Dechirp Processing Research Articles

Spectral-Efficient Frequency-Division Photonic Millimeter-Wave Integrated Sensing and Communication System Using Improved Sparse LFM Sub-Bands Fusion

Scheduling radar and communication functions in a frequency-division multiplexing (FDM) fashion is a classic solution to achieve mutual-interference-free integrated sensing and communication (ISAC) system. However, it is haunted by poor spectral efficiency (SE) in comparison to the spectrum-sharing scheme. In this paper, we propose a novel photonics millimeter-wave ISAC system reaping the FDM scheme while offering inspirations for significantly enhancing SE using super-resolution techniques. Using the coherent fusion processing (CFP) of sparse sub-band linear frequency modulated (LFM) radar signals, a small fraction of the total bandwidth can enable a full-band-equivalent radar range resolution and more spectrum resources can be allocated for communications, leading to an ultrahigh SE for ISAC system. Moreover, an improved sparse sub-band fusion algorithm is developed based on particle swarm optimization to enhance the accuracy of synthesized radar range profiles. In experiments, a 60-GHz super-resolution FDM ISAC system using photonics-assisted millimeter-wave signal generation and dual-parallel coherent de-chirping processing is established, wherein the idle spectrum between two widely-spaced LFM sub-bands is loaded with broadband orthogonal frequency division multiplexing (OFDM) communication signals. Our system can simultaneously achieve up to 18-Gbit/s communication data rate and a radar range resolution as high as 2.14 cm, by exploring the globally unlicensed 7-GHz spectrum around 60-GHz with 4.5-GHz communication and 2-GHz radar bandwidths. In addition, a 0.97-cm radar range resolution corresponding to 16-GHz equivalent bandwidth (EB) is experimentally obtained by using two 1.5-GHz LFM sub-bands. This renders only ∼9.4% EB loss (3 GHz/32 GHz), without significant performance tradeoff between sensing and communications.

Interference Countermeasure System Based on Time–Frequency Domain Characteristics

We investigate the issue of combating interrupted-sampling repeater jamming (ISRJ). Due to the advantages of miniaturization, lightweight, and flexibility, the ISRJ poses a great menace to radar performance through the fast sampling and forwarding of radar signals. Given this problem, we propose an electronic counter-countermeasure (ECCM) system based on the time–frequency domain. The system mines the information of radar echoes using de-chirping processing and the short-time Fourier transform (STFT). We introduce a binarization algorithm to achieve noise suppression and utilize two different features to guarantee the correct rate of target signal extraction. Simulation experiments show that our system can be effective against ISRJ. Moreover, our system still exhibits good interference suppression performance under the condition of multiple jammers, which effectively enhances the anti-jamming capability of the radar.

Radar coherent detection for hypersonic target based on shear transform and dechirp process

For hypersonic target detection, the range migration (RM) and Doppler frequency migration (DFM) often occur during coherent processing time, which would lead to integration loss and degrade detection performance. Aiming at these problems, this paper proposes a coherent integration method based on shear transform (ST) and dechirp process (DP), namely ST-DP. Specifically, the RM is eliminated by shearing the echo data plane, and then the DFM is compensated by dechirp process. Eventually, the Fourier transform (FT) is carried out in the slow time dimension to realize coherent integration. Compared with existed methods, the presented method has a coherent integration and detection performance close to that of GRFT without increasing the computational cost, while there is no blind speed sidelobe (BSSL) effect. Some simulation experiments are given to prove the practicality and efficacy of the presented method.

Optical Filter-Less Photonic FMCW Radar for Multi-Target Detection

Frequency modulated continuous wave (FMCW) radar monitors the instantaneous frequency difference between the transmitted and the received signal to determine the time of flight which is proportional to the range. Modern radar system necessitates multispectral sensing to enhance detection capability and anti-jamming efficacy that encourages the evolution from conventional electrical solutions to photonics-enhanced solutions. Photonics offers wide-bandwidth signal generation, parallel processing, and low propagation distortion. Most photonic-assisted radar models are based on optical filters to select the reference and reflected waveform to mix at the photodiode, which limits the system functionality due to the inability to operate in different frequency bands. We propose an optical filter-less photonics-based de-chirp processing of frequency-modulated continuous-wave radar waveforms that overcomes the issue of carrier frequency tunability and agility. A theoretical analysis is performed, followed by simulation, and validated by experiment. The proposed model is experimentally tested for detecting targets at different distances ranging from 150 cm to 210 cm. The system is emulated to detect multiple targets (up to four), showing a range resolution of <15 cm, and signal processing enables the estimation of the widths of different sizes of the targets.

