LFM Waveform Research Articles

HRRPs Reconstruction of Multiple High-Speed Targets With Large Time-Bandwidth Product LFM Waveform Based on Efficient SBL

HRRPs Reconstruction of Multiple High-Speed Targets With Large Time-Bandwidth Product LFM Waveform Based on Efficient SBL

Designing Interference-Immune Doppler-Tolerant Waveforms for Radar Systems

Dynamic target detection using LFM waveform is challenging in the presence of interference for different radar applications. Degradation in SNR is irreparable and interference is difficult to mitigate in time and frequency domain. In this paper, a waveform design problem is addressed using the Majorization-Minimization (MM) framework by considering PSL/ISL cost functions, resulting in a code sequence with Doppler-tolerance characteristics of an LFM waveform and interference immune characteristics of a tailored polyphase sequence (unique phase code + minimal ISL/PSL). The optimal design sequences possess polynomial phase behavior of degree <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q$</tex-math></inline-formula> amongst its sub-sequences and obtain optimal ISL and PSL solutions with guaranteed convergence. By tuning the optimization parameters such as degree <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q$</tex-math></inline-formula> of the polynomial phase behavior, sub-sequence length <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$M$</tex-math></inline-formula> and the total number of sub-sequences <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> , the optimized sequences can be as Doppler tolerant as LFM waveform in one end, and they can possess small cross-correlation values similar to random-phase sequences in polyphase sequence on the other end. The numerical results indicate that the proposed method is capable to computationally design chirplike sequences which prior to this work, were obtained by mimicking phase variations of LFM waveform. An application of the proposed method for the automotive scenario is also illustrated in the numerical results.

ALMA 2022 experiment : Source paramater estimation comparison betweenLFM and MLS waveforms

For underwater source localization, accurate estimation of the time of flight (or travel time) is required and the observed waveform may include Doppler (waveform dilation). The source waveform, thus, needs to be sensitive to estimate both time of flight and Doppler. In this study, we compare two waveforms : Linear Frequency Modulated sweeps (LFMs) and Maximum Length Sequences (MLSs) with identical center frequency (6 kHz) and bandwidth (2 kHz). During an at-sea experiment conducted in March 2022 near in the Mediterranean Sea, near the coast of Toulon, a mobile source transmitted the two waveforms, recorded by a 192-hydrophone moored bilboard array. The source-array distance ranged from 2.5 to 8 km, and the source was towed at a constant speed (∼1.5 m/s) and depth (∼50m). The analysis of the gathered data shows that the MLS waveform outperforms the LFM waveform for estimating time of flight (uncertainty reduced by a factor 4) and radial velocity (uncertainty reduced by a factor 20). Preliminary results focus on single hydrophone processing, but possible applications to 2D array processing are discussed for localizing the source in range-dependent environments.

Advanced orthogonal frequency and phase modulated waveform for contrast-enhanced photothermal wave radar thermography

Thermal wave radar (TWR) thermography is a high-efficient nondestructive testing technique to increase the signal-to-noise ratio (SNR) and to enhance target detection capability. However, the detection of subsurface defects, especially small-size defects, usually requires a distinctively high SNR and depth resolvability. This paper proposed an orthogonal phase-coded linear frequency modulated (OPCLFM) excitation waveform, which has significantly improved the SNR and depth resolvability of TWR compared to the LFM waveform. The pulse compression quality of the OPCLFM waveform was initially evaluated through a 1D thermal wave analytical model of carbon fiber reinforced polymer (CFRP) laminate. Results show that the OPCLFM waveform combined with the Kaiser window function compresses the largest sidelobe at least by 18.39 dB compared to the LFM waveform. Furthermore, the superior depth resolvability performance of the OPCLFM waveform was also validated by 3D finite element simulation. Finally, the effect of thermal conductivity on the depth resolvability performance of the OPCLFM waveform was evaluated quantitatively by a delaminated CFRP laminate.

