فصلنامه رادار
سال ششم شماره 1 (پیاپی 19، بهار و تابستان 1397)

In this paper, the problem of joint optimization of transmit–and-receive Beamforming vectors in array radar is considered. Here, our goal is to maximize the signal-to-interference-plus-noise ratio (SINR) for a point like target in the presence of multiple signal dependent interferences. Therefore, we propose an iterative algorithm, which is based on the convex optimization solution and in each iteration of it, the constrained transmit beamformer is designed by a relaxation and randomization technique. In addition to power constraint, the constant envelop constraint on the transmit beamformer is considered which is important in practice. Simulation results show that the proposed method is able to achieve a higher SINR in comparison with the conventional phased array radar. it is also possible to get close to the performance of non-constrained method by considering the constant envelop constraint.

In microwave oscillators, the quality factor of the resonators decreases with an increasing frequency. There exist some methods for compensating the losses in the resonators. The push-push configuration is able to improve the oscillator phase noise. In this work, a push-push voltage-controlled oscillator utilizing a parallel coupled resonator has been designed for the frequency range of 1200-1400 MHz. The push-push oscillator consists of two similar Collpits oscillators, which are designed for the frequency range of 600-700 MHz. These sub-oscillators are coupled by a phase coupling network and with 180 degrees phase difference. The in-phase combiner combines the even harmonics of each sub-oscillator and cancels the fundamental and odd harmonics. The overall oscillation of the push-push oscillator consists of the second and even harmonics of the sub oscillators. By this technique, the phase noise is improved 9 dB compared to the case of the stand-alone oscillation of each sub circuit at the output frequency.

Space-Time Adaptive Processing (STAP) is a fundamental technique in improving the performance of radars which are acted in the presence of severe dynamic disturbances such as clatter and jamming. STAP operation is based on very high sampling rates from signals received simultaneously from several antenna arrays and a number of pulses. Therefore, the volume of computation is very high and its implementation is difficult. In this paper, multi-vector method is proposed to reduce the amount of STAP calculations. Implementation results show that the proposed method, in addition to reducing the computational volume, leads to reduced hardware resources, reduced power consumption and increased maximum operating frequency. For example, the calculation volume for 6 antenna arrays, 10 receiving pulses and 200 range samples with a vector size of 17 is obtained 28.1 GFLOPS with maximum frequency of 278 MHz, and the computation time of 262 μs on the Arria 10 chip. Therefore, the multi-vectoring method can meet the real-time requirements of STAP weights. Angle-doppler adaptive pattern and the static test for the target detection indicate that using of the above method is very practical for calculating weights.

In this paper, a method is proposed to compensate the effect of mutual coupling in a Uniform Circular Array (UCA) with directional antennas. UCA is a suitable array geometry, because it leads to uniform precision of Direction of Arrival (DoA) estimation over the entire range of azimuth angles in [0,360] degree. However, the mutual coupling effect of a UCA can be much stronger than that of a uniform linear array (ULA). In this paper, we will show that using directive antennas will improve the accuracy of direction of arrival estimation. In order to reduce the effect of mutual coupling on the performance of direction of arrival estimation, an auto-calibration method will be proposed. In the proposed algorithm, direction of arrival and mutual coupling matrix are estimated using an iterative method. Simulation results show that a UCA with application of the proposed algorithm has a better performance in comparison to the array with isotropic antennas at the presence of mutual coupling.

In non-coherent Direction Of Arrival (DOA) estimation, the goal is to determine DOA based only on the magnitude of the received sensor array signal. The advantage of the non-coherent DOA estimation is its robustness against phase errors; despite phase errors present in both sensors and phase shifters, direction of arrival can be estimated. In this paper, DOA is estimated using a frequency estimation technique which can be simply implemented by discrete Fast Fourier Transform (FFT) methods. In addition, for removing the ambiguity and solving the nonlinear equations, a reference target with high power emission is used. Simulation results, in both linear and plane array cases show the efficiency and robustness of the proposed algorithm against phase errors.

Due to numerous capabilities of polarimetric radar images as a source of strategic information, their use in military and commercial applications is rapidly growing. One of the important topics of interest to researchers is the classification of these images. So in this paper, the structure of an ensemble of classifiers based on sparse representation technique is presented which classifies adaptively two sets of border and non-border pixels in a polarimetric image. In this plan, various aspects such as base classifiers and their diversity, ensemble of classifier structure, reliability, combination rule, polarimetric and texture features and contextual information are considered. For designing the combination rule of base classifiers, a method based on reliability parameter is proposed. Also, a post-processing stage is used for reducing the unwanted discretion in the output of the ensemble of classifiers. Implementation of the proposed algorithms on a benchmark PolSAR image, demonstrates their superiority over that of traditional techniques.