Time-frequency Synchronization Research Articles

Time-frequency synchronisation contrastive learning-driven multi-sensor remaining useful life prediction

ABSTRACT Deep learning techniques play a crucial role in predicting the remaining useful life (RUL) of mechanical equipment. Nevertheless, obtaining a substantial number of labeled samples is a challenge, and the prediction accuracy tends to decline when the labeled samples are insufficient. Moreover, existing RUL prediction methods usually extract the degradation characteristics only from one single domain, which is insufficient for a high-accuracy prediction. To address these challenges, a time-frequency synchronization contrastive learning-driven (TFSCL) multi-sensor remaining useful life prediction model is proposed. The proposed TFSCL utilizes a large amount of unlabeled data for model pre-training and key feature extraction, and it introduces a novel time-frequency fusion contrastive loss function to optimize the pre-training process. It employs a dual-channel structure at the sensor and timestamp levels, incorporating an attention mechanism that adaptively adjusts sensor feature weights, enabling more accurate extraction of critical information while effectively mitigating interference from irrelevant data. To validate the effectiveness of the proposed TFSCL, two case studies are conducted, with different labeling ratios being used. The experimental results demonstrate that even with lower labeling ratios, the proposed TFSCL model still achieves a satisfactory prediction effect and outperforms other advanced methods.

Multiple-Access Time and Frequency Transfer over Fiber and Free-Space Link Based on Optical Frequency Comb

We have demonstrated a multiple-access transfer of time and frequency signal over a fiber and free-space link based on an optical frequency comb (OFC). With this transfer technique, two time–frequency signals were disseminated separately from a master site to two slave sites over a 3.9 km fiber and 100 m free-space link for 10,000 s. The timing fluctuations and instabilities of the time and frequency transfer were measured, estimated, and discussed. The experimental results show that the total root-mean-square (RMS) timing fluctuation of the transfer from site A to B is about 119 ps, with a fractional frequency instability on the order of 3.3 × 10−11 at 1 s and 2.8 × 10−14 at 2000 s. The RMS timing fluctuation of the transfer from site A to C is about 59.5 ps, with a fractional frequency instability on the order of 3.0 × 10−11 at 1 s and 2.6 × 10−14 at 2000 s. These results indicate that the multiple-access transfer technique proposed in this paper can provide important support for the application of a large-scale time–frequency synchronization network.

Zadoff–Chu Sequence Pilot for Time and Frequency Synchronization in UWA OFDM System

In underwater communications for 6G, Doppler effects cause the coherent time to become similar to or shorter than the orthogonal frequency division multiplexing (OFDM) symbol length. Conventional time and frequency synchronization methods require additional training symbols for synchronization, which reduces the traffic data rate. This paper proposes the Zadoff–Chu sequence (ZCS) pilot-based OFDM for time and frequency synchronization. The proposed method transmits ZCS as a pilot for OFDM symbols and simultaneously transmits traffic data to increase the traffic data rate while estimating the CFO at each coherence time. For time–frequency synchronization, the correlation of the ZCS pilot is used to perform coarse and fine time and frequency synchronization in two stages. Since the traffic data cause interference with the correlation of ZCS pilots, we theoretically analyzed the relationship between the amount of traffic data and interference and verified it through computer simulations. The synchronization and BER performance of the proposed ZCS pilot-based OFDM were evaluated by conduction computer simulations and a practical ocean experiment. Compared to the methods of Ren, Yang, and Avrashi, the proposed method demonstrated a 6.3% to 14.3% increase in traffic data rate with similar BER performance and a 2 dB to 3.8 dB SNR gain for a 14.3% to 23.8% decrease in traffic data rate.

Phase Noise Analysis of Time Transfer over White Rabbit-Network Based Optical Fibre Links.

