Coherent Wideband Research Articles

Direction of Arrival Estimation of Coherent Wideband Sources Using Nested Array.

Due to their ability to achieve higher DOA estimation accuracy and larger degrees of freedom (DOF) using a fixed number of antennas, sparse arrays, etc., nested and coprime arrays have attracted a lot of attention in relation to research into direction of arrival (DOA) estimation. However, the usage of the sparse array is based on the assumption that the signals are independent of each other, which is hard to guarantee in practice due to the complex propagation environment. To address the challenge of sparse arrays struggling to handle coherent wideband signals, we propose the following method. Firstly, we exploit the coherent signal subspace method (CSSM) to focus the wideband signals on the reference frequency and assist in the decorrelation process, which can be implemented without any pre-estimations. Then, we virtualize the covariance matrix of sparse array due to the decorrelation operation. Next, an enhanced spatial smoothing algorithm is applied to make full use of the information available in the data covariance matrix, as well as to improve the decorrelation effect, after which stage the multiple signal classification (MUSIC) algorithm is used to obtain DOA estimations. In the simulation, with reference to the root mean square error (RMSE) that varies in tandem with the signal-to-noise ratio (SNR), the algorithm achieves satisfactory results compared to other state-of-the-art algorithms, including sparse arrays using the traditional incoherent signal subspace method (ISSM), the coherent signal subspace method (CSSM), spatial smoothing algorithms, etc. Furthermore, the proposed method is also validated via real data tests, and the error value is only 0.2 degrees in real data tests, which is lower than those of the other methods in real data tests.

Design of coherent wideband radiation process in a Nd3+-doped high entropy glass system

We discover that the spatially coherent radiation within a certain frequency range can be obtained without a common nonlinear optical process. Conventionally, the emission spectra were obtained by de-exciting excited centers from real excited energy levels to the ground state. Our findings are achieved by deploying a high-entropy glass system (HEGS) doped with neodymium ions. The HEGS exhibits a much broader infrared absorption than common glass systems, which can be attributed to be high-frequency optical branch phonons or allowable multi-phonon processes caused by phonon broadening in the system. A broadened phonon-assisted wideband radiation (BPAWR) is induced if the pump laser is absorbed by the system. The subsequent low-threshold self-absorption coherence modulation (SACM) can be controlled by changing excitation wavelengths, sample size, and doping concentrations. The SACM can be red-shifted through the emission of phonons of the excited species and be blue-shifted by absorbing phonons before they are de-excited. There is a time delay up to 1.66 ns between the pump pulse and the BPAWR when measured after traveling through a 35 mm long sample, which is much longer than the Raman process. The BPAWR-SACM can amplify the centered non-absorption band with a gain up to 26.02 dB. These results reveal that the shift of the novel radiation is determined by the frequency of the non-absorption band near the absorption region, and therefore the emission shifts can be modulated by changing the absorption spectrum. When used in fiber lasers, the BPAWR-SACM process may help to achieve tunability.

Interrupted-sampling repeater jamming suppression with one-dimensional semi-parametric signal decomposition

Protective measures against interrupted-sampling repeater jamming are vital for coherent wideband radar. This paper proposes a one-dimensional semi-parametric method for the interrupted sampling repeater jamming suppression by representing the target and jamming signal uniformly with semi-parametric models. Instead of utilizing the two-dimensional time-frequency analysis, we straightforwardly perform one-dimensional echo parameter estimation with sparse decomposition. Afterward, a modified k-means clustering method distinguishes jamming components from target components. The semi-parametric signal decomposition algorithm is of high efficiency and low complexity simultaneously by introducing fast operations. Extensions to suppress interrupted-sampling repeater jamming of synthetic aperture radar are also illustrated. Comparisons with traditional methods show the superiority of our proposal in actual working conditions.

