A vector space approach to direction finding in a coherent multipath environment

The superresolution methods of spatial spectrum estimation heretofore reported are known to break down when arriving signals from different directions are coherent. It is shown here that this deficiency can be overcome by replacing the direction-of-arrival (DOA) search vector ordinarily used with a linear combination of such vectors. Particular attention is here given to extending the multiple signal classification (MUSIC) method to cover such cases. A procedure for determining the proper linear combination is presented so that determination of the separate coherent signal arrival angles involves a multidimensional search in angle only. Such a search is demonstrated for coherent two-ray and three-ray multipath environments.     

High resolution wideband direction finding arrays based on optimumsymmetrical number system encoding

A new interferometer direction finding (DF) array architecture based on the optimum symmetrical number system (OSNS) is presented. OSNS arrays are capable of unambiguous high resolution DF over a wide bandwidth and field of view with as few as three elements, with multiple baseline options. A three element array was designed, fabricated and tested at 8.5 GHz to verify the OSNS concepts experimentally     

Method for estimating the number of signals in problems of wireless direction finding with a small observation period

A new algorithm for estimating the number of signals received by the antenna array has been proposed. As confirmed by simulation and field tests, this algorithm features smaller computational costs and ensures more accurate estimates at short observation times.     

Distributed versus centralized multiuser detection with antenna arrays in frequency-selective multipath channels

We have analyzed and compared two different multiuser detection receiver structures that are appropriate for code-division multiple access (CDMA) systems with multiple antennas in frequency-selective multipath channels. Distributed decorrelation detection (where signal decorrelation is performed at each receiving antenna) and centralized decorrelation detection (where decorrelation is performed once collectively) are compared. We show that they provide the same performance in additive Gaussian channels. Centralized decorrelation detection provides better performance in general frequency-selective multipath fading channels. Therefore, the hardware cost, complexity, and processing at the receiver can be optimized using centralized decorrelation detection. We employ decorrelation detection to demonstrate our results, although the work can be easily extended to other linear multiuser detection configurations.     

Performance analysis of direction finding with large arrays andfinite data

This paper considers analysis of methods for estimating the parameters of narrow-band signals arriving at an array of sensors. This problem has important applications in, for instance, radar direction finding and underwater source localization. The so-called deterministic and stochastic maximum likelihood (ML) methods are the main focus of this paper. A performance analysis is carried out assuming a finite number of samples and that the array is composed of a sufficiently large number of sensors. Several thousands of antennas are not uncommon in, e.g., radar applications. Strong consistency of the parameter estimates is proved, and the asymptotic covariance matrix of the estimation error is derived. Unlike the previously studied large sample case, the present analysis shows that the accuracy is the same for the two ML methods. Furthermore, the asymptotic covariance matrix of the estimation error coincides with the deterministic Cramer-Rao bound. Under a certain assumption, the ML methods can be implemented by means of conventional beamforming for a large enough number of sensors. We also include a simple simulation study, which indicates that both ML methods provide efficient estimates for very moderate array sizes, whereas the beamforming method requires a somewhat larger array aperture to overcome the inherent bias and resolution problem     

Direction finding in multipath environment using method of alternating projection

In a coherent environment the problem of direction finding using arbitrary antenna array geometries always seems to be a multivariate optimisation problem. An estimator derived using suboptimal eigen techniques and optimised using the method of alternating projection (AP) is shown to be superior to the maximum likelihood method.<>     

An experiment on antenna arrays with randomly spaced elements

An experimental investigation on the planar array with randomly spaced elements was made by using the holey-plate technique. Two sample arrays, each consisting of 210 elements over a circular aperture of about 56 wavelengths in diameter, were tested at 71.25 GHz. The measured sidelobe levels were -12.8 dB and -13 dB, in excellent agreement with the theory which predicted below -12.8 dB with 90 percent probability and -13.3 dB with 50 percent probability. Further, one may consider the pattern in each plane cutting through the antenna as that of a linear random array. Thus one may study the sample distribution of the sidelobe levels of as many linear random arrays as cuts. The results showed a nearly perfect agreement with the theory, despite the fact that in the theory the mutual coupling effect was neglected altogether. This experiment seems to suggest that when the average element spacing is of the order of a few wavelengths or more, the net coupling effect on the pattern of a random array is generally negligible.     

