Dechirp Processing Research Articles

Photonics-based simultaneous distance and velocity measurement of multiple targets utilizing dual-band symmetrical triangular linear frequency-modulated waveforms.

The distance and velocity measurement can be obtained by the round-trip time and Doppler effect on the down-chirp and the up-chirp of the linear frequency-modulated waveform (LFMW), but false targets will appear in a multi-target situation, resulting in erroneous detection. Here, we report a photonics-assisted approach to realize unambiguous simultaneous distance and velocity measurement in multi-target situations utilizing a dual-band symmetrical triangular LFMW. Dual-band observation invariance is proposed, to effectively resolve the false targets. The de-chirped signals can be obtained from parallel de-chirping processing to the dual-band echoes. By measuring and calculating the beat frequencies of the de-chirped signals in the two frequency bands, the actual parameter measurements can be acquired according to the authenticity criterion. In the experiments, detections to three targets are performed, and the distance and velocities are acquired without false targets. The absolute measurement errors of the distance and the velocity are less than 9 mm and 0.16 m/s, respectively. These results show the feasibility of the proposed approach.

Broadband Cognitive Radio Enabled by Photonics

Cognitive radio is considered as a possible disruptive force to improve the spectral resource efficiency through sensing and interacting with the environment. Traditional electrical technologies to implement the cognitive radio face challenges in terms of bandwidth, resolution, and speed. In this paper, the concept and architecture of broadband cognitive radio systems enabled by photonics are proposed. Key microwave photonic techniques for the architecture are reviewed, including the photonics-based spectrum sensing, the photonic arbitrary waveform generation, the photonics-based self-interference cancellation processing, and the microwave photonic dechirp processing and radar imaging. A preliminary demonstration of a cognitive radar system enabled by photonics is performed. The future possible research directions on this topic are discussed.

Novel RF-source-free reconfigurable microwave photonic radar.

A novel reconfigurable microwave photonic (MWP) radar has been proposed and experimentally demonstrated. At a transmitting end, a microwave signal with a large bandwidth and ultra-low phase noise is generated by a Fourier domain mode locking optoelectronic oscillator. At a receiving end, photonics-based de-chirp processing is implemented by phase-modulating light waves in a dual-drive Mach-Zehnder modulator and mixing the modulated light waves at a photodetector. Without the requirement of external RF sources, the developed photonics-assisted programmable radar is capable of generating and processing microwave signals with adjustable format, bandwidth and central frequency. The proposed radar working from X to Ku band with an instantaneous bandwidth of 2 GHz is demonstrated. The reconfiguration of the radar is theoretically analyzed. The tunability of radar bandwidth and central frequency is investigated. Microwave imaging of a pair of trihedral corner reflectors based on the developed MWP radar is achieved.

Ground clutter simulation for airborne forward‐looking multi‐channel LFMCW radar

Ground clutter is a crucial factor that affects the detection performance of the airborne forward-looking multi-channel linear frequency modulated continuous wave (LFMCW) radar, so the distribution characteristics of studying the ground clutter plays an important role in suppressing ground clutter and improving radar detection performance. In this article, the authors first use the Ward model to divide the effective clutter area into multiple independent clutter patches and calculate the echo signal of each clutter patch. Then the beat signal of each clutter patch is obtained by the de-chirp processing. Next, a Fourier transform is performed on the beat signal of each clutter patch to determine the range ring where the echo of the clutter patch is located, and the peak spectrum signal of the beat signal of the clutter patch in the same range ring is superimposed, thus the authors can realise the simulation of the ground clutter signal. Finally, the simulation results indicate the effectiveness of the proposed method.

