Conventional Phased Array Radar Research Articles

Colocated MIMO Radar Waveform-Array Joint Optimization for Sparse Array.

Colocated multiple-input multiple-output (MIMO) radar can transmit a group of distinct waveforms via its colocated transmit antennas and the waveform diversity leads to several advantages in contrast to conventional phased-array radar. The performance depends highly on the degrees available, and element spacing can be deemed as another source of degrees of freedom. In this paper, we study the joint waveform and element spacing optimization problem. A joint waveform and array optimization criterion is proposed to match the transmit beampattern, the suppression range, and the angular sidelobes, under the constraints of minimal element spacing and total array aperture. Meanwhile, the effect of receive beamforming on suppressing mutual correlation between returns from different spatial directions is also incorporated into the optimization criterion. The optimization problem is solved by the sequential quadratic programming algorithm. Numerical results indicate that with more degrees of freedom from array spacings, colocated MIMO radar achieves a better transmit beampattern matching performance and a lower sidelobe level, compared with a fixed half-wavelength spaced array, but the benefits from additional degrees of freedom from array spacing optimization have a limit.

FDA-MIMO radar detection for independent and nonidentically distributed fluctuating targets

Due to its range-dependent target response, frequency diverse array multiple-input multiple-output (FDA-MIMO) radar systems enable superior detection capabilities as compared with conventional phased-array radars. This paper proposes an incoherent detector for airborne FDA-MIMO radar to detect independent but possibly non-identically distributed fluctuating targets. For FDA-MIMO, variations in the target's flight attitude result in different distributions of the target radar cross section (RCS) for different pulses. In order to allow the FDA-MIMO RCS to vary due to the large frequency increments across the array antennas, we formulate a statistical FDA-MIMO RCS model for the reflected signal. We further derive the analytical detection probability for an FDA-MIMO radar measuring independent and non-identically distributed fluctuating targets. The resulting expression takes the form of an infinite sum; to enable evaluation in practice, an upper bound of the truncation error is also derived. The proposed method and all theoretical analysis are verified by numerical results, which provides theoretical basis and guidance for FDA-MIMO system implementation and design in independent and non-identically distributed fluctuating targets detection.

The deception effect of FDA counteracting amplitude-comparison monopulse reconnaissance

Conventional phased array radar can direct the high-gain beam, resulting in high visibility under the reconnaissance system and even direct exposure to the enemy. As an amplitude-based direction-finding system, the amplitude monopulse system can recognize the traditional radar signals. However, there are still some issues with the countermeasures of the monopulse system, so it is necessary to find better styles of jamming. Since the beam of the frequency diverse array (FDA) signal at a certain angle varies with the range, we studied the deception effect of the FDA against the amplitude-comparison monopulse direction-finding system. By analyzing the established FDA-based direction-of-arrival location deception model against the amplitude-comparison monopulse system, we found that the introduction of the frequency increment might cause the FDA signals to deceit the monopulse system in principle. Then, to indicate the superiority of our deception method, we obtain the instantaneous signal-to-noise ratio, Cramér–Rao lower bound, root-means-square error, and variation. Numerical simulation results demonstrate that the FDA can generate deceptive signals to counteract the amplitude-comparison monopulse direction-finding system.

