Pulse Compression Method Research Articles

Continuous Emission Ultrasound: A New Paradigm to Ultrafast Ultrasound Imaging.

Current imaging techniques in echography rely on the pulse-echo (PE) paradigm which provides a straight-forward access to the in-depth structure of tissues. They inherently face two major challenges: the limitation of the pulse repetition frequency, directly linked to the imaging framerate, and, due to the emission scheme, their blindness to the phenomena that happen in the medium during the majority of the acquisition time. To overcome these limitations, we propose a new paradigm for ultrasound imaging, denoted by continuous emission ultrasound imaging (CEUI) [1], for a single input single output (SISO) device. A continuous insonification of the medium is done by the probe using a coded waveform inspired from the radar and sonar literature. A framework coupling a sliding window approach (SWA) and pulse compression methods processes the recorded echoes to rebuild a motion-mode (M-mode) image from the medium with a high temporal resolution compared to state-of-the-art ultrafast imaging methods. A study on realistic simulated data, with regards to the motion of the medium, has been carried out and, achieved results assess an unequivocal improvement of the slow time frequency up to, at least, two orders of magnitude compared to ultrafast US imaging methods. This enhancement leads, therefore, to a ten times improvement in the temporal separability of the imaging system. In addition, it demonstrates the capability of CEUI to catch relatively short and quick events, in comparison to the imaging period of PE methods, at any instant of the acquisition.

Coded Excitation for Ultrasonic Testing: A Review.

Originating in the early 20th century, ultrasonic testing has found increasingly extensive applications in medicine, industry, and materials science. Achieving both a high signal-to-noise ratio and high efficiency is crucial in ultrasonic testing. The former means an increase in imaging clarity as well as the detection depth, while the latter facilitates a faster refresh of the image. It is difficult to balance these two indicators with a conventional short pulse to excite the probe, so in general handling methods, these two factors have a trade-off. To solve the above problems, coded excitation (CE) can increase the pulse duration and offers great potential to improve the signal-to-noise ratio with equivalent or even higher efficiency. In this paper, we first review the fundamentals of CE, including signal modulation, signal transmission, signal reception, pulse compression, and optimization methods. Then, we introduce the application of CE in different areas of ultrasonic testing, with a focus on industrial bulk wave single-probe detection, industrial guided wave detection, industrial bulk wave phased array detection, and medical phased array imaging. Finally, we point out the advantages as well as a few future directions of CE.

Defect Detection in Solid Timber Panels Using Air-Coupled Ultrasonic Imaging Techniques

This paper reports on investigations of the air-coupled ultrasonic (ACU) method to detect common defects in solid timber panels made of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.). The ACU technology is a non-contact method for nondestructive timber testing with quicker scanning rates compared to contact methods. A testbed was set up consisting of commercially available piezo-ceramic ACU transducers and in-house manufactured signal processing circuits. To demonstrate the suitability of the ACU technique, through-transmission measurement results are presented for samples with defects such as knots, wormholes, and cracks. Pulse compression methods (Barker-coded method) were used to improve the power of received signals based on cross-correction algorithms. Results showed defects of timber panels made of Chinese fir can be detected with a thickness of less than 40 mm. Defects larger than 3 mm in diameter could be detected with high precision. Applying the pulse compression method showed better results than using common sine signals as excitation signals since it increased the signal-to-noise ratio, which is especially important for air-coupled measurement of high-attenuation materials like timber materials. The measurement results on reference samples demonstrated that ACU technology is a promising method for timber defect detection, especially for the quality assessment of engineered wood products.

Transcranial Ultrasonic Focusing by a Phased Array Based on Micro-CT Images.

