Cross-correlation antenna systems with modified principal solution spatial frequency transfer functions

The principal solution type of spatial frequency transfer function (SFTF) for a linear antenna system, e.g., line source or linear array, is constant (flat) over its entire spatial frequency (SF) passband and drops abruptly to zero at the edges of the band. Such a SFTF is obtainable with cross-correlation (multiplicative) techniques. The associated power pattern, of thesin (Lu)/Lutype, has high sidelobes diminishing asymptotically asu^{-1}. Moreover, the weighting distribution for the line source has an inverse taper with end point singularities. A modified principal solution SFTF, flat over most of the SF passband but with a smoothly varying transition function of the quadratic-linear-quadratic type at the edges of the passband is introduced. This engenders a power pattern whose sidelobes diminish asymptotically asu^{-3}instead ofu^{-1}. The line source weighting distributions, moreover, are continuous and free of singularities, making physical realizability no longer a problem.     

Reconfigurable antenna solution for MIMO-OFDM systems

A reconfigurable microstrip dipole antenna solution for multiple-input multiple-output (MIMO) communication systems making use of orthogonal frequency division multiplexing (OFDM) is presented. When applied to closely spaced antenna arrays, this method can increase link capacity. The benefits of this novel antenna solution are demonstrated by channel capacity measurements taken in an indoor environment with a 2/spl times/2 MIMO system.     

Ground-based measurements of inflight antenna patterns for imagingradar systems

An approach is presented for determining the inflight antenna pattern in the cross-track direction for air- and spaceborne synthetic aperture radar (SAR) systems. In the 1991 Oberpfaffenhofen DC-8/E-SAR calibration campaign, ground-based measurement, equipment comprising 18 precision calibration receivers and nine polarimetric active radar calibrators, all operating in C-band, were tested. These instruments are capable of handling various pulse lengths and repetition frequencies, and they have a very high dynamic range. Together with precise internal clocks, these instruments are suitable for recording the actual radar transmit pulse shape for the later evaluation of the desired inflight antenna pattern. Lining up these devices in the cross-track direction, each receiver yields an azimuth cut of the three-dimensional antenna pattern. The elevation pattern was then obtained by time correlation of these azimuth cuts. Further results concerning pulse shapes, squint angles, and H-V pattern misalignment are presented     

A template for shaped-beam satellite antenna patterns

A modified template is proposed for shaped-beam satellite antenna patterns for use in orbit-planning studies of a fixed satellite service. Based on a previously discussed simple model, the template now includes the effects of beam scanning, aperture blockage, and surface errors. It is found to agree well with the measured data on current satellites and is a reasonable upper bound specification for future satellite antennas     

CAD for antenna systems

Computer methods for solving antenna problems are described and their advantages and disadvantages discussed. No one program will deal with all possible antenna problems and some expertise is required on the part of the user to judge which method is appropriate for a given problem. The better the computer facilities, the more complex the problems which can be solved, but there are some basic limitations in the underlying physics of all computation electromagnetic (CEM) software. There are still shortcomings in the ease of use and user-friendliness     

Realizable feed-element patterns and optimum aperture efficiency inmultibeam antenna systems

The results of an earlier paper by Y. Rahmat-Samii et al. (ibid., vol.AP-29, pp.961-3, 1981), regarding realizable patterns from feed elements that are part of an array that feeds a reflector antenna, are extended. The earlier paper used a cos^q &thetas; model for the element radiation pattern, whereas here a parametric study is performed, using a model that assumes a central beam of cos^q &thetas; shape, with a constant sidelobe level outside the central beam. Realizable q-values are constrained by the maximum directivity based on feed element area. The optimum aperture efficiency (excluding array feed network losses) in an array-reflector system is evaluated as a function of element spacing using this model as well as the model of the earlier paper. Experimental data for tapered slot antenna arrays are in agreement with the conclusions based on the model     

A joint-channel diagonalization for multiuser MIMO antenna systems

In this paper, we address the problem of improving the performance of multiuser space-division multiplexing (SDM) systems where multiple independent signal streams can be transmitted in the same frequency and time slot. The problem is important in multiuser multiple-input multiple-output systems where communication from one base station to many mobile stations can occur simultaneously. Our objective is to devise a multiuser linear space-time precoder for simultaneous channel diagonalization of the multiuser channels enabling SDM. Our new approach is based on diagonalizing the multiuser channel matrices and we use a variation of successive Jacobi rotations. In addition to the diagonalization, our approach attempts to optimize the resultant channel gains for performance enhancement. Our method is valid for both frequency-flat and frequency-selective fading channels but we assume that the base station knows all the channels and that they are quasi-stationary.     