A Photonics-Based Coherent Dual-Band Radar for Super-Resolution Range Profile

We propose and demonstrate a photonics-based coherent dual-band radar for super-resolution range profile in this paper. This radar cannot only generate coherent dual-band linear frequency modulated signals with large bandwidths, but also realize a paralleled coherent de-chirping processing to the dual-band echoes simultaneously based on a single compact photonics-assisted transceiver. The sharing of a single transceiver for signals with different bands ensures perfect stability and coherence among the waveforms, which makes it possible to realize a super-resolution range profile by fusion of dual-band signals without heavy computation on phase compensation. In the experiment, the radar operates in S -band with a bandwidth of 1.5 GHz and X -band with a bandwidth of 3 GHz, and results show its range resolution is up to 1.6 cm by coherent fusion processing, which is nearly corresponding to the resolution gotten by a linear frequency modulated wave with a bandwidth of 9.5 GHz ranging from 2 GHz to 11.5 GHz, much higher than the scenario when the radar is operated in the single-band mode. This technology would improve the accuracy of classification and recognition of the targets.

Coherent integration method for random pulse repetition interval radar based on non‐uniform keystone transform and non‐uniform fast Fourier transform

In order to achieve weak target detection, the range migration and Doppler frequency spread induced by the target motion within the coherent processing interval should be compensated to focus the target energy in a certain range-Doppler cell. This article proposes a new integration method based on non-uniform Keystone transform and non-uniform fast Fourier transform (NUFFT) for random pulse repetition interval (PRI) radar. In this method, non-uniform Keystone transform is applied to correct the linear range migration. After that, the acceleration is estimated by dechirp process and the Doppler frequency spread is compensated. Finally, the target energy is accumulated coherently via NUFFT. Compared with the random PRI generalised Radon-Fourier transform (RPRI-GRFT) and the moving target detection method for random PRI radar, the proposed algorithm achieves a good balance between the detection performance and the computational cost. Simulation results are provided to demonstrate the effectiveness of the proposed method.

Ultrafast and ultrahigh-resolution optical vector analysis using linearly frequency-modulated waveform and dechirp processing.

We propose and experimentally demonstrate an ultrafast and ultrahigh-resolution optical vector analyzer (OVA) using linearly frequency-modulated (LFM) waveform and dechirp processing. An optical LFM signal, achieved by modulating an electrical LFM signal on an optical carrier via carrier-suppressed optical single-sideband (OSSB) modulation, is separated into two portions. One portion (denoted as the reference signal) directly goes through the reference path, and the other (denoted as the probe signal) undergoes magnitude and phase changes by an optical device under test (DUT) in the measurement path. After balanced photodetection, the reference signal and the probe signal are mixed to perform a dechirp operation. A relatively low-frequency electrical signal is generated, which can be sampled by a low-speed analog-to-digital converter. As a result, the frequency responses of the DUT can be extracted at a high speed by post digital signal processing. Thanks to the large chirp rate of the electrical LFM signal and the dechirp processing, the proposed LFM-based OVA enables ultrafast measurement speed and ultrahigh frequency resolution. We perform an experiment in which a narrowband tunable optical filter is characterized. The measurement speed reaches 1ns/point, and the frequency resolution is 1.6MHz.

Photonic Generation and De-Chirping of Broadband THz Linear-Frequency-Modulated Signals

In this letter, we propose and experimentally demonstrate photonic generation and de-chirp processing of terahertz (THz) linear-frequency-modulated (LFM) signals using coherent photonic super-heterodyne based on frequency quadrupling and photo mixing. In the experiment, a 340–380-GHz LFM signal with 40-GHz bandwidth is successfully generated with a temporal period of $1\,\,\mu \text{s}$ . As a result, a range resolution of 5 mm is successfully enabled, which is close to the theoretical limit. Benefited from photonic de-chirp processing, the requirements of receiving instantaneous bandwidth and sampling rate are reduced. Moreover, the proposed scheme features high stability, and the measured performance of the system has not deteriorated within 2 hours in the experiment. Therefore, our proposed system has a great potential in THz ranging and imaging applications.

High-resolution phased array radar imaging by photonics-based broadband digital beamforming.

A photonics-based broadband phased array radar is demonstrated to realize high-resolution imaging based on digital beamforming. This photonics-based phased array radar can achieve a high range resolution enabled by a large operation bandwidth, and can realize squint-free beam steering by digital true time delay (TTD) compensation. In addition, the photonic dechirp processing applied in the receiver can alleviate the hardware requirements for data sampling and storage, and hence remarkably enhance the real-time signal processing capability. In a proof-of-concept experiment, target imaging by a photonics-based 1 × 4 phased array radar that has a bandwidth of 4 GHz (22-26 GHz) is demonstrated, of which the range and azimuth resolution is measured to be 3.85 cm and 2.68°, respectively. The proposed scheme provides good solution to overcoming the bandwidth limitation and implementing high-resolution imaging in a phased array radar.

