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Improving safety is of general interest in aviation. Flight transponders used in collision avoidance systems advise pilots with information of potential collisions and avoidance resolutions. Some transponders are capable of transmitting the instantaneous position of the vehicle which might be turned off. To prevent autonomous flights from collisions, the transponder signals shall be used to estimate the direction-of-arrival (DoA). Recently, multimode antennas have been investigated for communication and localization purposes. Multimode antennas provide several orthogonal ports on a single radiator surface. This leads to a reduction of space and weight, which is of interest in airborne applications. To perform DoA estimation as well as communication, a suitable channel model is required. In this contribution, a channel model fulfilling the requirements for both multiple-input multiple-output (MIMO) communication and DoA estimation is proposed. This channel model relies on the manifold matrix representation of the UAV under consideration. The derived description of a manifold matrix for the UAV use-case is used for an optimization to achieve a close to omnidirectional radiation pattern. Therefore, different optimization functions are derived and their performance is compared. The results of the optimization using the calculated manifold matrix are verified by measurements taken in an antenna measurement chamber. [ABSTRACT FROM AUTHOR]

The smart radio environment has become an eye-catching topic and is believed to own great potential in future wireless networks. Its essential demand lies in sensing and manipulating the electromagnetic (EM) fields accurately and simultaneously with the same aperture, which is hard to achieve using conventional methods due to the difficulty in integrating different systems. We propose a millimeter-wave space–time-coding digital metasurface (STCM) to realize simultaneous in situ direction finding and field manipulation. An analytic method of direction finding is first developed using the STCM modulated by a space–time-coding (STC) matrix while maintaining its original capability of EM field manipulations. In addition, three STC matrices corresponding to different EM functions are applied to the STCM to verify the feasibility and robustness of direction findings experimentally. Finally, a proof-of-concept prototype is built to exhibit different EM functions according to the incident angle detected simultaneously. The proposed concept sets the stage for the integrated implementation of the perception and response of the radio environment, which may find promising applications in future intelligent wireless networks and self-adaptive systems. [ABSTRACT FROM AUTHOR]

Traditionally, regularly spaced antenna arrays follow the spatial Nyquist criterion to guarantee an unambiguous analysis. We present a novel technique that makes use of two sparse non-Nyquist regularly spaced antenna arrays, where one of the arrays is just a shifted version of the other. The method offers several advantages over the use of traditional dense Nyquist-spaced arrays, while maintaining a comparable algorithmic complexity for the analysis. Among the advantages we mention: an improved resolution for the same number of receivers and reduced mutual coupling effects between the receivers, both due to the increased separation between the antennas. Because of a shared structured linear system of equations between the two arrays, as a consequence of the shift between the two, the analysis of both is automatically paired, thereby avoiding a computationally expensive matching step as is required in the use of so-called co-prime arrays. In addition, an easy validation step allows to automatically detect the precise number of incoming signals, which is usually considered a difficult issue. At the same time, the validation step improves the accuracy of the retrieved results and eliminates unreliable results in the case of noisy data. The performance of the proposed method is illustrated with respect to the influence of noise as well as to the effect of mutual coupling. [ABSTRACT FROM AUTHOR]

A new model for accurate direction findings is presented, and the superiority of the proposed model is demonstrated over the existing models for real system requirements. The proposed model is constructed based on the phase center contour of each element in the array. It is also shown that the accuracy of the proposed model will remain for tightly coupled arrays while other models fail. A novel algorithm according to this new model is also proposed. The proposed algorithm is proved by both simulations and measurements to be a viable solution for indoor applications where strong multipath interferences exist in the environment. Furthermore, a detailed explanation of accurate direction finding in harsh environments is presented using ultra-miniaturized tightly coupled arrays. In general, the direction-of-arrival estimation accuracy deteriorates using such miniaturized arrays, whereas we have shown that an excellent estimation accuracy is achieved using the proposed model and the detection algorithm. This statement is substantiated for any environments regardless of the strength of interference. To validate the developed model and theory, several practical prototypes of tightly coupled arrays are fabricated using the typical FR4 printed-circuit-board (PCB) fabrication process. The prototypes are measured in several indoor scenarios at the presence of multipath. The results are studied in detail. [ABSTRACT FROM AUTHOR]

