Clock Synchronization Research Articles

Evaluation of receiver calibration based on clock-steering for time–frequency transfer

Calibrating the global navigation satellite system (GNSS) time–frequency transfer system is crucial for accurate GNSS time–frequency transfer. The most commonly used relative calibration method relies on the common-clock to eliminate clock differences. However, it cannot eliminate the changes in clock signal reference point delay (REFDLY). The changes in REFDLY during calibration could not be ignored, especially for ordinary users in an unstable environment. To address this issue, we use the GNSS real-time single-difference time synchronization calibration based on clock-steering, which can detect the change in REFDLY. According to this method, the clock differences and REFDLY are eliminated simultaneously. We installed two GNSS antennas on the roof of the Xinghu Building of Wuhan University and designed a clock synchronization terminal to evaluate the performance of this method. The data were collected on Day of Year 152–156, 167–171, 222–226, 231–235, 244, 258, 260, and 262 in 2022. In accordance with the calibration results, the standard deviation better than 100 ps, and the stability reaches 10−15@10000s . Also, the time variance better than 60 ps, and the uncertainty uCAL better than 0.5 ns. Compared to the common-clock calibration, one of the most significant benefits is the elimination of REFDLY, which leads to decreased uncertainty and better stability over both the short-term and long-term.

Методика формирования комплексной системы синхронизации и доставки шкалы времени для крупной распределенной системы технологического назначения

Purpose: To develop scientific and technical solutions for constructing a comprehensive synchronization system and delivery of the time scale, which includes subsystems of network clock synchronization (NCS) and unified time (UT), as a unified timekeeping system addressing the challenges of frequency, time, and phase synchronization in large-scale distributed technological purpose systems. Methods: Research methods are based on the fundamental provisions of the theory of systems and the theory of decision-making. Results: A methodology for the formation of a comprehensive synchronization system and the delivery of a time scale is proposed, which is characterized by an integrated approach to building unified time and frequency systems in the communication networks of the technological segment, the independence of the types of generator equipment used and the means of delivery of NCS and UT signals, as well as meeting the requirements of potential consumers of frequency and time services. The proposed methodology for the formation of a comprehensive synchronization system and delivery of a time scale will allow to reasonably approach building systems for delivering unified time and frequency signals in complex heterogeneous communication networks. Practical significance: Solutions based on the presented methodology will allow the provision of an infrastructure independent of external communication operators to ensure the unity of network clock synchronization and unified time. This is necessary for the correct operation of telecommunication equipment of the technological communication network. The methodology will also meet the needs for providing a full range of synchronization and unified time signal services to existing and prospective consumers. Additionally, it will enable a gradual transition of the organization of communication networks from channel-swithching to packet-oriented technologies due to a flexible approach to the routes and transmission systems used for the distribution of synchronization signals and unified time.

Timing Matters: The Interplay between Early Mealtime, Circadian Rhythms, Gene Expression, Circadian Hormones, and Metabolism-A Narrative Review.

Achieving synchronization between the central and peripheral body clocks is essential for ensuring optimal metabolic function. Meal timing is an emerging field of research that investigates the influence of eating patterns on our circadian rhythm, metabolism, and overall health. This narrative review examines the relationship between meal timing, circadian rhythm, clock genes, circadian hormones, and metabolic function. It analyzes the existing literature and experimental data to explore the connection between mealtime, circadian rhythms, and metabolic processes. The available evidence highlights the importance of aligning mealtime with the body's natural rhythms to promote metabolic health and prevent metabolic disorders. Specifically, studies show that consuming meals later in the day is associated with an elevated prevalence of metabolic disorders, while early time-restricted eating, such as having an early breakfast and an earlier dinner, improves levels of glucose in the blood and substrate oxidation. Circadian hormones, including cortisol and melatonin, interact with mealtimes and play vital roles in regulating metabolic processes. Cortisol, aligned with dawn in diurnal mammals, activates energy reserves, stimulates appetite, influences clock gene expression, and synchronizes peripheral clocks. Consuming meals during periods of elevated melatonin levels, specifically during the circadian night, has been correlated with potential implications for glucose tolerance. Understanding the mechanisms of central and peripheral clock synchronization, including genetics, interactions with chronotype, sleep duration, and hormonal changes, provides valuable insights for optimizing dietary strategies and timing. This knowledge contributes to improved overall health and well-being by aligning mealtime with the body's natural circadian rhythm.

