i»?Phased-array antennas are known for their capability to electronically steer a beam with high effectiveness, but beam steering is fixed in an angle for all range cells. This paper reviews frequency diverse array (FDA) antennas. Different from a phased array, an FDA uses a small frequency increment, as compared with the carrier frequency, across array elements. The use of a frequency increment generates an array factor that is a function of the angle, the time, and the range, allowing the FDA antenna to transmit the energy over the desired range and angle. In addition to analyzing FDA factor characteristics, this paper investigates FDA potential applications in range-dependent energy control and technical challenges in system implementation, with an aim to call for further investigations on the FDA.

Frequency Diverse Array Antenna: New Opportunities

Time and frequency modulated arrays have numerous application areas including radar, navigation, and communications. Specifically, a time modulated array can create a beampattern with low sidelobes via connecting and disconnecting the antenna elements from the feed network, while the frequency modulated frequency diverse array produces a range-dependent pattern. In this paper, we aim to introduce these advanced arrays to the signal processing community so that more investigations in terms of theory, methods, and applications, can be facilitated. The research progress of time/frequency modulated array studies is reviewed and the most recent advances are discussed. Moreover, potential applications in radar and communications are presented, along with their technical challenges, especially in signal processing aspects.

An Overview on Time/Frequency Modulated Array Processing

Due to the spatial sparsity caused by severe propagation loss, mm-wave channel estimation can be performed by estimating the directions and gains of the paths that have significant power. In this paper, we apply the beamspace 2-D multiple signal classification (MUSIC) method to estimate the path directions (the angles of departure and arrival) and use the least-squares method to estimate the path gains. Different from its element-space counterpart, the beamspace MUSIC method may exhibit spectrum ambiguity caused by the beamformers. In this paper, we therefore analyze the sufficient conditions on the beamformers under which the MUSIC spectrum has no ambiguity which also leads to the maximum number of resolvable path directions. Moreover, based on the uniform linear array with half-wavelength spacing, we show that the discrete Fourier transform beamformers, which are naturally analog and often employed in the mm-wave communication systems with hybrid precoding structure, can avoid the spectrum ambiguity and maximize the number of resolvable path directions. Simulation results demonstrate that the proposed 2-D beamspace MUSIC mm-wave channel estimator significantly outperforms existing estimators that are based on beam training and sparse recovery; and in the meantime, it requires much less training slots than these existing methods.

Millimeter-Wave Channel Estimation Based on 2-D Beamspace MUSIC Method

Different from phased array providing only angle-dependent transmit beampattern, frequency diverse array (FDA) employs a small frequency increment across its array elements to provide range-angle-dependent transmit beampattern. This enables the array beam to scan without the need of phase shifters or mechanical steering. Since FDA has received much attention in antenna and radar signal processing societies, it is necessary to make an overview on this interesting topic. This study introduces what FDA is and why it could be exploited for radar and navigation applications from a top-level system description and appeal to the radar signal processing and system engineering communities for more investigations on this promising array technique. The status of FDA studies is overviewed and the most recent advances of FDA radar are discussed. The basic FDA system architectures are introduced, along with performance compared to a conventional phased-array. Next, guidelines for choosing good system parameters and typical implementation schemes are provided. Finally, potential applications in range and angle estimation of targets, cognitive FDA radar and low probability of identification FDA radar are discussed, along with several technical challenges.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-rsn.2015.0464

Overview of frequency diverse array in radar and navigation applications

Suppression of radar-to-radar jammers, especially the mainbeam jammers, has been an urgent demand in vehicular sensing systems with the expected increased number of vehicles equipped with radar systems. This paper deals with the suppression of mainbeam deceptive jammers with frequency diverse array (FDA)-multiple-input multiple-output (MIMO) radar, utilizing its extra degrees-of-freedom (DOFS) in the range domain. At the modelling stage, false targets, which lag several pulses behind the true target, are considered as a typical form of mainbeam jammers. To this end the data-independent beamforming is performed to suppress false targets by nulling at the equivalent transmit beampattern with an appropriate frequency increment. However, the suppression performance degrades in the presence of transmit spatial frequency mismatch, which could be induced by quantization errors, angle estimation errors and frequency increment errors. To solve this problem, a preset broadened nulling beamformer (PBN-BF) is proposed by placing artificial interferences with appropriate powers around the nulls of the equivalent transmit beampattern. In such a way, effective suppression of deceptive jammer can be guaranteed owing to the broadened notches. At the analysis stage, numerical results in a scenario with multiple unmanned aerial vehicles (UAVs) are provided to illustrate the effectiveness of the devised data-independent BF, and the signal-to-interference-plus-noise ratio is improved compared with the conventional data-independent BF.

