Precise Time Synchronization Research Articles

Underwater Wireless Communications and Networks: Theory and Application: Part 2 [Guest Editorial

The Earth is a water planet, two-thirds of which is covered by water. With the rapid developments in technology, underwater wireless communications and networks have become a fast growing field, with broad applications in commercial and military waterbased systems. The need for underwater wireless communications exists in applications such as remote control in the off-shore oil industry, pollution monitoring in environmental systems, collection of scientific data from ocean-bottom stations, disaster detection and early warning, national security and defense (intrusion detection and underwater surveillance), as well as new resource discovery. Thus, the research into new underwater wireless communication techniques has played the most important role in the exploration of oceans and other aquatic environments. In contrast to terrestrial wireless radio communications, the underwater channel poses serious technical challenges depending on the communications modalities (e.g., acoustic, optical, or RF/magnetic) employed. These include, but are not limited to, ambient channel noise, severe attenuation, propagation delay, multipath, frequency dispersion, bio-fouling, lack of access to precise time synchronization (GPS), and constrained bandwidth and power resources. These challenges also provide an opportunity for design of hybrid and adaptive transmission, such as the underwater acoustic and optical communications and networks, which have somewhat complementary properties, with potential for longer range and higher bandwidth networked communications in size- and power-constrained modems and mobile unmanned systems.

Precision time synchronization control method for smart grid based on wolf colony algorithm

Abstract In order to satisfy the high accuracy and high security requirements of time synchronization in smart grid, this paper has conducted a research on high-precision time synchronization control algorithm based on IEEE 1588 protocol. A slave clock model is built considering both phase offset and frequency drift. A novel approach using proportional–integral–derivative (PID) controller in cooperation with the wolf colony algorithm (WCA) is proposed to eliminate the offset of the slave clock with respect to the master clock. As a new swarm-intelligence algorithm, WCA is introduced for self-tuning the optimal parameters of the PID controller, which simulates the intelligent predatory behaviors and prey distribution rule of the wolf colony. The WCA-based PID control approach is compared with the method based on standard Particle Swarm Optimization (PSO) and Back-Propagation (BP) neural network. The experimental results show that the proposed WCA-based PID control approach in time synchronization for smart grid has higher precision, can adaptively adjust compensation and keep the master–slave clock synchronization effectively.

Simultaneously precise frequency transfer and time synchronization using feed-forward compensation technique via 120 km fiber link.

Precision time synchronization between two remote sites is desired in many applications such as global positioning satellite systems, long-baseline interferometry, coherent radar detection and fundamental physics constant measurements. The recently developed frequency dissemination technologies based on optical fiber link have improved the transfer instability to the level of 10−19/day at remote location. Therefore it is possible to keep clock oscillation at remote locations continuously corrected, or to reproduce a “virtual” clock on the remote location. However the initial alignment and the correction of 1 pps timing signal from time to time are still required, besides the highly stabilized clock frequency transfer between distant locations. Here we demonstrate a time synchronization based on an ultra-stable frequency transfer system via 120-km commercial fiber link by transferring an optical frequency comb. Both the phase noise compensation in frequency dissemination and temporal basis alignment in time synchronization were implemented by a feed-forward digital compensation (FFDC) technique. The fractional frequency instability was measured to be 6.18 × 10−20 at 2000 s. The timing deviation of time synchronization was measured to be 0.6 ps in 1500 s. This technique also can be applied in multi-node fiber network topology.

High speed fault tolerant secure communication for muon chamber using FPGA based GBTx emulator

