Linear Frequency Modulation Pulse Research Articles

Optically Delaying a Radio Frequency–Linear Frequency-Modulated (RF-LFM) Pulse Using Kerr Comb Carriers and Off-the-Shelf Concatenation of a Linearly Chirped Fiber Bragg Grating and a Chirped-and-Sampled Fiber Bragg Grating

We demonstrate a low latency delay of a radio frequency (RF)–linear frequency-modulated (LFM) pulse by modulating it onto optical carriers from a Kerr comb and sending the signal through a concatenation of off-the-shelf linearly chirped fiber Bragg gratings (LC-FBGs) and chirped-and-sampled FBG (CS-FBG). We characterize the frequency response and latency of the LC-FBG and CS-FBG. Then, experimentally, the LFM pulse performance is characterized by measuring the peak sidelobe level (PSL) at the output of the tunable delay system. The experiment, performed with an LFM pulse of 1 GHz bandwidth at a 10 GHz center frequency, shows a PSL better than 34.4 dB, attesting to the high quality of the buffer RF transfer function. Thus, the proposed optical memory buffer architecture, utilizing compact devices based on a Kerr comb and FBGs, offers several benefits for delaying LFM pulses, including (i) a larger tunable delay range, (ii) low latency, (iii) wide bandwidth, and (iv) high PSL.

Step-adaptive accelerated demodulation algorithm for LFM-pulse-based distributed acoustic sensing

We propose a novel, to our knowledge, step-adaptive cross-correlation algorithm tailored for distributed acoustic sensing systems based on linear frequency modulation pulses, aiming for rapid demodulation. This algorithm adjusts its step length through an adaptive “successive refinement” search strategy, which greatly improves computational efficiency by reducing the number of cross-correlation computations. Experimental results have shown that the demodulation time can be reduced by approximately 15 times compared to the conventional method, while maintaining the same demodulation result.

High-Resolution Wideband Waveform Design for Sonar Based on Multi-Parameter Modulation

The design of transmitting waveforms is an effective way to improve the detection performance of sonar systems. For the problem of high-range sidelobe when designing reverberation-resistant waveforms, this paper proposes a high-resolution wideband composite waveform design with reverberation suppression performance and a waveform parameter improvement method. Firstly, we propose a novel wideband waveform, which utilizes linear frequency modulation (LFM) as the fundamental pulse, referred to as multi-parameter coded modulation LFM pulse (MPCM-LFM). Additionally, we deduce the wideband ambiguity function for waveform design. Then, we deduce the constraint relations of the waveform parameters for different sub-band overlaps, and according to the mathematical expressions of the obtained range ambiguity function, we analyze in detail the effects of the waveform parameters on the range ambiguity function under different constraints. Secondly, on the basis of the analysis, we also propose a hopping carrier frequency constraint rule to optimize the spectral performance, and the range sidelobe is restrained effectively in significant measure by this parameter improvement method. Finally, we analyze the computer simulation results. It is obvious that our proposed waveform parameter improvement method leads to good results. The proposed improved MPCM-LFM signal shows a “near-thumbtack” ambiguity function, whose sidelobe suppression performance is superior to other classical waveforms in the desired region, and it can realize high-precision parameter estimation. In addition, the proposed improved MPCM-LFM signal possesses good performance in detecting stationary and low Doppler targets in the background of reverberation.

Nonlinear Through-the-Wall Detection Using DORT Applied With LFM Pulses

The detection of visually obstructed small nonlinear targets in a through-the-wall (TTW) environment is difficult because of linear scatterers and strong wall reflections. In this letter, harmonic-based nonlinear TTW detection is introduced as a method to effectively detect small nonlinear targets behind the wall. Nonlinear TTW detection allows for the suppression of linear scatterer responses and wall reflections by detecting only the harmonics generated from nonlinear targets. Here we apply the decomposition of the time-reversal operator (DORT) for the detection and separation of multiple nonlinear targets behind the wall. DORT extracts the eigenvalues and eigenvectors of nonlinear targets and allows for selective focusing (imaging) of each detected target. In addition, we utilize linear frequency modulated (LFM) pulses, which allow for the compression gain in the detection processing. To validate the proposed concept, we conducted simulation and measurements of various configurations of nonlinear TTW detection. The results demonstrate that nonlinear targets behind the wall can be effectively detected and located.