A Transmitter Design for the Multi-beam CC-OFDM Azimuth Scanning MIMO Radar

A conventional phased array radar provides a high gain by transmitting coherent signals over a large number of antenna elements. However, it does not provide a full time-on-target because it produces beams that scan the whole angular view of the radar. This paper describes an implementation of circulating codes (CC) in azimuth scanning MIMO radar that adopts OFDM waveform. The CC MIMO technique can produce a beam toward a certain angle by applying an appropriate delay difference between the MIMO antennas that transmit the same waveform across the azimuth. The adoption of OFDM waveform in the CC MIMO radar, gives the possibility to apply the phase difference for beamforming in the frequency domain of the OFDM signals. CC-OFDM MIMO with spectrum division into sub-bands and use of orthogonal codes, allow for simultaneous transmission of orthogonal OFDM symbols that generate orthogonal beams toward different angles. This strategy makes it possible for the radar system to have multiple and simultaneous beamforming to provide a full-time coverage. Furthermore, the transmission of a series of orthogonal OFDM symbols in each beam enables the detection of long-range targets. By taking as an example the design of long-range surveillance radar, the paper discusses further issues pertinent to the implementation of the CC OFDM MIMO radar, including the beam squinting problem and its remedy, arrangement of the multiple beams, the radar system structure, use of Golay codes for orthogonalization and PAPR mitigation, use of software-defined radar that eliminates the need of amplitude and phase control in each transmitter, transmit scheduling for multi-beam long-range target detection, and multidimensional ambiguity function analysis. The latter indicates that the CC OFDM MIMO radar can achieve a high resolution in angle, range, and velocity, with beam isolation of -30 dB for neighboring beams and co-channel beams.

Three-Dimensional Parameter Estimation of Uniform Circular Frequency Diverse Array Radar With Two-Stage Estimator

Frequency diverse array (FDA) radar has been studied by many researchers due to the range-angle dependent beampattern which is different from the conventional phased array radar. However, the contribution of previous studies has received little attention within the circular frequency diverse array radar and no known empirical research has focused on investigating the three-dimensional (3D) parameter estimation (azimuth, elevation, and range). In this paper, we proposed a two-stage estimation algorithm for 3D parameter estimation of uniform circular frequency diverse array (UC-FDA) radar and firstly demonstrate the capability and performance of 3D parameter estimation. The first step of the two-stage estimation is the estimation of azimuth and elevation from the beamspace which is transformed from the element space by the beamformer. The second step is to substitute the angle obtained in the first step into the transmitting steering matrix for the range estimation. Cramér-Rao bound (CRB) that is used to evaluate the 3D parameter estimation performance is derived to compare. Theoretical analysis and numerical simulation demonstrate that the UC-FDA radar has superior 3D parameter estimation performance and the effectiveness of the proposed two-stage estimation algorithm. The research findings can contribute to a better 3D localization method and represent an additional step towards developing multi-dimensional localization and interference suppression for UC-FDA radar.

Multi-Aperture Space-Time Transmit and Receive Design for MIMO Radar

We consider the joint transmit and receive design for multi-input multi-output radar with slow-time processing. The radar employs multiple transmit apertures to improve diversity. The design parameters include the spatial transmit code for each aperture, which varies from pulse to pulse to provide Doppler shaping, and the space-time receive filter, to jointly optimize the radar output signal-to-interference-and-noise ratio (SINR). To relieve the dependence on specific target parameters as required by some prior methods, we use the average SINR, averaged with respect to the target location/Doppler uncertainties, as the design metric. Simulation results show that our proposed multi-aperture solution outperforms a previous single-aperture based space-time transmit and receive design as well as the conventional phased-array radar.

Joint beam selection and resource allocation for cognitive multiple targets tracking in MIMO radar with collocated antennas

Compared with conventional phased array radar, the collocated multiple-input multiple-out (C-MIMO) radar is envisioned to offer greater freedom in waveform design and can simultaneously manage different beams to track multiple targets in the simultaneous multi-beam (SM) working mode. In this study, aiming at the multiple targets tracking (MTT) problem, a joint beam selection integrated with power and bandwidth allocation (JSPBA) scheme for the C-MIMO radar is proposed in the SM mode. By incorporating the modified particle filter, the predicted conditional Cramer–Rao lower bound (PC-CRLB) is calculated, which gives a measure of the best achievable performance for targets tracking. Then, the optimisation model is established with the aim of improving the worst case of PC-CRLB to achieve better performance of the worst-case tracking. Next, a three-step solution method is proposed to solve the JSPBA problem by converting the non-convex problem into a series of convex problems. At last, after the solutions are fed back to the control centre to guide the beam generation in the next tracking epoch, a cognitive tracking system is established. Simulation results confirm the superiority of the proposed method in improving the MTT performance in C-MIMO radar system.