In this paper, we utilize micro-computed tomography (micro-CT) to obtain micro-CT images with a resolution of 60 μm and establish a micro-CT model based on the k-wave toolbox, which can visualize the microstructures in trabecular bone, including pores and bone layers. The transcranial ultrasound phased array focusing field characteristics in the micro-CT model are investigated. The ultrasonic waves are multiply scattered in skull and time delays calculations from the transducer to the focusing point are difficult. For this reason, we adopt the pulse compression method and the linear frequency modulation Barker code to compute the time delay and implement phased array focusing in the micro-CT model. It is shown by the simulation results that ultrasonic loss is mainly caused by scattering from the microstructures of the trabecular bone. The ratio of main and side lobes of the cross-correlation calculation is improved by 5.53 dB using the pulse compression method. The focusing quality and the calculation accuracy of time delay are improved. Meanwhile, the beamwidth at the focal point and the sound pressure amplitude decrease with the increase in the signal frequency. Focusing at different depths indicates that the beamwidth broadens with the increase in the focusing depth, and beam deflection focusing maintains good consistency in the focusing effect at a distance of 9 mm from the focal point. This indicates that the phased-array method has good focusing results and focus tunability in deep cranial brain. In addition, the sound pressure at the focal point can be increased by 8.2% through amplitude regulation, thereby enhancing focusing efficiency. The preliminary experiment verification is conducted with an ex vivo skull. It is shown by the experimental results that the phased array focusing method using pulse compression to calculate the time delay can significantly improve the sound field focusing effect and is a very effective transcranial ultrasound focusing method.

Активно-пасивний імпульсно-шумовий радар діапазону 3мм та результати його попередніх випробувань

The subject of this manuscript is broadband noise signals and systems. The aim of this research is to show the results of a modern active-passive 3-mm waveband system consisting of a noise-pulsed radar and a radiometer. Noise radar systems (NRS) based on broadband signals are characterized by high resolution, accuracy, and information content when performing unambiguous measurements of the range and speed of targets, as well as increased electromagnetic compatibility and noise immunity. These distinctive features of NRS determine the relevance of their construction for practical tasks of short- and medium-range radar. Additional opportunities for ensuring secrecy, reliability of detection of objects, and their tracking are provided by the combination of active and passive location modes in conjunction with advancement to the short-wave part of the millimeter (MM) wave band (WB). The most important characteristic of any pulsed radar, which largely determines its potential for practical application, is the operating frequency, as well as the shape and width of the probing signal spectrum. The main idea of this work is to describe the construction scheme and the results of preliminary tests of the developed active-passive system in the 94 GHz band with a noisy 20–100 ns illumination pulse in the 5 GHz band. The obtained values of the energy potential of the system in the active location mode (-105 dB) and the achieved radiometer sensitivities in the passive mode (0.007K and 0.03K) make it possible to observe ground and air objects at a distance of several kilometres. Noted that the measured parameters of the radar, in the case of processing the received signal by pulse compression methods, make it possible to count on ensuring the resolution of targets in range at the level of 10-15 cm. A multiple (more than an order of magnitude) decrease in the interference fluctuations of the received signal, which is due to the facet nature of the backscattering of targets, has experimentally demonstrated using a noise probing pulse compared to a single-frequency pulse. The methods for further work on the development and practical application of the constructed measuring system are outlined.

Time Domain IRCI-Free Pulse Compression for OQAM-OFDM Radar System

In this article, a novel majorization-minimization (MM)-based time domain pulse compression method (MM-based TDPCM) is proposed for offset quadrature amplitude modulation-based orthogonal frequency division multiplexing (OQAM-OFDM) radar system. We first propose a TDPCM by constructing a general linear model from the time domain with the estimated weights of the OQAM-OFDM signal and solving the model with a whitening operation to complete the pulse compression. The proposed TDPCM greatly reduces the required number of subcarriers for the given range cells and achieves inter range cell interference (IRCI)-free pulse compression. Furthermore, the MM-based TDPCM is proposed to maximize the signal-to-noise ratio (SNR) of the TDPCM. By theoretically analyzing the SNR performance of the TDPCM, we develop an optimization model to design the transmitted weights of the OQAM-OFDM signal. Then, the MM algorithm is proposed to solve the optimization model, realizing the maximum SNR output. Finally, numerical simulation results verify the superior performance of the proposed MM-based TDPCM.

