Consider the problem of sampling signals that are nonbandlimited but have finite number of degrees of freedom per unit of time and call this number the rate of innovation. Streams of Diracs and piecewise polynomials are the examples of such signals, and thus are known as signals with finite rate of innovation (FRI) [3]. We know that the classical ('bandlimited-sinc') sampling theory does not enable perfect reconstruction of such signals from their samples since they are not bandlimited. However, the recent results on FRI sampling [3], [4] suggest that it is possible to sample and perfectly reconstruct such nonbandlimited signals using a rich class of kernels.In this paper, we extend the results of [4] in higher dimensions using compactly supported kernels that reproduce polynomials (satisfy Strang-Fix conditions). In fact, the polynomial reproduction property of the kernel makes it possible to obtain the continuous-moments of the signal from its samples. Using these moments and the annihilating filter method (Prony's method), the innovative part of the signal, and therefore, the signal itself is perfectly reconstructed. In particular, we present local (directional derivatives based) and global (complex-moments, Radon transform based) sampling schemes for classes of FRI signals such as sets of Diracs, bilevel and planar polygons, quadrature domains (e.g. circles, ellipses, cardioids), 2-D polynomials with polygonal boundaries, and n-dimensional Diracs and convex polytopes. This research has been promisingly explored in super-resolution algorithms [5] and distributed compression [6], and might find its applications in photogrammetry, computer graphics, and machine vision.
This paper presents a study of a planar antenna-array inspired by the metamaterial concept where the resonant elements have sub-wavelength dimensions for application in microwave medical imaging systems for detecting tumors in biological tissues. The proposed antenna consists of square-shaped concentric-rings which are connected to a central patch through a common feedline. The array structure comprises several antennas that are arranged to surround the sample breast model. One antenna at a time in the array is used in transmission-mode while others are in receive-mode. The antenna array operates over 2-12 GHz amply covering the frequency range of existing microwave imaging systems. Measured results show that compared to a standard patch antenna array the proposed array with identical dimensions exhibits an average radiation gain and efficiency improvement of 4.8 dBi and 18%, respectively. The average reflection-coefficient of the array over its operating range is better than S 11 ≤-20 dB making it highly receptive to weak signals and minimizing the distortion encountered with the transmission of short duration pulse-trains. Moreover, the proposed antenna-array exhibits high-isolation on average of 30dB between radiators. This means that antennas in the array (i) can be closely spaced to accommodate more radiators to achieve higher-resolution imaging scans, and (ii) the imagining scans can be done over a wider frequency range to ascertain better contrast in electrical parameters between malignant tumor-tissue and the surrounding normal breast-tissue to facilitate the detection of breast-tumor. It is found that short wavelength gives better resolution. In this experimental study a standard biomedical breast model that mimics a realhuman breast in terms of dielectric and optical properties was used to demonstrate the viability of the proposed antenna over a standard patch antenna in the detection and the localization of tumor. These results are encouraging for clinical trials and further refinement of the antenna-array.
The paper presents a technique to enhance the isolation between adjacent radiating elements that is common in densely packed antenna arrays. Such antennas provide frequency beam‐scanning capability needed in multiple‐input multiple‐output (MIMO) systems and synthetic aperture radars. The method proposed here uses a metamaterial decoupling slab (MTM‐DS), which is located between radiating elements, to suppress mutual coupling between the elements that would otherwise degrade the antenna efficiency and performance in both the transmit and receive mode. The proposed MTM‐DS consists of mirror imaged E‐shaped slits engraved on a microstrip patch with inductive stub. Measured results confirm over 9–11 GHz with no MTM‐DS the average isolation (S12) is −27 dB; however, with MTM‐DS the average isolation improves to −38 dB. With this technique the separation between the radiating element can be reduced to 0.66λ0, where λ0 is free space wavelength at 10 GHz. In addition, with this technique there is 15% improvement in operating bandwidth. At frequencies of high impedance match of 9.95 and 10.63 GHz the gain is 4.52 and 5.40 dBi, respectively. Furthermore, the technique eliminates poor front‐to‐back ratio encountered in other decoupling methods. MTM‐DS is also relatively simple to implement. Assuming adequate space is available between adjacent radiators the MTM‐DS can be fixed retrospectively on existing antenna arrays, which makes the proposed method versatile.
This paper presents empirical results of an innovative beam scanning leaky-wave antenna (LWA) which enables scanning over a wide angle from-35 o to +34.5 o between 57 GHz and 62 GHz, with broadside radiation centered at 60 GHz. The proposed LWA design is based on composite right/left-handed transmission-line (CRLH-TL) concept. The single layer antenna structure includes a matrix of 3×9 square slots that is printed on top of the dielectric substrate; and printed on the bottom ground-plane are Π and T-shaped slots that enhance the impedance bandwidth and radiation properties of the antenna. The proposed antenna structure exhibits metamaterial property. The slot matrix provides beam scanning as a function of frequency. Physical and electrical size of the antenna is 18.7×6×1.6 mm 3 and 3.43 ×1.1 ×0.29 , respectively; where is free space wavelength at 55 GHz. The antenna has a measured impedance bandwidth of 10 GHz (55 GHz to 65 GHz) or fractional bandwidth of 16.7%. Its optimum gain and efficiency are 7.8 dBi and 84.2% at 62 GHz.
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