White Rabbit (WR) is an optical fibre-based time-frequency synchronization technology typically used in timekeeping laboratories for distributing time-frequency signals from a reference clock to distant locations. The accuracy of the received signals at the user end can be affected by random noise processes present in the WR network due to the internal electronic components of WR devices. In this paper, we investigate the presence of random noise processes in the WR network. We then study their statistical properties and model the distribution based on experimentally recorded measurements. According to our study, the probability density function (PDF) follows a Gaussian mixture model (GMM) with varying distribution parameters, and the correlation analysis indicates a strong correlation of the phase noise process over the temporal samples. Furthermore, the developed phase noise models have also been verified by comparing them against additional experimental data. Finally, we present the methodology to generate the phase noise process using computer simulations with the PDF and correlation models developed in this work to help algorithm developers and equipment manufacturers make use of our results.

Adapt and Aggregate: Adaptive OFDM Numerology and Carrier Aggregation for High Data Rate Terahertz Communications

We propose a communication framework suitable for data rate maximization in the (THz) bands using adaptive (OFDM) numerology and carrier aggregation. OFDM is a widely adopted waveform due to the simplicity of its implementation and its effectiveness in combating frequency selectivity when the numerology is carefully chosen. However, it suffers from a multitude of limitations, including phase noise due to local oscillator inaccuracies, high peak-to-average power ratio, and is particularly sensitive to time-frequency synchronization errors, which can considerably impact its performance. This is especially relevant at THZ frequencies where larger-than-usual bandwidth is available, and the choice of the numerology should be carefully made given the intrinsic transceiver constraints. Moreover, the abundance of frequency resources in the THZ band imposes new design challenges that should be addressed, especially since the bandwidth usability at these frequencies depends on the communication distance. Hence, we propose a dynamic OFDM numerology adaptation mechanism, where the bandwidth of a CC covered by a single OFDM waveform is changed. For each (CC), the Component Carrier Data Rate (CCDR) is evaluated while considering the effect of both hardware impairments and the wireless channel statistics. We further propose the adoption of a dynamic distance-aware CC allocation such that the available frequency resources are fully utilized, and maximize an (ADR) through the aggregation of several CCs. Simulation results show that the proposed approach yields the highest ADR out of all possible setups.

AN ALGORITHM FOR AUTOMATED DETECTION OF DELAYED BRAIN ISCHEMIA INDICATOR FROM VIDEO-EEG MONITORING DATA

Abstract. In this work, we propose a new algorithm for detecting the indicator of delayed cerebral ischemia from video-EEG monitoring data. The proposed algorithm combines an algorithm for detecting the effect of interchannel time-frequency synchronization of wavelet spectrogram ridges of EEG signals and an algorithm for detecting motion artifacts in video recording frames. Using the EEG analysis algorithm, we identify an indicator that is used to predict the occurrence of ischemia. By analyzing the optical flow, we exclude time intervals in which erroneous fixation of ischemia indicators is possible. The developed algorithm was tested on the clinical data of video-EEG monitoring. The preliminary results obtained during testing confirm the fundamental possibility of detecting the ischemia indicators and the acceptable accuracy of detecting motion artifacts leading to an erroneous diagnosis.

High-precision time–frequency synchronization in Ethernet communication networks

Currently, many applications require not only high-speed communication, but also high-precision time–frequency synchronization. However, the simultaneous transmission and exchange of digital time signals with continuous analog frequency signals face challenges in large-scale Ethernet networks. Here we report a method to converge high-precision time–frequency synchronization and communication in fiber-optic distributed Ethernet networks. The time–frequency signals are multiplexed by wavelength division to the communication data with different optical wavelengths. The transmission error of the time–frequency signals was measured and controlled using a round-trip method. Network switching is implemented by add-drop multiplexing with a continuous wavelength selector switch and IP packets for the frequency signal and communication data, respectively. Experimental verification is carried out in a five-node network. In this network, high-precision time–frequency synchronization in Ethernet communication networks is realized. The frequency instability is less than 3 × 10−14@1 s, and the time synchronization offset is less than 70 ps with the communication rate of 1.25 Gbps. These results demonstrate the broad application prospects of the proposed method in distributed coherent detection fields with multiple nodes.