Robust Adaptive Pulse Compression Method Based on Two-Stage Phase Compensation

The conventional pulse compression method [i.e., matched filter (MF)] and adaptive pulse compression (APC) methods are based on the assumption that the echo sampling point is located in the target point which is in a range cell. If the echo sampling point is not located in the target point, it will result in sampling mismatch problem and the performance of MF and APC methods will be significantly reduced when using the continuous-time phase-modulated waveforms, such as linear frequency modulation (LFM) signal. Aiming at solving the sampling mismatch problem, an APC based on two-stage phase compensation (TPC-APC) and its dimensionality-reduced version-contiguous fast APC based on two-stage phase compensation (TPC-CFAPC) are proposed in this article. In TPC-APC/TPC-CFAPC method, the mismatch phase caused by the sampling mismatch is first compensated through the first-stage phase compensation, which suppresses the sampling-mismatch-induced range sidelobes; then the mismatch phase caused by the target’s Doppler frequency is compensated through the second-stage phase compensation to suppress the Doppler-mismatch-induced range sidelobes; finally, the APC is applied to suppress the range sidelobe. To further compensate the mismatch phases of different targets, this article proposes a reiterative TPC-APC method and its dimensionality-reduced version, which iteratively use TPC-APC or TPC-CFAPC to suppress the range sidelobes caused by the mismatch phases of different targets. The results of the experiments have shown that the proposed methods are more robust compared with MF and APC methods.

An Improved Chirp Coded Excitation Based on Compression Pulse Weighting Method in Endoscopic Ultrasound Imaging.

Chirp coded excitation is an effective method to improve the signal-to-noise ratio (SNR) and penetration depth of high-frequency endoscopic ultrasound (EUS) imaging. In coded excitation, pulse compression is applied to compress the elongated coded signals into a short pulse, which determines the final imaging performance, including spatial resolution and SNR. However, with the current pulse compression methods, it is hard to get high performance in the peak sidelobe level (PSL), image contrast, and axial resolution at the same time. To solve this problem, in this article, a new method named compressed pulse weighting method (CPWM) was proposed based on the combination of two kinds of pulse compression signals. A brief theoretical derivation proved the feasibility of method. The proposed method was evaluated by the simulation and phantom experiments. Compared with traditional method, the results showed that the proposed adaptive weighting method can provide increases of 32.42% in the penetration depth, 9.48 dB in the SNR, 5.60 dB in the contrast ratio (CR), 5.46 in the contrast-to-noise ratio (CNR), and 0.13 mm in the axial imaging resolution for 12-MHz EUS. Therefore, this method can effectively improve the ultrasound penetration depth and imaging quality, which made it have good potential for high-frequency ultrasound imaging.

Ultrashort pulse compression method in bulk BBO crystal using electro-optically tunable cascaded second-order process: a theoretical analysis

We propose an electro-optically controllable cascaded second-order optical process that mimics as a third-order nonlinear process which is utilized for the compression of ultrashort pulses. Effective compression of optical pulses has been achieved by making efficient use of frequency doubling into an extraordinarily advantageous crystal like beta-barium-borate which is capable of generating shorter pulses within a few cycles. In accordance with simulation, it endeavors ≈ threefold compression in femtosecond pulse, i.e., 47 fs has been achieved by applying a voltage of ∓ 6.2 kV which delivers a phase mismatch (Δk) = ± 117.236 m−1 and effective cascaded refractive index (n2cascade) of ± 3.045 × 10–20 m2/W. This competent compression leads to an increase in the value of peak intensity from 50 to 288 GW/cm2. On account of the decent control in peak intensity, this promising methodology can be utilized for advanced scientific research, and also implemented easily in a large variety of applications in solid-state physics and medical field.

Parallel implementation of pulse compression method on a multi-core digital signal processor

Pulse compression algorithm is widely used in radar applications. It requires a huge processing power in order to be executed in real time. Therefore, its processing must be distributed along multiple processing units. The present paper proposes a real time platform based on the multi-core digital signal processor (DSP) C6678 from Texas Instruments (TI). The objective of this paper is the optimization of the parallel implementation of pulse compression algorithm over the eight cores of the C6678 DSP. Two parallelization approaches were implemented. The first approach is based on the open multi processing (OpenMP) programming interface, which is a software interface that helps to execute different sections of a program on a multi core processor. The second approach is an optimized method that we have proposed in order to distribute the processing and to synchronize the eight cores of the C6678 DSP. The proposed method gives the best performance. Indeed, a parallel efficiency of 94% was obtained when the eight cores were activated.

