Synchronous Link Research Articles

A Fast Time Synchronization Method for Large Scale LEO Satellite Networks Based on A Bionic Algorithm

A fast time synchronization method for large-scale LEO satellite networks based on a bionic algorithm is proposed. Because the inter-satellite links are continuously established and interrupted due to the relative motion of the satellites, the topology of the LEO satellite networks is time varying. Firstly, according to the ephemeris information in navigation messages, a connection table which records the connections between satellites is generated. Then, based on the connection table, the current satellite network topology is calculated and generated. Furthermore, a bionic algorithm is used to select some satellites as time source nodes and calculate the hierarchy of the clock transmission tree. By taking the minimum level of the time transmission tree as the optimization objective, the time source nodes and the clock stratums of the whole satellite networks are obtained. Finally, the onboard computational center broadcasts the time layer table to all the satellites in the LEO satellite networks and the time synchronization links can be established or recovered fast.

Minimal synchrony for implementing Timely Provable Reliable Send primitive with Byzantine failures

Broadcast abstractions are among the most important concepts in the field of fault tolerant distributed computing. These abstractions are used by consensus algorithms as a fundamental building block for ensuring that all correct processes in the system decide the same value. The Timely Provable Reliable Send primitive is among these broadcast abstractions with which we guarantee that messages are delivered correctly and in a timely manner, even in the presence of faulty processes. In this paper, we present an authenticated algorithm implementing provable reliable send primitive with very few eventually synchronous links. In other words, this algorithm assumes that there is a -sink in the system. A -sink is a correct process where the number of incoming eventually timely links that connecting it with correct processes is (including itself). We also show that a -sink is the minimal synchrony assumption for implementing this primitive in a Byzantine system where an authentication mechanism is available.

Selection of Optimal Palmer Predictors for Increasing the Predictability of the Danube Discharge: New Findings Based on Information Theory and Partial Wavelet Coherence Analysis

The purpose of this study was to obtain synergistic information and details in the time–frequency domain of the relationships between the Palmer drought indices in the upper and middle Danube River basin and the discharge (Q) in the lower basin. Four indices were considered: the Palmer drought severity index (PDSI), Palmer hydrological drought index (PHDI), weighted PDSI (WPLM) and Palmer Z-index (ZIND). These indices were quantified through the first principal component (PC1) analysis of empirical orthogonal function (EOF) decomposition, which was obtained from hydro-meteorological parameters at 15 stations located along the Danube River basin. The influences of these indices on the Danube discharge were tested, both simultaneously and with certain lags, via linear and nonlinear methods applying the elements of information theory. Linear connections were generally obtained for synchronous links in the same season, and nonlinear ones for the predictors considered with certain lags (in advance) compared to the discharge predictand. The redundancy–synergy index was also considered to eliminate redundant predictors. Few cases were obtained in which all four predictors could be considered together to establish a significant information base for the discharge evolution. In the fall season, nonstationarity was tested through wavelet analysis applied for the multivariate case, using partial wavelet coherence (pwc). The results differed, depending on the predictor kept in pwc, and on those excluded.

Inter-satellite time synchronization and ranging link assignment for autonomous navigation satellite constellations

The BeiDou Navigation Satellite System (BDS) has equipped its new-generation satellites with inter-satellite link (ISL) payloads for the autonomous operation. In the autonomous operation mode, the navigation constellation realizes the time unification and orbit determination through inter-satellite time synchronization and ranging, so as to self-maintain the time-space reference. For solving the problem of inter-satellite time synchronization and ranging link assignment for autonomous navigation constellations, assignment algorithms of time synchronization links based on satellite layers, as well as ranging links combining satellite selection and weighting, were proposed. The ISL assignment simulation of the BeiDou-3 global constellation was implemented using our algorithms, and the assignment was completed almost within 5 cycles. According to the statistical results, using 56.6% of ISL timeslots, the constellation had achieved the optimal mode of inter-satellite time synchronization with the least skips; the number of inter-satellite ranging links established per minute of each satellite was not less than 10, which met the constraint of the minimum links; the average PDOP of ranging links of the MEO, GEO and IGSO satellite was about 1, 1.6 and 1.5 respectively, close to the lower reference limit. It was demonstrated that the ISLs were reasonably assigned for the demand of inter-satellite time synchronization and ranging with constraints of the TDMA system, narrow beams, the quantity of payloads and inter-satellite visibility.

