Asymmetric Side Lobes Research Articles

Optimization of Mismatched Filters With Asymmetric Side-Lobe Shape for Short-Range MIMO Radars

This paper presents the design of signal compression and separation filters with asymmetric responses for multiple-input multiple-output (MIMO) radars that use pulses longer than the maximum return time of the reflected signal. Such radars suffer from direct penetration of the transmitted signal to the receiver, appearing as a very strong stationary reflection in the zero range. After pulse compression, the far-side lobes of this crosstalk may compete with the reflections of distant weak targets and limit the maximum radar range. Hence, in addition to the maximum suppression of the direct transmitted signal penetration to the receiver, the far-side lobe reduction of this crosstalk with positive time lags is desirable. Asymmetric response filters allow optimization to be focused on the region where the maximum suppression of crosstalk between the transmitted signal and the receiver is desired. Extreme suppression was achieved by applying lower demand in other areas and other parameters. In this paper, the problem is described in more detail along with the mathematical apparatus by which asymmetric filter outputs can be achieved.

Window Function Design for Asymmetric Beampattern Synthesis and Applications

This letter proposes a novel window function design to synthesize asymmetric beampatterns while minimizing the gain loss for clutter/interference suppression as required in practical applications, such as airborne phased array radar (APAR). Compared with beampattern synthesis in terms of designing the weight vector directly, the proposed method can achieve asymmetric sidelobes for multiple steering directions resort to designing one window while circumventing repetitive computations. To tackle the resultant non-convex optimization problem, an iterative alternating optimization (IAO) algorithm is introduced where each iteration involves multiple subproblems that can be solved via either convex approximation (CA) approach or closed-form solutions. Finally, numerical simulations are provided to verify the effectiveness of the proposed method comparing with the classical adjustable window functions (CAWFs).

Multishape radio: new approach to utilizing the physical properties of electromagnetic waves

Usage of higher frequency bands enable a new possibility for the exploitation of the physical properties of electromagnetic waves such as bended propagation, nondiffractive propagation, and orbital angular momentum. We characterize new concept that takes full advantage of such physical properties a “multishape radio.” As a representative case, we explore the Airy wave that has the unique feature of bended propagation with asymmetric sidelobe distribution. We experiment using sub-THz frequency bands to demonstrate that Airy waves can be used for parallel transmission of multiple streams without causing harmful interference.

Processing of Very High Resolution GF-3 SAR Spotlight Data With Non-Start–Stop Model and Correction of Curved Orbit

A novel imaging algorithm is presented in this article for focusing the very high resolution spaceborne synthetic aperture radar (SAR) data of spotlight mode. The long synthetic aperture time in spotlight mode brings new problems, such as Doppler spectrum aliasing and curved orbit. The imaging results will be filled with ambiguity and would suffer from resolution reduction if Doppler spectrum aliasing is not handled. The error introduced by curved orbit will degrade resolution of the target and introduces asymmetric sidelobes in azimuth direction. The start–stop approximation is no longer applicable, since it introduces two effect, e.g., “fast-time” effect and “slow-time” effect, which will cause defocusing and range-dependent azimuth shift, respectively if not handled. The proposed algorithm combines the two-steps processing approach (TSPA) and the backprojection algorithm (BPA). First, the initial step of TSPA is used to get a high azimuth sampling rate which is higher than pulse repetition frequency to avoid the Doppler spectrum aliasing. Then, the “fast-time” effect of start–stop approximation is corrected in the 2-D frequency domain. Finally, the BPA is used to correct the error introduced by the curved orbit and the “slow-time” effect of start–stop approximation. The experimental results of spaceborne SAR data acquired by Gaofen-3 (GF-3) SAR system demonstrate the feasibility of the proposed algorithm.

Refocusing of Moving Targets in SAR Images via Parametric Sparse Representation

In this paper, a parametric sparse representation (PSR) method is proposed for refocusing of moving targets in synthetic aperture radar (SAR) images. In regular SAR images, moving targets are defocused due to unknown motion parameters. Refocusing of moving targets requires accurate phase compensation of echo data. In the proposed method, the region of interest (ROI) data containing the moving targets are extracted from the complex SAR image and represented in a sparse fashion through a parametric transform, which is related to the phase compensation parameter. By updating the reflectivities of moving target scatterers and the parametric transform in an iterative fashion, the phase compensation parameter can be accurately estimated and the SAR images of moving targets can be refocused well. The proposed method directly operates on small-size defocused ROI data, which helps to reduce the computational burden and suppress the clutter. Compared to other existing ROI-based methods, the proposed method can suppress asymmetric side-lobes and improve the image quality. Both simulated data and real SAR data collected by GF-3 satellite are used to validate the effectiveness of the proposed method.

