Huaizong Shao

Range-Angle Localization of Targets by A Double-Pulse Frequency Diverse Array Radar

Phased-array radar is widely used in localizating non-cooperative targets, but the range and angle of targets cannot be directly estimated from its beamforming output due to an inherent range ambiguity, i.e., two-dimensional localization of non- cooperative targets cannot be obtained directly from conventional linear phased-array radar beamforming peaks. This paper proposes a simple range-angle localization of targets by uniform linear array (ULA) double-pulse frequency diverse array (FDA) radar. The FDA transmits two pulses with zero and non-zero frequency increments, respectively. The azimuth angle and slant range of targets are then estimated directly from the beamforming output peaks. This approach can be interpreted as detecting the targets in angle dimension and then localizing them in range dimension by properly choosing the frequency increment. Moreover, multiple FDA radars can work as netted radar networks, in which distinct frequency increments or orthogonal waveforms are employed. The localization performance is examined by analyzing the Cramér-Rao lower bound (CRLB) and numerical mean square error (MSE). The effectiveness is demonstrated by simulation results.

Nonuniform Frequency Diverse Array for Range-Angle Imaging of Targets

Although phased-array antennas are widely used in communication, radar, and navigation systems, its beampattern is a function of angle only and thus there is no range information. To circumvent this limitation, we design a frequency diverse array (FDA) antenna system for range-angle imaging of targets. Our approach exploits the nonuniform FDA as the transmitter to provide range-dependent beampattern and the uniform phased-array as the receiver which results in angle-dependent beampattern. Range-angle imaging of targets is achieved from the cooperative transmit-receive beamforming. The imaging performance measures including spatial resolution and system processing gain are analyzed. In addition, several design specifications are discussed. The effectiveness of the proposed approach is verified by simulation results.

Range-Angle-Dependent Beamforming by Frequency Diverse Array Antenna

This paper proposes a range-angle-dependent beamforming for frequency diverse array (FDA) antenna systems. Unlike conventional phased-array antenna, the FDA antenna employs a small amount of frequency increment compared to the carrier frequency across the array elements. The use of frequency increment generates an antenna pattern that is a function of range, time and angle. The range-angle-dependent beamforming allows the FDA antenna to transmit energy over a desired range or angle. This provides a potential to suppress range-dependent clutter and interference which is not accessible for conventional phased-array systems. In this paper, a FDA radar signal model is formed and the range-angle-dependent beamforming performance is examined by analyzing the transmit/receive beampatterns and the output signal-to-interference-plus-noise ratio (SINR) performance. Extensive simulation examples and results are provided.

Frequency Diverse Array Radar Cramér-Rao Lower Bounds for Estimating Direction, Range, and Velocity

Different from phased-array radar, frequency diverse array (FDA) radar offers range-dependent beampattern and thus provides new application potentials. But there is a fundamental question: what estimation performance can achieve for an FDA radar? In this paper, we derive FDA radar Cramer-Rao lower bounds (CRLBs) for estimating direction, range (time delay), and velocity (Doppler shift). Two different data models including pre- and postmatched filtering are investigated separately. As the FDA radar has range-angle coupling, we use a simple transmit subaperturing strategy which divides the whole array into two subarrays, each uses a distinct frequency increment. Assuming temporally white Gaussian noise and linear frequency modulated transmit signal, extensive simulation examples are performed. When compared to conventional phased-array radar, FDA can yield better CRLBs for estimating the direction, range, and velocity. Moreover, the impacts of the element number and frequency increment are also analyzed. Simulation results show that the CRLBs decrease with the increase of the elements number and frequency increment.