A Novel Intrapulse Repeater Mainlobe-Jamming Suppression Method With MIMO-SAR

This paper deals with a novel transmitted scheme for a multi-subcarrier frequency MIMO-SAR system, which aims at suppressing the intrapulse repeater mainlobe-jamming and immensely improving the dynamic range of the receiver. To this end, due to the multi-subcarrier frequency transmission scheme, the mixed signal with true target signal and repeater jamming can be separated by a well-designed spatial-frequency filter. Thus, the range and direction of arrival (DOA) information is accurately estimated without interrupting the normal work of the radar. Furthermore, to further improve the orthogonality of transmitted waveform, the design of a phase-coded LFM waveform pair exhibiting both low cross-correlation energy (CCE) and low peak to sidelobe ratios (PSLRs), is considered. Besides, to handle the resulting nondeterministic polynomial (NP) hard problem, an alternating direction multiplier method (ADMM) based optimization method is employed. Compared with the traditional jamming suppression method, the proposed method improves the degree of freedom (DOF) from the carrier frequency domain, and it has the capability to suppress the intrapulse repeater mainlobe-jamming. Finally, detailed simulation experiments are carried out to verify the practicability and effectiveness of the newly proposed transceiver schemes.

Radar Waveform Design Based on Multi-Agent Reinforcement Learning

Under the actual combat background, prior information on radar targets has great uncertainty. The waveform designed based on prior information does not meet the requirements for the estimation of parameter. Thus, an algorithm for designing a waveform based on reinforcement learning is proposed to solve the above-mentioned problem. The problem on radar target parameter estimation is modeled as a framework for multi-agent reinforcement learning. Each frequency band acts as an agent, collectively interacts with the environment, independently receives observation results, shares rewards, and constantly updates the Q-network. The results of the simulation experiments indicate that the algorithm exhibits a significant improvement in terms of the mutual information obtained using the water injection method. In the case of simulation experiment, the SINR of the waveform designed based on multi-agent reinforcement learning is more than 3[Formula: see text]dB higher than that of LFM waveform. Under the condition of different time width and power, the mutual information obtained by the algorithm is better than that of water injection method. Moreover, such algorithm is also found to effectively improve the parameter estimation performance of target detection.

Research on Cognitive Marine Radar Based on LFM Waveform Control.

In this paper, the method of applying cognitive radar technology to marine radar is studied, and the cognitive marine radar structure and transmitted signal model with three control parameters are constructed. The selection method of waveform control parameters, which is based on the target spatial distribution and the reference target detection effect with the minimum emission energy as the criterion, is given. The transmission signal control selection method given in this paper can flexibly realize different emission signal groups of groups by independently setting the values m, n and p of three control parameters. It does not require radar hardware circuit reconstruction to meet the radar waveform changes. This is more convenient for the technical realization of cognitive marine radar. According to the method of this paper, a cognitive marine radar test system was constructed. The experimental results show that the proposed radar could reduce the emission energy by 15.9 dB compared with the traditional fixed-parameter pulse compression marine radar under the experimental conditions.

Transmit beampattern design for coherent FDA by piecewise LFM waveform

In transmit beampattern design, it is desirable to maximize the power around locations of interest. Multiple-input multiple-output (MIMO) radar transmits orthogonal waveforms, which is hardly achievable in practical applications. Moreover, the constant modulus constraint and the computational burden limit the system performance. In this paper, we propose an effective piecewise linear frequency modulation (PLFM) waveform for coherent frequency diverse array (FDA) radar to synthesize the desired transmit beampattern. By deriving the relationship between the beam direction and the transmit signal frequency, we find that different sub-band of transmitted signal corresponds to different angular sector. With the relationship between frequency band and mainlobe of beampattern, the beampattern synthesis is transformed into the design of waveform spectrum which is realized by designing the PLFM waveform in this paper. Compared with the state-of-the-art technique, the proposed PLFM waveform in coherent FDA enjoys the advantage of the constant modulus, low sidelobe level and reduced computation complexity. In addition, the multi-dimensional ambiguity function and the transmit beampattern response are derived to examine the properties. Numerical results are presented to validate the effectiveness of the proposed method.

