Suppression Of Fading Research Articles

Suppression of the interference fading in heterodyne Φ-OTDR with fast signal synthesis algorithm

Abstract With the increasing diversity of application scenarios, there is a growing demand for distributed acoustic sensing (DAS) systems to monitor the environment without false alarms. In DAS system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR), interference fading significantly lead to a large number of false alarms in the system, which affecting the system’s monitoring capability. Before the phase demodulation process, Φ-OTDR system generally requires additional time for interference fading suppression. Therefore, the rapid acquisition of a fading-free signal before phase demodulation benefits the use of Φ-OTDR in real-time demanding scenarios. In this paper, based on the time-domain stitching and complex-field moving average, a fast signal synthesis (FSS) algorithm is proposed, enabling the quick synthesis of signals while maintaining fading suppression performance. Experimental results show that the signal intensity obtained by FSS algorithm fluctuates within 27 dB, while the intensity of the conventional denoising algorithm fluctuates more than 50 dB. Meanwhile, this method is more rapid than the rotated-vector-sum (RVS) algorithm, and the time consumption of signal synthesis is only 26.58% of that of the RVS algorithm. FSS algorithm provides a new and faster way to synthesize signals for interference fading suppression. Benefiting from this, FSS algorithm further improves the real-time performance of the Φ-OTDR system.

Coherent fading suppression of distributed acoustic sensing based on time–frequency domain linkage algorithm

Abstract Due to the inherent characteristic of coherent fading phenomenon, the performance of distributed acoustic sensing system will be severely degraded, producing anomalous information and positioning errors. In order to suppress the influence of this phenomenon, the data array of demodulated results is obtained after processing the collected distributed Rayleigh scattering signals along the sensing fiber, which can be utilized to construct a two-dimension (2D) image with the time–distance domain information. Thus, this allows us to remove the abnormal information that are introduced by fading noise. Based on this principle, a morphology compositional analysis (MCA) scheme, an effective time–frequency domain linkage algorithm, is applied to the constructed 2D images to enhance the sensing accuracy. Compared with traditional methods including the 2D mean value filter (MVF) and 2D wavelet transform (WT), the proposed method makes full use of the independence of image source property and abnormity feature so that the abnormal information caused by the coherent fading can be eliminated without deteriorating the source information along the sensing fiber. Experimental results show that the anomaly rate of the time–distance domain information along the sensing fiber for the proposed method, the MVF method and the WT method can be reduced by 92.43%, 65.05% and 70.18% respectively. In addition, the isolation degree of the MCA, MVF and WT methods are 41, 16.02 and 23.35 times higher than that treated by traditional method, respectively. Also, the stability of the time-domain vibration can be improved and the spatial resolution would not be influenced by the proposed method compared with the traditional method. This scheme will promote the development of distributed acoustic sensor system with high-accuracy and high-quality without any hardware modification.

Fading suppression and noise reduction of a DAS system integrated multi-core fiber.

Multi-core fiber (MCF) has attracted increasing attention for application in distributed fiber sensing owing to its unique properties of independent light transmission in multiple spatial channels. Here, we report a distributed acoustic sensing (DAS) system integrated MCF to suppress coherent fading, which overcomes an inevitable challenge in DAS systems. Because the parallel spatial cores in MCF allow the use of space-division multiplexed (SDM) technology, we propose that fading can be effectively suppressed by merging different signals with the spatial rotated-vector-average (SRVA) method. We theoretically analyze the principle of SRVA in fading suppression, and identify that it can effectively reduce phase noise with preventing phase unwrapping failures. In our experiment, a DAS system with 2.58-km length MCF have been investigated, the fading rate of Rayleigh backscattered signals is effectively reduced by three orders of magnitude and the amplitude fluctuation range is decreased by 21.9 dB. Compared with the conventional spectrum extraction and remix method (SERM), SRVA reduces the noise level by 9.5 dB, which also shows excellent low-frequency signal recovery ability. Benefiting from its fading suppression, the false alarm of localization is mitigated and the phase recovery can be distortionless. The proposed and verified method is helpful for the application of SDM-based MCF in long-distance distributed fiber sensors and accelerates the progress of integrated sensing and communication.