Multi-antenna GFDM for resilient decentralized communications

Owing to the superior performance of generalized frequency division multiplexing (GFDM), in terms of enhanced spectral efficiency and lower out-of-band radiations, it is considered as a potential replacement of traditional orthogonal frequency division multiplexing (OFDM) for next generation wireless communication systems. However, non-orthogonal pulse shaping in GFDM gives rise to intrinsic self-interference complicating the receiver design. Moreover, its extension to multi-input multi-output (MIMO) designs for spatial diversity and enhanced reliability is also not straightforward as overlapping of transmitted symbols in time and frequency hinders the extension of conventional diversity techniques to MIMO GFDM. In this work, we consider a multi-antenna GFDM decentralized communication system and present a generic framework to achieve spatial transmit diversity along with a low complexity transceiver design. We extend our proposal by presenting a novel multi-antenna preamble that helps not only in acquisition of robust time and frequency synchronization but also in reliable estimation of time-varying frequency selective channels. Performance evaluation over realistic 3GPP simulation scenarios, confirms the attainment of full diversity order along with superior preamble-based time–frequency synchronization and channel estimation performance as compared to state of the art.

Distributed deep learning-based signal classification for time–frequency synchronization in wireless networks

Distributed deep learning-based signal classification for time–frequency synchronization in wireless networks

Review: Waveform Design for Improve Perform Analysis of 5G

We need to access very large bandwidths in order to accomplish the goal of ubiquitous mobile broadband, where radio access performance should not be a limitation for customer experience. As a result, higher frequency bands up to the millimetre wave region should be taken into consideration. In terms of mobility, energy efficiency, and cost efficiency, the design of the air interface, which involves waveforms, is a crucial element. Four candidate multicarrier waveforms are evaluated by comparing in this study based on a variety of parameter bit error rates (BER), which include orthogonal frequency division multiplexing, filtered orthogonal frequency division multiplexing, universal filtered multicarrier, and generalized frequency division multiplexing to decide which of them is a better option for 5G mobile. The waveforms significantly enhanced spectrum localization and produced suitable spectrum fragmentation, based on the results of the MATLAB simulation. These waveforms successfully solve the problem of time-frequency synchronization by being able to merge different traffic parameters. As a result, these waveforms have a significant amount of promise for orthogonally and synchronization and can manage multiple users without signal degradation. Additionally, they handle all multiple-input and multiple-output applications and scenarios. The simulation's results show the range of these waveforms and propose the optimal ones for use in 5G systems.

NB-IoT Random Access for Nonterrestrial Networks: Preamble Detection and Uplink Synchronization

The satellite component is recognized as a promising solution to complement and extend the coverage of future Internet of Things (IoT) terrestrial networks (TNs). In this context, a study item to integrate satellites into narrowband-IoT (NB-IoT) systems has been approved within the 3rd Generation Partnership Project (3GPP) standardization body. However, as NB-IoT systems were initially conceived for TNs, their basic design principles and operation might require some key modifications when incorporating the satellite component. These changes in NB-IoT systems, therefore, need to be carefully implemented in order to guarantee a seamless integration of both TN and nonterrestrial network (NTN) for a global coverage. This article addresses this adaptation for the random access (RA) step in NB-IoT systems, which is in fact the most challenging aspect in the NTN context, for it deals with multiuser time-frequency synchronization and timing advance for data scheduling. In particular, we propose an RA technique which is robust to typical satellite channel impairments, including long delays, significant Doppler effects, and wide beams, without requiring any modification to the current NB-IoT RA waveform. Performance evaluations demonstrate the proposal’s capability of addressing different NTN configurations recently defined by 3GPP for the 5G new radio system.

