Time Reference Signal Research Articles

Remote Radio Frequency Sensing Based on 5G New Radio Positioning Reference Signals.

In this paper, the idea of a radar based on orthogonal frequency division multiplexing (OFDM) is applied to 5G NR Positioning Reference Signals (PRS). This study demonstrates how the estimation of the communication channel using the PRS can be applied for the identification of objects moving near the 5G NR receiver. In this context, this refers to a 5G NR base station capable of detecting a high-speed train (HST). The anatomy of a 5G NR frame as a sequence of OFDM symbols is presented, and different PRS configurations are described. It is shown that spectral analysis of time-varying channel impulse response weights, estimated with the help of PRS pilots, can be used for the detection of transmitted signal reflections from moving vehicles and the calculation of their time and frequency/Doppler shifts. Different PRS configurations with varying time and frequency reference signal densities are tested in simulations. The peak-to-noise-floor ratio (PNFR) of the calculated radar range-velocity maps (RVM) is used for quantitative comparison of PRS-based radar scenarios. Additionally, different echo signal strengths are simulated while also checking various observation window lengths (FFT lengths). This study proves the practicality of using PRS pilots in remote sensing; however, it shows that the most dense configurations do not provide notable improvements, while also demanding considerably more resources.

Cabinet clock distribution network for low-frequency aperture array

Square Kilometer Array (SKA)-Low is the radio telescope operating in the lowest frequency band of the SKA, from 50 up to 350 MHz. It consists of 512 stations, each composed of 256 dual-polarization log-periodic antennas for a total of 262,144 independent signal paths. The low-frequency aperture array (LFAA) is the portion of the SKA-Low telescope including the antennas and the related electronics. Signal processing is hosted in a temperature controlled and shielded facility: the central processing facility (CPF), for all the core stations, or remote processing facilities (RPF), for stations in the array arms, to limit the maximum fiber length. Such a geographically distributed and interconnected radio telescope, spanning ∼65 km in diameter, requires that frequency and timing reference signals are distributed to the processing facilities with high stability and precision to ensure the required system performances. We present the realization of the clock and pulse per second distribution network inside the LFAA signal processing cabinet where subracks containing signal acquisition boards are housed. We describe the different parts of the chain, and we report on the total jitter introduced by this structure.

High resolution laser fringe pattern projection based on MEMS micro-vibration mirror scanning for 3D measurement

3D measurement of micro-sized components by Fringe projection profilometry (FFP) requires fringe patterns with high spatial resolution. In order to project high-resolution and fine fringe patterns, a MEMS scanning module is used to project the precise time-modulated line laser stripes into space to form high-resolution laser fringes. The micro-vibration mirror in the MEMS module works in a resonant state, and the internal position detection of the MEMS module generate precise time reference signal. The reference signal is used to project spatial fringe patterns and ensure the stability and high resolution. The high-resolution laser fringe patterns generated by MEMS scanning module are projected to micro-sized components for 3D measurement. The measurement experiment shows that the spatial resolution of the projected fringes can reach 0.47 mm and the phase shift resolution is 0.12 mm. The measurement results show that the tiny height changes of the measured components can be well measured and presented. The standard gauge block with a height of 1 mm is measured, and the root mean square error of the measured height is 0.037 mm. It has a good prospect to apply the proposed method to the 3D measurement of micro-sized components.

Setup and Characteristics of a Timing Reference Signal with sub-ps Accuracy for AWAKE

We describe a method to overcome the triggering jitter of a streak camera to obtain less noisy images of a self-modulated proton bunch over long time scales (~ 400 ps to ns) with the time resolution (~ 1 ps) of the short time scale images (73 ps). We also determine that this method, using a reference laser pulse with a variable delay, leads to the determination of the time delay between the ionizing laser pulse and the reference pulse with an error of 0.6 ps (rms).

Devices with Whispering Gallery Mode Optical Resonators: Current State of Research and Prospects for their Application in Time and Frequency Metrology

The development of highly stable, narrow-band laser sources and oscillators producing sets of equidistant narrow optical spectral lines (so-called optical frequency combs) constitutes one of the most urgent tasks in the metrological support of time and frequency measurements. These devices are necessary for the transmission of frequency reference signals, as well as for use in mobile sources of time and frequency reference signals in optical and microwave spectral ranges. Such devices are also used in precision, ultra-high resolution spectroscopy. A promising direction for the development of highly stable, narrow-band lasers and generation of optical frequency combs (OFCs) is the use of miniature whispering gallery mode (WGM) optical resonators. Recently, the properties of such resonators have been actively studied along with technologies for their production. This article presents a review of the current state of research into the use of WGM optical resonators in time and frequency metrology. The types, advantages, and disadvantages of WGM resonators, as well as methods for generating various signals, specifically microwave signals, are described. Examples of existing laboratory setups used by Russian and foreign scientific groups are given. The authors examine prospects for the further use of WGM resonators.