Interference fading suppression with a multi-subcarrier pulse in a distributed acoustic sensor

A low-complexity multi-subcarrier pulse generation scheme is proposed to suppress the interference fading in a phase-sensitive optical time-domain reflectometer (Φ-OTDR) based distributed acoustic sensor (DAS) with heterodyne coherent detection. The multi-subcarrier pulse is generated in the digital domain based on the proper clipping operation of a sine signal. The localization and recovery of the disturbance signal are realized by the spectrum extraction and rotated vector sum (SERVS) method. The experimental results show that the occurrences of interference fading can be significantly reduced. The intensity fluctuation is reduced from ∼75 dB to ∼25 dB. Multiple disturbance signals are successfully demodulated to verify the effectiveness of the proposed method.

Interference fading suppression with fault-tolerant Kalman filter in phase-sensitive OTDR

A multi-sensor information fusion algorithm based on fault-tolerant Kalman filter is proposed in phase-sensitive optical time-domain reflectometer (Φ-OTDR) system, for achieving fading-free distributed vibration sensing. Firstly, a fault-tolerant dual-core complementary array model is designed. The Rayleigh scattering signal denoising, and vibration existence judgment of localization points are carried out to obtain the differentiated frequency demodulation results of the sensing points of the dual-core fiber array. Then a fault-tolerant control strategy is used to determine the sensor weight coefficients and vibration judgment coefficients during data fusion processing, and array data fusion is carried out based on time series data using Kalman filter to realize error value identification and filling. The advantage of this method is the combination of redundant data in a complementary way to improve the system stability. The frequency response ranges from 10 Hz to 2400 Hz and the localization accuracy is 98.33%. The influence of key parameters on the frequency demodulation performance of fault-tolerant Kalman filter is discussed, and a standard deviation of 14.6 Hz and an average error of 7.6 Hz are obtained. The demodulation frequency data matrix obtained by the classical demodulation method has a demodulation error probability of 89.18%, which proves the widespread existence of demodulation errors in vibration signals. The fusion error of demodulation frequency is reduced to 0.25 Hz, the frequency demodulation accuracy reaches 100%, and the demodulation error caused by interference attenuation can be completely eliminated. This system based on fault-tolerant Kalman filter has the characteristics of simple multiplexing structure, interference fading resistance and stable demodulation performance.

Integrated Fading Suppression for Phase-Sensitive OTDR: An Algorithm Set

Integrated Fading Suppression for Phase-Sensitive OTDR: An Algorithm Set

Linear Anti-interference Algorithm for Digital Signal Transmission in Fiber Optic Communication Networks based on Link Analysis

In order to achieve accurate transmission of protection signals in fiber optic communication networks, it is necessary to perform channel balancing configuration of fiber optic communication networks and adaptive forwarding control processing of relay protection signals, the author proposes an accurate transmission method for relay protection signals in fiber optic communication networks based on time-varying multipath fading suppression and adaptive beamforming. The system analyzes the sources of wireless long-distance pain signal interference signals, introduces anti-interference technologies such as two-dimensional joint processing (STAP), provides anti-interference algorithms and related gain analysis, and conducts signal processing gain simulation using MATLAB. Based on the analysis of comprehensive simulation results, at a given symbol length, the signal bandwidth increases, and the processing gain infinitely approaches the given theoretical limit value, rather than increasing nonlinearly. The reason is that the channel is affected by noise, and the channel estimation value and signal conjugate multiplication produce a noise quadratic term. At this point, the estimated value of the coherent region channel is reduced by the influence of noise, and the signal-to-noise ratio loss caused by the noise quadratic term is reduced, so the processing gain increases. During the process of infinite increase in signal bandwidth, the input signal-to-noise power ratio of the receiver tends to decrease towards an infinite value, limited by the size of the coherent region. The channel estimation value increases under the influence of noise, and the noise quadratic term is the main factor affecting the output noise power. When the symbol length is greater than the coherent time, the smaller the maximum Doppler frequency shift and the larger the coherent detection area, the greater the processing gain.

Coherently parallel fiber-optic distributed acoustic sensing using dual Kerr soliton microcombs.

Fiber-optic distributed acoustic sensing (DAS) has proven to be a revolutionary technology for the detection of seismic and acoustic waves with ultralarge scale and ultrahigh sensitivity, and is widely used in oil/gas industry and intrusion monitoring. Nowadays, the single-frequency laser source in DAS becomes one of the bottlenecks limiting its advance. Here, we report a dual-comb-based coherently parallel DAS concept, enabling linear superposition of sensing signals scaling with the comb-line number to result in unprecedented sensitivity enhancement, straightforward fading suppression, and high-power Brillouin-free transmission that can extend the detection distance considerably. Leveraging 10-line comb pairs, a world-class detection limit of 560 fε/√Hz@1 kHz with 5 m spatial resolution is achieved. Such a combination of dual-comb metrology and DAS technology may open an era of extremely sensitive DAS at the fε/√Hz level, leading to the creation of next-generation distributed geophones and sonars.