Split PO for paging in B5G networks

Beyond 5G (B5G) wireless networks, imbued with several beams to support high-frequency spectrum, would require enhancements to the current paging monitoring mechanism in Idle mode Discontinuous Reception (IDRX) for improving UE’s energy efficiency. The lengthy pre-wakeup and processing overheads in directional IDRX for beam tracking and time–frequency synchronization in every IDRX cycle would result in higher UE’s power consumption. Recently, the functioning of Synchronization Signal Block (SSB) has been standardized for synchronization in New Radio (NR) 5G communications. In this article we first highlight inevitability and details of SSB measurements in every IDRX cycle of directional 5G/B5G communication. The UE’s power saving reduces substantially due to SSB measurements as the deep sleep duration in every IDRX cycle shrinks. Moreover, during the time gap between periodic SSB transmission(s), the UE can only transit to light sleep. An early indication that can inform UE whether to measure multiple SSB’s when paging is expected in an IDRX cycle or continue to deep sleep in absence of paging would be very beneficial in UE’s power saving. We propose a novel Split PO Paging (SPOP) mechanism that manifests early indication, avoids unnecessary SSB measurements and reduces UE’s pre-wakeup energy overheads. We use Markov chain modeling to analyze SPOP and validate our modeling through simulations. The Proposed SPOP can save more than 40% power compared to IDRX mechanism with elaborate pre wake-up and processing. • While the IDRX in legacy LTE/LTE-A networks gave UE’s longer opportunity for deep sleep, IDRX in 5G and B5G networks provides limited deep sleep. • 5G/B5G networks have long pre-wakeup in every IDRX cycle to accommodate SSB measurements. • An early indication on absence of paging in an IDRX cycle would allow the UE to avoid unnecessary long pre-wakeup. • We propose a novel paging concept, Split PO paging (SPOP) that manifests early paging indication in B5G. • SPOP optimizes paging monitoring and related SSB measurements by circumventing unnecessary processing. • Since power saving in SPOP would depend on the probability of paging arrival,we analyze SPOP using Markov chain modeling.

Overview and Comparison of Candidate 5G Waveforms: FBMC, UFMC and F-OFDM

The fifth generation (5G) technology standard, utilizing the Internet of Things, promises enhanced communication systems. However, the efficiency expected from such systems entails significant requirements, such as higher data rates and flexibility of the lowest 5G layer. Meeting these requirements in subsequent wireless communication systems is highly dependent on the use of waveforms capable of efficiently enabling multiple access. In other words, proper waveforms determine the effective handling of diverse traffic within a given band. In this study, four candidate multicarrier waveforms, namely filtered orthogonal frequency division multiplexing, filter bank multicarrier, universal filtered multicarrier, and orthogonal frequency division multiplexing, which is currently used in 4G systems, are compared based on multiple parameters. MATLAB simulation results indicate that the waveforms significantly improved spectrum localization and provided appropriate spectrum fragmentation. As these waveforms can mix diverse traffic specifications, they handle the problem of time-frequency synchronization effectively. Therefore, these new waveforms exhibit significant potential in terms of orthogonality and synchronicity and can support numerous users without dropping signals. In addition, they support all applications and scenarios related to multiple-input and multiple-output. The obtained simulation results confirm the suitability of such waveforms for 5G applications and systems.

Analysis and Design for Pilot Power Allocation and Placement in OFDM Based Integrated Radar and Communication in Automobile Systems

In this paper, we consider the orthogonal frequency division modulation (OFDM) based integrated radar-communication (Rad-Com) for automobile systems operating in millimeter wave band with imperfect time-frequency synchronization for communications. Since different pilot power allocation and placement strategies can result in different performance, we investigate the power allocation and placement design for the Rad-Com system. We analyze the effect of power allocation and pilot placement on the system performance. Based on the analysis, we provide, along with a channel estimation scheme, the pilot power allocation and placement designs that can optimize the radar and communication performance, respectively. Moreover, the tradeoff between radar and communication performance is discussed and a tradeoff design is presented. We conduct simulations to validate our analysis and evaluate the proposed designs. Results show that our analysis is valid and insightful and the proposed designs are effective.