Coherent OTDR with large dynamic range based on double-sideband linear frequency modulation pulse.

Rayleigh scattering-based distributed optical fiber sensors with long sensing distance and large dynamic range are highly desired for application scenarios such as vehicle tracking, structure health monitoring, and geological survey. To enlarge the dynamic range, we propose a coherent optical time domain reflectometry (COTDR) based on double-sideband linear frequency modulation (LFM) pulse. By utilizing I/Q demodulation, both the positive and negative frequency band of the Rayleigh backscattering (RBS) signal can be properly demodulated. Consequently, the dynamic range is doubled without increasing the bandwidth of signal generator, photodetector (PD), and oscilloscope. In the experiment, the chirped pulse with 10 μs pulse width and 498 MHz frequency sweeping range is launched into the sensing fiber. Single-shot strain measurement is achieved over 5 km single-mode fiber with a spatial resolution of 2.5 m and a strain sensitivity of 7.5 pε/H z. A vibration signal with 3.09 με peak-to-peak amplitude (corresponding to 461 MHz frequency shift) is successfully measured with the double-sideband spectrum, which cannot be properly recovered with the single-sideband spectrum.

Fiber-Optic Distributed Acoustic Sensing Technology Based on Linear Frequency Modulation Pulses

Fiber-Optic Distributed Acoustic Sensing Technology Based on Linear Frequency Modulation Pulses

ISAR Imaging for a Composite Target-Layered Rough Surface Scene Based on the Accelerated Time-Domain Scattering Algorithm

Most existing imaging studies are mainly used for a single target or a composite scene consisting of a target and a rough surface (TRS) below it, such as the target-ocean scene. This article focuses on inverse synthetic aperture radar (ISAR) imaging of a special composite scene consisting of a target and layered rough surfaces (TLRS) below it, such as a target and a snow-soil layered scene. Differently from previous imaging algorithms based on the frequency-domain (FD) method, this article combines the time-domain ray bouncing (TDRB) method with imaging algorithms to develop an accurate ISAR imaging framework to obtain composite images of the TLRS scene. To improve the simulation efficiency of the time-domain (TD) echo matrix for ISAR imaging, the multi-thread parallel is used to accelerate the TD scattering algorithm, then, the ISAR imaging formulas based on scattering field expressions in the TLRS scene are derived. Since the linear frequency modulation pulse used in traditional radar imaging has longer pulsewidth than the Gaussian, the matched filter is modified to realize pulse compression under modulating Gaussian pulse excitations to enhance the imaging efficiency. Simulation results show that the proposed ISAR imaging framework ensures the image quality and improves the simulation efficiency significantly. Finally, the ISAR images of the target-snow-soil scene with different parameters are simulated. This article addresses the correlation between the scattering mechanism and image features for ISAR images based on different scattering components, a topic that the literatures do lack.

Comparison between linear frequency-modulated ultrasonic excitation signals and pure tones for concrete applications

Non-destructive testing has supported prognosis and health management in the construction industry for decades. Among them, ultrasonic tests (UT) are increasingly used to assess the structural integrity of the concrete. Typically, the UT uses pure tones as the excitation signal. However, an alternative is to use a linear frequency-modulated pulse (LFMP) followed by compression. This process improves the signal-to-noise ratio, provides broadband frequency responses, and is less susceptible to propagation dispersion. This work compared the ultrasonic transit times (UTT) of LFMP and pure tones. Three nondispersive models for concrete materials were numerically simulated using the k-Wave MATLAB toolbox. Additionally, a sample of reactive powder concrete (RPC) was experimentally tested. The frequencies ranged from 50 kHz to 750 kHz for both simulations and experiments. The simulation results presented LFMP relative errors in UTTs ranging from 0.0 % to 2.7 % compared to the theoretical values, whereas for pure tones, the RE was 2 % to 63 %. Experimentally, as no theoretical value can be considered, the quantitative comparison was the measurement dispersion under repeatability conditions. LFMPs disclosed a standard deviation in transit time of 35 ns, whereas for pure tones, it ranged from 43 to 51 ns. This research reinforces the potential of LFMP excitation compared to traditional pure tones, providing more accurate and precise ultrasonic velocity measurements in heterogeneous environments.