Performance Analysis of Microwave Photonic Radar Receiver with De-Chirp Processing

Performance Analysis of Microwave Photonic Radar Receiver with De-Chirp Processing

Photonic Fractional Fourier Transformation Based on Discrete-Frequency Processing

Photonic fractional Fourier transformation (FRFT) based on discrete-frequency processing is proposed to overcome the limitations of processing latency, resolution, and tunability. Typical applications of FRFT, such as de-chirp processing, separation of two linear frequency modulated waveforms (LFMWs) in fractional domain and filtering of a LFMW from a noisy background, are experimentally demonstrated. The operation bandwidth and spectral resolution are 4 GHz and 60 MHz, respectively. Both the phase and amplitude can be measured by the proposal without relying on complex algorithms, while the processing latency is close to the minimum delay required in the FRFT. All optical devices employed in our system shows great potential in integration.

Random phase compensation method of coherent lidar based on symmetrical double harmonic signals

The coherent cumulative lidar based on the symmetrical double harmonic signals has been implemented and verified. In the system, the double harmonic signals are generated by the laser through an electro-optic modulator, and then the signal is divided into two paths, one of which is coupled to the transmitter as the signal light, and the other is used as the local light, which performs I/Q de-chirp processing with the echo reflected from the targets. The proposed lidar works at a wavelength of 1550 nm with a sweep bandwidth of 1 GHz, and a pulse period of 70us. This article respectively gives two processing methods in frequency domain and time domain to compensate for the random phase. Finally, the experimental results of coherent accumulation on stationary and moving targets show that the method can be excellently used to compensate for the random phase induced by atmospheric disturbance, vibration, and nonlinear effects of optical devices in the system, so that the phase can be stabilized within 0.4 rad in any complex environment, and the coherent accumulation efficiency can be improved to 90%. Therefore, this paper has great guiding significance for detecting the low detectable targets and improving detection distance.

Dual-Chirp Photonics-Based Radar for Distance and Velocity Measurement Based on Compressive Sensing

We proposed a dual-chirp microwave photonic radar based on compressive sensing for distance and velocity measurement. This radar can generate different chirp linear frequency modulated (LFM) signals using a dual-parallel Mach-Zehnder modulator (DPMZM). In the receiving part, a dual-drive Mach-Zehnder modulator (DDMZM) and a Mach-Zehnder modulator (MZM) are cascaded for the optical mixing and de-chirp processing with a pseudo-random bit sequence (PRBS). Then the mixed signal can be gathered by an analog-to-digital converter (ADC) at a sampling rate that is well below the Nyquist sampling rate. Using fewer sampling points, the reconstruction algorithm can recover the de-chirped signal accurately with a compression ratio of 8. A proof-of-concept experiment demonstrates that when the target is stationary, the distance measurement error is about 1.560 cm. The signal-to-noise ratio (SNR) of the recovery signal is enhanced to 30.725 dB. When the target is moving, the simulation results present that the maximum distance error is 1.2 cm, and the velocity error is below 0.140 m/s. This compressive sensing radar reduces the pressure of a massive amount of data storage or processing and guarantees the accuracy of signal recovery. At the same time, it breaks the limitation of operation bandwidth and increases the speed of operation.

Microwave Photonic MIMO Radar for High-Resolution Imaging

A microwave photonic multiple-input and multiple-output (MIMO) radar is proposed and demonstrated to implement high-resolution imaging. In the proposed system, multiple orthogonal linearly frequency modulated (LFM) signals are generated by heterodyning between two optical frequency combs, which enables a MIMO transmitting array with a simple and reconfigurable structure. The receiving array uses photonic frequency mixing to implement multiple channel separation and de-chirp processing simultaneously. This microwave photonic MIMO radar can have a large operation bandwidth and a large equivalent aperture, which helps to achieve high-resolution imaging in both range and azimuth directions. In the experiment, a microwave photonic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{4}\times \text{8}$</tex-math></inline-formula> MIMO radar is established with a 2-GHz bandwidth in each channel. Based on this MIMO radar, high-resolution back-projection (BP) imaging with a theoretical range resolution of 7.5 cm and azimuth resolution of 1.85° is demonstrated. The experimental results can verify the feasibility of the proposed MIMO radar, which is a good solution to high-resolution radar imaging by combining microwave photonic and MIMO technologies.