Photonic systems for antenna applications

For well over a decade, research groups around the world have been investigating photonic devices for use in antenna, phased-array, and electromagnetic-field-sensor systems. Proposed applications include EMC monitoring, EMP measurements, anechoic-chamber calibration, novel antenna and array designs, remotely located antennas, and smart skins. Photonic antenna links allow immunity from electromagnetic interference, provide an extremely wide working bandwidth, and allow long cable runs with a minimum of loss. Developments in laser technology allow photonic-based links to have very low noise figures and high dynamic range. Opto-electronic devices are now being built in III-V semiconductors (GaAs, InP), which will ultimately lead to opto-electronic integration with MMIC devices. Photonic systems will revolutionize the way in which electromagnetic-measurement and antenna systems will be designed and built in the future. The paper presents an introduction to the types of photonic links and modulator systems available to the antenna engineer     

Model based antenna array calibration for smart antenna systems

Model based antenna array calibration is used to remove the antenna array errors or predict the element patterns from the array manifold measured at a small number of directions. The experimental results show that this works very well for one sector angle region even in the presence of modelling errors     

On the efficiency and radiation patterns of mismatched shaped Cassegrainian antenna systems

Calculations are carried out with regard to the aperture illumination efficiency, spillover efficiency, and blockage efficiency of a shaped Cassegrainian antenna system whose feed pattern is not similar to that for which the antenna was designed. It appears that a maximum overall efficiency may be expected for antenna systems with a feed pattern having a slightly narrower beamwidth than those for which the systems were originally designed. It is further proved that the radiation pattern of mismatched systems is very unfavorable in the case of the beamwidth of the feed pattern being wider than the beamwidth of the feed pattern for which the system was designed. This is due to overillumination of the edges.     

A lower complexity antenna selection for DF relay mimo systems

Based on the existing incremental selection algorithm for Decode-and-Forward (DF) Multiple Input Multiple Output (MIMO) systems, a faster one which combines the incremental selection with the decremental exclussion is proposed. Firstly, the algorithm selects one antenna which makes the increment of the capacity greatest in every step. Then, excludes the antennas whose contribution to the capacity is less than μ% of that of the best antenna. Simulations show that the proposed algorithm achieves comparable performance to the existing one and has obviously decreased complexity under the appropriate threshold μ%.     

A semiblind method for transmit antenna arrays in CDMA systems

Using a transmit antenna array (TAA), the downlink beamforming is able to improve the performance of wireless communication systems. We propose a semiblind downlink beamforming method based on channel reciprocity of the wireless channels for code-division multiple-access (CDMA) systems equipped with TAA. In the proposed semiblind method, it is shown that the feedback delay can result in performance degradation under a time-varying fading channel environment. Hence, the performance of the semiblind method is satisfactory only when the maximum Doppler frequency is low or the feedback delay is not significant. To overcome this difficulty, the channel prediction is considered. Using the channel predictor, the semiblind method can provide a reasonable performance for a moderate maximum Doppler frequency (about a few hundred hertz). For theoretical performance analysis, upper, and lower bounds of the bit error probability under a fading channel environment are derived     

A receiver of simple structure for antenna array CDMA systems

In code division multiple access systems, the two-dimensional (2-D) RAKE structure had been proposed using multiple antennas to increase the capacity. It consists of multiple fingers of a beamformer and a correlator. In this paper, we consider a simpler receiver structure. We only use one finger, which consists of one pair of spatial and temporal filters to combine signals. It is shown that the performance of the receiver system is comparable with that of the 2-D RAKE receiver. Furthermore, it is observed that the proposed receiver and the 2-D RAKE receiver have limited near-far resistance by the generation of the space domain due to antenna arrays     

VHF antenna systems for spin-stabilized satellites

Lincoln Laboratory has developed two circularly polarized VHF antenna systems for use on spin-stabilized satellites, Both are described, and test results are discussed. One is a low-gain (3-dB) antenna with omnidirectional coverage in the equatorial plane. The second is a high-gain (10-dB) antenna with a despun beam electronically scanned to always be Earth directed. Development of the component dipole and slot array elements is explained, and performance data are given for the final flight configuration.     