Suppression of wavelength-dependent polarization fading using hybrid-polarization scheme in OFDR.

What we believe to be a new hybrid-polarization diversity scheme which can eliminate the polarization state variation caused by wavelength tuning of laser in optical frequency domain reflectometry is proposed in the paper. In the scheme, a 45° polarizer is used to maintain the polarization of signals. It decreases the polarization angle fluctuation to 2.81° and realizes a -145 dB test sensitivity with a 32 dB Rayleigh scattering signal-to-noise ratio in a 10 m fiber single test. The polarization fading suppression is achieved for tests with a large wavelength tuning range from 1480 nm to 1640 nm. Meanwhile, a 6 µm spatial resolution is also achieved. The proposed scheme can be applied to the structure measurement of high-precision optical fiber devices with high spatial resolution and sensitivity.

Interference fading suppression for distributed acoustic sensor using frequency-shifted delay loop

In order to suppress the interference fading of phase-sensitive optical time domain reflectometer (Φ-OTDR), a multi-frequency detection method based on frequency-shifted delay loop (FSDL) is proposed. A probe pulse train with up to five consistent frequency components are generated by periodic frequency-shifted, amplification, delay and filtering processing in FSDL. The piecewise aggregate approximation (PAA) is introduced to compress the amount of operational data. Wavelet energy spectrum analysis and support vector machine (SVM) are used to extract features and realize the fading classification. With the help of fading label making and restoration, the multi-frequency signals are intelligently aggregated using rotated-vector-sum (RVS). Experimental results show that PAA has applicability in data compression of beat signals. After 5-layer wavelet energy spectrum analysis, the fading binary classification accuracy can reach 96.77 %, and the fading signals identified after segmentation can be restored to the original data. The fading probability based on SVM output labels can be reduced to 1.89 %. The SVM-based classification output labels aggregation shows that the vibration positioning signal-to-noise ratio (SNR) can be increased to 13.52 dB, the phase demodulation curve is smoother, and the demodulation SNR can be up to 17.63 dB. Finally, the R-Square of 0.997 for the amplitude of demodulated vibration signal in repeatability experiment verifies the validity of this effective scheme for fading recognition and suppression.

Orthogonal time-frequency space (OTFS) modulation for underwater mobile acoustic communications

This paper investigates the Orthogonal Time-Frequency Space (OTFS) Modulation for Underwater Mobile Acoustic Communications where the communication channel suffers from severe multipath and Doppler effects simultaneously. Practical OTFS modulation schemes with different parameters are designed for acoustic transmission at a center frequency of 115 kHz and a bandwidth of 11.5 kHz. The schemes are tested in lake experiments where the transmitter was anchored and the receiver was towed by a boat at a speed of approximately 1 m/s or 3.6 km/h. The receiver utilizes low-complexity channel estimation and equalization algorithms, such as NLMS (Normalized Least Mean Squares) and IPNLMS (improved proportionate NLMS) algorithms. The results show some insights of the OTFS scheme for acoustic communications. First, the mobile acoustic channel characteristics are different in the 2D delay-Doppler domain than those in the 1D time and frequency domains for the Single-Carrier Coherent Modulation (SCCM) and the Orthogonal frequency division modulation (OFDM), respectively. Second, in mobile scenarios, the OTFS scheme receiver effectively and significantly reduces the accuracy requirements of the Doppler compensation algorithm and provides better frequency-selective fading suppression and Doppler effect robustness compared to SCCM and OFDM schemes. Third, the OTFS scheme has better anti-multipath performance and reduces multipath interference by effectively differentiating signals on different paths in the 2D delay-Doppler domain.

Phase Stability Control of Optical Fiber Partial Discharge Ultrasonic Sensing System

Optic fiber interferometers are highly sensitive ultrasonic sensors for partial discharge detection. However, low-frequency vibration and environmental noise will disturb the sensors in the field, and cause a phase fading suppression effect that reduces sensitivity. This paper analyzed the problems existing in the phase feedback control system based on PZT, and an improved scheme incorporating a high-frequency carrier phase demodulation is proposed. Based on an acousto-optic modulator, the proposed phase feedback control system overcomes the phase fading suppression effect. A test is carried out on an ultrasonic calibration platform and a transformer oil discharge platform. The test results show that the stability of the improved phase demodulation system has been significantly improved, and meets the requirements of field applications. Compared with the signal-to-noise ratio at the time of phase fading of the system before the improvement, the signal-to-noise ratio of the improved system is improved by 69 dB.

