Linear Frequency Modulated Continuous Wave Research Articles

Demonstration of Airport Runway FOD Detection System Based On Vehicle SAR

In the background of detecting Foreign Object Debris (FOD) on the airport runway with conventional radar systems or optical systems, to solve the problems that the current detection technology of conventional radar is high frequency and high cost, and the optical system is vulnerable to environmental factors, this paper uses linear frequency modulated continuous wave (LFM-CW) signal in Ka band to detect FOD based on the Synthetic Aperture Radar (SAR) airport runway FOD detection system. Firstly, it is proved that the operating frequency of 36GHz is more suitable. Then demonstration that the use of LFM-CW can effectively reduce the system power. Finally, through the demonstration of the main technical indicators of the system, it is proved that the airport runway FOD detection system based on vehicle SAR can effectively detect objects with a height of 3cm and a cross-sectional area of 0.00051m2 in a distance of 210 meters when the transmitting power is 25dBm and the running speed is 61.2m/s. At this time, the detection probability is 0.9999 and the false alarm rate is 10−12.

Radio Frequency Interference Cancelation in High-Frequency Surface Wave Radar Using Orthogonal Projection Filtering

The characteristics of radio frequency interference (RFI) are analyzed for high-frequency surface wave radar (HFSWR), which adopts a linear frequency-modulated interrupted continuous wave. If RFI enters the receiver, it contaminates the positive and negative frequency range bins simultaneously, whereas the negative frequency range bins contain the RFI and noise only. Moreover, RFI is always spatially structured over a short duration. Based on the above characteristics, a new orthogonal projection filtering algorithm is proposed for RFI cancelation. The proposed method uses the information of antenna channel and sweep dimensions in negative frequency range bins to estimate the interference covariance matrix. To suppress RFI, the signals of interest in the positive range bins are projected onto the interference subspace, which is obtained by eigendecomposition of the covariance matrix. Experimental results suggest that the proposed method can achieve effective RFI cancelation by using data collected by a multifrequency HFSWR.

Multitarget AOA Estimation Using Wideband LFMCW Signal and Two Receiver Antennas

Linear frequency modulated continuous wave (LFMCW) signals have been widely used in target detection and localization, thanks to their good detection sensitivity and high range resolution. However, estimation of the angle of arrival (AOA) of the LFMCW signal reflected from each target in a multitarget environment is still quite limited at present. In this paper, we propose a novel AOA estimation framework in which one transmitter antenna and two receiver antennas are employed to transmit a wideband LFMCW signal and receive the signals reflected from different targets, respectively. The signals received at two receiver antennas are processed in three steps. First, each of them is demodulated by mixing with the transmitted signal to transform the wideband nonstationary signal into superposition of a series of single-tone signals. Second, signals received at each receiver antenna from different targets are separated using bandpass filtering. Third, the corresponding AOA of each target is estimated using the phase difference between two receiver antennas. Different from the traditional subspace-based methods, the number of targets is not limited by the number of receiver antennas. The theoretical analysis and simulation results show that the proposed algorithm can achieve accurate AOA estimation for multiple targets even at a low signal to noise ratio.

An Active Detection Method for Buried Target Based on LFMCW Waveform

Underwater clutter is the main interference for active sonar to detect buried target. Anti-clutter waveform designing and processing method play a significant role in clutter suppression. This paper proposes a processing framework for detecting buried target. The linear-frequency-modulated continuous wave (LFMCW) signal is selected as the detecting waveform, and the properties of time-frequency distribution, spectrum, periodic fractional Fourier transform (PFRFT) and wideband ambiguity function (WAF) for LFMCW are analysed, then the waveform parameters is designed to well detect buried target. Furthermore, on the basis of PFRFT and WAF, this paper proposes a processing method for LFMCW. Both the theoretical analysis and simulation results validate the proposed method. Sine-square weighted LFMCW outperforms the rectangular weighted one.