Synthesizing Asymmetric Side Lobe Pattern with Steered Nulling in Nonuniformly Excited Linear Arrays by Controlling Edge Elements

In radar antennas, asymmetric side lobes are useful, where undesired signals such as noise and ground clutter should be minimized. Also, for practical implementation, the feeding network of such antennas should be efficiently designed. In this paper, a simple analytical method for synthesizing asymmetric side lobe pattern with a wide-angle steered null in the nonuniformly excited linear arrays is presented. In this method, the difference in the side lobe levels on both sides of the main beam is achieved by varying just the phase excitations of the two-edge elements. The major novelty of this paper lies in the fact that the required asymmetric side lobe pattern can be achieved by changing a single phase shifter resulting in a simple feeding network.

Optimal Monopulse Beamforming for Side-Looking Airborne Radars

The synthesis of the optimal excitations of the sum and the difference patterns in planar active electronically scanned arrays ( $\text{AESA}$ s) for side-looking airborne monopulse radar systems is addressed. Under the assumption that each array element is equipped with a single transmit receive module ( $\text{TRM}$ ), which is shared among all patterns, the complex (amplitude and phase) $\text{TRM}$ coefficients are optimized with a customized convex programming procedure to yield a sum beam with maximum directivity and difference beams with maximum slope along the boresight direction as well as asymmetric sidelobes envelope for noise suppression from half-space. Representative results from a synthesized $\text{AESA}$ devoted to a real airborne application are illustrated and discussed to give some insights to the interested readers on the features and the potentialities of the proposed synthesis approach.

Analysis of the Spectrum Distortions of Weak Fiber Bragg Gratings Fabricated In-Line on a Draw Tower by the Phase Mask Technique

Identical weak optical fiber Bragg grating (FBG) arrays can be fabricated in-line on a draw tower using the phase mask technique. On the FBG arrays, some grating spectrum distortions, for example, asymmetric side lobes or deformed side lobes, low extinction ratio of the main reflection peak and comb filter-type peaks, can be observed. We collected different distorted reflection spectra of the in-line-fabricated weak FBGs and compared them with the simulated spectra of weak FBGs. By comparing the experimental and theoretical weak FBG spectra, it is found that the different kinds of spectrum distortions are caused by phase shifting, asymmetric apodization and nonlinear chirp. This analysis is helpful to improve the FBG array fabricating process by phase mask in-line on a draw tower. And we got good FBG arrays which have good central wavelength consistency, good reflectivity consistency and minor spectral distortion.

Pencil Beams and Flat-Topped Beams with Asymmetric Sidelobes from Circular Arrays [Antenna Designer's Notebook

Given a circular antenna array, and a requirement for the sidelobes surrounding a pencil beam or flat-topped beam to have a specified ϕ dependence, it can be advantageous to optimize the array excitations for the best fit between the far field and a pattern synthesized for a continuous circular aperture of the same size.

Design of digitally controlled multiple-pattern time-modulated antenna arrays with phase-only difference

An optimization approach based on differential evolution (DE) is proposed to design multiple-pattern time-modulated linear antenna arrays (TMLAAs) with phase-only control by using 5-bit digital phase shifters. The synthesized multiple patterns include a pencil beam (PB), a flat-topped beam (FTB), and a cosec squared pattern (CSP). The function of the DE is to find, simultaneously, a common, i.e., fixed, set of continuous values of switch-on time durations and discrete static excitation amplitudes for 5-bit digital attenuators, and different sets of discrete excitation phase distributions for 5-bit digital phase shifters; to generate different power patterns in the far field of the TMLAA. By perturbing the static amplitude distribution of a 20-element TMLAA in the discrete search range of “(0.2–1),” the patterns are obtained by reducing side lobe levels (SLLs) to almost −20 dB and sideband levels (SBLs) to less than −25 dB. Towards the end, the same on-time and amplitude distribution is used to produce a symmetric side lobe (S-SL) and asymmetric side lobe (A-SL) CSP, and their usefulness under a noisy signal environment is discussed.