A high temporal/spatial resolution neuro-architecture study of rodent brain by wideband echo planar imaging

Latest simultaneous multi-slice (SMS) methods greatly benefit MR efficiency for recent studies using parallel imaging technique. However, these methods are limited by the requirement of array coils. The proposed Coherent Wideband method, which employs an extended field of view to separate multiple excited slices, can be applied to any existing MRI instrument, even those without array coils. In this study, the Coherent Wideband echo-planar imaging method was implemented on 7 T animal MRI to exhibit comprehensive enhancements in neuro-architecture, including diffusion tensor imaging (DTI) and functional MR studies (fMRI). Under the same scan time, the time-saving effect can be manipulated to increase the number of averages for DTI SNR improvement, reducing fractional anisotropy difference by 56.9% (from 0.072 to 0.041) and the deviation angle by 64% (from 25.3° to 16.2°). In summary, Coherent Wideband Echo Planar Imaging (EPI) will provide faster, higher resolution, thinner slice, or higher SNR imaging for precision neuro-architecture studies.

Micro-Electro-Mechanical System Fourier Transform Infrared (MEMS FT-IR) Spectrometer Under Modulated-Pulsed Light Source Excitation.

Miniaturized Fourier transform infrared (FT-IR) spectrometers suffer from limited optical throughput due to their tiny aperture size. Therefore, coherent wideband sources with high brightness can provide an advantage over the wideband thermal radiation sources. However, the former ones are available based on pulsed operation. In this work, we present and study a miniaturized FT-IR spectrometer with pulsed light sources including chopped thermal source, semiconductor optical amplifier, Q-switched and femtosecond mode-locked laser sources. A system model for the FT-IR spectrometer system under a modulated input light source is presented. The model accounts for the relatively high scanning speed of the micro-electro-mechanical system (MEMS) interferometer. The signal-to-noise ratio of the spectrometer, due to the light source modulation, is calculated at different values of modulation repetition rate ranging from 20 Hz to 2 MHz, and duty cycle values ranging from 1% to 50%. An analytical expression for the worst-case repetition rate for the spectrometer system is derived. The model results are verified by experimental measurements showing good agreement with the theoretical expectations. Spectroscopic measurements for CO2 gas with pressure ranging from 300 mbar to 700 mbar are also performed using a high-repetition rate source, and the measured spectra agree with the simulation results demonstrating the utility of the spectrometer.

Joint 2-D angles and time delay estimation of coherent wideband signals

Joint direction of arrival (DOA) and time delay (TD) estimation of wideband signals such as WIFI and mobile communication signals has to face the challenges aroused by multipath effect in indoor environments. New methods that can deal with coherent wideband signals with high estimation accuracy are necessary. To achieve localization when signals are coherent, this paper proposes a new joint estimation method which fully utilizes the subcarriers of wideband signals. This method could greatly extend the array aperture and accomplish the joint estimation through UCA-ESPRIT and one-dimensional (1-D) search. Simulation experiments are conducted to show the validity and efficiency of this method.

ISAR Imaging of a Rotating Asteroid Irradiated by Pulsar’s Electromagnetic Emission

Abstract The aim of the present study is imaging of moving objects, asteroids illuminated by continuous coherent wideband pulsar’s signals. As pulsars are located on more than thousands of light years from Earth, objects crossing pulsars’ emission beams are considered as second sources of electromagnetic waves, carrying object’s shape and velocity information that can be extracted by application pulsar emission-based inverse aperture synthesis. Inverse Synthetic Aperture Radar (ISAR) scenario, geometry and kinematics are analytically described. Models of pulsar signals and ISAR signals secondary emitted by asteroids are developed. White Gaussian noise of high level is added to the deterministic reemitted signal in order to approach the real signal scenario. Two-Dimensional (2D) Fourier transform for image extraction is applied. Special iterative noise removing procedure is suggested for asteroid’s image enhancement. To verify mathematical model and imaging algorithms numerical experiments are carried out.

Photonic-assisted radio frequency waveform generation for high-range resolution microwave radar

In modern radar systems, the target to achieve high-range resolution poses a high requirement on bandwidth and stability of probe radio frequency (RF) waveforms. We experimentally demonstrate the generation of RF waveforms with a low bandwidth electrical instrument and optical bandwidth multiplication method. Based on the high-order modulation-sideband of electro-optic modulators, wideband optical swept waveforms are generated from narrow band RF swept waveforms with a bandwidth multiplication factor of eight. By heterodyne beating the swept waveforms with a single tone frequency driven on the same dual-drive Mach–Zehnder modulator, we generate coherent wideband RF waveforms with linearly, V-shaped, and quadric frequency modulation, respectively. These waveforms exhibit high time-bandwidth products up to 2560 and high phase-noise performance. The utility of these generated RF waveforms for ranging radar system has been proven a pulse compression process.