Phase Noise Effects on Phase-Sensitive OTDR Sensors Using Optical Pulse Compression

We introduce a detailed theoretical, numerical, and experimental study of the effects of laser’s phase noise on the performance of phase-sensitive optical time-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">$\phi$</tex-math></inline-formula> -OTDR) sensors that use optical pulse compression (OPC). Pulse compression is a technique that can be used to improve the received signal amplitude by increasing the effective energy of the pulses that are launched into the fiber without degrading the spatial resolution of the measurements. Therefore, it is a valuable tool to extend the range of these sensors and mitigate fiber attenuation constraints. However, it has been observed that the limited coherence of the laser source has a degrading effect on the actual performance enhancement that this method can provide. Here, we derive a theoretical model that can be used to quantify this degradation for any type of OPC such as those based on either linear frequency modulation (LFM) pulses or perfect periodic autocorrelation (PPA) bipolar bit sequences. The model facilitates numerical estimation of the sensitivity of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\phi$</tex-math></inline-formula> -OTDR measurements. It also produces theoretical expressions for the mean and the variance of the phase-noise perturbed backscatter response. These results are validated via numerical simulations and experiments in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\phi$</tex-math></inline-formula> -OTDR setups using LFM as well as PPA OPC. Furthermore, we demonstrate the use of the model to investigate the basic trade-offs involved in the design of OPC <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\phi$</tex-math></inline-formula> -OTDR systems.

Transient Scattering Echo Simulation and ISAR Imaging for a Composite Target-Ocean Scene Based on the TDSBR Method

We propose an inverse synthetic aperture radar (ISAR) imaging algorithm for a composite target-ocean scene based on time-domain shooting and bouncing rays (TDSBR) method. To do this, we develop the TDSBR method to simulate the electromagnetic scattering echoes from a composite target-ocean scene. Then, we derive the ISAR imaging formulas based on electromagnetic scattering field expressions. The demodulation, pulse compression, and azimuth inverse fast Fourier transform are performed on the scattering echo data to obtain ISAR images. Linear frequency modulation (LFM) pulses are usually used in traditional radar imaging. However, its long pulse width leads to more computation time for simulations. To increase efficiency, we replace LFM pulses with modulating Gaussian pulses. As a result, we modify the matched filter to achieve pulse compression under modulating Gaussian pulse excitations. Simulation results confirm that this proposed method improves efficiency while ensuring image quality. This proposed technique fully incorporates the electromagnetic scattering mechanism and is accurate. It effectively combines the electromagnetic scattering algorithm and the radar imaging algorithm. Finally, we present ISAR images simulated by this method. These results confirm the feasibility and efficiency of this technique.

Experimental demonstration of forward scattering barrier for AUV intruder

The active detection of autonomous underwater vehicles (AUVs) is a challenging task because the echoes from targets are seriously weak under a monostatic configuration. In this work, a forward scattering detection scheme was developed for the active sensing of AUV intruders. A linear frequency modulation pulse was transmitted periodically for the acquisition of multipath propagation responses in a harbor environment. The least mean square algorithm after matched filtering was employed for direct blast suppression. A cumulative index was defined for detection on the basis of the error sequence of the adaptive filter. Sea experiments using an AUV as an intruder in crossing events were conducted to demonstrate the ability of the proposed scheme. The time of each crossing event was determined. The detection scheme was found to have a detection capability with a low source level. The performance was determined by the signal-to-direct blast ratio, which was correlated with the source–target–receiver geometry.

Parameter Measurement of LFM Signal With FRI Sampling and Nuclear Norm Denoising

Measurement of linear frequency modulation (LFM) signal is significant for radar, communication, and electronic reconnaissance fields. An LFM signal is a wideband signal whose frequency varies linearly with time, and traditional measurement methods require very high sampling rates and heavy processing to estimate parameters of the LFM signal. In this article, we propose a multichannel cooperative sampling (MCS) system based on the finite rate of innovation (FRI) theory to sample and estimate the parameters of the real-valued LFM pulse sequence (LFMPS). The MCS system consists of three parts: the autocorrelation sampling structure (ACSS), the time-delayed ACSS, and the quadrature time-staggered sampling structure (QTSS). These three parts can sample the LFMPS with sub-Nyquist sampling rate, and using the subspace method in time or frequency domain, the discontinuity locations (DLs), chirp rates, and initial frequencies (IFs) of the LFMPS can be estimated with the sub-Nyquist samples, respectively. The sampling rate of the MCS system is determined by the rate of innovation of the LFMPS, instead of signal bandwidth (BW). The minimum number of samples required for parameter estimation is proven by theoretical analysis. In addition, a nuclear norm denoising algorithm is proposed based on the low-rank property of the signal subspace, which significantly improved the performance of the measurement system in the noise environment. Simulation and hardware experimental results demonstrate the effectiveness and robustness of the proposed method.

