Abstract
Time synchronization plays an important role in the scheduling and position technologies of sensor nodes in underwater acoustic networks (UANs). The time synchronization (TS) algorithms face challenges such as high requirements of energy efficiency, the estimation accuracy of the time-varying clock skew and the suppression of the impulsive noise. To achieve accurate time synchronization for UANs, an energy-efficient TS method based on nonlinear clock skew tracking (NCST) is proposed. First, based on the sea trial temperature data and the crystal oscillators’ temperature–frequency characteristics, a nonlinear model is established to characterize the dynamic of clock skews. Second, a single-way communication scheme based on a receiver-only (RO) paradigm is used in the NCST-TS to save limited energy. Meanwhile, impulsive noises are considered during the communication process and the Gaussian mixture model (GMM) is employed to fit receiving timestamp errors caused by non-Gaussian noise. To combat the nonlinear and non-Gaussian problem, the particle filter (PF)-based algorithm is used to track the time-varying clock state and an accurate posterior probability density function under the GMM error model is also given in PF. The simulation results show that under the GMM error model, the accumulative Root Mean Square Errors (RMSE) of NCST-TS can be reduced from 10−4 s to 10−5 s compared with existing protocols. It also outperforms the other TS algorithms in the aspect of energy efficiency.
Highlights
Underwater acoustic networks (UANs) are important scientific observation platforms for internal observation, information interaction and sensing detection [1,2]
The lifetime of a sensor node equipped with an atomic clock is about one-fourth of that equipped with a crystal oscillator [9]
The second kind is based on the underwater acoustic communication where the time error can be measured by the sensor nodes with the Time synchronization (TS) algorithms, and it has the advantages of low energy consumption and error compensation
Summary
Underwater acoustic networks (UANs) are important scientific observation platforms for internal observation, information interaction and sensing detection [1,2]. There are mainly two kinds of methods for sensor nodes to obtain the reference time in UANs. The first is based on the individual atomic clocks, which are more precise than the crystal oscillator clocks but consume a lot of energy. The crystal oscillators with low energy consumption are widely used in the sensor nodes They suffer from temperature variation and the frequency is time varying, leading to nonlinear clock drift [20]. The problems of nonlinear variations in the clock skew and non-Gaussian noises cannot be ignored when the sensor nodes work for a long time under the sea. We propose an energy-efficient TS based on nonlinear cloc3kosfk1e4w tracking (NCST-TS), which aims to overcome the time-varying nonlinear clock skew issue for energyconstrained UANs. The enhancement lies in the following aspects. Where ηk denotes the system noise of the clock skew at step k and it represents Gaussian distributions
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