Exploiting Chirp Rate Estimation Methods to Improve Image Formation Quality of Synthetic Aperture Radar

A common assumption in SAR image formation and processing algorithms is that the chirp rates of the transmitted and received radar signals are exactly the same. Dechirp processing is also done based on this common assumption. In real scenarios, the chirp rate of the received signal is different from that of the transmitted signal due to several reasons. In case the difference between the chirp rates of the transmitted and received signals is obvious, the demodulation and compression of the received pulse are not carried out precisely and defocusing the targets and the output images of the SAR processor results. In the present paper, a new technique is proposed to improve the image formation quality of SAR by exploiting chirp rate estimation methods. Based on the proposed technique, the chirp rate of the received signal is estimated, and then, dechirp is carried out by using a time-reversed complex conjugate filter constructed based on the estimated chirp rate. In this stage, the existing chirp rate estimation algorithms can be used. The quality of the output image is assessed using PSLR as a quantitative criterion and the average number of point target extension pixels along the azimuth direction. Simulation results indicated that the smaller the average number of point target extension pixels along with azimuth and the higher the PSLR average is, the better the output image quality would be. Therefore, output images obtained from the proposed method by exploiting chirp rate estimation algorithms would have a better quality with a higher PSLR average (14.1 and 13.6) and also the lower average number of point target extension pixels along the azimuth directions (2.1 and 4.9) than the common method with PSLR average (8.3) and an average number of point target extension pixels along the azimuth direction (7.1).

Simultaneous detection of the distance and direction for a noncooperative target based on the microwave photonic radar.

For a noncooperative target, multiple-dimension detection is required to determine its spatial location. A microwave photonic radar is proposed for simultaneous detection of the distance and direction of targets. At the transmitter, a linear frequency modulation (LFM) signal with a large instantaneous bandwidth was obtained by frequency doubling. Meanwhile, the optical reference signal was supplied. At the receiver, two antennas with a certain baseline length were used to receive the echo signal. The dechirp process and fixed-length cable are combined to decouple the distance and direction and distinguish between the positive and negative directions of a target. Theoretical derivation has already verified that the distance and direction can be resolved simultaneously by dechirping of the echo modulation signal and reference signal. In experiments, an LFM signal with an instantaneous bandwidth of 2 GHz (2-4 GHz) was generated and launched as the detection signal. Targets at different distances with positive and negative angles were detected. The results show that the directional measurement ranges from -72.5° to 72.5°, and the error is less than 1.6°. The measured distance was found to be quite similar to the actual distance. The proposed approach can confirm the spatial location of a noncooperative target by combining the detection of both distance and direction. Furthermore, the baseline length need not be less than half the wavelength of the echo signal, and large antennas can be chosen to improve the detection distance and range accuracy.

Scalable distributed microwave photonic MIMO radar based on a bidirectional ring network.

A scalable distributed microwave photonic multiple-input-multiple-output (MIMO) radar is proposed based on a bidirectional ring network. The network is constructed with a fiber ring on which a local node and several remote nodes are distributed. In the local node, radar signals are generated over different optical wavelengths based on external modulation. Employing wavelength-division multiplexing, the radar signals are sent to remote nodes through the fiber ring. In different remote nodes, radar signals modulated on corresponding wavelength are utilized for transmitting or photonic de-chirp processing. Benefiting from the bidirectional ring network, the proposed radar is suitable for large-scale distribution. Together with the pluggable remote nodes, the scalability of the radar is enhanced. A proof-of-concept experiment is demonstrated to verify the feasibility of the system. Measurements of two-dimensional position and velocity of targets are realized. The position error and velocity error are better than 8 cm and 0.20 m/s respectively.

Three-Dimensional Interferometric ISAR Imaging Algorithm Based on Cross Coherence Processing.