Transmit Beampattern Synthesis for Frequency Diverse Array With Particle Swarm Frequency Offset Optimization

Different from the conventional phased array radar, a frequency diverse array (FDA) applies additional frequency shift across the array elements, yielding a range-angle-dependent transmit beampattern that provides a huge potential opportunity for target estimation and range-dependent clutter and interference suppression. Generally, the conventional FDA employs a linear small frequency increment, forming a time-varying “S”-shape range-angle beampattern, which is range-angle coupled. In this article, a range-angle transmit beampattern synthesis method for FDA based on frequency offset optimization is proposed. Particle swarm optimization (PSO) is adopted in the FDA element frequency increment design to achieve a dot-shaped transmit beampattern. Meanwhile, matched weights are designed for time-modulation compensation. Numerical simulation results demonstrate that the proposed approach achieves better time-invariant performance over other FDA frameworks.

Joint time-space resource allocation and waveform selection for the collocated MIMO radar in multiple targets tracking

Compared with conventional phased array radar, the collocated MIMO radar can effectively control the transmitting beam width by means of sub-array division. Therefore, different targets may be illuminated simultaneously with one beam in the collocated MIMO radar, providing greater freedom degree in resource management. The joint time-space resource allocation and waveform selection optimization model for the collocated MIMO radar is proposed in this paper, where the objective function that takes both the system resource and tracking precision into consideration is minimized under the guarantee of effective targets detection. Through solving the proposed optimization model, the corresponding joint optimization algorithm is obtained. The optimal sampling period, sub-array number, illuminated targets set, transmitting energy and transmitting waveform parameters combination is chosen adaptively, where the former four realize the time-space resource allocation and the last one realizes the waveform selection. Simulation results demonstrate that the validity and effectiveness of the proposed algorithm.

An Implementation Scheme of Range and Angular Measurements for FMCW MIMO Radar via Sparse Spectrum Fitting

The work presented in this paper is about implementing a frequency-modulated continuous wave (FMCW) multiple-input multiple-output (MIMO) positioning radar and a sparse spectrum fitting (SpSF) algorithm for range and angular measurements. First, we designed a coherent FMCW MIMO radar system working in the S-band with low power consumption that consists of four transmitter and four receiver antennas and has the ability to extend its virtual aperture; thus, this system can achieve a higher resolution than conventional phased array radars. Then, the SpSF algorithm was designed for estimating the distance and angle of the targets in the FMCW MIMO radar. Due to the fact that the SpSF algorithm can exploit the spatial sparsity diversity of a signal, the SpSF algorithm that is applied in the designed MIMO radar system can achieve a better estimation performance than the multiple signal classification (MUSIC) and Capon algorithms, especially in the context of small snapshots and low signal-to-noise ratios (SNRs). The simulated and experimental results are used to prove the effectiveness of the designed MIMO radar and the superior performance of the algorithm.

Adaptive simultaneous multibeam resource management for colocated MIMO radar in multiple targets tracking

Different from conventional phased array radar, the colocated multiple-input multiple-output (MIMO) radar can form multiple beams simultaneously through transmitting orthogonal waveforms by its sub-arrays. The illuminating mode of multiple beams can be changed with the sub-array number, providing greater flexibility in system resource management. In this paper, a novel resource management optimization model for the colocated MIMO radar in multiple targets tracking is proposed, where the average time and energy resource consumption of the system are minimized under the guarantee that the illuminated targets can be effectively detected. When solving the proposed optimization problem, the adaptive simultaneous multibeam resource management (ASMRM) algorithm for the colocated MIMO radar is obtained, where the sub-array number, the beam directions, the system sampling period, the transmit waveform energy and the working mode can be controlled adaptively. Simulation results demonstrate that the working parameters of the colocated MIMO radar can be changed effectively. Furthermore, in terms of the average system resource consumption, the proposed algorithm is superior to the algorithms with fixed parameters while realizing the effective multiple targets tracking.

