In this letter, we devise two moving sampling schemes for two-dimensional (2D) direction-of-arrival (DOA) estimation using sparse planar arrays. The proposed schemes are applied to two different kinds of sparse planar arrays on a moving platform. By designing the appropriate motion trail, the proposed schemes can joint the uniform rectangular parts of the synthetic difference coarray together to yield a larger uniform rectangular array (URA). Compared with the traditional virtual array construction, our schemes can generate a larger virtual URA, and thus result in a significant increase in the number of degrees of freedom (DOFs). Numerical results demonstrate the advantages of the proposed schemes in the number of uniform DOFs and DOA estimation accuracy.

In this letter, we develop an effective algorithm for direction-of-arrival (DOA) estimation of coherent sources with coprime arrays. Firstly, we generate a virtual uniform linear array (ULA) through coprime array interpolation. Subsequently, we derive an augmented noise-free covariance matrix that is constructed using the entries of the covariance matrix of the virtual ULA outputs, and recover the Hermitian Toeplitz matrix by solving an atomic norm minimization problem with multiple measurement vectors. Finally, the DOAs of coherent sources are estimated via MUSIC spectral search. Unlike the state-of-the-art approach, our algorithm is insensitive to the phase differences among sources. Numerical results demonstrate the superiority of the proposed algorithm over several existing techniques.

Range ambiguity is a serious problem existing in the wide swath SAR imaging due to the impact of high pulse repetition frequency. To address this problem, this paper proposes a new imaging configuration via using the frequency diverse array (FDA) antenna. By designing the frequency increment along array elements and adopting the sub-array technique, FDA can easily allocate its beam to the corresponding unambiguous block, which outperforms the conventional methods. The FDA generated range-dependent phase shift can be used to extract the signal from different range block, accomplishing range unambiguous focusing. Simulation results are provided to demonstrate the effectiveness of the proposed approach.

Frequency diverse array (FDA) employs frequency offsets along its array element, thus generating a range-dependent beampattern, giving it the capability to deal with the range-dependent deceptive jamming, which cannot be achieved by the traditional phased array. However, the range and angle are coupled in conventional FDA. To address this problem, the frequency offsets are optimized in this paper with particle swarm optimization to focus the beam energy to the interested spot, facilitating the following imaging process. The effectiveness of the proposed method is validated by numerical simulations.

In this letter, we develop a coarray interpolation method for direction-of-arrival (DOA) estimation using the coprime electromagnetic vector-sensor (EMVS) array. Firstly, we derive the coarray signal model of the coprime EMVS array, which can be viewed as a combination of multiple polarization components. Subsequently, we fill in zero elements in each polarization component and recover the corresponding low-rank covariance matrix by solving a nuclear norm minimization (NNM) problem. By exploiting the recovered covariance matrices, we eventually construct a larger covariance matrix to carry out DOA estimation. Numerical experiment results demonstrate the superiority of the proposed algorithm over conventional methods.

In this study, the authors develop a new algorithm for passive source localisation using time‐difference‐of‐arrival (TDOA) and frequency‐difference‐of‐arrival (FDOA) measurements. In this algorithm, the position and velocity of the target source are successively estimated by formulating two alone semidefinite programming problems, where the variances of the TDOA and FDOA measurement noises can be separated and neglected. Unlike the traditional methods, the authors’ approach does not require the prior knowledge of the TDOA/FDOA measurement noise power. Simulation results indicate that the proposed algorithm outperforms the existing methods under unknown noise power knowledge.

In this letter, we propose a sparse array design method for adaptive beamforming in the presence of interferences. Our solution is based on finding the beamformer weight vector such that maximum output signal-to-interference-plus-noise ratio is attained. To control the sidelobes of the beampattern, quadratic fractional constraints are also introduced to optimize the beamformer weights. We formulate the array design problem as real-valued quadratically constrained quadratic program (QCQP) with reweighted l1-norm to promote sparsity. Moreover, we adopt semidefinite relaxation (SDR) and linear fractional SDR together to solve the QCQP problem. The resulting array yields excellent beamforming performance and a beampattern with low sidelobes. Numerical results demonstrate the effectiveness of the proposed sparse array design.