Improvement of Intersatellite Measurements Scheduling to Refine the Accuracy of the Ephemerides of Modern and Prospective GLONASS Orbital Segments

The information technology developed by the authors to ensure the possibility of operation without downloading information by the ground control complex while improving the accuracy of the ephemerides of the existing global navigation satellite systems (GLONASS) midaltitude segment and its promising additions is described. Achieving the stated goals within the framework of the technology under discussion involves the formation of precise ephemerides by solving the following interrelated tasks: improving the accuracy of determining and predicting the ephemerides of navigation satellites in the inertial coordinate system; improvement of methods and algorithms for forecasting and updating the evolution of the Earth’s rotation parameters onboard satellites; and precise synchronization of onboard clocks. The primary attention is paid to the first of the listed tasks, i.e., prediction and refinement of the ephemerides of navigation satellites in the inertial coordinate system, including in the mode of operation of satellite constellations without updating the ephemerides-temporal information. As a base for its solution, it is proposed to use advanced onboard hardware for intersatellite measurements. The basic limitations related to the implementation of intersatellite measurements and their subsequent processing for both midaltitude and advanced space segments of the GLONASS are discussed. The results of the design-ballistic analysis and processing of real and simulated measurements, as well as the preliminary characteristics of the accuracy of the ephemerides estimates obtained in this case onboard, are presented.

Голографическая фотонная система фазовой синхронизации в спутниковом радиоканале

A necessary condition for the qualitative solution of telecommunication, infocommunication and navigation tasks in satellite communication systems is the presence of a high-precision synchronization system in satellite radio channels. It is indicated that the operation of the data exchange synchronization system in satellite radio channels is adversely affected by a number of external factors, such as, for example, the presence of the Doppler effect, errors in determining the relative speeds and directions of movement of artificial Earth satellites (ISS), the presence of a delay time of the transmitted signal due to significant distances between the nodes of the satellite communication network. The main disadvantages of modern synchronization systems used in satellite radio channels are given, and it is concluded that there is a relatively high probability of synchronism failure. It is indicated that the primary type of synchronization in a satellite radio channel is phase synchronization, the errors of which affect all subsequent types of synchronization (clock, frame and cycle). Analysis of the technical characteristics of modern phase synchronization systems in satellite radio channels allowed us to conclude that it is urgent to search for a new approach to the construction of these systems, which provides an increase in the speed and accuracy of their works. A possible way to solve this problem is the use of holographic photonics elements based on holographic sensing elements in measuring equipment that ensures the functioning and synchronization of satellite radio channels. The article solves the problem of constructing a holographic photonic phase synchronization system in a satellite radio channel. A technical solution is proposed to increase the speed, sensitivity and accuracy of the phase synchronization system by using a volumetric reflective Fourier hologram in the design of this system. A variant of the block diagram of the holographic photonic phase synchronization system is developed and the operation of this scheme is described. The constructive construction of optical elements of the developed block diagram in the form of electro-optical modulators and deflector optical systems is proposed. The individual features of the construction of these structural elements are indicated. The functional purpose of a volumetric reflective Fourier hologram in the design of a holographic photonic phase synchronization system is formulated. The regularities of changes in the parameters of optical fields in the holographic photonic elements of the phase synchronization system of a satellite radio channel are investigated. Concrete numerical examples demonstrate the possibility of providing high sensitivity and accuracy of a holographic photonic phase synchronization system.

Joint Synchronization and Range Estimation With Correlated Arrival Times in Molecular Communication Networks

Time synchronization is needed in information exchange and collaboration among nanomachines for molecular communication networks. The time of arrival-based synchronization method requires the range information between the transmitter nanomachine (TN) and the receiver nanomachine (RN) in order to improve the accuracy of clock synchronization. The range information between the nanomachines helps to choose the optimal transmission rate. In contrast to the existing methods, this article proposes a joint clock offset and range estimation with correlated arrival times based on the maximum likelihood estimator. The RN uses the molecules' transmission and correlated arrival times to estimate the clock offset and range between the TN and RN. Even with correlated arrival times, the individual clock offset estimation performance is comparable to independent arrival times performance. Notably, the joint estimation problem with correlated arrival times is generic and realistic for molecular applications.