Systolic blood pressure measurement by detecting the photoplethysmographic pulses and electronic Korotkoff-sounds during cuff deflation

Objective:The commonly used noninvasive techniques for blood pressure measurement, auscultatory Korotkoff-based sphygmomanometry and oscillometry, both have limitations in their clinical use. The former is relatively accurate but is limited to use in a physician’s office because its automatic variant is subject to noise artifacts. Automatic oscillometers can be used at home, but their accuracy is low. In the current study, we present two electronic techniques for systolic blood pressure (SBP) measurement that are more accurate than the available techniques, and can be automated.Approach:The cuff-based electronic techniques for SBP measurement used in this study utilize a finger photoplethysmographic (PPG) signal or electronically recorded Korotkoff sounds, and detection of the first appearance of PPG or Korotkoff-based pulses during cuff deflation. Most noise artefacts were eliminated by means of a time-reference signal—a PPG signal measured in the free hand.Main results:The two electronic techniques showed good agreement. Both techniques were found to be more accurate than the auscultatory Korotkoff-based technique, the gold-reference, in those examinations where large discrepancies were found between them, since the PPG and Korotkoff-based electronic pulses appeared before Korotkoff sounds were heard. The PPG signal in the free hand, that served as a time-reference signal, enabled both electronic techniques to eliminate artefacts—noise pulses that appeared outside the time intervals in which a blood-pressure-related pulses should appear.Significance:The two electronic techniques provide accurate systolic blood pressure measurements and can be implemented automatically, as was shown in a previous study on blood pressure measurement by PPG.Trial registration number of the institutional ethical committee of Shaare-Zedek Medical Center, Jerusalem is 0154-16-SZMC.

State of the art and perspectives of the devices with the optical WGM resonators in time and frequency metrology

One of the most important tasks in the metrological support of time and frequency measurements is a development of an extremely stable narrow-bandwidth laser sources and oscillators of the sets of the equidistant narrow optical spectral lines (the so-called “optical combs”). These devices are necessary for the transmission of the reference frequency signals and for use as part of mobile sources of time and frequency reference signals in optical and microwave spectral ranges. They are also required for precision ultra-high resolution spectroscopy. A promising direction for the creation of highly stable narrow-band lasers and optical combs is the use of miniature optical whispering gallery modes resonators. Recently, research has been actively performed on the properties and manufacturing techniques of such resonators, as well as devices using them. The article provides a review of current research into applications of optical whispering gallery mode resonators in time and frequency metrology. Main advantages and disadvantages of such devices and prospects for their further use are discussed. The authors review in detail some examples of existing laboratory setups of Russian and foreign scientific groups.

Jitter Characterization of a Dual-Readout SNSPD

To better understand the origins of the timing resolution, also known as jitter, of superconducting nanowire single-photon detectors (SNSPDs), we have performed timing characterizations of a niobium nitride SNSPD with a dual-ended readout. By simultaneously measuring both readout pulses along with an optical timing reference signal, we are able to quantify each independent contribution to the total measured jitter. In particular, we are able to determine values for the jitter due to the stochastic nature of hotspot formation and the jitter due to the variation of the photon detection location along the length of the nanowire. We compare the results of this analysis for measurements at temperatures of 1.5 K and 4.5 K.

Optimization of Clock Synchronization System Architecture in Nuclear Power Plant

Clock Synchronization System (CSS) can generate standard time, time and frequency signals, and provide various high-precision time reference signals for systems and equipment requiring standard time scale in nuclear power plants. With the rapid development of power system, the requirement of time synchronization is becoming more and more urgent, which requires accurate, safe and reliable clock sources. The structure of nuclear power plant clock synchronization system is constantly optimized to meet the requirements of time synchronization of various systems and equipment, to ensure the time consistency of real-time data acquisition, to improve reliability and to adapt to the interconnection of large power grids in China. At the same time, the interconnection between time synchronization system and time-service equipment or system, and the interconnection of first-level time synchronization system network and operation mode of time synchronization devices from different manufacturers are standard

A Modem with a Fiber-Optic Communication Line for Dissemination of Frequency and Time Reference Signals

The features of a multichannel modem that has been developed for transmission of frequency and time reference signals on a fiber-optic communication line with phase instability compensation are examined. Its capabilities for transmitting signals of reference frequencies of active hydrogen masers (such as the Ch1-1003M), we well as a 1 PPS signal with an error of synchronization no greater than 250 psec over 100 km without the use of optical amplifiers are assessed. The results of experimental verification of the metrological characteristics of the modem are given.

