Research on the Time Synchronization Technology of Wireless Sensor Network

Study of consensus-based time synchronization in wireless sensor networks

Wireless sensor networks require time synchronization, which is the coordination of events or actions to make a system operate in unison. In this work, real experiments and a theoretical analysis of the behavior of the clock sources, most used in wireless sensor networks, have been carried out. The experiments have been performed on two real platforms from two different manufacturers in real environments with sudden changes in temperature. Complementary metal-oxide-semiconductor oscillators have a low accuracy, bigger than 500 ppm, and a high dependency with temperature. External crystal oscillators have good accuracy, around 20 ppm, and are stable with temperature. Temperature-compensated crystal oscillators are very accurate, around 5 ppm, and the temperature has no influence in their drift. The use of phase-locked loop circuits minimizes the impact of temperature and stabilizes oscillators. We highlight and demonstrate the importance of the early stages of design, especially the selection of the clock source, because that decision has a great impact on the performance of the time synchronization in wireless sensor networks.

Time synchronization, aiming to provide a common timescale among distributed sensor nodes, is a key enabling technology for many applications, such as collaborative condition monitoring and localization detection. Due to the complexity of time synchronization in wireless sensor networks, the Discrete Event Simulator is recommended to adopt resulting from the feature that the behavior of a complex is represented as an order sequence of well-defined event in time. In this paper, the PCO clock is firstly implemented into the open-source software simulator OMNeT++ by using desynchronization mechanism under a realistic scenario, and it can also be simulated at an adjustable and higher resolution. The developed relay node, can either be a Full Function Device or a Reduced Function Device, enables the high scalability for multi-node and multi-hop simulation. In addition, the shared code of this project on the GitHub directly benefits the researchers and engineers in communication.

Time Synchronization in WSNs with Random Communication Delays: A Constant Gain Design

Time synchronization for wireless sensor networks (WSNs) has been studied in recent years as a fundamental and significant research issue. Many applications based on these WSNs assume local clocks at each sensor node that need to be synchronized to a common notion of time. Time synchronization in a WSN is critical for accurate time stamping of events and fine-tuned coordination among the sensor nodes to reduce power consumption. This paper proposes a bidirectional, reference based, tree structured time synchronization service for WSNs along with network evaluation phase. This offers a push mechanism for (i) accurate and (ii) low overhead for global time synchronization. Analysis study of proposed approach shows that it is lightweight as the number of required broadcasting messages is constant in one broadcasting domain.

Synchronization of tiny sensor nodes forming Wireless Sensor Networks (WSNs) is a challenging problem due to frequent topological changes, node failures and power, memory and computation constraints. These difficulties promote a self-organizing solution for the problem of time synchronization in WSNs to be quite desirable. Current self-organizing time synchronization protocols in WSNs have drawbacks: They either provide synchronicity but not a common notion of time for the nodes in the network or they demand keeping track of the time information of the neighboring nodes. The latter drawback becomes quite crucial especially on dense WSNs, which makes available self-organizing time synchronization protocols impractical. This paper provides a novel self-organizing time synchronization protocol for WSNs which does not require keeping track of the neighboring nodes. The main component of our protocol is a computationally light "adaptive-value tracking" algorithm which synchronizes the rate of each sensor node to that of all its neighboring nodes through successive feedbacks. We show by simulations that the proposed protocol achieves a tight synchronization which results in the desired property of global time notion after a finite amount of time on dense networks. Although there is no formal proof yet of the protocol's convergence, we anticipate this protocol could be used as a practical time synchronization approach.

Accurate time synchronization of ultrasonic TOF measurements in IEEE 802.15.4 based wireless sensor networks

Firstly, we show that in the unified structural model of the consensus-based time synchronization (CBTS) algorithm, the offset estimate is divergent in general while the drift estimate converges in a rate of $1/k$ , where $k$ is the discrete time. Then, we propose an offset compensation approach to guaranteeing the boundedness of the offset estimate for wireless sensor networks (WSNs) with random bounded communication delays. Two numerical experiments are presented to illustrate the availability of this algorithm and the better synchronization performance.

Localization and synchronization are two important issues in communication systems and wireless sensor networks. These two problems are addressed in many applications, and share many aspects in common. However, these two problems are traditionally treated separately. In this paper, we present a unified framework to jointly solve these two problems at the same time. The maximum likelihood joint estimation of location and timing is firstly derived. Then, a more computationally efficient two-stage least square method is proposed. The Cramer-Rao lower bound for the joint localization and time synchronization is also derived. Simulation results show that the performances of the maximum likelihood and two-stage least square estimators can both achieve the Cramer-Rao lower bound.