An Innovative Push-To-Talk (PTT) Synchronization Scheme for Distributed SAR

Bistatic/multistatic synthetic aperture radar (Bi-/M-SAR) has extensive applications, including cross- and along-track interferometry, multiangle imaging, and so on. However, these applications are affected by several factors, especially the time and frequency synchronization that this article focuses on. Here, the new insights about the focus are described, based on which an innovative push-to-talk (PTT) scheme is first put forward to solve the time and frequency synchronization problems of distributed SAR. It realizes unidirectional and long-distance synchronization through the two-segment frequency rate (TSFR) waveform composed of the two linear frequency modulation (LFM) signals with different frequency rates and achieves radar imaging and synchronization with a high rate ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{\mathrm{ syn}}$ </tex-math></inline-formula> ) at the same time by the frequency diversity (FD) waveforms and bandpass filters. To clarify the PTT scheme, its system framework and synchronization error models are detailed. Besides, the channel impulse response, multiple errors, the signal-to-noise ratio (SNR) of synchronous links, and time synchronization accuracy are analyzed. In addition, some simulations are carried out to evaluate its performance. Those results demonstrate that the PTT scheme can perform the long-distance synchronization with high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\mathrm{ syn}}$ </tex-math></inline-formula> , its synchronization accuracy increases with the improvement of SNR and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\mathrm{ syn}}$ </tex-math></inline-formula> , and it can measure the time and frequency deviations in different ranges accurately. In a word, the PTT scheme has the potential to be used in future distributed SAR missions.

FPGA Implementation of an NCO Based CDR for the JUNO Front-End Electronics

This article describes a design of an field-programmable gate array (FPGA) implementation of a clock and data recovery (CDR) system. The core will be integrated in the FPGA configuration for the front-end electronics (FEE) board of the Jiangmen underground neutrino observatory (JUNO) experiment. The front-end will be placed on the main detector, underground and underwater, making the electronics not accessible after installation. The timing and trigger system relies on a synchronous link connection over CAT5e cable (up to 100 m long) between the front-end and the back-end electronics (BEE), where a twisted-pair is dedicated to clock-forwarding. The robustness of the recovery clock system is essential for the stability of the FPGA firmware. The proposed project is intended to improve the clock recovery operation by increasing the immunity of the link to sudden electromagnetic interference (EMI). On top of this, the core allows to free a twisted-pair in the link, since the clock can be recovered from the data and there is no more need for a clock-dedicated transmission. This will optimize the link granting the possibility to implement other features. The design is based on two components: a numerically-controlled oscillator (NCO), in order to create a controlled frequency clock signal, and a digital phase detector (PD) to match the clock frequency with the data rate. NCOs are often coupled with a digital-to-analog converter (DAC) to create direct digital synthesizers (DDSs), which are able to produce analog waveforms of any desired frequency. In the presented case instead, the NCO generates a digital clock signal of an arbitrary frequency, while the PD manages this frequency by intercepting any shifting on the relative phase between the clock and the data. A phase aligner (PA) module guarantees that data are sampled in the middle of the eye pattern, which represents the optimal sampling point. The article presents an overview of the NCO-based CDR design and implementation, together with some tests and results in order to verify the CDR reliability. Moreover, in the last section, some other possible applications of the core are illustrated.

Implementation of a Phase Synchronization Scheme Based on Pulsed Signal at Carrier Frequency for Bistatic SAR.

Phase synchronization is one of the key technical challenges and prerequisites for the bistatic synthetic aperture radar (SAR) system, which can form a single-pass interferometry system to perform topographic mapping. In this paper, an advanced phase synchronization scheme based on a pulsed signal at carrier frequency is proposed for a bistatic SAR system and it is verified by a ground validation system. In the proposed phase synchronization scheme, the pulsed signal at carrier frequency is used for phase synchronization link, and it is exchanged by virtue of a time slot between radar signals. The feasibility of the scheme is proven by theoretical analysis of various factors affecting the performance of phase synchronization, and the reliability of the scheme is verified by the test results of the ground validation system.

Ground-to-satellite time and frequency synchronization link with active carrier phase compensation.