The Compressed Baryonic Matter (CBM) experiment is a part of the Facility for Antiproton and Ion Research (FAIR) in Darmstadt at the GSI. The CBM experiment will investigate the highly compressed nuclear matter using nucleus-nucleus collisions. This experiment will examine lieavy-ion collisions in fixed target geometry and will be able to measure hadrons, electrons and muons. CBM requires precise time synchronization, compact hardware, radiation tolerance, self-triggered front-end electronics, efficient data aggregation schemes and capability to handle high data rate (up to several TB/s). As a part of the implementation of read out chain of Muon Cliamber(MUCH) [1] in India, we have tried to implement FPGA based emulator of GBTx in India. GBTx is a radiation tolerant ASIC that can be used to implement multipurpose high speed bidirectional optical links for high-energy physics (HEP) experiments and is developed by CERN. GBTx will be used in highly irradiated area and more prone to be affected by multi bit error. To mitigate this effect instead of single bit error correcting RS code we have used two bit error correcting (15, 7) BCH code. It will increase the redundancy which in turn increases the reliability of the coded data. So the coded data will be less prone to be affected by noise due to radiation. The data will go from detector to PC through multiple nodes through the communication channel. The computing resources are connected to a network which can be accessed by authorized person to prevent unauthorized data access which might happen by compromising the network security. Thus data encryption is essential. In order to make the data communication secure, advanced encryption standard [2] (AES - a symmetric key cryptography) and RSA [3], [4] (asymmetric key cryptography) are used after the channel coding. We have implemented GBTx emulator on two Xilinx Kintex-7 boards (KC705). One will act as transmitter and other will act as receiver and they are connected through optical fiber through small form-factor pluggable (SFP) port. We have tested the setup in the runtime environment using Xilinx Cliipscope Pro Analyzer. We also measure the resource utilization, throughput., power optimization of implemented design.

RB Particle Filter Time Synchronization Algorithm Based on the DPM Model.

Time synchronization is essential for node localization, target tracking, data fusion, and various other Wireless Sensor Network (WSN) applications. To improve the estimation accuracy of continuous clock offset and skew of mobile nodes in WSNs, we propose a novel time synchronization algorithm, the Rao-Blackwellised (RB) particle filter time synchronization algorithm based on the Dirichlet process mixture (DPM) model. In a state-space equation with a linear substructure, state variables are divided into linear and non-linear variables by the RB particle filter algorithm. These two variables can be estimated using Kalman filter and particle filter, respectively, which improves the computational efficiency more so than if only the particle filter was used. In addition, the DPM model is used to describe the distribution of non-deterministic delays and to automatically adjust the number of Gaussian mixture model components based on the observational data. This improves the estimation accuracy of clock offset and skew, which allows achieving the time synchronization. The time synchronization performance of this algorithm is also validated by computer simulations and experimental measurements. The results show that the proposed algorithm has a higher time synchronization precision than traditional time synchronization algorithms.

DEVELOPMENT AND EVALUATION OF A UAV BASED MAPPING SYSTEM FOR REMOTE SENSING AND SURVEYING APPLICATIONS

Abstract. In recent years, unmanned aerial vehicles (UAVs) have increasingly been used in various application areas, such as in the remote sensing or surveying. For these applications the UAV has to be equipped with a mapping sensor, which is mostly a camera. Furthermore, a georeferencing of the UAV platform and/or the acquired mapping data is required. The most efficient way to realize this georeferencing is the direct georeferencing, which is based on an onboard multi-sensor system. In recent decades, direct georeferencing systems have been researched and used extensively in airborne, ship and land vehicle applications. However, these systems cannot easily be adapted to UAV platforms, which is mainly due to weight and size limitations. In this paper a direct georeferencing system for micro- and mini-sized UAVs is presented, which consists of a dual-frequency geodetic grade OEM GPS board, a low-cost single-frequency GPS chip, a tactical grade IMU and a magnetometer. To allow for cm-level position and sub-degree attitude accuracies, RTK GPS (real-time kinematic) and GPS attitude (GPS compass) determination algorithms are running on this system, as well as a GPS/IMU integration. Beside the direct georeferencing, also the precise time synchronization of the camera, which acts as the main sensor for mobile mapping applications, and the calibration of the lever arm between the camera reference point and the direct georeferencing reference point are explained in this paper. Especially the high accurate time synchronization of the camera is very important, to still allow for high surveying accuracies, when the images are taken during the motion of the UAV. Results of flight tests demonstrate that the developed system, the camera synchronization and the lever arm calibration make directly georeferenced UAV based single point measurements possible, which have cm-level accuracies on the ground.