Range Observation Method Applied to Linear Frequency-Modulated Continuous-Wave Synthetic-Aperture Radars

The sliding signal processing method interpreted as the range observation technique for syntheticaperture radar systems is discussed. The given method enables us to shift the observed ranges, thereby eliminating the maximum range restriction inherent to the standard de-ramping processing method. The operational capability of the method is demonstrated using radar images obtained from the single received implementation but exhibiting different detection ranges. It is revealed that the method under consideration is efficient upon reduction of nonlinear distortions caused by the nonideal operation of a voltage-controlled oscillator.

Assessment of Range and Radial Velocity of Objects of a Broadband Radar Station Under Conditions of Range Cell Migration

A method is proposed that makes it possible to increase the period of coherent accumulation of signals reflected from objects, in order to increase the operating range of a radar station with linear frequencymodulated continuous wave under conditions of range cell migration. The range and radial velocity of an object are calculated with the condition that the change in distance to it during the modulation significantly exceeds the resolution of the radar station at that range. An assessment of the potential accuracy of the estimates is conducted, and results of experimental verification of the proposed method are presented.

A C-Band FMCW SAR Transmitter With 2-GHz Bandwidth Using Injection-Locking and Synthetic Bandwidth Techniques

A synthetic bandwidth architecture utilizing an injection-locked oscillator to enhance the bandwidth of a linear frequency-modulated continuous wave transmitter is presented. A 500-MHz narrow bandwidth chirp from a direct digital synthesizer is upconverted to four adjacent carrier frequencies with 500-MHz spacing to obtain an effective synthetic bandwidth of 2 GHz at the C-band (43%). The four adjacent frequencies are obtained through the injection-locking technique to achieve fast frequency switching. This effectively results in an improved range resolution of 7.5 cm. Single-sideband mixing is employed to reduce in-band spurs and to avoid the use of switched filters. The architecture demonstrates ranging capability with a standard deviation of about 0.8 mm ( $1.36 \sigma_{\mathrm {CRLB}}$ , 26-dB SNR). Synthetic aperture radar images obtained from the proposed transmitter show sidelobe levels of less than −27 and −36 dBc in range and azimuth, respectively.

On fundamental operating principles and range-doppler estimation in monolithic frequency-modulated continuous-wave radar sensors

The diverse application areas of emerging monolithic noncontact radar sensors that are able to measure object?s distance and velocity is expected to grow in the near future to scales that are now nearly inconceivable. A classical concept of frequency-modulated continuous-wave (FMCW) radar, tailored to operate in the millimeter-wave (mm-wave) band, is well-suited to be implemented in the baseline CMOS or BiCMOS process technologies. High volume production could radically cut the cost and decrease the form factor of such sensing devices thus enabling their omnipresence in virtually every field. This introductory paper explains the key concepts of mm-wave sensing starting from a chirp as an essential signal in linear FMCW radars. It further sketches the fundamental operating principles and block structure of contemporary fully integrated homodyne FMCW radars. Crucial radar parameters like the maximum unambiguously measurable distance and speed, as well as range and velocity resolutions are specified and derived. The importance of both beat tones in the intermediate frequency (IF) signal and the phase in resolving small spatial perturbations and obtaining the 2-D range-Doppler plot is pointed out. Radar system-level trade-offs and chirp/frame design strategies are explained. Finally, the nonideal and second-order effects are commented and the examples of practical FMCW transmitter and receiver implementations are summarized.

High‐precision estimation of target range, radial velocity, and azimuth in mechanical scanning LFMCW radar

This study investigates the effects of antenna rotation on range, radial velocity, and azimuth estimation during coherent integration in linear frequency-modulated continuous wave (LFMCW) radars. When antenna scans during the coherent integration time, theoretical analysis indicates that the angle sensing with the traditional sum-difference amplitude-comparison (SDAC) monopulse method and the range, radial velocity estimation with interpolation method are different from that in staring mode. Simulation demonstrates that the estimation performance is heavily impaired by the antenna rotation. In this study, a generalised mathematical model of coherent integration for LFMCW radar is established. Furthermore, a high-precision estimation method, which consists of generalised SDAC monopulse method, radial velocity estimator with modified interpolation curves, and range estimator refined by Doppler frequency, is proposed. A complete signal-processing scheme is proposed for the accurate estimation of the target parameter in mechanical scanning LFMCW radar. A sub-optimal method is provided for practical implementation. Experimental and simulation results show that the proposed method achieves high accuracy and robustness.