In this letter, we devise an efficient approach for estimating the directions-of-arrival (DOAs) of coherent signals using coprime arrays. Specifically, we firstly derive an augmented uniform linear array (ULA) via coprime array interpolation. Subsequently, we define a Toeplitz matrix that is formed using the correlation information of the interpolated ULA theoretical outputs, and recover the Toeplitz matrix by solving a nuclear norm minimization problem. After the low-rank Toeplitz matrix is recovered, the coherent signals are well resolved by the MUSIC algorithm. Simulation results demonstrate the advantages of the proposed approach over various existing methods when dealing with coherent signals.

In this letter, we focus on the problem of direction-of-arrival (DOA) estimation with a nested array in the presence of unknown mutual coupling. Firstly, the measurement model of the nested array with mutual coupling is established, and the properties of the corresponding array covariance matrix are derived. Based on the obtained properties, the coarray output signal is then reconstructed with reduced mutual coupling. Finally, joint sparse recovery technique is employed to extract the DOAs. Our algorithm can achieve underdetermined DOA estimation with satisfactory performance under unknown mutual coupling. Numerical results demonstrate the effectiveness of the proposed algorithm.

Mosaic synthetic aperture radar (SAR) can flexibly steer its antenna beam, achieving high resolution wide swath (HRWS) imaging. Nevertheless, the conventional imaging algorithm may suffer from spectrum ambiguity and finally lead to defocused result due to data acquisition geometry. In this paper, the imaging geometry of mosaic SAR is introduced analyzed. An imaging algorithm based on the spectrum analysis technique is proposed to handle the mosaic SAR imaging. Simulation results validate the effectiveness of the proposed approach.

Compared to the monostatic geosynchronous synthetic aperture radar (GEO SAR), GEO bistatic SAR can provide finer spatial resolution and higher signal-to-noise ratio (SNR) with lower system complexity, which leads the GEO-LEO (low earth orbit) SAR be a hot research topic. In this paper, an imaging scheme using cooperative high-orbit and low-orbit satellites is proposed. Firstly, the bistatic range history is extended by Taylor series, and then two-dimensional non-uniform fast Fourier transform (2D-NUFFT) is presented to formulate a bistatic SAR image in the range-velocity domain. The effectiveness of the proposed imaging scheme for GEO-LEO bistatic SAR is verified by simulation data for multi-point targets.

In this letter, we devise a subspace updating algorithm for tracking directions-of-arrival (DOAs) using the difference coarray of a sparse array. Our solution is based on the projection approximation subspace tracking (PAST) framework, where we design new cost function and iterative process via spatial smoothing of the difference coarray output signal. Since the proposed spatial smoothing PAST (SS-PAST) algorithm exploits the aperture of the difference coarray, it attains higher DOA tracking accuracy than the conventional PAST method operating in a uniform linear array. Simulation results demonstrate the effectiveness of the SS-PAST algorithm.

Based on the synthetic aperture radar (SAR) geometric model, a novel method of jamming space-borne SAR by utilizing the array beam-pattern characteristics of frequency diverse array (FDA) is proposed. A small frequency increment, as compared to the carrier frequency, is applied between the FDA elements to produce multiple false targets in the final space-borne SAR imaging. The separations between the false targets are determined by the frequency offset. Based on the imaging geometry in side-looking mode, this paper derives a range-Doppler (RD) algorithm for the space-borne SAR imaging under FDA jamming signals. Simulation results demonstrate the effectiveness of the proposed approach.

Millimeter-wave (mmWave) wireless communication system allows a compact antenna array with a large number of colocated elements, which enables flexible array design and beampattern synthesis. However, security is also particularly important but challenging for the fifth-generation (5G) wireless communications, as the broadcasting nature allows any receivers to have a copy of the transmitted signals. In this paper, we propose a hybrid multiple-input multiple-output (MIMO) phased-array time-modulated directional modulation (DM) scheme for physical layer security in mmWave wireless communications. The hybrid MIMO phased-array enjoys the advantages of MIMO in spatial diversity without sacrificing the main advantage of phased-array in coherent directional gain. The essence is to divide the transmit array into multiple subarrays. Each subarray can be used to form a directional beam, while all subarrays are jointly combined to work as an MIMO for higher angular resolution capabilities. More importantly, a time-modulated DM scheme is applied for the phased-MIMO to achieve physical layer security for mmWave wireless communications, even without the knowledge of the eavesdropper's position information. The feasibility of the proposed scheme is verified by numerical results.