Dechirping Compression Method for Nonlinear Frequency Modulation Waveforms

This letter proposes a novel dechirp processing method for wideband nonlinear frequency modulation (NLFM) waveform which enables low sampling rate of data. As we know, NLFM waveform can easily realize optimal shape of power spectral density which helps to achieve better pulse compression result with low range sidelobes without signal-to-noise ratio degradation caused by weighting as we usually do for LFM waveforms. In the approach, hamming window is adopted in the generation of the NLFM waveform for better balance on the low range sidelobes and a relative small bandwidth. In dechirp operation, the NLFM radar echoes are mixed with a local reference LFM signal, and thus narrowband signals are resulted and only low sampling rate is needed. The dechirped NLFM signals are then up-sampled and inverse dechirped by software and finally matched filtering is conducted to complete pulse compression. Both numerically simulated data and real radar data are processed to verify the effectiveness of the proposed method.

Robust wideband beamforming method for linear frequency modulation signals based on digital dechirp processing

This study presents a robust digital beamforming (DBF) method for wideband digital array radar, which uses linear frequency modulation (LFM) waveforms. Firstly, received signals are digitally dechirped, followed by narrowband filtering and decimation. Then the authors generate time-variant factors to compensate phase perturbations induced by the dechirp processing. It is proved that the wideband LFM signals can be treated as narrowband signals after compensation. Finally, a robust adaptive narrowband DBF method based on uncertain set constraints and interference-plus-noise matrix reconstruction is applied to suppress the interferences. The proposed method is computationally efficient and feasible for real-time applications. It behaves robustly in the presence of direction of arrival estimation errors across a wide range of signal-to-noise ratios. Simulations demonstrate the effectiveness and feasibility of the proposed method.

Bistatic ISAR Sparse Imaging Method for High-Speed Moving Target Based on Dechirping Processing

Bistatic inverse synthetic aperture radar (ISAR) can increase the probability of tracking the high-speed target and provide more angle information than monostatic ISAR. However, bistatic ISAR suffers from a serious defocusing problem, resulting from the high-speed motion. Furthermore, the inherent geometry distortion and calibration problems make bistatic ISAR (B-ISAR) imaging more complex. In response to these problems, we propose a bistatic ISAR imaging method for high-speed moving target with geometric distortion correction and calibration based on dechirping processing. At first, based on the motion decomposition idea, the B-ISAR echo model of the high-speed moving target is established. Then, by analyzing the imaging mechanism of the Range-Doppler algorithm (RDA), we eliminate the phase items influencing the imaging quality with speed compensation and Doppler compensation. After that, the analytic formula of the geometric distortion factor and calibration factor are deduced, which helps transform the geometric correction and calibration problem into a parameter estimation problem. Finally, with the sparsity of the scattering points, the required parameters are solved using the expectation maximization (EM) algorithm based on the maximum a posteriori probability criterion. With the estimated parameters, a clear B-ISAR image of a high-speed moving target with geometric correction and calibration is obtained. The simulations show that the proposed method has a better resolution and simultaneously attains geometric correction and calibration of the image.

Photonics-Based High-Resolution 3D Inverse Synthetic Aperture Radar Imaging

A photonics-based radar for high-resolution 3D microwave imaging is proposed and demonstrated, in which high range resolution is achieved by employing a wideband linearly frequency modulated signal, and high cross-range resolution in both azimuth and elevation is realized by rotating the target around two orthogonal axes to compose a 2D inverse synthetic aperture. The proposed radar performs photonics-based in-phase/quadrature up-conversion and de-chirp processing in the transmitter and receiver, respectively, which features a compact structure with low-sampling-rate electronics in the receiver. An experiment is carried out. The established radar works in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -band, with bandwidth as large as 8 GHz. Captured echo signals at different rotation angles of the target are processed to a 3D image, in which the voxel values are scattering intensities at different spatial positions.

