Transmit Subaperturing for Range and Angle Estimation in Frequency Diverse Array Radar

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.

Frequency diverse array (FDA) radar provides new application capabilities and potentials due to its range-dependent transmit array beampattern. As radar ambiguity function (AF) is an effective tool to analyze the range (time delay) and Doppler resolution characteristics of various radar systems, this paper derives and analyzes the FDA radar AF characteristics in a general way. These characteristics provide some insights into optimal FDA parameters design and are helpful for comparing delay-Doppler performance of the FDA radar with conventional phased-array radar. Furthermore, this paper optimally designs the frequency increments for better FDA radar AF with the simulated annealing algorithm. The optimization object is to minimize undesired range-angle-dependent sidelobes, so that we can focus the transmit beampattern to desired range-angle sections and simultaneously null undesired interferences. All the derived expressions and methods are verified by numerical results. The results also validate that the FDA indeed offers advantages over traditional phased array in delay-Doppler radar applications.

Frequency diverse array (FDA) radar provides new application capabilities and potentials due to its range-dependent transmit array beampattern, but the FDA using linearly increasing frequency increments produces a range and angle coupled beampattern. This paper proposes a uniform linear array (ULA) FDA using Costas sequence modulated frequency increments to produce random-like energy distribution in the transmit beampattern to decouple the range-angle-dependent of targets. In doing so, the range and angle of targets can be solely estimated through matched filtering and subspace decomposition algorithms in the receiver signal processor. Numerical results show that the proposed scheme outperforms the standard FDA in focusing the transmit energy, especially in the range dimension.

Frequency diverse array (FDA) radar with non-uniform frequency increasing offset is the most potent method to decouple the range-angle dependent beampattern and form a dot-shaped beampattern. However, little known empirical research has focused on the joint range-angle estimation of FDA radar with non-uniform frequency offset. The aim of this letter is to investigate a decoupling model and propose a range-angle estimation algorithm of FDA radar with uniform or non-uniform frequency offset. First, the blind source separation algorithm is performed to process the observation signal and obtain the separation matrix. Then, the array manifold matrix of observation signal is estimated by separation matrix correction. Next, a decoupling model is formulated to separate the range and angle component from the estimated array manifold matrix. Finally, an independent range and angle estimation algorithm via pulse-pair technique is developed. The numerical simulations are performed to indicate the effectiveness and superior performance.

In recent years, frequency diverse array (FDA) radar with uniform interelement frequency offset has been proposed and investigated. It has been shown that the beampattern of this form of radar is range-angle-dependent, and it exhibits maxima at multiple range values. Due to the multiple maxima, a number of reflectors located at any of the maxima can interfere the target-return, thus deteriorating signal-to-interference-plus-noise ratio (SINR). In this letter, we have proposed FDA with logarithmically increasing frequency offset to achieve a beampattern with a single maximum at the target location. A transmitter for the proposed radar system has been described and analyzed mathematically. Furthermore, the transmit beampattern has been derived and plotted and compared to that of phased-array radar (PAR) and the existing forms of FDA employing uniform frequency offset.

Frequency diverse array (FDA) radar for joint range and angle estimation has drawn much attention, but Doppler estimation is often ignored in the literature. In this paper, we propose joint range, angle and Doppler estimation for FDA-multiple-input multiple-output (FDA-MIMO) radar. In order to reduce the computation complexity, the Doppler is independently estimated firstly by the extended invariance principle instead of maximum likelihood (ML) estimator. Next, the range and angle are estimated separately with the unstructured model. That is, the three-dimensional search problem of joint range, angle and Doppler estimation is simplified into three one-dimensional search problems. The superiorities of the proposed method over conventional ML algorithms are verified by numerical results.

Coherent receiver design and aperture expansion beamforming for frequency diverse array radar

To meet the urgent demand of low-observable moving target detection in complex environments, a novel method of Frequency Diverse Array (FDA) radar signal processing method based on Space-Rang-Doppler Focusing (SRDF) is proposed in this paper. The current development status of the FDA radar, the design of the array structure, beamforming, and joint estimation of distance and angle are systematically reviewed. The extra degrees of freedom provided by FDA radar are fully utilizsed, which include the Degrees Of Freedom (DOFs) of the transmitted waveform, the location of array elements, correlation of beam azimuth and distance, and the long dwell time, which are also the DOFs in joint spatial (angle), distance, and frequency (Doppler) dimensions. Simulation results show that the proposed method has the potential of improving target detection and parameter estimation for weak moving targets in complex environments and has broad application prospects in clutter and interference suppression, moving target refinement, etc..