Enhanced Paging Monitoring for 5G and Beyond 5G Networks

User experience, characterized by not only data rate and latency but also power consumption, is one of the major drivers for the success of 5G and Beyond-5G (B5G) networks. User Equipment (UE) power saving enhancements are at the forefront of ongoing discussions in 3GPP for next generation radio standards. Idle mode Discontinuous Reception (IDRX) is an important mechanism for UE’s power saving. However, IDRX in 5G/B5G networks is more challenging than in legacy networks. UE is required to measure Synchronization Signal Block(s) (SSBs) for Physical Downlink Shared Channel (PDSCH) and Physical Downlink Common Control Channel (PDCCH) reception in every paging occasion (PO) of IDRX cycle in 5G/B5G networks. Furthermore, PDSCH decoding is very sensitive to the time-frequency synchronization error and requires multiple SSB measurements before every PO. In this paper we present an Enhanced Paging Monitoring (EPM) for reducing the impediments on IDRX in 5G/B5G due to SSB measurements and related overheads. We also extend our EPM proposal by incorporating paging early indication. The proposed EPM can manifest up to 37% more power saving compared to existing system while considering multiple SSB measurements that are essential to 5G/B5G networks.

Distributed Deep Learning-Based Signal Classification for Time-Frequency Synchronization in Wireless Networks

Distributed Deep Learning-Based Signal Classification for Time-Frequency Synchronization in Wireless Networks

HF MODEM CLOCK AND FREQUENCY SYNCHRONIZATION ALGO-RITHM

The article proposes an algorithm for time-frequency synchronization of a high-speed modem operating under conditions of a highly dispersing channel, supplemented by an autocorrelation algorithm. It is shown that under the conditions of ray energy redistribu-tion, the proposed algorithm makes it possible to achieve satisfactory synchronization.

SYNCHRONIZATION OF THE OFDM MODEM WITH THE ANTENNA ARRAY IN HIGH-POWER INTERFERENCE CONDITIONS

The article proposes a method of time-frequency synchronization of an OFDM mo-dem, the input of which receives signals from the elements of the antenna array. The method is based on the combined action of correlation procedures for entering into communication and spatial selection, which clears the signal from interference based on the criterion of the minimum difference between the received signal and the test combination.

An Enhanced Method of PTP Fiber Cascade Fine Time-frequency Synchronization

In this paper, an enhanced method of PTP fiber cascade fine time-frequency synchronization is proposed. Based on the PTP synchronization technology, combined with the synchronous Ethernet clock transfer technology and the multi-level cascade fine clock synchronization technology based on the digital double mixing time difference method, the PTP technology is improved and enhanced. Then, based on this method, the multi-level time-frequency equipment fiber cascade is used to realize the multi node, large-span and networked time-frequency signal transmission and synchronous output can solve the problem that the synchronization accuracy deteriorates step by step under the condition of multi-level cascade, realizing the ns level system time synchronization accuracy, and ensuring the efficient synchronization and linkage of all parts of the system under the highly unified time scale. The feasibility and effectiveness of the method are verified by design and test.

Long-Distance Time Transfer in Optical Fiber Networks Using a Cascaded Taming Technology

In order to meet the time service needs of high-precision, long-distance, and multinode optical network, this paper proposes a new time synchronization solution, which combines the wavelength division multiplexing (WDM) technology with cascaded taming clock technology. The WDM technology is used for time synchronization between each pair of master-slave nodes. In the system, there are two wavelengths on the fiber link between the master node and the slave node for transmitting signals. 1 plus per second (PPS) signal, time code signal, and 10 MHz signal are, respectively, and successively, sent to the optical fiber link. By solving the one-way delay through analysis of error contribution and link characteristics of the time transmission process, time synchronization of the master-slave nodes pair is achieved. Furthermore, the authors adopt cascaded taming clock technology to ensure accurate time synchronization of each node. A 700 km long-distance time-frequency synchronization system is constructed in the laboratory. The system uses a cesium atomic clock as the reference clock source and transmits the signals through 8 small rubidium atomic clocks (RB clocks) hierarchically. Results from the experiment show that the long-term time stability is 47.5 ps/104 s. The system’s structural characteristics and the experiment results meet the requirements to allow practical use of high-precision time synchronization in networks. This proposed solution can be applied in various civil, commercial, and military fields.