It is important and challenging to achieve secure time synchronization in wireless sensor networks, which may be deployed in a hostile environment under various malicious attacks. The recently developed average-consensus-based time synchronization protocol (ATS) is a promising alternative as it does not depend on any reference node or network topology, which makes it robust to different kinds of attacks, e.g., denial-of-service, node destruction, etc. However, the in-network information fusion nature of average consensus makes the ATS vulnerable to the well-known message manipulation attacks. In this paper, we focus on how to defend the ATS protocol in wireless sensor networks under message manipulation attacks. We first investigate the impact of message manipulation attacks over ATS, and derive a necessary condition for ATS to converge. Then based on the obtained insights, we propose a novel adjusting parameter checking mechanism which exploits the two-hop neighboring information to dynamically constrain the attackers. We further incorporate all checking processes into the traditional ATS protocol to form a secure average-consensus-based time synchronization protocol (SATS). We prove that SATS guarantees the network time synchronization with an exponentially converging speed.

Impact of time synchronization error on the mode-shape identification and damage detection/localization in WSNs for structural health monitoring

A unified framework to jointly solve the problems of localization and synchronization at the same time is presented in this paper. The joint approach is attractive because it can solve both localization and synchronization using the same set of message exchanges, which is extremely important for energy saving in wireless sensor networks. The inaccuracy of anchor locations and timings is taken into account to provide accurate joint localization and synchronization. The anchor uncertainties are assumed to be bounded, but knowledge of the statistics of anchor uncertainties is not required. The problem is formulated into a linear model with uncertainties on both sides of the equation. A robust joint estimator is then proposed based on minimizing the worst-case mean square error and the solution is obtained by solving a semidefinite programming problem. Simulation results show that the proposed estimator outperforms the traditional least squares estimator at the cost of higher computational complexity.

The reduced-rank method exploits the distortion-variance tradeoff to yield superior solutions for classic problems in statistical signal processing such as parameter estimation and filtering. The central idea is to reduce the variance of the solution at the expense of introducing a little distortion. In the context of parameter estimation, this yields an estimator whose sum of distortion plus variance is smaller than the variance of its undistorted counterpart. The method intrinsically results in an ordering mechanism for the singular vectors of the system matrix in the measurement model used for estimating the parameter of interest. According to this ordering rule, only a few dominant singular vectors need to be selected to construct the reduced-rank solution while the rest can be discarded. The reduced-rank estimator is less sensitive to measurement errors. In this paper, we attempt to derive the reduced-rank estimator for the total least squares (TLS) problem, including the order selection rule. It will be shown that, due to the inherent structure of the problem, it is not possible to exploit the distortion-variance tradeoff in TLS formulations using existing techniques, except in some special cases. This result has not been reported previously and warrants a new viewpoint to achieve rank reduction for the TLS estimation problem. The work is motivated by the problems arising in practical applications such as channel estimation in wireless communication systems and time synchronization in wireless sensor networks.

Energy efficiency has received extensive attention in energy-constrained Wireless Sensor Networks (WSNs). As is known, radio communication accounts for major energy consumption of wireless device and the control of radio transmitter can make effective function with energy efficiency system. In this paper we test and analyse the relationship between precision and synchronization rate and we also give an initial model denote for the relationship among synchronous cycle, time error and energy consumption. The experiments and analysis in the paper show the feasibility of long cycle time synchronization for WSNs under certain limitation and the result will be helpful for the time synchronization method with low energy consumption.

In wireless sensor networks, efficient usage of energy helps in improving the network lifetime. Many power saving MAC schemes based on TDMA have been proposed in recent years, but none of them particularly discuss the problem of time synchronization. As there are many time synchronization algorithms have been proposed, but they can't be applied to these TDMA MAC schemes directly. In this paper, a lightweight and energy efficient time synchronization (LEETS) scheme that can be applied to all kinds of TDMA power saving MAC schemes was proposed. In the scheme, nodes compete the first slot of each time frame to broadcast local time information, which can be used to realize whole network synchronization. And guard interval is inserted in two consecutive slots to avoid collision may be caused by synchronization errors. The scheme has been simulated and the results show LEETS needs less message exchanges to realize synchronization than other time synchronization schemes. So, It is lightweight and energy efficient.