In this paper, a synchronization link between one ground station and one geostationary satellite is established. The ground station receives retransmitted signals from the satellite, measures phase delay along the propagation route, and actively compensates back to its sending signals, realizing real-time phase fluctuation compensation. The transmitted signal contains two frequencies to eliminate common-mode phase noise. The difference between their carrier phase delays is measured. Different modes of carrier phase variation are separated and compensated, achieving a remaining time jitter of ±200 ps. Major sources of error are analyzed, and potential methods for improvement are discussed. The proposed ground-to-satellite link and active compensation method has potential applications in frequency standard dissemination to remote receivers (including ground stations or satellites). These potential applications justify further study of this system.

Mapping individual behavior in financial markets: synchronization and anticipation

In this paper we develop a methodology, based on Mutual Information and Transfer of Entropy, that allows to identify, quantify and map on a network the synchronization and anticipation relationships between financial traders. We apply this methodology to a dataset containing 410text{,}612 real buy and sell operations, made by 566 non-professional investors from a private investment firm on 8 different assets from the Spanish IBEX market during a period of time from 2000 to 2008. These networks present a peculiar topology significantly different from the random networks. We seek alternative features based on human behavior that might explain part of those 12text{,}158 synchronization links and 1031 anticipation links. Thus, we detect that daily synchronization with price (present in 64.90% of investors) and the one-day delay with respect to price (present in 4.38% of investors) play a significant role in the network structure. We find that individuals reaction to daily price changes explains around 20% of the links in the Synchronization Network, and has significant effects on the Anticipation Network. Finally, we show how using these networks we substantially improve the prediction accuracy when Random Forest models are used to nowcast and predict the activity of individual investors.

Improvement of high‐voltage impulses in track circuits with Kasami and LS codes

SummaryTrack circuit systems based on high‐voltage impulses (HVI) have already been used in China to detect the state of track sections. With the demand for more reliable intelligent transportation systems in recent years, new advances, improvements, and innovations are being applied to their development. This work defines a HVI track circuit based on the encoding of the transmitted signal. Not only 15‐bit and 63‐bit Kasami sequences but also 19‐bit and 71‐bit loosely synchronized codes are applied to encode simultaneous HVI transmissions from 2 emitters. Train occupancy in different parts of the track section is also considered. The quality of the detection is evaluated by means of the correlation bounds. Furthermore, a simulation model for double‐track railways has been developed, which can be dedicated to the evaluation of a suitable signal processing in the track circuit under study. Compared to previous HVI track circuits, as a novelty, this work proposes the use of encoded transmissions to these track circuits, to improve their performance in terms of scanned distance and detection accuracy. The correlation properties of the sequences involved also make feasible to avoid any synchronization link between emitters and receivers, thus providing a significant advantage for later deployment.

A New Walsh-Like Near Orthogonal (WNO) Sequence for Asynchronous CDMA System

Present generation mobile communication system employs one of the most popular wireless access technologies called code division multiple access (CDMA). Design of CDMA spreading codes has drawn significant attention amongst the researchers over the last few decades. CDMA code family is generally categorized into purely orthogonal and non-orthogonal (near-orthogonal) members which have established their application in synchronous (downlink) and asynchronous (uplink) CDMA system respectively. Walsh code has been regarded as the most useful spreading code to be used in synchronous link because of its orthogonality property. However, the performance of Walsh code is significantly inferior in asynchronous surroundings. A number of codes have consequently been proposed with an aim to mitigate the shortcomings of Walsh code. This paper makes an innovative attempt to enhance the correlation properties of existing Walsh code through one simple yet powerful algorithm. Proposed code of length `N' has been generated from code sets of length `N/4' and thus makes the code generation algorithm recursive in nature. Performance of the proposed code has subsequently been compared with some existing orthogonal and semi orthogonal codes in terms of various performance metrics and finally the supremacy of our proposition has been established.

NaNet3: The on-shore readout and slow-control board for the KM3NeT-Italia underwater neutrino telescope

The KM3NeT-Italia underwater neutrino detection unit, the tower, consists of 14 floors. Each floor supports 6 Optical Modules containing front-end electronics needed to digitize the PMT signal, format and transmit the data and 2 hydrophones that reconstruct in real-time the position of Optical Modules, for a maximum tower throughput of more than 600 MB/s. All floor data are collected by the Floor Control Module (FCM) board and transmitted by optical bidirectional virtual point-to-point connections to the on-shore laboratory, each FCM needing an on-shore counterpart as communication endpoint. In this contribution we present NaNet3 , an on-shore readout board based on Altera Stratix V GX FPGA able to manage multiple FCM data channels with a capability of 800 Mbps each. The design is a NaNet customization for the KM3NeT-Italia experiment, adding support in its I/O interface for a synchronous link protocol with deterministic latency at physical level and for a Time Division Multiplexing protocol at data level.