OMNeT++을 이용한 스페이스와이어 네트워크의 타임코드 전송 지연 분석

SpaceWire is a standard for high-speed links and networks between spacecraft components, which was invented for better, cheaper and faster on-board data handling in spacecraft. The standard defines timecode and its distribution which can be used for time synchronization among the nodes in a SpaceWire network. A timecode output from the time master which provides standard time over a SpaceWire network travels through links and routers to reach every nodes. While traveling, a timecode suffers from transmission delay and jitter which cause some difference in time synchronization among nodes. In this work, a simulator was developed using OMNeT++ to simulate the operation of a SpaceWire network and some analyses were performed on the transmission delay and jitter accompanied with a transmission of a timecode. The result will be used in the near future for the research of a precise time synchronization technique over a SpaceWire network.

Final design of the Korean AC/DC converters for the ITER coil power supply system

The final design of the ITER TF, CS, CC and VS AC/DC converters has been completed to implement ITER requirements following the detailed design and refinements of the preliminary design. The number of parallel thyristors and the rating of fuses are coordinated to keep those devices within the explosion limit even under most severe fault conditions. The impedance of the converter transformer has been optimized taking into account the energization inrush current, short circuit current, reactive power consumption and the available DC voltage. To ensure system integrity, AC/DC converters are mechanically divided into transformers, AC busbars, 6-pulse bridges, DC interconnecting busbars and DC reactors, and then all subsystems are decoupled by flexible links. To provide stable real time network communication down to the converters, a one GbE link is deployed between master controllers and local controllers. IEEE 1588 is implemented to the embedded controllers for precision time synchronization. This paper describes the detailed solutions implemented in the final design for the ITER AC/DC converters with R&D results of converter prototypes.

A time synchronization circuit with sub-microsecond skew for multi-hop wired wearable networks

This paper describes and evaluates a fully digital circuit for one-way master-to-slave, highly precise time synchronization in a low-power wearable system equipped with a set of sensor nodes. These sensors are connected to each other in a mesh topology, with conductive yarns used as one-wire bidirectional communication links. The circuit is designed to perform synchronization in the MAC layer, so that the deterministic part of the clock skew between nodes is kept constant and compensated with a single message exchange. In each sensor node, the synchronization circuit provides a programmable clock signal and a real-time counter for time stamping. Experimental results from a fabricated ASIC (in a CMOS 0.35μm technology) show that the circuit keeps the one-hop average clock skew below 4.6ns and that the skew grows linearly as the hop distance to the reference node increases. The sub-microsecond average clock skew achieved by the proposed solution satisfies the requirements of many wearable sensor network applications.

Joint Time and Frequency Dissemination Network Over Delay-Stabilized Fiber Optic Links

A precise fiber-based time and frequency dissemination scheme for multiple users with a tree-like branching topology is proposed. Through this scheme, ultra-stable signals can be easily accessed online anywhere along the fiber without affecting other sites. The scheme is tested through an experiment, in which a modulated frequency signal and a synchronized time signal are transferred to multiple remote sites over a delay-stabilized fiber optic links that are over 50 km long. Results show that the relative stabilities are 5E-14@1s and 2E-17@10000s. Meanwhile, compared with each site, time synchronization precision is less than 80 ps. These results can pave the way to practical applications in joint time and frequency dissemination network systems.

DCE: A Novel Delay Correlation Measurement for Tomography with Passive Realization

Tomography is important for network design and routing optimization. Prior approaches require either precise time synchronization or complex cooperation. Furthermore, active tomography consumes explicit probing resulting in limited scalability. To address the first issue we propose a novel Delay Correlation Estimation methodology named DCE with no need of synchronization and special cooperation. For the second issue we develop a passive realization mechanism merely using regular data flow without explicit bandwidth consumption. Extensive simulations in OMNeT++ are made to evaluate its accuracy where we show that DCE measurement is highly identical with the true value. Also from test result we find that mechanism of passive realization is able to achieve both regular data transmission and purpose of tomography with excellent robustness versus different background traffic and package size.