A unique beam scanning array, neither applying spherical-to-plane wave transformation like a reflect array nor utilizing corporate feed network like a Butler matrix, is proposed and investigated in this paper. The governing equations of a common retro-directive array are revisited and generalized to fulfill a linear array having retro-beam scanning without using heterodyne technique. In the proposed scheme, the progressive phase difference between the adjacent antenna elements is controlled not only by the interconnecting line in-between but also by an extra phase term introduced by a newly added phase shifter. According to the theoretical prediction, the main beam of the retransmitted wave can be steered to an arbitrary direction irrespective of the direction of arrival (DOA) of the incident wave. A linear four-element quasi-Yagi array, controlled by 4-bit phase shifters, is applied as a preliminary proof-of-concept. An automated retro-beam scanning system integrated with a DOA estimation network and a low-cost microcontroller was then experimentally demonstrated to validate its capability to steer and re-transmit a randomly incoming oblique wave within ±30° from the normal incidence to a preset targeted direction without any manual control. The theoretical background, formulation, experimental results, and potential application scenarios are carefully discussed. [ABSTRACT FROM AUTHOR]

Accurate direction of arrival (DOA) estimation in compact terminal devices using small antenna arrays is challenging due to their limited radiating apertures. In this communication, we employ both the amplitude and the phase information to improve the estimation accuracy and eliminate phase ambiguity without increasing the size of the antenna array. The optimal amplitude ratio between the antenna patterns is theoretically studied using the theories of the Fisher information, Cramér–Rao bound, and ambiguity function. Criteria of proper amplitude ratio are summarized from the theoretical investigations. Following the guidelines, two antenna arrays are modified to build effective antenna patterns and improve the estimation accuracy. Specifically, the compact monopole-patch array and the dual-monopole array reduce the DOA estimation errors by over 60% and 20%–40%, respectively, under a medium SNR of 10 dB. The phase ambiguity has been eliminated in both arrays as well. The monopole-patch array requires phase calibration in the measurement, whereas the dual-monopole array does not. Experiments were carried out for the dual-monopole array in the anechoic chamber, achieving the estimation accuracy of 0.67° averaged over 360°. [ABSTRACT FROM AUTHOR]

A method for superresolution direction-of-arrival (DoA) estimation of correlated signals using a circular array is presented. This is accomplished in two steps: 1) the directions of arrival and amplitudes of signals with angular spacing larger than the array beamwidth are detected using the array signal segregation using an iterative approach (ASSIA) and 2) signals with angular proximity smaller than the array beamwidth are separated using a novel closely spaced nulls-synthesis method (CNSM). It is demonstrated that the proposed ASSIA-CNSM is superior to the other superresolution DoA estimation techniques such as maximum-likelihood method. The experimental results indicate that ASSIA-CNSM can be effectively used to distinguish the DoAs of the signals with angular spacing as small as 0.15 BW (where BW is the array half-power beamwidth) and with signal-to-noise ratio as low as 5 dB. [ABSTRACT FROM AUTHOR]

The direction of arrival (DoA) estimation performance of three different coarray structures, namely, the nested array, the coprime sampling array, and the sparse ruler array are presented and compared. The coarray concept makes it possible to detect the DoAs of much more sources than the number of physical antennas. Crucial is that for the first time, these coarrays are investigated based on real measurements conducted on a demonstrator platform. Based on the results obtained, the conclusion is clear. The MUSIC algorithm-based coarray concept with spatial smoothing is not suitable for DoA estimation in practical circumstances due to the unavoidable multipath phenomenon. [ABSTRACT FROM AUTHOR]

Crossed-loop (CL)/monopole antenna has been widely used in high-frequency ground-wave radar for ocean surface remote sensing owing to its compact size. The multiple signal classification (MUSIC) algorithm is commonly adapted to achieve a favorable angular resolution for this broad beam antenna. However, the direction of arrival (DOA) estimation is always troubled by the frequently occurring distortion of antenna pattern. Previous studies are mainly focused on the calibration by using the actual pattern or other assistant algorithms. In this paper, theoretical analysis is made to illustrate the reason for pattern distortion, which draws a conclusion that the distortion is essentially due to the different influence of environmental obstacles upon the electric field part and magnetic field part. This also implies that there is nearly no distortion between the two loops, which is validated by three different experiments. Based on this conclusion, the DOA estimation performance of the MUSIC algorithm is studied by creating a numerical relationship associating the estimation error with distortion level. Furthermore, a new method using ideal pattern is proposed to improve the estimation performance by removing the amplitude information of monopole and relying on the CL processing. Both simulation and experimental results are given to prove its validity. [ABSTRACT FROM AUTHOR]

