Synchronization in Wireless Sensor Networks: Parameter Estimation, Performance Benchmarks and Protocols

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1 Synchronization in Wireless Sensor Networks: Parameter Estimation, Performance Benchmarks Wireless sensor networks are set to play a key role in a wide range of civilian and military applications, with tiny sensors connected through wireless links performing various sensing, computing, communication, and control tasks in highly distributed systems. This book presents a critical element in the deployment of wireless sensor networks: the process of synchronization. It summarizes the most important clock synchronization protocols proposed for wireless sensor networks, with special emphasis placed on deriving efficient clock offset estimation schemes and performance benchmarks. Graduate students of electrical and computer engineering and computer science will find this a valuable resource, as will engineers who are interested in designing efficient clock synchronization algorithms and improving the performance of existing synchronization protocols. ERCHIN SERPEDIN is currently an Associate Professor in the Wireless Communications Laboratory at Texas A&M University, where he joined after receiving his Ph.D. in Electrical Engineering from the University of Virginia, Charlottesville, in His research interests lie in the areas of statistical signal processing and wireless communications. Dr. Serpedin has served as Associate Editor for numerous journals including IEEE Transactions on Wireless Communications, IEEE Transactions on Signal Processing, IEEE Transactions on Communications, IEEE Signal Processing Letters, andieee Communications Letters. QASIM M. CHAUDHARI was awarded his Ph.D. in Electrical Engineering from Texas A&M University in 2008 and is currently an Assistant Professor at Iqra University, Islamabad, Pakistan. Before entering academia, he worked with the SoC Tools Group of Communications Enabling Technologies, Islamabad, and later with the HSDPA performance test team of Qualcomm Inc., San Diego. His research interests include digital communications, estimation and detection theory in general and channel estimation and synchronization in wireless sensor networks in particular.

2 Synchronization in Wireless Sensor Networks: Parameter Estimation, Performance Benchmarks and Protocols Erchin SERPEDIN Texas A&M University and Qasim M. CHAUDHARI Iqra University

3 cambridge university press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi Cambridge University Press The Edinburgh Building, Cambridge cb2 8ru, UK Published in the United States of America by Cambridge University Press, New York Information on this title: C Cambridge University Press 2009 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2009 Printed in the United Kingdom at the University Press, Cambridge A catalogue record for this publication is available from the British Library Library of Congress Cataloging in Publication data Serpedin, Erchin, 1967 Synchronization in wireless sensor networks : parameter estimation, performance benchmarks, and protocols /. p. cm. Includes bibliographical references and index. isbn (hardback) 1. Wireless sensor networks. 2. Wireless sensor networks. 3. Synchronousdata transmission systems. 4. Timing circuits. I. Chaudhari, Qasim M. II. Title. tk7872.d48s dc isbn hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

4 To our parents and families, Zara, Nisa, Aisha, Nesrin and Maryam.

5 CONTENTS Preface page xi 1 Introduction page Wireless Sensor Networks Time Synchronization Importance of Time Synchronization History of Clock Synchronization Outline 6 2 Signal Models for Time Synchronization page Definition of Clock Design Considerations Delay Components in Timing Message Delivery 13 3 Time Synchronization Protocols page Pairwise Synchronization Timing-Sync Protocol for Sensor Networks (TPSN) Tiny-Sync and Mini-Sync Reference Broadcast Synchronization (RBS) Flooding Time Synchronization Protocol (FTSP) Network-Wide Synchronization Extension of TPSN Lightweight Time Synchronization (LTS) Extension of RBS Extension of FTSP Pairwise Broadcast Synchronization (PBS) Time Diffusion Protocol (TDP) Synchronous and Asynchronous Diffusion Algorithms 26 vii

6 viii Contents Protocols Based on Pulse Transmissions Adaptive Time Synchronization Rate-Adaptive Time Synchronization (RATS) RBS-based Adaptive Clock Synchronization Adaptive Multi-Hop Time Synchronization (AMTS) 30 4 Fundamental Approaches to Time Synchronization page Sender Receiver Synchronization (SRS) Receiver-Only Synchronization (ROS) Receiver Receiver Synchronization (RRS) Comparisons 41 5 Minimum Variance Unbiased Estimation (MVUE) of Clock Offset page The System Architecture Best Linear Unbiased Estimation Using Order Statistics (BLUE-OS) Symmetric Link Delays Asymmetric Link Delays Minimum Variance Unbiased Estimation (MVUE) Asymmetric Link Delays Symmetric Link Delays Explanatory Remarks 57 6 Clock Offset and Skew Estimation page Gaussian Delay Model Maximum Likelihood (ML) Clock Offset Estimation Cramer Rao Lower Bound (CRLB) for Clock Offset Joint Maximum Likelihood Estimation (JMLE) of Clock Offset and Skew Cramer Rao Lower Bound (CRLB) for Clock Offset and Skew Exponential Delay Model Cramer Rao Lower Bound (CRLB) for Clock Offset Joint Maximum Likelihood Estimation (JMLE) of Clock Offset and Skew 73