A Novel Smeared Synthesized LFM TC-OLA Radar System: Design and Performance Evaluation

This paper introduces a novel smeared synthesized LFM (SSLFM) time compression overlap-add (TC-OLA) radar system. The new system allows us to control the signal to noise ratio level, and, therefore, obtain a higher processing gain compared to the traditional LFM-PC radar systems. In addition, it allows us to control the signal spectrum spreading, making it more immune to noise jamming. The new SSLFM signal is obtained by either multiplying the LFM waveform with a complex unit signal with the random phase or by encoding the time compression signal with the random phase at the transmitter. A denoising processor, placed either before or after the OLA processor, is used to remove the random phase from the SSLFM and forward the resulted LFM signal to the rest of the conventional LFM-PC radar receiver system. The new SSLFM TC-OLA radar system enjoys a better low probability of intercept feature while maintaining the LFM time sidelobe and Doppler tolerance properties. Moreover, the additional modules in the new radar system do not require changing the core LFM radar components. Using TC-OLA and denoising requires a synchronization system (SS) to properly recover the LFM signal. We, therefore, offer three SSs. The performance evaluation of the new radar system shows its superiority over the traditional LFM, the wideband LFM, and the TC-OLA-based LFM radars, especially under powerful noise jamming. The synchronization system is implemented and tested experimentally using software-defined radar.

Waveform design with low range sidelobe and high Doppler tolerance for cognitive radar

This paper considers the unimodular Linear-FM Synthesized (LFM-Syn) waveform design for cognitive radar.Firstly, the synthesized waveform is formulated by a given phase-scaling factor which combines the random noise waveform and the conventional LFM waveform. Secondly, to achieve low range sidelobes and high Doppler tolerance of LFM-Syn waveforms, a novel template-optimizing objective function is established and then transformed into a two-variable non-convex optimization case with constraints on quadratic phase and random phase. Meanwhile, a Relaxed Alternating Projection Phase Control method (RAPPC) incorporating spectrum approximation relaxed alternating projection and quadratic phase amendment is proposed via an iterative way. RAPPC not only enhances local exploiting for desired waveforms but also keeps high Doppler tolerance and low range sidelobes. Lastly, simulations have provided evidences that the LFM-Syn waveform optimized by RAPPC 1) has high Doppler tolerance similar to conventional LFM, 2) has Low Probability of Intercept (LPI) similar to random noise waveform, and 3) has low range sidelobe levels for prescribed requirement. Additionally, simulations also highlight the superior performance of our proposed algorithms over several existing techniques.

FJB 파형을 이용한 SAR 영상 생성 기법 분석

Recently, the SAR-GMTI mode is becoming increasingly essential in airborne radar systems. While SAR requires wideband waveforms for high resolution imaging, GMTI requires narrowband waveforms for doppler processing, which makes general LFM waveforms difficult to use for SAR-GMTI. This paper analyses the FJB(Frequency Jump Burst) waveform, which is studied for the SAR-GMTI waveform, and presents the method for the pulse compression and SAR image formation using FJB waveforms. Simulation results show that there is little difference in performances between the FJB waveform and the LFM waveform.

A TOTAL QUALITY ASSESSMENT SOLUTION FOR SYNTHETIC APERTURE RADAR NLFM WAVEFORM GENERATION AND EVALUATION IN A COMPLEX RANDOM MEDIA

Abstract A Design, simulation and optimal selection of non-linear frequency modulation waveforms (NLFM) based on correlated ambiguity function (AF) quality analysis for the purpose of Synthetic Aperture Radar (SAR) is done in this article. The selected optimum CNLFM waveform in contribution with other waveforms are applied directly into a SAR image formation algorithm (IFA) and their quality metrics in comparison to other waveforms are derived and analyzed in a complex random media (CRM). The total quality performance analysis includes both the qualitative AF diagrams and the objective image quality metrics assessments. The simulation results not only verify the robustness of the proposed NLFM waveforms as a suitable alternative for LFM waveform but also introduce NLFM as a proper method of modulation for SAR in CRM.

