A standoff 3-D imaging radar for security body scan at 340 GHz has been described. This terahertz imager incorporates a multiple input multiple output (MIMO) array as an electronic beam former in a horizontal dimension and an elliptic cylinder as a focusing reflector in elevation. Frequency-modulated continuous-wave (FMCW) emission with 16-GHz bandwidth caused a 10-mm-range resolution after calibration for nonlinearities of the distorted radar signals. The horizontal MIMO array contains 4 transmitters and 16 receivers (4Tx-16Rx) to generate a 64-Tx/Rx full virtual array corresponding to an antenna size of approximately 128 mm, which caused a horizontal resolution of about 14 mm at a distance of 3 m. FMCW signals were transmitted from the 4-Tx sequentially and focused as narrow beam lines by the elliptic reflector to get a 12-mm vertical resolution at 3 m. A fast-scanning mirror swings in the light path to make these beam lines screen in the vertical direction. A 4-Hz frame rate is now possible for a personal screen with a field of view of 2 m × 0.6 m.

Based on a novel prototype terahertz screening system, its ability of fast three-dimensional (3-D) image reconstruction is demonstrated. We first give a brief introduction of the system. Then detailed analyses on signal model, system calibration, the influences of approximations, and the imaging performances are conducted. At last, proof-of-concept experiments are performed and preliminary 2-D and 3-D imaging results are presented. We show that the imaging problem can be resorted to a problem of spectral estimation finally. At this stage on this prototype system, a 180 cm$\,\times\,$90 cm region at 4-m range costs less than 0.1 s for 2-D imaging, 1 s for 3-D imaging and an extra 2 s for mechanical scanning. Faster speeds are on the horizon as hardware and software updates are ongoing. These results show its potential of real-time 3-D imaging for standoff concealed threats detection.

A reconfigurable sparse array radiates multiple patterns from a single array while using fewer array elements through the design of the array structure. To address this array design problem, a new consensus penalty dual decomposition (CPDD) method was proposed in this work based on the concept of consensus computation. The CPDD method is a double-loop iterative algorithm. It employs a virtual star-shaped topology graph network. The central node of the network corresponds to the global variable, i.e. the common array structure, while each of the edge nodes corresponds to a single beam synthesis task. In the outer loop, the algorithm collects excitation vector information from the edge nodes through the center node and then integrates the information to obtain a better common array structure. In the inner loop, each beam is synthesized locally in the corresponding edge node based on the information provided by the central node. Finally, all edge nodes achieve a consistent array structure. Some numerical simulations were performed to verify the effectiveness of the algorithm in different situations. They show that the proposed method CPDD has good performances. It can achieve a satisfactory radiation pattern with fewer antennas in different application cases.

In this work, a new two-stage penalty dual decomposition (TSPDD) method is devised for the power mask-constrained sparse array beampattern synthesis problem. Unlike the usual approximation of l ₀ -quasi norm with weighted l ₁ norm or l_p -quasi (0< p <1) norm, we directly utilize the objective function of l ₀ -quasi norm to solve this problem without any approximation, so it can obtain a sparser solution than some other method. The TSPDD solve the augmented Lagrangian problem via dual decomposition (DD) type methods. It is an iteration algorithm. In the first stage, we update the penalty parameters while in the second stage we update the dual variables. Furthermore, in order to further enhance the sparsity of the solution, we extended the algorithm TSPDD to E-TSPDD by discarding the array elements with smaller weights and replacing two close array elements with one. Finally, we considered the application of the proposed method in the sparse array synthesis problem with mutual coupling. Numerical examples show that the proposed methods have good convergence and performances. It can achieve satisfactory radiation pattern with fewer antennas than the existing techniques and can handle the mutual coupling effect in the sparse array synthesis task.

In this work, the synthesis of partially subarrayed linear array is studied to obtain as few subarrays as possible. For the partially clustered array (PCA), the amplifiers are placed at the subarray level while the phase shifters are placed at the element level. By performing a matrix transformation on the equivalent excitation amplitude vector, the original synthesis problem of PCA is reformulated as a minimization problem of l0-norm. It is nonconvex and cannot be solved by the existing convex methods. Then, a new penalty dual decomposition (PDD) framework is proposed to address the task. The new approach can directly optimize the subarray number and simultaneously guarantee that the elements of subarrays are physically contiguous, while the excitation matching (EM) methods cannot. Numerical simulations show that the proposed method can get as few subarrays as possible in the synthesis of PCA.

In this paper, the array imaging process with the presence of signal leakage between channels is derived and analyzed in short-range imaging applications. The echo wave of the system can be divided into two parts: the echo signals we expected, and the echo signal that resulted from the leaked signals. Signals leaked to the adjacent channels can lead to ghosts which can deeply deteriorate the system cross resolution. To solve this problem, a concise calibration method is proposed to remove the impacts of the signal leakages on the imaging quality. Furthermore, the results based on the simulation and the real-world data validate the effectiveness of the proposed method, which indicates that this calibration method can mostly cancel the influence of the signal leakages.