Unsupervised Deep-Learning for Distributed Clock Synchronization in Wireless Networks

One of the major factors which limits the throughput in wireless communications networks is the accuracy of time synchronization between the nodes in the network. Synchronization methods based on pulse-coupled oscillators (PCOs) have the advantage of simple implementation and achieve high accuracy when the nodes are closely located. However, such schemes tend to have poor synchronization performance for distant nodes, as well as in the presence of clock frequency offsets between the nodes. In this paper we present a novel PCO-based Deep neural network (DDN)-Aided Synchronization Algorithm coined DASA. We design DASA as a novel low-complexity and interpretable architecture by converting classic PCO-based synchronization into <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a trainable discriminative model</i> . To enable DASA to operate in dynamic settings, we propose a novel, unsupervised, distributed, fast <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">online</i> training scheme which is able to train DASA <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">within a few sampling instances, locally</i> , thereby avoiding the need for information exchange between the nodes or for a central node for coordination. DASDA is demonstrated to achieve an improvement by a factor greater than ten compared to the classic reference scheme. Lastly, we propose another novel, distributed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">offline</i> training scheme for DASA, which is demonstrated to offer a tradeoff between performance and simplicity of deployment compared to the online training scheme, yet, at the same time, DASA with offline training still achieves superior performance compared to the classic reference scheme.

Communications with guaranteed bandwidth and low latency using frequency-referenced multiplexing

Emerging cloud applications such as virtual reality and connected car fleets demand guaranteed connections, as well as low and stable latency, to edge data centres. Currently, user–cloud communications rely on time-scheduled data frames through tree-topology fibre networks, which are incapable of providing guaranteed connections with low or stable latency and cannot be scaled to a larger number of users. Here we show that a frequency-referenced multiplexing method can provide guaranteed bandwidth and low latency for time-critical applications. We use clock and optical frequency synchronization, enabled by frequency comb and signal processing techniques, to provide each user with dedicated optical bandwidth, creating scalable user–cloud upstream communications. As a proof of concept, we demonstrate a frequency-division multiplexing system servicing up to 64 users with an aggregate bandwidth of 160 GHz, exhibiting a data rate of up to 4.3 Gbps per user (240.0 Gbps aggregated capacity considering a 200 GHz wavelength band) with a high receiver sensitivity of –35 dBm.

A Modified Strong Tracking Adaptive Kalman Filter for Precision Clock Synchronization System

In order to reduce the clock offset caused by network delay jitter, network asymmetry, clock drift and clock frequency difference in the clock synchronization process of IEEE1588, a modified Strong Tracking Adaptive Kalman Filter (MSTAKF) algorithm was proposed. The Kalman filter model established by the algorithm is based on clock offset, network delay, frequency deviation and asymmetry ratio. The filter includes the modified strong tracking algorithm based on fuzzy logic and hypothesis test proposed in this paper, which can effectively eliminate the influence of outliers, and finally make the model tracking effect better. In addition, the modified Sage‐Husa algorithm based on Cholesky decomposition is also included, which makes the measurement noise more stable and ensures the positivity of the measurement noise covariance. The experimental results show that the proposed MSTAKF is smoother than Karman filter (KF), Sage‐Husa adaptive Kalman filter (AKF) and strong tracking Sage‐Husa adaptive Kalman filter (STAKF). Compared with these filter algorithms after testing in the LAN environment, the precision improvement and accuracy rate are significantly improved. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

DA-DMPF: Delay-aware differential multi-path forwarding of industrial time-triggered flows in deterministic network

The time-sensitive network (TSN) utilizes gating technology to achieve mixed traffic transmission across the common network, replacing dedicated networks with time division multiplexing, simplifying the interconnection architecture. TSN fundamentally changes the uncertainty of traditional Ethernet through clock synchronization, traffic scheduling, and network configuration. TSN can meet the requirements of industrial communication for low delay, low jitter, and high-reliable data transmission. It is also fully compatible with the existing Ethernet system and is an important communication platform supporting the development of the industrial Internet. Time-triggered flow is the key flow responsible for transmitting real-time data in time-sensitive networks. Its transmission should strictly follow the gating schedule. Selecting different transmission paths will affect the success rate of scheduling. To solve the delay optimization problem of industrial time-triggered (TT) flow path selection, this paper proposes a novel forwarding mechanism for industrial TT flow, namely delay-aware differential multi-path forwarding (DA-DMPF). This mechanism first designs a multipath forwarding index based on sending delay and processing delay by analyzing the end-to-end delay, then converts the multipath forwarding problem into a 0-1 integer linear planning problem, and finally solves the forwarding path set of the stream through differential multipath forwarding algorithm to achieve low delay parallel transmission of the stream. The simulation results show that compared with load-balanced multi-path forwarding and Hop-Load forwarding mechanisms, this scheme reduces the end-to-end average delay of industrial TT flows by 22.55% and 17.32% respectively, meeting the low delay transmission requirements of TT flows.