Piecewise LFM waveform for MIMO radar

To separate the received signals in a multiple-input multiple-output (MIMO) radar, the transmitted signals should be orthogonal or have a low cross-correlation level. Some other properties, such as low autocorrelation sidelobes, high range resolution, and good Doppler tolerance are also desired. However, they are difficult to achieve simultaneously. In general, an improved criterion is at the cost of another degraded one. In this paper, we propose a piecewise linear frequency modulation (PLFM) waveform for the MIMO radar. The transmitted signals are pulse trains, including diversified subpulses, which are divided into three segments with controllable durations and bandwidths. The duration sequence of the second segment is obtained by the genetic algorithm to optimize the cross correlation. Theoretical analyses and numerical results are presented to illustrate the properties of the proposed PLFM waveform. Compared with the polyphase coding and the discrete frequency coding waveforms, the PLFM waveform has a lower cross-correlation level and higher degrees of freedom. The relationship of the bandwidths among different segments can be used to adjust the sidelobe peak and the main lobe width of the autocorrelation function.

Real-Time Imaging With a 140 GHz Inverse Synthetic Aperture Radar

A summary of a 140 GHz inverse synthetic aperture radar (ISAR) is presented. High resolution and real time imaging are two key points in the design. Using the frequency-modulated continuous-wave (FMCW) radar technique with a 5 GHz bandwidth, 3 cm range resolution is achieved with assistance of software calibration compensating for nonlinearities of the LFM waveform. In cross-range region, synthetic aperture technique is adopted to improve imaging resolution. Real time imaging capability is enabled by using a hardware structure constructed with FPGA+CPU+GPU in signal processing unit. The final system was used to get ISAR image of a rotating target at 5 m. The 2-D (range and cross-range) imaging resolution is 3 cm × 3 cm with a real time image frame rate of 5 Hz.

Analog Encoded Chirp Transmission System Using Surface Acoustic Wave Filters

Linear frequency modulation is implemented in encoding analog signals. Using surface acoustic wave (SAW) dispersive delay lines, the transmission system described operates with overlapping chirps that are position-modulated according to the sampled data. Demodulation in the receiver requires pulse compression in a matched fdter. The principle of analog encoding and decoding is explained. Ex- perimental results are provided on the performance of a modem de- signed for handling baseband signals with a spectral range from 300 Hz to 400 kHz. SAW linear FM filters, operating at a center frequency of 60 MHz with a time-bandwidth product of 180, are employed. The system shows good amplitude and phase characteristics and a dynamic range greater than 40 dB. tude of the sampled information signal. Investigating the basic types of simple analog pulse modulation (6) , pulse-amplitude modulation (PAM), is normally excluded in spread spectrum communications. Because of its easy demodulation, it tends to be at cross purposes with the intent of these systems. Vari- ation of the LFM, pulse-duration modulation (PDM), thus varying the bandwidth at the same time, would cause ampli- tude and duration modulation of the compressed pulse in the receiver and a compressive gain depending on the modulating signal. A possible but more complicated approach is the varia- tion of the center frequency of each chirp according to the instantaneous sampled signal amplitude. Here the receiver re- quires a high time bandwidth product in its compression filter, yielding pulses whose time separation corresponds to signal amplitude value. These considerations and the narrow correla- tion peak of the LFM waveform suggest the use of pulse- position modulation (PPM) with its significant simplicity. In the following section of the paper, encoding and decoding in a system transmitting overlapping PPM-chirps is explained. Section 111 provides experimental results on the performance of the SAW filters and the overall system.

Effect of Particular Gain Changes upon LFM Sidelobes

The envelope variation of an LFM waveform due to transmitter droop or receiver STC tends to cause range sidelobes. A parametric analysis of the magnitude of the sidelobes has been performed. It is shown that the sidelobes can be quite high at the matched filter output, but are low at the output of the sidelobe reduction filter. 40-dB sidelobes can be achieved even with a 4-dB envelope droop. It is shown that these results are consistent with conventional paired-echo theory. Similar results are shown to hold for droop variations of the filter transfer function.