Abstract: We describe a secure communication scheme that uses the random fluctuation of the natural environment of communication channels. Only the transmitter and the receiver share the communication channel characteristics. From reciprocity between a transmitter and a receiver, it is possible for them to share one-time information of their fluctuating channel. This can provide a secret key agreement scheme without key management and key distribution processes. In this paper, we propose a new secret key generation and agreement scheme that uses the fluctuation of channel characteristics with an electronically steerable parasitic array radiator (ESPAR) antenna. This antenna, which has been proposed and prototyped, is a smart antenna designed for consumers. Using the beam-forming technique of the ESPAR antenna, we can increase the fluctuation of the channel characteristics. From experimental results, we conclude that the proposed scheme has the ability to generate secret keys from the received signal strength indicator (RSSI) profile with sufficient independence.

Abstract: A six-monopole circular antenna array for use in a MIMO system is considered. The authors show how to calculate the embedded element patterns, both by classical analytical modeling and by the method of moments. Thereafter, these are used to calculate the radiation efficiency of each embedded element, correlation and diversity gain, as well as the maximum average capacity of the MIMO system when the array is located in a rich scattering environment. The theoretical value for the capacity is obtained by numerically distributing many plane wave sources statistically uniformly over 4/spl pi/, letting them illuminate the calculated embedded element pattern and using Shannon's capacity formula on the received wave amplitudes. The calculated results are compared with measurement in a reverberation chamber, representing a similar scattering environment. The agreement is good.

Abstract: The goal of the Advanced Multifunction Radio Frequency Concept (AMRFC) Program is to demonstrate the integration of many sorts of shipboard RF functions including radar, communications, and electronic warfare (EW) utilizing a common set of broad-band array antennas, signal and data processing, signal generation, and display hardware. The AMRFC Program was launched in response to the growing number of topside antennas on U.S. Navy ships, which have almost doubled from the ships launched in the 1980s to those launched in the 1990s. The AMRFC Program seeks to develop and demonstrate a wide-band generic active array antenna architecture that has the ability to transmit and receive multiple simultaneous independent beams for radar, EW, and communication functions. This paper describes a proof-of-principle test-bed that is being developed to demonstrate the AMRFC.

Abstract: A novel approach to realize uniform amplitude time modulated linear arrays with both suppressed sidelobes and sidebands is proposed. The approach utilizes the direct optimization of the "switch-on" time sequence of each array element via the simple genetic algorithm (SGA). As compared to previous time modulated linear arrays, the new array has the advantages of having low sidelobe level (SLL), low sideband level (SBL) and uniform excitations simultaneously. Experimental results on an experimental prototype of a 16-element printed dipole linear array with the SGA optimized time sequences verified the approach.

Abstract: An optical processor for controlling a phased antenna array uses a frequency-shifted feedback cavity (FSFC), which includes a traveling-wave cavity. The FSFC incrementally delays and incrementally frequency shifts optical signals circulating in the traveling-wave cavity. Optical signals coupled out of the FSFC are separated by frequency, hence by delay, and processed to control either or both transmit and receive beam-forming operations. The FSFC provides a receiver with multiple receive signals which have incremental values of frequency. Each frequency corresponds to an incremental time sampling of optical signals input into the FSFC. Transmit signals coupled out of the FSFC have frequency and phase relationships that result in short time-domain pulses when combined. Controlling modulation and frequency of the transmit signals achieves carrier interference multiple access, a new type of spread-spectrum communications.

Abstract: Conventionally, the approaches to coherent direction-of-arrival (DOA) estimation are to eliminate the rank loss of the spatial covariance matrix. In this letter, a new technique is presented from another point of view, which reconstructs a special antenna array model based on the Toeplitz matrix whose rank is only related to the DOA of signals and cannot be affected by the coherency between them. Therefore, the signal and noise subspaces can be estimated properly. In conjoint with the improved estimation of signal parameter via rotational invariance techniques (ESPRIT) algorithm, it is capable of resolving the DOAs of coherent signals as well as uncorrelated signals without peak searching. The numerical examples demonstrate its validity.