A generalized probabilistic data association detector for multiple antenna systems

The Probabilistic Data Association (PDA) method for multiuser detection (MUD) over synchronous CDMA channels is extended to the signal detection problem in V-BLAST systems. Computer simulations show that the algorithm has an error probability that is significantly lower than that of the V-BLAST optimal order detector and has a computational complexity that is cubic in the number of transmit antennas.     

A spatial-temporal decorrelating receiver for CDMA systems withbase-station antenna arrays

We investigate multiuser signal detection with a base-station antenna array for a synchronous DS-CDMA uplink using nonorthogonal codes in Rayleigh fading channels. We have developed a new formulation for a spatial-temporal decorrelating detector using the maximum-likelihood criteria. The detector is shown to be near-far resistant. We propose to implement the spatial-temporal decorrelating receiver iteratively by applying the space-alternating generalized expectation-maximization (SAGE) algorithm. Simulation results show that the SAGE-based decorrelating receiver significantly outperforms the conventional single-user receiver and with performance close to that of a spatial-temporal decorrelating receiver with known channel parameters. We have observed that adding base-station antennas can actually improve convergence of the proposed iterative receiver     

A novel technique for the approximation of 3-D antenna radiation patterns

In this paper, a novel technique for the approximation of three-dimensional (3-D) antenna radiation patterns is presented. The proposed method combines the two principal cuts in order to acquire an adequate estimate of the 3-D antenna radiation solid. The absolute error from the theoretical solution is analyzed along with other statistical measures for various types of antenna structures, namely omni-directional and directive arrays. The performance of the method is compared against other existing extrapolation algorithms. The proposed technique exhibits low approximation errors and is easily integrated into 3-D radio propagation planning tools (such as ray-tracing algorithms).     

Synthesis of notched antenna patterns for wideband processing

Antenna patterns can be designed to have broad notches in regions where clutter and jamming are expected to be received. A number of narrowband pattern synthesis techniques exist to design notched antenna patterns, but they break down in the presence of wideband signals on arrays which employ a combination of phase and time steering. The authors derive definitions for wideband antenna patterns and present a new synthesis procedure which can be used to design notched patterns for signals having specified bandwidths on any type of array     

Realizable feed-element patterns for multibeam reflector antenna analysis

The radiation pattern of a feed element is approximately described by a simple function(cos theta)^{q}. For a given element spacing of the feed array, we give simple formulas for estimating the practical value ofqwhen the element is an open-ended rectangular waveguide, an open-ended circular waveguide, a pyramidal horn, or a cigar antenna.     

Use of Euler-rotation angles for generating antenna patterns

Antenna patterns are most naturally expressed in spherical coordinates, since the pattern angles represent directions in space. All patterns can be represented by circles on the coordinate sphere. Most patterns are either polar great-circle patterns or equatorial patterns, referring to the coordinate sphere, where all look directions lie in a plane. In some cases, however, the surface containing all of the look directions is a cone, with the origin at the antenna, and the axis along one of the Cartesian-coordinate axes. In general, all patterns are conical, known in the trade as “conics”, and the axes of the cones can be in any direction. Conical patterns are useful in cases where the beam is electronically scanned, and the sidelobes in the plane perpendicular to the plane of the scan therefore fall on a cone, instead of in a plane. Additional examples are patterns of constant range or constant Doppler shift, for airborne-radar antennas, which are all conical patterns. Calculating antenna patterns, in such cases, presents a problem, since the antenna-pattern-coordinate system will generally not be the same as the antenna- or the antenna-radome-coordinate system. The two systems may not even be "square" with each other. But by using the proper Euler angles, the desired antenna-pattern look direction can be easily converted to a coordinate system better suited to the calculations