Time Average Synchronization in Thermoelastic Stress Analysis

Thermoelastic stress analysis is noncontacting technique used to obtain stress field on the material surface by measuring changes of the surface temperature by infrared camera. Roughly 1MPa change in stress state causes a temperature change of 1 mK in steel. Because of extremely small temperature measurement it is necessary to reduce noise. Thermoelastic stress analysis expects adiabatic conditions so that the specimen is usually loaded by cyclic load. Lock-in principle is most frequently method used to extract signal from statistical noise. However, it is necessary to ensure a synchronization link between IR camera and loading machine. In the case of large displacement of the test object it is necessary to be eliminated. In the contribution is presented an algorithm which requires only knowledge of the temporal signal and identification of displacement by using Lock-in technology and time average synchronization to determine infrared field.

Converting unstructured into semi-structured process models

Business process models capture process requirements that are typically expressed in unstructured, directed graphs that specify parallelism. However, modeling guidelines or requirements from execution engines may require that process models are structured in blocks. The goal of this paper is to define an automated method to convert an unstructured process model containing parallelism into an equivalent semi-structured process model, which contains blocks and synchronization links between parallel branches. We define the method by means of an algorithm that is based on dominators, a well-known technique from compiler theory for structuring sequential flow graphs. The method runs in polynomial time. We implemented and evaluated the algorithm extensively. In addition we compared the method in detail with the BPStruct method from literature. The comparison shows that our method can handle cases that BPStruct is not able to and that the method coincides with BPStruct for the cases that BPStruct is able to handle.

Ppssm:push/pull smooth video streaming multicast protocol design and implementation for an overlay network

IP multicast is one of the best techniques for video streaming on the Internet. It faces issues with respect to address allocation, routing, authorization, group management, security, and scalability. By default, local Internet Service Providers did not enable IP multicast services, because of the cost incurred in using multicast-enabled routers. To solve these issues some of the IP layer functionalities have been shifted to the Application Layer, thus leading to Application Layer Multicast (ALM) protocols. However, ALM protocols face issues related to synchronous data delivery, scalability, link stress, link stretch and node failures. Some of the existing protocols are CoolStreaming, and mTreebone. A novel ALM protocol based Push/Pull Smooth video Streaming Multicast (PPSSM) protocol is proposed in this paper, to increase the throughput and reduce the packet loss rate. The PPSSM protocol involves three stages, such as tree-mesh construction, dynamic buffer management and network coding techniques. In the tree-mesh construction, a tree consists of stable nodes and a mesh consists of unstable nodes. The proposed PPSSM optimizes the stable nodes in the tree, which minimizes or eliminates the pull operations from the unstable mesh overlay nodes, by exploring the potential of the stable nodes. Dynamic buffer management is achieved by setting the optimal buffer threshold value, using the optimization of the sensitivity parameters, such as packet loss and packet workload/delay by the Infinitesimal Perturbation Analysis and Stochastic Approximation algorithms. In addition to the tree-mesh construction and buffer management, the introduction of the network coding technique will enhance the throughput and minimize the packet loss and delay. Finally, the performance of the proposed PPSSM protocol is compared with those of CoolStreaming, and mTreebone, and it shows improvement in respect of throughput, packet loss, and average decoding time.

An integrative synchronization and imaging algorithm for GNSS-based BSAR

In this paper, we develop an integrative synchronization and imaging algorithm for bistatic synthetic aperture radar (BSAR) using Global Navigation Satellite Systems as transmitters and a fixed receiver. Due to the lack of a dedicated synchronization link, the usage of navigation signals and the system complexity, traditional synchronization and imaging solutions are not applicable for GNSS-based BSAR. To resolve this problem, this paper focuses on the integrative signal processing algorithm for GNSS-based BSAR. Firstly, to remove the effect of navigation signals, synchronization pre-processing is developed. Then the principle of measuring errors neutralization is theoretically proved by considering the geometric peculiarity of the system. Based on this principle, an integrative synchronization and imaging algorithm is proposed. Finally, the validity of the developed algorithm is confirmed by experimentally obtained data.