Multicore Fiber Enabled Fading Suppression in φ-OFDR Based High Resolution Quantitative DVS

Coherent fading has long been regarded as a critical issue in phase-sensitive optical frequency domain reflectometry ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> -OFDR) based distributed fiber-optic sensing. Here, we report on an approach for fading noise suppression in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> -OFDR using multi-core fiber (MCF). By exploiting the independent nature of the randomness in the distribution of the refractive index in each of the cores, the phase fluctuations due to the fading phenomena can be effectively alleviated by applying a weighted vectorial averaging for the Rayleigh backscattering (RBS) traces obtained from each of the cores with their distinct fading distribution. The proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> -OFDR with a commercial seven-core fiber has achieved quantitative distributed vibration sensing with ~2.2 nm length precision and 2 cm sensing resolution along a 500 m MCF, corresponding to a resolution-to-range factor as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\sim }4\times 10^{-5}$ </tex-math></inline-formula> . Featuring long distance, high sensitivity, high spatial resolution, and fading robustness, it shows promising potential in various sensing techniques for a wide range of practical scenarios.

A Fast Data Transmission Method of Vehicle-Road Cooperative Communication Based on the Clustering Algorithm.

In order to improve the security and stability of data transmission in vehicle-road cooperative communication and reduce the error rate of data transmission, a fast data transmission method based on the clustering algorithm is proposed. First, the multisensor information acquisition method of the vehicle networking is used to realize data acquisition and data structure analysis of the vehicle-road cooperative communication, and a data transmission channel model for vehicle-road cooperative communication is constructed. The interference suppression in the data transmission process of the vehicle-road cooperative communication is realized by the matched filter detection algorithm. Then, according to the symbol characteristic distribution of the vehicle-road cooperative communication channel, the baud interval equalization method is used to realize the piecewise equalization adjustment of data transmission for the vehicle-road cooperative communication. With the adaptive error compensation and channel fading suppression, the K-means clustering algorithm is used to carry out the coordinated adjustment to data packets during the rapid data transmission of the vehicle-road cooperative communication. Finally, the adaptive control of data transmission for the vehicle-road cooperative communication is carried out according to the dynamic distribution characteristics of the clustering center so as to reduce the influence of channel fading and intersymbol interference caused by channel spread. The simulation results show that the bit error rate of this method is kept at about 0.05, and the data transmission rate continues to increase, most of which remain above 0.95. This method has strong anti-interference ability for the rapid data transmission of vehicle-road cooperative communication, with lower communication bit error rate, less end-to-end time delay, and higher stability and accuracy of data transmission.

Polarization fading suppression in distributed interferometric sensing by matched interference between polarization-switched pulses.

A polarization fading suppression technique is proposed for distributed interferometric sensing systems, based on matched interference between polarization switched pulses. For each individual sensor, two sets of interferometric outputs are obtained, one corresponding to the interference between two pulses with initially parallel polarization, the other corresponding to that between two pulses with initially orthogonal polarizations. As such, at least one output presents visibility no less than 2/2. By selecting the one with higher visibility for demodulation, the influence of polarization fading can be suppressed significantly, leading to distributed acoustic sensing with notably improved robustness and reliability.

Polarization Fading Suppression for Optical Fiber Sensing: A Review

Optical fiber sensors are polarization sensitive and generally affected by polarization fading. This paper contributes to the optimal choice of polarization fading suppression methods for different optical fiber sensors, by means of comparative analysis of the mechanism of polarization fading, the strategy of polarization state control, and the applicability of suppression methods. The comparison results show that the polarization diversity receiving method is suitable for remote multi-channels optical fiber sensing system. The polarization orthogonal pulse pair method is only suitable for optical fiber sensing systems based on the backward scattering light. Meanwhile, the Faraday rotating mirror method is only suitable for optical fiber sensing systems based on the forward light interference. The polarization pulse coding method is suitable for sensing systems with high sensitivity and stability requirements. The polarization control method may increase the control complexity of sensing system for simultaneous variation of multiple polarization states. The full and partial polarization-maintaining fiber methods can be used to keep the polarization state of sensing light along the optical fiber, but the additional system cost limits the application. The polarization scrambler method is currently only available for sensing systems based on Brillouin scattering light. This paper provides a reference for the feasible selection of polarization fading suppression schemes for different optical fiber sensing systems.

