Research On The Algorithms And Applications For Digital Beamforming In Wideband Digital Array

Multifunction digital array radar, which employs a single radar system to obtain the tactic functions of multiple radars, is very important for the development of future phase array radar. High range resolution is required to enable the multifunction digital array radar for target identification and target imaging. Thus, the operation frequency of the phased array radar should be wideband.Wideband digital array radar makes the realization of the multifunction phased array radar possible. In the wideband digital array radar, time delay elements are used instead of the phase shifters used in the traditional phased array radars, such that the aperture effect can be eliminated and wideband wide scan can be implemented. Further, the wideband digital array radar has the advantages of very high dynamic range, facilitating transmitting waveforms, and frequency agility.Wideband digital beamforming is the key technique for the wideband digital array radar. Wideband beamforming requires replacing phase shifters used in the traditional phased array radars by the time delay elements. On the other hand, to form a null in the direction of interference to improve the system performance, adaptive beamforming algorithms and techniques are needed, which are different from those used for narrowband adaptive beamforming.Towards the application of wideband digital array radar, the research on digital time delay, adaptive wideband interference nulling, and other related techniques are of significant theoretical and practical importance. Of course, they are emergent topics in the current research of array signal processing.This thesis studies the beamforming for the wideband digital array radar. The research includes:1. Wideband digital beamforming based on fractional time delay is investigated. When direct RF sampling is adopted, the beamforming scheme based on time delay is discussed. Three types of fractional time delay approaches are applied to the wideband beamforming. The performance and complexity of these approaches are analyzed and compares. The complexity of applying fractional time delay to all array elements is shown.2. Beamforming based on sub-array level is discussed. Although fractional time delay can achieve accurate digital time delay and obtain the desired array beam pattern, when the number of array elements is very large, as in some practical phased array radars, employing fractional time delay for each array element seems impossible due to the problems faced in the data transmission and storage and high speed filtering. In this case, we have to resort to the beamforming scheme in the sub-array level. The principle and approach for dividing subarrays are studied. The performance of wideband beamforming based on applying phase shifters within sub-array and fractional time delayer between sub-arrays is analyzed and illustrated by simulations.3. Wideband adaptive beamforming algorithms are studied. The performances of the wideband adaptive beamforming algorithms, particularly the ISM and CSM algorithms, used in the wideband digital array radar system, are analyzed, simulated, and compared. The performance of these algorithms under different operation frequencies, operation bandwidths, coherent interferences, and array configurations are investigated through simulations. The discussions on the impacts of the number of snapshots, sampling frequency, and FFT point on the beamforming performance are provided, which is useful for the test system design.4. The possible realizations of the wideband digital array radar and the corresponding beamforming methods are investigated. Three types of wideband signals are analyzed and compared, including instantaneous wideband signals, wideband linearly modulated signals, and frequency stepped signals. The advantages, difficulties and existing problems of the realization of wideband digital array radar system based on the direct RF sampling and IF sampling are studied. A wideband beamforming approach based on dechirping is proposed. Further, the impact of the multi-channel non-synchronized sampling on the beamforming performance is studied, which provides a theoretical reference for the hardware and waveform selection for designing the wideband digital array radar system. Additionally, the application of the array signal processing in MIMO communications system is studied in this thesis. MIMO technique is a hot topic in recent years, which is one of the key techniques of the next generation wireless communications systems (Beyond 3G).We focus on the application of the nonlinear detection algorithms in MIMO communications system. Multiple received signals in the MIMO system are combined using nonlinear algorithms. Taking the advantage of the higher-order information, the detection performance of the MIMO system is improved. To combat the drawbacks of the nonlinear algorithms in computational complexity, two methods based on incremental method and Fisher rate method are proposed for reducing the computation load of the nonlinear algorithms, which also lead to improved bit error rate performances than the traditional linear detection algorithms used in MIMO systems.