High-resolution W-band ISAR imaging system utilizing a logic-operation-based photonic digital-to-analog converter.

W-band inverse synthetic aperture radar (ISAR) imaging systems are very useful for automatic target recognition and classification due to their high spatial resolution, high penetration and small antenna size. Broadband linear frequency modulated wave (LFMW) is usually applied to this system for its de-chirping characteristic. However, nearly all of the LFMW generated in electronic W-band ISAR system are based on multipliers and mixers, suffering seriously from electromagnetic interference (EMI) and timing jitter. And photonic-assisted LFMW generator reported before is always limited by bandwidth or time aperture. In this paper, for the first time, we propose and experimentally demonstrate a high-resolution W-band ISAR imaging system utilizing a novel logic-operation-based photonic digital-to-analog converter (LOPDAC). The equivalent sampling rate of the LOPDAC is twice as large as the rate of the digital driving signal. Thus, a broadband LFMW with a large time aperture can be generated by the LOPDAC. This LFMW is up-converted to W band with an optical frequency comb. After photonic-assisted de-chirping processing and data processing to the echo, a high-resolution two-dimension image can be obtained. Experimentally, W-band radar with a time-bandwidth product (TBWP) as large as 79200 (bandwidth 8 GHz; temporal duration 9.9 us) is established and investigated. Results show that the two-dimension (range and cross-range) imaging resolution is ~1.9 cm × ~1.6 cm with a sampling rate of 100 MSa/s in the receiver.

On the Processing of Very High Resolution Spaceborne SAR Data: A Chirp-Modulated Back Projection Approach

A new image formation algorithm is proposed for processing very high resolution spaceborne sliding-spotlight synthetic aperture radar (SAR) data. Because of along-track antenna steering, the Doppler bandwidth of the received SAR data is expanded significantly beyond one pulse repetition frequency interval. Furthermore, the range histories become spatially dependent in both dimensions and cannot be expressed exactly by a hyperbolic model. In our approach, we first reduce the Doppler bandwidth by a novel azimuth dechirp processing method in the range frequency domain. The data are then processed by the standard $\omega $ – $\kappa $ algorithm with a fixed effective velocity. Thereafter, the chirp modulation concept is imported to rebuild new data with much shorter apertures. Finally, a standard back-projection algorithm is employed to accumulate the signal pixel by pixel along the newly built aperture. Thus, the balance between processing efficiency and precision can be controlled by adjusting the length of the new apertures. In addition, a more accurate 2-D spectrum derivation is employed to enhance the processing precision, and a novel range-splitting method is presented to accommodate the range dependence of effective velocities. Furthermore, when implementing the back projection, the image grid—the region and granularity level of which are user defined—is placed on the earth’s surface instead of on the slant-range plane, and the routine geometry projection processing thus becomes dispensable.

Dual-band dechirping LFMCW radar receiver with high image rejection using microwave photonic I/Q mixer.

In this paper, a photonics-based dual-band linear frequency-modulated continuous wave (LFMCW) radar receiver is proposed. The system core is a microwave photonic in-phase and quadrature (I/Q) mixer, whose inherent large bandwidth, high I/Q balance and favorable uniformity enable the receiver to operate over an extremely wide frequency range. An integrated dual-band waveform offers the possibility of independent detection, allowing the sharing of hardware resources and joint dechirp processing of dual bands. In the proof-of-concept experiment, the distance measurements of S- and C-bands are implemented, with a high and uniform image rejection exceeding 28 and 30 dB, respectively. The image rejections of the two bands can be further improved to 43 and 41 dB at least by digital signal processing (DSP). The proposed photonic-assisted receiver is thus able to simplify the architecture and improve performance for the multispectral sensing application.

Photonics-based broadband radar for high-resolution and real-time inverse synthetic aperture imaging.