Abstract: The far-field radiation characteristics of a two-dimensional (2-D) periodic leaky-wave antenna (LWA) constructed from a periodic array of metal patches on a grounded dielectric substrate is investigated. A simple dipole source is used as the excitation. Reciprocity together with a periodic spectral-domain method of moments is used to calculate the far-field pattern. Design rules for the scan angle, the substrate dielectric constant, and the periodicity are provided. Finally, a comparison of the 2-D periodic LWA and a dielectric-layer LWA is given to show the similar performance of the two antennas.

Abstract: Partial discharges (PD) generate wideband radio frequency interference and, consequently, can be detected using radio receiving equipment. Due to the advances in ultra-high-speed sampling equipment, it is possible to accurately measure the propagation of the PD wavefront as it passes through a 4 element antenna array. From these measurements, the three-dimensional position of the PD source can be calculated using an iterative algorithm. The locating equipment is suitable for use within the vicinity of energized high-voltage plant and can locate sources up to 15 m from the array. Results are presented showing the location ability of the equipment under laboratory and field conditions. A significant advantage is the ability to detect PD sources in energized plant without the need for outages or electrical connections.

Abstract: An effective method for optimizing the performance of a fixed current distribution, uniformly spaced antenna array has been to adjust its element positions to provide performance improvement. In comparison with the default uniform structure, this approach yields performance improvements such as smaller sidelobe levels or beamwidth values. Additionally, it provides practical advantages such as reductions in size, weight and number of antenna elements. The objective of this paper is to describe a unified mathematical approach to nonlinear optimization of multidimensional array geometries. The approach utilizes a class of limiting properties of sinusoidal, Bessel or Legendre functions that are dictated by the array geometry addressed. The efficacy of the method is demonstrated by its generalized application to synthesis of rectangular, cylindrical and spherical arrays. The unified mathematical approach presented below is a synthesis technique founded on the mathematical transformation of the desired field pattern, followed by the application of limiting forms of the transformation, and resulting in the development of a closed form expression for the element positions. The method offers the following advantages over previous techniques such as direct nonlinear optimization or genetic algorithms. First, it is not an iterative, searching algorithm, and provides element spacing values directly in a single run of the algorithm, thereby saving valuable CPU time and memory storage. Second, It permits the array designer to place practical constraints on the array geometry, (e.g., the minimum/maximum spacing between adjacent elements)

Abstract: This invention is in the field of base station antennas for wireless communications. The present invention refers to a slim multi-band antenna array for cellular base stations, which provides a reduced width of the base station antenna and minimizes the environmental and visual impact of a network of cellular base station antennas, in particular in mobile telephony and wireless service networks. A multiband antenna array comprises a first set of radiating elements operating at a first frequency band and a second set of radiating elements operating at a second frequency band, said radiating elements being smaller than λ/2 or smaller than λ/3, being (λ) the longest operating wavelength. The ratio between the largest and the smaller of said frequency bands is smaller than 2.

Abstract: Microstrip (patch) antennas usually strongly radiate in directions along the ground plane. This effect causes unwanted radiation patterns and increased coupling among array elements. Dielectric polarization currents are identified as physical sources of this radiation. A general technique is proposed to compensate these currents and suppress radiation in horizontal directions.

Abstract: A radio communication system including multiple antenna elements divided into subgroups of at least two antenna elements, and multiple line cards operable to employ spatial processing techniques. Each line card is coupled to a subgroup such that the line card may transmit and receive signals using the subgroup. A base station for use in a radio communication system includes one or more line cards. Each line card includes an antenna interface used to couple the line card to a subgroup of multiple antenna elements, a radio frequency component coupled to the antenna interface, and a signal processing component coupled to the radio frequency component such that the line card is operable to transmit and receive radio frequency communications. A method for providing increased capacity in a radio communication system includes dividing an antenna array, creating N subgroups of antenna elements, and for each of the N subgroups of antenna elements, coupling a line card to the subgroup of antenna elements. Each line card is operable to communicate using its coupled subgroup of antenna elements.