The data model for direction of arrival (DOA) estimation is revisited and studied with more attention to the practical implementation of the antenna arrays and verified through measurement results. A complete data model for a single-channel direction finding system that includes, for the first time, the effects of the switching mechanism among antennas is presented. In order to combat coupling among antenna elements and the radio frequency switch nonidealities, a comprehensive model is developed, which includes more accurate noise modeling. The effect of the phase center (PC) in DOA estimation is then discussed and an improved data model is presented, which includes the antenna’s PC movement. The Cramer–Rao bound for circular arrays including the newly introduced parameter for the PC is derived. The new model shows significant improvement in the estimation accuracy. [ABSTRACT FROM PUBLISHER]

We introduce a modified multiple signal classification (MUSIC) algorithm to estimate the directions of arrival (DOAs) of multiple radio frequency signals. We specifically consider an antenna array with an imperfectly calibrated array response. The modified MUSIC algorithm estimates the DOA of each signal individually after attempting to null the other signals that are present. We compare the new approach to the conventional MUSIC algorithm in simulations, and show that the new approach leads to improved performance. The new approach converges quickly and produces DOA estimates with smaller mean error and standard deviation than MUSIC over a range of incident signal separations. We recommend our approach for systems with challenging antenna placements, a time varying host platform, and limited processing capabilities. [ABSTRACT FROM PUBLISHER]

A direction of arrival (DoA) estimation technique for broadband microwave signals is proposed using a single ultrawideband antenna. It is inspired by the sound source localization ability of a human auditory system using just one ear (monaural localization). By exploiting the incident angle-dependent frequency response of a wideband antenna, the DoA of a broadband microwave signal can be estimated. The DoA estimation accuracies are evaluated for two antenna configurations and microwave signals with different signal-to-noise ratios. Encouraging the DoA estimation performance of the proposed technique is demonstrated in both simulation and experiment. [ABSTRACT FROM AUTHOR]

This paper presents a systematic approach to the optimum placement of direction finding (DF) antennas in a harsh platform environment. A circular array with omni-directional elements is considered for azimuthal direction of arrival (DOA) estimation based on the multiple signal classification (MUSIC) algorithm. The element positions are optimized by the use of a genetic algorithm (GA) in conjunction with the FEKO electromagnetic simulator. The proposed approach is first tested with a generic platform shape. It is then extended to a more realistic military aircraft model with a strong scattering object. The optimized results show a significant performance improvement over the uniform circular array in both platform environments. [ABSTRACT FROM PUBLISHER]

Multidimensional channel sounding aims to estimate the geometrical structure of multi-path wave propagation in terms of directions of arrival/departure, Doppler shift, time delay, and complex polarimetric path weights. Maximum likelihood parameter estimation based upon an underlying data model is used to achieve high-resolution of the path parameters and, thus, renders possible an antenna independent channel characterization. However, any mismatch of the underlying data model to physical reality imposes limits to accuracy and reliability of the estimation. To cope with the limited resolution capability of the setup we are using a propagation data model that does not only contain discrete deterministic components but also a non-resolvable stochastic part. Joint estimation of both components considerably enhances the estimation quality and finally allows the interpretation as specular and diffuse contribution of multi-path propagation respectively. However, besides of noise influence, the achievable resolution is further limited by the accuracy of the data model that describes the measurement setup. Since the antenna characteristics are very susceptible to calibration and modeling errors, the directional estimates are most error-prone. We refer to the antenna array calibration procedure and discuss common pitfalls in high-resolution multi-path direction estimation that are related to inaccurate and/or incomplete device data model. Depending on the type of the antenna array (linear, circular) this will inherently produce biased and artificially spread angular estimates. Only with precise knowledge of the model errors the stochastic part can be identified as diffuse propagation component vs. modeling error. [ABSTRACT FROM PUBLISHER]

This paper presents an improved two-antenna microwave passive direction finding system inspired by the human auditory system. By incorporating a head-like scatterer between two monopole antennas, both phase and magnitude information can be utilized to estimate the direction of arrival (DOA) of a microwave signal, thus eliminating ambiguities associated with phase wrapping at high frequency, just like human ears. In addition, better DOA sensitivity is demonstrated with the incorporation of the head-like scatterer. Both numerical modeling and experimental results of a simple X-band two-monopole configuration with symmetric and asymmetric scatterers are presented. [ABSTRACT FROM PUBLISHER]