Uncertainty Analysis of an Equivalent Synchronization Method for Phasor Measurements

This paper deals with the accuracy evaluation of a method for gathering synchronized phasor measurements of electrical quantities in nodes of power networks. Usually, this task is performed using commercially available instrumentation, for instance, the phasor measurement unit. They rely on a common time reference signal provided usually by the GPS system. After a brief recall of two methods already presented by authors in previous papers, which do not require a common time reference for providing synchronized phasors of voltages at the network nodes, the major sources of uncertainty in the measurement chain (lengths of the branches of the network; per-unit length reactance of the cables; and effect of the instrument transformers) are considered and investigation on their contribution, in one of the two methods, to the overall uncertainty performed.

Hypersampling of pseudo-periodic signals by analytic phase projection

A method to upsample insufficiently sampled experimental time series of pseudo-periodic signals is proposed. The result is an estimate of the pseudo-periodic cycle underlying the signal. This “hypersampling” requires a sufficiently sampled reference signal that defines the pseudo-periodic dynamics. The time series and reference signal are combined by projecting the time series values to the analytic phase of the reference signal. The resulting estimate of the pseudo-periodic cycle has a considerably higher effective sampling rate than the time series. The procedure is applied to time series of MRI images of the human brain. As a result, the effective sampling rate could be increased by three orders of magnitude. This allows for capturing the waveforms of the very fast cerebral pulse waves traversing the brain. Hypersampling is numerically compared to the more commonly used retrospective gating. An outlook regarding EEG and optical recordings of brain activity as the reference signal is provided.

Accuracy Evaluation of an Equivalent Synchronization Method for Assessing the Time Reference in Power Networks

This paper deals with the evaluation of the accuracy performance of an approach for assessing the phase displacement between voltages at power network nodes. This task is accomplished by processing asynchronous measurements taken at each node. This turns into an equivalent synchronization, which is, therefore, obtained without exploiting any synchronization signals, such as the ones provided by means of wireless (i.e., global positioning system) or wired technologies. As a matter of fact, distribution system operators will gain the possibility of deploying, at more affordable costs, wide area measurement system (WAMS) over their power networks for enhancing their stability and reliability. Phasor measurement units (PMUs) are the most common examples of such WAMS, but, besides their high cost, there are circumstances where providing a time reference signal to remote PMUs often becomes a difficult task. This paper aims at recalling the basic theoretical principles of the method and at proving its applicability in power network through a deep analysis of its metrological performance.

Time delay finite time control of unified power flow controller for power flow reference tracking

This paper presents a time delay finite time controller (TDFTC) applied to a unified power flow controller (UPFC) to achieve rapid and finite time reference signal tracking of power flow in presence of plant uncertainties. The TDFTC is a nonlinear control based on the time delay disturbance estimator and the Lyapunov's stability theory to estimate the disturbances and cancel them. The TDFTC overcomes the drawback of traditional linear PI control, which is typically tuned for one specific operating condition, and reduces chattering by using lower design gains. In this paper, first a state space complete dynamic model of UPFC is established based on Kirchhoff's equations and Park's transformation and then TDFTC law is developed for the UPFC system. The proposed control is validated via a detailed simulation on a two bus system. Simulation results show power, effectiveness, robustness and accuracy of the proposed controller.

Polarization effects in KAT-7 telescope optical fibre network: Towards the distribution of frequency and timing references in the MeerKAT telescope

Polarization mode dispersion (PMD) is one of the key effects in optical fibre communication that distorts the transmitted signal. The effect of birefringence on the polarization states of an optical signal leads to broadening of the pulses in an optical fibre. The difference in propagation time of the orthogonal modes in a fibre results in differential group delay (DGD). For proper functioning of a telescope, accurate and stable clock signals will be distributed via an optical fibre network to each of the dishes. This paper presents field PMD measurement that is essential in estimating the expected phase change of the clock signal. The average DGD of a 10.2km fibre link established to be about 62.1fs. This study serves to address the applicability of optical fibre network transmission in the distribution of frequency and timing reference signals in the MeerKAT and later phases of the square kilometre array (SKA) telescope.

Mapping the unmappable: local vector mapping.