External Modulation Method for Generating Accurate Linear Optical FMCW

Frequency modulation continuous wave (FMCW) lasers are key components in modern optical imaging. However, current intracavity modulation lasers do not exhibit low-frequency jitter rate and high linearity due to the inherent relaxation oscillations. Although this may be compensated in a direct modulation laser diode using an optoelectronic feedback loop, the available sweep speed is moderately small. In this letter, a special external modulation method is developed to improve the performance of FMCW. Since only the first sideband optical field is used during the entire generation process, phase noise is kept to a minimum and is also independent of the sweep speed. We demonstrate that the linearity and jitter rates do not deteriorate appreciably when the sweep speed is changed over three orders of magnitude, even up to the highest sweep speed of 2.5 GHz/ $\mu \text{s}$ .

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.

An Approximate Cramer–Rao Lower Bound for Multiple LFMCW Signals

In this paper we focus on deriving an approximate Cramer–Rao lower bound (CRLB) for the parameters of a multicomponent linear frequency modulated continuous wave (LFMCW) signal corrupted by complex additive white Gaussian noise. The approximation is necessary due to the discontinuities inherent in the mathematical model of the instantaneous phase of each LFMCW signal model. By comparing our approximate bound to a simulation of the maximum likelihood estimator (MLE) of the LFMCW parameters, we confirm our analysis. In general, the CRLB is a useful tool for feasibility studies or in evaluating the degree of suboptimality that non-MLE methods exhibit. For passive detection and estimation of LFMCW signals, the Generalized Likelihood Ratio Test and the associated MLE are difficult to implement in practice, primarily due to their large computational requirements. So, lower bounds on performance, such as those provided by the CRLB, are necessary to evaluate suboptimal methods that are more suited for practical implementations.

Linear frequency-modulated continuous-wave ladar system for synthetic aperture imaging.

A novel and high-efficiency linear frequency-modulated continuous-wave (FMCW) ladar system for synthetic aperture imaging is proposed and experimentally demonstrated. This novel system generates wide-bandwidth linear FMCW ladar signals by employing an electro-optic LiNbO3- in-phase and quadrature modulator with an effective bias controller. The effectiveness of the proposed system is experimentally validated. Optical synthetic aperture images are obtained by using two 0.41 cm aperture diameter telescopes at the distance of 1 km. The resolution of these images can reach to 4 cm. A resolution improvement by about 10 times is achieved when compared with the conventional real aperture imaging system.

Ground-Based 3D Radar Imaging of Trees Using a 2D Synthetic Aperture

Motivated by the desire to gain insight into the details of conventional airborne synthetic aperture radar (SAR) imaging of trees, a ground-based SAR system designed for short-range three-dimensional (3D) radar imaging is developed using a two-dimensional (2D) synthetic aperture. The heart of the system is a compact linear frequency modulation-continuous wave (LFM-CW) radar, a custom two-dimensional scan mechanism, and a three-dimensional time-domain backprojection algorithm that generates three-dimensional backscatter images at an over-sampled resolution of 10 cm by 10 cm by 10 cm. The backprojection algorithm is formulated directly in spatial coordinates. A new method for estimating and compensating for signal attenuation within the canopy is used that exploits the backprojection image formation approach. Several three-dimensional C-band backscatter images of different individual trees of multiple species are generated from data collected for trees both in isolation and near buildings. The trees imaged in this study are about 10 m in height. The transformation of the three-dimensional images to airborne SAR images is described and a sample result provided.

Rapidly and continuously frequency-scanning opto-electronic oscillator.