Generation of Asymmetrical Difference Patterns from Array Antennas

It is common to produce sum patterns from array antennas. These patterns are basically symmetric with one main lode in bore sight and symmetric sidelobe structure around the main beam. The antennas associated with the marine radars are required to produce patterns with asymmetric side lobes. Asymmetrical patterns are very useful for marine radars when ships sail in turbulent water where roll and pitch exists.In particular the sidelobes in one side are lower than those of the other side. It is also essential to produce difference patterns with asymmetric structure from the boresight direction. These patterns are required to have null in the bore sight and high difference slope in the same direction. When these patterns are required from marine radars, the side lobes associated with the difference pattern on the either side of the boresight is at the different heights. In the present work, such useful patterns are generated from the newly designed arrays. The amplitude and phase distributions are designed for small and large arrays. The radiation patterns are numerically computed and they are presented in udomain. Keywords—asymmetric patterns, arrays; array antennas; marine radars; amplitude and phase distributions.

선형 위상 배열 안테나의 비대칭 Sidelobe 레벨 제어 및 다중 Nulling에 관한 연구

본 논문에서는 선형 위상 배열 안테나 패턴 합성 문제에서, 주 빔 패턴 양쪽에 임의로 설정한 비대칭 sidelobe 레벨(SLL)들을 만족시키는 안테나 소자 가중치들을 계산하는 방법을 새로이 제안한다. 소자 가중치들을 배열인자로부터 직접 최적화하는 기존의 방법들과는 달리, 이 방법은 배열 인자를 표현하는 Schelkunoff 다항식에 내재된 복소근의 최적 섭동에 기본을 둔다. 제안한 방법으로부터 여러 개의 jammer들의 방향으로 다중 nulling도 가능하며, 이는 각 jamming 방향에 대응하는 복소근들만의 독립적인 섭동에 의해 이루어진다. 따라서 해 공간차원의 적절한 감소에 의해 수치적 절차가 간소화될 수 있다. 또한 배열 소자들의 복소 가중치들은 최적 섭동된 복소근들을 Schelkunoff 다항식에 대입함으로써 쉽게 계산된다. 몇 가지 예를 들어 검토하고, 도출된 가중치들을 배열 인자 방정식에 대입함으로써 타당성을 수치적으로 검증한다. This paper newly proposes a methodology towards computing antenna element weights which are satisfying asymmetric sidelobe levels(SLLs) specified arbitrarily on both sides of the main beam pattern, in the linear phased array antenna pattern synthesis problem. Opposite to the conventional methods in which the element weights are directly optimized from the array factor, this method is based on the optimum perturbations of complex roots inherent to the Schelkunoff's polynomial form which is described for the array factor. From the proposed methodology, the capability of nulling the directions of multiple jammers is also possible by independently perturbing only the complex roots corresponding to each jamming direction, hence allowing an enhancement of the simplicity of the numerical procedure by means of a proper reduction of the dimension of the solution space. The complex weights over the array are then easily computed by substituting the optimally perturbed complex roots to the Schelkunoff's polynomial. Some examples are examined and numerically verified by substituting the extracted weights into the array factor equation.

Frequency-swept directivity lobes—An emerging functional principle of biosonar beamforming

In all bat species, the spatial sensitivity of the biosonar system is mainly determined by the shape of the outer ears. In addition, a noseleaf surrounds the nostrils in many species and shapes the spatial distribution of the emitted sound energy. Predicting the beam-forming effects of these structures numerically from their surface geometry has enabled comparative studies of high-resolution beam pattern estimates across a large set of different bat species. The beam patterns obtained so far differ from common manmade transducers, such as loudspeakers and microphones. Strong asymmetric side lobes, which change direction as a function of frequency, are a distinguishing feature consistently seen in emission and reception beam patterns of the bats. Where investigated, the occurrence of these side lobes appears to be tied to the presence of flap-like structures (including the tragus of the pinna), which may be hypothesized to have evolved for this purpose. The presence of these side lobes suggests that bat biosonar has not been evolved to maximize the ensonification energy and sensitivity for a single small target unconditionally. Instead, the function of these strong side lobes could be explained as maximizing the information obtained in analogy to squinted beams used in manmade systems.