Distributed MIMO chaotic radar based on wavelength-division multiplexing technology.

A distributed multiple-input multiple-output chaotic radar based on wavelength-division multiplexing technology (WDM) is proposed and demonstrated. The wideband quasi-orthogonal chaotic signals generated by different optoelectronic oscillators (OEOs) are emitted by separated antennas to gain spatial diversity against the fluctuation of a target's radar cross section and enhance the detection capability. The received signals collected by the receive antennas and the reference signals from the OEOs are delivered to the central station for joint processing by exploiting WDM technology. The centralized signal processing avoids precise time synchronization of the distributed system and greatly simplifies the remote units, which improves the localization accuracy of the entire system. A proof-of-concept experiment for two-dimensional localization of a metal target is demonstrated. The maximum position error is less than 6.5 cm.

On Improving the Time Synchronization Precision in the Electric Power System

In order to improve the time synchronization precision in the electronic power system, the transmission of the timing information is demodulated based on FPGA. Based on the time service accuracy, the transmission timing information adopts IRIG-B code, the digital Costas loop is introduced in the process of FPGA demodulating IRIG-B code, which can extract the zero-crossing point information of IRIG-B code to avoid the problems of zero drift and pulse jitter in zero-crossing detection circuit. In this article, the time unifying terminal equipment has adopted the BeiDou satellite navigation receiver as the clock source and the output standard is IRIG-B code. The simulation results show that the algorithm reduces the synchronization errors of IRIG-B code, improves the timing synchronization precision, and fulfills the requirement of timing accuracy in the power system.

A New Timing and Frequency Synchronization Algorithm for Distributed MIMO-OFDM Systems

To solve the problems of time delay and different frequency offset between different transmit antennas and the same receive antennas for distributed multiple input multiple output orthogonal frequency division multiplexing systems. In this paper, a new timing and frequency synchronization algorithm is proposed. The search range of the fine timing synchronization is set with coarse time synchronization position and cyclic prefix, and the precise timing synchronization is obtained by utilizing the correlation between data information and training sequence. In the integer frequency offset estimation algorithm, the real component of the training sequence is taken as the real sequence, and then the differential relationship between the first and last samples is estimated accordingly. Theoretical analysis and simulation show that: (1) under multipath fading channel, the timing offset and frequency offset between different transmit and receive antennas can be effectively estimated with the synchronization algorithm proposed in this paper; (2) with signal noise ratio $$-$$-5Â dB, the probability of the correct synchronization is 95Â % and the precision of the frequency offset estimation can be as high as $$10^{-2}$$10-2, which is over the similar synchronization algorithm.

Timing synchronization based on mode-locked fiber lasers with attosecond timing jitter

Mode-locked fiber lasers output ultra-short pulse trains with extremely high temporal stability, showing great potential in systems that require precise timing synchronization, such as pump-probe experiments, high-speed analog-to-digital conversion, large-scale timing distribution and coherent combination. The fiber lasers are usually simpler, less costly, more efficient and more robust to the environment than solid state lasers, making them a better option for real-world applications. With the atto second temporal resolution of the balanced optical cross-correlation (BOC) method, timing jitter of mode-locked fiber lasers has been carefully measured and optimized over the last decade. However, due to the inherently large amplified spontaneous emission noise in the long gain fiber and broad pulse width inside the laser cavity, the quantum-noise-limited timing jitter of mode-locked fiber lasers is still much higher than that of the solid state lasers. In order to further optimize the timing synchronization of mode-locked fiber laser, larger locking bandwidth is required to suppress the low-frequency timing jitter, which contributes significantly to the total amount of residual timing jitter. In this work, tight timing synchronization between two mode-locked Yb-fiber lasers is achieved via a feedback loop built on an intra-cavity electro-optic phase modulator. Both lasers work in the stretched-pulse regime, which has been proven to support the lowest quantum-noise-limited timing jitter of mode-locked fiber laser. The output of the BOC system provides a timing error discriminator of 40 mV/fs, corresponding to 13 as resolution within the integration bandwidth. When the pulse trains from both lasers are successfully synchronized, the residual timing jitter can be measured with the same signal as that used for timing synchronization Based on the residual timing jitter measurement, the intra-cavity dynamics of the laser and the locking parameters of the feedback loop can be further optimized and a tight synchronization with 400 kHz locking bandwidth is finally achieved. When performing the integration from 1 Hz to 10 MHz, the residual timing error is as low as 109 as, corresponding to 77 as averaged timing jitter of each laser. A parallel out-of-loop single-arm cross-correlation measurement is also performed to test the validity of the in-loop results, and both measurements agree with each other.