Damage detection of thick plates using trailing pulses at large frequency-thickness products

Trailing pulses, which are Lamb waves generated at large frequency-thickness (fd) products, act like a list of longitudinal pulses with constant time interval. These longitudinal pulses with short wave length are a promising tool for detection of damage in thick plates. However, the waveform complexity of trailing pulses brings challenges. In this paper, a damage detection method for thick plates using trailing pulses at large frequency-thickness products is introduced. Firstly, the characteristics of trailing pulses, including slightly change of velocity and pulse splitting, are discussed from the aspects of ray tracing and approximate analytical solution. Then, a blind pulse compression method for trailing pulses is interpreted, which doesn’t need the prior knowledge of material properties. A compression item is designed for trailing pulses. Finally, an experiment is implemented on a thick aluminum plate with three Side Drilled Holes (SDHs), in which all the SDHs are recognized with accurate position estimation.

IRCI-free OQAM-OFDM radar pulse compression

In this paper, a pulse compression method is proposed with the offset quadrature amplitude modulation based orthogonal frequency division multiplexing (OQAM-OFDM) signal. By establishing the received OQAM-OFDM signal as a linear model form and without inserting any cyclic prefix (CP), we propose a concise and easily implemented pulse compression method. The proposed method realizes the inter-range-cell interference (IRCI) free pulse compression perfectly, which improves the power efficiency, and the scope of observation will not be restrained by the number of subcarriers. Besides, the signal-to-noise ratio (SNR) after the pulse compression is derived. It is proved that the SNR gain for the proposed pulse compression method can reach the same as that of the method using the matched filter. Finally, numerical simulation results are provided and discussed.

A New SAR Imaging Method for Sparse Field Traget

Satellite borne Synthetic Aperture Radar (SAR) has become an indispensable means of earth observation because of its ability of all-weather, all-weather and global observation. However, at present, the space borne SAR still has the characteristics of large volume and high cost. In recent years, the vigorous development of micro satellite field has also promoted the research of small, intelligent and distributed cooperative imaging field of space borne SAR. The traditional spaceborne SAR imaging method is to process the echo of the received LFM signal in the two-dimensional pulse compression domain. Due to the targets in the ocean are usually sparsed, it is an effective method to detect the region of interest and targets by analyzing the echo and transform domain. In this paper, the method of azimuth pulse compression is used to realize the fast perception and imaging of sparse field targets. Through comparative analysis, it can be seen that the method has a greater improvement in resource and time consumption performance compared with the traditional method.

Secure hybrid pulse compression as a technique against jamming in radar systems

In this paper, we propose a secure hybrid pulse compression method for radar waveform design. This method uses a confidential code as the phase-code for the binary-shift keying modulation. We also use another confidential code for the frequency-code. This code is replaced in the frequency control register of a direct digital synthesizer to perform the narrow-band frequency-shift keying. We exploit the output of an encryption algorithm, e.g. a block cipher, as the binary code, which fulfills security and confidentiality requirements of electronic warfare scenarios. In contrast to other proposals that use only confidential phase-codes, we propose a method that considers both the frequency and phase codes to be confidential. We evaluate the proposed method’s performance in the presence of a jammer for three cases: where the jammer has access only to the phase-code, only the frequency code, and has access to neither. Our method effectively decreases the probability of false alarms, while improving detection probability.

OQAM-OFDM Radar Approximated IRCI-Free Pulse Compression

This paper proposes a pulse compression method by using the offset quadrature amplitude modulation based orthogonal frequency division multiplexing (OQAM-OFDM) signal without inserting any cyclic prefix (CP). A modified interference approximation-based pulse compression method (MIA-based PCM) is proposed, by exploiting the demodulated real weights of the OQAM-OFDM signal, the inter-symbol and inter-subcarrier interferences to enhance the performance. The MIA-based PCM achieves approximated inter-range-cell interference (IRCI)-free pulse compression and perfect Doppler estimation, which has both the benefits of power efficiency and good Doppler tolerance. Finally, numerical simulation results are provided and discussed.

Radar Pulse Compression Methods Based on Nonlinear and Quadratic Optimization

This article addresses the problem of radar pulse compression. First, a matched filter at the receiver is assumed and a method to design a transmit waveform characterized by an autocorrelation function with a narrow mainlobe and low sidelobes is proposed. After that, a mismatched filter is considered and a method to design the optimum linear filter receiver is proposed. In both cases, the pursued objectives are minimum sidelobe energy and peak level through a strategy based on quadratic and nonlinear optimization. In this article, we focus particularly on the case of ground weather radars, where the problem of pulse compression is particularly challenging.