An opto-electronic oscillator (OEO) scheme which operates at "chirp oscillation" mode and generates low-phase-noise, frequency-swept microwave is proposed and experimentally demonstrated. This frequency-swept OEO is achieved by embedding a rapidly frequency-scanning microwave filter in an opto-electronic cavity. The filter has fixed passband while its center frequency scans rapidly and periodically at cavity round-trip time, covering a large frequency range (~GHz). Experimentally, the generated frequency-swept microwave is linear frequency-modulated continuous wave (FMCW) which centers at 7 GHz with 1-GHz bandwidth. Its instantaneous frequency varies linearly from 6.5 GHz to 7.5 GHz, back and forth, in a period of 12.8 μs, resulting in a frequency scanning rate of ~156 MHz/μs. The single-side-band (SSB) noise of the generated FMCW is -104 dBc/Hz at 10 kHz offset frequency, which is much lower than that from a commercial electronic arbitrary waveform generator (E-AWG). Improvement as large as 23 dB is experimentally reported.

A highly linear and fully-integrated FMCW synthesizer for 60 GHz radar applications with 7 GHz bandwidth

A highly linear and fully-integrated frequency-modulated continuous-wave (FMCW) generator based on a fractional-N phase-locked loop (PLL) that is able to synthesize modulation schemes in 57---64 GH...

An LFMCW detector with new structure and FRFT based differential distance estimation method

This paper describes a linear frequency modulated continuous wave (LFMCW) detector which is designed for a collision avoidance radar. This detector can estimate distance between the detector and pedestrians or vehicles, thereby it will help to reduce the likelihood of traffic accidents. The detector consists of a transceiver and a signal processor. A novel structure based on the intermediate frequency signal (IFS) is designed for the transceiver which is different from the traditional LFMCW transceiver using the beat frequency signal (BFS) based structure. In the signal processor, a novel fractional Fourier transform (FRFT) based differential distance estimation (DDE) method is used to detect the distance. The new IFS based structure is beneficial for the FRFT based DDE method to reduce the computation complexity, because it does not need the scan of the optimal FRFT order. Low computation complexity ensures the feasibility of practical applications. Simulations are carried out and results demonstrate the efficiency of the detector designed in this paper.

Terahertz Wide-Angle Imaging and Analysis on Plane-wave Criteria Based on Inverse Synthetic Aperture Techniques

This paper presents two parts of work around terahertz imaging applications. The first part aims at solving the problems occurred with the increasing of the rotation angle. To compensate for the nonlinearity of terahertz radar systems, a calibration signal acquired from a bright target is always used. Generally, this compensation inserts an extra linear phase term in the intermediate frequency (IF) echo signal which is not expected in large-rotation angle imaging applications. We carried out a detailed theoretical analysis on this problem, and a minimum entropy criterion was employed to estimate and compensate for the linear-phase errors. In the second part, the effects of spherical wave on terahertz inverse synthetic aperture imaging are analyzed. Analytic criteria of plane-wave approximation were derived in the cases of different rotation angles. Experimental results of corner reflectors and an aircraft model based on a 330-GHz linear frequency-modulated continuous wave (LFMCW) radar system validated the necessity and effectiveness of the proposed compensation. By comparing the experimental images obtained under plane-wave assumption and spherical-wave correction, it also showed to be highly consistent with the analytic criteria we derived.

Adaptive Threshold Detection and Estimation of Linear Frequency-Modulated Continuous-Wave Signals Based on Periodic Fractional Fourier Transform

The fractional Fourier transform (FRFT) has been used to detect and estimate the parameters of linear frequency-modulated continuous-wave (LFMCW) in low probability of intercept radar waveforms. The FRFT, which is optimal for single linear frequency-modulated (LFM) signals, becomes sub-optimal when applied to LFMCW signals because the observed waveform of this type of signal is composed of concatenated LFM pulses. A new signal processing method, called the periodic FRFT (PFRFT), is proposed for the detection of LFMCW signals. First, the discrete PFRFT is studied and the signal processing gain of this transform for LFMCW signals is analyzed. Second, an adaptive threshold detection and estimation algorithm for LFMCW signals is formulated after analysis of the test statistics of the squared modulus of LFMCW signals when using the probability density function in the presence of Gaussian white noise. It is then proved that PFRFT-based estimation is equivalent to maximum likelihood estimation in the detection and estimation of LFMCW signals. Finally, the results of both the theoretical analysis and verification simulations show that the PFRFT significantly outperforms the FRFT for LFMCW signals.