Arrhythmias are commonly mapped using a timing reference signal. There are a number of situations where mapping may be difficult or even impossible using conventional reference-based mapping. Wobbling arrhythmias or those who do not maintain a 1:1 relationship with the reference signal are problematic. Local vector mapping may be performed in these situations to map arrhythmias without the use of a reference signal.

A Generalized Decentralized Robust Control of Islanded Microgrids

This paper presents the fundamental concepts of a generalized central power management system and a decentralized, robust control strategy for autonomous mode of operation of a microgrid that includes multiple distributed energy resource (DER) units. DER units are divided into voltage-controlled and power-controlled DER units. The frequency of each DER unit is determined by its independent internal oscillator and all oscillators are synchronized by a common time-reference signal based on a global positioning system (GPS). The power management system (PMS) specifies the power and voltage set points for the local controllers of each DER unit. A linear, time-invariant, multivariable, robust, decentralized control system is designed to track the setpoints. Each control agent guarantees fast tracking, zero steady state error, and robust performance despite uncertainties of the microgrid parameters, topology, and the operating point. Existence conditions, control design procedures, eigenanalysis and robust stability analysis of the closed-loop system, and performance of the control strategy based on digital time-domain simulation studies in PSCAD/EMTDC platform, are reported. Performance of the control system is also verified based on hardware-in-the-loop (HIL) studies in the RTDS environment.

Decentralised ranging method for orthogonal frequency division multiple access systems with amplify‐and‐forward relays

In this study, a decentralised ranging method for uplink orthogonal frequency division multiple access (OFDMA) systems with half-duplex (HD) amplify-and-forward (AF) relay stations (RSs) is proposed. In the OFDMA systems with HD AF RSs, twice more resources and delays are required as ranging without RS. To reduce the required resources and delays for ranging, the authors propose a two-phase ranging scheme based on the decentralised timing-offset estimation at each ranging mobile station (MS). At the first phase, RS occasionally broadcasts timing reference signal, and at the second phase RS retransmits the collected ranging signals from the MSs. Then, each ranging MSs can individually estimate its own timing offset from the received signals. In the proposed ranging method, the base station does not need to send a timing-adjustment message, and the overhead associated with ranging in the downlink resources, and computational complexity can be significantly reduced without degrading the timing-offset-estimation performance. Moreover, the delay associated with ranging can be maintained as same as ranging without RS.

A Decentralized Robust Control Strategy for Multi-DER Microgrids—Part I: Fundamental Concepts

This paper presents fundamental concepts of a central power-management system (PMS) and a decentralized, robust control strategy for autonomous mode of operation of a microgrid that includes multiple distributed energy resource (DER) units. The DER units are interfaced to the utility grid through voltage-sourced converters (VSCs). The frequency of each DER unit is specified by its independent internal oscillator and all oscillators are synchronized by a common time-reference signal received from a global positioning system. The PMS specifies the voltage set points for the local controllers. A linear, time-invariant, multivariable, robust, decentralized, servomechanism control system is designed to track the set points. Each control agent guarantees fast tracking, zero steady-state error, and robust performance despite uncertainties of the microgrid parameter, topology, and the operating point. The theoretical concept of the proposed control strategy, including the existence conditions, design of the controller, robust stability analysis of the closed-loop system, time-delay tolerance, tolerance to high-frequency effects and its gain-margins, are presented in this Part I paper. Part II reports on the performance of the control strategy based on digital time-domain simulation and hardware-in-the-loop case studies.

High-Accuracy Reference-Free Ultrasonic Location Estimation

This paper presents a novel reference-free ultrasonic indoor location system. Unlike most previous proposals, the mobile device (MD) determines its own position based only on ultrasonic signals received at a compact sensor array and sent by a fixed independent beacon. No radio frequency or wired timing reference signal is used. Furthermore, the system is privacy aware and one way in that the receive-only MD determines its own position based on ultrasonic signals received from fixed transmit-only beacons. The MD uses a novel hybrid angle of arrival (AoA)-time of flight (ToF) with timing lock algorithm to determine its location relative to the beacons with high accuracy. The algorithm utilizes an AoA-based location method to obtain an initial estimate of its own location. Based on this, it estimates the timing offsets (TOs) between the MD clock and the beacon transmissions. The average TO and the known periodicities of the beacon signals are then used to obtain a second more accurate MD location estimate via a ToF method. The system utilizes wideband spread spectrum ultrasonic signaling in order to achieve a high update rate and robustness to noise and reverberation. A prototype system was constructed, and the algorithm was implemented in software. The experimental results show that the method provides 3-D accuracy better than 9.5 cm in 99% of cases, an 80% accuracy improvement over the conventional AoA-only method.