Resistance against spot jamming by using pulse compression based on quadru-phase and frequency codes in radar systems

In electronic warfare, enemies apply many attacks to the radar systems, and try to obtain their signal information. In one type of spot jamming, the jammer accesses the structure of radar's signal. This information increases the success rate of attacks. In this paper, we propose a pulse compression method that uses the output of a block cipher as phase and frequency codes. The proposed method resists against spot jamming as the codes generated by the cipher blocks are confidential. We also use quadru-phase codes, instead of binary codes. Thus, a required level of confidentiality can be satisfied by codes with a shorter length. We investigate the probability of false alarms, autocorrelation and integrated side-lobes ratio by mathematical equations and verify them by simulation results. Results show that the proposed method provides a high level secure system with ultra-low side lobes and false alarm probability.

Implementation of Stepped Frequency Modulation Pulse Compression on NI Suite

In Radars and Sonar, the pulse compression technique is used continuously to increase the range resolution, range detection and the signal-to noise ratio (SNR). This can be achieved by modulating the transmitted pulse and then correlating it to the received pulse with the transmitted signal. This transforms short pulse into long pulse and is used to increase long pulse bandwidth by some form of modulation such as linear frequency modulation (LFM), so that Range Resolution is not compromised. The proposed Stepped Frequency Modulation (SFM) is a common Pulse Compression Method, which is useful to increase the Radar Range Resolution without losing the capability of target detection. Step Frequency Continuous Wave (SFCW) is also one of the techniques used in the Pulse Compression Technique, where the return echo step is used to determine range and is used for different purposes. This type of setup is widely used in RADAR design and testing. This Paper proposes the implementation of various Modulation techniques such as LFM, SFM and SFCW for proposed Stepped frequency Modulation using NI suite hardware PXI system which has a configurable FPGA and RF front end to generate custom waveform in wide range of frequencies with bandwidth up to 1GHz design and testing.

Defect detection capabilities of independent component analysis for Barker coded thermal wave imaging

Active pulse compression favourable infrared imaging techniques are promising evaluation techniques among other thermal non-destructive testing and evaluation modalities for identification of subsurface defects in the test samples. This is due to their merits such as high defect detection sensitivity and resolution by using relatively low peak power heat sources and in a moderate experimentation time in comparison with widely used pulse and modulated conventional thermal wave imaging modalities. Various data processing methods have been developed to extract the thermal features of defects from the captured thermographic data. The cross-correlation based pulse compression method is recommended for Barker coded thermal wave imaging technique as it enhances the concentration of applied thermal energy into mainlobe and distributes the significantly less energy into the sidelobes of cross correlation data. In this paper, the stated property of Barker coded thermal wave imaging along with the independent component analysis approach has been utilised. The study in this paper is focused on the application of independent component analysis approach on captured thermal response data, entire cross correlation data and only the mainlobe of CC data. The results obtained clearly indicate that the application of independent component analysis on cross correlation data has improved the detectability, visibility, and contrast (interms of signal-to-noise values) of the defects. Further, the results obtained with the different methods have been compared by taking Signal to noise ratio as the figure of merit.

High sensitivity detection of ultrasonic signal for nondestructive inspection using pulse compression method

Semiconductor parts are currently being stacked and miniaturized, and as device structures become more complicated, nondestructive inspections are increasingly needed for improving yield and securing reliability. Scanning acoustic microscopy (SAM) is used to detect delamination and voids. However, when the frequency of the ultrasonic probe is increased to improve the spatial resolution, conventional SAM cannot detect defects in deep parts because ultrasonic attenuation increases. In this research, we developed high sensitivity detection technique of ultrasonic signal using a pulse compression method, examined a modulation method, and designed the waveform of the transmitted signal to achieve high defect detection sensitivity. Conventional and developed detection techniques of ultrasonic signal were applied to a 3-layer stacked specimen, and their ultrasonic images, noise reduction effect, and defect detection sensitivity were compared.