Detection performance analysis of hybrid MIMO radar

This study focuses on the detection performance analysis of a notional hybrid MIMO radar, which can be seen as a combination of the phased-array and distributed MIMO radars. The signal model of the proposed hybrid MIMO radar is re-derived using the Rayleigh target model. In the sense of Neyman–Person principle, the optimal detector is developed. Then, the detection performance bound is derived based on the relative entropy. Theoretical derivation and numerical examples show that, for most common signal-to-noise ratio (SNR) cases, the detection performance of the hybrid MIMO is better than the conventional phased-array and MIMO radars. Furthermore, for the high SNR case, the proposed system configuration with larger number of sub-arrays performs better detections, while in the low SNR case, that with smaller number of sub-arrays performs better detections.

Two-stage ESPRIT for unambiguous angle and range estimation in FDA-MIMO radar

Frequency diverse array (FDA) offers potential applications for joint range and angle estimation, but ambiguous estimates may be generated due to its range-angle coupling and time-variant beampattern. This problem can be addressed by jointly utilizing FDA and multiple-input multiple-output (MIMO) radar, but only multiple signal classification (MUSIC) algorithm was considered in the FDA literature. In order to avoid high computational complexity in the MUSIC algorithm due to the required 2-D peak searching, in this paper, we propose a two-stage estimating signal parameters via rotation invariance technique (ESPRIT) algorithm for FDA-MIMO radar to estimate both range and angle of targets, along with the proposed pairing method for unambiguous estimates. Moreover, closed-form expressions of the mean squared error (MSE) and Cramér-Rao lower bound (CRLB) for angle and range estimations are also derived. All proposed methods and derivations are verified by both theoretical analysis and numerical results, which show the superiority of FDA-MIMO radar over conventional phased-array radar and MIMO radar in target localization.

Multi-dimensional Ambiguity Function for Coherent Pulsed-LFM FDA Radar

Frequency diverse array (FDA) which employs a small frequency increment between adjacent elements can provide a range-angle-dependent beampattern with new application potentials. In this paper, the proposed coherent pulsed-LFM FDA scheme enables full spatial coverage with a single transmit waveform, wherein the transmit beamforming can be performed at the receiver with flexible signal processing means. In the coherent pulsed-LFM FDA radar configuration, the range sidelobe level (SLL) can be reduced significantly. Analyses of the multi-dimensional ambiguity function in terms of the low sidelobe characteristics, spatial coverage capability, and resolution properties in range, angle and Doppler domains are given by ambiguity function profiles. Comparisons with conventional phased array and MIMO (Multiple-Input Multiple-Output) radar are presented in the simulation. Results demonstrate the superiority of the proposed approach in angular coverage and sidelobe level with simplicity in engineering.

Time-Invariant Joint Transmit and Receive Beampattern Optimization for Polarization-Subarray Based Frequency Diverse Array Radar

We propose a polarization-subarray based frequency diverse array (FDA) radar with the subarray-based FDA as the transmit (Tx) array and the polarization-sensitive subarray-based FDA (PSFDA) as the receive (Rx) array. The subarray-based FDA has the capability to achieve a single maximum beampattern at the target location, while the PSFDA can provide an extra degree of freedom to further suppress the interference and, thus, to improve the radar's signal-to-interference-plus-noise ratio (SINR). The time-dependent frequency offsets are designed for the proposed radar to realize the time-invariant beampattern at the desired target location over the whole pulse duration. To further improve the target detection performance, the time-invariant joint Tx–Rx beampattern design is considered based on the output SINR maximization. To effectively solve the nonconvex output SINR maximization problem, a suboptimal alternating optimization algorithm is proposed to iteratively optimize the FDA Tx beamforming, the PSFDA spatial pointings, and the PSFDA Rx beamforming. Numerical experiments illustrate that the time-invariant and single-maximum joint Tx–Rx beampattern at the target location is achieved. Moreover, compared to the basic FDA and logarithmic frequency offset FDA as well as conventional phased array radars, the proposed polarization-subarray based FDA radar achieves a significant SINR improvement, particularly when the desired target and the interferences are spatially indistinguishable.