Abstract: In principle, the end-fire directivity of a linear periodic array of N isotropic radiators can approach N/sup 2/ as the spacing between elements decreases, provided the magnitude and phase of the input excitations are properly chosen. Thus, the directivity of a two-element array of isotropic radiators would approach a value of four, that is, 6 dB higher than that of a single isotropic radiator. We have conducted a theoretical, computational, and experimental study for a two-element superdirective array of resonant monopoles. In agreement with the theoretical and computational curves, the measured gain of the monopole array does indeed continually increase with decreasing spacing of the monopoles, provided the relative magnitudes and phases are maintained. However, for very small separation, maximum achievable gain is not reached due to the presence of ohmic loss.

Abstract: Array resolution limits and accuracy bounds on the multitude of signal parameters (e.g., azimuth, elevation, Doppler, range, cross-range, depth, frequency, chirp, polarization, amplitude, phase, etc.) estimated by array processing algorithms are essential tools in the evaluation of system performance. The case in which the complex amplitudes of the signals are unknown is of particular practical interest. A computationally efficient formulation of these bounds (from the perspective of derivations and analysis) is presented for the case of deterministic and unknown signal amplitudes. A new derivation is given using the unknown complex signal parameters and their complex conjugates. The new formula is readily applicable to obtaining either symbolic or numerical solutions to estimation bounds for a very wide class of problems encountered in adaptive sensor array processing. This formula is shown to yield several of the standard Crame/spl acute/r-Rao results for array processing, along with new results of fundamental interest. Specifically, a new closed-form expression for the statistical resolution limit of an aperture for any asymptotically unbiased superresolution algorithm (e.g., MUSIC, ESPRIT) is provided. The statistical resolution limit is defined as the source separation that equals its own Crame/spl acute/r-Rao bound, providing an algorithm-independent bound on the resolution of any high-resolution method. It is shown that the statistical resolution limit of an array or coherent integration window is about 1.2/spl middot/SNR/sup -1/4/ relative to the Fourier resolution limit of 2/spl pi//N radians (large number N of array elements). That is, the highest achievable resolution is proportional to the reciprocal of the fourth root of the signal-to-noise ratio (SNR), in contrast to the square-root (SNR/sup -1/2/) dependence of standard accuracy bounds. These theoretical results are consistent with previously published bounds for specific superresolution algorithms derived by other methods. It is also shown that the potential resolution improvement obtained by separating two collinear arrays (synthetic ultra-wideband), each with a fixed aperture B wavelengths by M wavelengths (assumed large), is approximately (M/B)/sup 1/2/, in contrast to the resolution improvement of M/B for a full aperture. Exact closed-form results for these problems with their asymptotic approximations are presented.

Abstract: In this paper, the use of a smart antenna system for the estimation of the directions of arrival (DOAs) of multiple waves is considered. An efficient method based on the support vector regression is proposed, in which the mapping among the outputs of the array and the DOAs of unknown plane waves is approximated by means of a family of support vector machines. Several numerical results are provided for the validation of the proposed approach, considering multiple impinging waves both in noiseless and noisy environments.

Abstract: The problem addressed in this study is how to design and test compact antenna arrays for portable Mulitple-Input Multiple-Output (MIMO) transceivers. Mutual coupling in an antenna array affects signal correlation and array radiation efficiency - both of which have dramatic consequences for MIMO channel capacity. Mutual coupling becomes more pronounced as array aperture shrinks and is therefore a critical issue in compact array design. Two novel compact arrays are designed and fabricated for use in MIMO enabled mobile devices. These arrays are extremely compact yet demonstrate acceptable mutual coupling and radiation efficiency because of the MIMO-specific criteria used during their design. An experimental methodology is presented for fair and meaningful characterization of MIMO arrays by field trial. This methodology addresses the issue of capacity normalization and quantifies how well an antenna array's radiation pattern interfaces with multipath propagation. Results are presented from an extensive measurement campaign in which a true MIMO transceiver testbed is outfitted with compact arrays and dipole arrays of various sizes. A comprehensive and fair comparison is made between the compact arrays and dipole arrays in a variety of indoor propagation scenarios. Design recommendations for compact MIMO arrays are given.

Abstract: The matrix method for array diagnosis is based on the reconstruction of the excitation from measured near-field data by solving the linear system relating the excitation coefficients to the field at the measurement points. In this paper the matrix method is applied to the diagnosis of element failures in high-performance conformal array radar. The results confirm the usefulness of this technique in the cases wherein the standard backward propagation technique cannot be applied.