Simulation and Performance Analysis of the IEEE1588 PTP with Kalman Filtering in Multi-hop Wireless Sensor Networks

As the wireless sensor networks (WSNs) find wider and wider applications, time synchronization has emerged as a key technology to improve network performance and enable WSNs in time-sensitive applications. The recently proposed IEEE 1588 Precision Time Synchronization Protocol (PTP) has shown its capability for the wired Ethernet, but its performance in multi-hop WSNs is still an open question. This is because of the non-negligible asymmetric delays caused by the wireless media access control protocol, low-cost crystal oscillators and time-stamping uncertainties due to limited resources in WSNs nodes. This paper studies the performance of the IEEE 1588 in a multi-hop WSNs by state-space modeling and realistic simulation. Furthermore, the Kalman filter is introduced to improve the offset and skew estimation. The realistic simulator was developed on the OMNeT++ discrete event simulation platform and the simulation results show that the proposed Kalman filtering improves the performance of time synchronization in multi-hop WSNs in terms of offset and skew estimation

Research on Clock Synchronization Scheme of Intelligent Substation Based on EPON Technology

With the development of digital and intelligent trend of substation, higher requirements on communication network were put forward. With features of high bandwidth, low cost, high reliability, easy maintenance, EPON has been widely applied in the field of distribution communication network. But precision of time synchronization is insufficient while bearing intelligent substation services by EPON. For providing sub microsecond time synchronization, IEEE 1588v2 can solve this problem. Based on depth study of EPON synchronization technology and IEEE 1588v2 protocol, a novel method to implement high-precision time synchronization based on EPON technology in intelligent substation is proposed. Through test, the proposed scheme has very good time synchronization performance. It can well meet requirements on precise time synchronization in intelligent substation.

도시형 지진방재시스템을 위한 네트워크 구성, 시간관리 및 데이터 저장 방법

In this paper, we propose a precise time management and time synchronization based on real-time data storage and transmission scheme in design of seismic data acquisition system for urban earthquake disaster preventing system (UEDPS). It is possible to improve the performance of the existing research results through the proposed methods. To evaluate the performances of the proposed methods, we implemented a prototype system(H/W & S/W) and performed some experiments with real seismic data and test equipment generated data as the input.

Implementation of MPLS Network Delay Measurement with 1588 Precision Time Synchronization Protocol

The right measurement of network time delay is the basis of a good network service. With the research on the IEEE 1588 precision time synchronization protocol and MPLS (Multi-protocol Label Switching), a method to realize the network time delay measurement in the MPLS network is introduced. It is also pointed out that this method has very important significance for the rapid deployment of network and the network quality evaluation.

Peak Diffusing Measurement Method for Estimating Thermal Diffusivity of Thin Films

A novel peak diffusing measurement method is reported. In this method, a pulsed Gaussian heat source is focused on the material’s surface, and a thermal camera is utilized for detecting the temperature evolution. Specially, the peak temperature timetmaxat different distance from the heat source center is extracted using the Singular Spectrum Analysis (SSA). Thermal diffusivity is then calculated by fitting multiple sets of extractedtmaxinto the derived regression equation. The advantage of this method is that it does not require the high precision time synchronization between the excitation source and detector. Also, the calculation process is very simple, which achieves an efficient in situ measurement of the thermal diffusivity. The measured radial thermal diffusivity of samples prepared from Ti and Ni is in good agreement with the reference data with a 1.8% and a 3.9% error bound.