Null Steering Technique Research Articles

A Small-Aperture, Ultrawideband HF/VHF Direction-Finding System For Unmanned Aerial Vehicles

A small-aperture HF/VHF direction-finding (DF) system is presented. The antenna aperture occupies a cubical volume with electrical dimensions of $0.0107 \lambda _{0} \times 0.0107 \lambda _{0} \times 0.0033 \lambda _{0}$ , where $\lambda _{0}$ is the free-space wavelength at its lowest frequency of operation and is used for DF over a 100 : 1 bandwidth. The antenna array is composed of four hexagonal-cone-shaped monopoles on a finite ground plane. The antennas are loaded with top hats and exploit the near-field parasitic elements to obtain the realized gain values close to the theoretical limits over their entire frequency band of operation. A simple, yet robust null-steering technique is used both on its own and also in conjunction with the multiple signal classification (MUSIC) algorithm, as signal-processing methods, to detect the direction of arrival of the wave over the entire 100 : 1 bandwidth. A scaled prototype of the antenna was fabricated and used to perform DF experiments in the field in the VHF-UHF band. The DF system demonstrates azimuthal bearing accuracies comparable to the Cramer–Rao-Lower-Bound values calculated for the measured received signal-to-noise ratios over the whole frequency range. The DF performance of the full-scale version of this antenna, when mounted on a representative unmanned aerial vehicle (UAV), is also examined through computer simulations. It is demonstrated that the proposed DF system can be effectively used in a typical UAV to perform DF in the HF, VHF, and low-UHF bands.

Novel sensing and joint beam and null steering based resource allocation for cross-tier interference mitigation in cognitive femtocell networks

Cognitive radios and femtocell networks are gaining much popularity due to the formers ability to carry out unlicensed transmission in licensed bands and the latter’s ability to increase the indoor capacity by bringing the transmitter and receiver closer to each other. The combination of the two provides us with a joint benefit of unlicensed transmission and increased system capacity. However, deployments of such cognitive radio based femtocell networks (CRFN) can pose some severe cross-tier interference towards the primary system. In this paper, we propose a novel spectrum sensing technique based on Uniform Linear array (ULA) combined with Genetic Algorithm to detect not only the presence of a Primary user but also estimate its parameters such as range, frequency, angle of arrival and amplitude. Furthermore, a joint beamforming and null steering technique is proposed to maximize the signal to interference plus noise ratio (SINR) towards the secondary user equipment (SUE) while minimizing the interference towards the primary Base Station (PBS). We are reutilizing the sensed uplink (UL) frequency of the Primary users for the downlink (DL) transmission; the cognitive femtocell base station (CFBS) calculates such weight vector that aims at sending the maximum signal power towards the femto user and null in the direction of PBS.

A processing scheme for LFM-based waveform MIMO SAR with digital beam-forming in elevation

Multiple-input multiple-output synthetic aperture radar (MIMO-SAR) is of great potential for future high-resolution and wide swath imaging. However, its development is limited as the condition of orthogonality among the multiple waveforms received simultaneously is hard to satisfy. In this paper, a class of linear frequency modulation-based MIMO waveforms is introduced to settle this problem and correspondingly an effective processing scheme is proposed based on digital beam forming in elevation. In this scheme, for separating the mixed signals, null steering technique is employed to extract one waveform from the others. The main advantages of the proposed scheme include transmitting simultaneously, good inter-orthogonality between different waveforms with identical frequency range, separating them with mutual interference cancellation and good imaging quality. The main procedure of this scheme is derived and described in detail in the paper. Furthermore, an example MIMO-SAR system is provided for numerical simulation and the simulation results are obtained to validate the proposed scheme.

Cognitive null steering in frequency diverse array radars

Null steering has been a challenge in radar communications for the past few decades. In this paper, a novel cognitive null steering technique in frequency diverse array radars using frequency offset selection is presented. The proposed system is a complete implementable framework that provides precise and deep null placement in the range and angle locations of the interference source. The proposed system is cognitive such that the transmitter and receiver are connected via a feedback loop. System extracts interference source location parameters from the radar scene using Multiple Signal Classification, a super resolution direction of arrival estimation technique. Neural networks known for minimum computation time, and good non-linear and non-parametric approximation have been utilized for prediction of next location of the interference source. Simulation results validate the proposed frequency offset selection by demonstrating precise and deep nulls at the desired locations.

Beampattern control of a microphone array to minimize secondary source contamination.

A null-steering technique is adapted and applied to a linear delay-and-sum beamformer in order to measure the noise generated by one of the propellers of a 1/8 scale twin propeller aircraft model. The technique involves shading the linear array using a set of weights, which are calculated according to the locations onto which the nulls need to be steered (in this case onto the second propeller). The technique is based on an established microwave antenna theory, and uses a plane-wave, or far field formulation in order to represent the response of the array by an nth-order polynomial, where n is the number of array elements. The roots of this polynomial correspond to the minima of the array response, and so by an appropriate choice of roots, a polynomial can be generated, the coefficients of which are the weights needed to achieve the prespecified set of null positions. It is shown that, for the technique to work with actual data, the cross-spectral matrix must be conditioned before array shading is implemented. This ensures that the shading function is not distorted by the intrinsic element weighting which can occur as a result of the directional nature of aeroacoustic systems. A difference of 6 dB between measurements before and after null steering shows the technique to have been effective in eliminating the contribution from one of the propellers, thus providing a quantitative measure of the acoustic energy from the other.