Abstract: This paper addresses the estimation of the code-phase (pseudorange) and the carrier-phase of the direct signal received from a direct-sequence spread-spectrum satellite transmitter. The signal is received by an antenna array in a scenario with interference and multipath propagation. These two effects are generally the limiting error sources in most high-precision positioning applications. A new estimator of the code- and carrier-phases is derived by using a simplified signal model and the maximum likelihood (ML) principle. The simplified model consists essentially of gathering all signals, except for the direct one, in a component with unknown spatial correlation. The estimator exploits the knowledge of the direction-of-arrival of the direct signal and is much simpler than other estimators derived under more detailed signal models. Moreover, we present an iterative algorithm, that is adequate for a practical implementation and explores an interesting link between the ML estimator and a hybrid beamformer. The mean squared error and bias of the new estimator are computed for a number of scenarios and compared with those of other methods. The presented estimator and the hybrid beamforming outperform the existing techniques of comparable complexity and attains, in many situations, the Crame/spl acute/r-Rao lower bound of the problem at hand.

Abstract: At least one antenna array including three mutually orthogonal antennas each sharing a common center point senses an electromagnetic signal emitted by a buried object such as a utility line, pipe or sonde. A circuit at least partially mounted in a housing is connected to the array and determines a location of the buried object by measuring signal strength and field angles in three dimensions without having to align the antenna array relative to the buried object while eliminating nulls and false peaks. A graphical user interface (GUI) has user-friendly icons, symbols, menus, numbers and graphical and auditory representation of signal strength. A plurality of different underground objects can be simultaneously detected and their different locations can be simultaneously indicated to a user via audible sounds and/or visual images on a display.

Abstract: Wireless networks consisting of compact antennas find applications in diverse areas such as communication systems, direction of arrival estimation, sensor networks, and imaging. The effectiveness of many of these systems depend on maximizing the reception of RF power and extracting maximum information from the incident electromagnetic (EM) wave. Traditionally, this has been achieved through multiple-input multiple-output (MIMO) systems employing a spatial array of antennas that enhance the channel capacity. In this paper, we report similar increases in channel capacity obtained through the use of vector antennas consisting of co-located loops and dipoles, which can respond to more than one component of the EM field. It is shown that systems with three- and four-element vector antennas at both the transmitter and receiver operating around the frequency of 2.25 GHz support three and four times more information, respectively, as compared to conventional systems consisting of sensors with single antennas. Comparison with a simplified theoretical model of a MIMO system with co-located antennas in a rich multipath environment shows good agreement.

Abstract: A communications device for separating source signals provided by M signal sources includes an antenna array comprising N antenna elements for receiving at least N different summations of the M source signals. A respective in-phase and quadrature module is connected downstream to each antenna element for separating each one of the N different summations of the M source signals received thereby into an in-phase and quadrature component set. A blind signal separation processor forms a mixing matrix comprising at least 2N different summations of the M source signals, with each in-phase and quadrature component set providing 2 inputs into the mixing matrix. The mixing matrix has a rank equal up to 2N. The desired source signals are separated from the mixing matrix by the blind signal separation processor.

Abstract: The synthesis and the optimization of three-dimensional (3-D) lens antennas, consisting of homogeneous dielectric lenses of arbitrary shape and fed by printed sources, are studied theoretically and experimentally at millimeter(mm)-wave frequencies. The aim of the synthesis procedure is to find a lens profile that transforms the radiation pattern of the primary feed into a desired amplitude shaped output pattern. This synthesis problem has been previously applied for dielectric lenses and reflectors. As far as we know, we propose, for the first time, to adapt and implement it for the design of substrate lens antennas. The inverse scattering problem is solved in two steps. In the first one, the geometry of the 3-D lens is rigorously derived using geometrical optics (GO) principles. The resulting second-order partial-differential equation is strongly nonlinear and is of the Monge-Ampe/spl grave/re (M.A) type. The iterative algorithm implemented to solve it is described in detail. Then, a surface optimization of the lens profile combined with an analysis kernel based on physical optics (PO) is performed in order to comply with the prescribed pattern. Our algorithms are successfully validated with the design of a lens antenna radiating an asymmetric Gaussian pattern at 58.5 GHz whose half-power beamwidth equals 10/spl deg/ in H plane and 30/spl deg/ in E plane. The lens is illuminated by a microstrip 2/spl times/2 patch antenna array. Two lens prototypes have been manufactured in Teflon. Before optimization, the measured radiation patterns are in very good agreement with the predicted ones; nevertheless, the -12 dB side lobes and oscillations appearing in the main lobe evidence a strong difference between the desired and measured patterns. This discrepancy is significantly reduced using the optimized lens.