Confocal microscopy using variable-focal-length microlenses and an optical fiber bundle

The use of variable-focal-length (VFL) microlenses can provide a way to axially scan the foci across a sample by electronic control. We demonstrate an approach to coupling VFL microlenses individually to a fiber bundle as a way to create a high-throughput aperture array with a controllable aperture pattern. It would potentially be applied in real-time confocal imaging in vivo for biological specimens. The VFL microlenses that we used consist of a liquid-crystal film sandwiched between a pair of conductive substrates for which one has a hole-patterned electrode. One obtains the variation of the focal length by changing the applied voltage. The fiber bundle has been characterized by coupling with both coherent and incoherent light sources. We further demonstrate the use of a VFL microlens array in combination with the fiber bundle to build up a confocal system. The axial response of the confocal system has been measured without mechanical movement of the sample or the objective, and the FWHM is estimated to be approximately 16 microm, with asymmetric sidelobes.

A numerical study of the role of the tragus in the big brown bat

A comprehensive characterization of the spatial sensitivity of an outer ear from a big brown bat (Eptesicus fuscus) has been obtained using numerical methods and visualization techniques. Pinna shape information was acquired through x-ray microtomography. It was used to set up a finite-element model of diffraction from which directivities were predicted by virtue of forward wave-field projections based on a Kirchhoff integral formulation. Digital shape manipulation was used to study the role of the tragus in detailed numerical experiments. The relative position between tragus and pinna aperture was found to control the strength of an extensive asymmetric sidelobe which points in a frequency-dependent direction. An upright tragus position resulted in the strongest sidelobe sensitivity. Using a bootstrap validation paradigm, the results were found to be robust against small perturbations of the finite-element mesh boundaries. Furthermore, it was established that a major aspect of the tragus effect (position dependence) can be studied in a simple shape model, an obliquely truncated horn augmented by a flap representing the tragus. In the simulated wave field around the outer-ear structure, strong correlates of the tragus rotation were identified, which provide a direct link to the underlying physical mechanism.

Phase-only synthesis of continuous linear aperturedistribution patterns with asymmetric side lobes

A method for synthesising linear aperture distribution patterns with different side lobe levels (SLLs) on both sides of the main beam is proposed. This phase-only control method was applied to a Taylor distribution as well as to a uniform amplitude distribution and achieved differences in the SLL on both sides of the main beam as great as 23 dB, improving on a previous procedure that had achieved a gap of little more than 10 dB.

Anomalous sidelobes and birefringence apodization in acousto-optic tunable filters

Integrated-optic acousto-optic filters have large asymmetric sidelobes that have not been explained in previous publications. We show that this is caused by waveguide birefringence nonuniformity. For the devices described here, the dominant cause of the nonuniformity is a thermal gradient associated with the generation and absorption of the surface acoustic wave. When the thermal birefringence gradient is included in the coupled-mode model, asymmetric sidelobes in good agreement with observations are predicted. In addition, we show that waveguide inhomogeneities also lead to birefringence nonuniformity and can be used to tailor the birefringence to supress sidelobes in a two-section acousto-optic filter.

The design of narrow beamwidth asymmetric sidelobe array antenna patterns using analytic signal concept

An analytic method for designing an asymmetric sidelobe array antenna pattern is presented. The fundamental principles applied are the analytic signal theory and the phase shifting property of the excitation of an array antenna. An example is given to demonstrate the feasibility and effectiveness of this design methodology.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A parabolic reflector with asymmetric low sidelobes

A technique for generating low sidelobes on one side of the main lobe for a parabolic reflector antenna is presented. The reflector system that generates such an asymmetric sidelobe pattern consists of a central parabola with two sections of offset parabolas situated at the top and bottom of the central parabola. By adjusting the positions and dimensions of the side reflectors, considerable suppression of the first few sidelobes can be obtained. Further, by similar adjustments it is possible to achieve suppression of sidelobes on both sides of the main beam, though this suppression is relatively less. The technique, though explained in detail with reference to a cylindrical geometry, is extended for a paraboloid of revolution. The analysis program utilized for the optimization of the side reflectors is based on physical optics current integration.

Design of circular apertures for narrow beamwidth and asymmetric low sidelobes

Taylor's earlier work on circular apertures is enlarged to include a class of radiation patterns consisting of a narrow central beam and asymmetric sidelobes. A synthesis technique applicable to such problems, and using Taylor patterns as building blocks, is formulated. One special case is treated in detail: a sidelobe topography which is 25 dB down in the \phi = 0 half-plane, smoothly rising to 15 dB down in the \phi= \pi/2 half-plane, remaining at that level until the \phi = 3\pi/2 half-plane is reached, and then smoothly subsiding to 25 dB again as \phi increases from 3\pi/2 to 2\pi . The complex aperture distribution is derived generally, and exhibited specifically for this case.