Partially adaptive array design using DOA estimation and null steering

The authors propose a novel approach to designing a partially adaptive array using direction-of-arrival (DOA) estimation and a null steering technique. The DOAs of the interferences are estimated using some selected outputs of the signal blocking matrix to form the ensemble signal blocking correlation matrix. The eigenvector corresponding to the minimum eigenvalue of the ensemble signal blocking correlation matrix is then used to form nulls in the spatial spectrum which are corresponding to the incident angles of interferences. Synthesised nulls are then placed in the mainbeam to cancel all the interference signals.

Normal mode wave-number estimation using a towed array

In the analysis of the null-steering technique of normal mode filtering presented by the authors previously [H. M. Chouhan and G. V. Anand, J. Acoust. Soc. Am. 89, 735–744 (1991)], it was assumed that the modal wave numbers km are known a priori. In practice, the waveguide parameters upon which the modal wave numbers depend are normally not known and are difficult to measure accurately. In order to use the null-steering technique of mode filtering, or in applications such as source localization and waveguide characterization, it is necessary to estimate the modal wave numbers. An estimation procedure using a towed horizontal line array is presented in this article. The method entails eigendecomposition of the range-averaged array signal correlation matrix. Range-averaging effectively decorrelates the normal mode signals and enables the use of high-resolution spectral estimation techniques such as MUSIC for estimating the modal wave numbers. Simulation results for the Pekeris model of the ocean are presented.

New GSC beamformer employing DOA estimation and null steering for linear array

In the Letter, a new linear array GSC beamforming structure employing DOA estimation and the null steering technique is presented. It has the advantages that the quiescent response of the beamformer can be closely maintained and that the adaptive dimension of the beamformer can be reduced to about half of that of the full processor. Theoretical justification of the proposed approach is given and the performance achievable is investigated via computer studies. >

A new technique of acoustic mode filtering in shallow sea

A new technique of normal mode filtering in an isovelocity channel using an equispaced linear array is presented. Filtering is achieved by steering nulls in the directions of arrival of the quasiplane wave pairs associated with the normal modes that are to be rejected. A theoretical analysis of the problem is presented for both horizontal and nonhorizontal arrays. The minimum number of array elements and their weighting coefficients are determined for each case. The null-steering technique offers several advantages over other techniques of mode filtering. The weighting coefficients are independent of the array depth. A horizontal or vertical array of 2M − 1 elements with real weighting coefficients or a horizontal array of M elements with complex weighting coefficients is sufficient to select one mode from a set of M modes. There is no constraint on interelement spacing. The filter performance is equally good under all types of bottom conditions. If a horizontal array is deployed, this method of mode filtering can be used in a horizontally stratified channel also. The null-steering method gives perfect filtering if the orientation of the array and the modal wave numbers are known exactly. An analysis of the sensitivity of the mode filter to errors in the assumed values of these parameters is also presented.

A new technique of acoustic mode filtering in shallow sea

A new technique of normal mode filtering in an isovelocitcyh anneal singe n equispaced line Array is presented. Filtering is achieved by steering nulls in the directions of arrival of the Quasi plane wave pairs associated with the normal modest hat are to be rejected .A theoretical Analysis of the problem is presented for both horizontal and non-horizontal arrays. The Minimum number of array elements and their weighting coefficients e determined for each Case. The null-steering technique offers several advantages over other techniques of mode filtering. The weighting coefficients are independent of the array depth. A horizontal or vertical array of 2M - 1 elements with real weighting coefficients or a horizontal array of M elements with complex weighting coefficient is sufficient to select one mode from a set of M modes. There is no constraint on imperilment spacing. The filter performance is equally good under all types of bottom conditions. If a horizontal array is deployed this method of mode filtering can be used in a horizontally stratified channel also. The null-steering method gives perfect filtering if the orientation of the array and the modal wave numbers are known exactly. An analysis of the sensitivity of the mode filter to errors in the assumed values of these parameters is also presented.

A Combined Programmed and Adaptive Null Steering Technique

The described method of antenna null steering makes use of calibrated and stored weight distributions for each solid angle increment of a radar search volume. Based upon direction measurement to an interference source, the appropriate weight distributions are calculated for each relative antenna position. In addition to the programmed part an adaptive inner loop is included for fine null adjustment. The major parameters and characteristics of the null steering technique are studied and evaluated through computer simulation programs.

Simple null-steering antenna for frequency-agile radars

A null-steering technique is described that is applicable to mechanically scanned antennas that can be synthesised as a number of identical subarrays. The process generates a substantially stationary polar pattern in space in which the nulls are locked to the directions of the interfering sources. The system is particularly suited to frequency-agile radars. The average system loss is 3 dB per source suppressed.