Frequency diverse array radar signal and data processing

The frequency diverse array (FDA) radar is re-explored with a special focus on how the FDA data should be correctly and efficiently processed to maximise its full potential and capacity of search and detection. Some features, such as its extraordinary Doppler tolerance and ability of resolving range ambiguity without using multiple pulse repetition frequencies, are first demonstrated. The extension of non-linear and random frequency increments is proposed in order to suppress range sidelobes and increase range resolution. Wide-area search and detection is achieved by processing the same received data set without a need of electronic scan and repeat pulse transmission and reception. This not only greatly simplifies the radar design and operation but also makes radar quieter and stealth. The limitation of the FDA radar is its sacrificed detection range. Therefore, in comparison with the conventional phased-array radar, the detection area is fan-area-like for the former and pencil-beam-like for the latter, making two radars (or radar operation modes) complementary.

FDA Radar Ambiguity Function Characteristics Analysis and Optimization

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.

FDS-MIMO Radar Low-Altitude Beam Coverage Performance Analysis and Optimization

Detecting and tracking low-flying targets at low-grazing angles is a difficult task due to the possible beam split and radar blind area. To alleviate this issue, we propose a frequency diverse subaperturing multiple-input multiple-output (FDS-MIMO) radar with optimized low-altitude beam coverage performance. A specular echo model is first formulated in the presence of multipath propagation together with a closed-form expression for the joint transmit–receive beampattern. Then, a perturbational echo model is developed for anomalous terrain. Moreover, a notional multipath mitigation region concept is defined together with the corresponding boundary conditions. The FDS-MIMO radar beam coverage capability is evaluated by the low observability rate. Furthermore, the FDS-MIMO radar low-altitude beam coverage is improved according to the solutions of boundary conditions, and an adaptive frequency offset design strategy is proposed for the changing environment. Both theoretical analysis and numerical results demonstrate that the optimized FDS-MIMO radar outperforms conventional phased-array radar and MIMO radar in terms of low-altitude beam coverage performance.

Radar wideband digital beamforming based on time delay and phase compensation

ABSTRACTIn conventional phased array radars, analogue time delay devices and phase shifters have been used for wideband beamforming. These methods suffer from insertion losses, gain mismatches and delay variations, and they occupy a large chip area. To solve these problems, a compact architecture of digital array antennas based on subarrays was considered. In this study, the receiving beam patterns of wideband linear frequency modulation (LFM) signals were constructed by applying analogue stretch processing via mixing with delayed reference signals at the subarray level. Subsequently, narrowband digital time delaying and phase compensation of the tone signals were implemented with reduced arithmetic complexity. Due to the differences in amplitudes, phases and time delays between channels, severe performance degradation of the beam patterns occurred without corrections. To achieve good beamforming performance, array calibration was performed in each channel to adjust the amplitude, frequency and phase of the tone signal. Using a field-programmable gate array, wideband LFM signals and finite impulse response filters with continuously adjustable time delays were implemented in a polyphase structure. Simulations and experiments verified the feasibility and effectiveness of the proposed digital beamformer.

Optimum Signal Shaping for MIMO Radar System

signal and hence, spatial diversity gain is obtained. A MIMO radar system, transmit via its diverse antennas multiple probing signals that is used to detect targets. The receiving antennas combined together form independent units. Their signals can be processed to make use from the diversity gain and better identifies the targets. In this paper, optimal probing signals are derived and its performance is simulated. MIMO radar can transmit different orthogonal signals that lead to signal diversity. Results showed good utilization of spatial diversity and better timing detection when using the optimal signals over the conventional phased array radar.