A Survey on In-Vehicle Time-Sensitive Networking

With the continued evolution of autonomous driving, the communication demand for in-vehicle networks has dramatically increased, and the traditional in-vehicle network solutions have become ineligible for the needs of new in-vehicle applications. Time-Sensitive Network(TSN) with the characteristics of determinism, low latency, high reliability, large bandwidth, and open industry standards, has been considered as the most promising technology for the next-generation in-vehicle networks(IVN). This paper summarizes the current state and methodologies of in-vehicle TSN research through the following five aspects: QoS(Quality of Service) strategy, reliability, clock synchronization, network planning, and network management. The corresponding research priorities and technical challenges are also addressed respectively.

ALOHA-NUI: A collision-free version of ALOHA using a Neighborhood-Understood Index

ALOHA with priority acknowledgments (ACK) is transformed into a collision-free channel access method by means of a neighborhood-understood index (NUI). Each node maintains the NUI, which allows nodes to remember all nodes that requested to share the channel and the order in which they should be allowed to transmit. The resulting protocol, ALOHA-NUI, adds signaling packets and NUI information in data packets to maintain the NUI, rather than just remembering that a data packet was sent successfully as in ALOHA. This results in ALOHA-NUI, which is compared with TDMA assuming a fixed transmission schedule, ALOHA with priority ACK’s, and CSMA with priority ACK’s analytically and by simulation. ALOHA-NUI is shown to attain the high throughput of collision-free transmission scheduling methods that usually require clock synchronization while maintaining most of the simplicity of ALOHA with priority ACK’s. ALOHA-NUI is also shown to be fair and to reach stable transmission schedules very quickly.

The light source of the TRIDENT pathfinder experiment

In September 2021, a site scouting mission known as the TRIDENT pathfinder experiment (TRIDENT EXplorer, T-REX for short) was conducted in the South China Sea with the goal of envisaging a next-generation multi-cubic-kilometer neutrino telescope. One of the main tasks is to measure the in-situ optical properties of seawater at depths between 2800 m and 3500 m, where the neutrino telescope will be instrumented. To achieve this, we have developed a light emitter module equipped with a clock synchronization system to serve as the light source, which could be operated in pulsing and steady modes. Two light receiver modules housing both photomultiplier tubes (PMTs) and cameras are employed to detect the photons emitted by the light source. This paper presents the instrumentation of the light source in T-REX, including its design, calibration, and performance.

Clock synchronization based on pulse with propagation delay eliminated in wireless sensor networks

AbstractClock synchronization is indispensable for numerous applications of wireless sensor networks (WSNs). When no common reference clock is available, the nodes must employ distributed synchronization techniques. This paper proposes, a distributed pulse‐based clock synchronisation approach, wherein the propagation delay is eliminated through signal ping‐pongs between neighbouring nodes. Such an approach can jointly estimate the clock skew and offset without requiring any reference clock. The whole synchronization process is completed at the physical (PHY) layer, effectively avoiding the random delay caused by packet queuing and retransmission. Simulation results show that the proposed approach can achieve higher synchronization accuracy compared with other existing methods.

The SCN-HPA-Periphery Circadian Timing System: Mathematical Modeling of Clock Synchronization and the Effects of Photoperiod on Jetlag Adaptation.

Synchronizing the circadian timing system (CTS) to external light/dark cycles is crucial for homeostasis maintenance and environmental adaptation. The CTS is organized hierarchically, with the central pacemaker located in the suprachiasmatic nuclei (SCN) generating coherent oscillations that are entrained to light/dark cycles. These oscillations regulate the release of glucocorticoids by the hypothalamus-pituitary-adrenal (HPA) axis, which acts as a systemic entrainer of peripheral clocks throughout the body. The SCN adjusts its network plasticity in response to variations in photoperiod, leading to changes in the rhythmic release of glucocorticoids and ultimately impacting peripheral clocks. However, the effects of photoperiod-induced variations of glucocorticoids on the synchronization of peripheral clocks are not fully understood, and the interaction between jetlag adaption and photoperiod changes is unclear. This study presents a semi-mechanistic mathematical model to investigate how the CTS responds to changes in photoperiod. Specifically, the study focuses on the entrainment properties of a system composed of the SCN, HPA axis, and peripheral clocks. The results show that high-amplitude glucocorticoid rhythms lead to a more coherent phase distribution in the periphery. In addition, our study investigates the effect of photoperiod exposure on jetlag recovery time and phase shift, proposing different interventional strategies for eastward and westward jetlag. The findings suggest that decreasing photic exposure before jetlag during eastward traveling and after jetlag during westward traveling can accelerate jetlag readaptation. The study provides insights into the mechanisms of CTS organization and potential recovery strategies for transitions between time zones and lighting zones.