Different from traditional phased array radar, frequency diverse array (FDA) radar uses a small frequency increment across the array elements; thus, the beampattern of FDA can be performed in both range and angle domains. Nevertheless, the transmit beampattern of conventional FDA is coupled in range and angle domains, which is not suitable for target detection. In this paper, we propose a multi-carrier FDA framework to generate the range-angle-decoupled beampattern. To focus the transmit energy at the desired position and suppress the interference at the undesired position, we then propose a beampattern synthesis approach based on Lagrange programming neural network, which can be implemented by hardware easily. Numerical simulation results demonstrate the effectiveness of the proposed method.

The range-angle coupling in Frequency Diverse Array (FDA) radar is uniquely solved. The array operates as a linear pulsed FDA in the transmission (TX) mode while it operates as a pulsed phased array (PA) in the receiving (RX) mode. The duration of FDA TX pulse is chosen as minimum. A novel radar concept is proposed. It is shown that FDA waveform can be further compressed by matched filtering (MF).

In this article, a novel technique is applied to achieve time-invariant spatial focusing performance for static and moving target by applying an irregular frequency modulation method to a frequency diverse array (FDA) waveform transmission system. The FDA radar can improve the anti-noise ability of the system by the means of concentrating the waveform energy on the required range or angle sector. However, the synthesized waveform has high sidelobes and is time-varying during the transmission waveform. In view of the above problems, a new waveform synthesis model of rang compensation frequency diverse array (TMRC-FDA) is established and the effect of parameter $$\varsigma$$ on the waveform synthesis is analyzed. A new accelerated particle swarm optimization algorithm is used to optimize the distribution of parameter $$\varsigma$$. The numerical simulation results show that the proposed TMRC-FDA waveform synthesis method achieves the goal of time invariance and reduces the waveform sidelobes. At the same time, it has better performance in terms of waveform focus compared to log-frequency offset FDA based on time-modulated and fixed parameters $$\varsigma$$.

Since a nested phased array cannot directly estimate the range of targets due to range ambiguity, this paper proposes a nested array with diverse time-delayers for target range and angle estimation. The essence is to construct a new array structure by systematically nesting two uniform linear arrays through diverse time-delays which yields a range-dependent receiving array beampattern. Using second-order statistics of the received data, it is capable of providing a dramatic increase of the system degrees-of-freedom which means more sources can be resolved. More importantly, target range and angle can be estimated by exploiting the range-dependent array beampattern. The performance improvements are evaluated by examining the Cramer-Rao lower bounds.

A novel high-frequency surface wave radar (HFSWR) has been proposed, which is based on the principles of synthetic impulse and aperture processing. It uses multiple omnidirectional transmit antennae to simultaneously transmit a set of orthogonal waveforms, and the echoed signals are received and processed by one or multiple reception arrays. Although the transmit beam pattern is omnidirectional, by proper processing, the received signal multiple equivalent directional transmit beam patterns can be simultaneously formed. The signal processing scheme to obtain equivalent directional transmit beam pattern is investigated in detail. Considerations for system parameter selection to achieve an overall best system performance are proposed and discussed. In particular, for the novel HFSWR, target range and angle estimates are coupled together due to the orthogonality of the transmitted waveforms. A necessary and sufficient condition on the transmit antenna array geometry and transmit frequencies, which ensures that target range and angle estimates are uncoupled, is presented.

Frequency diverse array (FDA) can provide a range-angle-dependent beam that can be controlled by using a frequency increment between adjacent elements, but the frequency increment is fixed in a basic FDA, which restraints the performance improvement in different environments significantly. In this letter, we propose an adaptive frequency offset selection scheme to control the transmit beam direction by choosing the frequency offset adaptively. In doing so, we design the frequency increment of transmit signal in each step by maximizing the output signal-to-interference-plus-noise ratio (SINR) criteria. Our proposal can adaptively adjust the beam direction to match the current target angle and range sector. The effectiveness of the proposed scheme is verified by simulation results.

This paper studies the optimal detection performance of the standard frequency diverse array (FDA) radar and FDA multi-input multioutput (FDA-MIMO) radar in Gaussian clutter and noise. Array signal processing scheme at the receiver is firstly designed to obtain the array steering vector containing range, azimuth, and frequency increment. For the two array configurations, namely, collocated transmit-receive and collocated transmit distributed receive, the likelihood ratio test statistics and the test statistic distributions are derived in the Neyman–Pearson sense. It is then investigated how the number of array elements influences the detection performance of various radar systems at low signal-to-noise ratio (SNR). Several numerical simulations are carried out to demonstrate that the performance improvement is hard for MIMO and FDA-MIMO by only increasing the number of transmit elements, while it is achievable for the FDA. The paper finally makes a comparative analysis for detection performances of five radar configurations under different SNRs.