Photonics-based coherent wideband linear frequency modulation pulsed signal generation.

A photonics-based scheme is proposed to generate wideband linear frequency modulation pulses with broadly tunable carrier frequencies for coherent radars. The approach integrates the concept of the microwave-photonic multiplication and coherent beating to enable reconfiguration of the bandwidth and carrier frequency of the generated pulses. The phase fluctuation between two beating arms is suppressed by a stabilization technique based on an optical phase-locked loop to maintain the pulse-to-pulse phase coherence. Further, we also demonstrate a coherent radar system, including the generation, wireless transmission and detection of the radar echo signal in the Ka-band. The coherent integration of several echo signals is achieved. About 8dB of signal-to-noise ratio improvement is obtained with every ten-fold of integration times. The range resolution of the radar system is ∼3.75 cm, which is close to the theoretical prediction.

Sparse representation-based DOA estimation of coherent wideband LFM signals in FRFT domain

In this paper, the method of direction-of-arrival (DOA) estimation for wideband signals based on sparse representation of FRFT domain is proposed by using the excellent convergence of FRFT to LFM signals. This method focuses the wideband signal to the reference frequency using FRFT, establishes the DOA estimation model and the array manifold matrix in the FRFT domain, and reconstructs the spatial spectral function based on the sparse representation of the spatial angles, so as to realize the wideband LFM signal DOA estimations. The method not only is suitable for non-coherent signals but also can process coherent signals directly without any decoherent operations; in addition, the number of signals is not necessarily known a priori. Simulation results show that the proposed method can achieve better estimation performance and higher angular resolution, especially under low signal-to-noise ratio (SNR) condition.

DOA estimation for wideband signals based on weighted Squared TOPS

This paper introduces a new direction-of-arrival (DOA) estimation method for wideband signal sources. The new method estimates the DOA of wideband signal sources based on squared test of orthogonality of projected subspaces (Squared TOPS) which is an improved method of TOPS. TOPS and Squared TOPS use the signal and noise subspaces of multiple frequency components of wideband signal sources. Although coherent wideband method, such as coherent signal subspace method (CSSM), performs high DOA estimation accuracy, it requires the initial estimate of signal source directions. On the other hand, TOPS and Squared TOPS can provide good performance of DOA estimation without the initial value of signal sources; however, some false peaks appear in spatial spectrum based on these methods. The proposed method, called weighted Squared TOPS (WS-TOPS), uses the modified squared matrix and selective weighted averaging process to improve DOA estimation performance. The performance of WS-TOPS is compared with those of TOPS, Squared TOPS, incoherent MUSIC, and test of orthogonality of frequency spaces (TOFS) through computer simulations. The simulation results show that WS-TOPS can suppress all false peaks in spatial spectrum and improve DOA estimation accuracy and also keep the same resolution performance as Squared TOPS.

Dual binary phase‐shift keying tracking method for Galileo E5 AltBOC(15,10) signal and its thermal noise performance

Coherent wideband processing for the Galileo E5 AltBOC signal encounters a great challenge because of its modulation complexity, multi-peaked auto correlation function (ACF) and large bandwidth. In this study, a new tracking method called ‘dual binary phase-shift keying (BPSK) tracking’ (DBT), which is derived from its reception models and double estimator technique (DET) methodology, is specially designed for the Galileo E5 signal. While this method can achieve the full potential of the Galileo E5 signal in ranging performance, it features a robust wideband processing technique, backward compatibility with conventional BPSK signal tracking, and easy implementation in hardware. More specifically, the DBT method makes use of the coherence of the lower and upper bands of the Galileo E5 signal and decouples sub-carrier phase and carrier phase through coherently combining correlator outputs of the two bands, and then implements independent tracking for code, sub-carrier and carrier based on the DET methodology. Furthermore, thermal noise performances of this new method are given and verified by processing both simulated and real Galileo E5 signals.