Abstract: A rigorous method for characterizing, in terms of a multimode scattering matrix, a finite array of antennas where each antenna can be described by means of spherical waves is presented in this paper. The procedure provides the impedance, coupling and radiating characteristics of the arrays and comprises two steps. First, the generalized scattering matrix (GSM) of each single antenna is numerically calculated over a wide band of frequencies. For this purpose we use a previously-developed methodology that combines the domain segmentation technique, the three-dimensional finite element method (3D-FEM), spherical mode expansion and a reduced order model obtained using a symmetric matrix Pade/spl acute/-Via-Lanczos (SyMPVL) algorithm. Second, the overall GSM of the finite array is calculated starting from the GSM of each antenna and using rotation and translation of spherical waves. A closed-form expression for the overall GSM is given and different examples are shown in order to validate the proposed method.

Abstract: Emerging radar applications require phased arrays that can operate over wide bandwidths to support multiband/multifunction operation. In response to that need, this paper presents a cost-effective implementation for extremely wide-band phased-array radars. Two designs are demonstrated, one operating from 3 to 12 GHz and the other operating from 8 to 20 GHz. These designs incorporate ultra-wideband antipodal tapered slot antennas, a novel cross-polarization suppressed array architecture, piezoelectric true-time-delay phase shifters, and broad-band high-power monolithic amplifiers. The resulting systems provide target detection and beam steering over the complete operating bandwidths. These results exceed the state-of-the-art for phased-array radars in terms of bandwidth and cost and should have direct applications in the development of ultra-wideband and multifunction radar systems.

Abstract: Two-dimensional aperiodic tilings are collections of polygons, devoid of any translational symmetries, capable of covering a plane without gaps and overlaps. Although aperiodic, these structures can exhibit order and symmetry in an extended sense. In this paper, we study the radiation properties of planar antenna arrays based on certain categories of two-dimensional aperiodic tilings that illustrate diverse aspects of aperiodic order. Background material on aperiodic tilings and their known electromagnetic properties is reviewed. Results are illustrated to highlight the effects of aperiodic order in the antenna array radiation properties. Potential applications are also envisaged

Abstract: A modified printed bow-tie antenna is designed to simultaneously cover the operations in the C and X-bands from 5.5 to 12.5 GHz. The presented antenna has an end fire radiation pattern that makes it suitable for integration in single and dual polarized phased array systems. The antenna exhibits small size and wide bandwidth of 91%. The radiation characteristics are presented for a single element and a linear array of this antenna.

Abstract: An adaptive MIMO communications system includes a multifunctional reconfigurable antenna with a selectively alterable effective physical antenna array configuration and radiation/polarization properties, which configuration and properties is a component in the optimization of the adaptive system parameters. The multifunctional reconfigurable antenna comprises a plurality of antenna components and a plurality of selectively controllable switches coupling selected ones of the plurality of antenna components together into a multifunctional reconfigurable subarray of antenna components. A processing unit coupled to the multifunctional reconfigurable antenna determines communication channel conditions for generating adaptive control signals to the plurality of selectively controllable switches to selectively apply a selected space-time coding protocol or a selected beam forming protocol together on the plurality of antenna components depending on channel conditions.

Abstract: An antenna array is provided for an RFID reader, which includes a first reader antenna tuned to operate at a first frequency and a second reader antenna tuned to operate at a second frequency different from the first frequency. The first and second antennas are preferably arranged in an overlapping arrangement or an opposing magnetic flux arrangement to reduce the effect of antenna self resonance.