A photonics-based radar with generation and de-chirp processing of broadband linear frequency modulated continuous-wave (LFMCW) signal in optical domain is proposed for high-resolution and real-time inverse synthetic aperture radar (ISAR) imaging. In the proposed system, a broadband LFMCW signal is generated by a photonic frequency quadrupler based on a single integrated electro-optical modulator, and the echoes reflected from the targets are de-chirped to a low frequency signal by a microwave photonic frequency mixer. The proposed radar can operate at a high frequency with a large bandwidth, and thus achieve an ultra-high range resolution for ISAR imaging. Thanks to the wideband photonic de-chirp technique, the radar receiver could apply low-speed analog-to-digital conversion and mature digital signal processing, which makes real-time ISAR imaging possible. A K-band photonics-based radar with an instantaneous bandwidth of 8 GHz (18-26 GHz) is established and its performance for ISAR imaging is experimentally investigated. Results show that a recorded two-dimensional imaging resolution of ~2 cm × ~2 cm is achieved with a sampling rate of 100 MSa/s in the receiver. Besides, fast ISAR imaging with 100 frames per second is verified. The proposed radar is an effective solution to overcome the limitations on operation bandwidth and processing speed of current radar imaging technologies, which may enable applications where high-resolution and real-time radar imaging is required.

Detection and RM correction approach for manoeuvring target with complex motions

This study addresses the coherent accumulation problem for detecting a manoeuvring target with complex motions, where range migration (RM) [i.e. range walk (RW) and range curvature (RC)] and Doppler frequency migration (DFM) occur during the coherent integration time. An efficient approach based on generalised keystone transform (GKT), radon transform (RT) and generalised dechirp process (GDP), i.e. GKT-RT-GDP, is presented to eliminate the RM and realise the coherent accumulation. More specifically, the GKT operation is first employed to remove the RC. Then, the RT is applied to estimate the trajectory slope for RW correction and velocity estimation. After that, GDP is introduced to obtain the estimations of target's acceleration and acceleration rate motion. Thereafter, the DFM caused by target's high-order motions can be compensated and then the coherent integration can be realised via Fourier transform. The advantage of the presented algorithm is that it can obtain a good balance between the computation cost and the detection performance, in comparison with the existing coherent integration algorithms.

CLEAN‐based coherent integration method for high‐speed multi‐targets detection

Coherent integration for high-speed multi-targets detection is a challenging task in radar applications. First, range migration (RM) and Doppler frequency migration (DFM) induced by target's complex motions (i.e. high speed, acceleration, etc.) would result in serious integration performance loss. Second, if the scattering intensities of different targets differ significantly, the weak one would be shadowed by the strong target and makes it difficult to achieve the coherent accumulation for weak target. Existing methods either perform poorly or are too computationally expensive for coherent integration of high-speed manoeuvring targets. Two contributions are made towards addressing these limitations. First, a method based on keystone transform, fold factor phase term compensation and generalised dechirp process is proposed to remove the migrations (RM and DFM) and realise the coherent accumulation. Compared with the generalised Radon Fourier transform algorithm, the proposed method can avoid the blind speed sidelobe and has a much lower computational cost. Second, two CLEAN techniques based on sinc-like point spread function (PSF) and modified PSF are presented to eliminate the strong target's effect and highlight the weak ones. By this way, the coherent integration of strong target and weak ones can be achieved iteratively. Several simulations are provided to demonstrate the effectiveness.

A low complexity coherent integration method for maneuvering target detection

This paper considers the coherent integration problem for detecting a maneuvering target with complex motions, where the velocity, acceleration and jerk result in respectively the range migration (RM), linear Doppler frequency migration (LDFM) and quadratic Doppler frequency migration (QDFM) within the coherent pulse interval. A new coherent integration algorithm based on keystone transform (KT) and generalized dechirp process (GDP), i.e., KTGDP, is proposed. In this method, KT and fold factor searching are first employed to correct the RM, and then GDP is applied to estimate the target's radial acceleration and jerk. With the estimated motion parameters, LDFM and QDFM can be compensated and the coherent integration can be achieved via Fourier transform. In addition, at the cost of some performance loss, a fast coherent integration method combing KT and cubic phase function (CPF), i.e., KTCPF, is also introduced to further reduce the computational complexity. Compared with the generalized Radon–Fourier transform (GRFT) method, the proposed algorithms can avoid the blind speed side lobe (BSSL) effect and have much lower computational burden. Finally, we evaluate the performance via some numerical simulations.