Fast DOA estimation for coherent wideband sources based on symmetrical spectrum

A new fast algorithm was proposed to solve the heavy computational load of coherent wideband signal DOA(Direction Of Arrival) estimation. Firstly,through Toeplitz technique of matrix,incoherent wideband signals were obtained without any division for the array. Secondly,according to the character of Hermitian matrix of received data,utilized the unitary transformation matrix,the real matrix could be acquired and the computation of eigendecomposition could be decreased. Finally,through the orthogonality technique of projected subspaces,new space spectrum was acquired with noise subspace and conjugate noise subspace,and the DOA could be obtained just through the angle search from zero to ninety degree in semi-spectrum according to the symmetrical spectrum,while the computational complexity for the search of spectrum peak was reduced by half. The theoretical analysis and simulation results show that the new algorithm has higher precision and smaller computational load,and it is effective for wideband coherent signals.

2-D DOA Estimation of Coherent Wide-Band Sources for Arbitrary Array Plane Geometry

Direction of Arrival (DOA) estimation is of big importance in Sonar, Radar and satellite systems. In this work, we estimated the 2-D DOA (two-dimensional DOA) of coherent wide-band sources by using a broadband focusing method, which was based on the separation of angle and frequency of the array manifold matrix. At first, we transform the element position of the plane array into the spherical coordinates. Then we adopted a generalized Fourier series expansion based on the spherical harmonics and the first kind of spherical Bessel functions to get the arbitrary plane array steering vector. In this vector, the angle and frequency of array manifold matrix was separated, so we could get the matrix of focusing transformation. Finally, we used the traditional narrow-band algorithm to estimate the 2-D DOA. The design is suitable for arbitrary array geometry models of coherent broadband signals, and avoids the angle pre-estimation in traditional methods. Simulation results demonstrate its validity and better performance.

FRFT Based Method to Estimate DOA for Wideband Signal

This paper presents a new direction-of-arrival (DOA) estimation for wideband sources, using fractional Fourier transform with fitting angle (F3A). Unlike other coherent wideband methods, the new method does not require any preprocessing for initial values and decomposing into narrowband components. This new technique estimates DOA by rotating the time frequency plate with the fitting angle to fit the time frequency distribution approximately. The algorithm can be applied to arbitrary shaped one dimensional or two dimensional arrays. The signal frequency can be higher than the frequencies in many wideband algorithms. The performance of this wideband technique is compared with that of the new method through simulations. The simulations show that this new technique performs better than others, while this algorithm does not apparently vary with signal-to-noise ratio (SNR).

Bayesian Focusing for Coherent Wideband Beamforming

In this paper, we present and study a Bayesian focusing transformation (BFT) for coherent wideband array processing, which takes into account the uncertainty of the direction of arrivals (DOAs). The Bayesian focusing method minimizes the mean-square error of the transformation over the probability density functions (pdfs) of the DOAs, thus achieving improved focusing accuracy over the entire bandwidth. In order to solve the Bayesian focusing problem, we derive and utilize a weighted extension of the wavefield interpolated narrowband generated subspace (WINGS) focusing transformation. We provide a closed-form expression for the optimal BFT and extend it to the case of directional sensors. We then consider a numerical computation scheme of the BFT in the angular domain. We show that if an angular sampling condition is satisfied then the angle domain approximation yields the optimal BFT. We also treat the important issue of robust focused minimum variance distortionless response (MVDR) beamformer. We analyze the sensitivity of the focused MVDR to the focusing errors and show that the array gain (AG) is inversely proportional to the square of the signal-to-noise ratio (SNR) for large values of the SNR, and highly sensitive to the focusing errors. In order to reduce this sensitivity we generalize the popular narrowband diagonal loaded MVDR to the focused wideband case, referred to as the Q-loaded focused MVDR wideband beamformer. We derive a closed-form analytic expression for the AG of the Q-loaded focused MVDR beamformer which depends on the focusing transformations. A numerical performance evaluation and simulations demonstrate the advantage of the BFT over that of other focusing transformations, for multiple source scenarios.