A Flexible Phased-MIMO Array Antenna with Transmit Beamforming

Although phased-array antennas have been widely employed in modern radars, the requirements of many emerging applications call for new more advanced array antennas. This paper proposes a flexible phased-array multiple-input multiple-output (MIMO) array antenna with transmit beamforming. This approach divides the transmit antenna array into multiple subarrays that are allowed to overlap each subarray coherently transmits a distinct waveform, which is orthogonal to the waveforms transmitted by other subarrays, at a distinct transmit frequency. That is, a small frequency increment is employed in each subarray. Each subarray forms a directional beam and all beams may be steered to different directions. The subarrays jointly offer flexible operating modes such as MIMO array which offers spatial diversity gain, phased-array which offers coherent directional gain and frequency diverse array which provides range-dependent beampattern. The system performance is examined by analyzing the transmit-receive beampatterns. The proposed approach is validated by extensive numerical simulation results.

Adaptive Frequency Offset Selection in Frequency Diverse Array Radar

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.

Optimal Frequency Diverse Subarray Design With Cramér-Rao Lower Bound Minimization

This letter proposes an optimal subarray design strategy for frequency diverse array (FDA) radar target localization via Cramér-Rao lower bound (CRLB) minimization. To decouple the range and angle response of targets, we divide the FDA transmit array elements into multiple subarrays. For a given number of array elements and subarrays, the optimal array division and frequency increments design strategies are formulated as a constrained CRLB minimization problem. Then it is converted into an unconstrained optimization problem and further resolved by iteratively utilizing the Nelder-Mead algorithm. Numerical results show that, under the same condition, FDA radar localization performance is significantly improved by employing the CRLB minimization strategy.

Dot-Shaped Range-Angle Beampattern Synthesis for Frequency Diverse Array

The frequency diverse array (FDA) using linearly increasing frequency increment yields an “S”-shape range-angle beampattern, which provides the potential capability to suppress range-dependent interferences. However, the FDA transmit beampattern is coupled in range and angle dimensions, which may degrade the output signal-to-interference-plus-noise ratio performance. In this letter, we propose a symmetrical FDA beampattern synthesis approach using multicarrier frequency increments and convex optimization, named convex-multilog-FDA, to achieve dot-shaped transmit beampatterns. Both single-dot and multi-dot shaped beampatterns can be synthesized. Numerical results show that, the proposed approach outperforms the existing log-FDA using logarithmically increasing frequency increments in transmit energy focusing, sidelobe suppression, and array resolution performance.

Two-Antenna SAR With Waveform Diversity for Ground Moving Target Indication

Along-track interferometry (ATI) and displaced phase center antenna (DPCA) are two representative synthetic aperture radar (SAR) ground moving target indication (GMTI) techniques. However, the former is a clutter-limited detector, whereas the latter is a noise-limited detector. In this letter, we propose a two-antenna SAR with waveform diversity for GMTI. The two antennas placed in the azimuth dimension simultaneously transmit two orthogonal waveforms, namely, the up- and down-chirp waveforms. In doing so, four independent transmit-receive channels are obtained for the receiver, and thus, an efficient GMTI is developed by cooperatively utilizing the ATI GMTI and DPCA GMTI processing techniques. This method first uses the DPCA technique to cancel clutter and then the ATI technique to suppress noise. Next, the fractional Fourier transform is employed to estimate the Doppler parameters and corresponding reference filters are designed to focus the moving targets. The proposed approach cooperatively utilizes the advantages of ATI and DPCA GMTI techniques and overcomes their disadvantages. It does not prerequire knowledge of the targets across-track velocity. The effectiveness is verified by simulation results.

Large time-bandwidth product OFDM chirp waveform diversity using for MIMO radar

Multiple-input multiple-output radar waveform diversity design has received much attention, but many of existing waveforms are lack of a large time-bandwidth product. This paper proposes an orthogonal frequency division multiplexing (OFDM) chirp waveform diversity design scheme to generate four waveforms with good correlation properties, ambiguity performance and Doppler tolerance. The method jointly utilizes classic chirp waveform and OFDM to conduct multi-carrier modulation. This scheme enables us to exploit full bandwidth for each waveform and thus has an high frequency utilization efficiency. The modulation and demodulation of the four OFDM chirp waveforms are presented. The performance of the designed waveforms is analyzed by the correlation and ambiguity functions. Simulation results validate that our proposal can generate more orthogonal waveforms without significant performance degradation.