An ISAR Imaging Framework for Large and Complex Targets Using TDSBR

An inverse synthetic aperture radar (ISAR) imaging framework is proposed to process transient scattered echoes into ISAR images. First, the time-domain shooting and bouncing ray (TDSBR) method is developed to simulate the transient scattering echoes. Then, based on the electromagnetic scattering echo expressions, the ISAR imaging formula is derived. In order to improve the imaging efficiency, we use Gaussian modulation pulse to replace the traditional linear frequency modulation pulse and propose an improved matched filtering method. Finally, some ISAR images are presented to demonstrate the accuracy and efficiency of the proposed method. Besides, by comparing with time-domain physical optics(TDPO) method, we also analyze the effect of multiple scattering contribution calculated by TDSBR on the image emphatically.

A Numerical Simulation Method of Radar Echo From a High-Speed Target

The radar echo signal of a high-speed target is simulated in this letter. Using the Lorentz transformation, the linear frequency modulation pulse radar signal is transformed from the laboratory coordinate system into a moving system. The near-zone scattered fields and the far-zone echo signals are solved by the finite-difference time-domain (FDTD) algorithm in the moving system that is kept the same velocity as the target. Subsequently, transform the echo signal back to the laboratory system. Meanwhile, this letter combines the graphic processing unit parallel technique with the Lorentz–FDTD method to realize the simulation of a high-speed target excited by a large time-width signal and extracts the one-dimensional(1-D) range profile of the target. The validity of the proposed method is verified by the analytical results of a high-speed target excited by arbitrary directions incident waves. The numerical examples show that the motion's effects on the range information of the radar signal are nonnegligible for high-speed objects.

External Modulation Optical Coherent Domain Reflectometry with Long Measurement Range.

Optical coherent domain reflectometry (OCDR) can achieve a high spatial resolution that is independent of the bandwidth of the receiver, but the measurement range is usually very limited. Here we propose an external modulation OCDR system, in which a pair of linear frequency-modulated pulses generated by one modulator are employed as the probe pulse and the reference, respectively. The spatial resolution is determined by the frequency modulation range of the pulse, and the measurement speed is boosted by orders because the proposed technology can simultaneously diagnose a section of fiber with each pair of pulses, while only a single point can be accessed at a time in typical OCDR. In the demonstrational experiment, a measurement range of up to 50 km is achieved with a spatial resolution of 1.4 m and a measuring time of less than 30 s.

High-resolution two-dimensional imaging using MIMO sonar with limited physical size

For an imaging sonar mounted onto a small sonar carrier (e.g., unmanned underwater vehicles), the angular resolution is strictly restricted by the limited physical size. To solve the problem, we propose a high-resolution imaging approach utilizing a multiple-input multiple-output (MIMO) sonar. The MIMO sonar is composed of two transmitting transducers and a receiving uniform linear array (ULA). The two transmitting transducers and the receiving ULA are located on two neighboring parallel straight lines, respectively. And the spacing between the two transmitting transducers is equal to the product of the hydrophone number and the inter-hydrophone spacing of the receiving ULA. Simultaneously, a pair of up- and down-chirp linear frequency modulation (LFM) pulses with the same frequency band (producing the same auto-correlation function for multibeam processing) and a large time-bandwidth product (ensuring low cross-correlation functions for range sidelobe suppression) are employed as transmitting waveforms. Based on the designed array layout and the transmitting waveforms, an improved hybrid beamformer combining the subarray processing, the phase shift and the shifted sideband beamformers together, is given, which ensures a fine imaging resolution and a low computation load. Via theoretical analysis, numerical simulations and a tank experiment, we show that the angular resolution of the proposed MIMO sonar imaging method doubles that of the traditional single-input multiple-output (SIMO) imaging method using the same array physical size and the same working frequency band.