Internet of things-blockchain lightweight cryptography to data security and integrity for intelligent application

The industrial internet of things (IoT) plays a major role in the growth of automation and increasing digital connectivity for machine-to-machine communication. The research community has extensively investigated the possibility of IoT and blockchain integration for the last couple of years. The major research is focused on the benefits of integrating blockchain with IoT. In this work, we first focus on the issue of integrating IoT nodes with blockchain networks, especially for non-real-time IoT nodes that do not have an in-built clock mechanism. As a result, they cannot establish communication with real-time blockchain networks. Another critical security issue is protecting data coming from IoT devices to blockchain networks. Blockchain is enough mature to protect the data in its ecosystem. However, information coming from outside of the world does not have any guarantee of data integrity and security. This paper first addresses the clock synchronization issue of IoT nodes with blockchain using a network time protocol and then proposes an IoT-blockchain light-weight cryptographic (IBLWC) approach to secure the entire IoT-blockchain ecosystem. This paper also presents the performance analysis of IBLWC as a suitable and cost-effective solution that incurs less processing overhead for IoT-blockchain-based applications.

Clock-Latency-Aware Fault-Tolerant DLL for Multi-Die Clock Synchronization

A Delay-Locked Loop (DLL) circuit is indispensable for clock synchronization in a chip incorporating several heterogeneous dice. It has been shown previously that a fault and soft-error tolerant DLL can be achieved by Triple-Module Redundancy (TMR) enhanced with a timing correction scheme. However, the prior work still has a severe limitation -it does not consider the latency of the clock tree, and this limitation will make it infeasible in realistic situations. We demonstrate in this paper that this limitation can be overcome by a new “clock-latency-aware” architecture, thereby making a fault and soft-error-tolerant DLL truly realistic.

Centimeter-level clock synchronization and space-borne timescale generation for BDS-3 using inter-satellite link measurements

Centimeter-level clock synchronization and space-borne timescale generation for BDS-3 using inter-satellite link measurements

EasyAPPos: Positioning Wi-Fi Access Points by Using a Mobile Phone

Determining the location of Wi-Fi access points (APs) is vital for various Wi-Fi-based applications such as localization, security, and AP deployment. Considerable effort has been exerted in the field of AP localization. In contrast to studies that require additional robots with specialized antenna arrays, we present EasyAPPos, a lightweight, always-on, and user-centered AP positioning solution that utilizes widely available mobile phones. We focus on addressing three challenges in AP positioning. First, the patch antenna on a mobile phone has a limited angular range due to its size, but our approach proposes a method for utilizing human natural rotation to enhance angular diversity. Second, our angle-based algorithm does not require synchronous clocks between the mobile device and the APs, in contrast to existing algorithms that require this synchrony to transform propagation delays into positions. Nevertheless, our algorithm can still utilize asynchronous delay information. Third, the low bandwidth of Wi-Fi beacon frames, which only provide limited capacity to counteract the effects of multipath, is addressed by performing AP positioning under challenging conditions. We validate EasyAPPos through simulations and experiments, which demonstrate its ability to achieve decimeter-level positioning accuracy even under harsh conditions.

Impact analysis and detection of time-delay attacks in time-sensitive networking

Time-sensitive networking (TSN) will be widely used in automotive industry and industrial automation because it can provide deterministic transmission. Most of the TSN traffic shaping mechanisms rely on clock synchronization between different devices in the network. However, Time-Delay Attacks (TDAs) can interfere with synchronization, which further reduces traffic transmission quality. Although some studies have proposed detection strategies against TDAs, they are oriented to traditional ethernet and restricted by network architecture and devices. Therefore, this paper first analyzes the impact of TDAs on traffic transmission quality under different combinations of Time-Aware Shaper (TAS) or Cyclic Queue Forwarding (CQF) with link redundancy. Then, this paper provides a distributed monitoring parameter using Per-Stream Filtering and Policing (PSFP) for the detection and localization of TDAs, in which a modified token bucket mechanism is applied. Finally, the paper models the application of TSN in automotive industry for verification and further evaluates the effectiveness of this parameter through simulation. The results show that TDAs can cause undesirable variations in traffic latency under TAS and CQF. While proper parameter configuration and link redundancy can mitigate the impact of TDAs, they cannot prevent them. Besides, the proposed parameter helps detect and locate TDAs in TSN effectively.