Low-sidelobe patterns from linear and planar arrays with uniformexcitations except for phases of a small number of elements

The use of genetic algorithms allows synthesis of linear and planar arrays with excitations of uniform amplitude that generate radiation patterns with sidelobe levels closely approaching the recognised minima for phase-only control, in spite of only a small number of elements having non-zero phase     

A mathematical theory of antenna arrays with randomly spaced elements

Various probabilistic properties of a large antenna array with randomly spaced elements have been studied. It is found that for almost all cases of practical interest the required number of elements is closely related to the desired sidelobe level and is almost independent of the aperture dimension, the resolution (or the beamwidth) depends mainly on the aperture dimension, and the directive gain is proportional to the number of elements used if the average spacing is large. As a consequence the number of elements required is considerably less than that with uniform spacings. Starting with a given number of elements and a given aperture size, it is possible to improve the resolution by a factor of ten, a hundred, or more by spreading these elements over a larger aperture with little risk in obtaining a much higher sidelobe level and a lower directive gain. In fact, this method offers a solution which is optimum in a certain statistical sense, i.e., all sidelobes are of equal level with equal probability. In addition, this analysis also gives a simple estimate of the sidelobe level of most nonuniformly spaced antenna arrays. In a number of such arrays studied by various investigators with high speed computers, the agreement found is remarkable.     

Mobile antenna gain in the multipath environment at 900 MHz

A program has been completed that investigated the effect of the urban multipath environment on mobile antennas at 900 MHz. The program involved the construction of several mobile antennas, careful measurement of these antennas on an antenna range to determine their characteristics in a controlled environment, and measurement of their characteristics in the multipath environment of cities. Measured results were compared to computed results and pattern range results were compared to field data to permit generalized conclusions to be reached. The results of the second phase of the program, measurements made in the mobile multipath environment, are presented. Analysis of this data shows that gain antennas have significantly less gain in areas where multipath propagation predominates and that the increased aperture of the gain antennas provides no significant smoothing of the received signals in the multipath environment.     

A compact flush-mounting antenna with direction finding and steerable cardioid pattern capability

A flush-mounting antenna is described which could be used, in lieu of present antennas, to provide direction finding and steerable directional pattern capability in airborne beacon transponder systems. The antenna is comparable in size to the existing, omni-directional antennas presently used for air-traffic control and military identification, friend or foe (IFF) functions. The design and performance of an experimental model of the antenna are discussed.     

Performance of radial-basis function networks for direction ofarrival estimation with antenna arrays

The problem of direction of arrival (DOA) estimation of mobile users using linear antenna arrays is addressed. To reduce the computational complexity of superresolution algorithms, e.g. multiple signal classification (MUSIC), the DOA problem is approached as a mapping which can be modeled using a suitable artificial neural network trained with input output pairs. This paper discusses the application of a three-layer radial-basis function neural network (RBFNN), which can learn multiple source-direction findings of a six-element array. The network weights are modified using the normalized cumulative delta rule. The performance of this network is compared to that of the MUSIC algorithm for both uncorrelated and correlated signals. It is also shown that the RBFNN substantially reduced the CPU time for the DOA estimation computations     

DESIGN OF VIRTUAL UNIFORM LINEAR ARRAYS FOR EFFICIENT DIRECTION FINDING

The problem considered in this paper is to interpolate a virtual uniform array froma real two-dimensional array with arbitrary geometry via an interpolation matrix. The key to thisproblem is how to arrange these virtual sensors. It is shown that the virtual uniform linear arrayshould have the same main-lobe beam-pattern as the real array over an angular sector of interest.Simulation results are presented to illustrate the application of virtual array in direction finding.     