Time reversal pulse position modulation communication in shallow water acoustic channels

The remote sensing and control of biomimetic robot fishes is an important but challenging task because the available underwater acoustic communication cannot accommodate their size, power consumption, and price. Hence, we propose a power-efficient, low-complexity time reversal pulse position modulation communication scheme, which partitions the available bandwidth of a transducer into several different sub-bands using different linear frequency modulated (LFM) pulses. The information is reflected by the position of the LFM pulses with different start and end frequencies for adjacent symbols. In the receiver, the Doppler factor is obtained by cross-correlating two hyperbolic frequency modulation signals transmitted before and after the modulated signal, and the channel response is estimated by match filtering the sync preamble. Furthermore, time reversal and bandpass filtering are performed to mitigate intrasymbol and intersymbol interferences caused by the multipath effects of underwater acoustic channels, respectively. The effectiveness and robustness of the proposed communication scheme are confirmed experimentally.

A Test Method for Linear Frequency Modulated Strong Pulse Field Using Time-Frequency Combination

In order to measure the linear frequency modulated (LFM) strong pulse field generated by high-power radars, a test method using time-frequency combination is proposed. On the basis of deriving the expressions of LFM pulse signal power, a measurement and evaluation method in time domain is given. According to the principle of root mean square (RMS) detection, a measurement and evaluation method in frequency domain is put forward. Through the combination of the two, the parameters of the LFM pulse field can be obtained such as the peak field strength, average field strength, pulse width, pulse repetition period and spectrum distribution. If the resolution bandwidth for the spectrum analyzer is greater than the occupied bandwidth of the LFM pulse signal, the measured results in the time domain and frequency domain can be mutually verified. The experiments are carried out to verify the method. The results show that the measured deviations of the peak field strength and average field strength of the LFM pulse field are not more than 2 dB, which indicates the test method is effective and feasible. It can meet the measurement requirements of LFM strong pulse field even if modulation parameters are unknown.

Optimal Signal Design for a New Physically Motivated Clutter Model With Applications to Ultrasonic Testing.

In this article, we will explore the optimal signal design problem for a new physically motivated clutter model. The reason to pursue the optimal signal design is that the model, even though it was simple, led to a robust detector that worked well on real ultrasonic data. Moreover, by using the model, an analytical solution for the optimal signal is obtained. In addition, the optimal waveform gives a new insight into the signal design problem that can be valuable for many applications. New proof has been provided for a finite data record that is lacking in the literature. We have found that the optimal signal is an impulse, and as a result, it has an impulsive autocorrelation sequence (ACS). Since an impulsive ACS signal is not realizable, in practice, a study for an alternative waveform is conducted. Waveforms from four different categories have been explored: linear frequency-modulated (LFM), nonlinear frequency-modulated (NLFM), phase-coded modulated (PCM) signals, and, finally, what we called other signals. Comparison analysis between these waveforms themselves and the most commonly used transmitted pulse in practice, which is the Gaussian amplitude-modulated sinusoid (GAMS) pulse, has been implemented. We show that the LFM pulse has a large advantage over the GAMS pulse in terms of detectability. In addition, a comparison between the LFM and the GAMS signals under a deviation from the single scatterer assumption, indicating a more complex target, using simulated noise is performed.

High-resolution and low-sidelobe forward-look sonar imaging using deconvolution

For the two-dimensional forward-look sonar imaging, the conventional beamformer (CBF) and the matched filter (MF) are often used due to the simplicity and robustness. However, the angular and range resolutions are limited and the sidelobes (SLs) are high. In this paper, we propose a deconvolution method to improve the resolutions and reduce the SLs. The proposed method uses a two-step Richardson-Lucy (R-L) algorithm to deconvolve the raw sonar image produced by CBF and MF, which is actually a post-beamformer processing method. In the first step the R-L algorithm is applied to the angular dimension, where the CBF beampattern is used as the point spread function (PSF); in the second step the R-L algorithm is applied to the range dimension where the auto-correlation function (ACF) of the transmitted linear frequency modulation (LFM) pulse is taken as the PSF. Furthermore, the non-ideal conditions including the low signal-to-noise ratio, the array manifold vector error and the echo distortion error are considered. Via numerical simulations and real data analysis, we show that the proposed method can improve the angular and range resolutions and reduce SLs significantly, while maintains a good robustness against these non-ideal conditions.