Interferometric inverse synthetic aperture radar (InISAR) has received significant attention in three-dimensional (3D) imaging due to its applications in target classification and recognition. The traditional two-dimensional (2D) ISAR image can be interpreted as a filtered projection of a 3D target’s reflectivity function onto an image plane. Such a plane usually depends on unknown radar-target geometry and dynamics, which results in difficulty interpreting an ISAR image. Using the L-shape InISAR imaging system, this paper proposes a novel 3D target reconstruction algorithm based on Dechirp processing and 2D interferometric ISAR imaging, which can jointly estimate the effective rotation vector and the height of scattering center. In order to consider only the areas of the target with meaningful interferometric phase and mitigate the effects of noise and sidelobes, a special cross-channel coherence-based detector (C3D) is introduced. Compared to the multichannel CLEAN technique, advantages of the C3D include the following: (1) the computational cost is lower without complex iteration and (2) the proposed method, which can avoid propagating errors, is more suitable for a target with multi-scattering points. Moreover, misregistration and its influence on target reconstruction are quantitatively discussed. Theoretical analysis and numerical simulations confirm the suitability of the algorithm for 3D imaging of multi-scattering point targets with high efficiency and demonstrate the reliability and effectiveness of the proposed method in the presence of noise.

Microwave photonic de-chirp receiver for breaking the detection range swath limitation.

A novel microwave photonics-based de-chirp radar receiver which breaks the limitation of the detection range swath is proposed and demonstrated. In the proposed receiver, a multi-channel time-division photonics de-chirp processing is implemented to increase the detection range swath. A linear frequency modulated pulse train is sent to multiple reception channels and temporally delayed in the optical domain to form reference signal replicas, enabling time-division photonics-de-chirp processing with echoes reflected from different distance regions so that the total detection range swath is increased and determined by the number of reference replicas. Hardware-in-the-loop simulation experiments are demonstrated and an inverse synthetic aperture 2D imaging is carried out, showing that the MWP radar with the proposed photonics de-chirp receiver is capable of achieving a detection range swath of 13km which is 20 times larger than that when employing a conventional de-chirp receiver with the same parameters.

Broadband array radar based on microwave photonic frequency multiplication and de-chirp receiving (Invited)

Microwave photonic radar enables the generation and processing of broadband radar signals, which can significantly improve the range resolution of the radar system. To improve the radar angle resolution and realize flexible beam control, combining microwave photonic radar technology with array radar technology is an inevitable development trend. Previously, the optical truth delay technology is intensively investigated to achieve squint-free beam steering in broadband phased array radars, which usually face the problems of high complexity, poor flexibility, and limited delay accuracy. In recent years, the broadband radar architecture based on microwave photonic frequency multiplication and de-chirp receiving has received extensive attention. The array radar constructed based on this technology has wide operation bandwidth while enabling real-time digital compensation and processing functions, which provides a new idea for the development of broadband array radars. In this paper, the research progress of the broadband array radar based on microwave photonic frequency multiplication and de-chirp processing was reviewed. After expounding the transceiver mechanism of microwave photonic broadband radar, the method for constructing broadband phased array radar and the performance of digital beam scanning and imaging were introduced. Then, the radar array was extended to MIMO architecture. The broadband microwave photonic MIMO radar based on optical wavelength division multiplexing technology was introduced and its performance in target detection and imaging was analyzed.

Maneuvering target detection in random pulse repetition interval radar via resampling-keystone transform

With good electronic counter-countermeasures capability, random pulse repetition interval (PRI) radars receive increasing attention recently. However, the problem of maneuvering target detection in random PRI radars is rarely studied yet. In this problem, the difficulty lies in not only the range migration (RM) and Doppler frequency migration (DFM) effects but also the non-uniform sampling pulses. This paper proposes a novel algorithm for this problem. In the proposed algorithm, we first combine the non-uniform resampling operation and the keystone transform to propose the resampling-keystone transform, which can eliminate the RM and resample the non-uniform sampling pulses into uniform ones in one step. Then, the dechirp process, whose implementation can benefit from the fast Fourier transform, is employed to accomplish coherent integration for target detection by compensating the DFM. The proposed algorithm is applicable for both single-target and multi-target scenarios. Besides, the available integration time and computational complexity of the proposed algorithm are analyzed. Finally, simulation results are given to show that the proposed algorithm can approach the optimal detection performance with a much lower computational cost than the well-known generalized Radon Fourier transform.

Microwave‐Photonic Radars: Chip‐Based Microwave‐Photonic Radar for High‐Resolution Imaging (Laser Photonics Rev. 14(10)/2020)

In article number 1900239 by Shilong Pan and co-workers, a chip-based microwave-photonic radar is proposed and experimentally demonstrated. Key optical devices in the radar transceiver are integrated on a single chip, which generates a flat linear frequency-modulated signal occupying the full Ku band (12–18 GHz) and performs wideband de-chirp processing of the radar echoes. Inverse synthetic aperture radar imaging with a range resolution of 2.7 cm is implemented. This work paves the way for applying microwave-photonic radars to miniaturized platforms such as UAVs and autonomous vehicles.