Direction finding receiver for UWB impulse radio signal in multipath environment

In this paper, we propose a direction finding (DF) receiver for ultra wideband impulse radio (UWB‐IR) signal in a realistic multipath environment. The receiver uses an array of antenna, where each antenna is connected to a proposed propagation‐delay estimation structure. The advantage of the proposed structure is that it outputs a trapezoidal signal whose amplitude reflects the propagation delay and thus relaxes the sampling rate requirement of the analog‐to‐digital‐converter (ADC). The angle‐of‐arrival (AOA) is estimated from the vector of propagation delays measured with respect to reference antenna. Because these estimated delays contain outliers, we propose a simple AOA estimation algorithm based on minimum fractional‐distance search. Experimental results based on simulation show that the proposed DF receiver achieves better performance compared with the minimum ?₁‐ and ?₂‐based (least‐squares based) distance search. Copyright © 2010 John Wiley & Sons, Ltd.     

Cumulant-based adaptive multichannel filtering for wirelesscommunication systems with multipath RF propagation using antenna arrays

A method based on high-order statistics is proposed to mitigate the performance degradation caused by multipath RF propagation in a mobile radio communication system using a linear antenna array at the base-station receiver. It is shown that an overdetermined system of linear equations (involving only cumulants of the received baseband digitized signal) can be obtained to perform noniterative deconvolution. An efficient adaptive algorithm based on square-root decomposition is proposed to avoid numerical problems when real-time tracking of moving transmitters is needed     

Construction of filled phased antenna arrays from thinned arrays with position-coded elements on the basis of the properties of prime and composite number moduli

A method for construction of filled phased antenna arrays (PAAs) by placing thinned arrays with elements whose positions are coded in a lattice of nodes (PAA-Cs) at equidistant nodes of the same lattice with the use of the properties of prime and composite number moduli p is proposed. It is shown that, for moduli p satisfying certain conditions, it is possible to fill asymptotically one-half of the lattice nodes with elements of different PAA-Cs constructed from all primitive roots of a particular modulus p so that the elements of different PAA-Cs coincide at only two lattice nodes. Conditions for prime modulus p and the number of corresponding primitive roots as well as all values of modulus p up to 1000 satisfying these conditions are presented. For several values from this number set, examples of the arrangement of the elements of all PAA-Cs in a lattice of equidistant nodes are given. For these configurations, resulting patterns of receiving PAAs in which the pattern is formed after amplifiers (receiving active PAAs) are calculated.     

Linear prediction approach to direction estimation ofcyclostationary signals in multipath environment

We investigate the estimation of the directions-of-arrival (DOA) of closely spaced narrowband cyclostationary signals in the presence of multipath propagation. By exploiting the spatial and temporal properties of most communication signals, we propose a new cyclic forward-backward linear prediction (FBLP) approach for coherent signals impinging on a uniform linear array (ULA). In the proposed algorithm, the evaluation of the cyclic array covariance matrix is avoided, and the difficulty of choosing the optimal time lag parameter is alleviated. As a result, the proposed approach has two advantages: (1) the computational load is relatively reduced, and (2) the robustness of estimation is significantly improved. The performance of the proposed approach is confirmed through numerical examples, and it is shown that this approach is superior in resolving the closely spaced coherent signals with a small length of array data and at relatively low signal-to-noise ratio (SNR)     

一种小型四阵元GPS天线阵列的设计

分析了GPS微带天线阵列阵元间的互耦特性,得出了满足互耦要求的合适的最小阵元间距;设计了一种小型四阵元GPS微带偏振天线阵列,该阵列采用传统右圆极化阵元舜口双线性极化阵元相结合,使得相同阵元的天线阵列的自由度提高;从仿真的结果可以看出,在不增加阵列面积的情况下,该阵列能够在干扰方向产生比传统微带天线阵列更深的零陷,很大程度地提高了阵列的抗干扰性能;最后给出了GPS微带双频偏振层叠天线的一些重要的设计参数.