Study On MIMO Radar Transmit Beampattern And Waveform Design Method

Multiple-Input Multiple-Output(MIMO) radar consists of multiple transmit antennas and multiple receive antennas. Each transmit antenna can freely choose the transmit waveform, which provide MIMO radar with the advantage of waveform diversity. In general, MIMO radar systems can be classified into two categories according to the configuration of antennas: MIMO radar with widely separated antennas and MIMO radar with colocated antennas. The traditional radar transmits invariable waveform and scans a single narrow beam in the interesting space. In the complex and dynamic environment, the working mode of the traditional radar cannot obtain the satisfactory performance, and cannot meet the requirement for multi-function and multi-task. The waveform diversity supplies more degrees of freedom to MIMO radar. As a result, MIMO radar can freely design the transmit beampattern and adjust the transmit waveform according the operating environment and the requirement. Therefore, MIMO radar transmit beampattern and waveform design become a meaningful research field. This dissertation focuses on the waveform design for MIMO radar, including transmit beampattern design, range sidelobe suppression of the synthesized signals, waveform design with multi-function, clutter suppression and interference rejection. The main content of this dissertation is summarized as follows.1. Transmit beampattern design for MIMO radar is studied in two aspects, i.e. the orthogonal waveform and the waveform with specific beampattern. Firstly, an orthogonal waveform cyclic design method based on gradient descent is introduced. The optimization criterion is to minimize the weighted integrated sidelobe level. By using the gradient information, each waveform is circularly optimized one by one. The performance of the proposed algorithm is similar to that of the sequential quadratic programming(SQP) based on the criterion of minimizing peak sidelobe level. Furthermore, the proposed algorithm can be applied to large scale array. For the beampatten with multiple beams, the minimum sidelobe beampattern design model is modified to obtain the desired beampatten and the low sidelobes. By changing the permutation of the transmit samples and optimizing the initial phases, the auto-correlation sidelobes and the cross-correlation can be suppressed effectively. Finally, a MIMO radar waveform design method based on joint spatial and temporal optimization is proposed. The optimization model is identical for orthogonal waveform and the waveform with specific beampattern. By using the proposed method, the desiredbeampttern can be realized approximately, and the auto-correlation sidelobes of the signals in the directions of interest can be effectively suppressed.2. MIMO radar waveform design for realizing simultaneous multi-function is preliminarily analyzed and studied. Firstly, a multi-mode waveform design method for MIMO radar is proposed. The optimization criterions include beampattern match, power spectrum match or frequency spectrum match at the beam directions. The optimized waveform can form multiple beams at the desired directions. Furthermore, the signals at the beam directions have different properties to simultaneously realize different functions(i.e. searching, imaging and tracking). In the scenario of multiple targets, the traditional radar adopts the approach of beam-switching for each target imaging. This approach may decrease the illuminating time on each target and lead to the degradation of Doppler resolution. Therefore, a wideband waveform design method for MIMO radar is proposed. This method can realize the desired multiple beams and the desired power spectrums at the beam directions. By utilizing the Chirp-Z transform(CZT) to evaluate the discrete frequency spectrum of the spatial synthesized signal, the computational cost is apparently decreased. To reduce the mutual interferences among the echoes from different targets without the loss of range resolution, the desired power spectrums at the beam directions are distributed over multiple unoverlap bands. Based on the designed waveform, multitarget imaging can be implemented by using sparse reconstruction methods.3. As targets informations change continuously in the actual scenario, the beam directions and the powers allocated to different targets should be adjusted timely. Therefore, a fast waveform design method based on iterative FFT is proposed. The waveform design procedure consists of two steps: the fast beampattern synthesis and the spatial synthesized signal optimization. Firstly, the waveform is designed to meet the desired beampattern based on the concept of iterative FFT. Then the initial phases are optimized to improve the auto-correlation properties of the spatial signals based on the criterions of minimizing the integrated sidelobe level(ISL) and weighted ISL(WISL). The dominated computations of the proposed algorithm can be complemented by FFT/IFFT operation, and thus can be performed in real-time.4. To suppress the range sidelobe of the spatial synthesized signal, two approaches are proposed, i.e. the pulse train coding and the mismatch filter design. For the static or lowspeed target, the approach of pulse train coding can effectively suppress the range sidelobe without changing the beampattern. For the high speed target, a MIMO radar mismatch filter based on the instrumental variable is adopted. Under a certain constraint on SNR loss, this mismatch filter can minimize the range sidelobe with Doppler frequency shift near zero. This approach can widen the suppression scope(in Doppler dimension) for range sidelobe and is robust to Doppler frequency.5. Aiming at interference rejection and clutter suppression, the cognitive transmit for MIMO radar is preliminarily studied. An approach of adaptive transmit nulling is proposed to adaptively null the power toward the interferences. Based on the received data, the interference subspace can be constructed. By adjusting the waveform matrix to be orthogonal to the interference subspace, the beampattern can be adaptively nulled at the interference directions. For the rapidly moving interference, the derivative constraints or the covariance matrix taper approach can be used to extend the interference subspace and then widen the nulls. To suppress the clutter in the non-homogeneous environment, a beampattern design method based on a priori information is proposed. Firstly, an orthogonal waveform is transmitted to acquire the clutter distribution information. Then, the clutter power at the receive array can be approximated by the average correlation between the transmitted waveform and the echo of the orthogonal waveform. Finally, the waveform is optimized based on the model of maximizing the SCNR(Signal-to-Clutter-plus-Noise Ratio). The numerical results show that the optimized waveform can efficiently increase the SCNR of echoes in the spatial non-homogeneous environment of strong clutter.