Bistatic Forward-Looking SAR MP-DPCA Method for Space–Time Extension Clutter Suppression

Echoes of bistatic forward-looking synthetic aperture radar (BFSAR) disperse to multiple range cells and exist spatial frequency extension as well as Doppler spectrum extension (i.e., namely space–time extension). Furthermore, the characteristics of BFSAR clutter are strongly nonstationary and spatial variants. Because of the abovementioned issues, clutter and moving targets are fully overlapped in the initial 3-D space–time–range raw data domain, and the clutter cannot be suppressed effectively. To solve this problem, a BFSAR multipulse displaced phase center antenna (MP-DPCA) method is proposed in this article. First, keystone transform without Doppler ambiguity is applied to remove the coupling between the space–time and range domains. Hence, the overall complex 3-D processing in the space–time–range domain is reduced to independent 2-D processing in each space–time domain. Subsequently, a spatial-dechirp processing is applied in the space–time domain to eliminate the spatial frequency extension. Meanwhile, Doppler parameters of clutter point scatterers are equalized by nonlinear chirp scaling processing in the frequency and time domains. Accordingly, the Doppler spectrum extension of point scatterers can be eliminated by a uniform azimuth dechirp processing. After aforesaid three steps, the clutter and moving targets are separated in the space–time domain. Finally, based on the linear phase difference of clutter between the channels, a multipulse canceller can be designed to suppress the clutter. Compared with the existing space–time adaptive procession (STAP) and DPCA methods, this method not only overcomes the nonstationary problem in BFSAR but also conquers the strict application conditions of DPCA. Simulation results are given to verify the effectiveness of the proposed method.

A Novel Ship Imaging Method with Multiple Sinusoidal Functions to Match Rotation Effects in Geosynchronous SAR

Geosynchronous Synthetic Aperture Radar (GEO SAR) has a very long Coherent Processing Interval (in the order of hundreds of seconds) compared with other SAR platforms. Thus, the current methods of rotation effect matching and ship imaging that operate within a relatively short Coherent Processing Interval (in the order of seconds) are obviously not applicable. To address this problem, a novel ship imaging method with multiple sinusoidal functions matching for rotation effects is proposed for GEO SAR. Firstly, the influence of the rotational motion of a ship on the slant range is analyzed. It can be matched with the sum of multiple sinusoidal functions, and the signal model of a ship with rotational motion is given. Then, multiple sinusoidal functions for the matching-based ship imaging method are proposed, and their procedures are presented as follows: (1) The Generalized Keystone Transform and Generalized Dechirp Process (GKTGDP) is modified to compensate for the range migration and phase caused by the motion of GEO SAR. Then, the signal is focused at the frequencies of sinusoidal functions, and the frequencies can be matched. (2) From the matched frequencies, the other parameters of sinusoidal functions can be matched by parameter searching. (3) Based on the matched results, the Back Projection Algorithm (BPA) is used to take an image of the ship with rotational motion. Finally, the effectiveness of the proposed method is verified by numerical experiments.

Microwave photonic radar with a fiber-distributed antenna array for three-dimensional imaging.

A microwave photonic (MWP) radar with a fiber-distributed antenna array for three-dimensional (3D) imaging is proposed and demonstrated for the first time. Photonic frequency doubling, wavelength-division multiplexing and radio-over-fiber techniques are employed for radar signal generation, replication, and distribution. Based on the delay-dependent beat frequency division, parallel de-chirp processing is completed in the center office (CO), leading to multi-channel 2D ISAR imaging and further 3D reconstruction. The influence of the fiber transmission delay is discussed and the phase noise caused thereby is compensated in 3D imaging algorithm, improving the coherence between channels. An experiment of a Ku-band MWP radar with a transmitter (Tx) and 16 equivalent receivers (Rxs) is conducted and 3D imaging of three trihedral corner reflectors is achieved with a range resolution of 7.3 cm, a cross-rage resolution of 5.6 cm and an elevation resolution of 0.85°. The results verify the capability of MWP radar in high-resolution 3D imaging.