7 Contents ix 7 Computationally Simplified Schemes for Estimation of Clock Offset and Skew page Using the First and the Last Data Sample Gaussian Delay Model Exponential Delay Model Combination of Clock Offset and Skew Estimation Simulation Results Fitting the Line Between Two Points at Minimum Distance Apart Simulation Results Computational Complexity Comparison Pairwise Broadcast Synchronization (PBS) page Synchronization for Single-Cluster Networks Comparisons and Analysis Synchronization for Multi-Cluster Networks Network-Wide Pair Selection Algorithm (NPA) Group-Wise Pair Selection Algorithm (GPA) Comparisons and Analysis Energy-Efficient Estimation of Clock Offset for Inactive Nodes page Problem Formulation Maximum Likelihood Estimation (MLE) Cramer Rao Lower Bound (CRLB) CRLB for the Clock Offset of Inactive Node ˆφ q CRLB for the Clock Offset of Active Node ˆφ p Simulation Results Some Improved and Generalized Estimation Schemes for Clock Synchronization of Inactive Nodes page Asymmetric Exponential Link Delays Best Linear Unbiased Estimation Using Order Statistics (BLUE-OS) Minimum Variance Unbiased Estimation (MVUE) Minimum Mean Square Error (MMSE) Estimation Symmetric Exponential Link Delays Best Linear Unbiased Estimation Using Order Statistics (BLUE-OS) Minimum Variance Unbiased Estimation (MVUE) Minimum Mean Square Error (MMSE) Estimation 155

8 x Contents 11 Adaptive Multi-hop Time Synchronization (AMTS) page Main Ideas Level Discovery Phase Synchronization Phase Network Evaluation Phase Synchronization Mode Selection Determination of Synchronization Period Determination of the Number of Beacons Sequential Multi-Hop Synchronization Algorithm (SMA) Simulation Results Clock Drift Estimation for Achieving Long-Term Synchronization page Problem Formulation The Estimation Procedure Joint Synchronization of Clock Offset and Skew in a Receiver Receiver Protocol page Modeling Assumptions Joint Maximum Likelihood Estimation (JMLE) of the Offset and Skew Application of the Gibbs Sampler Performance Bounds and Simulations Robust Estimation of Clock Offset page Problem Modeling and Objectives Gaussian Mixture Kalman Particle Filter (GMKPF) Testing the Performance of GMKPF Composite Particle Filtering (CPF) with Bootstrap Sampling (BS) Testing the Performance of CPF and CPF with BS Conclusions and Future Directions page 211 Acronyms page 214 References page 218 Index page 227

9 PREFACE The clock or time synchronization problem in wireless sensor networks (WSNs) requires a procedure for providing a common notion of time across the nodes of WSNs. In general, clock synchronization is viewed as a critical factor in maintaining the good functioning of WSNs due mainly to their decentralized organization and timing uncertainties caused by the imperfections in hardware oscillators and message delays at the physical and medium access control (MAC) layers. In addition, synchronization of the nodes of wireless sensor networks is crucial for implementing fundamental operations such as power management, transmission scheduling, data fusion, localization and tracking, and security protocols to name only a few applications. The aim of this book is to provide an introduction to the clock synchronization problem of WSNs from a statistical signal processing viewpoint. Therefore, most of the topics presented in this book deal with building efficient clock offset estimation algorithms and performance benchmarks for general synchronization approaches that rely on sender receiver and receiver receiver timing packet exchange mechanisms. A summary of the key features of the most representative protocols proposed for clock synchronization of WSNs is also presented, together with some interesting open research problems. Synchronization of WSNs is currently a very active research field with a large number of results and very diverse contributions coming from an equally diverse body of researchers: computer scientists, electrical engineers, mathematicians, statisticians, etc. Despite the deployed efforts, the general problem of building efficient global synchronization protocols for large-scale wireless sensor networks is still open and the proposed results are still introduced in a quite ad-hoc manner, lacking comprehensive design and optimization studies to assess and improve their performance in a systematic fashion. Although herein we will not solve these general and very important problems, this book assumes the modest task of building efficient synchronization algorithms and performance benchmarks for simple sender receiver and receiver receiver timing packet exchange based xi

10 xii Preface synchronization protocols. Despite its unambitious goal, this book will be of interest to all practitioners who are looking for techniques to improve the performance of existing protocols such as network time protocol (NTP), time protocol for synchronization of sensor networks (TPSN), reference broadcast synchronization (RBS), pairwise broadcast synchronization (PBS), etc., and to develop novel synchronization protocols. This book consists of 15 chapters. Chapter 1 is a short introduction to the time synchronization problem, its history, and importance. Chapter 2 presents the main constraints that must be taken into account when designing time synchronization protocols for WSNs. Chapter 3 focuses on the most representative synchronization protocols for wireless sensor networks by outlining their main features, while Chapter 4 discusses three general packet-based synchronization approaches for WSNs. Chapters 5 14 propose a variety of statistical signal processing algorithms for improved estimation of clock phase offsets and assessing performance benchmarks. A series of novel results, extensions, and interesting relationships between different estimation schemes are reported. At first, the emphasis is put on developing efficient clock offset estimators for the general scenarios when the network delays are normally or exponentially distributed. However, observing the facts that the distribution of network delays in general cannot be predicted accurately, and that the estimators reported in the literature are not robust with respect to the unknown possibly time-varying distribution of network delays, we address in Chapter 14 the problem of building clock offset estimators that are robust to the distribution of network delays. Finally, Chapter 15 concludes this book with some open research problems. This work would have not been possible without the support and encouragement of Dr. Bruce Suter from Air Force Research Laboratory (AFRL), Rome, NY, and the help of our students. Our heartfelt thanks go to Bruce, and to our families and students (Dr. Kyoung-Lae Noh, Dr. Jangsub Kim, Dr. Yik-Chung Wu, Mr. Ilkay Sari, Mr. Jaehan Lee, and Mr. Sabit Ekin). Despite our efforts to contain all sources of errors and misunderstandings, we believe that inconsistencies and errors might show up. Therefore, we are asking our readers kindly to us their feedback to: eserpedin@gmail.com. Any feedback is welcome to improve this work. This book webpage will be maintained at serpedin/.

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