Sampling Rate Enhancement and Fading Suppression of Φ-OTDR With Frequency Division Multiplex Technique

To capture vibration waveforms whose frequencies exceed the limitation set by sensing fiber length in Φ–OTDR distributed vibration sensing (DVS), we propose a sampling rate enhancement technique based on frequency-division-multiplexing (FDM). Our proposal, unlike conventional FDM-based sampling rate enhancement methods, directly eliminates inherent distortion by using complementary frequency concept with new probe pulse sequence. The proposed method can suppress the distortion of arbitrary waveforms in regular fiber without prior knowledge of the vibration. The proposal can accurately measure vibration waveforms and fully characterize the vibration on a standard single-mode fiber; it determines not only the frequency but also the correct amplitude and the exact shape of the vibration. Furthermore, our distortion compensation scheme incorporates a state-of-the-art FDM-based fading suppression technique, yielding high sampling rates and high sensitivity simultaneously. We demonstrate the validity of the proposal by proof-of-concept experiments in which the number of multiplexed frequencies is set to three for both sampling rate enhancement and fading suppression. In creating the experimental setup, we take the great care with the optical pulse shape and the carrier frequencies to avoid crosstalk. We successfully measure vibration waveforms with frequencies ranging from 1 kHz to 9 kHz on a sensing fiber with length of about 12 km.

Phase Tunable Microwave Source With Chromatic-Dispersion-Induced Power Fading Suppression

A frequency and phase tunable microwave source based on a single sideband modulated optoelectronic oscillator (OEO) is proposed and demonstrated. A dual-drive Mach-Zehnder modulator (DDMZM) is utilized to achieve single sideband modulation in the OEO loop. Phase reconfigurability is realized by phase shift element, which is composed of a phase modulator (PM) and a fiber Bragg grating (FBG). The phase of the generated signal can be tuned by adjusting the bias voltage of the PM. The single sideband modulated optical signal can overcome the chromatic-dispersion-induced power fading which can be used for long-distance transmission. In the experiment, the generated signal has a frequency range of 2–14 GHz and a phase range of 0–360°. The phase noise of the generated signal at an offset frequency of 10 kHz is −105 dBc/Hz.

A linearized optical single-sideband modulation RoF link with tunable optical carrier-to-sideband ratio

In this paper, an optical single-sideband (OSSB) modulation radio over fiber (RoF) link with tunable optical carrier-to-side-band ratio (OCSR) and simultaneous third-order intermodulation distortion (IMD3) and power fading suppression is proposed. In the proposed link, a phase-modulated signal and an optical carrier are polarization multiplexed using a Sagnac loop. The periodic power fading caused by chromatic dispersion is avoided by filtering the lower sideband. By adjusting the polarization state, the IMD3 is suppressed and a tunable OCSR can be achieved. Compared with the traditional Mach–Zehnder modulator (MZM) based link, the spurious free dynamic range (SFDR) of the proposed link is improved by more than 18 dB with different RF frequencies. In the broadband characteristic test, the proposed link exhibits lower error vector amplitude (EVM) and higher adjacent channel power ratio (ACPR) in a larger input RF power range.

Interference fading suppression in φ-OTDR using space-division multiplexed probes

We propose and experimentally demonstrate a novel interference fading suppression method for phase-sensitive optical time domain reflectometry (φ-OTDR) using space-division multiplexed (SDM) pulse probes in a few-mode fiber. The SDM probes consist of multiple different modes, and three spatial modes (LP01, LP11a, and LP11b) are used in this work for the proof of concept. Firstly, the Rayleigh backscattering light of different modes is experimentally characterized, and it turns out that the waveforms of the φ-OTDR traces for distinct modes are all different and independent. Thanks to the spatial difference of the fading positions for distinct modes, multiple probes from spatially multiplexed modes can be used to suppress the interference fading in φ-OTDR. Then, the performances of the φ-OTDR systems using a single probe and multiple probes are evaluated and compared. Specifically, the statistical analysis shows that the fading probabilities over both the fiber length and the time scale are reduced significantly by using multiple SDM probes, which verifies the significant performance improvement on fading suppression. By introducing the concept of SDM to φ-OTDR, the proposed novel interference fading suppression method avoids the complicated frequency or phase modulation, which has the advantages of simplicity, good effectiveness and high reliability.