The knowledge of the directions of arrival (DOAs) of the signals impinging on an antenna receiver enables the use of adaptive control algorithm suitable for limiting the effects of interferences and increasing the gain towards the desired signals in order to improve the performances of wireless communication systems. In this paper, an innovative multi-resolution approach for the real-time DOA estimation of multiple signals impinging on a planar array is presented. The method is based on a support vector classifier and it exploits a multi-scaling procedure to enhance the angular resolution of the detection process in the regions of incidence of the incoming waves. The data acquired from the array sensors are iteratively processed with a support vector machine (SVM) customized to the problem at hand. The final result is the definition of a map of the probability that a signal impinges on the antenna from a fixed angular direction. Selected numerical results, concerned with both single and multiple signals, are provided to assess potentialities and current limitations of the proposed approach. [ABSTRACT FROM AUTHOR]

A tree structure algorithm using one-dimensional (l-D) multiple signal classification (MUSIC) algorithm is proposed to estimate the two-dimensional direction of arrivals (2-D DOAs) of coherent signals impinging on a uniform rectangular array. The basic idea of the proposed algorithm is to successively apply several times of the 1-D spatial smoothing MUSIC algorithm, in tree structure, to estimate the azimuth and the elevation angles independently. To optimally separate the receive signal, constrained spatial beamformers with adjustable null width are exploited in conjunction with the 1-D spatial smoothing MUSIC algorithm to decompose the received signal into several signals each lead by its own 2-D DOA. Performance analysis is provided to investigate the estimation bias caused by the residue signal propagating in the tree structure. [ABSTRACT FROM AUTHOR]

This paper presents the use of a minimal resource al- location network (MRAN) for the direction of arrival (DoA) estimation under array sensor failure in a noisy environment. MRAN is a sequential learning algorithm in which the number of hidden neurons are added or removed based on the input data and produces a compact network. The training for MRAN is done under no failure and no noise case and the trained network is then used when there is a failure. Thus, the need for knowing the element and the time of its failure, as. required in other methods is eliminated. MRAN's performance is compared with the conventional MUSIC algorithm and also the radial basis function neural net- work scheme developed by A. H. El Zooghby et al. under normal and failed cases. In normal case, different antenna effects like mutual coupling, nonuniform array and unequal source power have been studied under different signal to noise ratio (SNR) values. Results indicate the superior performance of MRAN based DoA estimation scheme under different antenna effects, failure conditions and noise levels. [ABSTRACT FROM AUTHOR]

We present a methodology for the direction of arrival (DOA) estimation using the induced voltages that are measured at the loads connected to electrically small tuned dipole antenna arrays illuminated by the signal of interest (SOI). The matrix pencil method is applied directly to the induced voltages to estimate the DOA of the various signals. Using electrically small tuned antennas can be advantageous as they can be placed in close proximity of each other saving the real estate and, thus, making it possible to deploy phased arrays on small footprints. When dealing with closely spaced tuned electrically small antennas, it is necessary to use a transformation matrix to compensate for the strong mutual coupling that may exist between the antenna elements. The transformation matrix converts the voltages that are induced at the loads corresponding to the feed point of the array operating in the presence of mutual coupling and other near field scatterers to an equivalent set of voltages that will be induced by the same incident wave in an uniform linear virtual array (ULVA) consisting of omnidirectional isotropic point radiators equally spaced and operating in free space. For any given incident field, the open circuit voltage developed across the feed-point of the small dipole will always be less than the open circuit voltage developed across the feed-point of the half-wavelength dipole. The difference is in the voltage developed across the loads connected to the dipole's feed-point. With the small dipole antenna, the voltage developed across the load impedance will be orders of magnitude greater than the voltage developed across the load connected to the half-wavelength dipole, even though the power captured by any conjugately matched antenna is approximately the same irrespective of their lengths. Three different scenarios are presented to illustrate the methodology. First, we consider resonant dipole elements spaced half wavelength apart, electrically small tuned an- - tenna elements spaced half wavelength apart and electrically small tuned antenna elements placed in close proximity of each other to reduce the footprint without affecting the performance of the phased array. In addition, we consider the possibility of DOA estimation using a combination of different type of electrically small antennas both uniformly and nonuniformly spaced. Numerical examples are presented to illustrate the principles of this methodology [ABSTRACT FROM PUBLISHER]

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 FROM AUTHOR]

It is shown that in the paper by N. Tayem and H. M. Kwon, the Esprit-like algorithm was derived from a rather unrealistic assumption, and therefore, this conjugate Esprit algorithm will have little utility in practice. [ABSTRACT FROM AUTHOR]