parameter estimation algorithms. Recently, studies dedicated to the estimation of synchronization parameters, such as clock offset and skew, have begun to emerge in the literature. The aim of this article is to provide an overview of the state-of-the-art clock synchronization algorithms for WSNs from a statistical signal processing point of view. This article focuses on describing the key features of the class of clock synchronization algorithms that exploit the traditional two-way message (signal) exchange mechanism. Upon introducing the two-way message exchange mechanism, the main clock offset estimation algorithms for pairwise synchronization of sensor nodes are first reviewed, and their performance is compared. The class of fully-distributed clock offset estimation algorithms for network-wide synchronization is then surveyed. The paper concludes with a list of open research problems pertaining to clock synchronization of WSNs.

Highlights

  • A wireless sensor network (WSN) is a group of spatially-distributed autonomous sensors, which monitor physical or environmental conditions, such as temperature, humidity, speed, pressure, etc., and transmit the recorded data to a central computing unit for analysis

  • Based on the classic two-way message exchange mechanism, we describe several robust estimation methods namely the bootstrap bias correction [30], the composite particle filtering [38] and the least squares estimator [39]

  • Wireless sensor networks can be applied to a variety of applications, and most of these applications require synchronization among the sensor nodes

Read more

Summary

Introduction

A wireless sensor network (WSN) is a group of spatially-distributed autonomous sensors, which monitor physical or environmental conditions, such as temperature, humidity, speed, pressure, etc., and transmit the recorded data to a central computing unit for analysis. Industry [8]: machine health monitoring; waste monitoring; data logging Most of these applications require the clocks of network nodes to be synchronized, because performing a joint task requires all of the nodes to operate on a common time scale. Some nodes act as a root (in the spanning-tree structure) or as a gateway (in the cluster structure), and the failure of such nodes may lead to the failure of a large number of nodes connecting to it To tackle this problem, several algorithms based on a fully-distributed communication topology have been proposed. The focus of [10,16] lies in the clock synchronization protocols that are applied in WSNs. The survey paper [17] provides a technical overview of the history, recent advances and challenges in distributed clock synchronization for distributed wireless networks, while this article presents synchronization algorithms with emphasis on the mathematical and statistical techniques employed for clock synchronization parameters estimation. Only pairwise synchronization methods are investigated in [9], while our paper discusses both pairwise and fully-distributed synchronization algorithms

System Model for Pairwise Clock Synchronization
Clock Offset and Skew
Two-Way Message Exchange Mechanism
Pairwise Clock Synchronization under Gaussian Delays
Pairwise Clock Synchronization under Exponential Delays
Maximum Likelihood Estimator
Best Linear Unbiased Estimator
Minimum Variance Unbiased Estimator
Comparison of Estimators
Joint Estimation of Clock Offset and Skew under Exponential Delays
Removing Nuisance Parameters
Direct Joint Estimation of Clock Offset and Skew
Confidence Interval for Clock Offset
Pairwise Clock Synchronization under Unknown Random Delays
Bootstrap Bias Correction
Composite Particle Filtering
Least Squares Estimators
Fully-Distributed Clock Synchronization Algorithms
System Model
Fully-Distributed Clock Synchronization Algorithms under Gaussian Delays
Belief-Based Synchronization Algorithms
Consensus-Based Synchronization Algorithms
Fully-Distributed Clock Synchronization Algorithms under Exponential Delays
Conclusions and Open Problems
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call