Improved TOPS about Direction of Arrival Estimation for Wideband Signals

The study about the direction-of-arrival (DOA) estimation for wideband signals is a basic topic of Signal Processing. The method called test of orthogonality of projected subspaces (TOPS) does not require initial values to form focusing matrix compared with the coherent signal subspace method (CSSM), but it has poor performance for coherent wideband signals at low signal-to-noise ratio (SNR). Based on the lack of the conventional TOPS, this paper reconstructs a set of new matrices with the narrowband correlated matrices coming from the DFT decomposition of the array output and modifies the method frame of the conventional TOPS. So we get a set of reconstructed matrices with full rank corresponding to each frequency bin, and which have no array aperture loss. The improved TOPS can estimate the DOA for coherent wideband sources at low SNR, and enhances the performance of the conventional TOPS. Simulation results show that the improved TOPS is effective, and the total performance of which is similar to the rotational signal-subspace (RSS) method.

Rapid Rotary Scanner and Portable Coherent Wideband Q-Band Transceiver for High-Resolution Millimeter-Wave Imaging Applications

Millimeter-wave imaging techniques, based on synthetic aperture focusing (SAF), have been successfully used for nondestructive testing (NDT) of various composite and aerospace structures. Most current imaging mechanisms utilize raster scanning platforms, whereby the imaging system is scanned in a rectangular grid over the structure-under-test (SUT). Most raster scanning platforms, although relatively simple in design and construction, are inherently slow. Furthermore, SAF techniques necessitates the use of vector-measuring instruments such as a vector network analyzer (VNA), which are typically: 1) bulky; 2) cannot be mounted on scanning platforms; 3) are not suitable for in-field use; and 4) expensive. These factors limit the effectiveness of these millimeter-wave imaging techniques in applications where frequent and rapid inspection of large structures is required. Hence, there is a great demand for rapid mechanical scanning systems combined with portable wideband transceivers in order to increase the utility of these imaging techniques, and provide a real solution to many practical NDT applications. To this end, a unique rotary scanner system, capable of scanning a relatively large area in a relatively short span of time, was designed and constructed. In addition, a custom-designed portable transceiver system operating in the frequency range of 35-45 GHz (Q-band) was developed and incorporated into the rotary scanner system for producing coherent (amplitude and phase) and accurate data suitable for synthetic aperture imaging and the 10-GHz bandwidth allows the generation of relatively high-resolution millimeter-wave holographical images. This paper presents the design of the rotary scanning system, the associated Q-band transceiver and the integration of the two systems via a custom-designed software. To illustrate the efficacy of the complete imaging system, SAF of several complex structures produced using the proposed system, are presented and discussed.

TOPS: new DOA estimator for wideband signals

This paper introduces a new direction-of-arrival (DOA) estimation algorithm for wideband sources called test of orthogonality of projected subspaces (TOPS). This new technique estimates DOAs by measuring the orthogonal relation between the signal and the noise subspaces of multiple frequency components of the sources. TOPS can be used with arbitrary shaped one-dimensional (1-D) or two-dimensional (2-D) arrays. Unlike other coherent wideband methods, such as the coherent signal subspace method (CSSM) and WAVES, the new method does not require any preprocessing for initial values. The performance of those wideband techniques and incoherent MUSIC is compared with that of the new method through computer simulations. The simulations show that this new technique performs better than others in mid signal-to-noise ratio (SNR) ranges, while coherent methods work best in low SNR and incoherent methods work best in high SNR. Thus, TOPS fills a gap between coherent and incoherent methods.

Asymptotically perfect wideband focusing of multiring circular arrays

The concept of coherent focusing of sensor arrays, introduced by Wang and Kaveh, led to the development of high-performance and computationally efficient algorithms for wideband direction finding and beamforming. Nonetheless, the quality of focusing depends on the understanding and the proper exploitation of specific array manifold properties. Circular arrays exhibit uniform (isotropic) performance over the entire azimuthal range and allow the use of fast algorithms, originally developed for uniform linear arrays, by decomposing the manifold into circular harmonics. Coherent wideband focusing of circular arrays suffers from ambiguities, noise warping, and numerical ill conditioning. In this work, it is shown that a fast orthonormal beamspace transformation combining the responses of two concentric rings can perfectly focus wideband sources at all azimuth angles in the circular harmonic domain in the limit of infinite number of sensors. The information loss after focusing is minimized through an analytical approach. The proposed focusing scheme is computationally very efficient and can be directly extended to multiring circular arrays that are arranged into a set of nested two-ring subarrays to cover very large bandwidths.