Reconstruction of Coherent Pairs of Synthetic Aperture Radar Data Acquired in Interrupted Mode

A number of synthetic aperture radar (SAR) systems might work in interrupted operation for different purposes. Examples are cooperative bistatic SAR systems with a synchronization link between the transmitter and receiver or multistatic systems operating in receive-only mode, among others. As a direct consequence, the acquired raw data contain missing echoes presented in a periodical or random pattern. Since the missing raw data introduce artifacts in the processed images, recovery methods have to be applied. Usually, spectral-estimation-based interpolators can be used to recover data. Although such algorithms show good performance for pointlike targets, their efficiency is decreased for distributed scatterers. In this paper, we propose, for a coherent pair of SAR images, the use of the common information in one image to reconstruct the other and vice versa. The conditions required for the proper use of the approach are discussed, and the method is verified using simulated data. One special case of study is the TanDEM-X mission, where the cooperative nature of the bistatic operation requires the periodic exchange of information between the satellites in order to gather information for calibration and synchronization, creating a periodic missing data pattern in the raw data. For this case of study, the reconstruction methods based on spectral estimation are analyzed, and the proposed reconstruction using cross-information is validated.

Adaptive yielding scheme for link scheduling in OFDM-based synchronous device-to-device (D2D) communication system

Compared to asynchronous contention-based random access, e.g., carrier sensing multiple access, synchronous and distributed link scheduling for orthogonal frequency-division multiplexing (OFDM) systems is a viable solution to improve system throughput for device-to-device (D2D) ad hoc network. In particular, spatial spectral efficiency can be improved by scheduling as many concurrent D2D links necessary to satisfy individual signal-to-interference ratio (SIR) requirements. In this paper, we propose an adaptive yielding mechanism that can further improve the spatial spectral efficiency by allowing for more concurrent D2D links whenever more interference can be accepted, e.g., when the instantaneous bandwidth efficiency requirement is less than the current link capacity. Even if the system throughput varies with the link density, it is shown that the average system throughput can be significantly improved by the proposed yielding mechanism.

Design of Layered Aerospace Time Synchronization Architecture and Time Synchronization Links Budget Comparison

The rapid development of various kinds of aerospace application systems requires the appropriate high-accuracy time and frequency standard. In order to establish suitable time and frequency standard in aerospace, based on the establishment of aerospace satellite visual model, we simulate the satellite visual time of 3-layer satellite constellation, including GEO satellite(Geostationary Earth Orbit), IGSO (Inclined GeoSynchronous Orbit)satellites and MEO(Medium Earth Orbit) satellites. The visual features of this satellite constellation have been gained. Combining with the major influencing factors of satellite clock offset error, we study the layered aerospace time synchronization architecture and give the advantage of the architecture and the choice of time synchronization method. Starting from the establishment of the time synchronization link demand, we simulate the variation range of intersatellite distance of the layered aerospace time synchronization architecture, and give budget results of GEO-GEO link in laser band and GEO-IGSO and GEO-MEO links in S-band and Kuband. The results show that GEO-GEO link can achieve to more than 3Gb/s data transmission rate on condition that the transmitting power is 0.5W, the antenna diameter is only 20cm; if the S-band transmission rate is in excess of 2Mb/s, when the antenna is 1m in diameter, the transmitting power needed is about 50W. In the Ka-band, 1m antennas only need 1W transmitting power to provide an intersatellite data transmission rate higher than 2Mb/s. Research results serves as reference for the establishment of layered aerospace time synchronization architecture and improvement of time synchronization precision.

Phase synchronization processing method for alternating bistatic mode in distributed SAR

In the distributed synthetic aperture radar (SAR), the alternating bistatic mode can perform phase reference without a synchronization link between two satellites compared with the pulsed alternate synchronization method. The key of the phase synchronization processing is to extract the oscillator phase differences from the bistatic echoes. A signal model of phase synchronization in the alternating bistatic mode is presented. The phase synchronization processing method is then studied. To reduce the phase errors introduced by SAR imaging, a sub-aperture processing method is proposed. To generalize the sub-aperture processing method, an echo-domain processing method using correlation of bistatic echoes is proposed. Finally, the residual phase errors of the both proposed processing methods are analyzed. Simulation experiments validate the proposed phase synchronization processing method and its phase error analysis results.