/pdf/suppression-of-mainbeam-deceptive-jammer-with-fda-mimo-radar-2773mf46b4.pdf

Suppression of Mainbeam Deceptive Jammer With FDA-MIMO Radar

Traditional phased array-based space-time adaptive processing (STAP) suffers from severe performance degradation in the presence of dense false-target jamming, especially mainlobe deceptive jamming. In this paper, an enhanced 3-D joint domain localized STAP method, incorporated with deceptive jamming pre-whitening in the transmit–receive spatial domain, is proposed to deal with this problem. It exploits the characteristic that the target, clutter, and deceptive jamming are discriminable in the range-angle-Doppler domain of a frequency diverse array multi-input multi-output radar. The designed pre-whitening filter has excellent performance in deceptive jamming mitigation, since it is constructed by the target-free jamming samples which are obtained by a novel strategy of spectrum feature-based jamming sample selection. As a result of taking the advantage of differences among the 3-D spectra of the true target, clutter, and deceptive jamming, the proposed sample selection strategy can extract unpredictable deceptive jamming samples efficiently even in the presence of strong true targets and heavy ground clutter. Signal-to-interference-plus-noise-ratio loss analysis and numerical examples are given to demonstrate the effectiveness of the proposed method in the dense deceptive jamming scenario.

Enhanced Three-Dimensional Joint Domain Localized STAP for Airborne FDA-MIMO Radar Under Dense False-Target Jamming Scenario

As training samples are not always identically distributed with the clutter in the cell under test (CUT) in heterogeneous environments, the estimated clutter covariance matrix for space-time adaptive processing (STAP) is not accurate, which degrades the performance of STAP To improve the performance of STAP in heterogeneous environments, this letter proposes a novel training sample selection algorithm to estimate the covariance matrix Based on the subaperture smoothing techniques, subapertures' covariance matrices are estimated, which are used to measure the similarities between the clutter covariance matrix of the CUT and the clutter covariance matrices of the training samples Training samples whose clutter covariance matrices are similar to that of the CUT are selected, leading to a better estimation of the clutter covariance matrix, and the performance of STAP improves Experimental results confirm the performance of the proposed algorithm

Training Sample Selection for Space-Time Adaptive Processing in Heterogeneous Environments

Direction finding with automatic pairing for bistatic MIMO radar

Clutter suppression for airborne FDA-MIMO radar using multi-waveform adaptive processing and auxiliary channel STAP

Space-time adaptive processing (STAP) techniques have achieved good performance when applied to side-looking airborne radar (SLAR) where the ground clutter is relatively stationary, but due to severe range dependence, the performance is not so good in non-SLAR particularly when the pulse repetition frequency is high enough to induce range ambiguity. Because the short-range clutter Doppler for non-SLAR varies rapidly with range while the long-range clutter Doppler varies gently, at the presence of range ambiguity, the clutter in different ambiguous ranges does not coincide. The clutter range dependence mitigation for the case of range ambiguity is the major problem that this letter intends to solve. A subarray synthesis algorithm with prefiltering in elevation is presented. By this technique, the ambiguous short-range clutter is eliminated before STAP; therefore, the clutter range dependence is alleviated, and then, the STAP performance will be improved greatly.

Jianxin Wu

Papers

Enhanced Three-Dimensional Joint Domain Localized STAP for Airborne FDA-MIMO Radar Under Dense False-Target Jamming Scenario

Training Sample Selection for Space-Time Adaptive Processing in Heterogeneous Environments

Direction finding with automatic pairing for bistatic MIMO radar

Clutter suppression for airborne FDA-MIMO radar using multi-waveform adaptive processing and auxiliary channel STAP

Short-Range Clutter Suppression for Airborne Radar by Utilizing Prefiltering in Elevation